pkboatplans.com Yacht Design Blog

The STP 65 Class

2012-02-24T05:47:00.002-08:00

The STP 65 class of racing yachts are truly impressive boats. I saw one of these out of the water recently and I have to say the most distinctive feature is the very deep draft with kelp cutters on the leading edge of both the keel and rudder. They are designed and built to a box rule with upper and lower limits on hull and rig measurements. My reading of the 2006 edition of the box rule indicates the box is tight enough to make the STP a virtual one design class. A true one design class, like the Star, has tolerances built into the class measurements for hull, keel, rudder, and rig that give the builder some wiggle room. I remember when building Star boats in the 1970's we would always hit the upper or lower limit of the measurement tolerances to maximize the speed potential of the boat. The same thing will happen under a tight box rule, and all the boats built under the box rule will end up with the same hull and rig dimensions. Because of this, I believe just about any competent naval architect can survey the existing fleet and design a competitive STP 65 to the box rule after consulting with the usual experts in the areas of structural, appendage, rig, and hydrodynamic design, even with no prior experience in designing racing yachts. I think the builder is faced with a much larger challenge than the designer because the boats must be built to very tight tolerances with state of the art materials and techniques to be competitive.

One of the objectives of the STP 65 box rule is "to create state of the art rule parameters that maintain tight restrictions with regard to construction and design, yet allow some flexibility for each owner to customize the yacht in their own image." An apparent Freudian slip revealing that yacht racing at the highest levels is, was, and always will be characterized by megalomania of biblical proportions.

Husband and Wife Spinnaker Sailing

2012-02-22T20:23:00.006-08:00

Sailing with traditional symmetrical spinnakers with spinnaker poles and associated afterguys, foreguys, sheets, and topping lifts just isn't done much anymore, and I will probably give in to the simplicity of an asymmetrical spinnaker sans pole on my next boat, now under construction. However, I always enjoyed sailing downwind with the main all the way out and spinnaker pole all the way back. My previous boat (sister to the Marie J) was set up so my wife and I could set, gybe, and take down the spinnaker with just the two of us relatively easily. I had a unique setup that allowed us to use the spinnaker when shorthanded or with novices on board that worked very well. Anyway, what follows is how a husband and wife sailing team can sail with a traditional spinnaker without ending up with a sail handling fiasco and/or divorce proceedings. This will work on masthead boats up to about 30 feet and fractional rigged boats up to about 35 feet.

SETUP. The headsails need to be set with hanks. Headfoils are no good for shorthanded sailing because the jib luff is not contained when lowering or raising the sail, and roller furling is ... well, it's just wrong in my opinion. Halyards and pole lift need to be led aft to the cockpit through rope clutches. The foreguy should be double ended and led to both sides of the cockpit. Use lazy guys and sheets. This is the most important element for gybing with just two people. On our boat we led the afterguys through the jib sheet leads, which had wide sheaves, to the primary winches. The mainsheet was double ended and led to the winches on the cabin top that also doubled as halyard winches. When the spinnaker was up, the spinnaker sheets went to the cabin top winches with the mainsheet and halyards stopped with rope clutches. The spinnaker itself should be packed in a "turtle" that can be clipped to the forestay and placed forward of the forestay in the bow pulpit.

SETTING THE SPINNAKER. All maneuvers involving the spinnaker with the husband and wife team should be done with boat sailing dead downwind. Everything is easier that way. Set up for a "bear away" set with the spinnaker pole to windward. The foredeck person sets the turtle in the pulpit and hooks up the sheets, guys, and spinnaker halyard. The foredeck person connects the pole to the mast and connects the pole lift, making sure the windward jib sheet is over the top of the pole and forward of the pole lift. The afterguy and foreguy are connected. The helmsman raises the pole from the cockpit. At this point the boat should be headed dead downwind. The foredeck person comes back to the cockpit and raises the spinnaker while the helmsman brings the pole back and trims the spinnaker sheet. The foredeck person releases the jib halyard, and the jib falls on the deck in the lee of the main. This is why you need hanks. The foredeck person tidies things up forward and the helmsman surveys everything to make sure no lines are fouled.

GYBING THE SPINNAKER. With lazy sheets and guys, gybing end for end is easy. The helmsman controls the gybe by keeping the boat headed absolutely dead downwind throughout the process. I like having a tell-tale on the backstay just over my head so I can check wind direction by glancing upward. The rule of thumb for keeping the boat headed downwind is to "follow the chute" and keep the bow under the spinnaker. The gybe begins with the helmsman taking up the lazy sheet and easing the after guy. Sometimes it makes things easier if the pole lift is lowered a bit. The spinnaker is now being flown with both sheets and there is no load on the afterguys. The foredeck person disconnects the pole from the mast and drops the lazy guy into the end, taking care to pass the pole under the leeward jib sheet in the process. The foredeck person takes out the now slack after guy and hooks the new end to the mast. While the foredeck person is gybing the pole, the helmsman steers the boat by-the-lee and brings the main across. The helmsman takes up the after guy and eases the old, now lazy, sheet. What used to be the lazy sheet is already trimmed as the new spinnaker sheet and the old after guy has already been eased and is the new lazy guy. The pole lift is raised if need be.

TAKING DOWN THE SPINNAKER. As with other husband/wife spinnaker maneuvers, the boat should be headed dead downwind for the take-down. Raise and trim the jib. The helmsman eases the afterguy until the pole is just off the headstay and lowers the pole as needed to put the end within reach of the foredeck person. The foredeck person pulls the pin on the afterguy shackle, releasing the spinnaker in the lee of the jib. The foredeck person moves aft and on the lee side near the companionway hatch, pulls the clew in with the lazy guy, and gathers the foot of the spinnaker before the halyard is released. There is no load on the spinnaker as long as the boat stays headed downwind and the spinnaker is in the lee of the jib. Once the foot of the spinnaker is gathered, the helmsman releases the spinnaker halyard and the foredeck person shoves the spinnaker down the companionway hatch. With a little practice and proper timing of the halyard release, the husband/wife team should be able to douse the spinnaker without getting it wet. The foredeck person hooks the spinnaker halyard to the leeward lifeline, taking care not to foul the jib sheet. The helmsman lowers the pole end to the deck. At this point the boat can be brought up to a higher point of sail, and the boat can be tacked if necessary since the windward jib sheet has been kept above the pole and forward of the pole lift. Once the boat is settled on the desired course, the foredeck person stows the pole and cleans up the spinnaker gear.

The key to success with the two person crew is doing things one at a time in sequence and keeping the boat dead downwind during the spinnaker manuevers. My wife was a natural foredeck person, having been a competitive board sailer. However, with the lazy guy approach there is no load on the spinnaker pole during gybes, and she was able to execute the manuever while eight months pregnant with ease. We took pride in our sail handling, and even though we were only casual racers, we never raced in a "no spinnaker" division.

The Bird Shot Keel

2012-02-10T04:30:00.000-08:00

Keels cost too much. I read that one builder budgets $2.75 a pound to have a lead keel made plus $1.75 a mile to have it shipped, and I tend to believe those numbers. This is more than I want to spend for my own boat, so I looked into alternatives. Cast iron is out because of the rust issue and the cost of having a foundry produce the keel. Quite a few amateur builders have cast their own lead keels, having collected tire weights and scrap lead for the purpose. These keels are invariably long and shallow lead shoes bolted to the bottom of a traditional full keel. Casting a tall foiled fin of the type I need without incurring twist, warp, or bulging in a backyard setting would be a challenge. And good luck obtaining scrap lead in California.

However, reclaimed lead shot is easy to get and its density is 425 pounds per cubic foot which increases to 450 pounds per cubic foot when combined with epoxy resin which is the same as cast iron which is do-able. An idea that has been proposed is to make a styrofoam plug, build a fiberglass shell over the plug with epoxy resin, melt away the styrofoam, then fill the fiberglass shell with lead shot and epoxy. The keel can be finished by glassing over the top, drilling and tapping the top for the keel bolts, and setting the keel bolts with epoxy. No melting the lead or welding the keel bolt cage. Simple and easy. The lead shot is not cheap, but there are no manufacturing or shipping costs, and the fiberglass shell can be filled in several "pours" over time.

Then there's the design issue that involves a bit of math and naval architecture. To achieve the same stability as a boat with a lead keel, the replacement "bird shot" keel must have a lower center of gravity because it has greater volume. The increased volume of the bird shot keel lowers the center of buoyancy and floats the boat a bit higher, both of which reduce metacentric height, or GM (a basic measure of stability). My boat has a GM of 643 mm in the light ship condition. A bird shot keel must have a center of gravity that is 88 mm lower than the lead keel to get the same metacentric height.

I'm going with a retro 1970's Columbia Yachts style fin just because it allows me to have a 39 degree leading edge sweepback and still get the center of gravity in the right place.

The Tiller Head

2012-02-07T20:08:00.000-08:00

You can get anything you want on the Internet. Not only can you get it, but it can be delivered to your front door in a few days. Anything you want ... except a Lewmar tiller head. I wanted to buy a Lewmar tiller head, which is shown among the products on their web site, but I knew it wouldn't be easy getting a relatively obscure small part from a big company. So after determining such a thing is not available online (you can still get heavy bronze tiller heads, and Schaefer used to make a nice one, but no more), I called Lewmar and was told to contact Sailing Supply to order one. No problem. I asked Sailing Supply if they could get me a price for a 35 mm Lewmar tiller head. No problem.

Apparently the tiller head in question is not actually being produced (no surprise at all), but Lewmar will only give Sailing Supply a price to make one if I agree to purchase it without actually knowing what they will charge for making it.

Update Feb 10, 2012: Sailing Supply stayed on this and got me a price and lead time from Lewmar. It's a bit pricey, as expected, but not out of the question. I may wind up getting one yet, cash flow permitting.

Keel Bolts

2012-01-28T19:10:00.000-08:00

The default material for keel bolts attaching the lead ballast to the bottom of a sailboat is type 316 stainless steel. The reason for this is out of all the corrosion resistant metals, stainless steel is the cheapest. It's definitely not the best. Stainless steel is subject to crevice corrosion when it is deprived of oxygen, which is the case when it is underwater. If stainless steel is allowed to sit in stagnant water, it will slowly corrode and eventually fail. Although stainless steel keel bolts have an excellent track record, I believe the primary cause of rudder and rig failures on cruising sailboats is crevice corrosion on stainless steel components.

I'm using C6550 alloy bronze for the keel bolts, rudder stock, and chainplates for my own boat. It's not susceptible to crevice corrosion, it's closer to lead than stainless on the galvanic scale, and it's stronger than type 316 stainless steel. Using bronze in lieu of stainless will cost me hundreds of extra dollars on a project that's on a tight budget, but it is worth it for the piece of mind and reduced likelihood of failure over time.

I've read from several sources that keel bolts should never be put on centerline and I don't agree. The ABS scantling rule for offshore racing yachts explicitly allows for keel bolts on centerline, and they are not precluded by the ISO scantling rule. Principles of Yacht Design has a good explanation of the implementation of the ABS requirements for keel bolts. The engineering principles behind the ABS keel bolt algorithm are very simple, but are best understood by completing a second year statics course followed by a third year mechanics of materials course in an ABET accredited engineering program.

John Phillip Sainty

2012-01-21T07:12:00.000-08:00

"Pearl" designed and built by John Phillip Sainty around 1820

In the late 1700's and early 1800's yacht designing and building began to emerge as a distinct branch of naval architecture and ship construction with the aristocracy of the day ordering boats to be built exclusively for their own pleasure. One such person was the Marquis of Angelsey, who had a fast cutter yacht built by John Phillip Sainty sometime before 1809.

Sainty honed his designing and construction skills on the Essex coast of England by building fast cutters for the profitable smuggling trade made possible by the quality of Dutch gin. Around 1820 the Marquis found himself in need of a larger, faster yacht and returned to Sainty only to find he had been locked up for his smuggling activities. The Marquis had enough influence with the authorities to spring Sainty as well as his son John Phillip, Jr. and his brother Robert, who were also imprisoned and considered indispensable for the construction of the new yacht.

The new yacht built by the Saintys was named Pearl and enjoyed great success racing against the fastest yachts of the time. The Marquis was so pleased with the Pearl he paid Sainty a pension of 100 pounds each year on the condition he build no more yachts, thus keeping Pearl the fastest yacht afloat. Sainty disregarded the no-build condition and went on to build at least six more yachts. John Phillip Sainty died in 1844 at the ripe old age of 93.

The Nautor Shipyard

2012-01-11T04:59:00.000-08:00

About 20 years ago I had the opportunity to tour the Nautor yard in Pietarsaari, Finland where the Swan line of sailing yachts are built. At the time, the Swans were among the best built fiberglass sailboats anywhere, and the shipyard was certainly the finest boat building facility I had ever seen. Everything was done in house by Nautor from the bottom of the keel to the top of the mast. All the shops were spotlessly clean. I went through the facility just after quitting time and there was not a drop of resin or a spec of sawdust anywhere. Nautor did not employ a "clean up" crew; the workers cleaned up after themselves.

The one and only time I sailed on a Swan was on board a sloop rigged Swan 65, a big powerful heavy displacement Sparkman and Stephens design, during the Swan Pacific Cup in 1983.

The H-28

2012-01-09T06:03:00.000-08:00

L. Francis Herreshoff wrote a series of articles for Rudder magazine explaining how to build several of his designs, including the H-28 cruising ketch. The "How to Build the H-28" article was later republished in the book, Sensible Cruising Designs with complete plans for the boat. Many variations of the H-28 have been built by professional, amateur, and production builders with the intent of improving a design by one of the true masters of the art.

I sailed several times on an H-28 that was reasonably close to the original design, and that boat had a more comfortable motion in a seaway than any boat I have ever sailed before or since.

Star Boats

2012-01-05T01:55:00.000-08:00

The design of the Star class is credited to Francis Sweisguth, who worked as a draftsman for William Gardner. The first 33 Stars were built in 1910 - 1911, and there are over 8,000 boats in the class today. I worked at Gerard Boat Company in Santa Barbara in the 1970's where we built several Stars that included world champion boats for Dennis Conner and Buddy Melges. We built just the hull and deck for the Melges boat which was finished out in the Melges shop in Zenda.

Prismatic Coefficient

2011-12-31T10:42:00.000-08:00

The prismatic coefficient is defined as the ratio of the immersed volume of the hull to a volume of a prism with equal length to the ship and cross-sectional area equal to the largest underwater section of the hull (midship section). A low or fine Cp indicates a full mid-section and fine ends, a high or full Cp indicates a boat with fuller ends.

The optimum value of the prismatic coefficient is one of the most misunderstood concepts is sailing yacht design. There seems to be a core belief among non-naval architects that the optimum prismatic coefficient for a given speed length ratio can be selected from a table in a book about yacht design, and that value is valid for all types of hull forms. I believe it's a bit more complicated than that, and different hull forms have different optimum prismatic coefficients for a given speed length ratio.

The ISO Scantling Rule

2011-12-26T08:15:00.000-08:00

The International Organization for Standardization (ISO) has developed standards for just about everything, including small boat construction. ISO has published a scantling rule for small craft, "Hull Construction and Scantlings, Part 5: Design Pressures for Monohulls, Design Stresses, Scantlings Determination". Production boats sold in most European countries are required to meet the ISO standard, and the ISO standard has been adopted for most offshore racing yachts.

The ISO scantling rule uses basic engineering to determine sizes of structural components. Plating is sized based on flat plate theory, and internal framing is based on beam theory, with uniform pressure and fixed ends. The process of designing a boat using the ISO standard is to calculate design pressures at various locations on the hull surface, and determine stresses on the shell plating and internal framing. The structure is adequate if the stresses are below the design stress of the material used. For composite materials, a "stack analysis" is required to determine the stress in each material used

I uploaded some examples of ISO scantling calculations here, including those for my own boat now under construction.

The Windermere 17

2011-12-22T07:31:00.000-08:00

The Windermere 17 is a restricted racing class developed by the Royal Windermere Yacht Club and dates back to 1904. These boats race exclusively on Windermere, the largest natural lake in England.

Dimensions are restricted to:

    Waterline Length               17'-0"      5.2 m
     Length Overall                  25'-6"       7.8 m
     Minimum Beam                5'-10"       1.8 m
     Maximum Draft                 4'-0"        1.2 m
     Sail Area                         300 ft^2   27.9 m^2
    Minimum Hull Weight       770 lb      343 kg

There are other restrictions that produce a full keel sailing yacht with relatively heavy displacement, low freeboard, and long overhangs.

The 1988 America's Cup

2011-12-14T18:28:00.000-08:00

The America's Cup was successfully defended by San Diego Yacht Club in 1988 with the catamaran Stars and Stripes sailing against the 90 foot monohull KZ-1 (the "Big Boat") from the Mercury Bay Boating Club. The racing itself was nothing more than a court case settlement with the real competition having already taken place between lawyers in the courtroom.

The catamaran easily beat the monohull - by a lot - in two races. I was on the committee boat for both races, so I had a great view of the boats finishing one at a time about twenty minutes apart. Aside from all the hard feelings produced by the court battles, this Americas's Cup marked the end of sailing in heavy displacement monohulls which was not a good thing in my opinion.

Naval Architecture is Engineering

2011-12-10T06:12:00.000-08:00

There are some misconceptions of what the term "Naval Architect" means. Here are some basic facts:

Naval Architecture is an engineering discipline, like civil engineering or electrical engineering. Naval architects are not architects for boats, they're engineers. Engineering is the application of special knowledge of the mathematical, physical, and engineering sciences. Engineering is not operating and maintaining machinery or equipment.

You can not learn Naval Architecture from a yacht design school. In the United States, you need to go to a college or university with an ABET accredited engineering program to learn Naval Architecture. There are many practicing naval architects with degrees in mechanical or civil engineering. The core subjects of these disciplines are similar with naval architecture, so it is not a hard transition to make.

Generally, in the U.S., a professional engineer's license is required to practice naval architecture, unless the work is done directly for the manufacturer or done under the supervision of the license holder.

There are many talented and successful yacht designers who are not naval architects. There are also some yacht designers who claim to be naval architects, but are not.

The Cruiser/Racer Rule

2011-12-08T05:25:00.000-08:00

International Cruiser/Racer 8 meter class

One of the more obscure rating rules is the International Cruiser/Racer rule that was implemented by the International Yacht Racing Union in 1950 to enable cruising yachts to race against each other without a handicap system. The Cruiser/Racer rule divided boats into five classes rating 7, 8, 9, 10.5, and 12 meters, the most popular being the 8 meter class, not to be confused with the International 8 meter which is a different type of boat entirely. Some fine examples of the 8 meter Cruiser/Racer class were built by Mcgruer & Co. of Scotland in the 1960's.

The Cruiser/Racers had limits on length and displacement to keep the proportions similar between boats and to encourage hull forms suitable for cruising. The base displacement of the Cruiser/Racer rule is somewhat heavier than what Norman Skene called "normal displacement" used by the International and Universal rating rules.

The formula for the Cruiser/Racer rule is:

Rating = (L+S-F+B+D+P+A+H+C-K)/2*Pf
L = Rated Length
S = square root of measured Sail Area
F = measured Freeboard
B = Beam coefficient
D = Draft coefficient
P = Displacement coefficient
A = bow overhang coefficient
H = underwater profile coefficient
C = stern overhang coefficient
K = coefficient for iron ballast
Pf = Propeller factor

The format of the Cruiser/Racer rule is similar to that of the CCA rule.

The Self Sufficient Yacht Designer

2011-12-02T04:31:00.000-08:00

I define the self sufficient yacht designer as one who can design, build, and sail a boat without relying on the expertise of others. While many designers fit this definition, I can only think of a few in recent memory who could design, build, and sail at the highest levels of yacht racing.

Encore by Tom Wylie

Tom Wylie designed and built some beautiful cold molded racers in the 1970's with a characteristic clear finish and flush deck with no regard for interior headroom. I remember the 30 foot "Gemini Twins" and the 40 foot Lois Lane, built in 1977. The first time I saw Lois Lane was before the start of a race on San Francisco Bay in 1978, and it was also the first time I had seen a three spreader rig. Lois Lane had a fractional rig and a bendy three spreader mast at a time when many racing yachts had stiff single spreader masthead rigs, starting a trend of trimming the mast with running backstays and checkstays that would continue for the next twenty years or so.

Infidel / Ragtime by John Spencer

John Spencer was designing and building fast light displacement boats in New Zealand in the 1960's, most notably the 62 foot sloop Infidel which was first to finish in the 1973 and 1975 Transpac races as Ragtime, ushering in a new era of very fast lightweight boats in the downwind races to Hawaii and Mexico. Ragtime has since undergone numerous refits with changes made to the hull, keel, rudder, and rig. I had the opportunity to sail on Ragtime in its original configuration in 1973.

Paper Tiger by Charley Morgan

Charley Morgan designed and built some of the most competitive ocean racers in the 1960's, including the 40 foot yawl, Paper Tiger, winner of the 1961 and 1962 SORC. Morgan has the best eye for proportions and form of any designer of the CCA era, in my opinion. Morgan designed, built, and skippered the 12 meter Heritage in the 1970 America's Cup trials, eventually losing the right to defend the Cup to the Sparkman and Stephens designed Intrepid.

The IOR Rule

2011-11-30T05:04:00.000-08:00

NM 43 "High Roler" (now "Secret")

The International Offshore Rule (IOR) came into being around 1969 -1970 to replace the CCA and RORC handicap rules in use at the time. The idea was to have a single international handicap system. Although the basic hull measurements used in the IOR are from the RORC rule and had been in effect for over 30 years, it wasn't until the IOR came into being that designers started connecting the dots between measurement points to optimize the rating, which lead to annual loophole plugging by the rule writers and more business for yacht designers.

NM 68 "Swiftsure"

The IOR produced some decent boats in the 1980's. While working as a draftsman at Nelson Marek Yacht Design, I drew the hull lines (under the direction of Bruce Nelson) for the 43 foot "High Roler" which was part of the 1985 Admiral's Cup team. At about the same time I did most of the drawings for the NM 68 "Swiftsure" which was the overall winner of the 1985 Transpac race. The Swiftsure drawings, all done by hand, were put on display at the La Jolla Museum of Contemporary Art as part of an exhibit featuring the work of local yacht designers.

The basic formula of the IOR is:

IOR Rating = (0.13(L*S)/(B*D)^0.5 + 0.25L + 0.2S + DC + FC) * EPF * CGF

L = measured length
S = square root of the sail area
B = measured beam
D = measured depth
DC = draft correction
FC = freeboard correction
EPF = engine and propeller factor
CGF = center of gravity factor

The format of the IOR is similar to the RORC rule and is a departure from the CCA rule.

The Cal 40

2011-11-28T07:34:00.000-08:00

The Cal 40 is naval architect C. William Lapworth's signature design, and perhaps the most successful production ocean racer ever built. Lapworth was a graduate of the University of Michigan's naval architecture program and one of the first of many successful California yacht designers with naval architecture or engineering degrees.

Much has been written about the success of the Cal 40, but what interests me is how Lapworth optimized the design to be competitive under the CCA rule in effect when the boat came out in 1963. At a time when the top ocean racer designers were "beating" the CCA rule with long keel heavy displacement centerboard yawls, the Cal 40 was a departure from the norm. The CCA rule takes a base length measured at 4% of LWL above LWL and adds penalties or subtracts credits for various characteristics of hull and rig. Lapworth put the Cal 40 transom right at the aft end of the "L" measurement, maximized the freeboard credit, took credits for beam, draft, and displacement, and a penalty for sail area. Contrary to popular belief, the Cal 40 is really not a very light boat for its length.

I met Bill Lapworth at a boat show almost 30 years ago, which for me was like meeting a rock star backstage.

The Delft Series Polynomial

2011-11-23T06:09:00.000-08:00

The source for approximating upright residual resistance in many velocity prediction programs (VPP's) is a long term study carried out by the Shiphydromechanics Laboratory at the Delft University of Technology in the Netherlands. The study compiled model test data from an extensive series of hull forms known as the Delft Systematic Yacht Hull Series consisting of 50 models.

The data has been boiled down to a relatively simple algebraic expression, or polynomial, that is easily plugged into a computer program or a spreadsheet. The parametric terms of the polynomial have evolved over several years of testing, and the coefficients have been determined for a number of different Froude numbers using a least squares fit through the measured data. Papers on the Delft study have been presented at the International HISWA Symposium on Yacht Design and Yacht Construction, and can be found online with a little searching.

An excellent summary of the Delft study and presentation of the polynomial and table of coefficients can be found in Principles of Yacht Design.

Images are from "Approximation of the Hydrodynamic Forces on a Sailing Yacht based on the Delft Systematic Yacht Hull Series" by Keuning and Sonnenberg

The Design Spiral

2011-11-20T16:57:00.000-08:00

The design spiral is a way of organizing the tasks of designing a ship or boat, and is presented in more detail in Principles of Yacht Design. The more complex the design, the more relevant the design spiral is. The key elements in the design spiral for a sailing yacht are the weight estimate, structural calculations, stability, and sail plan. All these items need to be considered as revisions are made to the design. Examples of procedures and calculations used in small sailing yacht design can be found here.

Years ago, when racing yachts were still lofted by hand, it was not uncommon for a designer to make changes to the hull shape on the loft floor. Changes to the rig to optimize the rating may have been made as the yacht was being completed, and the center of the design spiral was not reached until launching.

The MORC Rule

2011-11-18T10:50:00.000-08:00

The MORC Rule is the rating rule of the Midget Ocean Racing Club, founded in 1954 to promote offshore racing in small yachts. The MORC rule was originally created for small sailing yachts up to 24 feet, with the length limit being increased to 30 feet in 1958. In 1998, the club increased the length limit to 34 feet, but most people, myself included, consider the MORC Rule as applicable to boats 30 feet and under. The MORC rating formula effective as of April, 1998 is:

Rating = CLFAC * L *(1.0+BMCR+DRCR+DISCR+FBCR+SACR+BALCR+LBCR+PROP+PEN+MTCR+EMCR) * VC

CLFAC is a class factor
L is based on LWL and the 4% waterline length
BMCR is a beam correction
DRCR is a draft correction
DISCR is a displacement correction
FBCR is a freeboard correction
SACR is a sail area correction
BALCR is a ballast correction
LBCR is a live ballast correction
PROP is a propeller correction
PEN are penalties for excessive adjustments to the rig
MTCR is a mast thickness credit
EMCR is an engine moment factor credit
VC is a volume credit

The MORC Rule includes its own time allowance tables.

The MORC Rule is similar in format to the CCA Rule, and produces a type of boat that does not have severe hull distortions, is of moderate displacement, with plenty of interior volume. This type of boat is a capable offshore racer that can be used for cruising after its competitive racing days are over, and it is unfortunate the MORC Rule has lost popularity. I believe large offshore racing yachts would resemble the modern MORC boats if the CCA had been allowed to continue, rather than die a sudden death upon implementation of the RORC based International Offshore Rule.

Using Pine as a Boatbuilding Material

2011-11-16T17:49:00.000-08:00

The knock on pine as a boatbuilding material is it is soft, weak, and susceptible to rot. However, it is also readily available, inexpensive, and glues well. I think a good use for pine is in laminated structural members on small boats. Pine must be encapsulated in epoxy (negating the "soft" and "rot" issues), and be sized according to its strength (negating the "weak" issue). I'm using it for laminated keel floors and web frames in my own boat. The size of the frames is determined by an engineering analysis that includes the contribution of fiberglass hull sheathing and a fiberglass cap over the floors extending about 6" over the pine laminated member.

Cipper Ships

2011-11-14T09:12:00.000-08:00

The reign of the clipper ships was brief, lasting from about 1849 to 1869, with the glory days of the extreme clippers lasting from 1850 to 1856. The clipper ship era produced the finest in sailing vessel design, construction, and seamanship in an effort to maximize speed and the expense of cargo capacity and economy of operation, enabled by the boom economy of the California gold rush.

Donald McKay is perhaps the most well know of the clipper ship designers and builders, having built some of the most famous ships including Lightning and Flying Cloud. Less well known is one of McKay's competitors, William H. Webb, who has been called America's first true naval architect. Webb was a founding member of the Society of Naval Architects and Marine Engineers, and established the Webb Institute, which offers an undergraduate degree in naval architecture and marine engineering (and nothing else).

Graduates of Webb Institute include many top sailing yacht designers, which is appropriate since the founder was a major contributor to the search for speed under sail.

Small Yachts

2011-11-12T06:09:00.000-08:00

The full title of the book is Small Yachts, Their Design and Construction Exemplified by the Ruling Types of Modern Practice. Originally written in 1885 by C. P. Kunhardt, Small Yachts "is intended to cover the whole range of type in small yachts, but embracing only those which have received sufficiently wide recognition to entitle them to public confidence." Kunhardt notes the "division separating small yachts from large has been drawn between craft requiring one owner and friend with one paid hand for ordinary control and those demanding the shipping of crew and professional skipper."

Kunhardt was the yachting editor at Forest and Stream from 1878 to 1884. He was a strong advocate of deep heavily ballasted yachts of the English type and an equally strong critic of the beamy shallow yachts popular in America at the time. Kunhardt engaged in a heated war of words with supporters of wide and shallow boats through letters to the editor of the New York Times, The Spirit of the Times, and Aquatic Monthly under the nom de plume "Big Topmast".

In an odd twist of events, Kunhardt made an 1800 mile singlehanded cruise in a 21 foot by 9 foot beam catboat, the very antithesis of the type he favored, starting in November 1885 and completed in July 1886. C. P. Kunhardt was lost at sea in 1889, having been on board the steamship Conserva when it broke up and sank after a collision with another vessel.

Electric Auxiliary Power

2011-11-10T05:11:00.000-08:00

I've decided to add an electric auxiliary motor to the boat I'm building for myself. Keeping a boat in southern California requires maneuvering in and out of tight spaces on regular basis, which is my primary motivation for adding auxiliary power. Also, I can't resist building my own system from one of the kits available from Thunderstruck Motors, Electric Motor Depot, and others. The system I'm using is designed to power the boat at a maximum speed of 4 knots for no more than 2 hours. "Hull speed" for my boat is around 5.7 knots. A top speed of 4 knots requires less than 1/3 the power of 5.7 knots. I'm using a reduction ratio of 1.8:1 rather than the more typical 2:1 with a small diameter low pitch 2-blade propeller. The idea is to have a system with minimal battery weight that is capable of regenerating power at relatively low speed while under sail with a freewheeling propeller.