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What the authorities say should be
guiding principles of boat design


'Science is first in the requisites for consistent success, but it must be blended with natural genius, imagination and much practical handling' Norman L Skene


  • Simplicate, and add more lightness. Henry Ford.
  • The first requisite of any practical boat is safety, the second comfort, and the third speed. Edson B Schock
  • Other things being equal, the faster boat is preferable. The hell of it is, other things are never equal. John G Hanna
  • Unless a boat, however small and simple, is built in a shipshape manner , it is better not built at all. William Atkin
  • Cinch up your belt, roll up your sleeves, and go ahead and build it, regardless of hell, hurricanes, and high prices. It'll always be worth the cost, as it always was. John G Hanna
  • A man needs a foot of boat waterline for every year of his age. Traditional, repeated by Uffa Fox.
  • Originality, based on one's own study and experimental work, is really the keynote of success Norman L Skene.
  • The centre of bouyancy should be just aft of half way between bows and stern - John Teale
  • Flat-bottomed vessels need slightly more centreboard area; round bottomed ones can get away with slightly less - John Teale
  • The beam of a flat bottomed boat should be no more than half the length; if it is to be rowed or sailed, the length should be two and a half to three times the beam - HV Sucher
  • In wall-sided boats, making the curvature of the sides equal to the curvature of the bottom ensures that the water pressures at each surface are equal, and reduces eddying and wake, and therefore drag. - Philip C Bolger
  • In a ballasted craft, the weight of ballast should be 40 to 50 per cent of the all-up weight - universal wisdom.
  • In clinker-on-frame boats, there should be Ďthree strakes or three framesí of separation between joints - unknown source.
  • Don't concentrate ballast amidships in a ballasted boat, or the craft will move unpleasantly, but donít spread it too far, as that will turn it into a diver. A happy medium is likely to be to distribute it over about the middle third of the distance from stem to stern - T Harrison Butler
  • In small and medium-sized cruising vessels, make the least freeboard about a tenth of the LWL - T Harrison Butler
  • For an inshore racer in primarily light air conditions it might be wise to go to a prismatic coefficient of .525 Cp, while an all around cruising yacht would benefit from a higher Cp, say... .54-.55 and an ocean racer from higher yet, perhaps .56-.57. In any case, it is best if the Cp is a bit on the high side since the penalty for having too high a Cp at low speeds is less detrimental to performance than having too low a Cp at high speeds. As well, the high Cp should be achieved by fullness aft, not forward, as full bows have an adverse effect on performance. Ted Brewer
  • 'The curves of areas for the hull heeled at 25 or 30 degrees for an easy bilged boat should lie roughly parallel to the curve of areas upright, and fairness of diagonals is important. John Illingworth.
  • Wineglass sections throughout the hull will give slack diagonals in a chunky boat - and that's good!  Pete Culler
  • Suitable beam in a round-bottomed cruising boat should be found from the formula  square root beam=cube root LWL - T Harrison Butler
  • To minimise wave-making, (i) the length on the waterline should be as great as possible; (ii) there should be no parallel body; (iii) the curve of areas of cross section at the waterline should increase smoothly to a peak just aft of amidships and then decrease similarly smoothly, tailing off at both ends; (iv) there should be no abrupt changes in the shape of the of the cross sections, although planing vessels are an exception to this; (v) in sailing craft, conditions (i) to (iv) should be met also at heeling angles of up to 15 to 20 degrees. John F Sutton
  • 'It is only when the topsides stand high above the water that your straight-sectioned boat looks 'boxy.' William Atkin
  • To be successful, a self-draining cockpit should be at least eight inches above water level... The minimum leg room is held to be 15 inches. Dennis Sleightholme  
  • Expense: Never use expensive materials to build a boat designed to be cheap; also it is bad business to design a cheap looking boat that will be expensive to build. Philip C Bolger
  • A straight, nearly upright stem with a good depth of forefoot produces a dry bow. Howard Irving Chappelle
  • Issues of form. To be dry, keep the weight away from the ends of the craft. Also, to be dry a bow should pick up bouyancy smoothly as it submerges and only needs a moderate flare - in fact a straight-sided Vee-section will do. There is no advantage to a hollow entrance and a great width at the sheerline. Carrying flare aft can contribute significantly to dryness, however. Hollow should not be considered in anything but a relatively long craft with a length of no less than four times the beam. In craft with a large square stern, a raked transom with provide lift when hit by a wave. Howard Irving Chappelle  
  • The most seakindly boats have between three to five beams to their length, and a draft of one quarter to one half of the beam. they also had keels that were significantly deeper at the stern than at the bows to prevent any tendency to broach in a following sea. Howard Irving Chappelle 'In his experience as a designer, this 'drag' in a keel reduced the tendency to broach even in very square sterns,' he added.
  • Balance is important - for example a narrow and deep hull combined with a wide and shallow stern is undesirable - but in motorised fishing vessels, at least, this balance of the hull neet not be precise and does not on its own justify a canoe stern. Howard Irving Chappelle
  • Full-ended boats may seem dry, but this is only because they cannot be made to move quickly in a seaway. Howard Irving Chappelle
  • Flat-bottomed wave-bridging boats may be more seakindly than other types due to an air cushioning phenomenon. To be effective they should bridge three or more wave crests. Figure the length between wave crests and build the boat at least three times longer than that. Philip C Bolger
  • Length should be at least 6 times beam. 10 times beam is better... at about 14 times beam the energy saved in smaller immersed cross section begins to be exceeded by increasing surface friction James Wharram
  • For unballasted small craft design, try this Start the design as a flat-bottomed double-ender, making LOA/beam= 4/1. Other types can be developed along the lines of Matryoshkas (Russian dolls). A dory can be created by taking the double-ender and replacing the stem aft with the triangular 'tombstone'. On this basis, a 16ft double-ender becomes a 15ft 3in dory (LOA). A skiff can be created by removing a quarter of the overall length of the double ender from the stern and plugging the hole with a transom board; this makes the LOA/beam 3/1. The punt can be developed from a skiff by lopping 1/8 LOA fore (again measured from the original double-ender), with the resulting hole plugged by a bow transom. A dinghy is a punt shortened still further from the bows. A pram is a still more shortened dinghy. These figures are not absolute. The trick for each development is to stay in the confines of the same sheerlines of the original double-ender - that is, the beam for the pram is still 4 ft if the original Double-ender is 16 ft. Oh, I nearly forgot. Make sure that sheer line of the original D-E is a NATURAL curve, that is part of a circle arc. For hard-chined hulls, the circle arc segment is set on the LOA as chord. The (maximum) height of the arc is the hypotenuse of a right triangle of which the long leg equals the half-Breadth at Beam (1/8 LOA). The short leg equals the Profile height at Beam. The acute angle between the long leg and the hypotenuse equals the flare angle. (Gardner, John 'The Dory Book' page 43). Barend Migchelsen
  • The Dinghy Cruising Association, a largely British association of people into cruising small open boats, has its own set of criteria for boats regarded as suitable for this purpose. The criteria that apply to boat design are as follows: (i) The boat should carry sufficient crew - one stone (14lbs or approximately 6kg) for each foot of waterline length is suggested as a minimum. (ii) The boat should be stable enough to allow the recommended weight of crew to sit on the gunwale without dipping it under or the craft capsizing. A beamy hull is advised, minima to aim for with 12ft, 14ft and 16ft dinghies would be 4ft 4in, 5ft 3in and 5ft 10in respectively. (iii) The boat should carry sufficient positive buoyancy to support itself together with stores and partially immersed crew, plus a reserve of not less than 112lbs. This buoyancy should be so disposed that it is possible for the crew to put the boat back into sailing condition after capsizing or swamping. It should be stressed that capsizing is not an acceptable proposition in a seaway due to the long period of exposure that may ensue and the possible loss of stores and equipment. In rough seas, recovery may even become impossible. (iv) The vessel should have at least a foredeck. (v) Mast, rigging, fittings etc. must be strong enough to withstand capsizing forces. (vi) Consideration should be given to having the mainsail canvas a grade heavier than on a similar-sized racing dinghy; at least this will ensure longer wear. The mainsail should be capable of being reefed while at sea and the possession of a storm jib is desireable on sloops. The Dinghy Cruising Association
  • A moderate amount of rocker helps in waves by getting the center of gravity and the center of the boat down low in relation to where the bow or stern enters the wave. This keeps the bouyancy in the bow and stern up high and it allows the boat to rock fore and aft easily in response to waves. The downside of rocker is in loss of speed, however moderate rocker is acceptable for most boats. Tracking is also affected. To compare extremes, guideboats are dories optimized for rowing and salmon fishing on the Columbia river, which has a nasty sandbar and surf where it meets the Pacific. These boats have a great deal of rocker and are made for playing in the surf. A rowing scull is built for flatwater and speed, with a hull approaching a semicircle profile and no rocker whatsoever. This hull has very low resistance and is extremely fast but unstable in waves. Paul Van den Bosch, proprieter of The Guide to Sailing and Cruising Stories  
  • A 23' catboat is a one-portlight-per-side size; two would be an affectation. William Garden
  • Double the fiberglass overlay on chines centerline and sheer to reinforce these stress points. Over-zealous sanding on the easy-to-do corners is a major cause of glass overlay cracks. William Garden
  • In selecting a rowboat, remember that a 265-pound Whitehall is too much for pleasure rowing... About 100 pounds is the limit for enjoyable rowing. William Garden
  • The first rule of economy is deletion, and the second rule is substitution.William Garden
  • Nail where you can, screw where you must, and bolt where you have to.RD Culler  
  • A boat without flare is like a ship painter's pontoon, and about as handy underway.RD Culler
  • Creating the perfect line means using the eraser again, and again, and again. F S Kinney
  • My experience is that the initial drawings should be hand drawn, but then I am a technophobe when it comes to the electronic drawing board, and chewing on a mouse is not nearly as satisfying as a pencil. John Welsford
  • A small boat of traditional flavour is in reality a caricature , to get the features that say 'trad' and give the boat its flavour there needs to be subtle exageration and this can be hard to achieve without either over or under doing it. The designer has to pick out those characteristics in the 'parent' that make it memorable or induvidualistic and translate those into a form that says to the viewer, 'look at me, I'm going to be like my daddy when I grow up!' John Welsford
  • The keel, stem and stern post of a lapstrake boat should be twice the thickness of the planking, plus the thickness of the stem bolts, plus 1/8 inch.RD Culler
  • Never glue the strakesof a lapstrake boat. RD Culler He's referring to traditional lapstrake, not to the epoxy plywood lapstrake we see today.
  • The Trapezoidal Rule for finding the wetted area of a hull: Divide the underwater hull into any convenient number of slices, and measure the length of each of the lines. Then add the lengths of the lines together, remembering to halve the value of the first and last of these lines. To a fairly close approximation, the surface area will be this total multiplied by the distance between the slices. Traditional wisdom Outside of racing (and maybe rowing), I have yet to understand why this is thought to matter so much.
  • (About a 16', 8mph power boat) The deadrise is approximately contstant throughout the bottom sections, a feature that, assures (all values being equal) an easy and comfortable boat in rough water. William Atkin
  • A 7-in coaming coaming for even small boats is not too high. John Illingworth
  • Regardless of the volume of the (self-bailing) cockpit, the drains should be large enough to drain it completely in three minutes after it is filled to the coaming. To prevent flooding of the cabin should the cockpit be filled, the sills of the companionways should be no lower than the lowest point of the coaming. C William Lapworth
  • To be successfully self-draining, a cockpit should be at least eight inches above water-level, yet in must not be so shallow that it is virtually a box for the feet. ... The drains to empty it must be of at least 1 1/4-in. pipe. Des Sleightholme Des S is still around and writing entertaining columns for a couple of yachting magazines.
  • It must be mentioned that built-in buoyancy should never be more than a last resort. First comes design, and a hull that just cannot be swamped in a knockdown.Des Sleightholme He's talking about trailerable cruisers, not dinghies. I very much like his 1963 book 'Pocket Cruisers: A New Approach'. I take it that 'just cannot be swamped in a knockdown' means a structure designed so that any openings in the boat are likely to be well above the waterline in a knockdown.
  • 'Through the kindness of MV Brewington Jnr, the Bay rules for building skipjacks are available. They are as follows: the greatest beam is one-third of the length on deck, and is located between one half and two-thirds of the length on deck abaft the stem. The width of the stern is about three-quarters of the greatest beam. The flare of the sides varies from 2 inches to 3 inches for each foot of depth amidships, according to the practice of the builder. The mast step is located one fifth or one sixth of the length on the waterline, abaft the stem, varying somewhat with the size of the boat, and the mast rakes about 75 degrees to the lwl, the masthead coming directly over the greatest beam. The length of the mast is equal to the length on deck plus the greatest beam. The length of the bowsprit outboard is equal to the greatest beam. The length of the boom is equal to the length of the hull on deck. The length of the centreboard is one-third the length on deck, and is placed in the middle third of the aforementioned length; the board does not appear above the deck.' Quote from Howard L Chappelle
  • Side decks should be a minimum of 15 inches in width. With anything less it may be wiser to put the deckhouse right out to the side of the hull. F S Kinney
  • Metals used in wet conditions may create electrolytic actions that may eat away important hull components - the only way round this is to use together only metals that are close to each other in the galvanic series. Starting at the most noble end of the range, the series runs: mercury and mercury paint, vanadium, gold, silver, monel, nickel, passive stainless steel, silicon bronze, copper and copper paint, red brass, aluminium bronze, gun metal and Admiralty brass, yellow brass, phosphor bronze, manganese bronze, tin, lead, active stainless steel, cast iron, wrought iron, mild steel, aluminium, cadmium, galvanised iron and steel, zinc, magnesium. A truth discovered and rediscovered through thousands of unfortunate experiences.
  • You can complete your project on time, or or under budget. Never both. John Welsford

  • The centreboard should be about 4 per cent of the sail area. Marchaj
  • There should be about 1sq ft of centreboard area to 40sq ft of sail area; in light craft the rudder area should be half this, while in heavier craft it should be still smaller. John F Sutton  
  • A rudder area of between 8 and 10 per cent of the total lateral plane or underwater profile is the desirable size for a sailboat. F S Kinney
  • A foil should reach its maximum thickness about a third of the way from the front edge. unknown
  • On foils, I have experimented with all sorts of stuff and come back to this simple formulae. For a non racer centreboard, the thickness should be 12% of chord, the leading edge radius should be 1/6th the thickness, the maximum thickness should be at 40% back from the leading edge, the trailing edge should be square across 5% of the maximum thickness. Make a nice fair curve from leading to trailing edge. This gives a high lift section with a very high stalling angle which will assist the boat to tack reliably in very light or very rough conditions when the low drag sections tend to stall and lose lift. It is also not prone to damaging the leading edge if you hit anything. It is a thick section, though, and on some boats would not fit the centrecase and you may have to scale the proportions down to suit the thickness available. John Welsford
  • In general, a centerboard is not a desirable feature unless a boat is to be used in an area that requires shoal draft. Also, the rudder on a centerboard boat should not extend below the maximum draft of the rest of the boat with the centerboard raised. James A McCurdy

  • Any rig her owner likes to work is the best rig. John G Hanna
  • No boom should be twice the beam to avoid tripping the boat up. Philip C Bolger
  • Boomless sails need at least 10 degrees of beam to work, preferably 12. Philip C Bolger
  • The effective centre of effort of boomless sails is somewhat aft of where an equivalent boomed sailís would be . Philip C Bolger
  • Donít make the narrow angle at the top of a triangular sail too narrow. If itís less than 27 degrees, the sail maker will not be able to get any shape into it. Philip C Bolger
  • Cutting a small convex curve shape into the luff of a square-shaped sail is a quick and cheap way of getting a good shape. Jim Michalak
  • Sail area can be estimated using the following formula:
    (Ballast in tons x distance in feet from the centre of gravity to the metacentre)/
    (Sail area in sq ft x height of centre of effort in feet above the metacentre) = R
  • where R is some value in the range 16/10,000 to 24/10,000. T Harrison Butler, attributed to Admiral Alfred Turner
  • The sail area of a 14 foot cruiser should be 150sq ft; for a 16-footer 325sq ft; and for a 20 footer, 525 sq ft. From a graph published in Elements of Yacht Design by Norman L Skene
  • Stability is the engine of the sailing vessel. Des Sleightholme
  • The appropriate sail area of a vessel varies with its displacement in tons squared and then cube-rooted times a fudge factor constant. A vessel with 0.25 tons displacement will have a sail area of 100sq ft, one of 0.5 tons 150 sq ft, 0.75 tons 190sq ft, 1 ton 270sq ft. John Teale  
  • On a sloop the headsail should have about 50 per cent of the area of the main, and on a cutter the headsails should have about 70 per cent of the area of the main. John Teale
  • The centre of effort of a triangular sail will be found on a vertical line about a third of the distance from the mast along the foot of the sail. The height of the centre of effort will likewise be on a horizontal line third of the way from the foot of the sail to its head. This only works if the rake is negligible. Lew, of the boatdesign mailing list  
  • Good balance can be achieved by placing the centre of gravity of the sail plan directly over the centre of lateral resistance, or perhaps an inch or two ahead of it. John F Sutton
  • For balance, The lead of the centre of effort over the centre of lateral resistance should be 12-14 per cent of the waterline length in the case of a shallow hull, fin keel or centreboard craft; about 10 per cent for deeper, more traditional yachts; and about 8 per cent for cruising yachts of classic form. John Teale
  • Balance: for racing machines of the scow type the lead of CE over CLR should be 5 to 15 per cent of the waterline length; for shoal, full-ended centreboarders, the lead lies between 7 and 11 per cent. For full-ended keel boats the lead is generally a little less; for cruising boats of normal form it is about 6 per cent. Norman L Skene
  • To achieve balance, the correct lead of CE over CLP (=CLR?) as a percentage of LWL is 7-12 per cent for a schooner; 11-14 per cent for a ketch; 12-15 per cent for a yawl; and 13-17 per cent for a sloop or cutter. He seems to be talking about conventional designs in conventional sizes (say over 10 feet LOA).Dave Gerr (The Nature of Boats, p301)
  • In flat-bottomed boats, putting the CE directly over the CLR seems to achieve good balance just about every time; however it is different in boats with a deep vee-section near the bows. Jim Michalak.
  • L Francis Herreshoff used to place the sailplan centre of effort of a conventional sloop to produce a lead of about 7 per cent ahead of the centre of lateral resistance. However, he listed several factors that might affect a sailing craft's balance.
    (i) Hull shape. A deep, vee-shaped forefoot will obviously increase lateral resistance near the bows. However, in a yacht in particular you should also consider the lateral resistance of the heeled hull shape - it may be, for example, that at 15 or 20 degrees the bows have no grab, but the stern is beginning to put up significant lateral resistance.
    (ii) Sharpness of the bows. As a sailing boat heels, there may be a large flow of water from the lee-side to the weather side of the bows. This tends to turn the bows up to the wind - by how much depends upon the sharpness of the bows.
    (iii) Due to the shape is presents to the water, a wide shallow boat will tend to head up to the wind on heeling, while a deep narrow boat will tend to turn off the wind.
    (iv) Increasing amounts of draft in a sail will tend to move the effective centre aft. (This is especially noticeable in the case of ice yachts, apparently.)
    (v) Where the sail plan is divided into many sails, as in the case of a schooner, the effective centre of effort moves far less than when there is a single large sail, as in the case of a catboat.
    (vi) The height of the sail plan. As a boat with a high rig heels, the centre of thrust on the rig moves outboard, again tending to turn the craft up wind.
    How much allowance should be made for each of these factors? Think about each one and employ a sense of proportion, says Herreshoff, helpfully, for this is not mathematics but pure art.  
  • Balance, againThe shape of the leading edge of the keel is one of the chief factors in determining the balance of a boat. If the edge is sharp the forward part of the boat tends to bite into the water and hold its position, whereas a a rounded or blunt leading edge will tend to slide off sideways when a boat is sailing to windward - so what we might call the true centre of resistance is further forward inn a boat with a sharp leading edge than it would be in the case of one with a blunt leading edge. In other words, we can't really know where the centre of balance is likely to be without doing tank tests. F S Kinney .
  • The diameter of a traditional solid wood round boom should be 0.02 of the boom length. For a loose footed sail, it should be 0.022 maximum, tapering to 0.02 at the ends. The diameter of a solid wood gaff should be 0.017-0.019 of gaff length. John Leather.
  • The best proportions for a gaff mainsail are luff 1.0, head 0.833, leech 1.73, foot, 1.02. The angle of the gaff to the centerline of the mast should be 30 degrees. The rake upward of the boom should be 6 degrees. If a topsail is to be carried, then the angle of the gaff should be eased to about 42 degrees. John Leather.
  • The simplicity of the cat rig is a recommendation in itself. William and John Atkin .
  • For singlehanding a large boat and therefore obliged to split up the rig, I would have a ketch with the area of the mizzen about two thirds of the mainsail, and the mizzen mast stepped about as far forrard as practicable. There would then be some chance of the vessel working to windward under mizzen and headsails in heavy weather. F B Cooke .
  • The permanent backstay should be designed to clear the end of the boom regardless of how high the latter may rise in jibing. William and John Atkin .
  • I believe only a solid stick (mast) has any place on a gaff-rigged boat. Oliver Tantanen .
  • A good figure to aim at for important shrouds is to have the cap or upper shrouds make an angle of 14 to 15 degress with the centre-line of the mast. John Illingworth.
  • A cook's first need is somewhere to put things down. Des Sleightholme.
  • Sail area is modest in the small cruiser simply because the buyer cannot be trusted to reduce sail to a safe area when it begins to blow. Des Sleightholme.

    TheOars etc


  • The length of an oar should be of half the beam (from lock to lock) times three, plus 6 inches West Marine
  • Alternatively, the length of an oar from lock to grip should be half the beam times three, plus 2 inches John Vigor
  • The calculation of oar length for one of my boats is based on a movement at the hands of only 700mm. The theoretical cruising speed of the boat is then worked out by taking the square root of the boat's waterline length in feet (Imperial units of measurement are good for some things) and multiplying the result by a figure between 1 and 1.4... A short fat heavy boat will be close to 1 while a really long light slippery boat will be at the other end of the scale. This jiggery pokery tells me how fast the water will be moving in relation to the boat, and by applying a slippage factor appropriate to the oar type, about ten to twelve per cent for the narrow blades I use, I can work out the shaft length needed to move the blade at the right speed when the handle is stroked through 700mm25 times per minute. John Welsford See John Welsford
  • In the context of small boats, the oar length should be about 7ft - at that length, in beamier boats the 'gearing' changes appropriately. Jim Michalak See Jim Michalak
  • In a boat designed for adults, rowlocks should be about a foot astern of the thwart or seat that will be used for rowing. Unknown
  • A length of 5 inches is about right for the grip. Never use a keg shape, but give the grip a taper of about 1 1/4 inches at the inboard end tapering to about an inch next to the swelled part of the oar. RD Culler



  • Build settee berths with a slight hollow, and 1in fall outboards. This will do wonders to keep the tired mariner off the cabin sole. William Garden
  • Key internal dimensions The minimum possible headroom for any cabin is 4ft, to maintain sitting headroom on a 12in seat. The minimum headroom over any seat is 3ft. This is measured from the top of the cushions. The height of any seat from the top of the cushion to the floor should be between a maximum of 19in and a minimum of 12in. The minimum width of any seat should be 12in, the usual is 16in. The usual dimensions for seats to sleep on, or for berths, are 78in in length and 30in in width. The minimum is 72in in length and 21in in width. Sam Rabl
  • For cabin ventilation, the total intake area in square inches = beam x waterline length (in feet). Rod Stephens
  • The cramping of hatches, skylights and companionways only tend to emphasize the smallness of one's craft, whereas if these openings are held to comfortable proportions or even accentuated in size, they suggest the ship rather than the tabloid cruiser. Federic A Fenger
  • It is important to assume, I think, that at one time or another your boat will be completely submerged and/or capsized, and, to be extra safe, that it will be filled with water. So you need a boat capable of coping with each of these possibilities. Robert Manry, skipper of Tinkerbelle



  • The minimum power for the eighteen-footer is about 1.5 hp giving around four knots in smooth water; this is barely sufficient if she is to punch a tide against a head wind in order to get back up-river to her mooring, but under sail and motor it is practicable. Des Sleightholme  



  • Murphy's law for boatbuilders is that you won't find your mistake until the glue has set, and Welsfords law for boatbuilders is that the mistake that can't be fixed with lots of epoxy and fibreglass hasn't been born yet. Jon Welsford
  • In glue and dust, and God we trust. Traditional prayer of boatbuilding apprentices
  • Cut on the wastewood side lad, cut on the wastewood side! And always measure twice and cut once. Traditional