The purpose of this project is to design and mass-produce kits for a floating tiny house that can sail. It combines high-tech modeling and fabrication and low-tech assembly that can be carried out DIY-style on a riverbank or a beach. This boat is a four-bedroom with a kitchen, a bathroom/sauna, a dining room and a living room. The deck is big enough to throw dance parties. It can be used as a river boat, a canal boat or even a beach house. It's rugged and stable enough to take out on the ocean.

Tuesday, January 6, 2015

Hull shape

Click to enlarge
The main trick for keeping costs down is to keep the hull square, with a flat bottom, flat, slightly flared (10-13º) sides, and a flat, flush deck. Any additional complexity to the shape—compound curves, rounded chines, multiple chines—involve additional labor, molds, lofting, scrap and general complexity. A square hull complies with the KISS principle (Keep It Simple, Stupid!). The bottom should have rocker, the sides some flare. That's it!

In order to cut smoothly through the water, the lengthwise curve of the bottom (called “rocker”) must exactly match the curve of the sides, giving the water no excuse to want to flow across the chines which join them, causing turbulence and drag.

This rule can be relaxed somewhat toward the transom; making the transom wider provides a bit of weather helm, which provides built-in safety: if the steering gear fails, the boat noses up into the wind and stalls instead of falling off to leeward and maybe capsizing.

The chines should have lips that extend out horizontally, called “chine runners.” The chine runners provide enough lateral resistance to make it possible to sail without extending the centerboard(s) except when going to windward. In a tight spot, they even make it possible to sail to windward over shallows, where there is not enough draft to extend the centerboard. Chine runners work so well that one successful microcruiser design (Matt Leyden's PARADOX) doesn't even need a centerboard.

This sort of hull is easy to construct out of plywood sheets screwed and glued together and sheathed with fiberglass, and it sails just about as well as anything. My first boat, HOGFISH, which was a 32-foot square boat, could do 7.5 knots out on the ocean in moderate conditions, and that's good enough. Anything done to improve on that I would consider gold-plating, especially if it detracts from the boat's usefulness as a houseboat.

But I am improving on the traditional square boat hull shape somewhat. HOGFISH was a sharpie, with a sharp stem at the bow that joined the bottom just below the waterline, whereas QUIDNON is a scow and has a bulbous, rounded bow that bounces over the waves or, when that doesn't work, punches through them like a battering ram. Both designs are effective, and have even won races, but the scow can have much wider beam, providing lots of living space, which is essential for a houseboat.

Square boats have numerous advantages beyond low cost and ease of construction. Perhaps most importantly, they can go aground safely. Some boats go to windward well, and that's laudable, but hardly any of them go aground worth a damn. Keelboats flop on their side, bounce around in the surf and get destroyed. Washing up on the beach is a calamity for virtually every sailboat, and even “touching bottom” in calm conditions is considered an emergency of the highest order.

A square boat, with its flat bottom, can be beached, rolled ashore over round bits of driftwood, and relaunched later on without much fuss at all. Such a boat doesn't require a crane with slings, a Marine Travelift or a marine railway with a cradle to haul out or launch—just a ramp dug into a beach, some logs and a winch.

It can also settle upright at low tide, making it possible to anchor it in shallow water. Running aground in a square boat is generally a non-event: I ran aground numerous times in HOGFISH, and just waited for the tide to float me off. If that didn't work, then I “kedged off” by setting an anchor in deeper water by rowing it out with the dink, then winching it in.

Square boats make better houseboats because they have much more storage space. Using simple math, you can calculate a round-bilge boat has 20% less cross-section than a square hull of the same dimensions, but that's deceptive, because most objects you want to store are rectangular in cross-section and fit well a square hull but not a round one.

Square boats can also carry a lot more weight: because of the large surface area of the flat bottom, it takes a lot more weight to submerge a square hull below its design waterline than a round hull of the same dimensions. They also track better in the water: the long, hard chines where the bottom meets the sides prevent them from yawing around when buffeted by wind and waves, and they maintain the ability to steer a straight course all the way down to a fraction of a knot.

Lastly, they are much more comfortable in big waves: they don't corkscrew down waves like round-bilged boats do, and there is no keel to trip up on waves (once the centerboard is pulled up). Also, they are slow to heel in a gust because of the large quantity of water that has to traverse the chines, and in all but the most severe conditions square hulls adopt a certain level of heel and just stay there, while round hulls rock back and forth continuously. Even when lying ahull in big waves square hulls refuse to build up much angular momentum because a lot of the energy is dissipated by water swilling over the chines, and this limits the rocking.

And, so I can shave a very large amount of the cost off simply by declaring that the hull will consist of exactly 5 planar surfaces, 3 of them simple curves (bottom and sides) and two perfectly flat (deck and transom). It won't look like a yacht, but it will still look like a boat, and that's good enough for me.

The biggest complaint about square boat seems to be their lack of sexy, sleek lines. Now, being a family man, I am not overly concerned about such matters, but in case you are, I have never found women to be particularly concerned about the lack of sexy, sleek lines in a boat—especially the sexier, sleeker women. They are mostly concerned about whether the toilet flushes, whether there is plentiful hot water for the shower, whether there is enough room for all their shoes and hanging lockers to hang their evening gowns, and whether there is enough flat surface on deck for them to stretch out and model their bikinis. And so the problem with the lack of sexy, sleek lines should be easy to remedy through the usual means, without resorting to trying to sculpt them out of plywood. Seems commonsense to me.


  1. Hi Dmitri,
    did you see Yann Quenet's "Skrowl 900" ?
    I'm working with him on this project. My personal project is a bit different, with a "Tjotter bow" (look at a - french - page I made : )

    Main problem with your design is about chine runners : they need a 30-45° heeled boat to work, and this will be very difficult to obtain with a scow. Chine runners are OK with narrow boats like the Paradox, I think (but its only my opinion, I never sailed a boat with chine runners)...

    Eric, from France

    1. Hi Eric,

      Interesting discussion of hull shapes on your site! I'll have to take another look later. Français, ce n'est pas le problème pour moi.

      I have looked at SCROWL, and it's quite interesting, although much too small for my purposes.

      Important point about the chine runners: If chines are to be used to go to windward, it is important to induce a lot of heel, you are right. But my 32-foot HOGFISH had rather minimal chine runners, and I sailed a few thousand open ocean miles in it, with the board up, on a beam reach, healed over 10-15º, making at most 3º leeway and 7kts and being extremely comfortable. With going to windward, the hull has to oppose a lot of force to leeward, which means that a large quantity of water has to be displaced slightly to leeward. That's where a lot of heeling becomes important—to grab more water. On a beam reach, the force is lower, and chine runners are sufficient. The point at which they become overpowered is quite obvious: suddenly your leeway goes from very small to very significant. That happens somewhere on a close reach, depending on conditions. Also, inducing a lot of heel in a scow with a 100-120 sail/displacement ratio (which is what I am looking at) may be easier than you think.

    2. What is the planned length of your Quidnon (and other specifications) ?
      I guestimated ~15m / ~50 feet...

    3. Here are the provisional numbers. The displacement number is almost a guess.

      LOD = LOA = 36 ft
      WLL: 34 ft
      Beam: 16 ft
      Headroom: 6 ft Displacement: 15,000 lbs
      Draft (boards up): 2.5 ft Draft (boards down): 7.8 ft Sail Area: 1000 sq. ft. Masthead above DWL: 47 ft Power: inboard outboard, 35hp

    4. OK, it's not that big ... (a very different boat, but not to far from Yann's Skrowl 900 = 30' : the layouts are quite different indeed, but not the principle of the hull).

      To answer Koho's comments, David Raisons scows winn all races (even with wind from the wrong direction), and Yann's tiny (14 feet) Skrowl sail very well upwind (even with a sail shape very far from perfection)...
      I don't think your boat will be tender at all, the Skrowl 14 shows an incredible stability.

      How tall are you, to want a 2m headroom ? (the man on your sketch is 4' 3'' !!...). RWL is 6' 5"", doesn't have head room on his So It Goes, and says its OK for him.

    5. Yes, the figure in my drawing is a bit short. 5'5" actually. The interior headroom (down in the cabin) is 6'. In the pilot house it's lower, more like the cab of a minibus, which is not designed for people to stand around. This seems normal, since I intend to outfit the pilothouse with couches and tables, with just a single station for the wheel, the instruments and the boxes of line. I am 5'6", as was Chris Morejohn who built HOGFISH, and the fact of the matter is, if we built boats for ourselves as if we were building for tall people we'd be wasting time and money. I do have tall friends on board periodically, but there is a limit to how much I am willing to do for their periodic convenience.

  2. Not being a naval architect - can runners be added to the bottom, as like in a rowboat?

    Does this help? Also, will they help keep the hull in better shape when beaching?

    1. Chris Morejohn experimented with adding rails. HOGFISH has them, HOGFISH MAXIMUM does not. They don't do a thing for lateral resistance. Also, Chris found that heavily glassing the bottom is far more important for beaching than having those rails. All they do is concentrate forces in a small area. I spent a long time epoxying up cracks and divets in them during haul-outs. If the forces were distributed over a larger area, the fiberglass would have held up better.

  3. This fall I finished a little 12 scow, an Ooze Goose. In a few days I'll be hauling down from NH to TX then FL. It will give me the chance to see how a flat bottom scow performs in a varity of conditions. The few time I had it out on the lake before the freeze it behaved really well.

    My lovely wife informs me that there will be no boat building next summer, but I'm hoping to change her mind. If the 12 foot boat performs well maybe I can convince her to let me build a bigger one.

    1. That's excellent! Good luck with your sea trials, and please let us know how it goes. I had thoughts of building a scale model of QUIDNON first—just big enough for me to stretch out inside it—and sailing it around Boston Harbor, to see how the hull shape behaves, and to shake the bugs out of the NC tool paths on a small-scale job before feeding lots of plywood through the NC machine.

  4. Dimitri, what you have designed is a rather attractive sailing barge which has a long and distinguished history in naval architecture. These boats are suitable for shallow broad waters like estuaries and bays and broad rivers. Yours is probably most accurately called a scow barge. Like any barge they excel in load carrying capacity, storage and living space and their broad beam confers stability. I assume you are not considering taking such a boat into the ocean . Sailing barges can have a long and useful life as long as they stay out of the ocean. Square boats can sail after a fashion downwind and reaching but they cannot beat into the wind and while your junk rig has an even longer history, its ability to go into the wind is limited. The high sides have a lot of windage which will make them difficult to control as the wind and wave increases. The high center of gravity means that the barge is stable upside down as right side up. A square boat has marginal longitudinal stability which can be mitigated by bargeboards or centerboards if the water is deep enough. Your statement that the main trick for keeping the costs down is to keep the hull square is a questionable statement. For one thing the main thing that determines cost is the displacement of the design. The complexity of the hull shape is probably the next most important, but having built and designed many boats you should be aware that the hull is only 10-20% of the ultimate cost of the boat. There is really nothing new in boat design that hasn't been tried by someone somewhere in the last 5000 years. We have more durable materials, adhesives, fasteners and the like of course but ship design is all about compromises. You never get something for nothing. There are other problems of unconventional designs among them obtaining insurance, liability issues, hoity toity waterfront owners and marina operators who don't want anything out of the ordinary within their view. There are other obvious disadvantages of course such as a high wetted surface(drag), associated increased cost of bottom paint, haulout issues, narrow entrances to some marinas and canals etc. as well as the necessity of a very powerful engine in a vessel with high drag, windage and displacement.Resale of unconventional designs should be an issue . How well I remember Ferrocement designs which were a bust for most of their designers/builders. In boat design as in life(and I repeat) you never get something for nothing. Sailing scows will absolutely be an increasingly dominant class of craft(once again!) going forward in the next century after oil is scarce or gone in the bays and estuaries of the world as commercial craft or floating housing. For open ocean sailing and voyaging they are entirely unsuitable and unseaworthy.Boat designers for the past 4 or 5 millenia have taken as inspiration nature’s designs. There are very few square birds, reptiles. Very few square fish or whales . Of course there is the turtle…. So maybe I am missing something here after all.

    1. Contrary to what you say:

      1. Square boats point as high as a sloop. My HOGFISH tacked through 90º.

      2. Square boats have crossed oceans more times than I know of. My HOGFISH has been from northern Maine to the Virgin Islands and back. They handle great out on the ocean. Its bigger cousin HOGFISH MAXIMUS did a lap all the way around the North Atlantic once, and has sailed between Florida, Bahamas and Grenada a dozen or so times.

      3. Scows are some of the newest racing designs and are winning impressively.

      4. Junk rigs have been used to sail around the world lots of times, by such notables by Annie and Peter Hill, Tom Colvin and many others. Junks go to windward just fine, especially if the panels have a bit of camber or the battens a bit of flex.

      5. The center of gravity of my design is barely above the waterline. The design will be self-righting.

      ...and so on and so forth....

      6. Resale of unconventional designs: when I was selling HOGFISH, there was a feeding frenzy. A year later I was still getting phone calls about the listing. Clearly, there is a shortage of affordable square boats that are ocean capable.

      I disagree with pretty much everything you have to say. This blog is for refining a design. Sorry, but I don't see you as a valuable contributor.

    2. I will make a brieft rebuttal to you posted comments but I enclose this general statement pulled from Wikipedia which perhaps states better the point I was trying to make:

      "Sailing scows have significant advantages over the traditional deep keel sailing vessels that were common at the time the sailing scow was popular. Keelboats, while very stable and capable in open water, were incapable of sailing into shallow bays and rivers, which meant that to ship cargo on a keelboat required a suitable harbour and docking facilities, else the cargo had to be loaded and unloaded with smaller boats. Flat bottomed scows, on the other hand, could navigate shallow waters, and could even be beached for loading and unloading; this made them very useful for moving cargo from inland regions unreachable by keelboat to deeper waters where keelboats could reach. The cost of this shallow water advantage was the loss of the seaworthiness of flat bottomed scow boats in open water and bad weather." " It's a pity you cannot respond in a civil fashion to well meaning constructive criticism from someone with an abundance of design and sailing experience, primarily in the North Pacific. I certainly never meant to demean someone with a paucity of sailing, design and building experience. Everybody has to start somewhere after all. If I were you I would abandon the fatuous term "square boat" although what you have drawn comes very close indeed to resembling such a term. Calling your 32 foot sharpie a square boat is inaccurate.. Sharpies are well established fine coastal cruisers and they no doubt perform as you have stated. I clearly stated that sailing scows can be amazing sailboats and The inland lake scows I sailed on 50 years ago in the great lakes had astounding speed. I have also sailed on 2 junk rig boats. One of them a colvin design . What you have drawn is nothing more than a nearly square scow which will have dangerous and unpredictable sailing qualities and if you persist in persuading yourself that shallow draft beamy boats can safely cross oceans, you will be putting yourself and your loved ones at great risk. Since you have no background in naval design, I would hope you would in time submit your design for review by an established office or perhaps to a talented amateur like Steven Dashew who has designed and built more boats and that have crossed more oceans than anyone I can think of. I would also consider renaming the boat "QUID" as well. Good luck anyway. The only way to avoid mistakes is to gain experience. The only way to gain experience is to make mistakes.

    3. Thank you, but I think your considerable wisdom and experience are a bad fit to the task at hand, and no amount of cutting and pasting from wikipedia into blog comments (instead of posting links) is likely to change that. Calm seas and fair winds to you.

    4. s/v koho, as another experienced sailor/designer/builder (50+ years), I must say that I agree with both your posts nearly 100%. At first I was excited by the prospect of contributing to this blog, but after seeing your smack-down, not so much. Oh well.
      Fair winds!

    5. This is completely expected. Most sailors and boatbuilders would want nothing to do with this project. This is par for the course when it comes to square boats. I wish you all well, and don't expect anything from you except that you let us get on with working out the design instead of trying in vain to impress us with your considerable acumen. Thank you.

  5. Dmitry, remember - for yourself and for any following your trail-blazing - that the right way (the way that stays put for twenty-plus years and gives you dusty-dry bilges) to sheath a hull with GRP is the way the Vaitses boatyard developed several decades back, in New England.


    The essence of it is that you lay up half your plies of GRP, then, whilst it's still chewy, you through-pin, at fairly close intervals, never much more than six to eight inches, the whole slab onto the timber substrate with ring-nails or heavy staples (preferred), and then carry on the rest of the lay-up to the design thickness. This stops the sheathing from unsticking and tearing off, as timber and GRP go through differential swelling and shrinking according to weather, atmospheric conditions and temperature.

    Allan Vaitses gives numerous individual accounts of how this method has held up very well for many years, even with wooden boats which were dying of age when they were so treated. Modern resins and directional fibre mats will, it seems, increase the value of this treatment even further.

    There has been some controversy about this technique, but the fact remains that old wooden boats have had major life extensions because of it: twenty or thirty years, and counting. Note that you need to get Allan Vaitses' book, and follow his method exactly; though updating the resins and fibre material seems to be OK. Just remember that it's the through-pinning that is important.

    1. That's the way HOGFISH was laid up. Fiberglass matt was nailed to the plywood underneath with bronze annular nails every couple of inches throughout, then saturated with epoxy. In 30 years, it never developed any delaminations or moisture issues. I am going to follow the same recipe with QUIDNON. That's one thing I am *not* going to experiment with: a proven method is a proven method.

  6. Hi Dmitry,

    RE Rocker: We found it much harder than expected to move a beached hull on wooden rollers. Longitudinal skids help, greased, or to keep rollers clear of soft or rocky ground.

    Rocker and rollers didn't work great together (LUNA's was, however, more than QUIDNON's). It was uphill and down, often squirting the rollers out of position, then bottoming out. SLACKTIDE, whose bottom is half deadflat, eliminated these tendencies.

    If this procedure will be a frequent Occurance, you might consider a short, mid-ships dead flat.

    When you get to modeling, though, a fun couple of hours in the sandbox should give a good idea of your situation. 8)

    Dave Z

    1. Hi Dave,

      I tried to make the rocker have a flat spot in the center, but had lots of trouble fairing the line, so it's a fraction of a degree away from perfectly flat. I've never done the exercise of hauling out this way, but I imagine I would start by lashing together a "railway" out of fairly fat sticks, and move the boat over a raft of round sticks closely spaced. If it becomes a regular occurrence (as opposed to a once-in-a-lifetime act of settling down on some island somewhere) then I would consider building a carriage that rolls over a ramp.

      Does anyone else have experience with this style of "hauling out"?

  7. RE Heeling/Chine Runners: As I recall QUIDNON will draw considerably more than HOGFISH, which will immerse the CRs deeper and increase the lateral plane (of the immersed hull) accordingly.

    As I understand it, efficiency comes not so much from heeling, per se, but from the extra area heeling brings into play.

    In other words, Q with little heel might have the equivalent area of a shallower hull more aggressively heeled?

    Dave Z

    1. Right, it's all a question of how much water the chine runner can prevent from escaping across the chine.

      I expect QUIDNON to be very tender in the middle and progressively more stiff as she heels. With one chine clear of the water, the other should be buried to about 160% the design waterline (wherever that ends up). So, the chine runners may not be so effective on a beam reach in a light breeze, but will get more effective as the wind picks up.

      I just crunched the numbers (should have before) and the draft as shown is a bit too much for a nicely ballasted hull. Reducing it by half produces a sail area/displacement ratio of 16, making it a cruiser-racer. leaving it as is lowers is to 10, making it a cruiser. So, with 30k lbs of ballast it will be able to still sail when loaded with 30k lbs of additional stuff! I'll have to work out the actual ballast on a model.

      I haven't presented my idea of how to create 30k of ballast yet...

    2. Scow displacement numbers DO add up fast!

      Ballast ratio rules-of-thumb from standard hulls can be greatly reduced in sailing scows and barges, I believe, thanks to high form stability and massive reserve stability from high sides and cabin.

      Plus, you've got 'skitter theory' going for you in spades (boards up = less absorption of wave energy, and little tendency to broach or bury).

      NZ scow schooners sailed (at least) the S Pacific islands... Anyone have leads to their ballast practices? SPRAY might offer clues, as well (Bruce Roberts does great, informed assessment in his book on the boat and follow-ons).

      Dave Z

    3. I've been looking at ballast numbers that a "normal boat" would use, and realized a few things. First, the standard sail area/displacement metric doesn't mean much in this case. Boosting the ballast to the point where the boat qualifies as a racer/cruiser as opposed to a ridiculously high-performance racer would make the boat unnecessarily heavy. Second, it seems that what's important is the righting moment. With HOGFISH, I could take her out in a gale, put up full sails, sheet them in, let go the tiller and it would wallow 90º to the wind at 45º angle of heel until something broke. Chris said that when he first did sea trials on her, he went out in a full gale with a marine architect (and empty water tanks) and they did everything they could to capsize her—and couldn't. That's the effect I am looking for. So, I suppose what I should be doing is wind force calculations/righting moment calculations.

    4. Wind will never capsize a ballasted monohull sailboat, big waves will.

    5. I have trouble believing that all those knockdown stories are fake. Never say never. Go out and try it. Find a hurricane, put up all sail, sheet it all in, and see what happens. Report back. I am sure you will succeed :)

  8. Dimity. I have a Paradox here in the UK which I've sailed for 6 years.
    Some points .
    The leeway resistance is due to the entire design and not only the chine runners. The hull shape gives a lifting effect to windward and the more the boat heels the greater this effect. Also the centre of effort of the sail plan is further aft than is usual which is balanced by the large rudder. So tyhe rudder does duty as a centre board in addition to steering. The chine runners play a part in the whole design but aren't the only reason it sails well.

    Also the narrow beam contributes greatly by allowing the hull to heel so the design can go to windward and helps to make the boat unstable when upside down.
    The Paradox is an unusually heavy boat for its length which ensures sufficient hull immersion for the lifting hull and chine runners to work
    I can attest that the boat sails very well and is very well designed. A larger junk rigged version using the same hull form would be viable I think though I'm not so sure of your Scow type bow and wide beam and light weight would cut the mustard.



    1. Hi Jim,

      Yes, the Paradox is one tweaked design. I am shooting to make something a lot less paradoxical, so I am including two big centerboards in my design for going to windward. Using the rudder for lateral resistance works for smaller designs, but for a 36-footer I would prefer a nicely balanced rudder and a balanced rig to match, so that the rudder is only responsible for counteracting a token amount of windward helm, plus course adjustments.