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.

Wednesday, January 7, 2015

Ballast and stability

Dave and I have been puzzling over this all day (among other things): How much ballast should QUIDNON carry? Its beam and hard chines will make it very stiff for very little ballast, but how much is enough, and under what conditions?

To answer that question, let's ask another one: Take your typical cruising sailboat if you take it out into a near gale, a fresh gale, a whole gale, or a strong gale, corresponding to 7, 8, 9 and 10 on the Beaufort scale (we won't do this exercise in a whole storm because that would be completely stupid in any boat). What happens if you then put up all the sails (in this case, 1000 ft of canvas), sheet them in as tight as you can, and... turn the boat sideways to the wind?

Answer: instant knock-down. Right?

Wrong! I did this with HOGFISH in a near gale, and what it did was wallow at a 45º angle, gently drifting to leeward. Her builder, Chris Morejohn, said he did this same exercise on purpose during her sea trials, with his friend who is a naval architect, in a gale, and found that doing so had a similar effect. He said that she is impossible to capsize, and I believe him.

I want QUIDNON to behave similarly well, so I did some math. Here is a little table I put together:

The thing being calculated is the amount of ballast needed to achieve this effect under various conditions. Now, 40,000 lb of ballast is borderline reasonable. Above that we get into the somewhat ridiculous territory; the boat would still float fine, but would be fully loaded with ballast in order to make it safe for a completely incompetent, borderline suicidal skipper. And that just isn't me.

Keeping in mind that 1000 sq. ft of canvas is quite a lot, and that this exercise is about as extreme as I can imagine (don't try this with your boat, please!) I am tempted to just dial in the ballast at around 20,000 lbs and leave it at that. After all, the Junk rig is so easy to reef when the wind picks up.


  1. i would love to get into the cargo olive etc. transportation business in the mediterian area here in europe with a cargo trilo boat any buddies out there to serious join me in the bih-serb-ro-bg region (more eastern mentality not so western)?:

    1. Putting a triloboat on ebay—now there's a cute idea! I hope it sells! Best of luck to you with this wonderful endeavor.

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  3. Maybe I'm missing something: 20,000 lbs of ballast on a 15,000 lbs displacement boat?

    1. I made some preliminary calculations, and it looks like the hull shape can displace as much as 60k lbs and stay within the waterline I drew. My current concern is that it will float too high and that the motor will cavitate. But the ballast can be dialed up more or less arbitrarily.

      This hull is very wide, and with its flat bottom it submerges 1 inch for every 2500 lbs. So 30000 lbs of ballast would equate to 1 ft of additional draft. If the boat weighed 15k lbs, it would draw 1.5 ft. Adding 30k lbs of ballast would make it draw 2.5ft.

      The final ballast number will be determined by testing a model. The point of the exercise here is to illustrate just how stable this design is going to be, not to fix the exact number.

  4. Dmitry, I have Dave's triloboat plans and am awaiting a chance to build a "barge" sailboat as soon as I can work out exactly what I want. Obviously stability is a major concern for anyone engaging in this, is there any chance you could detail how you worked out the stability figures so we can all learn how to do it for ourselves or at least point us in the direction of what we should be looking for so we can teach ourselves

    Thanks Steve

    1. Keeping in mind that the thought experiment I am performing is completely ridiculous and would make most boat designers run away screaming...

      To calculate wind force F_w, take your sail area, reduce it by heeling angle (that's just a bit of high school trig), and plug it into the standard wind force formula together with your wind speed (you can google for that). That gives you wind force in lbs.

      To calculate wind lever arm L_w, take the center of effort of your sail plan (you can use cardboard cutouts and balance them on a pin if you like, or use math), and reduce it by heeling angle same way. It's measured from the waterline up.

      To calculate the ballast lever arm L_b, first calculate the center of buoyancy of your hull when heeled over (for a Triloboat, when it's heeled over, you just work with the submerged triangle and find the center of that. Cut it out of cardboard and balance it on a pin if you have to, but it's generally pretty a foot or so in from the side. The lever arm is the distance from the center of buoyancy to the center mass of the boat (right in the center of the beam, unless it was listing to start with) reduced by the healing angle.

      To calculate the force exerted by the ballast, F_b, assume that, at a 45º angle of heel (which is what I am using) half the ballast has been hefted out of the water and is resting on the submerged triangle.

      The fulcrum formula is F_w * L_w = F_b * L_b

      Solve for L_b, and calculate ballast from that.

      Keep in mind that the final ballast number will be based on a model, not on a calculation.

  5. Not a naval Architect but I believe your design will have massive initial stability somewhat like a catamaran but once you are heeled over to a certain angle you will loose some percentage of that righting force because of the flat bottom.

    A keel extends that righting force out increasing stability at higher angles.

    The other factor it seems to me is how much freeboard you have vs overall weight. In other words as the boat heels will the freeboard be forced under or will it be buoyant and the boat will stand on its beam.

    Just some thoughts.

    1. Not one of those "navel architects" either, but I do seem to have a bit of a clue about such things, as do you, Jef! The topsides on QUIDNON have about half the surface area of the bottom, and a similar curve. So, standing on its beam-end, QUIDNON will draw twice as much as sitting on her lines. Since the design draft is 2.5 feet or less, it will draw 5 feet (out of 16) when heeled 90º. That's not considering the pilothouse, each side of which will also act as part of the hull when floating on the beam end because all the openings in it will be above the 5-foot "waterline." The lever arm of the ballast will be about 1/2 of the height of the topsides, or 3.5-4 feet, and 1/2 of the ballast will be lifted out of the water. I haven't done all the calculations yet, but my feeling is that with some reasonable amount of ballast the boat will right itself just fine from a 90º capsize, and will eventually right itself from a complete inversion because it will be unstable floating upside-down.