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Submitted by: Anonymous

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Hi Keith, I'm thinking about building a barge to lift boat moorings, if the total load including mooring, lifting gear, hull material, motor, etc, etc, (everything) came to perhaps 20 tonnes. If I want the waterline to be half way up the flat bottom hull, based on what you've said, I presume I'll need a hull volume of 40 cu meters, have I understood you correctly? Thank You.

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Submitted by: Scotty

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My 2 cents worth....whatever houseboat you build will be heavier than what you had planned! My aluminum 35ft estimate was 5.5 Tons and I ended up with 7 Tons. A friends steel 45ft is 17 Tons. I have a 16 ft beam and his is 17 ft. Both are probably "well furnished" so hopefully this will give you a calculation. Also think of the water traffic where you intend to use your houseboat- it's pretty easy for a large cruiser to send a wake in your direction that is 3 ft high. To not have the wave come on board, you need to make sure you have enough freeboard. To my mind you need at least 2 ft out of the water. Good practice is to have maximum of half your pontoon diameter in the water, which means 4 ft minimum pontoon diameter. Don't forget to be realistic in your live load, EX: people, food, and water/fuel.... Say 1 week onboard- 700 liters of water, 150 liters of fuel, 200 liters of sewage you didn't pump-out last trip, 80kg of food, dishwash liquid, wine, beer and potatoes (you weigh your groceries next time!). And 3 people on board, plus the 4 friends who stop in... all of this adds up to say about 1.8 Tons on top of your estimated hull weight to be kept in reserve. Hull volume (m3)= max weight(Tonne) x 2 to have your hull or pontoon half submerged at max loading. If you said 10 Ton of boat load max (say 40ft alloy or 35ft steel) you need 20m3 of hull -2 pontoons of around 1m square or a flat bottom hull 0.5 m deep.

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Submitted by: Beached

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I'd like to add my 3 cents worth of opinion here. I am aware that these prefab aluminum tubes are popular to provide floatation in pontoon boats and houseboats. Popular because I presume they are factory made, cost effective and your floatation calculations are simple. I can see them in application of a dock and in the smaller pontoon boats, but would not consider them in any sort of serious houseboat. Any floatation design that has reduced buoyancy at a certain point of the immersion is not something I would want to depend on. A slab sided box maintains the same carrying capacity for every inch of immersion. All other hull types increase the capacity to carry weight for every inch of immersion with the exception of a cylinder that lies sideways on the water. Once you are past midpoint of diameter your ratio decreases at an accelerated level, where at a certain point a very small increase of weight will result in a sinking. This is also an alarming situation when the vessel is subjected to a sudden shift of weight to one side as for example people on the upper level rushing to one side because of some interesting situation. The total weight on the boat may very well be within the carrying capacity as long as it is relatively equally distributed and supported by both tubes but will become quickly dangerous when the load shift to mostly one tube. All boats will heel under that situation and can be made to capsize, however the houseboat with circular tubes is especially prone to that situation. If I would need to use something other then a single hull similar to a barge I would recommend a U or V cross shaped unit.

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