Weight and Displacement of a Houseboat

by Keith Fung

Calculate weight and displacement of houseboat pontoons

Calculate weight and displacement of houseboat pontoons

What is the average weight of a houseboat per square foot in plan? I plan to design and build my own houseboat and for starters I need to calculate how much displacement I need.

Keith Fung from Trinidad.

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Reply - Answer

Great question Keith, since the displacement of a houseboat is important to calculate before building.

I don't have the figures right off the top of my head, but I remember seeing a chart that showed the average weight of some sample houseboat sizes and equipment.

What I can help you with is the buoyancy and displacement that twin pontoons of different sizes and diameters can hold. The chart shows houseboat pontoon diameter by length, and by capacity in pounds.

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Lastly, hopefully some of our readers and visitors will share and post comments about their sample weight per square calculations and their houseboat building experiences and tips.

Feel free to use the "Click here to post comments." link found near the bottom of this page.

Thanks again for sharing, IAN from all-about-houseboats

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Aluminum Pontoon Houseboat Weight
by: Anonymous

I am new to houseboating and recently purchased a 21m x 6.10m, with 1m Dia pontoons, which is an aluminum twin pontoon houseboat, single deck ht of 3m and open party top deck made by Sea Spray.

The year of manufacture is 2009. The company here in the UAE has gone defunct and i am trying to get the weight specifications, and any other specifications as i wish to have it lifted to dry dock for anti-fouling.

I was reviewing the chart which is published on your forum regarding calculating weights etc, however I could not identify anything longer than 60ft?

Is it possible through this website that somebody can point me in the right direction to calculate the overall weight for lifting.

Many thanks and appreciation in advance, Keith

Re: Caveat
by: Anonymous

The comment posted by beached is one of the best articles written about pontoon buoyancy. It shows a thorough understanding of the issues involved.

Anyone thinking of using round section pontoons needs to be aware of this. I would urge anyone to also Google what is known as the PONTOON EFFECT ....

Archemedes Princible to calculate weight of houseboat
by: Anonymous

The weight of a floating object is equal to the weight of water displaced. I calculated the volume of one tube and divided the sum by 2 so the tubes will sit at half submersion. See Archiemedes Princible.

To calculate the volume of the toon I used (Volume = pie times Radius squared times length.) For each cubic foot of volume you can pack on 62.4 lbs of weight.

A question for Keith?
by: Anonymous

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.

Weight and displacement of some Houseboats
by: Scotty

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.

by: Beached

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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