Trailerable houseboat on pontoons

[Big Aussie]

Hi everyone!

I am thinking of building a trailerable houseboat in plate aluminium for use on smooth inland waters (fresh water) only. I have built a houseboat previously where one has to step down to the floor of the boat. This time I am considering pontoons.



The length of the houseboat will be 24 feet (7500mm) and the beam will be 8 feet,(2400mm) The internal height from floor to ceiling will be 6 ft 8 inches (2000mm). The total height will be 10 feet (3000mm) including pontoons. There will be no access to the roof.



I would greatly appreciate your help in deciding whether I should go with option a) or option b)

(a) Catamaran-type with square (flat) pontoons each feet 33.5 inches (800 mm) wide, 2 feet ( 600mm) high by 22 feet (7000mm) long with a gap of 12 inches (300mm) between them. The total weight of the pontoons will be about 700 lbs (315 Kgs)

(b) A barge-type punt made out of 2 inch (45mm) thick hollow planks , welded together and then an aluminium floor installed on top, to completely stop water from entering the barge. (in other words, one large reasonably water-tight pontoon!). The barge will be 25 ft (7500mm) long, 3 ft ( 600 mm ) high and 8 ft (2400mm) wide. Water tanks weighing about 440lbs (200 Kgs) will be permanently fixed right in the centre of the barge floor. The barge will weigh about 800 lbs (400 Kgs)



Above the floor of either the square pontoons or the sealed floor of the barge, I would construct the walls and roof of the houseboat. Furniture, sitting on the floor, consisting of shower cubicle, fridge, furniture and other items will weigh aproximately 880 lbs (400Kgs). I expect to carry 6 persons,weighing a total of 1000 lbs (480Kgs). Top wall cabinets will be installed below the ceiling and with personal effects etc is expected to weigh , say 220 lbs (100Kgs.)

At floor level the beam will be 8 ft (2400mm) wide tapering to 6 1/2 ft (1950mm) at roof level in an attempt to make the vessel more aerodynamic.



Cost is not a consideration as both options would cost roughly the same.



The total weight of the houseboat is estimated at being 4000 lbs (1800Kgs) without passengers. The houseboat would be propelled by a 25HP outboard motor weighing about 130 lbs (60 Kgs.)



I would greatly appreciate if you would comment on the following:



1) What would be the chances of it capsizing in calm conditions with either a) or b)?

2) In winds of 20 knots?

3) Possibility of it sinking completely, with a) or b)?

4) Do you consider it to be top heavy?

5) What would be the effect, (on stability) of making the pontoon/barge 6 1/2 ft (1950 mm) wide instead of 8ft (2400mm) so it may fit between the wheel arches of car trailer?



I very much appreciate your time in reading this long post, and I thank you in advance foryour forthcoming comments.

Cheers.!

 

[bajansailor]

This is an interesting post Big Aussie, and I am sure it will generate many responses.
However your questions would be a lot easier to understand if you could include some sketches of your proposed designs, rather than trying to describe them in words.
I presume that you do have some rough sketches already?
It doesn't matter if they are on the back of a napkin - as the saying goes, 'a sketch is worth a thousand words'.

 

[Big Aussie]

Good idea Bajansailor. Attaching a rough diagram now. Thanks.

 

[fallguy]

Start off by calculating the displacement of each.

4000# boat/
62.4#/cuft (weight of fw)
Is
64.1 cubic feet

64.1 cubic feet divide the area of each shape

Option A
33.5"/12"•22'•2 =122.8 square feet

Option B
25'•8'=400 square feet

64.1 cubic feet / 122.8 square feet = 0.5 feet

64.1 cubic feet / 400 = 0.16 feet

The toons sink over 6" in the water and the barge sinks 2". So, the barge offers about 3x the displacement.

Then you can continue with stability calculations.

But it is a bit obvious the narrow pontoons are going to be a problem.

 

[Heimfried]

The idea of narrowing the underwater body for road transport reasons is quite dangerous.
Look at the pics. in completely calm conditions the boat has nearly no stability. The first pic shows her on equilibrium, the second one after just two persons (160 kg) changed from starboard to portside (2000 mm), the resulting heel is nearly 12 degree. Assumption: cgz = 1300 mm above base plane, about 600 mm above floor.

 

[fallguy]

The idea of narrowing the underwater body for road transport reasons is quite dangerous.
Look at the pics. in completely calm conditions the boat has nearly no stability. The first pic shows her on equilibrium, the second one after just two persons (160 kg) changed from starboard to portside (2000 mm), the resulting heel is nearly 12 degree. Assumption: cgz = 1300 mm above base plane, about 600 mm above floor.
View attachment 169689 View attachment 169690

Thank you for posting a better reply than me pointing out what is obvious. I genuinely hoped someone would.

Pontoon trailers here typically span the bottom deck of the pontoon and this allows the toons to be far enough apart, but requires the construction of the deck to support the toons on the road.

 

[Big Aussie]

Thank you gentlemen. I have to spend some time to get my head around it.
But what about the stability of option B)?(full width)
and does weight come into it?, I see barges carrying very heavy loads above deck.

 

[bajansailor]

If you must go for a catamaran hull form, then at least try to have the two pontoons as far apart as possible within the given 8' beam limitation.
Is this limit etched in stone? Or could you possibly increase it?
A 24' x 12' cat houseboat would open up a lot more possibilities for you, and it would be a lot more stable.

If you go for a monohull barge form, if you spend US$ 72 on a set of plans for the Glen-L 24' Water Lodge, then I think that this would be money well spent, even if you just use them for reference and for ideas re the design of your own boat.
20' & 24' Water Lodge - flat bottomed houseboats-boatdesign https://www.boatdesigns.com/20-24-Water-Lodge-flat-bottomed-houseboats/products/899/

 

[Big Aussie]

Thank you bajansailor.
I am more in favour of using a barge as a base , because the barge is much tougher and would be less prone to mechanical damage, given that I would be navigating in mostly shallow waterways.

But can someone please explain to me why a 2-pontoon system would be more stable than a flat-bottom barge? Thanks.

 

[bajansailor]

But can someone please explain to me why a 2-pontoon system would be more stable than a flat-bottom barge? Thanks

All else being equal, a cat hull form should be relatively more stable than a monohull barge of the same length and breadth.
One formula for calculating stability is BM = I /V
Where I is the second moment of area of the water plane, and V is the volume of displacement.
With a barge, you have a lot of volume of displacement in way of the centreline of the vessel - but you do not have a lot of 'I' as I is dependent on the distance away from the neutral axis (ie the centreline of the vessel).
Whereas with a catamaran you have no volume of displacement in the middle to reduce you BM, and lots of 'I' with the hulls spaced far apart.
Yes, this is a very simplified way of explaining it, but I hope it makes sense.

 

[Big Aussie]

Yes, it is starting to make sense, thanks.

 

[Heimfried]

Regarding "plan B" the full width is far better. To compare both look at the pics but take notice that the ordinate in the diagram is different.
At heel of 10 deg. the narrower setting has a righting lever (GZ) of 0.06 m, the wider one a GZ of 0.32 m which is a large difference.

This is not exactly your boat but an blunt cut prism of 6 m lenght. It should work to give you an impression of the conditions. cgz is set to 1,3 m above base plane, weight 2280 kg and beam at 2,4 m.

 

[Heimfried]

Additional the barge to illustrate the formula bajansailor gave.
The GZ maximum is lower (0,32 m) and comes at 14° while the Kat provides 0.42 m at 17° heel angle. The area under the curve from 0° to 40° (AVS) is greater - that means it requires more energy to knock over the vessel.

 

[Big Aussie]

Thank you everyone for your comments,

However , as I am not an engineer, I dont fullye understand the graphs and formulae etc that you have been kind enough to send me.
But I am still confused. One thing I have understood is that if I went with the pontoons, I would need to place them as far apart as possible.

So in conclusion, I would appreciate if you could look at the attached diagram and let me know which system you would reccommend.

Again, thank you all for your assistance.

 

[Heimfried]

As bajansailor said, "A" of your sketch (Kat) is more stable.