Newbie with a houseboat question

[Matt25]

Hello all, I have a houseboat pontoon question I hope you can help me with, my maths finished the day I left school and now it has come back to haunt me as I need to work out the weight my pontoons will be able to take. I managed to get hold of 3 X 10.8m long 830mm X 660mm polyurethane pontoons, each pontoon is made up of 2 pods which have been welded together so there are 6 pods making up my 3 pontoons (hope your all still following) I have drawn a picture of the cross section of them so you can see their shape.

Could someone please work out how much weight in kg's I could place on these pontoons.

Thank you in advance. Matt

[CTMD]

390kg per meter with 75mm Freeboard.

I must be in a good mood.

[alan white]

Sorry---- I tried to edit the first comment. I come up with a displacement of about eight tons less the weight of the pontoons for a reserve bouyancy of twice the displacement.

[Matt25]

sorry, second stupid question! is that 390kg per m of all 3 pontoons when they are side by side or is it per m in each pontoon?

Thanks in advance. Matt

[CTMD]

per metre per pontoon.

[alan white]

Matt, I think the section shown was figured as if a single hull, meaning you would multiply that figure of 390 kg by the total length of 10.8 m x 3 (32.4 m).
This would give 12,636 kg if you allow a freeboard, or above-water exposure, of 75 mm.
You can allow a greater freeboard and come up with 8000 kg as I did. Possibly less, as I figured the section as if rectangular.
What shows above water is considered reserve bouyancy, the amount of which should be enough to ensure safety, good performance, and dryness.
At 50% of sectional area submerged, you might be looking at more like 7000 kg.
I'm not sure what reserve freeboard is usually considered enough for a house boat. The 50% figure would be typical for a smaller pontoon boat. A larger boat, especially with squarish sections could definately sink deeper.

[Matt25]

Thank you for your help, my fiancee mind is now at ease and I can feel the presure off my shoulders. Out of the 3 pontoons 1 is longer than the other 2, they have been modified to take out board brackets, the 2 are 10.8m and the other one is 11.6m. The deck size will be 11.6m X 4m and the height off the deck will be 2m. I am using 75X50mm treated framing for the deck structer with 15mm marine ply this will all be sealed underneath. The wall's will be 75X50mm studs with 400mm centers then covered with 9mm marine ply. The roof will be 150X50 again with 400 centers with a slight curve to allow water run off, this will be covered by 12mm marine ply.

What do you think? Am I on ther right track? any info would be greatly received.

Matt

[rwatson]

Sounds like you are building a real house. The two big enemys are weight and wind resistance.

For example, you can reduce the number of studs simply by making decent woodworking joins and gussetting where they join the other members

Also you wouldnt build a lot of eaves, to reduce windage, and you would want to streamline the 'shed' as much as possbile. You get a lot of value by rounding off corners as much as possible.

I would advise doing your best to measure how heavy the structure will be before building, in case you make the whole thing well and truly overweight. Dont forget, planing or sawing the studs to smaller sections will probably leave them strong enough for a boat, but lose a lot of weight.

[Matt25]

Shed.... Thats what im trying to avoid the look of, Ive been a cabinet maker/installer for a few years now so hopefully my design wil make it look slightly 'off box' shape. I have been considering the structure make up and did wonder if what I was doing was over the top... hence why I put it out to you guys for constructive criticism. I could easily opt for 50X50mm studs and go 600mm centers which would bring the weight down, I just want peace of mind that if we decide to use the roof space at a later date the structer will be safe enough to support it.

I have done some rough calculations for weight of structure based on
9mm ply 2440X1220 = 17kg per sheet
12mm ply 2440X1220 = 22kg per sheet
15mm ply 2440X1220 = 27kg per sheet
1m 75X50mm treated framing = 1.5kg (not exact, just off cut I had in garage)

Obviously I have got a long way to go before I start construction and hopefully with help it will go a lot easier.

[Matt.D]

Hi Matt R u going to glass over the ply? I dont believe u can successfully glass over the treated timber very well. and how much free board r u going to have (a lot more than 75mm i hope) Where r u going to be using ur boat? I would b inclined to use something a lot more substantial then 75x50 for ur deck beams more like 150x50 and use a hardwood. and maybe increace ur hight as 2m is only the height of a doorway and will b very hot little boat with such a low ceiling and i u take off the thickness of ur roof beems of 150mm only give u 1850mm head hight and that is before u line the ceiling! And is there a reason why only 4m wide u have three hulls u might want to look a going out a little futher u can go as wide as 6.1 and still fit in a single birth at a marina. and it will make it a much roomier boat. Im about to start my second house boat up here on the gold coast it is 6m x 11.7.

[Matt25]

Sorry I have not been very clear... This deck will be made up of 15mm ply then the 75X50 then another layer of 15mm ply, the underside will be glassed over to stop the water penetrating. The inside height we were trying to achieve was 2m not outside height. We have only gone 4 m wide because of the marinas here around Auckland charge by length and width and also because of transporting the thing. We could look at going maybe 5m? I'm just trying to sort the weight issues.

This is mainly going to be used to live on, we do not intend to take it out of the marina that much, however it will have twin Johnson 50's on the back this is also due to marina rules where each vessel has to be able to enter and leave under it's own power.

Please keep comments and suggestions coming. Matt

[rwatson]

One of the things you can think about is cutting out as many sharp corners as possible. Open water is invariably windy, and its a lot more peacefull if you can radius the square edges. If you were doing much cruising, the rounded edges would make the windage a lot less.

The other thing you probably should get some local advice on is the size of the main support beams. Three pontoons, say 5 metres wide ,gives you a fair old area to support, especially if you get any kind of swell. The bouyancy of the pontoons, and their flexibility means you should have some substantial beams to keep the whole structure rigid.
If you have lots of ply, as it appears, you could go for an 'egg crate' space frame construction for a light rigid platform. Think of 'hi-beams' as they are known in the building trade, for example

[Matt25]

I like the idea of rounding off the edges, anything to loose the box shape is great in my eyes. im drawing up new deck plans as we type (multi tasking at it's best)

Is this how you constructed your first one? how did you do your framing and walls?

[alan white]

It might be better to cantilever the deck a little to reduce overall weight (by reducing the structure below) and to limit twisting forces. Twist is usually not a problem on a squarish barge-type hull, but a structure like the one you describe is going to need to be very stiffly made above to avoid twist when the water becomes rough.
One means of stiffening would be to glue and screw the house frame, and then bolt the framed walls to the deck structure. Another would be to do the same to the deck frame. Internal walls can also contribute. The idea is to create squarish areas of wall above to create diagonal stiffness. More stiffness can be designed in by going much taller on your athwartship joists (frames) by web-trussing.
I would recommend more height than would provide span stiffness. 100mm would be good. The web trusses are lighter and have the added benefit of providing storage for a sump and access for running wires, pipes, etc..
The connection of each web truss to the longitudinal side frames should be glassed to further stiffen the joint.
Also, regarding the roof frames, you can lighten up those members by making your own web frames having a curved top and flat bottom, with 20mm top and bottom members (laid flat, maybe 50mm wide), and the vertically oriented member (the web) being 12mm ply (there being a groove in both top and bottom pieces). Then the crown could be about 150mm high, the sides maybe 100mm.

Alan

[Matt25]

Thanks for everyones help so far, I have took it all in and are now doing some more drawings and working out my weight again, I'll keep you posted with how I get on.
Matt