Weight and displacement question about pontoons and house

Hello everyone,

I have just finished construction of our pontoons and have a question about how much weight the pontoons can take. Basically what I need to know is how heavy can my house part of the boat be to safely stay afloat. The pontoons roughly weigh 4000kgs. Please if you can assist that would be great. Attached is a pic of the completed pontoons. Many thanks.

 

Welcome to the forum houseboatuser

Did you build the pontoon from plans, or just from imagination?

If you built it from plans, that means there is a "Lines Plan". Do you have the Lines plan in electronic/dwg format that you can share...as this will give the answer to your question. Because the Lines plan is used to establish the hydrostatic and stability characteristics of your vessel.

 

Hi Ad hoc

Yes from plans using google sketch up. Attached are the pontoon plans.

 

There are two issues you really need to address

1) Hydrostatics
and
2) Stability

#2 is not so much of an issue since you have two hulls separated by some distance, that provides the stability

#1, if you take just half the depth of your hexagon shaped hull that gives you an area of 1.344m^2. Your straight section, or parallel middle body section is 10.76m long. You have bit fore and aft of this, but for simplicity lets call the overall length 11.50 to account for these bits.

So you have 1.344 x 11.50 = 15.46m^3 x 2 = 30.92 x SW density = 31.70 tonne.

So if your pontoons weight roughly 4.0 tonne, you can add roughly up to 27 tonne of weight before you sink the pontoons to half their depth. If you go over that she wont move very far or fast and you will also then start to compromise #2, the stability. As a very rough guide.

The plans don't show any WTBs. I would recommend you put in at least 3 WTBs in each hull.

You also need to be very careful about trim. The pontoon must always remain at level trim and when you use consumables, the trim should remain as level as possible, or trim slightly aft.

 

I would think SketchUp would provide rudimentary volumetric information, what about this info are you having difficulty? As John has mentioned, 1/2 the total volume is the usual rule for these types of hulls, so make the house scantlings within these parameters, preferably a fair bit lower, so you have some margin and can overload a bit every once and awhile.

 

Houseboatuser is a bee-keeper perhaps ? Looks like a whole lot of work went into this.

 

Hi Adhoc and Par,

Thanks for the reply.

WTBs? If you mean watertight compartments there are four in each pontoon.

27 tonne thanks that should give me a good ball park figure to work with and from what you are saying PAR it is recommended to construct something 1/2 that volume to account for other things that would overload the boat such as tanks, people, furniture, etc.

 

Hehe yes a lot of work. Its a dream so just been trying to get my head around everything considering I dont have a builders background but it is a passion. I grew up around the beach and lake fishing but after having an accident that left me paralyzed it was difficult finding boats that were wheelchair accessible so I thought why not try and build one

 

I hope you get a lot of pleasure out of it. You certainly seem to have plenty of latitude with what weight you can pile on it. Where I live, an entire timber home with a corrugated iron roof weighs about 20 tons. Or so a removalist told me.

 

With a slight variation, the hulls could have been made to resemble giant pencils, right down to a sharpened lead point !

 

Houseboatuser, the weights should be estimated, before you finalize your pontoon volumes, not after. My comments are simply that ideally, you'll want the pontoons half submerged with a full load. This is full up, with as many guests as practical, full tanks, full stores, everything, with the 'toons still only half submerged. Of course this means you need the weights of things and their arrangement around the boat, so you can calculate how it'll trim come launch day. There's no real guessing at this, you do the math or pay the price. It's a little tedious, though a spreadsheet can help a lot.

 

Don't forget freeboard. This may be the real problem, not transversal stability.

 

And a final note: Resist the temptation to use the pontoons as tanks. The free surface effect could be disastrous for stability, absent a serious calculation.

Chris

 

Also is there a way to calculate the centre of gravity of the pontoons given the plans? A naval architect I've approached needs the weight of the pontoons (which I've already worked out) and centre of gravity to plug into his model analysis. Thank you all for your advice its much appreciated.

 

I would think SketchUp would be able to provide this information as well?

Is this the design you hope to get 20 knots from? Good luck with that.