# 3 pontoon houseboat convert to 2 pontoons

Discussion in 'Boat Design' started by flyinwall, Feb 23, 2018.

1. Joined: Feb 2008
Posts: 26
Likes: 0, Points: 1, Legacy Rep: 10
Location: Cooroy Queensland Australia

### flyinwallJunior Member

we have a 10m houseboat that currently has 3 600mm aluminum pontoons on it (which leak), we are looking to replace the pontoons but to try and keep costs down a bit i have been thinking of replacing the 3 pontoons with 2 800mm pontoons approx 12m long...
can someone tell me if it is going to have enough buoyancy with the 2 pontoons, the current dry weight of the houseboat is approx 5t
thank you in advance for all responces of assistance...

2. Joined: Nov 2003
Posts: 19,133
Likes: 473, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

### PARYacht Designer/Builder

Well the math is quite simple and you've obviously not tried it yet. Do the math for the total displaced volume of what you currently have (typically 50% of the 'toons diameter), so you'll know what you need to support with the new 'toons.

3. Joined: Feb 2008
Posts: 26
Likes: 0, Points: 1, Legacy Rep: 10
Location: Cooroy Queensland Australia

### flyinwallJunior Member

sorry PAR but maths has never been something i am good at... (sometimes it honestly confuses me)

4. Joined: Sep 2011
Posts: 5,861
Likes: 218, Points: 73, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Can you confirm the following?
-at present there are three pontoons, aluminum, of 3600 mm in diameter and 10 m in length.
- you want to place 2 pontoons, aluminum, of 2800 mm in diameter and 12 m in length.
- what minimum freeboard are you obliged, or want, to have?

5. Joined: Feb 2005
Posts: 3,818
Likes: 156, Points: 63, Legacy Rep: 971
Location: Coastal Georgia

### SamSamSenior Member

Assuming they are round shaped, the formula for finding the area of a circle is Area = pie (3.14) times the radius squared.
For the smaller pontoons the numbers are 300 x 300 = 90,000. Then 3.14 x 90,000 = 282, 600 mm area. Then times 3 pontoons = 847,800 total area.
For the larger pontoons the numbers are 400 x 400 = 160,000. Then 3.14 x 160,000 = 502,400 mm area. Then times 2 pontoons = 1,004,800 total area.
So if three 600mm pontoons worked, two 800mm pontoons supply that much more flotation.
If you take those area calculations and times them by length, you can figure the volume and from there you can figure out the displacement, which depends on whether it's in fresh or salt water.
I'm not too conversant with metrics, but compared to what we use, I'm pretty sure they are based on reasonable common sense. Something about a cubic meter of water weighs a metric ton or something. Maybe a thousand kilograms.

6. Joined: Sep 2011
Posts: 5,861
Likes: 218, Points: 73, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

That is correct only if the pontoons are supposed to be totally submerged. When they are not, which would be the right thing and sensible for there to be a reserve of buoyancy, the thing is not so simple, although we can not say that it is complicated. In addition, we must bear in mind that the weight of pontoons, in each case, will be different.

7. Joined: Feb 2005
Posts: 3,818
Likes: 156, Points: 63, Legacy Rep: 971
Location: Coastal Georgia

### SamSamSenior Member

Well if you want to half submerge them, halve the numbers and it still comes out better. I don't see where it changes anything.
I didn't do any volume calculations or displacement calculations as I'm not that nice of a guy.
Usually boats aren't terribly technical or as finicky as rocket science.

Last edited: Feb 24, 2018
8. Joined: Sep 2011
Posts: 5,861
Likes: 218, Points: 73, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Consider, if you feel like it and have time, when the pontoons of 3 m are submerged, for example, only 2/3 of their diameter. In that case, how much would the 2.8 m pontoons be submerged ?.
If you want to go to the real case, how much are the current pontoons submerged to support the 5 tons that, apparently, weighs the whole ?. It is not rocket science but it's not that easy, is it?

9. Joined: Feb 2005
Posts: 3,818
Likes: 156, Points: 63, Legacy Rep: 971
Location: Coastal Georgia

### SamSamSenior Member

Ok, I get it. 30 meters of 600mm pontoons vs 24 meters of 800mm pontoons. Submergence of different sized cylinders. Other stuff too, I'm sure.
You are right, it is not rocket science but it is also not that easy.
I have considered it and decided the time is not an issue but I just don't feel like doing all the math or to get all that involved in the problem right now. Probably later.

10. Joined: Nov 2012
Posts: 759
Likes: 39, Points: 28, Legacy Rep: 41
Location: Delta BC

### JSLSenior Member

apart from the pontoons, how about the cross beam structure. They may have to be 'beefed up' to account for the greater span. This may add weight which requires more floatation =bigger pontoons. The project might???? be more than a 'bit of math'.
The other matter is to look at waterplane area and the immersion rate ( Tons per Inch (TPI) or lbs/inch immersion.
Have you checked with the original builder for any of his comments. or.. repair the current pontoons?

11. Joined: Sep 2011
Posts: 5,861
Likes: 218, Points: 73, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

It is probable that, by appropriately changing the position of the small pontoons, it would not be necessary to act on the cross beams, but, of course, it would be convenient to do some "naval architecture" calculation.
Wait to have more information about the problem because with what we have now, nothing useful can be done.

12. Joined: Oct 2007
Posts: 568
Likes: 124, Points: 43, Legacy Rep: 37

### bajansailorMarine Surveyor

Let us assume that the pontoons are simple cylinders - ie they do not have pointy ends on them.
For your current 10 metre long boat with 3 x 600 mm diameter pontoons, assuming simple cylinders. we have total buoyancy (if totally immersed) of 8.48 cubic metres.
This is from cross section area (Pi times D squared / 4), times length x 3.
Hence (3.142 x 0.6 x 0.6) /4 times (10 x 3)

But we do not want to immerse the cylinders totally, as then they do not have any reserve buoyancy. So let's say that we do not want to go beyond halfway, ie at maximum diameter.
Hence we have a volume at half immersion for your current boat of 4.24 cubic metres, which in sea water is approx 4.35 metric tonnes.
If your current boat's weight is about 5 tons (or metric tonnes?) then she is probably floating a bit deeper than half immersion.

It would be useful to know the total surface area of these 3 cylinders - for one cylinder, the surface area is Circumference (Pi times D) times length (I am ignoring the surface area of the end caps, just to simplify things a bit) .
So for one cylinder, the surface area is 3.142 x 0.6 x 10 = 18.85 square metres.
And for 3 cylinders it is 56.55 square metres.
If the cylinders are 2 mm thick (for example), then the weight of them would be 305 kg.
If they are 3 mm (approx 1/8") thick, then the weight is 50% more, so approx 457 kg.

Now lets consider your new proposal, where you have two 800 mm diameter pontoons, each 12 metres long.
Using the same formula above, the total buoyancy in these two hulls is 12.06 cubic metres.
But we do not want to go more than halfway immersion, so at a draft of 0.4 m, we have a buoyancy of 6.03 cubic metres.
Or a displacement of 6.18 metric tonnes in salt water.
Even if the overall weight of this new boat is a bit more than 5 tonnes, then she will still be floating at less than half immersion, which is good.

If we calculate surface areas again as above, the total surface area of these two pontoons (again excluding end caps for simplicity) is 60.3 square metres.
For 2 mm thick aluminium this gives an aluminium weight of 325 kg.
Or for 3 mm thick pontoons, the weight is 487 kg.

In other words, although you are gaining an extra 40% of buoyancy with your new hulls, the extra weight is only 30 kg, or 6%.
Hence it sounds like a good idea.

You might also find that the resistance of two slightly wider hulls is less than the resistance of 3 slightly narrower hulls, depending on how far apart the hulls are (but if so, this will probably be fairly slight).

Last edited: Feb 24, 2018
13. Joined: Sep 2011
Posts: 5,861
Likes: 218, Points: 73, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

I wonder if the OP wants to know, more or less, what it can do or know in more detail how far it can go. One thing that can be deduced from the interesting calculations of @bajansailor is that the length of the new pontoons could be reduced. So, I think, it would be worthwhile to do some more detailed calculations.

14. Joined: Jan 2006
Posts: 3,001
Likes: 202, Points: 63, Legacy Rep: 1279
Location: Lakeland Fl USA

Two larger pontoons will provide better stability numbers than three smaller ones..........Arguments welcome

15. Joined: Nov 2017
Posts: 135
Likes: 6, Points: 18
Location: Sweden

### Magnus WSenior Member

Who knows why they built it with three pontoons to begin with. Deck height, redundancy and availability are possible reasons that come to mind.

All other arguments aside a twin design will give you nice space between the pontons that could be used for holding tanks, storage space, ballast tanks, whatever.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.