24' pontoon Houseboat

I have 19" pontoons on a 24' long pontoon boat. I have weight the almost finished structure and it weighs 2760 pounds. When finished I expect it to weigh approximately 3600 pounds. I am planning to add a middle pontoon about 18-20'. Will this plan support the added weight?

 

If I figured it right, half immersed, the 2 - 19 x 24' pontoons would displace around 2900 lbs so you would def need the center one and would be nice to have some extra buoyancy.

 

Marginal at best. The third toon would get you to about 900 more or less pounds capacity at half immersion. Not nearly enough safety factor unless you are always in calm water which can never be relied upon. Do consider the shifting weight. Passengers move from side to side of the boat and that will matter a lot if you are at, or near, the optimum immersion of your toons. which you are exceeding. The optimum immersion here is half the diameter or 9,5 inches. Past that immersion level the added buoyancy is seriously not a lineal function of immersion. Rectangular pontoons do have lineal displacement per inch of immersion, round ones do not.

Do your own math.....The area of a circle is the radius squared multiplied by pi (3.1416) 9.5 x 9.5 x 3.1416. Multiply by the length of the toon times... 228 inches You will get 64,688 cubic inches for total immersion of the toon. You want half or less immersion so half of 64,688 is 32322 cubic inches.......Now multiply that number by the constant 0.03611....you get 1176 which is the pounds of flotation (displacement of fresh water) at half immersion. Keep in mind that you have to subtract the weight of the toon itself from the calculated load capacity. Lets say that the toon might weigh......276 pounds....that leaves you with a useful displacement, or weight bearing, of 1176 minus 276 = 900 pounds.

The constant 0.03611 is merely a convenient number to help assess pounds of displacement.......One cubic foot of fresh water weighs 62.4 pounds and a cubic foot contains 12" x 12" x 12" inches = 1728 cubic inches....divide 62.4pounds by 1728 cubic inches and you get the 0.03611 pound per cubic inch of potential displacement.,. That's where the constant number comes from. I hope that helps simplify your calculations.

The bottom line is that your load and your potential three pontoon safe displacement is a no-no. You would need a fourth pontoon to get to the almost safe load capacity that you are contemplating.....sorry about the evil displacement math that thwarts your perfectly good intention.

 

This is not a good direction you are heading in.
Adding a third pontoon is by no means a safe way to deal with your lack of buoyancy from the original two pontoons.
Your waterline should be at 1/3 buoyancy, not 1/2.
What you have is a recipe for disaster.
Either lighten your load or get larger diameter pontoons for your catamaran.
I accept you wont like my post; reality sucks.

 

It sounds like it's going to be top heavy and easily rolled over.

 

So, each of your two pontoons displace about 3000 pounds of water.
At 1/3 static loading, that's 1000 pounds each ( catamaran configuration ).
So, 2000 pound boat, total, including everything.
People, fuel, water, provisions, mud, snow, rain soaked carpet, etc, etc, etc.
The reserve 1000 pounds is for wind load, wave inertia interaction, etc.
This may seem ridiculously conservative... but it's not.
Good luck.

 

I appreciate everyone's comments it has caused me the re think my project. I have began inquiring in my area what pontoons are available to replace my existing 19" pontoons so far I have found several sets but none of which I think would solve my problem. The largest so far is a set of 20" D shape pontoons but I am sure there would not be large enough solve the problem . Could anyone out there give some advise on what size would be sufficient to handle the load that I am expecting on my houseboat? Again thank you for all your input.

 

These would displace about 5300 lbs at half immersion, I would still go with a third one or larger diameters. You have to think about the live loads as well, what happens if one day you have a party, 20 friends come over and you add another 1,000- 2000 lbs
... I don't know if these are any good but the price is nice.

Boat Pontoon Log | 25 Foot x 25 Inch Tube Float w/ Strakes (2 PC Set) http://www.ebay.com/itm/Boat-Pontoon-Log-25-Foot-x-25-Inch-Tube-Float-w-Strakes-2-PC-Set-/192212699804

 

27" X 24' pontoons would safely float your 24 foot boat based on 3600 pounds total weight.
1/3 static loading.
What is not included in this calculation is the added weight of larger pontoons...
28" pontoons would likely cover the additional weight of the larger pontoons.
If you're starting over you may want to consider a flat bottom hull with a couple of skeggs.

 

Bluebell, be aware that the displacement of a round tube at different levels of immersion is not a lineal function. That is to say that a tube capable of displacing 3000 pounds total , will not support 1000 pounds when immersed at one third the diameter. ... more like 870.

The nineteen inch tube at one third would have a waterline beam of 17.9 inches and the 24 foot tube would then support about 715 pounds. Alternatively, A 26 inch tube at one third would displace about 1350 pounds if 24 feet long. At 26 inch diameter, 24 feet long, would displace about 2300 pounds at half immersion.

My post above used shamefully sloppy math. My bad. Let me try again. When I multiplied the area of the section by the length, I carelessly used 19 feet of length which gave me a smaller displacement. Done more carefully the 19 inch by 24 foot tube would displace a total of 2450 pounds. Half immersed 1225 pounds. In either case the fact that some displacement is lost if the front end of the toon is pointy. In these calculations that loss was ignored and supposed, for simplicity, that the tube was continuous end to end.

The arithmetic for a rectangular pontoon is dead simple, not so for round ones. In the case of the rectangular one, pounds per inch of immersion is in fact a lineal function. The rectangular toon continues to produce the same pounds per inch at any level of draft. The round one begins to provide progressively less flotation when immersed beyond half way.

 

Messabout: did you say that 3-19" logs would support 3675# minus point part of the tubes.

One last question is it possible to use a larger tube for the center for added safety say 22 or 23" diameter?

I did locate a couple of 25" D tubes x 25' long, they probably would not be enough with out a third >

 

A larger toon in the middle would give it more of a rocky motion side to side if the tops are level and the middle one sticks down below the side ones.
The larger 25" U shaped toons sound much closer to a solution for your boat. They come close to doubling your weight capacity.
How tall are the U shaped toons?
Are the rounded parts a true circle or more of a flatter, oval shape?

 

Messabout, I am well aware of the variables, that's why one should use 1/3 loading vs 1/3 immersion.

Hikerman, sorry, I missed the D shape comment earlier.
I'll assume you're referring to cross-section.
That is a game changer here.
Are they tapered at each end?
Just one end?
What do they weigh?
Have you included everything in your load calculation?

Going by your original post, yes, two D-shaped 25" X 25' pontoons would be adequate providing they are closed / sealed at the top.

 

Yeah. Now we are in better territory for you project. A pair of 25 inch D sections will do the job. I am assuming that the section looks like the letter U with or without a cap on top. That means that the rounded part at the bottom will have a radius of 12.5 inches. Using the whole 25 feet, the pontoon when immersed up to its point where it begins to be a straight side is actually a half round vessel. At 12.5 inches draft the displacement would be 2,654 for each pontoon. That would give you, for the pair 5300 pounds of flotation at only a tad over a foot of draft.

Let's assume for the sake of investigation that the skin is one eighth (0.125) thick. The wrap around length of the skin, without a top, is 64 inches. One eighth aluminum weighs 1.76 pounds per square foot, therefore each foot of the toon will weigh 9.38 pounds. If 25 feet long it will weigh 234 pounds. If it has a one eighth top then we must add 3.66 pound per foot or 91.5 more pounds. which makes the whole thing weigh 325 pounds. Subtract that from the displacement of the section at twelve and a half inches draft and we have 2654 - 325 = 2329 pounds of useful displacement two pontoons will then handle 4658 pounds with out reaching the dangerous depth of immersion of the round types of pontoons.

There is more to be pleased about the D section. After the round part is fully immersed, you have a reserve buoyancy of 3385 pounds before the section is sunk completely. The part above the rounded part is twelve and a half inches by 25 inches wide. That gives us a now linear progression of displacement about 270 pounds for each inch of additional immersion.

The D section is the most favorable section although the plain rectangular one, the one with a flat bottom, is a little more buoyant. There are structural considerations. The rounded part will need few, if any, interior frames. Whereas a flat bottomed section might need some reinforcement, or need to use thicker aluminum plate.

Let's say that you have a house that uses up the buoyancy of the rounded part of your pontoons.......somewhere around 4500 pounds of house, dwellers, supplies, and all that other stuff that accumulates so quickly. (not to forget the requisite beer or other libations.... there is always the weight of the beer and the cooler) Now you have only 12.5 inches of space between the water and the floor of your house. Maybe better to elevate the floor of the house above the top of the pontoons by some amount that will depend on how high the waves are when a storm arrives. You can be sure that a storm of some kind will arise.....plan ahead.

If you arrange a some secure hatches at the tops of the D section, you will have a useful storage space inside the toon. Another plus for that configuration.