Designing Stainless Steel Pontoons for a Big Houseboat/Floating House

I am on a freshwater irrigation lake in the San Joaquin valley. I have a 2 story, 61ft x 27ft houseboat that needs to come out of the water this year. The pontoons are currently fiberglass over wood and approximately 25 years old. With new gel coat and lots of sanding they could work but I would prefer to switch them to metal. I have looked at Stainless, Aluminum and Mild Steel with "Tar". I will be building these myself since I have a welding shop and spool gunning either AL or SS will be fine. However, how do I determine sheet thickness? Placement of bulkheads? Placement of structural support for pulling in and out of the water? I could copy the current pontoons but I would like to have the new ones built before pulling the boat from the water. I would love help with either links to certifying agencies, government agencies, engineering guidelines, design shops, or just information from personal past experiences.

 

It is a more complicated job than it might seem at first analysis. It is not only a matter of calculating the bulkheads, thicknesses, and reinforcement of the new metal pontoon, but also the assembly, which will have a different weight and CoG than the previous one, must float in the same conditions as the existing one, with the same freeboard (I suppose) and with the same safety and stability conditions.
Another point to take into account is whether the regulations applicable 25 years ago are the same as the current one.
Good luck, you're going to need it.

 

I feel you!! The boat has 3 pontoons and I am going to change all 3 at the same time so there is less of a concern about perfectly matching CoG, height, or other specs of the old pontoons.

The skirt will also be replaced given its age and damage, so the freeboard is not a huge concern as long as it accommodates my sterndrive and its appropriate depth. With 3 pontoons and added adjustable ballast chambers, it seems leveling the boat is more than doable. It gained a marked list (1-1.5") just from removing the previous owners furniture and archaic fridge. Given the size of both the boat and the pontoons, CoG movement isn't a large concern. the boat could use a lower CoG and heavier (SS) pontoons will help with that.

The current pontoons (3) are 58.5' x 4' x 2'. The 2 sides are just pontoons, no compartments or anything. The center contains my battery bank, black water tank, stern drive, and 2 large rope storage compartments. Regulations are strict given the black water tank and the engine pod but I can not find any regarding the actual construction. It mostly appears that they simply need to pass an inspection. The research I find calls for 0.125" Al 6061-T6 sheet but I can't find a similar recommendation for SS, I feel 316L is the best alloy given its "ease" in welding and marine corrosion resistance. 304 is often floated around our lake but I am concerned about galvanic corrosion on the aluminum stern drive housing.

 

Welcome to the forum, by the way.
I was more concerned about the longitudinal and transverse position of the CoG than its height. But since there is the possibility of adding fixed ballast, the hull can be balanced perfectly, you are right.
Another issue is that, as the metal pontoons are heavier, although the CoG may decrease, the submerged volume will also increase, which will decrease the initial stability. Surely this will not be a problem but it would be advisable to make sure that it is not.
The boat will surely have to pass an inspection. It's about finding out, in time, what is going to be inspected according to any new standards that may exist, if any.

 

Thank You, I feel this will be a very helpful environment.

I have been in touch with my inspector and there is a list of safety points, lake worthiness, sewage, fuel, battery bank and engine bay. As far as pontoons, all these boats are custom so its a visual inspection (free of rust and defects) and a pressure test. Hold 3-5 psi overnight. So I'm at a loss, it appears that I just build them like marina or barge pontoons with a little style and some angles at the front and then we are good to go.

 

Stainless steel is a poor choice for a boat hull. You will likely have crevice corrosion and fatigue cracks near the welds. In fresh water steel with a proper coating will last for many decades. Tar is not a good coating. Sandblasting, priming and painting is much better. If you want to design the hulls, a good guide would be Gerr's book: https://www.amazon.com/Elements-Boat-Strength-Builders-Designers/dp/0070231591

 

"The current pontoons (3) are 58.5' x 4' x 2'. "
So are you planning to make the pontoon rectangular then?
If so, you will need internal stringers to stiffen the 4 foot panel depth. ( assuming the 4 foot is the vertical measurement)

Alternatively, with proper design, you could form the pontoons, into a hexagonal shape to create stiffness in the pontoon. which will save some longitudinal distortion. Aesthetics mainly
You could have a shop forms lengths up to 20 feet long quite easily and have only a couple of butt welds per pontoon, though a couple of longitudinal ones but the would more than likely be
top and bottom and not noticeable.

"The research I find calls for 0.125" Al 6061-T6 sheet"
That is interesting research. Most aluminum boats and structures are made of 50xx series sheet. Easier to weld, form etc. The T-6 temper will disappear in the HAZ
when you weld and it is highly unlikely that you need the additional strength offered with 60xx series sheets.

In 30 odd years of purchasing aluminum for our fab shop, (not all boat hulls) I cannot recall seeing 60xx sheets. But then I was not looking

 

I was looking to use surplus oil transporting pipes over in Thailand a number of years ago. (actually they were gas pipelines)

and I would take a serious look at large diameter PVC pipes

You might have a look thru asome postings I made about a floating structure over in Thailand
Floatation Cottage, Restaurant in Thailand https://www.boatdesign.net/threads/floatation-cottage-restaurant-in-thailand.69082/#post-960544

Retirement Houseboat or Floating Home https://www.boatdesign.net/threads/retirement-houseboat-or-floating-home.23987/page-13#post-693034

Retirement Houseboat or Floating Home https://www.boatdesign.net/threads/retirement-houseboat-or-floating-home.23987/