Advice RE basic considerations for pontoon houseboat design?

[Coracular]

I am comfortable with buoyancy calculations etc, but looking for the "unknown (to me) unknowns" around structural integrity of the platform. Looking at thick-wall HDPE pipe for the pontoons for reasons of maintenance and resistance to winter ice so I will assume zero structural contribution from those (just flotation), and I want freedom in the future to mess with the superstructure so I don't want the structural integrity to depend on that either. Not meant to be a racing boat, just putter along slowly or stay at anchor, all in relatively calm inland waters.

So essentially I am looking to build a floating dock that is sufficiently stiff that sag, twist etc over waves won't be a problem for the superstructure. I'll probably start with a 20' x 8' prototype but may eventually want to join 4 of them together to make a 40' x 16' barge. I know it will have to resist racking (one corner hits a rock), sag (big wave dead ahead), twist (big wave off the bow) - but am concerned about what I might not have thought of.

So:
1) Is there a checklist or anything similar of "categories of forces to confirm the design can resist?"
2) Is there a standard way of assessing / addressing their magnitude - for example "design the hull to be able to resist the forces experienced when fitted with hardpoints at 25% and 75% of length and lifted by a crane, and it will be strong enough to deal with waves up to a certain size"?

Thanks!

 

[fallguy]

How do you intend to connect the hdpe to the platform?

 

[gonzo]

What you are asking for is a whole course in Naval Architecture. I suggest you find something similar to what you want, make sure it successfully operates in the conditions you require, and follow the same design. If you find nobody uses the system, first question why. The answer will most likely be that either it is not good material or design, or it is too expensive.

 

[Coracular]

How do you intend to connect the hdpe to the platform?

The most common way to do it seems to be to buy the pipe with flanges welded onto it: Pontoons - NyDock Floating Docks & Pontoons PipeFusion in Huntsville, Ontario, Muskoka https://nydock.com/products/pontoons/

However since even at that thickness the HDPE is not providing a lot of stiffness, and thermal expansion is not trivial, I suspect the best answer is going to be "lots of contact area".

 

[Coracular]

What you are asking for is a whole course in Naval Architecture. I suggest you find something similar to what you want, make sure it successfully operates in the conditions you require, and follow the same design. If you find nobody uses the system, first question why. The answer will most likely be that either it is not good material or design, or it is too expensive.

Use of HDPE in this application seems a fairly recent innovation, and I understand the more common steel / aluminum pontoon builds lean pretty heavily on a structural contribution from the pontoons themselves. I realize this stuff can get complex quickly for complicated hull designs etc, but this is essentially just a rectangular truss. Is there a basic list (naval architecture for dummies, chapter 1, table 1.1 sort of thing) of the forces a catamaran has to deal with?

 

[fallguy]

The most common way to do it seems to be to buy the pipe with flanges welded onto it: Pontoons - NyDock Floating Docks & Pontoons PipeFusion in Huntsville, Ontario, Muskoka https://nydock.com/products/pontoons/

However since even at that thickness the HDPE is not providing a lot of stiffness, and thermal expansion is not trivial, I suspect the best answer is going to be "lots of contact area".

here is some good reading for you that at least explains basic considerations

like Gonzo says, there is quite a bit to it

if they sell them; surprised they don't have specs for the boat?

here is the reading, yeah a ship, but the ideas transfer to small boats as well ... for a pontoon, one wave crest and another wave troughed and your pontoons will be moving in opposite directions which is what you are asking about

What Is The Purpose Of "Torsion Box" In Ships? https://www.marineinsight.com/naval-architecture/torsion-box-on-ships/

 

[TANSL]

I am not very sure that the concept of torsion box and what it intends to solve is applicable to a pontoon made up of several pipes joined by a platform.

 

[Coracular]

here is some good reading for you that at least explains basic considerations

like Gonzo says, there is quite a bit to it

if they sell them; surprised they don't have specs for the boat?

here is the reading, yeah a ship, but the ideas transfer to small boats as well ... for a pontoon, one wave crest and another wave troughed and your pontoons will be moving in opposite directions which is what you are asking about

What Is The Purpose Of "Torsion Box" In Ships? https://www.marineinsight.com/naval-architecture/torsion-box-on-ships/

I spoke to the company, they sent a lot of photos (including right up to floating house type things) but mostly seemed to be all about flotation. I think they are coming from a dock perspective, and one of the reasons I want to get an idea of the numbers myself is that I'm not sure how far into it they have gone beyond "will it float?"

 

[gonzo]

There are two basic approaches. The first is a fairly stiff structure that will have high stress at the connection points. The second is a structure with low stiffness that will move up and down with the waves and will have relatively low stress at the connection points. The second option does not work with a stiff superstructure.

 

[fallguy]

I am not very sure that the concept of torsion box and what it intends to solve is applicable to a pontoon made up of several pipes joined by a platform.

He asked about understanding forces on the pontoons, and the link I found explains it a wee bit is all.

He could use a torsion box for the platform, but I am not qualified to spec it.

 

[TANSL]

@fallguy, I am not going to argue with you, respected friend, but to solve any problem it is essential to make a correct analysis of it, to apply the correct solution. Not all solutions are valid for all problems.

 

[Coracular]

He asked about understanding forces on the pontoons, and the link I found explains it a wee bit is all.

He could use a torsion box for the platform, but I am not qualified to spec it.

Once I get an idea of the forces involved, I am hoping I can find standard dock or stage-truss modules with the necessary characteristics. After seeing a bit about the Dinsenbacher method, I am thinking the place to start is by designing a platform that will not deflect by an unacceptable (TBD) amount under load when supported under the two diagonally opposite corners, and then build it and see...

(so not exactly a torsion box, but could end up reasonably similar)

 

[Ad Hoc]

... Is there a checklist or anything similar of "categories of forces to confirm the design can resist?"...

Yes...but in order to do that, you need drawing of the general arrangement of what you are proposing.
And drawn to scale, i.e 1:1.

That forms the basis of how you will begin to analyse the structure.

 

[Coracular]

Yes...but in order to do that, you need drawing of the general arrangement of what you are proposing.
And drawn to scale, i.e 1:1.

That forms the basis of how you will begin to analyse the structure.

Working on that in Fusion 360!

It'll be very simple overall geometry though. This is just a random illustrative image off the internet, but this general category of structure (minus the legs), with appropriate anchor points for pontoons and superstructure (but treating both only as load sources for structural purposes).

Edit to add another image. I wouldn't build a yacht like this, but for a light-weight pontoon barge my thinking is that to a very rough first approximation rectangular load bearing things are rectangular load bearing things, and there's lots of work done on standardized solutions to that problem.

(my apologies to all the marine engineers and architects that statement causes to spontaneously combust - my work is on microscopic extremely precise systems, so my approach to recreational boat-building tends in the opposite direction e.g. Cicada | Boat Design Net https://www.boatdesign.net/gallery/albums/cicada.4546/ )

 

[Ad Hoc]

Working on that in Fusion 360!...

Well, when it is done, then there is an opportunity to comment and advise.
Without which, it is just speculation of what is or if or whatever etc....in which case it is just a bar room chat.