What hull will be most efficient between 8-12 kts?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Oct 9, 2021.

  1. fallguy
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    fallguy Senior Member

    There is some truth and some fallacy in this question. Yes, if you make the system stronger you would not worry about the pylon area failing (save for the hull shearing away. But there are tradeoffs with weight, and aluminum is special because it does not tolerate bending forces well. It really does not bend back, for lack of better language on my part.

    I am no expert and cannot show you the problems with analysis and for that I apologize. But I am a foam boat builder and I can see where your plan falls apart as drawn. It is just too weak at the pylon.

    It could be that simply using better foam or some marine foam and more glass around the pylon would make the boat safer, but I cannot propose the means.

    And your load analysis must account for stresses the sea gives; not simple weights.

    I do not think it is far off, but you need to understand the twisting forces better. I will try to find some links, although what could have been a great thread on the subject devolved into name calling and insults.

    I also believe somewhere someone sent me the typical force they use for slamming loads which you would also encounter and I can look for that. Again, the extent of my support will not be enough, but enough to say that will break...

    Another resource you ought to read is @garydierking .
  2. bajansailor
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    bajansailor Marine Surveyor

    I am noticing a trend here, same as with Dustman's previous thread, which also got to 5 or 6 pages, and was then going around in circles.
    @dustman if your design idea was viable, somebody would have already done it - it is very difficult to try to re-design a wheel.
    You have received a lot of good advice above, and you cannot underestimate the force of the sea.
    I recall seeing somewhere in a previous post you had mentioned a calculated factor of safety of 1.6 - this is not very much really, when you start to think about all the different variables that might come into play.
  3. fallguy
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    fallguy Senior Member

    I believe Dierking's larger amas are designed with a more vertical shear web. This is also a bit of a red flag for me to see the square tube versus something that would resist bending from the force of the sea better.

    And this may feel like piling on, but the 35 psi shear strength of the 2# foam along the shear web is quite weak. I believe Dierking uses glue or epoxy around the web, but haven't read him in awhile. And, of course, shear of the foam is still shear; despite the glue. Somewhere else, I read that another type of shear web was used...pyramid shaped. I will try to get you some reading links later.
  4. dustman
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    dustman Senior Member

    By pylon I assume you mean the part of crossmember that enters the hull. I'm attempting to convey that the union between the pylon and the fiberglass or the union between the pylon and foam is of little to no significance in the strength of the overall structure. In my mind it would be better if this interface were to remain very flexible. I even considered using another flexible material here for this reason. If the fiberglass were to make contact here then it would eventually crack and fall apart, unless it were made extremely strong. If I were to do that then that union would be the primary force carrier between hull and frame and I would have to make the fiberglass skin strong to distribute the load throughout the hull, at this point I'd have a conventional fiberglass hull with point loading at the interfaces. The purpose of my design is to distribute loads over large areas via the internal "skeleton". Loads imparted to the foam from the frame/skeleton would be distributed, and loads imposed on the hull would be distributed over a larger area of the skeleton. The crossmembers and the joint with the spine of the hull are the only structures that would take the full force of any load. And the plates at this joint would make it extremely rigid and distribute load to the spine. I'm not sure you guys are understanding that premise of the design. Furthermore, any part of the hull(foam or fiberglass) could be compromised without significantly affecting the rest of the structure. The only catastrophic failure would be the crossmember or its interface with the spine breaking.

    What concerns me based on what you guys are telling me is that I'm not fully appreciating the constant twisting that the spines in the hulls will be undergoing, and that has the potential to dissociate the foam from the spine, not catastrophic but not good. Not sure if this would happen because the foam has some flexibility. Also, the long term fatigue of the constant twisting could compromise the spine. It seems to me that increasing the rigidity of the spine is the solution to both these issues. I'm actually surprised that no one has questioned the strength of the crossmembers in this whole discussion.

    Any information that could help quantify potential loads would be helpful. This information is hard to come by.

    I have much more I'd like to address but have to get to work. Fortunately my job is flexible but got to get things done.

    Thank you guys for taking the time to work on this with me, and for not hurling insults. Good day!
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  5. gonzo
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    gonzo Senior Member

    The pylon and fiberglass is the area with the maximum stress. The foam is not going to "distribute" the forces. It will compress, collapse and break. Unless you can do a structural calculation, the 1.6 claimed safety factor is only a fantasy. Nobody questioned the strength of the crossmembers because the hulls will fail first. Professional Boatbuilder magazine had an article recently on how to calculate loads on multihulls (maybe issue 182). The problem with answering your questions, is that you are lacking the fundamentals of structural design. Start at the beginning with statics and then progress to dynamics. Boat structures are rather complicated and difficult to design from first principles. The majority of design uses proven methods, shapes, etc. Completely new designs are only done as proof of concept or to address some high performance or completely new situations. You want a boat for a rather mundane application. In this case, a new and exotic design would only be for a proof of concept. That is really interesting, but expensive too. In general prototyping only gives you an idea of what works and what doesn't. That means that you need to afford to throw it away and continue with several iterations until, with luck, a good design emerges. In a large percentage of times, the useful knowledge is that the design does not work, but gives direction to future projects.
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  6. fallguy
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    fallguy Senior Member

    Gonzo has answered well. The reason we didn't go down the road of discussing the crossmembers or their elbows is the blatantly obvious problem with the foam skin at the hull crossmember entrance.

    I am only a builder with enough intuition to tell you if something is really wrong. And that you need to appreciate and you seem to be a bit.

    The issue is pretty simple. Take four pencils and lash them together with rubber bands. Then twist one hull in one direction and the other the other way. Watch the intersections of the 'beams', you will see them flex and open and that flex (extrapolated to a 24' power boat) will he greater than what 2# pour foam and 6 oz glass can tolerate.

    You are doing some rationalizing now which is not good. You don't get to waffle between allowing flex or not allowing it. Aluminum won't. Light foams and light glass won't tolerate compression or tension.

    I will find you a couple links. Slamming loads for you are about 100kn/m^2 with some safety factor based on something I learned in another thread here.

    edited for clarity
    Last edited: Oct 14, 2021
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  7. fallguy
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    fallguy Senior Member

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  8. fallguy
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    fallguy Senior Member

    Also, read Ad Hoc March 6, 2009 post about fiberglass and aluminum and designing for aluminum and fiberglass and how designing for deflection is imperative.

    Anyhow, the thread I linked is actually really good. But be careful to read through the polemics. The one fellow there does not do well with the facts and starts down the road of using vessel weights to establish the structure. A major mistake because the sea does not care what the weights are...nor does it care which direction the hulls or beams point..

    I believe I have probably overspoken some here. I'll shut up a bit. I have another thread where I asked about slamming loads if you'd like the link.
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I feel guilty for once making a cuddy cabin for a boat out of PU foam glassed both sides, I would think using it as a structural element of a hull for offshore use, I would have resorted to the hair-shirt.

  10. S V
    Joined: Jan 2019
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    S V Junior Member

    I am definitelly not NA, but work in construction from time to time. This idea is exeptionally bad. The reasons not to make foam as structural element:
    1. Is is WAY too light. You have to go with heaviest one at least to have some strength. 2# foam is ok as a SECONDARY supporting member, such as to fill gaps for the doors which are already fixed in place with bolts. It is simply very weak.
    2. I will take the same example with the doors: if the doors are light, it is ok to support them only with foam, but there is a caveat - the foam is working as a GLUE with lots of CONTACT AREA for the force. In your construction there is not enough contact area between alluminum part and the foam for the forces your boat will experience. It does not matter if the foam is brittle or rubery, it will weaken during time. Only way this may work if you make contact area like 50x bigger than in your picture, which is not realistic as there are other ways to make strong connections
    3. You are protruding GRP (plastic in general) with alluminum. Those two have totally different coeafficient of thermal expansion and totally different specific heat. Last and not least - different strength and cracking tendencies. To translate in simpler words - the place where the alluminum protrudes GRP will weaken and crack during time. If it is made flexible by design and sealed with some kind of silicone sealant - I am not sure if it is better... The same principle applies to alluminum and rigid brittle foams.
    4. Rigid foams are very very good in constructing composite SANDWICHES. Those composite sandwiches are as strong as long as they are undamaged. In your construction you are protruding the sandwitch in the construction phase... You can see an example of how such sandwitch works in simple drywall plasterboard sheet: it is made from two very weak materials - paper and gypsum, but is hard to break it unless you cut a paper with knife. In your case I attach alluminum to the outside of the skin without protruding it at all. Again you will need to make attachment area much much wider and put extra glass and scaffolding to it. My wild guess scaffolding has to go longitudinally at least for 1-2 meters (it depends on the skin strength)...
    5. Related to 4: by making connection points outside of the "pontoon" you can easilly make your boat easy to assemble and diassemble even on the beach. I would go for it! You can make it almost any beam you need, store it in a cheap shed during offseason and so on.

    Someone offered you to buy couple of alluminum pontoons - imho it is a very good idea. You can fill them with foam, even the lightest one which will do many good things at once: reserve bouyancy in case you will damage pontoon, sound dampening(!!!!) and some extra rigidity - the sandwitch principle will work here perfectly as there are lots of contact area between alluminum skin and foam. It will not weaken in lifetime.

    Not related: all the construction you described is more or less similar to the pontoon boat - I would go with trimaran for the extra stability. Side amas could be filled with foam for safety, and the central could hold all the heavy stuff: water, fuel, batteries, even could sleep in it if you widen the top part a little. Side amas could be made smaller and narrower too. Also it will solve that "double engine problem" as you will need only one main engine and some 3-5Hp emergency one preferrable without electronics.

    Appologies for my english I just could not resist to comment about foam usage.

    p.s. I just understood that ther is longitudinal aluminum beam INSIDE pontoon... Whyyyy???? Just put it outside and you solve many problems at once :)
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