21ft tunnel hull design questions.

Discussion in 'Boat Design' started by Mhall, Aug 26, 2013.

  1. Mhall
    Joined: Jun 2008
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    Mhall Junior Member

    I am designing a 21ft tunnel hull or Mod VP style flats boat and I have a few questions. First off the bottom is designed using Liberator 21 specs or very close with no major changes as of yet. This boat will be built from plywood covered with glass/epoxy inside and out. I am at the stage where I need to ask a few questions before going any farther.

    1. Should I use thicker plywood without stringers on the padded surfaces of this design or, should I use stringers and thinner plywood? I was thinking about .5" plywood without stringers for the hull bottom or possibly go with .25" plywood and just have a generous amount of stringers to help stiffen it up. I would think the thicker the core material the more ridged the panel but stringers and thinner wood may create nearly the same strength. Any thoughts on this would be great.

    2. What fiberglass layup scheduled would you recommend for such boat? I was leaning towards 1708 x 2 layers on bottom and one layer inside. Is that overkill or sound about right? One thing to remember is this is a very high performance boat so being stiff is important. Maybe I should go with more layers? Any one care to share their thoughts on what glass to use and what kind of scheduled to go with?

    3. Anyone know how close BS 1088 marine plywood is to the advertised thickness? The way I am going to design this boat the material needs to be uniform in thickness. Would a .5" plywood be consistently .5" thick or is there a great deal of variation? As much as this stuff cost, i would hope its very consistent.

    Thanks
    Mike
     

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  2. boatbuilder41
    Joined: Feb 2013
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    boatbuilder41 Senior Member

    I would put stringers for sure. And personally ...I wouldn't glass inside and out. I would glass only the outside. And vent the inside very well. If you glass both sides the wood can not breathe . Just a little pin hole in the fiberglass will eventually saturate the plywood . Once this happens your boat will become extremely6 heavy and can never dry out. I don't know it all. I learn every day myself. But I can tell you about my past experience. Lifes most valuable. Lessons learned are learned from failure.... not success... I already made these mistakes years ago. Just vent bilges really well to prevent premature failure of wood
     
  3. tunnels

    tunnels Previous Member

    One thing to remember is this is a very high performance boat so being stiff and this is important !!

    ARE your sure its important ??? why ??? and how fast is fast ?? :confused::p
     
  4. Mhall
    Joined: Jun 2008
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    Mhall Junior Member

    Stiff may be the wrong word to use. I can understand a flexible hull being ideal to a point but, the bottom of this boat (padded running surfaces) needs to retain a very straight and smooth surface over the life of the boat. I do not want to bottom to become wavy over time which may be more likely with thinner plywood over stringers. This boat should be capable of 100mph (with correct setup) yet, I will be pleased if I reach low 90s with the power I plan to use. To be realistic, the boat that I am basing my design on is a 105mph boat with a 300xs and my design should be lighter with the nice smooth top deck which would be more aerodynamic. In theory my boat could be faster due to greater lift ablity with the top deck design on top of the lighter weight hull. I will have Aeromarine Researches software here soon to start my analysis of my design so maybe I will be pleased with the calculated results.

    Mike
     
  5. tunnels

    tunnels Previous Member

    getting THAT soft ware is a really good choice . once your in the 60mph plus you are entering aerodynamics and the shape of thing will have a huge baring on if you fly or not !! never go faster than angles can fly !!
    What kind of flowers do you like ?? :eek:send a address so I can post them !! :)
     
  6. rwatson
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    rwatson Senior Member

    If you only layup the outside of the plywood, you lose a lot of strength and rigidity, and leave the plywood sitting in water - and it will rot.

    If you use epoxy (not Polyester resin) with the fibreglass, for both sides of the plywood - total encapsulation - you get superior rigidity with a big increase in strength, and no plywood rotting problems.

    Epoxy is extremely waterproof - any other 'goo' is not.
     
  7. TANSL
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    TANSL Senior Member

    It appears that what for some is the perfect solution for others is nonsense. And I, I'm very candid, I wonder, is it that all shoddy work creates problems?. Is it good any solution provided it is implemented correctly?.
    Or, perhaps, is that I have not understood some of the answers. The issue seems very important, therefore, anyone clarify me, please?. Thank you.
     
  8. rwatson
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    rwatson Senior Member

    I dont understand what you are asking ???
     
  9. TANSL
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    TANSL Senior Member

    I'm sorry, I have not explained well:
    boatbuilder41 says: I would not glass inside and out. I would glass only the outside, and explains his reasons.
    rwatson says: with the fiberglass, for Both Sides of the plywood , and explains his reasons.
    My question, very simply, is: who is right?
    Thank you and sorry for my limited facility to explain.
     
  10. rwatson
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    rwatson Senior Member

    OK - thats easy

    If you apply fibreglass to the outside - the plywood is not waterproofed from inside. As a result, it will eventually obsorb water, and rot. This is made more likely if Polyester resin is used, which is not waterproof. If the inside is well ventilated, that will help keep the unprotected plywood drier, but it will not last. I bet boatbuilder41 used Polyester resin, though he doesnt say so.

    If you use Epoxy resin, which IS waterproof - and totally encapsulate the plywood ( inside and out) , no water can get to the plywood, and it will stay intact. The inner layer of fibreglass will also contribute greatly to the strength of the structure.

    This is not my pet theory - it is a very well documented fact. Search for the words 'encapsulate' and 'epoxy' in this forum, and you will be inundated by the amount of information about it, especially if you use PAR as the writer.
     
    Last edited: Aug 27, 2013
  11. michael pierzga
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    michael pierzga Senior Member

    You can consider plywood to be the core. In this case you would need to balance the laminates on both sides of the core.
     
  12. SamSam
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    SamSam Senior Member

    I guess at 90 mph you would want to be sure that more lift is what you want.

    Adding layers of glass will add weight to your boat but not much stiffness.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Well, I have worked on boats which have never been encapsulated with fiberglass, yet have an outstanding track record of longevity, like historical Riva models. ;)
    The bilge of these boats is simply treated (after the proper cleaning, sanding and drying) with several layers of primer (like UCP by International) followed by layers of the grey resin-based varnish for bilges (like Danboline). Sure, these boats are precious and are hence hauled out and stored in a dry place after every use - a kind of treatment many small boats don't enjoy. But the fact that so many Riva boats have more than 40-50 years of service and are still in a shiny condition shows that with a proper care of the hull the encapsulation with fiberglass is unnecessary.

    TANSL,
    I suspect that many problems related to the total (inside+outside) encapsulation of the wooden hulls arise from the non-respect of one fundamental rule in the wooden boat construction - the use of properly dried wood. The wood (either solid or ply) used for boat building shall have the moisture content of around 10-12% in order to assure the proper mechanical resistance, flexibility and resistance to rotting.

    The same is valid for the works of restoration of existing wooden hulls, where it is necessary to dry the hull to the proper moisture content (10-12%) prior to varnishing (or encapsulating it, if really necessary - but I wouldn't do it in this case). If this simple rule is respected, and the bilge is properly ventilated and kept dry, the wood doesn't need fiberglass cloth inside, unless there are structural reasons for doing it, like Michael has noted.

    Cheers
     
  14. TANSL
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    TANSL Senior Member

    Thanks daiquiri and michael, I am now able to understand things and I think I can reiterate what I said previously, a job well done, encapsulated or not, does not have to give any problem. Each job has its own technique and you have to apply it correctly. And, of course, some layers used to seal the wood need not add appreciable resistance to the hull.
    Your comments explain, at least so it seems to me, why two of our best experts in this forum could have such different opinions.
     

  15. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    I agree but please let me rewrite your phrase by adding an important part: A job should be done correctly and then properly maintained, in order to avoid problems. :)

    Wooden hulls require regular and correct maintenance. People don't like that part associated to the ownership of a boat, that is one of main reasons why plastics has been so successful in replacing it for general boating. The answer to the problem of wooden boat maintenance was the invention of various methods of encapsulation of wood with resins, (with or without reinforcing it with the glass). It might be a correct approach on some constructions and for some construction methods, but it is certainly not a holy grail of woodworking.

    Some woods bond better than then some others with the epoxy, and that can be one of important reasons why you will hear differing opinions (even between experienced boatbuilders) about hull encapsulation. Teak and white oak are two examples of commonly used woods which are more difficult to bond with epoxy than the others. So few unsuccessful (because not properly done) encapsulations of structures made of these species can create a bad reputation to the whole method, even where it would be perfectly indicated.

    Cheers
     
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