Building hull to “what was there” spec

Discussion in 'Boat Design' started by Bigtalljv, Apr 18, 2022.

  1. Bigtalljv
    Joined: Aug 2019
    Posts: 52
    Likes: 9, Points: 8
    Location: California

    Bigtalljv Junior Member

    Hi,

    For those of us that don’t have any data to backup what we are doing Ive heard the generally accepted route is to build to “what was there”. I am working on the hull extension for my old boat. I say old because it appears to be 1/2” thick solid glass hull from the mid 70’s. Now it was designed as commercial boat with an expected cargo of urchins up to 4000lbs, a big block I/O, and 20kts.

    Do I just carry on and head for the 1/2” or do I get stupid and start “thinking”, trying to ask if modern epoxies and biax are stronger than the stuff from the 70’s, if thats even a question or something similar.

    It’s a 24’x8’ hull. I’m 6 layers in. 1/2” of 1708 is 14-16 layers? hand layup and epoxy.

    thanks
    Jason
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    How long are you extending the boat by?
    Have you cut off the transom and built a mould to lengthen the boat including a new transom, or have you simply moved the cut off transom further aft, and are now filling in the bit in between?
    Do you have any photos that you can post?
    Re the 1/2" of solid glass, that is probably mostly CSM, maybe with a layer of two of rovings sandwiched in between somewhere (?)
     
    BlueBell likes this.
  3. Bigtalljv
    Joined: Aug 2019
    Posts: 52
    Likes: 9, Points: 8
    Location: California

    Bigtalljv Junior Member

  4. bajansailor
    Joined: Oct 2007
    Posts: 2,732
    Likes: 1,049, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    It might be easier in the long run to have your questions above in your first post incorporated in the original thread that you have linked to in your second post (?)
    Maybe @Boat Design Net Moderator could work some magic here?
     

  5. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    I'd say stick with what went before.For one thing,the cost of epoxy is a lot higher than polyester and then there is the reality that even if you made the switch,you would still need to get to the same sort of thickness to achieve panel stiffness.I have seen examples where a laminate with better theoretical tensile strength had less stiffness than the less sophisticated alternative.It may not be the prime consideration now but if you ever wanted to sell the boat and move on to another project,it would be less of an obstacle to a sale if the boat looked to be of uniform construction throughout.Think from a prospective purchaser's point of view-"Why did they change the construction for that bit?Is the rest a bit sub-standard and not up to the job?".What would your own reaction be to a survey that mentioned a new hull part made in a different way? If the original was good enough for over forty years that included commercial work it wasn't in any way bad.You just have to endure the smell of styrene.Incidentally,get hold of some woven rovings as it will build up the laminate thickness quite rapidly and strive for a balanced laminate.An added bonus will be that if you use something other than cloth for the initial layers there will be less chance of the weave printing through to the surface of the gelcoat.
     
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