Sheathing choices for plywood plank on frame race hull?

Discussion in 'Wooden Boat Building and Restoration' started by DSR, Jan 31, 2018.

  1. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi everyone,

    In the midst of hammering out some of the final construction details on my project and I'd like to gain some insight on the best course of action for sheathing the hull?

    Being here in the US, I have a ton of choices in materials available (almost too many), and I'd like to get some input from the people that have used these materials to see what my best option, or options would be?

    The overview of the project is a 12' 6" plank on frame tunnel hull using 6mm Okoume BS1088 plywood, epoxy resins, and capable of 80+ mph speeds.

    I'm looking for a material that would provide the best abrasion protection for the weight, money and time invested? It's a small hull so I'm not afraid of spending more money for better material for the job (to a point :)), and weight of the layup is a concern also. With it being a tunnel hull with several edges, good drapability would be a definite bonus.

    I'm currently leaning towards 8.5 oz. 2x2 twill weave e-glass and vacuum-bagging the layup. I've also given 4 oz. Xynole some consideration due to the better abrasion resistance, but concerned with the extra weight involved with sheathing in Xynole on a relatively lightweight hull?

    Any thoughts on this would be greatly appreciated!

    Thanks,
    Dave
     
  2. gonzo
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    gonzo Senior Member

    What kind of abrasion are you trying to protect the hull from? It this something you are going to run in shallow water and be running aground at high speeds?
     
  3. DSR
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    DSR Junior Member

    Hi Gonzo,

    Sorry for using such generic terms, that's my bad :rolleyes:.

    I'm not planning on jumping sand bars or anything like that. I'll be running in established waters with known depth. I guess I'm more focused on abrasion from impact with floating debris (sticks, etc.) While at speed. No matter how careful I was at assessing the water I was running in with my previous boats, they never failed to pick up an occasional scratch or gouge in the hull (sometimes through the gelcoat into the glass). It will also be "beached" on sand on occasion I'm sure.
    The finish will be an automotive 2K urethane, which I have no problem repairing/refinishing. I'm more concerned with breaching through the sheathing and into the wood.

    Thanks Gonzo,
    Dave
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You'll want as light a cloth as practical, say 3.7 E 'glass which is pretty commonly available. Any thing heavier is just weight, which you don't need and not much more abrasion resistance. Bagging it down is a good idea, which also will save some weight and improve abrasion resistance. Xynole is a good choice, but your boat is too small to really take advantage of it's weight/abrasion resistance properties. Simply put, it's too heavy for this application, though if the boat was a little bigger, where you can afford the extra weight, a better choice than regular cloth for abrasion resistance.

    Some 5 ounce Kevlar is another option on your boat. Great puncture resistance, which might be finical for you too. In all honesty, having raced boats like yours, the real competitive guys don't sheath their boats, just keep up after nicks and dings as they happen. The simple logic is they can't tolerate the extra few ounces the sheathing adds. We all inspected the boats pretty carefully after each race or race day, noting the places things got hammered or dicked up. These would be patched, filled and puttied up before the next race day and the world wasn't save until we busted into pieces son a rock. If you're a more casual racer, then sheath 'er up, but if you're "serious", well . . .
     
  5. feunatz
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    feunatz Junior Member

    Hi Dave,

    EDIT: PAR was faster
    no idea what could be done to harden the gelcoat to damage from floating objects, but you could add some top layers of aramid/kevlar fiber to protect the fiberglas. At least this should lessen the gouge left by sharp objects
     
  6. DSR
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    DSR Junior Member

    Hi PAR and feunatz, thanks for the quick replies.

    Well PAR, you confirmed my suspicions regarding Xynole. I figured it wouldn't be an appropriate solution for this particular build. It's funny that I was originally planning to use 3-4 oz. cloth when I started the project, but I started thinking that it was too light. The boat won't be a strictly race boat, more of a pleasure boat with race performance, so I'm definitely willing to trade a few ounces for more protection and less maintenance (although with the way I am, I'll be going over it with a fine-tooth comb after every outing....).

    I do see 3.25 oz. twill-weave e-glass cloth available also, any benefit to this over plain-weave?

    And I hadn't seriously considered Kevlar. If it will be a better bang for the buck, I wouldn't have a problem spending the little extra that it would cost the cover a hull this small. I'm assuming the 5 oz. Kevlar would be replacing the glass, not in addition to?

    Thanks guys,
    Dave
     
  7. feunatz
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    feunatz Junior Member

  8. gonzo
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    gonzo Senior Member

    You could use 3.5 or 4 oz S-glass which is about 20% higher in tensile and compression strength.
     
  9. DSR
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    DSR Junior Member

    Hi Gonzo,

    I hadn't thought of using S-glass but now that you mention it, I did find 4 oz. S-glass in a plain weave at US Composites. From what I'm reading, it looks like it might very well be the best bang for my buck. It looks like a better material for my application with no negative attributes compared to E-glass?

    I did a quick refresher on the Kevlar and while it is definitely tough stuff and the price didn't scare me, it sounds like it would be difficult to repair if something did damage it, unless I missed something in what I've read?

    Thank you sir!
    Dave
     
  10. gonzo
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    gonzo Senior Member

    Kevlar is hard to work with. It doesn't wet well. Also, it float in the resin. If you vacuum bag it is OK, but laminating by hand is really frustrating.
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Any light cloth will do. There's not much difference between an E or S 'glass sheathing for abrasion resistance (it is measurable, but in these weights . . .) and you'd need a fair bit more to really improve things, likely enough to warrant Xynole instead, because the weight will be similar.

    Simply put you have thin Okoume for a reason. If you go too far over the top, you might as well just use Meranti, which is about 20%, but also stiffer, though at the same thickness, just as prone to impact and abrasion damage, just with a loss of top speed (weight). In the end, you're going to have to draw a line. How hard are you going to run, how much abuse will you tolerate, how much maintenance are you up to, etc. If you're like most home builders, you'll treat it like your first born for a few years, then settle into what you personality can tolerate. Some are more fastidious than others, but given you're thinking about all this now, I'll bet your boat sees much more care than the average one, so going light, with minimal sheathing is something you could deal with, while others, less careful will likely need a heavier build and sheathing schedule, just to cope with their lack of a reasonable maintenance routine. Lastly, forget about Kevlar, you'll just wish you hadn't bothered in too long. The only time you'd need this is for an ultimate, light weight build and the Kevlar goes on the inside, not the outside. On a build like this you might also use 4mm plywood instead of 6, just to gain this advantage too. Yeah, she probably wouldn't last as long, but she'd go like stink until she broke. I guess my point is go light, which means light sheathings too or go heavier, knowing you'll lose something, but it'll be a little tougher (it's still just 1/4" plywood you know) and better able to cope with some dings, with the also heavier sheathing.
     
  12. DSR
    Joined: Mar 2017
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    DSR Junior Member

    As with any boat design and build, this is just another in a series of compromises to get as close to a realistic SOR as possible....... :)

    I'm more than happy with 6mm Okoume and 4 oz. cloth to get reasonable protection at a reasonable cost and weight for this build.

    And maintaining the boat won't be a problem, I assure you. Before building this boat, I spent a lot of time building hot rods and doing frame-off restorations. After spending so much time, effort and money on those, my OCD won't let me do anything but keep after it, because anything worth doing right in the first place is worth keeping nice (and I truly enjoy doing it).

    Thanks for all the help guys, I truly appreciate it!!!

    Dave
     
  13. messabout
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    messabout Senior Member

    DSR you have not mentioned any of the construction details such as framing intervals and such. At anything like 80MPH, the dynamic loading on the panels will be pretty serious. I trust that you have those things considered and have the structural elements and their placements well accounted for .

    Panel loading is a function of the square of the velocity and all that jazz. Just thinking out loud here..................
     

  14. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi Messabout.

    I really appreciate you thinking out loud. It's one of the biggest reasons that I decided to join this forum when I started my project. I've really enjoyed diving into the technical aspects of boat design and construction, and I hope for much more thinking out loud as I progress.

    Aside from the specifics of frame 2 in the design which we talked about in a previous post (and that I will be building and testing to verify results.....)

    Question regarding laminated frames? https://www.boatdesign.net/threads/question-regarding-laminated-frames.59349/

    I'm very confident with the final ititeration of the design and I would be happy to share any specifics if it would be something you would be interested in looking at.

    Thanks again!
    Dave
     
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