Prototype cedar boat hull? Mold?

Discussion in 'Boatbuilding' started by Cabinetguy74, Oct 30, 2017.

  1. Cabinetguy74
    Joined: Oct 2017
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    Location: Largo, Florida

    Cabinetguy74 New Member

    i will scrap it
     
    Last edited: Oct 30, 2017
  2. alan craig
    Joined: Jul 2012
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    alan craig Senior Member

    Hello Cabinetguy, an amateur builder here but I can't help chipping in:
    1 the panels are so flat you could have built in plywood.
    2 all the money you saved on wood will be spent on carbon and kevlar - use glass fibre.
    3 fit some interior framing and it should be strong enough.
    4 I think that if you install enough power to plane, it might fall apart or loose control in turns.
    5 finish it and tell us how it goes!
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Scary to think you imagine a 350CID engine going in there. It looks like a 25 hp outboard would be about the limit, if the structure can be brought up to some degree of watertight integrity, by whatever contrivances.
     
  4. jorgepease
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    jorgepease Senior Member

    Def glass it and add frames, I also think forget about that big motor, a lightweight outboard is better idea. I like the look though, put a little tunnel in it and you should be able to get skinny too.
     
  5. Canracer
    Joined: Aug 2009
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    Canracer Senior Member

    Come on; no way that an 18 foot skiff hull worth of clear Cedar costs $2,500.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    With some additional reinforcement (fabrics, frames, bulkheads, etc.) you can make this float, but a SBC and a jet (900 - 1,000 pounds) shouldn't be considered. It's not that you can't and an 18' boat wouldn't hold it, it's just that you'll need a fair bit of engineering to make this work, without swamping the back end on launch day. The hull form will easily plane off, but will also ride like crap and likely dig a chine in maneuvering, causing the boat to "trip". Her shape looks to make a better sailboat than a powerboat. A 20 - 25 HP outboard (depending on weight) will push her as fast as you'd want, with reasonable comfort. Much more power and speed and she pound unmercifully.

    Given all the defects in the planking on that hull, you'd be best advised to sheath the whole shooting match with a minimum of 6 ounce, preferably heavier 'glass, if only to seal the seams and hold all those knots in place. The problem with planks with the number of defect you've used is it's very weak, compairtivly (clear, straight grained stock). The defects (knots, pith, grain run out, sapwood, etc.) will be the places the planking will lose its longitudinal stiffness (read break, crack, split, etc.) and this isn't a good thing, farther from shore than you can swim back to.

    Agreed plywood, seeing the boat's shape, could have easily been used and she'd have been stronger and stiff for it too. Instead of the SBC, consider a much smaller engine, one more typical of a small boat, like the plentiful offerings from Yamaha, Kawasaki and Honda at a fraction of the weight of a SBC or even an LS.
     
  7. gonzo
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    gonzo Senior Member

    That rocker at about 1/3 aft of the bow will likely make the boat oversteer at planing.
     
  8. PAR
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    PAR Yacht Designer/Builder

    This appears to be the classic "all by eye" without a real good eye, with the adage "how bad could it be" tucked on at the end of the sentence. With modest power, it'll work at modest speeds, though not well or comfortably. Having the bow "root" around is also likely as Gonzo points out, but generally it's just poorly shaped for several reasons, given the limited SOR we have.
     
  9. gonzo
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    gonzo Senior Member

    That is an adequate shape for a speed of 4 MPH or so. It will do fine with less than 5HP.
     
  10. PAR
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    PAR Yacht Designer/Builder

    She'll plane with modest power, if trimmed out properly and she'll likely carry the bow most of the time too, though she still has plenty of issues to consider, before mounting a small block or even an outboard. Again, if power and speed expectations are modest, it'll work, just not as well as most similar length powerboats.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    There appears to be no rake angle to the transom, which will make placing a outboard there difficult. It would have been a better idea to enquire about the suitability of the shape/construction method prior to commencing to build it. It now being largely a fait accompli, not much useful can be said.
     
  12. fallguy
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    fallguy Senior Member

    5/16 cedar is typically what is used on a canoe, well, 1/4" actually - I made my canoe with 3/16", so when I did some sanding I was probably down to 0.150" in some areas. Very thin. You aren't much thicker, sorry to say, but dead true. I spent 8 hours at the lumber yard looking for clears, so I understand the difficulty and I understand the costs of clears. If you have any holes in the knots, you'll need to fill and sand them before glassing.

    I am not smart enough to give you a glass recommendation, but it is going to be tricky to glass around the chine. I would say 6 ounce glass is too light, even 3 layers. I would probably try something that would give the hull more strength. Expect a bit of air pockets; you can inject them later with a syringe, but if you don't, water will get in and rot the cedar.

    My hull reflects waves through it. It has been well tested with 2 layers of 6 ounce cloth bottom both sides, but I can't imagine how quickly a thin hull would fatigue if you had the hull moving from trying to go 60 mph with that thing. You'd need something much better for powerboat.

    The other thing is you really must consider a small keel. That would help with the walking that I can see would happen. Maybe one of the smarter fellow here can advise the size of the keel, but I think you need one for sure. I built a canoe with hard chines and a flat bottom and it would NOT track without the keel. The shape of the bottom was surprisingly similar, but of course, mine was only 3' wide and had no stern. When I would get into a wind alone, the bow would be up and turn into a perfect sailboat, only it was a sailboat without anything to stop slip. A good guess on the keel would be some hardwood, like ash, perhaps 2" wide by 2" deep. You could cut it to a vee shape and taper it in the front for drag. The ash also doubles as a hull protector.

    I think someone needs to explain the chine walking a bit better for you. But basically it gets really bad in certain hull designs, especially when they have too much horsepower. I have experienced chine walk, but it was never at speeds over 25 or I backed off or was limited by horses.

    PAR delivers very solid advice. Take it.
     
  13. fallguy
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    fallguy Senior Member

    Ah, yes, this would be another issue for the poster. He built a boat suitable only for outboard for an inboard. But, perhaps he could cut out the transom and put in a 12-15% cut into it. Most likely, the transom is underbuilt as well, for even a 25 hp motor, or even an inboard engine.
     
  14. gonzo
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    gonzo Senior Member

    Outboards are designed for a transom from about 1 1/2 to to 1 3/4 inch thick. At the very least he will have to put a pad on it. However, the outboard will not be able to be trimmed properly. The brackets are designed for a nominal angle of 14 degrees.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Again, there's a lot wrong with this boat, though nothing that couldn't be partly or nearly wholly addressed, if speed expectations and power requirements are kept within reason. Once a boat shaped like that approaches 30 MPH, it'll pound enough to knock out your dental work and start to dance uncontrollably. A skeg could help, but isn't necessary with outboards or jets. With the bottom rocker and so shallow an entry V, bow steering is probable, though the forward chines could be raised to improve this a bit. The transom could be notched, a bracket employed or even wedged to get the outboard at an appropriate angle. A jet wouldn't need this, though you'll need to develop an inlet that'll work. The hull shell we just don't know enough about, but a biax laminate schedule could stiffen things up considerably, particularly if coupled with athwart stiffeners (frames, bulkheads, ring frames, etc.). This all may be more effort and cost than this project warrants, but if some of the shape changes are made, the results could be used as a plug for a GRP mold.

    We've beat up the cabinet builder enough for now. Let's see what he wants and where he'd like to go. It would be helpful to get an accurate set of dimensions, weight and centers for this puppy, so effective options could be provided. On the other hand, if he simply shims up a 20 HP outboard, to get a reasonable angle, he can go fishing and tell us all to screw it, assuming the transom/structure can tolerate the loading transfer.
     
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