Mixing Wooden Boatbuilding Techniques

Discussion in 'Wooden Boat Building and Restoration' started by Gone Ballistic, May 5, 2014.

  1. Gone Ballistic
    Joined: Feb 2004
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    Gone Ballistic Junior Member

    Hi Guys,

    I'm looking at building a 18ft runabout / skiff / commuter, along the lines of the PT Skiff / Eco18 Marissa. I'm designing the hull and will be creating the plywood CNC files to 'kit' the design, primarily to make it easier for me to build. I have lots of experiencing of 'kitting' aluminium boats as it was my old job, so that bit is OK.

    However the design is 'experimental'. It has a vertical stem and deep forefoot developing aft to 12 degrees at the transom. The shape is therfore 'undevelopable'.

    I would like to make a self jigging kit of frames and longs to add the shell to. I anticipate the frames, longs and topsides being ply, however to get the shape I would like the bottom to be strip cedar.

    I have two specific questions:
    - Can anyone see any problems with mixing methods e.g ply and strip cedar?
    - If the frames are permanent is it ok to 'tab' them to the cedar shell and glass in the squares between framing? There will be no continuous inner shell glass under the lines of frames and longs?

    Any advice would be appreciated.

    Cheers

    Andrew
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I have seen many people mix strip planking and stitch-and-glue on the same hull. It is common on smaller boats like sea kayaks, I see no reason it could not be used on a skiff as well.
     
  3. MoePorter
    Joined: Nov 2012
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    MoePorter Junior Member

    "There will be no continuous inner shell glass under the lines of frames and longs?"
    not sure if I understand the question...If the cedar strip is "sheathed strip" with epoxy/glass both sides there would be no problem - if the inner side of the cedar strip is unsheathed there's the issue of the cedar absorbing water & swelling - which would overpower the mechanical bond of the ribs/framing. The question would be can the cedar be sealed enough to stay dry without a glass layer. The answer is maybe with epoxy but not with polyester resin.
    For your plan to work the cedar strip must behave like the ply in terms of expansion/contraction & sheathed strip construction is a way to do it.
    Here's a Phil Bolger design with an epoxy bonded sheathed strip/ply hybrid construction. Moe
     

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  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Add fillet.
    Cut the squares a bit wider so the rise inch or two on the frames. After both sides are sheated cover the frame with barrier coat preferably with glass.
    BR Teddy
     
  5. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Perhaps what you need to do is create the bottom as a separate part of the kit and "stitch" it to the rest once you have built the glass/cedar/glass sandwich. Leaving the inside unsheathed is not advisable, as the true strength of the combo is the outer skins, with the central cedar playing a minor roll. You could do the same with a foam type core too. Doing as you suggest and patching the visible sections is a compromise which may work and may not. Personally I would lay up the bottom as a whole separate piece, then do the rest as a stitch and glue project, including the bottom as one of the parts.
     
  6. messabout
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    messabout Senior Member

    Not sure if I understand Steve's entry. I do quarrel with the notion that the outer skin is the more significant. In a skin load analysis, the inside skin, in tension, is the more important where beam or plate strength is involved. Consider the direction of loading on the outer surface of the boat. If, and only if, the criteria is puncture resistance then the outer skin is the more important.

    The OP will need to consider the possibility of "hard spots" where the frames/bulkheads are. If interior skin is sufficiently substantial, then that will not be a problem if the interior skin fails to continue under the frame /bulkheads.
     
  7. lewisboats
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    lewisboats Obsessed Member

    What I mean is... oversize the frames slightly... a couple of mm, then do up the bottom in strips, not permanently attaching the strips to the frame. Once the strips are in place, glued up, faired and glassed, then pop it off the frames, glass the inside, plane down the frames and re-mount the bottom and continue with the build, permanently mounting the bottom to the frames etc. It wouldn't be much extra work but the results will be stronger and last longer.
     
  8. LP
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    LP Flying Boatman

    I believe that the OP wanted to piece in the interior sheathing between the frames so I believe TD hit the mark on that one. On a wooden strip build, the strips are structural in most builds IMHO. There are designs with heavy sheathings that are structural and these do carry more of most of the loads. I believe that a lot of strip builds utilize the sheathings primarily for abrasion and moisture resistance.

    If the OP is doing structural sheathings, then Steve's plan may be in order. For abrasive and moisture protection. I'd piece in the sheathings.
     
  9. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Mixing a strip plank bottom hull to ply upper is a technique used on lots of small dinghies. Especially in the 12 to 16' range, think International 14s' prior to the all carbon stuff. An alternative if the double curvature or non developable part is relatively local, is to cold mould that specific small area. This can be built onto a scarf in the adjoining ply. It is a technique I use on certain double chine designs in the bow/forefoot area to get a sweeter entry.

    The second part about removing the frames and retabbing them later should work OK as long as you put 1-2mm of polythene, polyprop, or other non stick surface on top of the frames. Then the glue will not give you a 'stick out' problem. Additional polythene sheet and masking tape also help in certain areas.
    Don't forget any limber holes you may need to get good drainage though from any areas that cannot be easily accessed and bailed properly. Good luck with the project.
     
  10. Gone Ballistic
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    Gone Ballistic Junior Member

    Hi guys,

    Thanks very much for the really useful responses. That Bolger boat is remarkably similar to what I had in mind shape wise. Moe, is it yours? Do you know the boat?

    My original intention was to avoid temporary having temporary frames, particularly as I wanted to use the (ply) chine flat to tie the jig/egg box/mould together. The outside could be completely glassed but the inside skin would have to break under every frame as it can't be glassed continuously with frames on the shell. The hard spots are a very good point. The glass/epoxy is primarily for moisture control but also helps controlling thickness of wood, it's kind of a hybrid core-glass/ and monocoque technique anyway. If I was to pop it off the jig then foam or balsa would be lighter.

    It would be possible to pop the bottom off the jig turn it over and glass inside. My gut instinct is that the thickness of the glass, say 450gsm, would not be enough to worry about trimming the frames? Any opinions on that?

    As for could moulding a section, I did consider it, say fwd 3rd of bottom, but my perception of cold moulding is that it's the most difficult of the 3 techniques?

    Thanks everyone.

    Andrew
     
  11. lewisboats
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    lewisboats Obsessed Member

    Not difficult but labor intensive and messy... more so than stripping.
     
  12. MoePorter
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    MoePorter Junior Member

    "On a wooden strip build, the strips are structural in most builds IMHO. There are designs with heavy sheathings that are structural and these do carry more of most of the loads. I believe that a lot of strip builds utilize the sheathings primarily for abrasion and moisture resistance."

    "The glass/epoxy is primarily for moisture control but also helps controlling thickness of wood, it's kind of a hybrid core-glass/ and monocoque technique anyway."

    I used "www.macnaughtongroup.com/publishingown.htm‎
    MacNaughton Associates Scantlings Rule Book (includes the ... Scantlings Rule for Sheathed Strip Construction $10 ... by Thomas A. & Nannette McNaughton"
    on the build - it's a hybrid in that the strips are critical longitudinal structures with the glass sheathing run unidirectional 90 deg to the strips, the idea being the glass provides the cross grain strength replacing frames & the strips do the job planking has always done...It's not a cored structural sandwich where the loads are all in the skins . I like the method because it uses basically traditional wooden boat building techniques with less glass & epoxy than a true skin stressed sandwich would use for the same strength. The maintenance/longevity benefits of the glass/epoxy skins are a bonus. I ended up using 90/45/45 oriented glass in this build because I had a bunch of it...

    It's my build & I still have her.Moe
     
  13. DCockey
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    DCockey Senior Member

  14. Gone Ballistic
    Joined: Feb 2004
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    Gone Ballistic Junior Member

    Moe, I really like your boat, my design is only 5.5m but is a practice for something a bit bigger, a lot like yours, weekender style.

    My stem is a little more vertical and develops the deadrise all the way aft with a half metre run at 12degrees to the transom. There is a chine flat to add lift and help ensure the ride is dry. She's narrow on the waterline, but flares in the topsides.

    I'm trying to combine a really fine entry for good ride in chop, with lots of lift aft. Downside is a lot of wetted area.

    I guess your boats great in a chop? How much power does she have and what's cruising speed? I'm looking at a 40hp outboard, and would like 30kts at WOT, I'm just a bit concerned that all the wetted area will slow her down too much if I keep the forefoot in the water. When your running fast does the bow come out?

    I think I'm going to go with releasing the bottom shell from the frames, then turning it over, glassing the inside skin and then filleting/tabbing the internals back in. Trouble is, then the internals are all secondary bonds.

    Cheers

    Andrew
     

  15. MoePorter
    Joined: Nov 2012
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    MoePorter Junior Member

    "I guess your boats great in a chop?" Great? Yes. She likes to go right into it...San Francisco Bay has all kinds of wind & tide chop - we only slow down for the ship wakes. 95% of our trips are out through the Golden Gate into exposed coastal waters with Pacific swells, out there we slow down when we start to get airborne. We can keep 16 + kts 90% of the time. Very comfortable ride at these speeds.

    "How much power does she have and what's cruising speed?" Volvo 41 diesel w/duoprop.200hp(150kw) 188shp(140kw) at the prop. Top is 27 kts, Volvo says I can sustain 24kts, these days I keep it to 16-17kts & get between 3.75 & 4.1 kts/gal. (4kts per US gal = 1.95 km/liter if I figured it right...) Current running weight is about 6500lbs (2950kg)

    "I'm looking at a 40hp outboard, and would like 30kts at WOT, I'm just a bit concerned that all the wetted area will slow her down too much if I keep the forefoot in the water."Too many variables to answer...but in Phils design there's very little wetted surface in the bow...it's all knife...It's up in front to "start the cut". The thing to think about is down wind/wave control. In big swells (over 6ft -182cm) coming from the stern quarter I have to apply "vigorous steering inputs" to overcome the sharp bows desire to head 10-15 deg off course from time to time. It's never uncontrolled & never a broach in 14 years but it requires attention. In Phils design I question the use of a fixed shaft in these conditions - I wonder if the lack of trim & rudder steering would make handling less controllable?

    "When your running fast does the bow come out? " (see pic @ 18kts) No - Phil got the balance just right, also the outdrive trim adjustment let's you fine tune the entrance angle. There's no change in attitude from 9kts on up to 27kts & very little from rest to planing.

    "half metre run at 12degrees to the transom. There is a chine flat to add lift and help ensure the ride is dry" As you can see Phil does an interesting flattening of the bottom arc towards the transom - easy in strip building. I added 2" -5cm chine flats starting maybe 2/3rds back from the bow (not in the plans) but they just add lift - the dry ride comes from the bow shape as you can see in the running pic. Phil was all about reducing drag & the flats add some.

    "PT Skiff / Eco18 Marissa" I always thought 505 was a big version of these designs & if I built smaller the Pt Skiff would be my starting point (might be my finish point too...)It sounds like an interesting project - In a smaller version I'd lose sleep over getting the trim/balance right. You want the sharp bow "just there"...too deep & the down wind steering gets squirrelly plus the drag goes up - too high and it stops helping you with pounding in chop and your ride comfort goes south...That's illustrated in the third pic - (my first design...) great little skiff - 30hp Yamaha - but we just endured the chop...In the first year I strongly urged my crew to sit in front to smooth out the ride...didn't help of course & my credibility has never really recovered...Moe
     

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