GERONIMO PLANKING - ANOTHER OPTION

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bobinpowayca
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GERONIMO PLANKING - ANOTHER OPTION

Post by bobinpowayca »

Well there's probably been too many posts from me obsessing about planking my Geronimo, trying to find ten foot plywood, where to place butt joints, etc., and finally deciding I needed to learn to make a scarf joint - but here's another idea I'd like to see what other folks might think about.
I was talking to a guy who works as a boat carpenter and he said he uses LAP JOINTS. I will be using 3/8" thick sheets of 4x8 marine plywood. So say I wanted to join two sheets I could take a router and a straight edge and rout down exactly 3/16" on each end to be joined, maybe 4 or 6 inches in from the ends depending on the width of the joint I want. Then epoxy the ends together in a lap joint.
Sounds good to me - I've never heard of this so far on the forum or the plywood book, only butt joints or scarf joints are discussed. What say, shipmates? Thanks, Bob
Bob
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Built the Glen-L 17 (1988), Geronimo (2018)
PBR support (1968)

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rbrandenstein
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by rbrandenstein »

A lap joint might be okay on solid lumber, but I would not do it on plywood.
All the stress is reduced to one ply joint and would be very weak.

Thing about it.. if you made a long thin piece with a lap joint and tried to bend it in an arc, it would split easily at the middle ply.
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bobinpowayca
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by bobinpowayca »

Yeah Bob, that makes sense, the ol "too good to be true" - didn't seem right this isn't in the boatbuilding with plywood book! I like your blog by the way, I refer to it a lot. I did buy a Bosch power planer which worked well for the keel and chine - if anyone has used one of these for making the scarf cut on plywood I would appreciate hearing about how you did it. Thanks! Bob
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Bob
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Built the Glen-L 17 (1988), Geronimo (2018)
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Moeregaard
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by Moeregaard »

We scarf-joined the 1/4"-ply planking for our Zip build and have had no problems. We used a router fitted with a very wide base, and a 3/4" face-cutting bit. The plywood has to be held absolutely flat--and I mean really, really flat--and you have to take small bites to avoid tear-out. Shoot for a 12:1 ratio and you should be fine.

-Mark Shipley
A boat is just a wooden box with no right angles.

bobinpowayca
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by bobinpowayca »

Thanks Mark
Bob
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Built the Glen-L 17 (1988), Geronimo (2018)
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jprice
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by jprice »

I still like the butt joint best.
No it's not as clean and pretty as a scarf joint, but it's just as strong and easy to do.
Plus, you can plank in pieces. Wielding 16-18' pieces of ply in my tight garage would have been a pain to say the least. With the butt joint, you never have to handle more than 8' at a time.
viewtopic.php?f=2&t=13567&start=45#p116185

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darthplywood
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by darthplywood »

when i got to this point in my build i obsessed over what was the right thing to do....i was afraid to scarf and i didnt want a butt joint (i don't remember why at this point). I was (still am) as amateur as they come. I bought a nice circular saw....a couple of really nice high end blades and the scarffing jig from west system. It ended up being way easier then i had originally thought. Check it out:

http://www.westmarine.com/webapp/wcs/st ... system+875

I will always scarf plywood on future builds, its not as difficult as you'd think....be prepared though, making those cuts along the plywood dulls the blades quick..and without a sharp blade it can make the edges rough...invest in good blades.
Built the 17' Glen-L "Sea Knight"
yet to come...11' Glen-L "Utility"

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AaronStJ
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by AaronStJ »

rbrandenstein wrote:A lap joint might be okay on solid lumber, but I would not do it on plywood.
All the stress is reduced to one ply joint and would be very weak.
I'll play Devil's advocate. Butt joints are a very well accepted type of joint on this board (and for boatbuilding in general). Don't butt joints also put all the stress on on ply of the plywood (namely the outer ply that the doubler is glued to)?

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galamb
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by galamb »

Before I every even thought about building a boat I did a lot of work with plywood.

I think if you built a lap joint and supported it the same way as recommended for a butt joint it would be "stronger" than a typical butt joint and here is why.

In the boat building with plywood book (and my plans from Mr. Hankinson) it suggests that you butt your two panels together and (block) them on the underside, between any longitudinals with plywood, either the same thickness of the panels being joined or in no case, less than 1/4" thickness. You are to extend these (backers) at least 4" back from the joint (on a 1/2" panel - which is about the length a scarf would be).

So on the butt, the "butted" edges (should) be supported underneath the entire joint by either a plywood (patch) or the lumber of the longitudinal (this method was well in place before the time of epoxy so the actual joint itself would have had glues "sub" structural strength) so the entire joint is screwed together (through the panel into the patches, lumber, whatever is underneath.

The top of the joint, unless covered by cloth, was at the mercy of the strength of the "non-epoxy" glue joint. And while I realize while "afloat" the pressure of the water, against the joint (at least on the bottom) would keep that joint closed tight (even if there was some flex or lack of "continuity"), on the side that would not be the case and after repeated flexing that joint could open up.

If you "lap" the joint, such that say 4" of lap existed and 1/4" of a 1/2" panel was glued down to 1/4" thickness of the panel being jointed to AND the underside was still "blocked" as a typical butt joint (extending beyond the extreme edges of the lap) logic says you should have a stronger joint.

The bottom (exposed) joint of the lap would be backed by blocking - the joint itself would only be 50% the depth of a typical butt - ditto on the exposed (outside) - now the joint is only a 1/4" deep (on a 1/2" panel) before it hit's (solid) wood. Any flexing would distributed throughout the joint and because it's over a larger area it will be absorbed to a drastically greater degree before the structure is compromised.

So while a lap joint is significantly more work than a butt (have to route out the laps) and requires more material than a scarf (more glue, backers etc) I would suspect that it is in fact stronger than either.

Think of how builders restore old boats with "birds mouth" scarfs when replacing ribs - that certainly looks like a scarf (angle) shaped "lap joint" to me.

Just my opinion on that one...

(ps - on my build I scarfed the side panels for "cosmetic reasons" and butt joined the bottom because it was a mix of 1/2" panels in the aft portion and two layers of 1/4" in the bow - which were "offset" so the joints didn't line up - I guess you could call them big "lap" joints)
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travis24
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Re: GERONIMO PLANKING - ANOTHER OPTION

Post by travis24 »

No such thing as a "Boat Carpenter" Your either a Carpenter or a Boatbuilder one may think there the same but they are actually totally different trades.

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