Glued laps vs Riveted laps

[ssor]

I have read the posts that I could find on this subject but I am still left with the question, "tradional riveted lapstrake construction requires rather closely spaced ribs or frames, does this schedule hold for glied lapstrake construction also? Or does the nature of plywood used in glued laps allow fewer ribs?
Inquiring minds want to know.

[ 02-20-2005, 06:38 PM: Message edited by: ssor ]

[StevenBauer]

Yup, way fewer frames. Once you glue the ply laps it's kinda like one solid plywood structure. So they say.

Steven

[Steve Lansdowne]

The plans for a particular glued-lap boat will show where frames go and how many there are.

[ssor]

Steve, I am adapting a John Gardner version of L Francis Herrshoff's 10foot skiff. His plans call for frames on 7 1/2 inch centers.

[Jamaica Mike]

Originally posted by ssor:
...Or does the nature of plywood used in glued laps allow fewer ribs?
Inquiring minds want to know.I have not built a glues lap boat yet, but am doing the research preparatory to doing so. The expectation is that I will get a tight, stiff hull with fewer ribs - and in less time - than with traditional methods.

JM

[Bob Smalser]

The expectation is that I will get a tight, stiff hull with fewer ribs... I've very limited experience with plywood and goo....but the 20-footer I'm refitting now is built that way and it's much, much more flexible than a traditionally-planked, riveted boat with a third more frames would be.

Too flexible IMO....set that Gardner-drawn, glued-batten-seam dory centered on one sawhorse with a man at either end and you can twist and bend it full inches, instead of eighths with the thwarts removed. 3/8 planking and 7/8 bottom, too.

The thwarts may well be the only thing keeping that boat from breaking its back crossing a bar.

http://pic3.picturetrail.com/VOL12/1104763/6035832/77494165.jpg

[ 02-20-2005, 09:46 PM: Message edited by: Bob Smalser ]

[JimConlin]

In traditional lapstrake construction, ribs were needed because the planking stock supplied very little cross-grain strength or stiffness. Plywood planking stock supplies that very well and the glue joints between the planks are strong and stiff, too.
Look at he Iain Oughtred boats and you'll see no transverse framing except floors, seat knees and bulkheads. It's a good thing, too because the pockets between ribs and lapstrake planking are very hard to clean and paint and are a notorious place for rot to start.

[ssor]

Thanks for your input. Very well stated. I guess that besides holding the planks together the ribs/frames also resist crushing forces at the gunwales. I don't believe anything was put in needlessly.

[JimConlin]

In a dinghy, that function can be done by the thwart knees. Don't leave out a post under the center of thwarts.
This is the LFH pram in cedar strip composite. No ribs.

http://www.conlin-boats.com/lfh_1_detail.png

[ssor]

That's Beautiful! smile.gif smile.gif smile.gif

[dmede]

I built a 15' glued lap ply canoe. No frames. Once the lap is epoxied together it is very strong and acts like longitudunal frames, sort of. Because the ply used can be thinner than the wood for a solid plank version it can deffiently be more flexy but thwarts and grails stiffen it right up. My boat has no noticable flex along it's length nor can it be twisted.

Dave

[ErikH]

I think it depends somewhat on the lap method used and certainly depends on boat design.

One aspect is the 'stringer effect', where the laps essentially produce full-length stringers which provide some rigidity. Laps which have a relatively high total thickness at the lap (of combined planks) are going to be stronger than those which do not. And because stiffness increases radically as thickness goes up (small thickness differences can make big stiffness differences) i think the lap overlap might make a fair bit of difference. I've read stuff somewhere confirming this but can't for the life of me recall what it was.

It's also got to have something to do with the included angle of the lap joint. A glued chine that's 165 degrees is going to be weaker than one that's 145, is it not?

[bainbridgeisland]

Bob, neither the method used to attach the laps or the frame spacing has much to do with the torsional rigidity of a boat. Diagonal framing, decking, thickness of the planking and size of the sheer clamp on open boats is far, far more influential.

Glued lap plywood boats need fewer frames simply because plywood is less likely to split than solid timber of the same thickness. On riveted lapstrake boats, the frames improve transverse strength so planks are less likely to split and also improve transverse rigidity so the laps are not overloaded. Enough frames or bulkheads must be retained on glued lap boats to hold the transverse shape of the boat so lap joints are not overloaded.

With an open boat, the easiest way to improve torsional rigidity is to add material to the sheer clamp or rub rail. Doubling the size of this member increases torsional rigidity carried by it about 16 times. On many open boats, the sheer clamp/rub rail accounts for about 1/2 the total torsional rigidity of the boat.

[JimConlin]

You can over-think this.
After you've gotten the shell, rails, quarter knees, breast hook and thwarts done, then judge what additional interior framing (seat knees, spreaders, or ribs) is needed.

OR

Cheat. Follow the scantlings and details of similar boats of the same construction method.

[hikingchrs]

I concure my Skerry was scary flexiable untill the rub rails were installed
Chris