How big a boat can built with glued lap construction ?

I've handled 30' long plywood planks for use in lapstrake repairs. Being 5' 9" tall I can handle 24' planks alone, though these usually 1/2" thick strakes, require I hold them over my head as I walk around with them. Longer planks require I have longer arms or a helper. Scarfing on the boat isn't as much trouble as you might think and Payson butt joints, much easier, so no real need to use full length stuff.

The upper limit would be plywood thicknesses, unless these were doubled on the bench (eliminating scarfs and butts), then hung. I don't see a technical limit to boat length, though there is a practical limit with plywood thickness. Picture a 90' yacht with 2 layers of 1" thick plywood planking, glued together with staggered seams. A skilled crew could hang planks like this, but the practicality could be questioned.

 

The staggered seams idea is good , despite doubling the work .It would certainly be strong .I'm giving serious thought to having the planking ply pressure impregnated to eliminate ....or at least strongly reduce the likelihood of rot in future years .

 

Wow! My boats are not that big - canoes mostly, my problem is the thin ply is fragile. A trick I use is to glue the longitudinal parts like inwales to the plank while it is flat, makes it easier to handle ...

 

The idea of Payson butt joints and butting the planks on the boat makes a lot of sense Par. Because of the reinforcement of the lap above and below the butt there is very little force acting on it. Scarfing is way overkill for this construction and it's so much easier to handle shorter planks.

 

I have a picture some place of me carrying a 24' plank, scarfed to length in 1/2" Douglas fir. It wasn't all that heavy, maybe about 20 pounds, but I had to hold it fully over my head to keep the ends from dragging on the ground. A longer plank and I'd have to have put wheels on one end. Scarfing in place is the common method of making repairs. Butt blocks often don't leave the strake run fair. The same can be true of the Payson joint, but you can knock it fair if necessary, though good support and joint prep can solve this unfair tenancy.

 

I've scarfed a sheet to a half sheet to build an 11' skiff but I'm not sure how I would approach a larger one. Chris Craft built a line of 30 or so foot lightweight off shore bass boats in the 50s and 60s with MDO plywood. The resin paper overlay made them very easy to get a good finish on and they seemed to last well when I did repairs to a couple in the 90s.
A cautionary note on taking a boat in the wrong conditions.
1968 on the SF entrance bar's extensive north shoal called the Potato Patch (remember half the drainage of California comes out this narrow opening on a strong ebb) I was crew on a 44' CG boat searching for three missing sportfishermen. They had gone out after salmon in one of the above mentioned CC lapped ply 30' boats.
As we crashed over and into a long line of breaking seas we came upon half of the missing boat. Yes, half of it, split right down the keel at the floor timbers, the other half with engine and probably the missing guy we never found. We rolled the hulk over with boat hooks to make sure there wasn't a body attached so I got a good look but it was brief because we were in a very bad place and had to keep moving or roll over ourselves.
The CG helicopter picked up the other guy and then we found the third one clinging to the floating pilot house top, put him in our warm cabin and treated for shock and bleeding and headed back.
After a mile we got a call "30' Chris Craft broken up on the Potato Patch" and said we got that guy and they called back and said, "no this is another one". No bodies were ever found from the second boat though we looked the usual two days.

 

That is a very good cautionary tale about lapstrake. Sounds like one of the garboards detached from the keel. Perhaps the laps in the 2 halves of the hull were stronger than the garboard-to-keel joint, or bilgewater may have weakened the joint. Worth bearing in mind during construction ...

 

Originally Posted by BATAAN
. I was crew on a 44' CG boat searching for three missing sportfishermen. They had gone out after salmon in one of the above mentioned CC lapped ply 30' boats.
As we crashed over and into a long line of breaking seas we came upon half of the missing boat. Yes, half of it, split right down the keel at the floor timbers ...

It is not so much about lapstrake per se.
It is more about inattention to structural continuity of boat.
Plywood is close to isotropic -it is strong both in longitudinal and in transversal directions, somewhat less strong in diagonal.
Keel, on the other side, whether solid or laminated, without a layer of plywood laminated in, is only strong in longitudinal direction. All of us know how easy it is to split a plank along the grain.
Now, look at the bottom of plywood hull with bottom sheets connected via timber keel, but not overlapped to each other, plus floors, spanning the connection. Imagine a boat, which has just landed on the wave. What we have here? We have static+dynamic water pressure at bottom, (maybe pressure is higher closer to chines, according to classic planing bottom pressure distribution) pushing up, and heavy engine trying to continue it's travel down. Bottom structure has to resist it. Now, if we bend a floor + plywood , we have a strong and homogeneous structure; if we bend floor + 2 sheets of plywood, connected at mid-span via timber keel... timber keel is an obvious breaking point, as strength of timber in transversal direction is nothing to compare with that of plywood. Additional breaking point via stress concentration could be created by limber-holes (or some rot so easily initiated here) in the floors, exactly at mid-span, where bending moment is greatest, and exactly at same location where discontinuity in bottom planking is in form of keel joint!
I do not know if keels of those boats had a layer of plywood laminated in or not. Keeping in mind when they were built, my guess would be rather no. Anyone more knowledgeable on the subject, please clarify.

 

I have used chine logs in my small plywood boats. I start by gluing a test piece of ply to a sample of the wood I plan to use for the log (or keel) so when I break it I can see if the ply breaks first. If it does I conclude I have sufficient joint width and log section. Other than a flat-bottomed boat, I regard the keel joint as a butt joint between 2 ply panels, it should be stronger than either of them. If ply breaks due to an impact it typically pushes a triangular piece in, it can be forced down and fothered as a get-me-home fix. But if the chine log or keel splits, once started the split may continue to grow along the grain like a zipper opening. So it pays to have a good margin of safety.

 

Noted !

 

The garboard plank on any method of built up construction, is the one that is most highly loaded. It's the plank that is replaced most often for this reason, typically at twice the rate of the broads. Planking is a consumable element, just like an oil filter. The garboards are most likely first to require replacement, typically with partial refastening on other planks. Then the broads, though usually by this time the whole bottom needs to be replaced and the topsides completely refastened. Lastly of course, are the topside planks, which usually also have major frame, floor, stringer and other structural elements repair needs as well. This is normal and typical "life cycle" events for built up construction methods.

I'm reminded of when they found "Stormy Weather" and hauled up half the boat, of course parted along the rabbit on the carvel hull. It's quite normal and not an indication of anything more, then the logical load concentration area on most "built up" construction methods.

 

This is on of the reasons for my desire to use my current keel timber ,a 20 foot stick of 9x6 Oz hardwood .It would take some considerable force to split it and it holds screws marvelously .I am being councilled by most ,but not all ,to use a laminated oregon (douglas fir ) keel .I am quite confused as a result .

 

A laminated keel is more dimensionally stable and can be encapsulated. A solid 9x6 can't be encapsulated and is subject to huge amounts of movement, checks, splits, etc. Given a choice, a laminate is better.

 

Now this is the bit I don't understand !

1. Why can't a solid keel be encapsulated ?
2. This particular timber is extremely unlikely to split , twist and will move less than fir .
3. Why is a laminated oregon / Douglas fir keel less likely to move ? It will ,in the course of time get wet inside .Everything gets saturated at a few years ,even the inside of plywood .The expansion numbers on this particular hardwood are less than fir and it is FAR less likely to rot .

 

You can't encapsulate large timbers because of the internal stresses and expected amount of movement you get from large timbers. Hardwoods move a lot more then softwoods as a general rule too. Large timbers (anything over 1" thick is subject) need to be cut into "lifts" and each encapsulated, before incorporation into a laminate. This stabilizes each piece at a specific moisture content, within the laminate, so it doesn't move and challenge the shear strength of the coating.

Properly encapsulated pieces don't "get wet inside", but remain dimensionally stable, including plywood.