Bedding garboard in classic Herreshoff pram

[rmidgley]

I am building my first boat. An oak keel is now fastened (SiBr screws and 3M 4200) to a keel batten and I am about to position the garboard tightly against the keel and then rivet the garboard to the keel batten. While the Northern white cedar garboard (9/32" thick) fits tightly to the keel batten, a variable small crack exists along the edge between the garboard and the keel (up to 1/32" wide). Would I be wise to a) keep planing to reduce the variable space or b) use some type of bedding compound between the keel and the garboard and perhaps between the keel batten and garboard riveted to it? If so, what bedding would be best? I am considering BoatLIFE Life Caulk or 3M 4200. Reactions? Better suggestions?

[Bob Smalser]

I read that to mean the keel batten is cracked. What's the grain orientation of your keel batten? Vertical or horizontal? As the full length of the garboards are fastened to it every three inches, the part is too critical to allow the crack to either work in use or get worse if the crack matches both wood grain and stress moment.

If the transoms aren't on yet, we aren't talking about either a big board or much time wasted fastening. If there is any doubt, replace it.

And there are cheaper, less adhesive goos than 4200 that will last just as long and may be a better idea.

[rmidgley]

Sorry for the confusion, Bob. The garboards are fine. It is just that the boards do not fit perfectly tightly to the vertical keel but clearly will be pulled down tightly to the keel batten. Would I be wise to use some bedding compound to fill the teeny gap between the nearly horizontal garboard and the vertical keel? The transoms are on and I plan to use rivets every 1-2/3" as recommended in John Gardner's book (with oak timbers sprung in after every 5" when they too will be held in place by longer rivets through the planks and the timbers).

Some people watching the construction tell me that the joints between the planks that I will be riveting also should include bedding compound. I keep protesting that the wood will be pulled together tightly with 3/4" overlap and that bedding is not needed. Who is right?

[Bob Smalser]

Bedding compound between laps is strictly optional. You'll hear that it perfects any minor flaws in plank fit but compared to the laps swelling when they hit the water, all it really does is leave a mess between your laps difficult to make neat. If you use it, use very little of it between your laps and under no circumstances use a compound that is even slightly adhesive. Those lapstrake planks can crack if you take away the "give" of the rivets by using glue.

Between your garboard and keel is different problem however, and wouldn't hesitate to bed it throughly. But here I'd also use a nonadhesive bedding like Dolphinite or some other oil-based compound.

[rmidgley]

Perfect. Exactly the type of advice I was seeking. I sensed that the keel-garboard union might require some type of bedding that is not needed for the rest of the planks. Thank you!

[rbgarr]

It sounds like you're off to a good start on a nice design. Congratulations on your choice. Walt Simmons' guides to lapstrake boatbuilding may be helpful, too: http://www.boatbuildingbooks.com/books.html

[oakman]

All I will offer to add to the advice given is to seal any wood that will get bedded.
This will prevent the wood from drawing off the oil in the bedding compound and rendering it useless. Shellac is purported to be the best sealer around.

Oakman

[rmidgley]

One more query. The garboard-keel junction needs to be bedded well and Dolphinite or an equivalent works well with one concern: over time it can dry and fall out. Does this concern suggest that other products including those that become rubbery (e.g. Teakdecking Systems TDS Teak Caulking) might be even better?

[Eric Hvalsoe]

I have used Boatlife LifeCaulk, it does not dry and fall out. The only place I tend to use Dolfinite is bedding deck hardware. A number of products out there will do the job - marine grade, non or only moderately adhesive (thus elimating 5200). Actually I've heard someone recommend tub and tile caulk for the laps - can't personally back that one up. Use to always plank dry - excluding garbard and hood ends which one should always bed. In recent years I've used Sikaflex, and Boatlife Lifecaulk and Lifeseal in the laps. No doubt other products will work as well. Applying caulk does somewhat run counter to all that time you've spent carefully beveling the laps. I also run just a touch of half oval along the lap with a rasp. Bedding the laps adds considerably to the messyness factor, clamping becomes more important, consider going without and concentrate on your craftsmanship. You may get just a little weeping, if you've done a good job she will seal up quick.

[Bob Smalser]

One more query. The garboard-keel junction needs to be bedded well and Dolphinite or an equivalent works well with one concern: over time it can dry and fall out. Does this concern suggest that other products including those that become rubbery (e.g. Teakdecking Systems TDS Teak Caulking) might be even better?

Visit any large collection of lapstrake craft still in use like at CWB Lake Union and it's surprisingly easy to find cracked strakes. Not all are fatal, but all are certainly eyesores and the weeping often spoiling the experience for inexperienced passengers.

Pick one reason or a combination of many. The strakes are too wide. An unstable species was used. Flatsawn wood was used. Defects in the stock were ignored. The rivets were set overly tight. The laps were inadvertently glued together. The planks were left unpainted or poorly painted and the boat baked in the sun for months, causing greater seasonal shrinkage than usual.

You'll have to find the right compromises for all of that, depending on what you have to build with:

http://pic20.picturetrail.com/VOL12/1104763/7534648/98698786.jpg

I happen to like the poly plumber's caulk my jobber Tacoma Screw carries. Cheap, lasts forever and is nowhere as near adhesive as 3M marine sealants. But it's still slightly adhesive so where I'm concerned about seasonal movement, I prime, paint and sometimes even wax the wood or metal components I bed with it.

Would I use it or something similar on your garboards? If they are narrow and vertical grain I might. If they are wide and flatsawn I'd use something less adhesive or even nothing at all but red lead primer.

[Clencher]

A view from across the pond:

What you have is exactly right. There should be a caulking seam between the edge of the garboard and the keel in lapstrake/clinker construction. In fact IMHO this should be the only seam in a lapstrake clinker boat that is caulked.

The reason you don’t want the edge of the garboard tight against the keel is that when the plank swells it has ‘nowhere to go’ so to speak, it may cup and then shakes may develop when the boat dries out. With a caulked seam against the keel, the plank can swell on immersion without distorting. Others have given excellent advice on suitable flexible caulking materials.

Bob S will correct me if my reasoning is wrong.