Replies to This Discussion

Permalink Reply by Randy Dersham on January 23, 2009 at 4:20pm

At Tatman boats we use a 12:1 scarf on all sides and bottoms. It is actually a wavy step scarf cut by a computer controlled controlled router. I have heard that people have used as low as an 8:1 scarf but don't recommend it.

I/2 inch bottom should be a six inch long scarf and 1/4 in sides should be three inches. The scarf should land in between frames for the least stress on the joint.

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Permalink Reply by Mike Baker on January 23, 2009 at 5:11pm

My first couple of boats were scarfed 8:1 with a West system scarfer attachment for a circular saw, it worked well enough. Then I started hand planing them at 12:1 or sometimes even 14:1 with a big ol 22" jack plane, Much Better!
Mike

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Permalink Reply by Dave Z on January 24, 2009 at 8:15pm

12:1. Wouldn't recommend anything less for either 1/4 or 1/2 inch. You need that surface area. I suppose you could go less on the bottom, depending on if you were sheathing in glass.

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Permalink Reply by Ross Duncan on January 25, 2009 at 5:16am

10:1 for me. I lay out both sheets staggered, scribe pencil lines every 1/2" for parallel indicator, and carve away with a power plan. Not very elegant, but fairly quick amd I have not had any failures.

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Permalink Reply by E. Landis on January 26, 2009 at 7:52am

Thanks for all of the replies. This is my first boat. So much to learn. If you guys have any more tips on cutting the scarfs or planing the scarfs by hand that would be greatly appreciated.

Any hints for laying out the joints? There are going to be two eight foot sheets and then a smaller section. Do you guys usually put the smaller section in the middle of the boat or at the ends?

This site is a great resource. Thanks!

EKL

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Permalink Reply by Dave Z on January 26, 2009 at 11:39am

EKL,

For laying out the joints, I like to put the joints in places on the boat where there aren't tight curves. This will be up front on a standard 16' or towards the back between the #2 and #3 frames. (considering a standard 16' double ender with transom) I have put them on frames, and in between frames. The reason I like to keep them in the areas without alot of curve is because the scarf will be stiffer than the surrounding wood. Placing this between frames in an area that is pretty straight seems to work nicely. I am fairly convinced that once a boat is built around the frames, that wood would retain a memory even if you removed the frames (look at S&G boats, right?). The scarf will be the strongest part once done right.

Also keep in mind that when you lay out your scarfs, that they won't be in the same place for each side. That is, if you are using one 16' scarfed sheet to make both panels, the front of one is the rear of the other. The scarfs are ging to end up opposite too. Two ways to do this:
1. don't worry about it.
2. Plan it on paper, draw it out and model it. Can be done with construction paper, 1" = 1'. On a separate piece, draw and cut out your side panel, draw your frame stations and lay it on your "fake" sheet of scarfed plywood. Line up where you get the scarfs in place where you want them. This will tell you where to put your sheets when you scarf them.

In case you are curious, I used method #1 without failure once. On the second boat I planned it out to match up the fir plywood grain. Did it for cosmetic reasons only. But I could be persuaded to layout more strategically for structural integrity.

I would be more concerned with getting a good mix of epoxy and a thorouoghly wetted out joint. A starved scarf joint is more likely to fail during building the boat than once it is on the frames.

To cut scarfs, I tried to use a circular saw and a homemade jig, but found it cumbersome. What I ended up doing was a laying the wood down, clamping it tight, ("A" or good faces together) and scribing lines, as someone suggested earlier. I used a Stanley #220 low angle block plane. (Must be razor sharp) and took my time. The effort is well worth the end result. I did see a nice router jig that cuts some perfect scarfs somewhere on the internet, but I'm too lazy to build one!

The old belt sander method works, but you will need to be very careful. Those things can chew off too much material when you are getting close to finishing.

Good luck,
Dave

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Permalink Reply by Ainslie French on April 17, 2009 at 12:02am

I am building a Merlin Rocket Hazardous 170 clinker plywood racing dinghy as my first boat.

I have glued up 6 scarfed 6mm 5 strata marine plywood sapele faced panels with west system epoxy in a stack. Most panels have come out satisfactorily but a couple of scarf edges are a little proud, 1/2mm. Is it safe to plane and sand even or should I redo the scarfs ?

Any advice would be welcome.

Ainslie French

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Permalink Reply by Richard on April 19, 2009 at 6:27pm

From the reading I have done on this it depends where the joint will be tested. Your 1/2 mm is pretty insignificant as long as you stuck to the 12:1 ratio. The rule of thumb is not to reduce the thickness of the scarf joint in relation to the rest of the pieces being attached, thus sacrificing the integrity of the joint. I would sand and if need be apply a thin film of epoxy over the joint to fill in any dips. You can always sand down the epoxy after.

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Permalink Reply by Richard on April 19, 2009 at 6:13pm

Stick to the 12:1 ratio as it is tried and proven. There are so many forces at work on the joint with a boat that unless you can predict them then don't stray from tested engineering. You will find it quite easy to control your joint face with plywood, as the glue lines from the laminations will guide you. I used a belt sander and that worked very well. Hand planning works as well as long as you use a jointer or fore plane (minimum 20" sole). The joint preparation is pretty much the same for either. Greg Rossel's book "the Boatbuilder's Apprentice" is a good book to have on hand and speaks about scarf joints.

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