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Discussion Starter #1 (Edited)
Hi folks. I'm new here and this is my first post.

I'm building a boat, a whitewater dory, and need some advice on how to cut the arched top of the transom. It is 3 layers of 1/2" marine plywood, laminated together with epoxy resin, plus some thin layers of E and S Glass fiberglass on each outside surface. So total thickness of a little over 1.5". It is already mounted to the boat, so a band saw is not a good option :)

I need to cut an arched top from one edge of the boat side panel to the other. The person who has been giving me some building advice says he uses a circular saw, but it seem to me the radius of the cut (the transom is approx 24" across the top) will be too tight and the blade will bind and either kick back or destroy the wood.

Other options include a jig saw, with some long blades. Or maybe a recip saw. My plan with any saw was to cut just short of the final cut line, and then hand finish with a plane. I don't see where a router can safely do much here but open for good ideas.

Thx for any advice, and thx for a great resource.

Jon
 

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I've made accurate circle cuts with my table saw quite a few times, with no binding, you just start out with a shallow cut and keep raising the blade after completing each circuit. A router and circle jig will produce a symmetric arch, however when dealing with plywood it can be a challenge regardless of the tool used. The more fibrous the ply the greater the chances are for shredding. You could pre draw the arch both sides of the transom then score the lines with a "sharp" utility knife or scalpel and or then add tape over the scored cut. You may even consider applying a pre coat of the resin along the line of the cut both sides of the transom to help set the fibers.
 
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Make a template out of 3/8 plywood. Then use a reciprocating saw to rough cut the transom about 1/4" from the template. Then use a top bearing long flush cutting router bit running against the template to finish the cut.

Charley
 

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Hi Jon and welcome to the forum. Don't try the circular saw. Your assumptions are correct. Even using a keyhole saw would be a better option than that. A jig saw with a long enough blade is your best option. That's the kind of cuts they were made for.
 

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I agree with CharleyL. Make a template, draw the line, cut outside the line with a jig saw, then use a trim bit with a bearing to make a smooth cut. An inch and a half, plus the template thickness is going to take a pretty good sized bit, so I'd go easy on how fast I'd move the bit during the cut. I also would cut a little closer than the quarter inch. But that's just me. Stick made a good point about using a Bosch blade. I find they cut better than any other brand.
 

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Discussion Starter #7
Thanks for the advice. Much appreciated. If I were to go with the flush trim router option, how to attach the template? With two sided tape? Really no way to clamp since I need to leave the top arch free for the router pass and the sides and bottom of the transom are attached to the boat hull leaving little point for clamping. I suppose I could rig up some clamps on the side panels and then run a board across the template, and then strap to each side panel clamp.

I have to admit I'm a little nervous considering a router bit big enough for 1.5"+ of wood, handheld sideways in a router.
 

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clamp the template on with extra clamps..
remove the clamp that is in the way of the router and place it behind the router where you have already cut...

or...

place your clamps ...
trim between clamps...
stop the router...
move the clamps to the trimmed sections...
finish trimming where the clamps were...
 
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Go with the jig saw..use your template with it...sand smooth after.

Did the same thing with wooden clam boat...

I agree with you for not using the router...
 

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Discussion Starter #10
Here's an early pic of the transom. I left the top part proud since it would help protect the dory when it was turned over.

PS> If anyone wants to check out this build I have a blog about it (my first blog so nothing professional!):

www.westtavaputs.com
 

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Excellent blog Jon!!!

once completed, this boat ought to be the source of an awful lot of fun while heading down the Colorado!!!
 

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Go with the jig saw..use your template with it...sand smooth after.

Did the same thing with wooden clam boat...

I agree with you for not using the router...
There are 2 types of template bits, one with the bearing at the bottom which is normally referred to as a flush trim bit and the other with the bearing at the top on the shaft of the bit which is normally referred to as a pattern bit. I certainly would not recommend a pattern bit that long or in that situation without using an offset base on the router (use our Community Search for that if you aren't familiar with them). Normally only about 40% of the routers base is supported on your work which would make it easy for the router to tip and the tip of the bit could dig in and you lose control. If you do use a router then use a flush trim bit which will put your template on the far side of your work from you. Nick may be right in suggesting that you just jigsaw as close as possible and the sand to finish. It probably doesn't have to be perfect, it just needs to be good enough to fool the eye.
 

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Welcome to the forum Jon.
 

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Nope, doesn't have to fit anything. Just please me.
I would cut it with a jigsaw like suggested and if you can get a hold of a belt sander to finish off the cut they are easy to control and will do a nice job. I imagine you are going to coat it with resin anyway.

Looks like you are doing a good job, I have built several prams and row boats and they are fun to make. All plywood with fiber glass over.

Herb
 

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Discussion Starter #17
Thanks all. I appreciate the warm welcome and all of the good advice. Seems like a nice place.

I do have a belt sander, and a power planer. My kids like to make fun of my love of my shop and tools. Their favorite quote (which I don't completely recall actually saying) is a reply once to their question of where I was going, "out to the garage to look at my tools".

Working on setting up my NYW style router table and Woodpecker top and lift, after retiring my old table and 690 Porter Cable. It was a good Christmas.
 

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Thanks all. I appreciate the warm welcome and all of the good advice. Seems like a nice place.

I do have a belt sander, and a power planer. My kids like to make fun of my love of my shop and tools. Their favorite quote (which I don't completely recall actually saying) is a reply once to their question of where I was going, "out to the garage to look at my tools".

Working on setting up my NYW style router table and Woodpecker top and lift, after retiring my old table and 690 Porter Cable. It was a good Christmas.
Your kids aren't allowed on the forum then. We don't appreciate having people make fun about tool addictions.:smile:
 

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