TOPIC: Shimming a curved transom

My boat has a slightly curved transom, not flat. The transom was replaced by a PO and is very solid. The Merc 650 I removed had no problems being mounted directly with 2 transom clamps and two lower bolts. The 70 Rude will be through bolted with 4 bolts. I see in my Evinrude manual that they recommend shimming the curve so the motor will have a flat area to bolt to. Anyone done any of this shimming work? Curious how you created the shims, or if you found a source to purchase, or if shimming is really needed.

Have you tried using a simple compass and pencil to scribe the curved shape on a flat board on edge ?

The way it works is to clamp a flat board at the crown of the transom (center of transom)
You will see the open air gap at the ends of the board (however wide for motor clamp spacing is)
Take the compass open it to the widest gap on the end.
Place the pin end of the compass against the transom
The pencil end on the top edge of the board (shim)
Run the pin from port to starboard or visa-versa.
The pencil will draw the arc of the transom on the top edge of the board
Do the same to the bottom edge of the board
You can also make a thin template to use when removing the curved waste on the board (shim)
If this explanation doesn’t make sense ?
Search on “scribe perfect fit with compass”

Norm,

I'm not going to say what you should do in your particular case, I but have seen many boats that had curved transoms and none of them had any sort of shim for the motor clamps.


My Skagit Skimaster had a curved transom and no shims were used. The boat originally had a Flying Scott, then a Evinrude Starflite 75, A Homelite 55 (till I blew it up) and a Gale V4-60. The engines all clamped up without problems.

If your motor uses the two bolt-on brackets that the motor inserts into, you could have a machine shop mill the appropriate bevel into the backside of the clamps. If the motor clamp is directly bolted to the transom, you can try to bolt it up & see how it goes. If there is a gap at the outer edges of the clamp, (even after tightening the bolts) tightening the bolts more to close the gap could distort or break the castings. A few companies used to make shims - T&H Marine and Westwind being 2 of them. In order to get it perfectly flat, you would likely need to have a piece machined for it. But the generic shims that were installed vertically on the right & left sides worked good enough.

You could try just shimming the places where the bolts go through the transom using SS fender washers. They come in different thickness and Dia..(I got a bunch me but I'm always needing them so can't part with what I have) Look for something in 2 inch ..(I have them in 1/8th and 1/16th thick), but you might only get some with 3/8ths hole. Yes, they can be drilled out, but as SS its not gonna be easy unless held tight in a drill press, or best is in a lathe like I do. Highly suggest NOT using a hand drill, as its grab and twist you wrist off. Just a suggestion to get ya thinking a bit outside the box!
Fast search link

www.amazon.com/Stainless-Steel-Fender-Washer-1-1/dp/B00AGK8D7Q

OK, found what I was thinking..

www.boltdepot.com/Product-Details.aspx?product=2936