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Catalina/Capri 25/250 Sailor's Forums

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Motor mount backing plate

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RichardG
Admiral

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USA
990 Posts

Initially Posted - 03/30/2003 : 01:42:59

I'm preparing a backing plate while Garhauer rebuilds my old mount. There was no backing plate previously, just 1" washers on the 4 bolts. I cut a piece of 3/4" exterior plywood (about 12" x 9") and put on a coat of thined satin varnish. I decided to do a trial fit before I put more coats on. But it seems the inside of the transom is not perfectly flat, as the board wobbles a little (maybe 1/8") when placed back there (starboard quarterberth). The board wobbles the same when I turned the board over. The transom appears dry and solid.

Anyone else run into this one?

RichardG 81 C25 SR/FK "Sanity"

Edited by - RichardG on 03/30/2003 01:58:34

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eric.werkowitz
Captain

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USA
283 Posts

Response Posted - 03/30/2003 : 08:18:11

Richard,

I installed a backer, but don't recall if the liner in that area was perfectly flat or not. I'd suggest using a belt sander to remove material from the high spot until it quits rocking. When you tighten the bolts it should draw down some anyway. I also coated the inside of the bolt holes with epoxy to seal the ply there. I bedded the thing with lots of polysulphide caulk and used large SS fender washers under the nuts on the inside. So far, so good.

Eric Werkowitz
C25 #4969

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Dennis Pierce
1st Mate

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USA
59 Posts

Response Posted - 03/30/2003 : 09:02:10

I used t-nuts on the backing plate for an easier installation instead of nuts and washers.

Dennis Pierce
Gypsy Witch #1719

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/30/2003 : 14:09:44

Richard: Because the transom is not flat--particularly inside--a very stiff backer can put more stress on the liner than no backer. As you tighen the mounting bolts, all of the pressure may be applied to the liner at the corners of the backer. I suggest:

1. Split the backer so the halves can conform better to the curve.

2. Round each corner to a 2" radius curve to reduce point pressure at the corners.

3. ALthough you've already made the inside backer, I'd suggest a thinner material, such as 1/2" or 3/8" plywood to allow some bend. 3/4" is actually too strong, and can damage the fiberglass instead of spreading the load.

4. Add a 1/2" Starboard backer to the outside of the transom to distribute the load of the bracket base. On our boat, the bottom of the PO's bracket was digging into the fiberglass on the outside of the transom.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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RichardG
Admiral

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USA
990 Posts

Response Posted - 03/31/2003 : 11:10:57

Thanks for the tips!

RichardG 81 C25 SR/FK "Sanity"

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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.