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 Backing plate when replacing motor mount?		  New Topic  Reply to Topic
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AJN77
Deckhand

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USA
7 Posts
Initially Posted - 02/23/2021 :  09:24:47  Show Profile  Reply with Quote
I am getting ready to replace the original motor mount with a 4 spring model. For anyone who has upgraded the mount, did you install backing plates inside the quarterberth, or is it unnecessary?

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts
Response Posted - 02/23/2021 :  09:58:07  Show Profile  Reply with Quote
For a week's worth of reading on this subject (which has come up "a few times"), try our Search function (top-right), looking for the words "bracket mount backing" in Catalina Specific Forum, Entire Message, including Archived Posts. You'll also see discussions of "fronting plates" (on the outside of the transom), which for heavy 4-strokes might be more important.
Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge
Edited by - Stinkpotter on 02/23/2021 10:20:48
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OLarryR
Master Marine Consultant

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USA
3367 Posts
Response Posted - 02/25/2021 :  17:04:05  Show Profile  Visit OLarryR's Homepage  Reply with Quote
Whether a backing plate or front plate outside of transom or both, it is adding some stiffening/reinforcement to the transom. In my case, I added Starboard to the outside of the transom and I had purchased a steel reinforcement plate that I was going to add to the inside but it did not fit/holes did not align and so giving it some thought, I did not put any reinforcement inside the quarterberth. I did the install about 15 years ago and transom has not bowed out or any ill effects noted. The install is detailed on my website.
Larry
'89 Robin's Nest#5820, Potomac River/Quantico, Va
http://catalina25.homestead.com/olarryr.html
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Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts
Response Posted - 02/26/2021 :  23:07:29  Show Profile  Reply with Quote
Fender washers inside are probably sufficient, but if you wish to go further, I'll suggest the upper bolts are the issue. The leverage of the motor (in forward gear) pushes in on the bottom of the bracket and actually pulls out somewhat on the top. In Reverse, I suspect the pull of the bracket is more equal between top and bottom. For my 108 lb. Honda, I added a small backing plate for the two top bracket bolts inside, and (I believe) used fender washers below. I made a "fronting" plate for the outside from Starboard, which may have made the rest unnecessary. But I suspected that heavy, high-thrust, XL shaft motor probably might put more stresses on the transom than Catalina anticipated, especially if I thought I might hit a dock. (As I've said, it could stop the boat as if it had run into a small pillow.)

Using a material like Starboard or plywood on the inside, round the edges and corners a little to reduce point-pressure against the slightly concave surface of the liner.
Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge
Edited by - Stinkpotter on 02/26/2021 23:20:18
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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.