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Thread: Engine mounting bolts have sunk into transom. Need repair?

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    Member cube01's Avatar
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    Default Engine mounting bolts have sunk into transom. Need repair?

    Hello all,
    I recently bought my first boat (with help from a previous thread.) I'm pretty happy with my purchase, but the engine mount bolts have counter sunk into the transom a bit... Me being new at boat ownership, I'm not sure if this is normal or if I should replace the transom. (I've already re-wired it and made a few small modifications, so a transom replacement sounds like a good excuse to do some more tinkering.)

    The engine still feels very firmly attached - there is no sogginess or 'give' anywhere. Here is what is looks like:

    IMG_2861.jpg

    Also, for anyone from the other post, here is what she looks like after getting her cleaned up:

    IMG_2856.jpg

    Thanks in advance!
    "If our father had his say, nobody who did not know how to catch a fish would be allowed to disgrace a fish by catching him." -A River Runs Through It

  2. #2

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    It is hard to tell. It looks weathered and seems to have things growing in it in places. It is probably marginal at best. However, without taking the bolts out and seeing what kind of wood is right there, it is a tough call. I replaced mine on my river boat a few years back and it looked about like that. It finally cracked and that was the end. No, the boat didn't sink......

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    Member JR2's Avatar
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    I look at it this way, there are still a couple of months until boating season, you appear to have a fully equipped garage... I say why not. If your asking on here, your concerned about it. Fix it now and live worry free.

    On a side note, my dad fixed a similar problem with a couple small pieces of 1/8th diamond plate. Put it on both sides and bolted thru it.
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    You could get a piece of aluminum cut to the right shape and put on the inside of the transom to sandwich the wood, re-drill your mounting holes from the outside and bolt the motor back on. I would probably replace the wood at the same time, it doesn't look all that great and you want something solid in there. You could use new wood, or some builders are using plastic materials in place of wood, "trex" type material or UHMW or something similar that won't rot. If you have a hoist of some type to hold the motor up while you work, it shouldn't take too long to have a good solid transom on your new(to you) boat.
    Casey
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    Wood looks pretty old to me, I would use some 1/8 or 3/16 aluminum and use as a sandwich.

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    Supporting Member Old John's Avatar
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    Quote Originally Posted by Akfish1 View Post
    You could get a piece of aluminum cut to the right shape and put on the inside of the transom to sandwich the wood, re-drill your mounting holes from the outside and bolt the motor back on. I would probably replace the wood at the same time, it doesn't look all that great and you want something solid in there. You could use new wood, or some builders are using plastic materials in place of wood, "trex" type material or UHMW or something similar that won't rot. If you have a hoist of some type to hold the motor up while you work, it shouldn't take too long to have a good solid transom on your new(to you) boat.
    ditto what Akfish1 said.. you got a couple months before you can put that boat in the water.. I'd replace that transom wood now. and if you want a piece of 1/4" aluminum to sandwich the transom, I got a piece that came off of a similar arrangement on another boat. the bolt holes might even match up as it was on a Johnson OB. I'd probably let the aluminum go for a 6 pac of Alaskan Amber.

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    I'll bet you dollars to donuts the previous owner just drilled the holes and slapped it on. That's an issue because now you almost certainly have rot starting in there, or at least pre-rot punk. Aluminum sandwich (aka lots bigger washers) is a short term fix, but sooner or later you're going to be replacing. I'd do it sooner, because the more the punk or rot, the more of the job the aluminum has to do. Start flexing aluminum like I've seen in similar setups, pretty quick it looses its flex and starts to crack. Then you have a real issue on your hands.

    With this much time on hand, I'd replace as a pretty high priority. The sandwich would be an "emergency" fix for when you don't have time, but that doesn't appear to be an issue right now.

    BTW- When you drill the new transom, smear the inside of the holes real good with silcon seal before putting in the bolts. I go so far as squirting silicon seal into the hole to fill the gap after the bolts is shoved through. Gotta keep the water out from between the plies.

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    Another vote for take the time to fix it right, i.e. replace the wood. I'd also add that a plate on the inside of the transom to spread the forces of the bolt is a good installation practice. And make sure to well and goodly goop the bolts with marine bedding compound, don't use silicon sealer it isn't nearly as tough and tenacious as a polyurethane.
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    Quote Originally Posted by Paul H View Post
    And make sure to well and goodly goop the bolts with marine bedding compound, don't use silicon sealer it isn't nearly as tough and tenacious as a polyurethane.
    Sounds like a PERMANENT engine mount job to me.

    We change engines out now and then, so I'll stick with the silicon.

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    It seems like an easy fix, but a couple of words of caution, I would not use Trex, and I would watch sandwiching wood between metals due to accelerated rot issues. I would also consider using treated lumber, but make sure what you use will not cause corrosion issues with the metal it touches.
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    Member Alaskanmutt's Avatar
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    +1 JRogers. Trex has zero structrual properties. Mights as well buy the cheap particle board.
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    Member sayak's Avatar
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    And yet another "replace the wood" vote.
    Slapping on a piece of aluminum would be a bandage.
    Cheap man's approach: see if you can score some green wood (chunk of vert. grain 2x10 or 12 from a contractor, then treat the cut sides with preservative. Routering the edges would be good at this point, and a marine grade paint would make it even better. A plate made of aluminum or stainless makes a good "pad" which can be attatched with stainless pan head screws. Use large, distributive stainless washers and bolts bedded in a marine grade sealant to mount your motor. Crude compared to more sophisticated treatments, but it will last a long time. I've done this on several boats.
    However, on a Monark jon boat, I could not get the rotted wood completely out, so I ended up gouging out what I could, treating the rest of the wood with a liquid restorant, and then filling all the gutted areas with a mixture of MarineTex and fiberlass flakes. It has worked well.

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    Quote Originally Posted by jrogers View Post
    It seems like an easy fix, but a couple of words of caution, I would not use Trex, and I would watch sandwiching wood between metals due to accelerated rot issues. I would also consider using treated lumber, but make sure what you use will not cause corrosion issues with the metal it touches.
    Trex may not be the right stuff, but just a question. Is there any concern about the chemicals used to treat the lumber being corrosive? They are bound to leech out of the wood onto the aluminum. I think I would be concerned about a reaction similar to copper bottom paint on an aluminum boat. Again, I don't know for sure, but the question occured to me, so I thought I'd get it out there for discussion from someone that might know. I like the idea of the longevity of a treated or weatherized product, but maybe the green wood would be better.

    I would give the aluminum sandwich a +1 too, just wondering what the best solution is to go inside the sandwich.
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    Member Alaskanmutt's Avatar
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    Most treated woods are a copper sulfate or arsenate. I do not think there is enough copper in it to react. A good grade marine ply or CDX should work just as well, as they have used it for deck boarding for years

    A friend of mine who builds aluminium boats (for fun) sandwiches oak between aluminium plates
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    Quote Originally Posted by Akfish1 View Post
    Trex may not be the right stuff, but just a question. Is there any concern about the chemicals used to treat the lumber being corrosive? They are bound to leech out of the wood onto the aluminum. I think I would be concerned about a reaction similar to copper bottom paint on an aluminum boat. Again, I don't know for sure, but the question occured to me, so I thought I'd get it out there for discussion from someone that might know. I like the idea of the longevity of a treated or weatherized product, but maybe the green wood would be better.

    I would give the aluminum sandwich a +1 too, just wondering what the best solution is to go inside the sandwich.
    You are right treated wood can be very corrosive. I doubt it would attack the aluminum it comes in contact with, but any hardware needs to be stainless or hot dipped galavnized, electro plated will not do. But motor mount bolts are big and will take a while before they go away.

    If the plywood is still solid (not rotten) and just crushed some I doubt its a issue and would probably fabricate and install a aluminum U shaped sleeve over the top, if you have the room? If the plywood is soft, its rotten and no longer has any structural integrity.
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    Member AKDoug's Avatar
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    Most of the treated lumber available in Alaska uses ACQ as it's preservative. It should NOT be used against aluminum.

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    Did not know that Doug, I hardly ever use treted wood, makes lousy looking cabinets and trim pieces .. lol
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    You have a interesting problem you needs to replace the plywood and we do not know what the condition of the aluminum is behind the plywood? If you use marine plywood it will not deteriorate when wet, it will cause corrosion in aluminum unless protected. You can use normal plywood and put a coating on it to keep it from getting wet. No mater what you use you will still have to keep the water from getting between the plywood and aluminum.

    With all the precaution about protecting the aluminum you would think there would be problem. This may not be true because in Alaska it's cold and if the boat is used in fresh water there may not be any problems in your boat.

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    I dont think marine plywood has the same corrosive properties as AWW. In any case for this boat any good grade of plywood with some good paint would last a long time, 3/4 AC or 3/4 T&G would be fine, If you wanted something thicker you could get a damaged sheet of 1-1/8 T&G from SBS, or even some laminated rim joist material, depending how wide of a piece you need.

    Boise Cascade makes laminated versa rim from 1-1/16" to 1-5/16" in thickness and 9-1/2" to 20" width. These are normally sold in long lenghts but you could probably scrounge a "cut-off" from the lumeryard, cut it, paint it bolt it in and be done.........IMHO

    I would not use any type of OSB or particle board type products......
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    If you are worried about contact between aluminum and wood (particularly all weather wood), put a piece of the self adhesive bituthene or ice dam between them. Personally, I doubt that there is any real problem in the form of a chemical reaction between the copper sulfate in AWW and aluminum. I've used it as floorboards in my skiff for years with no apparent problems. However, I do pre-paint it and attach it to stringer with SS screws.

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