Making fiberglass shims for outboard bracket

[brownoarsman]

It's meager. The transom thickness is between 1/4 of an inch and 1/3 of an inch, and appears composed of woven glass rather than chopped mat. The two backing plates are 1/8" 304 stainless plate, cut to extend an inch beyond the bracket base on all sides.


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[Scout 30]

Flexing is not good. There's a lot of stress here & it's important that it be very strong. I'd attach 3/4" fir exterior grade or marine grade plywood the full width & height of the transom with thickened epoxy. make a template out of cardboard & cut some 2x4 braces to hold it in place while it cures. I like things strong so I'd epoxy filet & glass tape the edges to the sides of the boat. Then paint & backing plate. If 1 piece is too big to get in there I'd piece it together with 2 layers of 3/8" plywood with the edges offset. If you have to piece it I'd epoxy/glass tape the seams also.

[Pete7]

Agree with Scout, that transom wasn't built to take a outboard. However, 3/4" ply is going to be heavy and you already have the weight of the outboard on the transom. As its a convexed surface it will be tricky too. Instead laminate with epoxy a couple of layer of glass cloth, then a layer of that foam board stuff the name of which I have forgotten, then another couple of layers of cloth. The result will be light and strong.

Pete

[Scout 30]

You have to be careful using foam boards for this application. You need compression strength which means density. Typically composite boards this dense are not lighter than plywood. They are also much more expensive than plywood. If you do go that route you would not need to add glass to the transom. You're making a sandwich & you already have the outer layer so just glue whatever board you use & then glass the inside. It's true that if there is significant camber to the transom you will have a difficult time bending 3/4" plywood. In that case 2 layers of 3/8" would work. I like plywood because it's very strong, easily obtainable & cheap. Baltek Airex PXc Boards

[chris95040]

Yeah, I agree that it seems like it needs beefing up.

0.25" fiberglass is certainly pretty tough, I think fiberglass tensile strength is like 20,000PSI even for just-ok layups, but I don't really know how to interpret that, maybe someone who does can chime in.

It seems like a lot of strength, for example if your backing plate is 6"x6", thats 6" x 0.25" x 4 sides x 20ksi = 120,000 pounds to rip that plate through along it's border.

But that's not how it works, the load isn't perfectly distributed along that border at all times, and once stuff is flexing and moving, tiny problems start, it can move more, it breaks more.... I end up going with my gut on this stuff, and it seems like this "quick workaround" for your busted inboard is turning into a structural engineering project.

It seems like something spanning the length of the transom is a good idea. But doing this inside is going to suck, in that cramped space, dealing with making a fiberglass layup lying on your back looking up at the inside of the transom.

Maybe a couple 1x2s, stacked one on top of the other, and glued in place - they'll individually be easy enough to flex to fit the shape, but when they are glued together they'll be nice and stiff. Put two horizontal sets of them, one where your top engine mount bolts go, one where your bottom engine mount bolts go. That way you are at least done in a day.

Me, I'd just focus on the inboard...

[Scout 30]

If the fiberglass is flexing it will crack. After it cracks you'll be re-enforcing the inside plus repairing the outside.

[brownoarsman]

Boats, aren't they fun? Even getting the backing plate in was tough work; it's cramped down there! I won't be able to fit one piece of ply, as the rudder tube is in the way, so will look into beefing up the transom. It seemed that hauling the engine out down here would have been the bigger project, but now I see the error of my ways. Thanks for the advice, much appreciated.
Dan


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[Tashtego]

As a single handed boat monkey I often have to tighten through bolts without a helper. I use locking pliers (Vise Grips). Insert the bolt on the inside of the boat. Clamp the locking pliers on the head of the bolt. Go outside and put the nuts on. You may need something sticking out on the inside to prevent the pliers from turning. I usually put one of the bolts in from the outside so that it sticks out on the inside and will block the pliers. A philips screwdriver of the right diameter will also do the job. A little blue painters tape will hold the pliers against the stop if necessary. It works the other way too - pliers on the nut and turn the bolt but if the pliers are gripping really hard it may make it a little harder to turn the bolt. Locking pliers are my third hand and longest arm.

[brownoarsman]

A much better idea than mucking up the threads with channel locks!


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[Scout 30]

The good news is it doesn't have to be pretty. Use slow curing epoxy & thicken it a little to fill gaps. I'd coat the transom & plywood. Put the biggest piece you can get in there in the middle. Offset the seams on your second layer. Don't worry about glassing the plywood except around the edges & seams on the second layer. Don't mix up to big a batch at one time. It can get too hot & kick too quick. Once you spread it out it doesn't get hot. Dry fit your pieces including your braces before you glue.

[brownoarsman]

Time to get cozy with some new west system videos.
Scout-you suggested curving the plywood to the transom. Would it also be okay to just use more thickened epoxy to bed the plywood against the curve?


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[Scout 30]

If this is just too much I would at least epoxy in the biggest piece of plywood you can get in there in the middle. If there is a camber use 1/2" plywood. Not the best way to do it but it will help distribute the load.

[Scout 30]

Originally Posted by brownoarsman

Time to get cozy with some new west system videos.
Scout-you suggested curving the plywood to the transom. Would it also be okay to just use more thickened epoxy to bed the plywood against the curve?


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You need to get the plywood to match the curve of the transom. If you have a 1" gap between the plywood & the transom it would take a massive amount of epoxy to fill the space. 3/8" plywood will bend pretty easily so you should be able to make that work. I'd brace against the rudder post. Just don't damage it.

[DeepFrz]

IMO you don't need 3/4 in. plywood. 1/4 in. would do you. Bed it in polysulfide like 4200 and bolt the thing together. Leave it for a day and you should be good to go. Make the plywood full height if you can and just wider than the stainless by as much as you can. This is only temporary after all. I wouldn't bother epoxying. Coat the holes and bolts with 4200 and get someone to hold the bolts or tighten the nuts, whichever way you insert them. Just my opinion.

Actually if it were me I would just head out, keep and eye on things and take it as easy as you can. Again, just my opinion.

[jaybird1111]

Just go to Mcmaster.com (McMaster-Carr) and order yourself a sheet of structural fiberglass (look up 'fiberglass sheet' in their search engine)

we call it 'strong back' down here but i don't think that's anything other than a local term