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1999 Beneteau 50
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Discussion Starter · #1 · (Edited)
Should I be concerned about this? It only happens on the port side, bottom mount, which is the one that bears most of the load. I think there are two things causing this. First, the transom seems thinner on the port side, perhaps 3/8 inch. Second, the inside backing is uneven so there is no metal backing plate to transfer the load - just washers. And even the washers don't lay flat against the backing. I tried shimming a backing plate made out of cutting board material but when I twist the davit side to side, it still seems to flex.

I know that fiberglass can be flexible and yet strong, but I worry about what repeated back and forth motion might do to the transom.

I'm not sure what more can be done to correct this, short of building up the surface with many layers of fiberglass.
 

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Not to worry. It will probably take several seasons of flexing before the transom fractures completely. Me, I'd consider building up a goodly sized patch of glass mat /epoxy .Remove bolts, Grind area clean Jam some TP in holes to prevent epoxy from filling them (easier redrilling from outside, longer bolts probably) Backing plate good. could be glassed on and drilled thru at same time. Maybe have to remove weight on davit to prevent building a depression in the transom
 

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First, the scary thought. I hope the penetrations for the davits didn't leak into the core and the flexing is due to delamination. Might be worth pulling off and checking. Hope not!!

If it simply isn't backed well enough, I'd get some G10 fiberglass board and make a thickened epoxy (peanut butter thick) and adhere it behind the attachments. Make it as thick and wide as you require. Drill through and remount. (Bed those penetrations well, even consider potting the holes, if it's cored).
 

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Personaly i would be worried. It isnt so much that flexes as much as its going to cyclic. I would definetly be looking at reinforcing and spreading out the stress but installing large backng kates.
 

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Discussion Starter · #6 ·
It's an 8' Nelson Sailing Dinghy with no motor. I think it weighs about 80 lbs. The boat is 20 yeas old and the PO had a Zodiac w/ 9.9 Yamaha. I actually think the core is solid glass.
 

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It's an 8' Nelson Sailing Dinghy with no motor. I think it weighs about 80 lbs. The boat is 20 yeas old and the PO had a Zodiac w/ 9.9 Yamaha. I actually think the core is solid glass.
If an 80lb dinghy is flexing the back of a 50ft Beneteau, something is wrong. A pic of the davits might help. Is there any bracing to the pushpit?
 

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...the inside backing is uneven so there is no metal backing plate to transfer the load - just washers. And even the washers don't lay flat against the backing. I tried shimming a backing plate made out of cutting board material but when I twist the davit side to side, it still seems to flex...
That does not sound like adequate backing at all. I'm with @Capt Len and @Minnewaska. Build that area up with a few layers of glass mat to reinforce and spread the load as far as possible, then add a backing plate. You could epoxy a G10 plate fairly easily, or you could use an aluminum plate with a good adhesive like 5200. The plate adds some strength and ensures that the washers are cleanly seated on a flat surface.
 

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If an 80lb dinghy is flexing the back of a 50ft Beneteau, something is wrong. A pic of the davits might help. Is there any bracing to the pushpit?
Its much more than 80 pounds if its a dink and donk. 80 pounds each

I had a bit of arch wobble.
SS welder wanted $3,000 to fix!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Solved by a retired engineer who dabbles in welding for fun. 2 little struts in the right places and problem solved $200


Mark
 
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Discussion Starter · #10 ·
If an 80lb dinghy is flexing the back of a 50ft Beneteau, something is wrong. A pic of the davits might help. Is there any bracing to the pushpit?
I have yet to even mount the dinghy since we back into our slip. When I pull the arm side to side, the transom flexes behind the bottom mount - not a lot - perhaps a few mm. So I'm thinking the weight of the dinghy supported by a rocking shifting boat would put more stress on it. We're going cruising in a week so I guess I will just secure the dinghy to the foredeck or tow it around.

139576
 

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I assume the vertical poles are the davits. The model isn't familiar to me. They look secure at the gunnel and the pushpit. If they are still straining the transom attach point, you're going to need to reinforce it, from behind. I don't think you need to glass and use G10. G10 is fiberglass board, just epoxy it down. It's ridiculously strong, even hard to cut. You can wear out a brand new jig saw blade. BTDT.
 
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