Force on transom from Sterndrive

utskicat

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Dec 15, 2015
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Quick question about force/stress on transom with Merc stern drive. Is there a lot? Since the rear engine mounts are part of the tramsom plate, it seems to me that most of the force from the leg is transferred to the engine and then the front engine mounts that are anchored into hull stringers. Is this logic correct?

My 1975 fibreglass skiboat had all stringers, deck and foam redone about 10 years ago and west epoxy system was used for the reno.

The transom was not replaced at that time due to no signs of rot and complexity of removing fibreglass topcap. There are still no soft spots or signs of rot on the transom, but a 40 yr old transom concerns me. I'm just curious how much stress it carries and if there were unseen rot, what would happen in a worse case scenario?
 
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JASinIL2006

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That's a very interesting question. I always assumed the transom took a fair amount of stress. When just sitting, it supports part of the weight of the engine and most/all of the weight of the outdrive. When the drive is pushing the boat, the transom has to be taking a good bit of the force. Other than connections to the transom, the only point of contact between the engine and the boat are the front mounts. I can't see those two points transferring the bulk of the drives force to the boat. (With an outboard, it's clear the transom is the doing the hard work of transferring force from the drive to the boat.)

Regardless, if the structure of the boat really is in good shape, I don't think the age alone would worry me. Lots of wood-only structure last much longer than 40 years, and fiberglass-encased wood should be much stronger. All that assumes there is not unseen rot, though...
 

Bondo

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Ayuh,.... The transom take All the force, none is transmitted through the drive to the motor,....
 

Scott Danforth

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if you have a phobia on old wood, you must be terrified by old buildings and homes.

seriously though, if there is no rot or moisture, you are good.

as far as loading, prop pushes drive, drive pushes transome plate, transom plate pushes transom/stringers/hull/ and everything in the boat. as bondo said, it takes all the loading
 

utskicat

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The stringers, floor and foam (but not transom) were done a few years ago. I had a small leak I hoped was a rusted y-tube. Turns out the transom was mush. Engine is being pulled and transom will be replaced. Core samples show that aft ends of stringers are dry so rot hasn't transferred to them.
 

airshot

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The transom is sandwiched between to plates that transfer all the power from the outdrive directly to the transom. I would definetly check things out for sure as a transom failure would be catastrophic, literally pushing the back of the boat inside.
 

utskicat

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Dec 15, 2015
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The transom is sandwiched between to plates that transfer all the power from the outdrive directly to the transom. I would definetly check things out for sure as a transom failure would be catastrophic, literally pushing the back of the boat inside.

Ummm...like I said, the transom is right g replaced as we speak.
 

H20Rat

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The transom is sandwiched between to plates that transfer all the power from the outdrive directly to the transom. I would definetly check things out for sure as a transom failure would be catastrophic, literally pushing the back of the boat inside.

I'm thinking an actual failure might cantilever the transom. The thrust angle of the drive is well below the mounts, so the bottom of the transom assembly is trying to push in, but the top is actually trying to get away from the boat as hard as it can! Just depends which area rots out first I guess.

(tried to google image for a failed I/O transom, nadda... You would think there would be some pic of a failure at some point! Lots of pics of rotted transoms after people tore into it)
 
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airshot

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You are probably correct there smokingcrater, however no matter which way that transom goes, I wouldn't want to be in that boat !!! Not only that.....what will the insurance company say about that....negligence on the owner??
 
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