Mercruiser Sterndrive Transom Stress Crack

[Iroc-Sea]

Hey all,

Have had my ‘83 Cobia Sportster for 13 years now. It has always been dry docked aside from the past 3 years, it’s been wet docked from May-October in Lake Hopatcong NJ.

Overall, it’s in great shape. Always maintained, engine is strong, in process of re-upholstering the interior. I noticed when I pulled it this season, a few hairline cracks on the transom. I’ve attached a few pictures. They are hard to see and are very thin but I zoomed in to make them more visible.

I’m wondering if this is due to a rotten transom or from running a ground on some rocks this summer (rolled end-blades on prop). Given the age of the boat I expect the transom to be in less than perfect shape but I wanted to access whether it is safe/will give me a few more seasons. Here’s what I’ve done so far to verify:

- Core drilled from inside hull in two locations (high up on transom and down in bulge). The higher location was lighter in color. Down in the bilge, the shavings look dark drown. They were both dry (no water or anything). I attached pictures of both holes and one with shavings from both.

- Tapped around with hammer. From what I can hear, down low and around outdrive it sounds firm. It is more ‘thud’ sounding up top but I’m not sure if I can really tell.

- With drive in trailer position, I stood on and shook outdrive, putting my weight on it. Boat was bouncing on trailer- 0 flexing or movement in transom.

- Bought a cheap moisture meter on Amazon, it indicated 60-80% moisture all over the transom. This freaked me out so I tried two other boats of much newer vintage. Those both were around the same or worse (they were wrapped though so not sure if they had water in bilge?).

I know cracks are not good, and those moisture readings make me uncomfortable (though I don’t trust the meter). But if I were to actually have a significant transom issue, wouldn’t it feel less solid / flex?

Also, does that chip in my drain hole seem concerning or do you think the plug would still seal?

Sorry for all the questions just looking for some guidance. Thanks!

 

[alldodge]

I think its mainly old bones, but does appear your getting water in and its just starting. Looks like its from the drain hole. There should be a brass drain tube. The link is the type not necessarily the correct length. It requires a special tool to install.

That said before its installed it would need to be dried out first and with some 3M 5200

https://www.amazon.com/SEACHOICE-19...ords=transom+drain+tube&qid=1674046807&sr=8-5

 

[Iroc-Sea]

I think its mainly old bones, but does appear your getting water in and its just starting. Looks like its from the drain hole. There should be a brass drain tube. The link is the type not necessarily the correct length. It requires a special tool to install.

That said before its installed it would need to be dried out first and with some 3M 5200

https://www.amazon.com/SEACHOICE-19...ords=transom+drain+tube&qid=1674046807&sr=8-5

Thanks so much for the detailed info. Will definitely do some more research and install the brass fitting. Makes me curious why my boat never had one?

Would you repair/fill the cracks? I was thinking of bottom painting with a barrier coating (and maybe vc-17m for a smooth finish) since it will be in the water again for 5 months a year.

 

[alldodge]

I would fix the drain and probably leave the rest alone, unless your ready to get into the lower portion of the transom. If the rot can be stopped I think it will last several more years. The issue is I'm not sure antifreeze can be injected far enough to get it all

Agree barrier and bottom painted will fix the issue coming in thru the hull. Fix the cracks that will be under water to help even more

 

[JASinIL2006]

Thanks so much for the detailed info. Will definitely do some more research and install the brass fitting. Makes me curious why my boat never had one?

Would you repair/fill the cracks? I was thinking of bottom painting with a barrier coating (and maybe vc-17m for a smooth finish) since it will be in the water again for 5 months a year.

Mine had nothing like that, either. I think it's because manufacturers look for any possible place to cut costs.