transom integrity

[bobsbikini]

How or what is the best way to check the integrity of the transom on my current project. It's a 1971 Silverline 15'. I Pulled out the rotten stringers and noticed the transom was just as wet. The stringers where not of a quality timber but from what I can see of the exposed transom it looks like plywood. What would be a good alternative that won't rot, thats not too expensive.

[TollyWally]

The plywood isn't really the problem. It's the penetrations that are poorly done that let in the water. Sadly, poorly done penetrations are the standard.

Odds are you don't really have to check the integrity of the transom, you already have.

[bobsbikini]

So letting it dry out would be a upgrade from OEM. Wasn't impressed with any of its structural design. Have several ideas for improvement upon assembly. How does the hp rating equate from. Displacement?

[Hunter25]

I've had my transom replaced, drying it out will not help if it's rotten which is sound like it is. Replace the transom core, use good epoxy techniques. In 1971 the hull shell will provide the majority of the hull strength. It is thick and heavy with a few stringers. The lumber used and plywood was not sealed well and it rotted. If you seal the wood with epoxy it will not rot.

[bobsbikini]

Seems how I've gone this far, it doesn't look like it would be that hard to repair what I have. It doesn't show any stress cracks in the gel coat so most of what has rotted is from sitting. There is plenty of good glass to work with so I must press onward. Thanks

[alan white]

You'll find threads about transom core replacement using the search feature.
Plywood is still a viable way to go, but as said epoxy should be used this time around---- to seal the plywood including edges, drain holes, bolt holes, etc..
Do it right and it will last a long time, longer than the factory job.

[GG]

A good substitute for wood that wont ever rot is a core material called Coosa .

[marshmat]

bobsbikini,

Sounds like you've got a full-on transom replacement on your hands along with the stringer work.... nothing too complicated, but it will be a tedious and messy job. There's plenty of old advice on here about this exact repair (use the forum search, or use google with site:boatdesign.net after your query).

An outboard boat's power rating is based on a number of factors- hull volume, motor well type, transom beam, tiller vs. remote steering, etc. that are combined, according to formulae set out by the appropriate regulatory agency, to produce the power and cargo/passenger capacity ratings. It's assumed that the structure will be designed accordingly. (In reality, on most small boats, the load from the engine bouncing around while the boat's on the trailer is usually more of a problem than the operating loads.) As a rough approximation (based on Canadian standard TP1332 sec. 4.2.5b, US standards give similar results):

Displacement at static float plane = displacement (in kilograms) at which the boat is just barely floating, ie. total hull volume below static float plane (not including motor well)

Gross load (kilograms) = One fifth of (displacement at static float plane, minus dry weight of boat), minus weight of outboard engine and its accessories

N = Gross load, multiplied by transom width (metres), divided by 1.382

For a V-bottom hull with remote steering, 4.75 to 6 m long, transom beam more than 1.22 m, with N > 600, max HP = (18 + 0.071*N ) / 0.745

For a V-bottom hull with remote steering, transom beam N between 250 to 600, max HP = (11 + 0.056*N) / 0.745

For anything else, max HP = (0.04*N) / 0.745

If your boat doesn't have an original capacity plate, you really ought to check the calculation and/or submit the appropriate government documentation, based on the standards applicable in your state- if you overpower a boat and someone gets hurt, you'll be fighting off lawyers for years. But it sounds like you've got a ways to go before that point.....

[gonzo]

You got a transom that lasted forty years with indifferent sealing and probably a lot of holes added through the years. That is, in my opinion, a good design. Make it the same and seal every hole well.