Mounting Volvo transom shield on wavy 1/4” transom question

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Mounting Volvo transom shield on wavy 1/4” transom question


Post by ElizaIsland87 »

The transom has a bit of a wave and am afraid hat it’s going to have a slow leak once the Volvo DP-c transom shield is mounted. What are my options besides caulking the crap out of it? Some sort of backing plate on the inside of transom?

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Re: Mounting Volvo transom shield on wavy 1/4” transom question


Post by Chaps »

Yup, have a thick flange made for the perimeter of the hole, weld in place while clamping the wavy transom plate to it.
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Re: Mounting Volvo transom shield on wavy 1/4” transom question


Post by ElizaIsland87 »

🤯 damn was hoping for an easier method. Tried to find a piece of 3/4” plate all day today.

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Re: Mounting Volvo transom shield on wavy 1/4” transom question


Post by kmorin »

Eli', Chaps, readers of the AAB.Forum,

the 'best' transom panel for an I/O is not a single plate of aluminum which can be, depending wholly on the design of reinforcement, subject to weld distortions and 'waves'.

In 1978 I was faced with my first I/O stern and after looking at two other 30'er's with I/O installs, I concluded that it was worth the effort to make a dedicated I/O stern plate so the make up and install would be sound.

I used the transom shield/mount/bezel template from the manufacturer to draw the outline of the two flat plates I used (and have since then when faced with I/O installs) to make a pair of 1/4 plates 3/4" wider on all sides than the transom bezel pattern. [ I think one of the BMW I/O's I did in the late 80's required a bit more boundary?? but don't recall exactly.]

After making the two cuts on each plate, inside profile and outside offset as 3/4" wider than the pattern, I drilled all the holes to the install pattern. Then I over drilled all these holes to the next pipe size OD- the holes were nominally 1/2" and I drilled them to the OD of 1/2" pipe.

Then I fitted pipes to each hole and keyhole welded the ends (TIG) to the holes, and lines the outside of the 2" thick plate with flat bars, bent to the curves and butted at the angle points. It did take some time to bend the 1/4" or 3/16" flat bars to the inside and outside profiles of the hollow box. The outside profile or shape could be left more faceted but the inside needed to all the shaft and sometimes the stub of the engine gear housing to fit inside. So the biggest time contribution to these boxes was shaping the bars that became the 1-1/2" deep "edges" so they conformed to the two cutouts- inside and outside of the transom mount pattern.

All welds were done with TIG, with entire hollow panel strapped to a thick steel bench and welded one side to match the other to reduce distortion. This resulted in a hollow (air tested) structure that held the transom bezel on the outside of the hull, and the 'ring' or washer bezel on the inside of the hull.

By tipping this 2" thick assembly to the factory recommended angle, and butting the transom materials to the outline along the 1/4" plate lining the two faces (fore and aft) of the hollow assembly; I always bolted to a dead flat box, where both faces were parallel, all bolts cold be bedded in sealant and the clamping forces would be carried on 1-1/2" long 1/2" pipe stubs welded between the two 1/4" plates.

Works like a charm, never has a leak or problem; but it does take a bit of pre-planning that maybe passed from the OP's circumstance once the transom plate is warped?

In our area (South Central AK, Kenai River mouth on Cook Inlet's East side) there was one builder of welded boats who used a single plate to mount I/O's. Over a period of time from about 1977 to 1988 I replaced about half dozen transom mounts on that builder's boat due to the newer bolt patterns when the boats were repowered. I offered my hollow box transom method of replacement as the design of these remodels. As near as I know they all worked satisfactorily? Volvo has big cross section O-Ring seated into the outer transom housing (during that period) and this would aid the seal of the cast aluminum transom housing to the hull's transom plate.

just a reference for those who may be reading; to add to the discussion- I don't think you need a single 3/4" plate to mount an I/O on welded boats? However, if you're already passed the point of cutting the transom for a mount, and are now just trying to flatten it ?? maybe just adding some thick plate is the simplest cure for the weld contractions in the mount area?

Kevin Morin
Kenai, AK

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