Transom mod

Hello all.

Thought I'd post this here as I've not had much of a reaction from my thread in the Powerboats section, and I'm sure some of you must have an opinion, but maybe didn't see the thread.

When I built my boat up from a neglected hull a few years ago I found a really good deal on an XL shaft Suzuki DT200EFI. The transom had already been cut down for a long shaft which put the engine very low in the water, so I looked at ways of raising the transom to suit the XL engine.

Rather than rebuild the transom in wood and fibreglass I added a 15mm thick Aluminium plate bolted to the transom and filled the original cutout in the transom with a piece of 4 x 2 Ali box section, mainly to fill the gap visually.

The two images below show the drawing I made beforehand and what it ended up like.

The spacers pass through a hole slightly larger than their diameter in the outer side of the box section, and finish flush with the outside edge. So the box is clamped in position by the inner edge of the spacer but is purely a gap filler to make the transom look the same thickness all the way up.

I'm going to whip the engine off for some tidying up in a few weeks, and for various reasons I am considering putting it back together without the 4 x 2 box section, so the top engine bolts will just be through the 15mm Ali plate.

Am I making a mistake? I don't think it will have any effect on the strength of the installation but will simplify it.

Am I right in thinking the 15mm Ali plate is strong enough on it's own to handle a 200kg 200HP engine?

I'd be interested to hear what any of you who are far more experienced than me think.

Thanks

Nasher.

 

I really couldn't understand how tight the fit is with the rectangular tube and the transom space. You need to use compression tubes where the engine mounting bolts pass through. Do the bolts pass through your spacers?

If you have the ability to do so, I'd notch the bottom of the rectangular section and weld in two sections (1.25 x 2.5 x 1/4wall???) to work as compression tubes. A pair of motor bolts run through each. I'd fit the face flush to the 6mm plate and have the boss aft to land on the 15mm plate. It is easier to tweak that way. Bolt the 6mm plate to the transverse tube with 4 3/4 long 3/16D bolts near each end. The washers on the motor mounting bolts need to be heavy duty and spread the load out to the compression tubes, or a thicker plate needs to run across the bolt pattern and extend 1/4 beyond the compression tubes.

No picture of inside? Bolts sucking in? I'd have used a heavier plate inside.

Leave as much of the tubing as you can at the top. You want a strong flange at the top.

 

Philsweet hi, and thanks for replying.

Sorry, I didn't describe it very well.
Yes the motor mounting bolts currently pass through the spacers, otherwise the mounting bolts would crush the rectangular tube.
They are as you say - compression tubes.

The tube, spacers etc are however just there to make it look like the transom is the same thickness all the way up.

The point of the thread, and what I'm asking about, is doing away with the rectangular tube and compression tubes all together, and just bolting the motor to the 15mm plate.

I'm convinced the 15mm plate is strong enough to deal with a 200kg 200HP motor on its own without the small additional help it gets from the 6mm plate on the inside via the spacers.

What do you think?

Nasher.