Putting a long shaft engine on a short shaft transom

[Pablo]

Hi,
i've just bought a Humber Assault (advertised as an Attaque!) and it turns out that the transom has been cut for a short shaft engine. The boat currently has a blown up "standard" shaft engine, which I am told is 18", on.

The seller tells me that Humber explained that the transom could easily be bulit higher by inserting a section of the same thickness of marine plywood and screwing it diagonally into the existing transom ? What do you think ? Would this be enough to support a 75hp 2-stroke Yamaha or 90hp Mercury ?

[Into The Blue]

You might want to PM Nasher - I seem to remember that he had a similar experience
Or maybe wait a bit and he'll be along later

[Hightower]

Depends on whether the the Engine mounting holes go though the new (extended) piece of transom or not, if they do then suitable reinforcement is required. This I would guess could be in the form of a stainless plate mounted on the outside of the transom and bridging original and new parts. Perhaps one on the inside too, this would provide a sandwich effect and would be a much stonger soloution.

There are other options, but none as simple and as strong as this one and certainly not easy for DIY.

[Nasher]

Pablo

Hi, as ITB says I had a similar issue when I brought a cheap XL shaft Suzi to put on my Ocean that was cut for a Long shaft. I ended up doing the following.

I cut a large Alloy plate 15mm thick that bolts to the transom with 8 M12 bolts, and spreads the load out way past the original cut out. The engine then bolts through this into the original transom, and only the top engine mounting bolts are above the original transom height.

The actual transom is built up to the same height as the 15mm plate with a section of 2' x 4' Alloy box. This has tube inserts for the top two engine bolts to pass through. These stop the box section crushing.
Inside the whole lot is another Alloy plate 6mm thick.
Everything was finished off with white sealant.

I was told by a reputable boat builder that the 2' x 4' box etc was not needed as the 15mm plate would take all the engine could throw at it, but went with it for a neater finish.

The two images below show the inside and outside before the transom was painted and proper engine bolts used etc, so excuse the state it looks.

Nasher

[landlord]

Nice job Nasher - looks very good

[Pablo]

Looks very good indeed !

Thanks everybody for your help. I think Nasher has given me THE solution. I should be able to get some stainless steel to make the sandwich.

[9D280]

Pablo,

Would you be easier (& quicker?) to find a running version (any shaft length) of the one on there now, and swap the middle legs over? You'd be surprised how long some of these bits were carried across to "new" models - My 1993 Suz shares parts with the current DF30! You'd need a microfiche to suss out if it would fit or not.

This would then solve a lot of mucking around & adding weight to the hull.

Even easier - find a short shaft engine? (But I guess you have one lined up already - in which case could you find a siezed short shaft version & swap the bits?

[Pablo]

As the boat is an Assault rather than the Attaque I was hoping for I think I want to set it up to use it one season and then sell it. The seller made a genuine mistake, he advertised it as an Attaque which I automatically / hopefully transfered into an Ocean Pro "Attaque Classic" 2m beam. As I got it at a bargain price and an Assault is still alot better than what I use at the moment then i'm at least going to give it a try.

It will sell better as a long shaft transom as most people here are fans of four stroke. Its almost impossible to find a short shaft 4 stroke with more than 50hp. I've got two engines lined up, a 90hp Mariner or a 75hp Yamaha, both old 2 strokes and dirt cheap. When I sell the boat I may have to sell it without the engine and so i'll put it on the Ocean Pro "Attaque Classic" or 5.4m SeaRider i'll get next year.

I'm only going on what i've read on this forum, I may even find the Assault will do me.

[Pablo]

I ended up doing it slightly differently. I've built up the transom with a piece of oak (at least 60 years old). It's going to be sikaflexed on and then a sheet of s/steel is going to be folded round.

[Pete7]

Pablo, surely it would be stronger if it was held in with GRP or epoxy and then glassed over. Andy Hightower had this done on his South Coast Rib andGary did a really good job of it.

Pete

[Dirk Diggler]

Originally Posted by Pablo

I ended up doing it slightly differently. I've built up the transom with a piece of oak (at least 60 years old). It's going to be sikaflexed on and then a sheet of s/steel is going to be folded round.

That really hasn't got a hope in hell of working!

[martini]

IMHO that piece of oak should have been an inch or so bigger and machined along with the transom to form a lap joint. Then you could epoxy the joint, glass over the whole thing then wrap it in stainless.

I may have the exact same problem on my next project only it will involve twin 250hp O/B's. Frankly I'm sh***ing myself. I've already lost one 150hp O/B off the back at WOT, a somewhat unpleasant experience I hope not to repeat!

[the bandit]

Had it done on a Revenger San marino with twin 225. Launched on the thames made it to France then in a force 5 / 6 it started to break.

So yes your right to be sh***ing yourself

[The Garfish]

Originally Posted by Pablo

a piece of oak (at least 60 years old).

duzz yorr muvver noe yoov nikt de dore ofn hir anteek looey de forteenth ritin tabul

[Nick Hearne]

If that bit of Oak wants to move (and it probably will!!) it is going to & you can not stop it! Marine ply is the way to go!

[Dirk Diggler]

Originally Posted by Nick Hearne

Marine ply is the way to go!

Buying the correct length motor is "the way to go". Failing that, a proper transom mod, costing a couple of days labour is the alternative!

[Pablo]

You've had me panicing all weekend and I haven't been able to do anything about it.

It's a pro who's doing it. I phoned him this morning to explain what I understood he was going to do and he laughed and told that it would not stand a chance in hell of working.

What he's going to do is glue the wood onto the transom with some special resin glue, he's then going to screw the wood into the transom with some big stainless screws. The 6mm sheet of stainless steel is going to be folded around the transom and then bolted on. The bottom bolts of the engine are going to be bolted to the original part of the transom and the top ones through the wood bit.

All this for a 60hp Yamaha. I have confidence in this guy (I have to as I know stuff all). He prides his work. Please tell me that that's enough.

[Nasher]

Pablo

With the 6mm SS either side it should be enough for thye 60HP you have.

thats just my opinion of course.

Nasher.

[Pablo]

Thanks, i'm relieved. I'll be happy when it's in the water and i'm testing.

[9D280]

I do have to ask here - add one lump of oak to a couple of square metres of 6mm stainless - that's a LOT of extra weight to add. Surely a siezed short shaft engine for a leg swap would provide you with a far cheaper and lighter alternative?