Mounting 90HP to transom with old holes - not lining up?

[Franknstein2019]

Hi All
I'm trying to mount my 1998 Mercury 90HP to a fibreglass hull (circa 1985) - its an 18ft planing hull made by Shetland in England.

My Mercury seems to have the standard mounting hole pattern.

I've just checked the holes in the transom, and the two lower ones are good at 9 7/8" apart.
However the top two holes are at 12 3/5" apart instead of the standard 12 7/8" !
That means I cant mount it without filling and re-drilling I assume.

I'm trying to establish why this is. They are the original holes, and theres no evidence of multiple drill holes so I assume these were drilled from new and it had a motor on it for its life before I got it (without motor).

Is there an older style motor with a 12 3/5" mounting hole distance on the top two holes?

I dont know what motor it had on it before, but if its like most of these hulls in the UK, they're generally fitted with 30HP or thereabouts.

So my next question would be how should I go about filling these holes so I can re-drill them to the correct location? I'll probably re-check the locations of all four holes using drafting techniques on the transom.
My concern is that the difference between the old and new holes are close enough that'd I'd be re-drilling through half of the 'old-hole repair'.

:disgust:

 

[Firefly625]

You can over engine a boat, you may just want to check first if it has a hull plate stating maximum engine power. More power you may think more fun.. but the hull may not have been designed to run at 30+ knots which it might do with a 90hp on it. Also the transom may not have been made to hang 150kg off it?

So once you have established that bit, if it were me.. I would fill the old holes with a Glass Fibre filler, buy 1.5mm to 2mm thick stainless steel plate circa 300mm square and cut into shape & attach inside and outside of the transom where the outboard is mounted, drill holes and mount/bolt your engine through SS plate which will act like a huge washer, spreads the load of a heavy engine on a 33yr old transom and covers repaired holes with a smart SS plate.

I'm sure others will have suggestions, but that's what I would do!

 

[Franknstein2019]

You can over engine a boat, you may just want to check first if it has a hull plate stating maximum engine power. More power you may think more fun.. but the hull may not have been designed to run at 30+ knots which it might do with a 90hp on it. Also the transom may not have been made to hang 150kg off it?

So once you have established that bit, if it were me.. I would fill the old holes with a Glass Fibre filler, buy 1.5mm to 2mm thick stainless steel plate circa 300mm square and cut into shape & attach inside and outside of the transom where the outboard is mounted, drill holes and mount/bolt your engine through SS plate which will act like a huge washer, spreads the load of a heavy engine on a 33yr old transom and covers repaired holes with a smart SS plate.

I'm sure others will have suggestions, but that's what I would do!

Thanks for your thoughts.
Yes the maximum rated motor for my boat is 90HP, so I'm within the specification. Seen this motor on many others too.
I've already made some stainless load evening plates for the inside of the transom, not done the outside but just might do.
Just curious as to how the holes are just 7mm too close together!

 

[Franknstein2019]

You can over engine a boat, you may just want to check first if it has a hull plate stating maximum engine power. More power you may think more fun.. but the hull may not have been designed to run at 30+ knots which it might do with a 90hp on it. Also the transom may not have been made to hang 150kg off it?

So once you have established that bit, if it were me.. I would fill the old holes with a Glass Fibre filler, buy 1.5mm to 2mm thick stainless steel plate circa 300mm square and cut into shape & attach inside and outside of the transom where the outboard is mounted, drill holes and mount/bolt your engine through SS plate which will act like a huge washer, spreads the load of a heavy engine on a 33yr old transom and covers repaired holes with a smart SS plate.

I'm sure others will have suggestions, but that's what I would do!

Thanks Firefly625.
Its a Shetland Family Four, and literature I've found all suggests 90HP as maximum, so I'm ok there. I've seen a few others with similar size, and a few with 115HP even.

I was curious more than anything as to why the holes would have been just 7mm closer together than standard, and the other holes all within standard spec. Doesn't look like they've been drilled skew either as centre to centre is identical inside and out of transom.

I've already made two stainless backing plates in 2.5mm, for the inside of the transom as large load bearing washers, they're separate strips as the splashwell separates the mountings so cant use a single sheet inside. I could do one on the outside, and might do, but in a way I cant really see the point of a stainless plate to the outside of the transom as the motor bracket is a rather large surface area itself.

Yes I think filling the holes with fibreglass bog will be my only solution. Was going to use polyester resin, chopped strands and cabosil - ream the holes our slightly larger to clean the wood out a bit, shove the bog in there and tape the inside and outside while it dries. Then a couple of small bits of biaxial on front and rear of hole. Someone chime in if this won't work. I'm guessing I'll get some suggestions of epoxy resin being better here...

 

[Seastoke]

You can over engine a boat, you may just want to check first if it has a hull plate stating maximum engine power. More power you may think more fun.. but the hull may not have been designed to run at 30+ knots which it might do with a 90hp on it. Also the transom may not have been made to hang 150kg off it?

So once you have established that bit, if it were me.. I would fill the old holes with a Glass Fibre filler, buy 1.5mm to 2mm thick stainless steel plate circa 300mm square and cut into shape & attach inside and outside of the transom where the outboard is mounted, drill holes and mount/bolt your engine through SS plate which will act like a huge washer, spreads the load of a heavy engine on a 33yr old transom and covers repaired holes with a smart SS plate.

I'm sure others will have suggestions, but that's what I would do!

I would also put tubes nbetween the transom from plate to plate a tad short of the width so when you tighten the bolts they close on the tubes ,but maybe it should have a 60 on it,

 

[Firefly625]

Thanks Firefly625.
Its a Shetland Family Four, and literature I've found all suggests 90HP as maximum, so I'm ok there. I've seen a few others with similar size, and a few with 115HP even.

I was curious more than anything as to why the holes would have been just 7mm closer together than standard, and the other holes all within standard spec. Doesn't look like they've been drilled skew either as centre to centre is identical inside and out of transom.

I've already made two stainless backing plates in 2.5mm, for the inside of the transom as large load bearing washers, they're separate strips as the splashwell separates the mountings so cant use a single sheet inside. I could do one on the outside, and might do, but in a way I cant really see the point of a stainless plate to the outside of the transom as the motor bracket is a rather large surface area itself.

Yes I think filling the holes with fibreglass bog will be my only solution. Was going to use polyester resin, chopped strands and cabosil - ream the holes our slightly larger to clean the wood out a bit, shove the bog in there and tape the inside and outside while it dries. Then a couple of small bits of biaxial on front and rear of hole. Someone chime in if this won't work. I'm guessing I'll get some suggestions of epoxy resin being better here...

if you do all that it will be fine.

got no idea why holes different, but matters little really, they are so irritatingly have to just deal with it.

 

[Franknstein2019]

Thanks for all the replies.
I've figured it out!
As I was placing my stainless backing plates over the holes, I was checking on the INSIDE of the transom. I have measured the holes yesterday evening on the outside of the transom and they're drilled at 326mm apart, which is about right. So it seems to me that whoever drilled the holes in the transom was inept at the task and drilled them skew as the inside of the transom measured 320mm centre to centre and outside was 326mm centre to centre!
Anyways, I'm just going to ream the holes out from 1/2" to 13mm to give a little more slack in them and run with it.
Good idea on the spacing sleeves inside the transom, I'll see what I can find - I guess some stainless steel round tube cut to length would do. Found some on eBay, 16mm Outside diameter and 13mm inside diameter (1.5mm thick). I'd need to drill the holes to 16mm then?

 

[Bigplumbs]

Sounds a good plan...……….. There is a product called CT1 that is an amazing mastic. You might like to use this as a seal around the holes and also between the tube and the hole in the transom