modify transom

cjj216 · 17 May 2011, 13:18

After much consideration regarding whether to mount aux on bracket or direct to transom on my Humber Ocean Pro.
I have decided to mount direct to transom. I have had a stainless "u" shaped transom plate fabricated to slip over top of transom protect the surfaces. Now i need to trim the top off the transom as it`s slightly curved.
I have a long shaft aux, but still i may need to cut up to 50mm to nothing off the transom top to avoid cavitation as the transom is high either side of the main motor.
My question is how to repair the top of the transom before fitting the plate, bearing in mind there may be exposed ply.
I have used fibreglass in the past but many years ago.
Any advice, regarding methods, types of material etc would be appreciated.

Courageous · 18 May 2011, 10:47

I'd be varnishing the exposed edge of the ply with a cuppla coats just to make sure and then fill the U shaped guard with sikaflex. Screw this down and then make sure that sikaflex squeezes out and then screw it in place. Blend the exposed sikaflex away with your finger initially then with a rag soaked in a solvent to tidy the job up. Care with the solvent. Rubber gloves!

That's what I would be doing anyhow. Shame the bracket idea doesn't appeal to you as filling holes after you take one of those off is so much easier!

Pikey Dave · 18 May 2011, 10:58

Quote:

Originally Posted by cjj216

After much consideration regarding whether to mount aux on bracket or direct to transom on my Humber Ocean Pro.
I have decided to mount direct to transom. I have had a stainless "u" shaped transom plate fabricated to slip over top of transom protect the surfaces. Now i need to trim the top off the transom as it`s slightly curved.
I have a long shaft aux, but still i may need to cut up to 50mm to nothing off the transom top to avoid cavitation as the transom is high either side of the main motor.
My question is how to repair the top of the transom before fitting the plate, bearing in mind there may be exposed ply.
I have used fibreglass in the past but many years ago.
Any advice, regarding methods, types of material etc would be appreciated.

I suppose it all depends on how far you want to go & what your skills are upto. As a minimum I'd be sealing the cut edges with West Epoxy. If it was me I'd be grinding back the surrounding GRP & glassing in the cutout & blending it into the original GRP. Gel / flow coat it in / grind & polish. A bit of a faff but long term better for it.

ee lad, tha can't educate pork

9D280 · 18 May 2011, 12:21

The other thing to remember is that as your aux will be shoving you along at displacement speeds, there will always be water filling up the gap behind your hull faster then the aux can push it out the way.

As your hull is what mine evolved into, there should be enough rake on the transom to not bother if the aux prop does not have a totally clear view ahead. Mine's sat on it's outboard pin, and has never had a problem shoving the boat along. Before I got the current one, I had a short shaft 2 that also worked as long as you didn't walk too far forward in waves - it's prop was above the bottom of the hull!

Where I am going here is do you really need to cut the transom at all?

cjj216 · 18 May 2011, 13:50

Pikey Dave - I`ve not used epoxy before. Do you bed fibre matting in to it as with polyester resin or do you simply build up the epoxy in layers without any matting/cloth of any sort?

9D280 - You may have a good point. I have offered the aux up to the transom whilst at sea and the prop is fully submerged but only just.
I suppose that on the move it could possibly be ok. However i need to trim some off the top of transom as it`s curved so the motor won`t sit vertical. In view of your comments i will trim the bare minimum off and see how it sits in the water before digging in to the ply. After all it`s easier to take off than put back on!

Pikey Dave · 18 May 2011, 14:06

Quote:

Originally Posted by cjj216

Pikey Dave - I`ve not used epoxy before. Do you bed fibre matting in to it as with polyester resin or do you simply build up the epoxy in layers without any matting/cloth of any sort?

You can use the epoxy simply as a varnish/sealer to keep out water, or use instead of polyester resin for laying up mat, this makes for a very strong repair. As already mentioned in another thread, epoxy is much more adhesive than polyester & bonds to substrate more easily. Mixed with colloidal silica, epoxy forms a very powerful putty like paste which will just about stick owt to owt. West epoxy is also less sensitive to moisture during curing. All in all it's good stuff

cjj216 · 18 May 2011, 16:25

Thanks Pikey Dave. Will take your advice and use Epoxy and matting/fabric.
I`ll also search ribnet forum for as much info as possible on epoxy.

Pikey Dave · 18 May 2011, 16:33

Quote:

Originally Posted by cjj216

Thanks Pikey Dave. Will take your advice and use Epoxy and matting/fabric.
I`ll also search ribnet forum for as much info as possible on epoxy.

Make sure you follow the instructions on the West packaging, especially the bit about using a wide shallow container for mixing. Get the mix wrong & use a deep container & you can soon have a fire (been there done it

)

hamster · 18 May 2011, 21:39

I'd use epoxy to seal the exposed ply as it's lots more waterproof than varnish or polyester resin and also a lot stronger bond to the wood.
Only problem would be if you tried to use a polyester flow coat to make it look pretty again as it won't stick to the cured epoxy very well. May be better painting it if you epoxy first.

spartacus · 18 May 2011, 22:07

Quote:

Originally Posted by cjj216

Thanks Pikey Dave. Will take your advice and use Epoxy and matting/fabric.
I`ll also search ribnet forum for as much info as possible on epoxy.

Before cutting the transom, are you sure you can't fit a bracket? I don't mean the adjustable brackets from Plastimo - which frankly rattle like a tool box, but something like this for example: outboard bracket | eBay UK

I know it's old, but a new ply section and the cast-aluminium bracket powder-coated white and it would look good and be as solid as a rock.

BogMonster · 19 May 2011, 00:23

If it's a Humber and you need the transom modified, fit a lighting pole and it will modify itself...

What shape is your transom? My Destroyer transom was lower in the middle where the main sat on it, but I fitted a long shaft aux straight on and it worked fine with no modification at all to the fibreglass?

Courageous · 19 May 2011, 01:08

If the prop is barely below the surface in a calm sea you will make no headway at all in a sea. Try it and see if you need to but surely it's obvious no? Varnish is impermeable and when covered from the effects of UV by a sikaflex filled channel it's not going to let water in really is it? Complicate it if you really must but it's not rocket science........

ashbypower · 19 May 2011, 10:49

Quote:

Originally Posted by BogMonster

If it's a Humber and you need the transom modified, fit a lighting pole and it will modify itself...

Or fit twins, ask the commercial guys that had one in Blackpool

Motors still not been found !!!

cjj216 · 23 May 2011, 09:05

Bogmonster - It`s an ocean pro. The transom is curved and high either side of the main motor. It would have needed trimming either way to take my aux.
Thanks for comments everyone. I took a saw to the transom of my newly purchased rib at the weekend

. Following Pikey Daves advice i repaired the exposed ply using west systems epoxy and biaxial mat. Not sure how many layers i should be using but i`m on 3? Shouldn`t matter really as its more to seal the exposed upper edge rather than anything structural.
Will upload step by step pics when i get chance.