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Old 17 May 2011, 08:18   #1
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modify transom

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.
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Old 18 May 2011, 05:47   #2
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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!
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Old 18 May 2011, 05:58   #3
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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
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Old 18 May 2011, 07:21   #4
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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?
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Old 18 May 2011, 08:50   #5
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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!
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Old 18 May 2011, 09:06   #6
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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
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Old 18 May 2011, 11:25   #7
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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.
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Old 18 May 2011, 11:33   #8
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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)
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Old 18 May 2011, 16:39   #9
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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.
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Old 18 May 2011, 17:07   #10
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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.
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