bonding epoxy to stainless steel

I would think that SS is much the same as aluminium to bond to. So yes no problem provided the surface is really clean and rough. Clean with acetone ensuring that each succesive wipe is with clean cloth and acetone. Aircraft Ali wing skins are often bond repaired with epoxy in a substrate.
Drilling holes in the SS where the fillet will cover will help a lot. What you could do is get kevlar strands to go from one side of ss through the hole and back through another hole facing both forward and aft. Like a lassoo through 2 holes with say 50mm of tag each side. It might be an idea if you do this to chamfer the holes edges so they can't cut the kevlar. If you can't get kevlar then glass strands will help. Use carbon fibre strands in the fillet for stiff strength.
Drilling SS is no problem witha bench drill press where a lot of pessure can be rought onto a slow turning bit. Make sure you get positive swathes of ss cut out. A larger drill bit is easy to sharpen provided you don't blunt it too much and carefully look at the cuttting angle to reproduce it. good luck olewill
 
Ian_Edwards said:
The application is to bond in a 10mm SS plate which will take the thrust loads from a P60-K Python Flexible drive.

The engine is a Yamaha 4JH4E producing a maximum of about 55bhp (on a good day).

The boat weighs about 15tonnes fully loaded.

So taking a few wild guesses, if the boat stops from 5knots (~2.5m/sec) in say 30 seconds, i.e. going full astern and I've not tried to measure this but it seems about right. Using Newton's F=MA I get a load of about 250kg. This gives a rough order of magnitude of the load, it might be twice this or perhaps half.

The plate will be approximately 460mm wide and 200mm tall, a mini bulkhead if you like, and the load will be applied around about the center of the plate. I don't think much torque will be applied, just the friction from the thrust bearing, which should be small.

I intend to Epoxy the plate in with multiple layers of bi-axial glass to the hull and the longitudinals (3 sides), see the sketch, which I hope makes sense. The longitudinal are 200mm x 75mm.

I'll ensure that the GRP is ground back to sound laminate and fully cleaned with acetone before I start, I'm confident about this bit having done it successfully many times before.

I am concerned about the SS, I don't think the bond will be heavily loaded, it will effectively be pushing against a heavy fillet of epoxy glass, and I've guessed that a 10mm SS plate will not bend that much over 230mm or so, so there shouldn't be a tendency to peel.

Comments?
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I think that's all pretty fair. Presumably, it's a 10mm stainless plate with a hole in it for the shaft to pass through though, and the bearing is in some sort of carrier that is then bolted to the 10mm plate? If so, the plate is going to work like a beam with a hole in the "worst" bit, built-in at both ends. Intuitively, I can't see it going anywhere. The nice thing is that even if the bond between the epoxy and the stainless fails, you're not going to get a catastrophic failure. I take it that if the bearing seizes, the plate can't "spin" anyway because of it's shape and the sides of the hull?

I think your force estimate is very much "worst case" because a fair bit of the force making it stop from 5 knots will be the resistive forces that the boat would have to overcome anyway (which will be significant at 5kt and drop off to almost nothing as you drop below 1 kt). Presumably whatever thrust it can manage was taken by your 4 rubber engine mounts working in shear originally? Maybe, if you can get some big scales, a better estimate would be to tether the boat to a bollard with the scales " "in-series" and go full ahead? (or astern if the gearbox gives the pro better "bite" that way)?

I've done something very similar on Avocet (fitted a flexible drive) and all the thrust is taken by a couple of 5mm grub screws in the inner bearing race, digging into the prop shaft. The bearing is bolted to a mild steel carrier which, in turn, is bolted to a 1/2" plywood "mini bulkhead" where your stainless one would be. OK, Avocet only has 12 HP (and I think a couple of those might be dead!) and only weighs 3 tons, but it's never gone anywhere. Maybe, a better way (if you have space) would be to make a couple of marine ply "mini bulkheads" with an oversized hole in the middle of each of them and sandwich your stainless plate between them so that the three are bolted together in a few places, then epoxy each of the wooden bulkheads to the hull? That way, the thrust loads would be taken by the bond between the epoxy and the wood, working in shear (where it will be brilliant), and the stainless plate is just a mechanism for transferring the load from the centre of the plate, to the outside edges (a whacking great thrust washer, in effect). Frankly, done right, you'd probably be able to pick the boat up by it's prop and dangle it off that lot!
 
Thanks Avocet, I did the same thing some time ago with an Aquadrive on a Parker 275 fitted with a 1GM10 (probably about 8bhp at the shaft) and this design is very similar in concept, on the Parker I used two bits of 10mm ply bonded together and then glassed into the boat. That didn't move, but the Southerly is lot heavier and has around 55bhp and I wasn't happy to use plywood to take the thrust, that's why I decided to use SS. I can get SS cut to any profile locally so that bit is quite easy.
You're correct in that currently these loads are taken by the engine mounts, but the new mini bulkhead will be a lot more ridged and the Python Drive only has two elastomer bushes concentric with the securing bolts. I guess these are designed to provide some compliance as well as some acoustic isolation. I expect the new stiffer system to see higher loads than the engine mounts.
I think I now have enough confidence to give it go, I'll drill a few holes in the SS plate to allow through bonding, but not too many as I don't want to weaken the SS plate.
 
Ian_Edwards said:
Thanks Avocet, I did the same thing some time ago with an Aquadrive on a Parker 275 fitted with a 1GM10 (probably about 8bhp at the shaft) and this design is very similar in concept, on the Parker I used two bits of 10mm ply bonded together and then glassed into the boat. That didn't move, but the Southerly is lot heavier and has around 55bhp and I wasn't happy to use plywood to take the thrust, that's why I decided to use SS. I can get SS cut to any profile locally so that bit is quite easy.
You're correct in that currently these loads are taken by the engine mounts, but the new mini bulkhead will be a lot more ridged and the Python Drive only has two elastomer bushes concentric with the securing bolts. I guess these are designed to provide some compliance as well as some acoustic isolation. I expect the new stiffer system to see higher loads than the engine mounts.
I think I now have enough confidence to give it go, I'll drill a few holes in the SS plate to allow through bonding, but not too many as I don't want to weaken the SS plate.
Click to expand...

The loads would still be taken by the stainless plate - it would carry the load from its centre to it's outside edges. The plywood is just a way of getting a very big bond area to the hull, but it wouldn't be carrying any of the bending load, it would just be like a backing pad for a skin fitting. I'd have thought this system would take ALL the load off the engine mounts though? Presumably the thrust goes from the propeller, up the propshaft, into the bearing and from there to the stainless plate and out into the hull?
 
Avocet said:
The loads would still be taken by the stainless plate - it would carry the load from its centre to it's outside edges. The plywood is just a way of getting a very big bond area to the hull, but it wouldn't be carrying any of the bending load, it would just be like a backing pad for a skin fitting. I'd have thought this system would take ALL the load off the engine mounts though? Presumably the thrust goes from the propeller, up the propshaft, into the bearing and from there to the stainless plate and out into the hull?
Click to expand...

Mine is mild steel rather than stainless, 5 mm thick. I drilled a few holes to assist with bonding. It has been there for well over ten years now, no signs of movement. Photos at http://coxengineering.sharepoint.com/Pages/Aquadrive.aspx
 
Yes, I agree the thrust bearing will take all the prop loads directly into the hull via the mini bulk head and there engine mounts will only take the weight of the engine as it rumbles around.
I understand the concept of making a SS ply sandwich to improve the bonding to the SS plate, my comment about not trusting plywood was referring to the possible use of an all plywood mini bulkhead. I'll have a think about it and see if I have room to use a SS plywood sandwich.
Thanks for your input.
 
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