Keel bolt backing plate replacement
- Thread starter Ronn
- Start date
The waterjet idea is a good one in this instance - I'd forgotten about that. They will need a CAD file which is easily created. Most waterjet houses in the US will supply the material as part of the service. And you can have round corners if you like.
In my case I just sent them a dimensioned drawing. (I used Sketchup and just exported it as an image) As its such a simple shape, most places should be able to accept something like that.
My bolts are M20, but I had the holes made 22mm, that way they fit easily. And the holes are still a closer tolerance than they were before.
- Thread starter
- #23
Thank you for all the advice.
I had considered using thickened epoxy to bed them but was concerned whether doing so would leave sufficient threads showing on the bolts (if it results in some sort of flat bridge between the curves in the bilges then the middle part will be higher than at present): I'm stuck at home with an illness at present but I'll check once I get mobile again - its the bending of thick steel (esp stainless) plates that I couldn't get my head around. I also wasn't sure that the thickened epoxy wouldn't crack under the strain, but if you have done it and it works then fine.
I'm sure that's its correct that the rust has expanded under the nuts, perhaps that shows that I don't have to worry about over torquing as I may be taking some strain out of the system. Someone in the boatyard was fairly confident that 'tapping the nuts with a hammer' might help break them loose (hold a lump hammer on one side, tap with a ball preen (sp?) on the other, I might ask for a demo first). The yard has a torque multiplier that I can borrow.
Will make some templates when I'll definitely outsource the manufacture of the plates £300 odd is something I'll happily pay (I'm in SE England) to avoid trying to do it with my tools (and skills).
Re abandoning the large plates and replacing with smaller ones, I wondered if an engineer might have said that from a physics pov the larger plates aren't adding any value?
I had considered using thickened epoxy to bed them but was concerned whether doing so would leave sufficient threads showing on the bolts (if it results in some sort of flat bridge between the curves in the bilges then the middle part will be higher than at present): I'm stuck at home with an illness at present but I'll check once I get mobile again - its the bending of thick steel (esp stainless) plates that I couldn't get my head around. I also wasn't sure that the thickened epoxy wouldn't crack under the strain, but if you have done it and it works then fine.
I'm sure that's its correct that the rust has expanded under the nuts, perhaps that shows that I don't have to worry about over torquing as I may be taking some strain out of the system. Someone in the boatyard was fairly confident that 'tapping the nuts with a hammer' might help break them loose (hold a lump hammer on one side, tap with a ball preen (sp?) on the other, I might ask for a demo first). The yard has a torque multiplier that I can borrow.
Will make some templates when I'll definitely outsource the manufacture of the plates £300 odd is something I'll happily pay (I'm in SE England) to avoid trying to do it with my tools (and skills).
Re abandoning the large plates and replacing with smaller ones, I wondered if an engineer might have said that from a physics pov the larger plates aren't adding any value?
slug zitski
Super Anarchist
The typical precision template is a piece of clean 6mm ply about the correct size
place on top of bolt pattern, tap with a hammer to make some bolt head dents, drill oversized holes on the dent marks …much bigger diameter than the bolt .
fit the crude , oversized bolt hole , template …if all looks ok double side tape the template to the substrate to prevent moment ..then fit large fender washers of the correct inside diameter over the keel bolts and superglue the washers to the wood template
this will be very accurate ..mark the perimeter shape of the template before you remove so that it looks correct
a machine shop will be able to rapidly produce the finished product
place on top of bolt pattern, tap with a hammer to make some bolt head dents, drill oversized holes on the dent marks …much bigger diameter than the bolt .
fit the crude , oversized bolt hole , template …if all looks ok double side tape the template to the substrate to prevent moment ..then fit large fender washers of the correct inside diameter over the keel bolts and superglue the washers to the wood template
this will be very accurate ..mark the perimeter shape of the template before you remove so that it looks correct
a machine shop will be able to rapidly produce the finished product
SloopJonB
Super Anarchist
I do that on all backing plates I make up. I also radius all the edges and polish them just to make them purty.
Not much extra work in the greater scheme of things for a big jump in quality of workmanship.
Needless to say, I also clock all the fasteners that go through them as well.
Good idea to round the bottom EDGES (just break the edge with a grinder) to avoid a sharp edge cutting into laminate. The rounded corners are nice.
If a washer is "thick" then bigger may help. Thick means relative to the fastener diameter so somewhere (thickness >1/3 x diameter) you start thinking it's "thick". For this boat using 8mm or 10mm plate as the backing plates would be good. At least as thick as the original steel because we know that has worked.
If it is thin (i.e. 5-6mm plate with 24mm fasteners) then it will deflect locally and a larger washer doesn't help you. The outer part of the washer just flexes upward (in tiny amounts that aren't really visible unless the mating surface is rough or bumpy)
A washer big enough to fit snugly against the laminated sides of the keel sump if it exists is ideal. Or against any transverse floors.
Oh, I'm a N.A./ mech. engineer by the way. You can probably trust me with most of the advice I give.
If a washer is "thick" then bigger may help. Thick means relative to the fastener diameter so somewhere (thickness >1/3 x diameter) you start thinking it's "thick". For this boat using 8mm or 10mm plate as the backing plates would be good. At least as thick as the original steel because we know that has worked.
If it is thin (i.e. 5-6mm plate with 24mm fasteners) then it will deflect locally and a larger washer doesn't help you. The outer part of the washer just flexes upward (in tiny amounts that aren't really visible unless the mating surface is rough or bumpy)
A washer big enough to fit snugly against the laminated sides of the keel sump if it exists is ideal. Or against any transverse floors.
Oh, I'm a N.A./ mech. engineer by the way. You can probably trust me with most of the advice I give.
These work surprisingly well on stainless plate. Go slow, keep lubed (Anchor Lube), maintain pressure.
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slug zitski
Super Anarchist
You still need a metal shopThese work surprisingly well on stainless plate. Go slow, keep lubed (Anchor Lube), maintain pressure.
Rotabroach Hole Cutters | Blair Equipment
Rotabroach Annular Cutters, cut clean, burr-free, round holes fast, in sheet metal and thin plate materials.
Plate cutting tools , center finder, cutting paste
A slow speed drill press , de burr , grinder,, vise , bench ..
best to bring it to a machine shop
if it was normal steel you could do it ..hands and knees in the boat yard ..stainless is trouble
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This is the key, but saying is easier than doing. 316 in the annealed state is softer than mild steel, and will cut more easily. Unfortunately 316 is a badly work hardening material, so if you don't cut around 0.002" or more per edge, it will work harden and the next edge runs into hardened material. To keep even a 1/2" drill cutting properly takes around 200+ lbs of pressure - impossible with a hand drill. That's why people try stepping the hole diameter up using larger drills, at each step the edge engagement is small and you can get enough pressure, but it causes other problems. The annular cutter has less edge engagement as it is just a thin ring, but still takes good pressure or the same things apply.
With a hand drill you are lucky to get a few holes with a decent bit before it dulls as it is always cutting hardened material. In the CNC machine with controlled feed and flood coolant, even a cheap cobalt drill will drill hundreds of holes in 316 before I need to sharpen it.
Got to agree with DDW here.
Especially given the little the OP has told us, drilling stainless is a royal PITA. Especially the thickness we're talking.
For a one off, easier to just pay to have it cut, there are tons of places in the UK that can do this service, even the local stainless guy subcontracts stuff like this to somewhere with a CNC cutter.
@Ronn If your boat is currently out the water then its probably worth removing one of the backing plates, assuming it fits correctly then you can take measurements off it on the bench, no need to make a template if its already a good fit. You can do that one by one, replacing them once you've had them off.
The thickened epoxy is just to bridge small low spots, in the laminate, not to cover large troughs.
Once you get the plate off then you can see the shape of the hull underneath, I would be quite surprised if there is significant hull curvature under the plate.
Especially given the little the OP has told us, drilling stainless is a royal PITA. Especially the thickness we're talking.
For a one off, easier to just pay to have it cut, there are tons of places in the UK that can do this service, even the local stainless guy subcontracts stuff like this to somewhere with a CNC cutter.
@Ronn If your boat is currently out the water then its probably worth removing one of the backing plates, assuming it fits correctly then you can take measurements off it on the bench, no need to make a template if its already a good fit. You can do that one by one, replacing them once you've had them off.
The thickened epoxy is just to bridge small low spots, in the laminate, not to cover large troughs.
Once you get the plate off then you can see the shape of the hull underneath, I would be quite surprised if there is significant hull curvature under the plate.
- Thread starter
- #32
Thanks for all the tips.
I went to the boat and had a quick look today and the curvature of the plates isn't nearly as bad as I remembered, there is a bit of an optical illusion going on, it is not an issue. The plates were probably 10 mm when they were new (the rust and subsequent work makes it difficult to be precise) and I'll put in 10 mm. I'm torn between DIY in mild steel 1 at a time vs get them made up in stainless - if the latter I'd have to send away and get them all done at once, I'm not, yet, aware of a local suppler. Will proceed in the new year.
I went to the boat and had a quick look today and the curvature of the plates isn't nearly as bad as I remembered, there is a bit of an optical illusion going on, it is not an issue. The plates were probably 10 mm when they were new (the rust and subsequent work makes it difficult to be precise) and I'll put in 10 mm. I'm torn between DIY in mild steel 1 at a time vs get them made up in stainless - if the latter I'd have to send away and get them all done at once, I'm not, yet, aware of a local suppler. Will proceed in the new year.
??? You have enough bolt sticking proud of the nut to make templates for all of them. Send them off and get SS replacements. Replace one at a time. I'd probably sell the boat before I'd go through all that trouble and then replace with mild steel. Drilling 24mm holes in mild steel will be no picnic either.
"There's never time to do it right, but always time to do it again..."
"There's never time to do it right, but always time to do it again..."
SloopJonB
Super Anarchist
I did it on a 43' years ago. I had a machine shop drill the S/S "floors" simply by providing the center spacing of the bolts (10"). I think you'll find the bolt spacing will be a nice round number and all the pairs will be the same distance apart but measure all of them to be certain.
It's really a very simple process and not that expensive. It can even be done in the water if you do one pair at a time - there is a huge safety margin on those things so simple static loading is a very minor part of what they can take.
Mine were big pieces of S/S channel, not just the usual backing plates but it's the same process - here are before & after pics.
- Thread starter
- #35
An update:
1. A local waterjet cutting place is making up replacement backing plates in 316.
2. I started with the easiest looking one, used a 5' length of scaffolding pole to extend the 1/2 inch socket universal joint bar and applied pressure, after 2+ foot of deflection I was expecting the universal joint to explode but there was a snapping sound and the nut released.
3. The backing plate came off easily with the help of a chisel, the stale sealant didn't put up a fight.
4. The condition of the backing plate is interesting:
a) It has a curve.
b) The galvanising around the hole is in perfect condition, and remains so a few inches out, but then gets rusty - I conclude that it wasn't tight up against the hull once it got a distance from the nut holding it down tight.
Whoever installed the backing plate (my guess is the original builder) didn't fair the hull underneath where the plate would sit, as recommended here. A flat edge placed in the area of the backing plate becomes a bridge.
It isn't clear to me whether the backing plate was curved as it tried to conform to the hull or whether it might be showing the 'cupping' that is normal for washers?
The way the backing plate was sitting, apparently not watertight to the hull, might suggest that the majority of the plate was just adding weight rather than doing anything useful?
The 316 is likely (I believe, I might be wrong) to be less malleable than the mild steel it is replacing.
The advice above has been to use thickened epoxy underneath the new backing plate. Current plan is:
1. use plasticine/putty around the bolt to stop epoxy from entering the area of the studding/hole in the plate.
2. putting down CSM in epoxy and, whilst it is still wet
3. placing a waxed/taped backing plate down on the CSM with light pressure to try and get a flat and fair surface. Wait for epoxy to cure.
4. remove the plate, remove plasticine, put sealant around the bolt, replace plate, add anti-seize, torque.
The reason I was thinking of CSM, rather than thickened epoxy, is that it seems as if it might be going to have to do more work than just fill in some scratches and dinks, it will be transferring the whole load to the hull?
I thought I'd post this here before continuing work to give an opportunity for those who know more to comment before proceeding.
Picture of underside of backing plate attached. The album at https://photos.app.goo.gl/T1fZ5E1oXXYEjxNu2 has more.
1. A local waterjet cutting place is making up replacement backing plates in 316.
2. I started with the easiest looking one, used a 5' length of scaffolding pole to extend the 1/2 inch socket universal joint bar and applied pressure, after 2+ foot of deflection I was expecting the universal joint to explode but there was a snapping sound and the nut released.
3. The backing plate came off easily with the help of a chisel, the stale sealant didn't put up a fight.
4. The condition of the backing plate is interesting:
a) It has a curve.
b) The galvanising around the hole is in perfect condition, and remains so a few inches out, but then gets rusty - I conclude that it wasn't tight up against the hull once it got a distance from the nut holding it down tight.
Whoever installed the backing plate (my guess is the original builder) didn't fair the hull underneath where the plate would sit, as recommended here. A flat edge placed in the area of the backing plate becomes a bridge.
It isn't clear to me whether the backing plate was curved as it tried to conform to the hull or whether it might be showing the 'cupping' that is normal for washers?
The way the backing plate was sitting, apparently not watertight to the hull, might suggest that the majority of the plate was just adding weight rather than doing anything useful?
The 316 is likely (I believe, I might be wrong) to be less malleable than the mild steel it is replacing.
The advice above has been to use thickened epoxy underneath the new backing plate. Current plan is:
1. use plasticine/putty around the bolt to stop epoxy from entering the area of the studding/hole in the plate.
2. putting down CSM in epoxy and, whilst it is still wet
3. placing a waxed/taped backing plate down on the CSM with light pressure to try and get a flat and fair surface. Wait for epoxy to cure.
4. remove the plate, remove plasticine, put sealant around the bolt, replace plate, add anti-seize, torque.
The reason I was thinking of CSM, rather than thickened epoxy, is that it seems as if it might be going to have to do more work than just fill in some scratches and dinks, it will be transferring the whole load to the hull?
I thought I'd post this here before continuing work to give an opportunity for those who know more to comment before proceeding.
Picture of underside of backing plate attached. The album at https://photos.app.goo.gl/T1fZ5E1oXXYEjxNu2 has more.
Your plan is good. SS is more, or about the same malleability as steel. 316 annealed is about the same as hot rolled mild steel, a bit softer than cold rolled steel. You can just wrap the bolts with packing tape, epoxy won't stick to it. Same with the bottom of the new backing plates. I would not bother with CSM, the steel plate should be way stiffer than the CSM (unless it is very, very thick). You just want even pressure between the SS and the uneven hull surface. The compressive strength of filled epoxy is perfectly adequate for this.
SloopJonB
Super Anarchist
I'd use filled epoxy rather than CSM - you want it to take up a minor variability in the surface which a fabric will do less well than a "putty".
It's only there to level things and is only in compression so a lot of strength is not needed - mill fiber in resin will be lots for such a thin application.
It's only there to level things and is only in compression so a lot of strength is not needed - mill fiber in resin will be lots for such a thin application.
Marty Gingras
Mid-range Anarchist
Sure looks like 'rust jacking' discussed early in the thread. How deep is the divot in the hull, especially compared to the relevant cup in the plate?
Third Reef
Member
Is there any reason why G10 material isn’t used for keel bolt backing plates ?
Could be used. Generally though, you want something stiffer than what the bolts go through to spread the load over more of it. The keel stub is (or ought to be) pretty thick solid laminate to start with. Adding a little G10 will not do much to spread the load unless very thick. SS is about 15 times stiffer than G10 so to do the same job is needs to be 2.5x thicker. You'd still want small steel backing plates to spread the nut load to the G10.
