Stacking keel backing plates?

[nickpitcher]

My current keel backing plates are 3mm thick, which I believe to be inadequate at spreading the load with 3/4" bolts over GRP. See photo attached.

So my question is, rather than going through the effort of re-bedding the plates, would it be ok for me to simply stack an additional plate on top of each of the existing plates to make up for it?

Or are my 3mm plates sufficient?

Cheers

[neilpride]

Why? previous problems? cracks?

[Lou-In-NJ]

I can't find ballast weight on your particular boat, but since Sailboat Data lists your total displacement at 4400 lbs., I'd guess your keel s nearly 1000 lbs. So I would have the same trepidations as you; I would get a single 7mm (1/4") plate, and have it extend at least an inch (25mm) further in all directions than what the two square plates currently cover. Spreading the forces out is the key increasing the margin of safety and reliability.

[Muaddib1116]

It doesn't look like there are any problems there, at least from the photo, but if you are going to go through the trouble of making new backing plates, why not go larger (surface area) as well as thickness?

[nickpitcher]

Originally Posted by neilpride

Why? previous problems? cracks?

I replaced the old plates, washers and nuts due to corrosion. The old hardware was very well bedded so the bolts were in great condition.

However upon reflection the old plates were approx 4-5mm thick (probably 3/16"), which I didn't really think about at the time as I was more concerned about bedding them with the right sealant, torquing them correctly, etc. Plus my local bolt distributer only had them in 3mm.

Anyway this is where I'm at now, so my question is, if I stack an additional 3mm plate (or thicker if necessary) on top of each of the existing 3mm plates, will that make up for it?

[nickpitcher]

It's also probably worth mentioning that it's a fin keel and the bolts are in a side-by-side configuration. There are 12 bolts in total, and all but the fore and aft bolts are in pairs as per the photo. The center 6 bolts are 3/4” and the remaining 6 are 1/2”.

I’m also not exactly sure of the thickness of the laminate, I’ll try measure it today. However I can say that it’s considerably thicker down the centreline. It’s raised by approx 3/4”.

[neilpride]

Originally Posted by nickpitcher

I replaced the old plates, washers and nuts due to corrosion. The old hardware was very well bedded so the bolts were in great condition.

However upon reflection the old plates were approx 4-5mm thick (probably 3/16"), which I didn't really think about at the time as I was more concerned about bedding them with the right sealant, torquing them correctly, etc. Plus my local bolt distributer only had them in 3mm.

Anyway this is where I'm at now, so my question is, if I stack an additional 3mm plate (or thicker if necessary) on top of each of the existing 3mm plates, will that make up for it?

Yes , but better to found a single plate of maybe 5 or 6 mm thick.

[nickpitcher]

Originally Posted by neilpride

Yes , but better to found a single plate of maybe 5 or 6 mm thick.

Agreed, ideally I'd replace/re-bed each of them with 5-6mm plates, but that'll require another haul out. So for now anyway, is there any major downside to just stacking on additional plates?

[wyb2]

Originally Posted by nickpitcher

I replaced the old plates, washers and nuts due to corrosion. The old hardware was very well bedded so the bolts were in great condition.

However upon reflection the old plates were approx 4-5mm thick (probably 3/16"), which I didn't really think about at the time as I was more concerned about bedding them with the right sealant, torquing them correctly, etc. Plus my local bolt distributer only had them in 3mm.

Anyway this is where I'm at now, so my question is, if I stack an additional 3mm plate (or thicker if necessary) on top of each of the existing 3mm plates, will that make up for it?

Yes, you can absolutely stack additional washers. Two 3mm washers will not be as stiff as a single 6mm washer, but will be stiffer than a single 3mm. The crossover point is probably four 3mm washers to roughly equal one 6mm.

Someone above said you probably have a 1000 lbs keel. The bolt pattern you’ve described sounds pretty robust for that size keel. I wouldn’t worry too much.

[nickpitcher]

Originally Posted by wyb2

Yes, you can absolutely stack additional washers. Two 3mm washers will not be as stiff as a single 6mm washer, but will be stiffer than a single 3mm. The crossover point is probably four 3mm washers to roughly equal one 6mm.

Someone above said you probably have a 1000 lbs keel. The bolt pattern you’ve described sounds pretty robust for that size keel. I wouldn’t worry too much.

Thanks. For peace of mind I think I'll stack an additional 5mm plate on at least each of the 3/4" bolt plates. Seems like that'll be overly adequate

[stormalong]

Originally Posted by nickpitcher

Agreed, ideally I'd replace/re-bed each of them with 5-6mm plates, but that'll require another haul out. So for now anyway, is there any major downside to just stacking on additional plates?

You posted that there are 12 bolts. There should be no reason that you could not remove one nut and plate at a time while the boat is in the water.

But also, just adding a thicker plate on top of the existing plate would be fine too.

[nickpitcher]

Originally Posted by stormalong

You posted that there are 12 bolts. There should be no reason that you could not remove one nut and plate at a time while the boat is in the water.

But also, just adding a thicker plate on top of the existing plate would be fine too.

Yes I could do that, however that'd take at least 12 days as I'd have to wait 24hrs for the sealant (Sika 291) to cure before torquing each bolt, one after another.

[stormalong]

Originally Posted by nickpitcher

Yes I could do that, however that'd take at least 12 days as I'd have to wait 24hrs for the sealant (Sika 291) to cure before torquing each bolt, one after another.

Well, you could do at least two at a time, not next to each other. And you could do these when you take the boat out. If you have a one ton keel, you have an average load of a mere 167 pounds per bolt and a lot of redundancy with twelve bolts.

[nickpitcher]

Originally Posted by stormalong

Well, you could do at least two at a time, not next to each other. And you could do these when you take the boat out. If you have a one ton keel, you have an average load of a mere 167 pounds per bolt and a lot of redundancy with twelve bolts.

That's some great insight thanks. And that's good to know as I cant imagine my keel is any more than 1000lb seeing as its only 26ft, so max 83lb per bolt, even more redundancy

I think what I'm gonna do for now is add additional 5mm plates to the existing plates, then re-bed them properly on my next haul out.

[wyb2]

Originally Posted by nickpitcher

Thanks. For peace of mind I think I'll stack an additional 5mm plate on at least each of the 3/4" bolt plates. Seems like that'll be overly adequate

Sounds like a good plan and you’ll end up with something stronger than what was there originally. I would bed the new 5mm plates to the tops of the existing 3mm to prevent bilge water from getting trapped in between them.