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Old 21-10-2021, 16:46   #1
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What material to use for 1/8" thick on-deck mounting?

Hey all,
I'm coming up short on what material to use to make mounting plates for my davit legs. Currently they are sitting on 3/4" teak plates that the previous owner installed. They are far too thick and have skewed the whole davit system downward, so I need to reduce the plate size to somewhere between 1/8" and 1/4". These are the forward legs of the davits (see pics) so the only compression the plates will receive is from the mounting bolts - the weight of the dinghy bouncing around will actually reduce the load on the plates. This makes me think that it would be ideal if the material was something with a tiny bit of give, so that it can slightly expand and contract with movement. Because it's on deck, it also needs to be UV tolerant and it also needs to be something that will work with one of the normal bedding compounds (butyl tape, 4200, lifeseal, etc). Starboard was my immediate thought, but my understanding is that no bedding compounds will stick to it. I thought about G10, but that has no give and apparently is not UV stable. Are there any extremely dense rubber products that would fit the bill? Any other ideas?
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Old 21-10-2021, 17:28   #2
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Re: What material to use for 1/8" thick on-deck mounting?

For something not UV stable, just keep it painted white.
"Give" isn't going to be critical, as I don't see how this is much different than any metal component bolted directly to the deck. A bedding compound should be flexible enough to keep it sealed against any small displacements from temperature changes, etc.

Personally I'd choose painted G10 or a thin machined piece of stainless, with the G10 being a lot easier to fabricate for most. There are some UV resistant polymers, maybe someone has some experience with one long term, or one that's easy to paint and somehow better than G10.
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Old 21-10-2021, 18:12   #3
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Re: What material to use for 1/8" thick on-deck mounting?

Do the new pads need to be tapered?
Are there plates underneath as well?
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Old 21-10-2021, 18:47   #4
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Re: What material to use for 1/8" thick on-deck mounting?

why can't you just take those circular teak plates out, and cut, sand to the desired thickness you want ?
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Old 21-10-2021, 19:36   #5
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Re: What material to use for 1/8" thick on-deck mounting?

You are going to be surprised at the loads which will be imposed on those mounts. The loads will be compressive on thefront legs when the boat hits bottom going down a wave, and under tension when the boat pitches forward going over a crest. Then there will be a twisting motion as the boat rolls and goes over a wave at the same time. I see cracks already.

Your mounts look to be about 8" apart fore and aft. The mechanical disadvantage will immense if you are in waves with a dingy on the davits, worse by far if the motor is on it.

I've seen similar structures break the fiberglass.

So my advice is put large (6x8) stainless plates 3/16" top and underneath, through bolted, on the forward legs, and something equivelently strong on the aft legs.

Secondly, build (weld) diagonal braces from 3' up the vertical legs down to the side deck outside of the hatch and forward 18" and put equivilent plates and back-up plates there too. This will support both the fore and aft direction and the side to side direction.

Best of all, do not carry a dingy back there when going outside of very sheltered waters.
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Old 21-10-2021, 20:21   #6
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Re: What material to use for 1/8" thick on-deck mounting?

Certainly looks like the davits are an integral part of the rail. If that is the case, you're stuck with what you have; you can't change any of the mounting dimensions without inducing stresses that will ultimately lead to broken welds or worse.

If you want to change the material of the existing .75" shim it's not really important, starboard will 'work', and will certainly 'give some give'.

I'd 'certainly' not recommend it though; what's there now is pretty good, though perishable. G10 is an option, but it is not UV stable. Aluminum would be acceptable, except for the 'battery quotient'...

Providing your hull scantlings are sufficient the SS pads you have now are adequate. Whatever spacer you use should be corrosion and compression compatible.

My main concern, again, providing the hull scantlings are adequate, would be the size and material of the backing plate on the large flange. It should, ideally, be about twice the surface area of the upper flange, with no 'hard' edges; i.e. the surface edges in contact with the F/G should be broken (radiused).
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Old 22-10-2021, 02:42   #7
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Re: What material to use for 1/8" thick on-deck mounting?

I’m going to question the entire premise.

Why are you doing this project? The dinghy appears to sit ok in the davits.

Also, are you sure the previous owner installed those? If so, why did he install them?

They look an awful lot like original equipment having the same appearance as the teak cap rail does.

I’d use polyester or vinyl sheet if you absolutely must change these. Has no issues with UV.

In this installation the pad is doing essentially nothing. It’s just a spacer. It’s not even spreading out the load. It hardly matters at all what you put there. You really need to be sure of what your backing plate situation is. That’s the main concern.

Edit: I see stress cracks around the base. You DEFINITELY need to upgrade things like the backing plate and maybe the spacer (widen it). This is through bolted, right??
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Old 22-10-2021, 05:14   #8
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What material to use for 1/8" thick on-deck mounting?

All the pressure in those particular legs is underneath the deck. The strength and size of the deck spacer is largely irrelevant.

Why not just sand the teak spacers. But why go at it at all
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Old 22-10-2021, 05:39   #9
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Re: What material to use for 1/8" thick on-deck mounting?

Quote:
Originally Posted by markxengineerin View Post
For something not UV stable, just keep it painted white.
"Give" isn't going to be critical, as I don't see how this is much different than any metal component bolted directly to the deck. A bedding compound should be flexible enough to keep it sealed against any small displacements from temperature changes, etc.

Personally I'd choose painted G10 or a thin machined piece of stainless, with the G10 being a lot easier to fabricate for most. There are some UV resistant polymers, maybe someone has some experience with one long term, or one that's easy to paint and somehow better than G10.

That's a thought. I can fabricate the plates in G-10 easily and if painting them with awlgrip or whatever solves the UV problem, easy.
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Old 22-10-2021, 05:40   #10
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Re: What material to use for 1/8" thick on-deck mounting?

Quote:
Originally Posted by Spot View Post
Do the new pads need to be tapered?
Are there plates underneath as well?

The current pads are tapered slightly, but that's because they're too tall and are contributing to the davits being tilted aft/down.


I had some new giant stainless backing plates fabricated because the davits had been installed with rinky dink fender washers below deck. Those will be swapped in as part of this process.
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Old 22-10-2021, 05:43   #11
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Re: What material to use for 1/8" thick on-deck mounting?

Quote:
Originally Posted by MicHughV View Post
why can't you just take those circular teak plates out, and cut, sand to the desired thickness you want ?

I don't believe the teak will be able to hold together if I plane it down to ~1/4" or less. It's already got a lot of exposed grains, so I expect it would just split.
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Old 22-10-2021, 05:48   #12
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Re: What material to use for 1/8" thick on-deck mounting?

Quote:
Originally Posted by wingssail View Post
You are going to be surprised at the loads which will be imposed on those mounts. The loads will be compressive on thefront legs when the boat hits bottom going down a wave, and under tension when the boat pitches forward going over a crest. Then there will be a twisting motion as the boat rolls and goes over a wave at the same time. I see cracks already.

Your mounts look to be about 8" apart fore and aft. The mechanical disadvantage will immense if you are in waves with a dingy on the davits, worse by far if the motor is on it.

I've seen similar structures break the fiberglass.

So my advice is put large (6x8) stainless plates 3/16" top and underneath, through bolted, on the forward legs, and something equivelently strong on the aft legs.

Secondly, build (weld) diagonal braces from 3' up the vertical legs down to the side deck outside of the hatch and forward 18" and put equivilent plates and back-up plates there too. This will support both the fore and aft direction and the side to side direction.

Best of all, do not carry a dingy back there when going outside of very sheltered waters.

I agree with your assessment of the poor load distribution due to the very close fore and aft support. I didn't design these and certainly wouldn't do it again in the same way. With that said, while I understand your thought about the additional braces, but I'm not going to do that as they will make a mess of the aft deck. My approach is not carrying the dinghy in the davits outside of protected water, as you suggest. I also never leave the outboard on.



Giant stainless backing plates are fabricated and going on below deck.
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Old 22-10-2021, 06:01   #13
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Re: What material to use for 1/8" thick on-deck mounting?

Quote:
Originally Posted by Chotu View Post
I’m going to question the entire premise.

Why are you doing this project? The dinghy appears to sit ok in the davits.

Also, are you sure the previous owner installed those? If so, why did he install them?

They look an awful lot like original equipment having the same appearance as the teak cap rail does.

I’d use polyester or vinyl sheet if you absolutely must change these. Has no issues with UV.

In this installation the pad is doing essentially nothing. It’s just a spacer. It’s not even spreading out the load. It hardly matters at all what you put there. You really need to be sure of what your backing plate situation is. That’s the main concern.

Edit: I see stress cracks around the base. You DEFINITELY need to upgrade things like the backing plate and maybe the spacer (widen it). This is through bolted, right??

The premise is wanting to resolve the downward tilt of the davit arms. See attached. It is clear the current installation is bending the stern rail aft/outboard. You can see in the first image that the aft legs are tilted slightly and that bears out when measured with a level. My theory is not that the original davit fabricators were idiots, but that when the teak decks were removed a few years ago, the yard added these overly thick teak spacers because they didn't properly account for the change in deck height, causing or exacerbating the tilt.

And as stated above, massive new backing plates are going on.
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Old 22-10-2021, 06:29   #14
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Re: What material to use for 1/8" thick on-deck mounting?

The davit is attached to the rail. Is the rail also canted aft at approx. 7degrees? If the vertical rail stanchions are not canted back, then lowering the pads will cant them forward.

The pads look like they give the stanchion flange a flat mounting surface. Common since decks are rarely flat.
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Old 22-10-2021, 07:34   #15
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Re: What material to use for 1/8" thick on-deck mounting?

Quote:
Originally Posted by Shrew View Post
The davit is attached to the rail. Is the rail also canted aft at approx. 7degrees? If the vertical rail stanchions are not canted back, then lowering the pads will cant them forward.

The pads look like they give the stanchion flange a flat mounting surface. Common since decks are rarely flat.

Yes, the rail is canted back roughly 7*, corresponding with the horizontal arms.



The real question is why the designer of the davits originally aimed for a perfectly horizontal or maybe slightly downward tilted angle. If anything, the arms should the arms should be designed with an upward trajectory. It really messes up the lines of the boat, giving it a saggy butt.
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