What material to use for 1/8" thick on-deck mounting?

[Sailing August]

I would do 316 stainless steel plates with backing plates. You would be surprised how much load this connection will take. You can buy the plates out of the Grainger or McMaster-Carr catalogs on line.
Also check out Metals Supermarket" on line. Best prices for anything you need.

https://www.metalsupermarkets.com/

[MicHughV]

I notice from the original OP pic that the aft leg of the stanchions is attached to cap rail.
The aft and forward leg of the davits are connected to each other.
I can't tell if the aft leg is thru' bolted to the caprail or if that is just a lag bolt. I don't see any washers, so I'm thinking maybe just a lag bolt.
Nonetheless, another option would be to raise the aft leg. It likely won't need much raising, as a small movement here will translate into a much large movement on the davit arms.
It may not be necessary to remove the aft pulpit, but loosen those connection points and slide something between the stanchion base and the caprail. A strip of rubber...a piece of SS plate...washer....etc...
something along the lines of 1/8" to 1/4" thick might solve your problem.

[thinwater]

Originally Posted by wingssail

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.

Excellent points. I would go a little farther.


As long as you are accessing the inside, lay up additional layers of glass, perhaps another 4-6 layers of 1708. Maybe something different, after I saw the structure from the inside, but with the goal of making the hull and deck stronger and spreading the load farther and more intimately than you can with a backing plate.


Not hard to do. Clean and sand, then pre-wet the laminated on a plastic sheet, no more than 2 layers at a time (exotherm and can fall off if overhead) and no larger than about 8x10" in one sheet (too hard to handle--overlap sheets ~ 1" if you need a larger area). Roll in place firmly. It's probably in a locker or under a liner, so cosmetics are not a factor.

[Thumbs Up]

Cast the base in place with thickened epoxy. This is what West system recommends, See casting an epoxy basehttps://www.westsystem.com/instructi...ners-hardware/
I have used this method extensively on all of my stanchion and pulpit bases, as well as the bonding hardware with epoxy techniques outlined. With the judicious use of masking tape, a base for these can be created without even removing the hardware by loosening the fasteners (good time to add backing plate), chiseling out the old teak base, masking, then slathering thickened resin (fibers added if you like). Tool it off then pull the tape. Once set, clean it up with some sandpaper then prime and paint. If you ever need to remove it apply heat to the screw heads with a soldering gun. This method is superior and should never crack nor leak. If you have a cored deck you can over drill the holes thus sealing the core in the same operation.

[Pillroller]

Are you sure it is not to give an extra inch or so clearance for the dink and stern? Just askin.
R.

[jhulmer]

I have used a lot of this FRP board for such things. Sand it well and it will take paint and gelcoat.

https://www.mcmaster.com/fiberglass/...rs-and-strips/

[Chotu]

Originally Posted by jhulmer

I have used a lot of this FRP board for such things. Sand it well and it will take paint and gelcoat.

https://www.mcmaster.com/fiberglass/...rs-and-strips/

To be clear, these are exactly what I meant also when I said polyester sheet. As opposed to G10, an epoxy sheet.

[Cheechako]

Originally Posted by MondayNever

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.

Not sure why you are thinking of doing this? It looks to be working fine?
If you change those circular pads dont you have to cut and reweld the rectangular pads on the bulwark cap?
Those circular pads/bolts are in tension under load as the pivot point is the rectangular pad on the toerail. The weight is pulling the circular pad up, no real load on the deck.

If you want to just fill the gap and get rid of the wood just use Starboard. As I said, there is almost no load on it. The leverage of the davit is pulling it up.

[guyrj33]

Remove davit.
Decore deck under mount.
Use fiberglass sheet to build top plate larger than the stainless foot.
https://www.mcmaster.com/fiberglass/...rs-and-strips/
Epoxy the fiberglass top plate to the deck. Increases strength and seal out water).
Chamfer the mounting holes in the top plate.
Use butyl tape around the bolts to seal the holes.
Use a large backing plate.

[Cadence]

Originally Posted by MondayNever

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?

I'm not sure of your intent. If you reduce the thickness of that "shim" will it not cause the othe legs mounting plate to be at an angle to it's present mounting surface? Maybe that was intent of the previous owner. The fabrication of the davits may not have been perfect.

[wingssail]

Originally Posted by Cheechako

Not sure why you are thinking of doing this? It looks to be working fine?

He mentioned he does not like the looks ("Saggy Butt") but there is another issue which he does not seem concerned about, and that is the lack of strength in the current set-up.

If you change those circular pads dont you have to cut and reweld the rectangular pads on the bulwark cap?
Those circular pads/bolts are in tension under load as the pivot point is the rectangular pad on the toerail. The weight is pulling the circular pad up, no real load on the deck.

Cheechako, what you are speaking of is the static load, just sitting at the dock. There is going to be substantial load as this boat moves through a seaway, both up and down, and on the deck pads and the cap rail pad. And that load will also manifest itself as a twisting load as the boat goes diagonally over waves.

If you want to just fill the gap and get rid of the wood just use Starboard. As I said, there is almost no load on it. The leverage of the davit is pulling it up.

I can think of at least four supposedly well engineered, modern, boats I am personally aware of with structures apparently better supported than this boat which all failed (broke) after only moderate weather at sea.

To the OP, please don't be offended, but your boat is always going to look "butt" heavy, in my opinion, as long as you have those davits and a dingy hanging out there. To improve the looks and the performance, and the stability, and reduce windage, and reduce hobby horsing, and eliminate the strength issues, lose the whole structure.

[David B]

Originally Posted by MondayNever

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?

The last thing you want there is 'give'. With the davit legs so close together (I don't like the design - should be further apart), there are very significant loads on the deck.

First thing is to check the size and thickness of load distribution plate under the deck. I see cracking in the photo indicating the stress that the deck is being put under. That stress needs to be distributed via a decent sized 8 or 10mm stainless steel plate under the deck (and you could make life easier by threading the plate).

As for the pad, ideally a larger stainless steel one replacing the current base. Otherwise, use Starboard - UV is not an issue.
When bedding the whole arrangement, do not fully tighten, until the sealant is cured. That way you don't squeeze all the sealant out.

[OS2Dude]

I find old frying pans to be a good source of aluminum for backing plates. The curved sides can be pounded flat for smaller/thinner plates.

[Shaneesprit]

Originally Posted by wingssail

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.

Wingsail always has good advice - if you think the Davits are not stressed, just try picking your dingy up and holding it at arms length. Oh you can’t can you? I was only surprised buy how small the mounts were.