Swim platform bending

I have a Beneteau Swift Trawler 34 that I have owned for 1 year. PO installed a Sea Wise manual davit for dinghy on the fiberglass swim platform. Boat was 3 years old when I got it. At first, I didn't have the dinghy on the boat but later last summer took some trips with the dinghy on the davit/swim platform.

I noticed the caulk line between the swim platform and the transom develop a crack on the starboard side which is where the davit lift is and the heavy motor end of the dinghy. This crack flexes open when pulling up the dinghy and gets bigger/smaller going over waves when carrying the dinghy. I took dinghy off and had the yard look at this after haulout last fall. They installed backer plates on the inside of the transom and tightened up the platform. But this Spring, same problem again. It appears the fiberglass platform above the transom bolts is bending or flexing under the load. Not good.

The dinghy is a 10.6 AB with console 20 hp motor and total weight is about 400 lbs. The surveyor signed off the this installation as ok at the time of purchase but obviously something is not right. I have attached a picture of the setup.

I'm thinking of moving the two side arms of the hoist from an attachment on the platform to attach to the transom. Also could add another support rod from transom to aft starboard platform where the dinghy couples on. Or could get a much lighter dinghy. Maybe all 3.

Anyone else have a problem like this? Any thoughts or solutions are greatly appreciated.

Attaching the side arms to the transom would make a big improvement to the sturdiness of the davit system. You may want to look a brackets underneath the transom as well.

I had a Sea Wise on a previous boat. Mine had the support arm to the transom. It looks like yours is designed so that you can walk between the davit and the transom, on mine you couldn't walk between due to the support arm going to the transom. We has a teak platrorm that was not the sturdiest in the world, but I had no problems with the Sea Wise. Call Sea Wise and ask for Stuart as he was the owner at the time I bought mine. He may have some other suggestions.

Greetings,
Mr. 17. As an adjunct to Mr. AC's suggestion of moving/relocating the braces to the transom, I would suggest placing the new mounting points as high on the transom as possible. That way, addition of weight to the platform would actually cause the platform to be forced inward toward the hull.

When the PO installed a SeaWise on the swim platform it was initially installed with the support brackets going to the transom. The PO then had them changed to go where yours do. I believe to allow access behind the davit on the starboard side. We have not had any issues but my swim platform is part of the hull mold, not just bolted on.

I can't tell how your swim platform is mounted. As Comodave mentioned, supporting the davit from the transom would help but I am not sure that it would help enough.

FWIW, the PO had left the mounting plates and bolts on the transom in place when they move the supports to the swim platform surface. I re-purposed those to hold my hard shell Lifesling case.

400 lbs is a lot of weight on a swim platform. I agree that if you relocate those struts so they attach to the transom, you will stiffen it up a lot, particularly if most of the weight is on that side. And another strut on the other bracket also attached to the transom should probably do the trick.

Your surveyor made a structural pronouncement of the swim platform's integrity with that weight? Where did he get his NA training? It is worth about as much as the BS you get on this forum .

David

Without seen what bracing is under the swim platform Id also be guessing .

Even with the existing struts, that's a lot of "moment" (or more accurately, torque) out at the edge of the platform. Add your own weight - or even two people - on the platform and I'm guessing Beneteau would say it wasn't built for that.
Tying back to the transom would help transfer some force but the weight is still there.
Maybe fill the dinghy with helium?
(Yes. Of course I'm kidding.)

The platform is already compromised by the additions, bracing back to the transom between which the movement is occurring seems the answer if you want to retain the existing set up. I think you just have too much load on the platform. Best check the strength of the transom to carry its part of the braced load before proceeding.

You might be pushing the weight limit at 400#. Short term you should think about removing the outboard. Diagonal straps to hold the dinghy rigid might help minimize movement of the dinghy.
Long term you need to transfer the weight from the swim platform davit base to the transom via brackets below davit base. You will need to have them custom fabricated to make sure the load goes into something structural.
See the AFI Swim Platform Mounting Bracket for typical right triangle design.

Swim platforms that have to carry weight are under considerable stress. Here is what I consider the best answer.

eyschulman said:

Swim platforms that have to carry weight are under considerable stress. Here is what I consider the best answer.

Click to expand...

That is by far the best answer. The bonus is extra speed and efficiency. Although not so easy to retrofit.

Thanks to all for the good advice. I will look into moving the bracing struts to the transom but I agree the dinghy is just too big and too heavy. I think I will look into an ultralite dinghy like the AB Alumina with a smaller motor. (170 lbs) This will also allow me to keep the dinghy long term on the boat deck since I could then be able to lift it with my trawler boom.

I would add two additional brackets under the platform that have struts or gussets to distribute the load better.

Bluefish17: I didn't say this in my earlier post but might be worth considering re-mounting the bases of the davits much closer to the transom. I realize that your dinghy would then be partially over the swim platform (when it's not deployed) but that would make for much less pressure out at the edge, especially with short tie-backs to the transom itself. You might have to slide the dinghy off the platform when launching but that's seems do-able.
An added advantage would be the shorter overall length of your boat, if you're ever renting a transient slip.
Just a thought...the lighter dinghy is a good solution as well.

This is how the Sea Wise is mounted on Great Laker.

Besides the winch being attached to the transom, there is a strap from a dinghy bow ring near the front seat to the transom (not shown). This keeps the bow from moving. With the four attachment points, the dinghy stays perfectly still and stable under high seas.

Also, the transom has a door that provides access to the platform. If we dock on the starboard side, I can remove the strap and use the platform for boarding. Also, when not cruising, the dinghy can be easily docked on the davits (not raised) and be ready for use at any time.

The dinghy is a Livingston 10 with a cat styled hull.

That is how I mounted the one on my previous boat. Never had any flexing problems.

Thanks again. Appreciate the photo of your setup Laker.
I found an old photo from the original listing showing the dinghy on and I see the PO did have a ratchet strap going to the bow of the dinghy.

Greetings,
Mr. 17. I don't know if it's just the angle of the picture but if your swim platform is as deep as it appears, it's no wonder you have a separation problem. Looks to be about 4'. LOTS of torque if so. I don't think extra brackets underneath will eliminate your problem. As I noted in my previous post, you may be best off to add support from high on the transom to the outside edges of the platform.

Another pic I found. Still wonder about the size and weight. And yet there was no crack in the caulk or obvious evidence of flexing when I got the boat after 3 years of use.

Thanks RT.
The swim platform is 2' 10" deep. I agree looks longer in the top down photo.
Yes, probably best to have extra support from above. Brackets below would be partly in the water and ? have to attach lower part below the waterline so not good.

Most bolt on swim platforms have brackets that attach underwater without any problems if secured well and properly caulked.

Sad to say (as an architect and an old fart), that's what engineers are for. Trouble is that the dynamic loads are far more impressive than simply lifting the dinghy up and precious few engineers have that knowledge/expertise. Have it engineered. Or have the manufacturer 'sign off' on the installation; they should have the engineering already done.

Our Walker Bay 10'-4 RIB and Honda 15 hp is much the same problem, but it's carried on and much closer to the transom and not on the swim platform. It flexes the transom and/or maybe the davits themselves. The mfr stands behind the installation.

Supports under would definitely carry the weight better. And as Comodave said, if they're properly installed and sealed, there is no problem with them being under water.

Assuming the center of the dinghy's weight is at the end of the swim platform, you have a constant 1133 ft/lbs of force trying to rotate the swim platform out of the transom. So its no wonder that its pulling away at the boat.

Ken