Swim Platform Construction

[CascoBay]

Hi everyone. I have a 1984 Carver Mariner 28 that had a 20" wide swim platform on the transom. The platform was soft, the core was rotten, and it was not large enough for our uses (dinghy storage and general boarding). Here is a photo of where I started:

Here is a sketch of the supports I plan to install, a total of 5 that will through bolt to the transom with large backing plates on the inside:

The question I have right now is, which method is stronger? There is a lot of stress on the upper bolt and transom using method A. I am wondering if I should go to method B, as I feel more pressure would be transferred through the diagonal.

Any thoughts?

Here is the framing for the supports mocked up.

Stainless steel framing and fittings. Will be using starboard on top and around the perimeter to give it some depth.

On paper this seemed a solid approach, I am just wondering what your thoughts all are. The 5 supports will have bracing going between them to provide support for the starboard. This will extend 36" out vs the old 20" platform.

 

[alldodge]

That's the right idea, I would suggest making them out of some thick material and therefore you won't need to go so far down to get the same support. Use plates on the inside to tighten against.



These are a bit on the thin side but still good enough depending on how wide the platform will be.

 

[CascoBay]

Thank you! My concern is that the platform will be 36" wide which will create a lot of leverage, which is the reason I have such large brackets. I do understand what you are saying about spreading the load with the wider material. If I went with option B on the drawing I would simply enlarge the pads where the brackets hit the hull.

 

[alldodge]

Agree wider with pads but more importantly is the thickness of the material being used

 

[CascoBay]

The framing I am using is very strong, but the contact area against the hull is small which increases load to the boat. I think you are saying to use a wide pad but if I am missing something can you clarify?

 

[alldodge]

My comment was to keep the lower attaching point from going so low, so the brace would need to be made with thicker stock. Yes use a wide base to allow for more attaching points and also to reduce side loading.

 

[CascoBay]

Understood, thank you.

 

[Scott Danforth]

going bigger on the swim platform also means the forces on the transom increase. you may need knee braces inside the transom to tie back to the stringers futher forward. would hate to see a boat sunk from a transom failure from a swim platform install.

 

[CascoBay]

Good point, www.swimplatforms.com installs 36" wide platforms onto this boat without internal modification. I do like the idea of tying to the stringers and I think I may look into doing that on the center 2 supports, as the stringers should line up with these supports. Here is a photo of their install, the shorter supports are where the factory platform ends.

 

[alldodge]

The factory tried to keep the braces above or the water line, doing this helps prevent water intrusion. I hope the lower ones which were installed sealed correctly. The lower attaching points just need to keep the brace in place, the weight of folks pushing down against the hull and bolts needed if a wave comes under it.

 

[CascoBay]

I know my factory braces were well under the waterline, that didn't bother me because I re-bed my fittings and keep a close eye on things. There's also no wood in the transom so that's another plus.

 

[CascoBay]

Here's one someone else built:

 

[sogood]

If you want extra support, without the need for a ton of braces extending so far down the transom, what about the possibility of hanging the swim platform from above? Giving additional support, using two braces, one at either corner of the platform, even using removable chains, which would act like your braces, only above the platform as opposed to below it. Just a thought.

 

[proshadetree]

Sogood most of the time there is not a lot of structural area above the swim platform. Ie thin glass. Also you introduce a trip hazard to wherever you install the braces. I know we jump from finger to platform to the next finger when we dock to tie up. Beats going over the railing.

 

[Scott Danforth]

If you want extra support, ......even using removable chains.

thats just redneck

 

[CascoBay]

I have some aluminum plate that I was thinking about using to make the braces, how does AL and stainless play together under water?

 

[alldodge]

I have some aluminum plate that I was thinking about using to make the braces, how does AL and stainless play together under water?

They will eat each other up in no time

 

[CascoBay]

That's what I was figuring.

 

[sogood]

thats just redneck

​We don't have "rednecks" here in Ireland. That would be an American thing.

 

[CascoBay]

I'm Irish! May be a touch redneck though....

Here's what I'm debating currently. Which option is better?



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