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Old 07-13-2016, 09:48 PM   #1
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Swim platform load on transom

Ok, I have searched by just about every keyword I can think of and I can't find the answer to this.

We currently have a 24" swim platform mounted to L brackets through bolted through the transom with backing plates. There are three brackets currently with 45 degree angle supports. I guess that technically is a triangle bracket. With a total length of about 14 feet these seems like a large span between brackets to me. So at a minimum I would plan to add two more.

Anyway, we are looking to replace the swim platform in the near future and are considering a wider one. Something around 36". My question is, how do you determine the structural strength of the transom to support the moment forces of the extra length. As I said, I would add two more brackets but that is more for support of the platform span, it doesn't necessary add support for the moment forces on the transom connection points.

Is there a way to determine the structural capacity of the transom based on glass thickness? This appears to be a solid layed up glass transom integral to the hull so I don't believe there is any core material. Would additional reinforcement pads at the bracket connection points add strength or would the entire transom width need thickening?

Or is the current thickness (unknown at this time) as it came from the factory sufficient?

Any input is much appreciated. The Internet is full of swim platform add on kits and gives information on load capacity of the platform but I couldn't find any resource that discussed the transom load capacity.

Thanks,
David
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Old 07-13-2016, 10:31 PM   #2
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Do you have a dingy on the swim step? If not extending it would most likely not be a problem. If you are concerned though you could certainly add a piece of mahogany or something similar as a back/load spreader.

We have about 400lbs of dinghy and motor bouncing around when the RIB is aboard with no issues.
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Old 07-13-2016, 10:51 PM   #3
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Being able to store a dingy on the platform is certainly a consideration, but not the primary one. We would just like a larger platform. Of course, that will just be inviting more people to occupy it at the same time. So I imagine people load would be every bit as much as dingy load.
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Old 07-13-2016, 11:39 PM   #4
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Starsman:

I have done exactly what you propose. My original platform was too skinny, as the davits are mounted to the outside of the bulwarks, so walking past the davits reduces the available space by 6", reducing the effective width of the platform from 20" to 14". Not nearly enough.
The supports were 5, bronze L shaped supports with a curved brace.
I took the platform off and added 13" to the inside, put Oak 2x6 onto the original supports to extend them out 13" and put it in service. One of the supports lasted only a couple of outings, so I had 5 new stainless supports fabricated. Much stronger than the old bronze ones. 1/4" x 2.5" stock, 1.5" pipe angled between the vertical and horizontal pieces.
I now have a 33" deep x 14' platform, adequately supported to hold all the weight I have ever placed on it, several heavy guys, my 750lb dinghy, no problem.
The transom in aprox 1/4" inner, 1/2" Airex core, 1/4" outer, I used fender washers on the inside of the top attachment bolts, and after 10 yrs or so, see no stress cracks, nothing to indicate any weakness in the transom construction.
If you can't find any information on the construction of yours, drill a test hole (from the inside, to see if yours is as robust as mine. If so,then go for it.
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Old 07-14-2016, 12:34 AM   #5
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I suggest you check to make sure it will not be in the water at various (all) RPMs, especially the higher ones. When we did this we had to raise ours to about 18 inches above the water line to avoid this when the boat stern went down at higher RPMs. If it ends up in the water tremendous up loads can occur. Maybe you could clam 1x2s mid and outer points on you existing platform out to the length of the planned platform and do some runs and see if the 1x2s hit the water. Off shore in larger seas your platform can get swamped..... Ours works but off shore I need to be careful.
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Old 07-14-2016, 05:30 AM   #6
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Increasing the maximum point of leverage by 50% could have a huge effect on loading force. What percentage of the platform is perforated to allow water to go through? On the new platform, what percentage of the outer 12" is perforated to allow water to go through?

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Old 07-14-2016, 06:11 AM   #7
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I take it the boat builder doesn't exist any more? Nor the identity of the architect or name of the yard where it was built? Barring that, a good boatyard can figure this out, it is not an uncommon modification. You just could pay them for the advice/opinion and inspection if you are determined to DIY.
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Old 07-14-2016, 06:47 AM   #8
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I would add a large backing plate of aluminum or better yet SS. It would cost almost nothing and give you the piece of mind that is always a good thing.
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Old 07-14-2016, 07:12 AM   #9
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Kolivar,
Thank you for the detailed description of your modification and your trials with it. I am totally convinced that the existing three stainless bracket are not nearly enough support. I am pretty sure the transom is solid glass at the point the the exiting platform bolts penetrate but I will test that. There isn't much technical information available as the manufacturer is indeed no longer in business.

Rogerh,
Great suggestion. I will try to do that test if we take her out this Saturday.

O C,
None of the existing platform is prorated. I plan for the new one to have perforations and drainage.

Caltexflanc,
Nope, they are out of business. The plan is to perform this upgrade at the haul out later this year. My plan is to definitely get a professional assessment of the transom strength prior to this install, wether from the yard, new platform manufacturer, or someone else. Wether I DIY install will depend on the strength assessment and how much if any reinforcements are necessary.
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Old 07-14-2016, 07:15 AM   #10
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Daddyo,
The existing brackets have a rectangular backing plate for each bracket. I will try to get some pics of the existing install tonight and post them.
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Old 07-14-2016, 07:30 AM   #11
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I added 7 inches to my swim platform using teak strips from another platform I bought. For support I simply added stainless angle iron on top of the existing brackets (5) to support the extra width.
That has seemed to work out fine. My thin Albin transom seems ok with it.
There is no issue when underway with flooding the platform.
The only "issue" I have found is in close quarter handling specifically in various locks or against walls, docks, etc. The few inches makes a difference when turning away with a single screw. Luckily I have a stern thruster to help but I've scraped a couple of times while with the original platform I would have cleared.
Hard to explain. Something you will have to get used to.
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Old 07-14-2016, 07:47 AM   #12
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Jleonard,
Thankfully SOS is a twin screw and I've been practicing my dockside maneuvering. The first time I tried to pull off a side dock I thought for sure I had smacked the platform against a piling. Luckily I had just bumped the rub rail on the side of the boat. I haven't yet gotten her to move truly sideways but I have been able to walk her out enough to clear the dock before making the turn away. Of course that has been in nice calm protected water with minimal wind so far. Probably gonna be a different story when I have to try that in more adverse conditions.
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Old 07-14-2016, 08:59 AM   #13
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One other suggestion is contact Butler, who knows the retrofit platform business better than anyone. They may even have the template for your boat (I saw their sales handbook at the Miami Boat show once and it was amazing how many models of all sorts of vintages they had in there), or at least advice on what will be needed. I've been impressed by their units on boats I've seen. There was a time when we considered putting a wider platform on our Hatteras as it got used all the time for docking the Whaler as well as swimming. Never got around to it.

Butler Marine Products-Manufacture teak and fiberglass transom platforms and anchor pulpits, boat ladders and boat accessories

Another consideration is how low to the water it will be when underway. Will it drag at certain speeds? Too high and it becomes inconvenient, including potentially the dink's gunnels getting caught under it.
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Old 07-14-2016, 09:12 AM   #14
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I already have Butler saved in my bookmarks and we are heavily leaning towards them for the replacement.
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Old 07-14-2016, 09:22 AM   #15
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Quote:
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Jleonard,
Thankfully SOS is a twin screw and I've been practicing my dockside maneuvering...
Stars...

We do a lot of locking and have to watch getting off the wall -
On my previous twin screw I found the following very effective in getting away from a dock / lock wall - give a try and see if it works for you

with fenders alongside - & wheel centered
Rev Port - may or not need a delay - then fwd stbd ( swings the bow out w/o any fwd or rev motion) -
then neutral both (letting any momentum continue to swing) -
Then fwd port - delay - then fwd stbd - to pull away - you can add wheel / rudder once off the wall

Try altering timing between moves based on how your boat reacts

I now have a single but thrusters both ends - easy peasey

Back to transom strength...
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Old 07-14-2016, 12:38 PM   #16
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I already have Butler saved in my bookmarks and we are heavily leaning towards them for the replacement.
Have you discussed you load bearing concerns with them? They'd have a lot of expertise in that regard.
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Old 07-14-2016, 04:20 PM   #17
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I haven't contacted them yet. Was trying to do some research on my own first. But no luck there. Will contact them pretty soon though as I have to get this platform stuff nailed down before scheduling the haul out.
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Old 07-14-2016, 04:21 PM   #18
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Bacchus,
I will give that a try. Our Marina pullout is a perfect place to practice because it is side dock, well protected, and minimal traffic.
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Old 07-14-2016, 07:00 PM   #19
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Bacchus,
I will give that a try. Our Marina pullout is a perfect place to practice because it is side dock, well protected, and minimal traffic.
I think he means pump out. Lol
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Old 07-14-2016, 07:02 PM   #20
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yes, I meant pump out. i really need to remember to proof read.
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