Boat Builder Central - Builder Forums

One thing I don't like is that if another boat ever comes up on your swim platfrom; the knees will likely puncture the hull below the waterline and with the weight of the inboard, might sink the ship.

For this reason, it would be good to make sure the knees can break. Consider a weak point that would be a structural failure point if another vessel/dock comes up on top.

You may already have done it and the margin between the flat and triangle might be fine. It looks really strong to me. (Too strong perhaps?)

I am an overbuilder as well; so this would be really hard for me and I would want to build as you have shown.

The guy that built the tw28 in Turkey had coined in on the subject. I don't profess to be the expert.

Like I said before, I really love this design and still wonder about building it someday; so am very tuned into the details.

The work you are doing is superb Reefie.

Here is a picture of one with a failure/breakaway; although in bronze which might not break; so they added mahogany as well.

Of course the risk is in heavy seas; underdesign breaks off, so it is challenging; hopefully Ilker will coin in.

Also, if you metal bracket to the transom; the brackets can be sized to fail..

Sorry if I am a pain..
I mean metal bracket, not like the picture, but like commercial platforms where they are designed to crumple when the boat hits something.

I have seen guys back into slips with swim platforms. The smart guys back into inflatable dinghies.

This is the bracketing I mean. It would be a lot less fairing. A couple pieces of 316 tubing; not too costly; you can probably get the mounting hardware for the angles, too, or get them made.

I think you pad the area of the metal bracket on the transom with high density core so the bracket doesn't pierce your hull.

I don't know the crumple strengths of the various tubing, but 3/4" ss tube used for railing has a good gut feeling for me.

You can probably purchase the same railing as you may? use for the top railings.

First that is some beautiful work you are doing I think three supports looks about right also. For the trim tabs it is my understanding they will be much more effective mounted closer to the outside edges. It looks like you have plenty of room to mount the outboard of the supports. Others may know more

Thanks Fuzz and Fallguy. The design of the swim platform (and I must admit that I am no engineer , and I also tend to over engineer just to be sure) has been something that I have given considerable thought to with regard to the concerns that have been raised. Here is my take on the design;
1. The transom itself is constructed of a very robust lamination schedule … 6 layers of 450 gsm layers of alternating double bias (+45 and -45) and
450 gsm biaxial (0 and 90) on the outside. There will be 4 layers of alternating 450 gsm double bias and 450 gsm biaxial on the inside. The
transom core is 275kg/m3 paulownia.
2. The swim platform will be sheathed in 2 layers of 450gsm double bias top and bottom (overlapping at the bezel) and a single layer of divinycell
(80kg/m3) for the core, and the bezel itself will also be strengthened with a further 2 layers of 450gsm double bias tape.
3. The knees I plan to cover in 2 layers of 450gsm double bias.

In the scenario put forward by Fallguy in which sufficient downward force was put on the swim platform (for example anoher boat running up on to it, a massive wave breaking on it, coming up under a dock etc…) I can see that there will be three possible break points;
1. The knees pushing through the transom
2. The knees buckling/breaking
3. The swim platform pushing past the knees (i.e. the knees piercing the swim platform)

Given that the transom construction is magnitudes more robust than the swim platform deck, I see the deck giving way long before the transom ever would, and it is most likely (given the shape of the knees) that they would also buckle/break before piercing the transom.

I guess we only know for sure if, heaven forbid, it gets put to the test in a real life scenario.

I have also contemplated using stainless struts in lieu of the knees but I’m not a big fan of fasteners below the waterline if they can be avoided. I also prefer the look of an integrated swim platform.

With regard to the placement of the trim tab actuators, and to keep them below the swim platform, the length of actuators dictates how far outboard they can be placed – which is about where the black texta line is marked. It would be preferable to have them further out towards the chine, but like so many aspects of boat construction, there are always compromises. Also, at this stage I do not plan on installing trim tabs unless sea trials deem them necessary (but I want to plan for them just in case).

I really value the input of forum members as it acts as an external QA method (a benefit previously stated by Fallguy), and also may raise issues and ideas that alone may not be considered.

Please keep up the input guys.

Happy building,
Cheers, Reefie

A convincing reasoned response to concerns raised.

Hope your next crop somehow defies the drought.

Happy farming and boat building,

This sunseeker was drunk and backed into me at night and pushed my TW28 to harbour wall
My bow opening the back of Sunseeker



My Swim platform after impact from both sides

My design to make this 2 tier

outside is disposable in case of impact and as you can see, no damage to hull, only to swim platform.

Build on Reefie!

It is beautiful and I eagerly await the next bits.

Hope you remember the rubber dinghy trick if you ever need it.

Thanks GB,
While our district has been experiencing a decline in rainfall, we are definitely not in drought. All our irrigation dams are full for the summer ahead
Farming for me is my passion - I am (nearly) always happy


Thanks Fallguy - the rubber dinghy trick is now locked into my memory bank

And thanks Icelikkilinc - it was in response to your post on your unfortunate swim platform incident that made me modify my plans

Happy building,
Cheers Reefie