Swim Platform Extension Idea for Review/Help

Discussion in 'Boat Design' started by Duradog, Apr 18, 2018.

  1. Duradog
    Joined: Apr 2018
    Posts: 3
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    Location: Grand Rapids, MN

    Duradog New Member

    Greetings!

    I have a 1993 Regal Valanti 203 SE that I recently purchased. I would like to add an extension to the existing swim platform and wanted to run my idea by the group. My plan is to build it myself to keep costs low. I came up with this idea last night and would like someone to help determine feasibility and engineering factors to consider. I will do my best to explain.

    The boat has a 19" deep integrated swim platform that is uniformly 3" thick (See photos). My thought was to place 1/2" thick plywood on top of the existing platform and have it extend another 29" to get a total length of 48". Support would come from "stringers" attached underneath the plywood and extend UNDER the existing platform towards the transom. The plywood would be bolted through the existing platform down to the stringers. None of the typical angled bracing/tubing that is typical of aftermarket swim platforms. Basically it will be a plywood top, with stringers underneath that would be slid over the top of the existing platform (like a sandwich) and bolted together. The plywood and stringers would be glued and screwed together and fully enclosed in epoxy/glass.

    Questions:
    • Is there enough strength in the existing platform (of which I do not know the internal construction) to manage the additional forces generated by the weight of 1-3 kids/adults (500+ lbs) at 29"?
    • Does the plywood with stringer support provide enough strength/rigidity to hold up the additional weight/stresses?
    • What else do I need to consider?
    I have not fully defined the dimensions of the stringers or the fiberglass layup schedule - hoping that can be a part of the discussion.

    I was also hoping we could consider the options prior to just jumping to the "add the bracing and be done" answer. If not for the aesthetics but for the challenge.

    Hopefully my rudimentary/blocky/not to scale drawing (MS Word) is helpful. The blue is the existing platform on the boat.

    Thanks in advance for your help. Please let me know if you have questions or if I missed something.

    Cheers,
    David

    Platform (1).JPG Platform (2).JPG Platform (3).JPG Drawing.JPG
     
  2. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    Freeboard looks about 9" - 12" so when at rest, any waves or movement are going to have quite a loud 'slapping' noise and a fair loading. Could even compromise the existing structure.
    You are also going to add some weight (and weight capacity: people, stuff, following waves) which will make it worse as well as possibly affect the running (and docking) of the boat.
    I saw a similar idea some years ago: the extension (not as big as yours) was hinged and deployed when needed.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Your idea will work, but your engineering approuch will not. The upper portion of the existing platform is part of the deck cap, while the lower section is part of the hull shell. In between there's nothing, possibly some foam, but don't bet on it. When you go to tighten up those through bolts, it'll just crush down and deform (pretty dramatically if you aren't careful), long before you feel they are tight. You'd be best advised to attach just to the liner portion of the step, from inside the engine compartment, which unfortunately isn't a pleasant place to work, with everything in the way (engine, steering, side panels, batteries, pumps, etc.). Ideally you'd glue a significant backing plate under the fasteners, say 3/4" plywood, spanning the area and then you'd through bolt to this, with some hefty washers also employed, under the bolts on the inside. This means you wouldn't drill right through the existing platform (it'll just leak anyway), just though the deck cap. Fasteners will be a pain in the butt to reach in some areas, particularly near the boat's centerline, but doable. Plywood is the cheap choice, though it can rot in time, if not encapsulated, so consider Cossa or other inert material, such as StarBoard. These materials are more costly, but can't rot. Fasteners should be a good quality stainless steel, countersunk or oval headed. These holes should be "bonded" to prevent leaks and provide a hard landing area too.

    As for the portion of the addition that cantilevers over the existing platform, consider some aluminum brackets. These can be bonded and/or through bolted to the hull shell, much the same way as the upper part. You don't need nearly as many as you have shown if sized meaty enough. I'd make three, likely from 3" aluminum structural angle stock. I'd taper the bottoms a touch to prevent drag and the same at the outer ends, much like you've drawn. The angles taken off would be welded in, with possibly an additional diagonal brace on the two outer supports. These would be through bolted to the transom, again over bonded fastener holes and bedding.

    This is a much easier project if the engine and drive are pulled first, but still possible. Given the looks of that Alpha, I'll bet it needs a "transom" service kit, if not more, so pulling it (fairly easy) may not be a bad idea. Pulling the engine isn't all that hard on that hull either, though you do need some tools to do it safely.
     
  4. Duradog
    Joined: Apr 2018
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    Location: Grand Rapids, MN

    Duradog New Member

    Thank you JSL and PAR for your timely and thoughtful responses.

    PAR - I want to make sure I was reading you correctly. For the configuration of the upper portion you said to "attach just to the liner portion of the step" and add a "backing plate under the fasteners, say 3/4" plywood, spanning the area and then you'd through bolt to this". I wasn't sure if you meant add the backing plate into the horizontal space/gap between the deck cap and hull shell in the existing swim platform (See Pic 1) or a vertical backing plate on the transom inside the engine compartment (See Pic 2). I am curious to see how the deck cap and hull shell look from inside the engine compartment.

    I also drew up my interpretation of what you mentioned for the lower portion - 3" alum angle brackets. I did not add the diagonal brace which I assume would be a piece of angle welded in at a 45 deg angle for additional support for the outboard brackets.

    Thanks again for your help on this.

    -David

    Option 1.JPG Option 2.JPG
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The upper is how I figure it should go. The portion of the bracket has a lot more room to live on the transom, which will provide much more leverage on the cantilevered section, once a diagonal brace is installed. I'd make the backing plate at least several inches across. I'd also shape the horizontal bracket to land on the plywood, instead of shimmed as you've shown. This will make for some odd angles, but easy enough to mock up in cardboard or thin plywood.
     
  6. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    You may want to estimate the weight of this "addition" and simulate it on the boat when floating. This "addition" will contribute to total weight, trimming moment, but not contributing to buoyancy.
    ps:
    whatever your weight estimate, add at least 10% as a safety margin.
     
    Last edited: Apr 19, 2018
  7. Duradog
    Joined: Apr 2018
    Posts: 3
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    Location: Grand Rapids, MN

    Duradog New Member

    PAR-
    Ok, for the upper portion we we will be inserting the backing plate inside the existing swim platform from the engine compartment and bolting down through the new plywood platform, deck cap of existing swim platform then backing plate. Stainless fasteners, large washers, lots of sealant.

    The lower portion (if you squint at the rudimentary sketch it looks like AutoCAD) would have the alum angle thru bolted to the transom with a stout backing plate. The bracket would extend past the existing swim platform and angle up to directly contact/support the new platform. Also, add a diagonal brace. I would also think the vertical portion of the bracket would be a little taller than depicted in the sketch.

    How we doing so far? Tracks with what you're thinking?
    JSL-
    I agree that the additional weight is a concern. Once I get the final design outlined I will look at material selection and get a better sense of additional weight.
    Thanks again for your help!!

    Option 3.JPG
     
  8. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    One other item: access to & viewing the I/o leg (ie: fouled prop, etc) will be more difficult- unless you fit a hatch or want to go diving each time. If the latter, make sure water is dead calm and no other marine traffic wash or you could get 'bonked' on the head. And yes, it hurts
     
  9. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    You should be able to bolt a couple of flat brackets to the two protruding flat parts on the transom; the ones where the u-bolts are. I think that the extra wide platform may be a hazard in following seas. I would make it so it folds forward and is out of the way when not in use.
     

  10. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    I’m guessing you want the additional swimstep width to make boarding from the water easier?
    Check out jet ski sleds, they are very versatile, and lately I’m seeing some cool custom items on the transoms of boats.
    They normally attach to the stern tow eyes, at water level. Can be flipped up when not in use.
     
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