Making up a swim platform - with a twist.

Discussion in 'Fiberglass and Composite Boat Building' started by missinginaction, Nov 28, 2015.

  1. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    I'll post a few photos as I know they always help.

    It's time to do something about the back of my boat. We get quite a bit of exhaust in the aft cockpit and I'd like to add an auxiliary outboard to this boat. I've thought about how I could kill two birds with one stone and build a solid swim platform that would help with the exhaust issue AND incorporate a removable center section that would allow the use of a small outboard bracket. I'd like to modify the existing small swim steps that are part of the original boat but I'm concerned about stress on the transom. Here are the facts:

    1. 25 1/2 foot Cruiser, 10 1/2 foot beam, weight (I'm guessing) 8,000 lbs +/-.

    2. Auxiliary outboard proposed, Yamaha 9.9 HP High Thrust mounted on vertically adjusted bracket. Weight of bracket and OB = 125 lbs +/-. OB to be located on boat center line. OB locked in position for most use, steering through the boats rudder. I may incorporate a remote for starting and throttle on the OB.

    From the photos you can see that Silverton installed aluminum swim pads on the back of the boat when it was built. They're perfectly functional but as you can see they're not doing much for my exhaust problem. The pads themselves are 24" wide by 12" fore and aft and extend 18" back from the transom. These pads are 9" above the waterline and 24" above the bottom of the "V".

    My plan is to fabricate two pads from 1/2" exterior grade plywood. These pads would butt to the transom and extend aft 24 to 30 inches. They would be 30 inches wide (6 inches wider than the existing pads) thus widening the usable area. Each solid pad would be bolted to the existing slotted pads with the inside edge even with the existing design. This would leave an opening on the center line of the boat that would be about 15 inches wide.
    Since the outboard is to be mounted on the center line it would be lowered between the pads during use. When not in use the the outboard is raised for stowage and a removable center board would be installed into the 15" wide area between the two pads to keep the exhaust fumes out of the boat.

    The best way to picture these pads is to think of two rectangular or square pieces of ply. The ply is to be wrapped or "picture framed" with mahogany using a tongue and groove technique. Of course everything is epoxied, three coats and a light fiberglass cloth (4 or 6 oz.), plus primed and painted and through bolted over the existing slotted pads. The removable center section slides into the space between the two sections. The same T&G method would be used to secure the center section and a couple of pins could be used to keep it in place.

    Questions:

    That transom is about 1/4 thick solid glass. As it exists today it flexes a bit when I step on the pads. There are small backing plates inside the transom (about 6" x 9") where it bolts through, fender washers are used to spread the load out a bit. I'm thinking that I should expand the transom backing plates considerably, perhaps planing down some 5/4 mahogany boards to 1/2 or 3/8" and attaching them to the transom (inside the hull) with removable screws and bonding with epoxy. How far is reasonable to go with this? How thick?

    Should the OB be mounted on the center line in the first place?

    Should I consider stainless steel tubing as a support option for the platform pads? I'd prefer not to go this route but if it is necessary so be it.

    As it sits the transom is functional. How much additional reinforcement is reasonable given a modest increase in what the transom will be asked to carry?

    Thanks in advance for any advice or opinions that you'd like to share.

    Missinginaction
     

    Attached Files:

    Last edited: Nov 29, 2015
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The weight of the additional pads is of no importance, but if you make them 30 inches long and somebody stands on the edge the load on the aluminum brackets almost doubles.
    I would go no further than 20-22 inches without additional support tubes.

    And I would install much larger backing plates. If the transom flexes under your weight you don't feel secure even when you know from experience nothing will happen.
     
  3. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Thanks CDK. I was wondering about the cantilever effect going longer fore and aft so perhaps a few stainless support tubes would be a good idea. Haven't heard anything else so I suppose I'm on the right track.

    Regards,

    MIA
     

  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Whether the extensions affect the exhaust fume circulation is subject to experimentation, wind direction, and a few other variables. Part of the deal is the speed that you are running. At idle speed the fumes gotta' go somewhere. Your idea seems worth a try.

    Perhaps it would be worth while to attack the problem at its' source. If you have excessive exhaust fumes, your engines may not have optimal fuel/air ratios which can be adjusted at the source, the carburetors or the injector programs. If the fuel mixture is too rich, hot but unburned fuel is discharged at the exhaust outlet and is pretty stinky. Not only that, the fuel consumption figures will be unfavorable in such a case.
     
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