Custom Extended Swim Platform

Discussion in 'Boatbuilding' started by tpenfield, Jan 8, 2019.

  1. tpenfield
    Joined: Dec 2016
    Posts: 236
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    Location: Cape Cod, MA

    tpenfield Senior Member

    Quick update on the exhaust riser extension block issue. . .

    It has been a couple of years and all is well with the swim platform. As noted earlier in this thread, the exhaust risers seem to be marginal as per the Mercruiser spec. I've had the boat out in various conditions and load without issue, but I did want to bring the exhaust system fully into spec, which is 13" of riser height WITH a full load on the boat. Currently the measurement is about 2" shy of the spec.

    Fast-forward to 2021, having used the boat/platform for 2 years. I purchased a set of riser extension blocks as shown below.

    Free-Flow-Riser-2.png

    It took a bit of searching, as I wanted to make sure the extension blocks were the kind that would allow 'cross flow' for coolant between the main ducts of the riser. These seemed to be the correct type, but were in short supply. I was able to obtain 2 sets.

    The engines are closed cooling with sea water going from the heat exchanger to the exhaust elbows, which are stainless steel . . (so they hold up well to the salt water). In adding the extension blocks, I wanted to maintain coolant flow through both the exhaust manifold and the extension block since they are iron. the new configuration is to look something like this . . .

    Merc-Exhaust-Stack.png
    Currently the 'block-off '(separator) gasket is between the manifold and the elbow, since there is no extension block. The new configuration would have the separator gasket between the extension block and the elbow. The gasket between the manifold and the extension block would now be a full flow gasket.

    There is a bit of re-engineering needed, but it looks like everything will work out. The extension blocks (once they arrived) turned out to have internal channels between the main ducts in addition to the surface channels that are visible. So, lots of cross flow.

    Here are a few pictures of putting the blocks in place for test fitting.
    IMG_0554.JPG
    .
    IMG_0557.JPG
    .
    Still a few fittings and some new hoses to be installed, but it looks like this will work out to add 3" of riser, making the exhaust height above the static waterline 18" without load and about 14" with full load on the boat.

    I'll post a few final pics when done.
     
    Last edited: Mar 25, 2021
  2. tpenfield
    Joined: Dec 2016
    Posts: 236
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: Cape Cod, MA

    tpenfield Senior Member

    Quick Update . . .

    Here is the port engine with the riser extension blocks installed..

    IMG_0610.JPG
    .
    IMG_0612.JPG

    The 'down elbows' were in rough shape, so I have a new set on order . . .

    the starboard engine exhaust looks to be in much better shape. . . probably had some prior maintenance.
     
  3. tpenfield
    Joined: Dec 2016
    Posts: 236
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: Cape Cod, MA

    tpenfield Senior Member

    All set, less a few clamps and the down elbows that are on order.

    IMG_0615.JPG

    Everything fits well 'under the hood'.
     
  4. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Not sure if Penfield is active. I never quite understood the attachment to the original platform and whether you designed it for a certain failure weight. A friend on another forum is building one of these and I wondered how you calculated the attachment hardware and if they are calculated to fail if say, another boat drives up on you in the marina.
     
  5. tpenfield
    Joined: Dec 2016
    Posts: 236
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    Location: Cape Cod, MA

    tpenfield Senior Member

    Greetings . . .

    Sorry to hear about your friend who had his swim platform driven up upon in the marina. Which marina was it? and was your friend or anyone else hurt?

    Which forum? Perhaps I can take a look . .
     
  6. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    that did not happen, but am wondering how you calculated the fastener need
     
  7. tpenfield
    Joined: Dec 2016
    Posts: 236
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    Location: Cape Cod, MA

    tpenfield Senior Member

    Good to know that everyone is OK.

    Not sure you would design a swim platform to survive the weight and force of a boat crashing into it, but certainly enough to support all your beer belly friends and maybe some 'bumper boats' around the dock.

    I had 1,000 lbs. in mind for a load, as shown in the earlier pages of this thread and planned the supports accordingly. Based on the geometry of the platform, the loads at the fastening points to support 1,000 lbs. at the extremity of the platform were not very high and therefore basic principles prevailed.

    I'd be interested to see what your friend is designing. Is there a forum or thread that I can check out?
     
  8. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member


  9. tpenfield
    Joined: Dec 2016
    Posts: 236
    Likes: 21, Points: 18, Legacy Rep: 10
    Location: Cape Cod, MA

    tpenfield Senior Member

    OK - thanks. I had to register over on boatbuildercentral, so I'll IM you my thoughts
     
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