Motor Mount Attachment to Hull
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Motor Mount Attachment to Hull

  1. #1
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    Default Motor Mount Attachment to Hull

    I am rebuilding my 1976 Sleekcraft Aristocrat. The new engine will be a fairly mild 489.

    The question that I have concerns the attachment of some new aluminum motor mounts to the hull. The original motor mounts were just a block of wood with a layer of roving over the top and along the hull to keep them in place. Of course, these mounts weren't in the right place for the new engine so I cut them out and have had some aluminum motor mounts fabricated.

    I would like to bolt the mounts to the hull, but the fiberglass isn't thick enough to allow for this and I would prefer to not shim the mounts farther inboard by installing a false set of stringers. At the moment, the best option that I can think of is to glue the mounts into the boat using 3M 5200. According to 3M's data, 5200 has a minimum bonding shear strength of 362 psi and even more in tension. Between the two mounts, I have 192 sq. in. of bonding surface. This would give me nearly 70,000 lbs of shear strength.

    If anyone has a better idea, please let me know.

    I have two pictures of what I'm dealing with. One is where the old wooden mount was removed. The other shows one of the new mounts.
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    Senior Member OFFSHORE GINGER's Avatar
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    Not to be rude but could you please post up some better pics ?

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    Senior Member Sleek Freak's Avatar
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    its hard to tell if you have a v or a tunnel. most v bottoms came with 1/4 stringers.
    Sleekcraft SST Tunnel Jet
    Last edited by Sleek Freak; 03-22-2016 at 08:17 AM.

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    Sleek Freak, the boat is a tunnel hull.

    Here is a sketch of what the original motor mounts looked like. Unfortunately, I don't have any better pictures right now.
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  7. #5
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    I found the original construction sketch that I gave to the fabricator. Perhaps this will help explain what I have to work with.

    If someone was serious about offering help, they are welcome to call me at 832-286-2930.
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    Last edited by boatbroke; 03-24-2016 at 05:31 AM.

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    Now that the temperatures are dropping, I'm going to be able to start making some serious progress on the build again. Does anyone have any recommendations for how to attach the motor mounts to the hull?

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    Just thinking out loud here - I might try bonding them to the hull with either Epoxy or 5200, and then encapsulate them with roving or other fiberglass material using Epoxy resin for that added bond and surface area to spread the load.

    just my $.02

    Doug

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    Thanks Doug. I hadn't thought about encapsulating them with glass too. That's a good idea.

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    Senior Member OFFSHORE GINGER's Avatar
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    How much thicker is the mount at the base of the stringer or where it is located and could the base be machined down to be flush on both sides of the stringer ( height ) leaving the top thicker . just an idea ............................

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    Offshore, I'm not sure I understand what you're suggesting. It sounds like you're saying I should remove fiberglass so that the aluminum will be flush with the fiberglass.

    Is this correct?

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    Senior Member OFFSHORE GINGER's Avatar
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    I have an idea put I need to know if the motor mount is flush with the stringer from side to side towards the bottom or width ( thickness ) of the stinger .
    Last edited by OFFSHORE GINGER; 10-14-2016 at 06:11 AM.

  14. #12
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    I'm interested to hear your idea. Feel free to call me if it would make things easier. 832-286-2930

    The engine will be supported by some aluminum supports as shown in the hand sketch provided above. The motor mounts will bolt to the top of these supports and include a single threaded stud sticking up. A bracket bolted to the engine block sticks out and has a hole that the motor mount threaded stud will pass through.

    The aluminum supports are not intended to be flush with the surface of the fiberglass at the current time. The current idea is to have them butted up against the existing fiberglass without removing any of the fiberglass. I do believe that the fiberglass is thick enough that I might be able to mortise the brackets into the fiberglass.

    The boat doesn't have stringers in the classical sense. Instead, the bilge is framed on the two sides by a vertical section of thick fiberglass to make the two tunnels. The computer sketch above shows the interior surface of the fiberglass that you see in the pictures as well as the bottom of the boat. The trailer bunks pass through the two tunnels you see.

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