Mounting a Yamaha t9.9 high thrust on a new swim platform

Discussion in 'Fiberglass and Composite Boat Building' started by missinginaction, Apr 15, 2016.

  1. missinginaction
    Joined: Aug 2007
    Posts: 1,097
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Hi Folks,

    I'm looking for some input as I build a new swim platform for my restored 1973 Silverton Sedan. As you can see from the photos below the existing set up (from 1973) leaves a bit to be desired (upper left) in the modern world.
    I mocked up a new platform, this one will be about 7' wide and 30 inches fore and aft. You can get the idea from the two center photos. I have access to a wide variety of hardwoods and will build this out of Philippine Mahogany, I'm used to working with it since the rest of the boat is made largely of it. I'll have to splice together some boards. I may spline them or plane them down to 1/2" and make up a laminated platform 1" thick (two layers of wood with the seams oriented 90%) 3 coats+ epoxy and covered in 6 oz. cloth. My plan is to place the new platform right over those two aluminum "pads" and bolt it down.

    The little engine weighs about 120 lbs. the swim ladder weighs about 20 lbs and a couple of people back there could raise the weight to about 500 lbs total. I need to reinforce the transom as it is mainly just 1/4" fiberglass.

    If I glass in 1/2 inch MDO plywood to the inside of the transom, AND add 3 or 4 supports underneath the transom (stainless steel tubing), will that be sufficient reinforcement? I have access to 12 or 17 oz. double bias cloth but am unsure as to how many layers might be necessary. It's dry back there and the light reinforcements there now have been in place for 40 years and are sound.

    Regarding the mount for the outboard. I see some from Garelick and Panther, and some 1/4" aluminum plate mounts welded up on e-bay but I wonder about strength.

    Is there a reason why I couldn't just use 1/2" 6061 aluminum angle with 6" legs. I mocked up this arrangement using some scrap pine boards. With a solid 1" platform, I'd through bolt two pieces of angle (sandwich the platform) and then bolt a solid board for the aft face (where the outboards bracket would rest). It would seem to me that this would make for a secure mount, but I've never seen it done this way, so I'm wondering if I'm missing something.

    I saved my money for some time to buy this little outboard and would hate to see it take a swim.

    Thanks in advance for any comments you may have,


    Attached Files:

  2. helluvaboater
    Joined: Jul 2015
    Posts: 50
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: Maui

    helluvaboater Junior Member

    Wish I could tell you for sure and I doubt you will get any definite answers on here.

    I would say you will be OK as long as you beef up the inside with more plywood. I'd tab the transom in more on the inside with a few layers of biaxial all around. You definitely need 45's that run up from the bottom of the transom.

    I am planning on doing something similar on my boat. It's a single-screw 27 foot diesel, but I am going to make a box similar to the attached pic but without the platform on the sides. Those high thrust Yamaha 9.9's are sweet little motors that put out a lot of thrust. Good choice on the engine.

    Attached Files:

  3. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    I had an emergency get home type setup for an outboard on the platform of my olde Mainship. It was made of 2 x 4's and plywood and clamped to the existing platform.

    Didn't work too well for me, might for you as you're building new and I was adapting to existing platform. I was trying to use short shaft dingy motor and the pitching caused too much aeration. I also had a real eye opener when I was how thrust was transferred to my cobbled together bracket.

    The outboard thrust is at the propeller, and is trying to twist the top of whatever bracket is on the platform aft. Be sure the bracket and the area around the bracket is up to this twisting affect. I had the same engine on a sailboat and the thrust is substantial, as is this twisting affect.

    From there the rest of the platform has to transfer thrust onto the transom. If you plan to steer than those thrust lines will be all over the platform. If you could position your brackets so they line up with the stringers and then install knees or braces inside from the transom to the stringers so the thrust is driven from the brackets to the transom through the inside braces to the stingers, it would help to make this more bulletproof rather than random spacing. Maybe 3 additional braces between the 4 brackets that will attach the platform to help with vertical support, transom reinforced at each place.

    Just a few thoughts

  4. missinginaction
    Joined: Aug 2007
    Posts: 1,097
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Thanks for the responses. I'm still laying out this platform. I ended up buying an outboard platform bracket made by Panther Marine. A little pricey, but it looks like it will hold up. Rather than relying solely on wood I'm backing up the platform in high stress areas with some 1/2" 6061 aluminum plate. These stiffeners will keep the platform from flexing and transfer the stresses to the transom, which gets reinforced with 1/2" to 1" of MDO/fiberglass cloth/epoxy and reinforcements where the transom intersects the stringers.

    Amateur engineering, fun, fun, fun.

  5. missinginaction
    Joined: Aug 2007
    Posts: 1,097
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    After thinking about it for awhile I decided to go with a manufactured outboard platform mount rather than design my own. Once I actually had this mount in my hands, I realized I had made a good choice.

    The platform is now completed and just needs some drainage openings and finishing. Weight of the wood is about 60 lbs. Dimensions are about 70" wide by 32" fore/aft.

    I decided to recess the outboard mount into the platform for a couple of reasons. I thought that reducing the moment-arm or torque on the platform and transom was a good idea, and I also liked how it looked!

    I haven't seen many posts regarding projects like this so perhaps this will help someone else as well

    I'll post a few photos below and add more as I go along. Next part of this project is some transom reinforcement.

    Regards, MIA

    Attached Files:

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.