Building a motor mount transom

[lakshmi369]

I am working on a rebuild of what I think is an early '70s mini hydro. It was a stripped down hull when I got it. Originally it had a motor mounted to a mechanism with a rubber skirt that put the outboard motor right through the bottom of the hull. All this hardware is long gone so I have fiberglassed the opening in the hull closed and am going to build a transom style mount which would be more traditional. The problem is that there was no true transom due to the original design and there is only 1/4" or so of fiberglass shell where the transom would normally be. See picture. Anyone tackled this one before? Just FYI this boat will be used for recreational fun I think with a small 15 outboard on it.

[johnsonm50]

Could you post more pic's from different angles of the area. I think it would be best to flatten the FG inside the transom as much as a piece of plywood fitted in could fit flat to spread out the stress as a start. I'd advise 1/4" okuome [?] for this to keep weight under control. Then you have a good base to build a proper transom. Are there any stringers in the bottom? You should somehow structurally attach 'knees' between the new transom & bottom to stiffen the angle. Another thing to consider is why was the well set up used to begin with? It wouldn't be too difficult to re-fabricate something like it. Researching the hull would be a good idea, it has a factory appearance & there might be some info available.

[lakshmi369]

Thanks Team Tower. I could find no maker info info and the work inside looks homemade. Already committed to the transom mount. No stringers through the bottom of the hull. Am thinking I will need to build a framework of supports where the motor used to sit. More pics to follow.

[lakshmi369]

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[Guest]

The transom looks like it's only 1/16" or 1/8" thick. Did this boat have provisions for throttle and steering? Are you sure it wasn't used on some kind of "Kiddy Ride"?
If it a real boat, it needs a beefed up transom for sure. If it was mine, I would epoxy marine plywood to one or both sides of the existing transom to a thick of at least 1" thick or more. I would also add gussets from the top of the reinforced transom to the top side of the boat's bottom surface. If the bottom was thin I would also add a 1/4" plywood to this same top side to stiffen it up.

[lakshmi369]

Great ideas. I like the gusset idea to cut down on weight. On the bottom the builder had incorporated a sheet of aluminum on the outside of the fiberglass floor. The only "stringers" run under the seat from side to side. Two fiberglass covered plywood pieces. I will post pics as work gets done.

[johnsonm50]

!st, it's a cool project. fiberglass boats almost [if not always] have wood as backup re-enforcement. Reason being to make the glass itself strong enough it would be awful heavy. I know you're not going all out racer with this so you'll probably do fine. If once you get the 'go fast on h2o bug' & want to race... we'll be here.
The pictures help so now you can get good input. I'd go with what I wrote before about starting with a 1/4 piece covering as much of the transom interior as it will set flat on. This will help disperse energy avoiding a future failure line at the main transom/motor mount. I agree with ProHydroRacer about doing the floor the same way at least enough to overlap the estimated planing area. It is important that the adheision be near 100%, standing the boat so the transom is down & putting weight on the glue up will help. Then I would add stringers, 2 main ones that you ride between & where the knees/gussets attach. If the bottom seems too flexible then smaller sub stringers will help.
Where the motor mounts: I would advise about a 14" to 16" wide main transom at about 1-1/2 thick, The outsides of this should align with the outsides of the stringers for knees & any deck support gussets as well, You could combine the knee/gusset as one piece, might be easier, stronger & look cool too. As far as ply on the outside, I'd put one piece a bit more than the size of the main transom, again to disperse energy avoiding a break point. Keep an eye on weight as you go, we're not talking heavy timber here, just enough to beef it up some.
The boat is by far most likely polyester resin /glass, good news being either poly or epoxy resin will work with it. You can smell it when you cut into it, the poly is strong & offensive, the epoxy is different, more subtle. If it turns out to be epoxy then use epoxy-not poly.
As far as technique & how to's think of things as dual or better function, consider motor set up, steering, throttle.. all at the same time as you think about structure. I had some skills when I built my 1st but knew little of race boating. It's still among my favorite boats, hope this works out like that for you. Good Luck.

[lakshmi369]

Thanks guys! I appreciate all the insights. To answer the earlier question. It was a full blown racer back in the day. Gas pedal on the floor, pulley and cable steering, etc... I have been unable to find any press article but reportedly crashed/sunk on its last run. Been stored away and canibalized over the years. Former owners nephew only had sketchy info. Having a good time bringing it back to some sort of fun life. This is my eighth boat resurrection but has some unique challenges.