Transom bracket: Am I doint this right?

[Gunnerbill]

Am I doing this right? I am building a cold molded,16' cabin day cruiser using clear oak frames and bs1088 marine ply. The 1940's design features a "retrousse" or reverse transom. In order to power the semi-v planing hull with a 30-50 hp outboard, I needed to change the angle from +5 to -12 degrees in order to obtain the "tuck" required without significantly adding setback. I intend to incorporate this frame into the transom construction and add additional transom and 1/4 knees to transfer thrust & torque to the vessel's frames. The frames will be finished with the same hull marine ply (3/8), and glassed. A photo is included and your comments are welcome.

[Landlubber]

...yep, that sort of thing ios what is required, some make them filled in some make em open design like yours, but at the end of the day it is required.

[Gunnerbill]

Thanks, Landlubber. I was worried that I was way off on my thinking. When it's done, it will be enclosed (taking the form of the hull shape) and filled with closed cell foam for rigidity and buoancy. I'm glad you answered.
PS Not sure how Brisbane celebrates but Happy Holidays from Toronto!

[viking north]

Gunnerbill-- why such a large engine (30 to 50 hp) on a sailing craft.?? I know i'm old school but will a 16ft. handle that much weight and HP. If so and this is as per the design specs. I recommend not building the bracket out of wood but do it in welded aluminium and if weight is not a factor maybe in steel (have it hot dipped galvanized) or stainless. If the photo shows your proposed design I fear it will not withstand the stresses of a 30 to 50 hp. engine--Geo.

A yacht is not defined by the vessel but by the care and love of her owner---

[m3mm0s rib]

Something more full of it would be better

[Gunnerbill]

Hi viking north, thanks for replying with your thoughts & detailed info. Allow me to make a slight correction if you will...she's a cabin cruiser day boat, not a sail boat (day sailer) and meant for engine power. I apologize if my initial description led you astray. Her full dims are as follows: LOA:16'91/2" LWL: 15'6" Beam: 6'4" Hull: Semi - V Planing. Think of her as a runabout with a wee little fore-deck, a small cabin for two and cockpit aft.
Yes, the photo is my proposed design. It is currently as depicted: incomplete, all gussets not there, not glued nor fiberglassed yet, missing marine plywood all around and no motor mount yet. If I go this way, the finished bracket will take the shape of a hull and look like one of those after-market, bolt-on transom extensions with swim platforms you see on the internet...I hope.
You make a sound argument for aluminum or steel and indeed may be the way I go as I have also considered the torque factor. Perhaps a consensus on this thread will sway me...

[viking north]

Gunnerbill --see now I incorrectly read into your post it was a day sailer-- The framework you have constructed if well bolted to the transom and boxed in with ply, epoxy glued and screwed should be very strong but my preference for these motor hanging extensions has always been in metal, usually a welded boxed arrangement of 1/2 in. alum. plate or at least welded alum. framed.It's hard to say if your design is sufficient without being on site to look over the transom construction makeup at the attachment area. Once a sufficiently strong enough motor mount extension unit has been built the key areas to pay attention to are that the attachment fasteners be of sufficient size and numbers as well as be carriage heads or seated in big washers on the frame and a backing plate with big washers, locking nuts, locktite, on the inside of the transom. What you have built could be ok but it might be a good idea to have an experienced/qualified person have a look at it. If for nothing else, peace of mind of not losing a 50hp. motor to the deeps. ---Geo.

[PAR]

I'll second the comment of Viking and add you'll need at least 1.5" of thickness on the bracket's engine clamp pad. If expecting to use bigger then 60 HP then 1.75" to 2" would be required. Bond you fastener holes and make the assembly a monocoque structure with the old transom, with substantial tabbing, preferably biax.

[Gunnerbill]

Thank you everyone! This has been most helpful with the project. You have recognized possible weak points that I didn't consider. It seems that todays motors are SO powerful that traditional methods are insufficient in this regard. I will take it under advisement that I should seek out a suitable fabricating company and have them assemble a bracket to my specs. Question: When a frame is manufactured from, say, aluminum, is it possible to attach the marine ply in order to create the "hull" shape I described? If so, how?
Thanks again...

[viking north]

Gunnerbill if the frame you have made up in wood allows you to sheet over and blend in with your hull/transom -no reason why it's exterior shape can't be duplicated in welded alum. box. tubing and likewise sheeted. I am a collector of disgarded fiberglass pieces that contain ample areas of flat surfaces. From these i use the flat surfaces in place of marine ply. I simply taper the edges to allow corners and joins where necessary, treating it as taped seam construction. This saves the labour intensive and messy steps of glassing over ply. I recently scrounged 40pcs. 2ft.x 10ft flat panels 1/4 in. thick for $5 a piece from a scrapped "Merry Go Round". I will make ample use of these on my build. In your case you would simply have to fasten such with say 1/8 stainless pop rivets along the edges and glass over the heads with the taped seams. Where a fastener has to be visible accent it using a low profile head stainless screw. ---

A yacht is not defined by the vessel but by the care and love of her owner--

[Poida]

The question I would like to ask here is, if you attach an aluminium bracket to the transome because the timber bracket may not be strong enough. Wouldn't the torque of the motor be transfered to the transome that is made of timber anyway. The theory that a chain is only as strong as its weakest link.

Would it not be better to cut some holes in the transome so you can pass some battens through to screw and glue to the existing battens inside the boat, and extend out to form the motor bracket.

It can then be finished off to suit the existing hull shape.

I feel that this construction would transfer the torque of the motor over a larger area and make the extension an integral part of the boat.

Poida

[PAR]

The previous transom (assumed) should be capable of handling the loads imposed by an outboard. In which case attaching to it is sound. Cutting through this transom is just silly, invites another leak point, possably introduces stress risers, etc.

A wooden extension will work, again if well tabbed to the transom. An aluminum bracket can work too and the manufacture will likely supply hefty backing plates for the mounting bolts.

[Gunnerbill]

Intersting thoughts, Poida. I know what you are saying by suggesting the bracket frames be part of the transom frames - and in a way, you are right. I thought of that too. I would hate, however, to cut the transom frames just to add more wood. Whether or not I use the wooden frame (as in the photo) or the recomended aluminum frame, I would still need to use through hull fasteners as viking north and PAR suggested. As a result, I need good, sound stringers to pass the bolts through. (yes PAR, even though the transom angle is reversed, it is still fully framed and kneed and is capable. I like the "monocoque"...I had to look it up...lol)