Transom Construction

I am building a 16.5 ft fibreglass boat with a deadrise of 18 degrees. I have completed the mold and am about to lay up the finished product. When complete, I would like to fit a Honda 75 hp/90 hp 4-stroke motor with a 25 inch leg. Obviously, I want to make sure the transom has sufficient strength to carry and operate the motor. I believe there are alternative materials that can be used but I am not sure what core material thickness or glass layers to use. Can anyone help me with information on the design or construction of a suitable transom? Thanks.

 

You can try a couple sheets of marine ply in 3/4 inch thickness. I've rebuilt a number of transoms like this and put an older 65 hp Merc on them.

There is a place in Edgewater FL that sells a pour in foam for transoms. Might be called "pour a transom" or something like that. They may be willing to give you some detailed information.

 

All outboard brackets are designed for a transom between 1 5/8 and 1 7/8 thick.

 

no rot transom

i mix saw dust and vinylester and pour the transom.the most power to date is 2 250s and no problems.

 

Seacast

The pourable material is Seacast and is available at http://www.TransomRepair.com I have a 1981 18' Sea Star with a rotted transom and that is the material I will be using to replace the wood.

 

Its weight an issue for you. I have 380horse going through a 70mm transome. The point I wish to make is that a good transome will evently load the stress into your surrounding hull by the floor and sides.

My transome is a glass/1' marine ply/glass/1' marine ply/glass sandwich.

So in my opinion, a glass/1' marine ply/glass transome will be fine as long as you laminate it into te floor and sides with loads of UD glass.

 

It has been touched on, but must be reinforced. The transom must not only be strong and watertight, but also spread the loading expected to the structure, usually through the use of stern knees, gussets, stringers, keel and other structural members. Only in this way can the loading be transmitted to the hull. If a transom is just tabbed into the hull's after end, the laminate must be quite beefy. Most manufactures solve this by using a substructure, under the sole or in the well (including the well shape itself), spreading out and forward to increase the load bearing surfaces to the hull and/or by using a deck cap to increase the tendency of the stern to twist, bend and other wise lose some of it's force transmission to the hull.

You'd be surprised on the amount and types of loading that occur by hanging an outboard on a transom. The leverage of the bracket spacing the engine just aft of the transom creates surprising additional static loads just hanging there let alone when powering up to plane.

What design are you building? Got plans? What's the designer's suggestion and why do you want to differ from it?

 

DIAB (the makers of Divinycell) have new product that we are specifying for our transoms - its called Renicell. Its not quite as expensive as H130 in 2". I am going to be using it for the transom of a 16 foot flats boat Im working on. I plan on putting between 50 and 90 h.p. on it, so its about the same as what you are looking for. Hope that helps.

Joel
Boatbuildercentral.com

 

Transom Laminate?

I am about to rebuild the transom of my 1972 Mako 19. The boat has a Mariner 150 hp. engine which is heavier than original spec. so I want to keep the stern light.

I have calculated that if I use a 12 lb/cuft high density foam, I can save about 40 lbs. over a plywood transom. So....

1. Does anyone have experience with Divinycell H130 on a transom with this large an engine?

2. What about Rehicell? It conveniently comes in a 2" thick panel, specifically for outboard transoms but I am not sure it is meant for engines this big.

3. I am planning on using Knytex DBM1708 fabric for the skin laminate. Two layers should be about 0.12 in. thick (approx 1/8 in.). Will this suffice?

4. I am thinking I will still use plywood in the core for the area immediately under the engine mount, just to take the compression of the mounting bolts. Does this make sense or can the high density foams handle the bolt loads?

5. I am going to rebuild the aft lockers, make them wider (inboard faces colser to centerline), make them deck height, and tab the inboard faces to the transom and hull bottom to help support the transom. This should help minimize flexing which can break down the core.

Any feedback would be GREATLY appeciated.

Thanks

Sean

 

Hold your horses

Sean,

Although you will have a significant weight saving by using foam, think about the differiences in terms of mechanical properities. During operation of your boat, the motor will excert a considerable bending moment onto your transome. The resultant giving rising to huge shear loads. A glass/foam/glass sandwich will normally fail with a inter-core shear failure, provided your skins dont fail first.

Although you can calculate the mechanical properties of your core, personally if that was me, I would make a sandwich panel, and test it to destruction. Its more reliable. Perhaps make a test panel and take it to your local university engineering department. Ask them nicely to test it on their instron machines via a 3 or 4 point bend method.

Hope this helps as you dont want your outboard jumping off at sea.

Buckle