Transom reinforcement question

Why an outboard bracket?
Generally, not always, outboard jets are mounted on the transom and include a bottom tunnel to be able to lift the shoe, (intake of the jet) up a bit for shallow running. (note tunnels can be a problem when turning tight
as they often ventilate- another topic)
Is your intent to keep the intake below the main hull bottom then, certainly doable and will eliminate the ventilation problems with a tunnel.
You say " the transom is about 1/8" inch. About or is it 3/16? Most jet boat builders use 1/4 for the transom but I have seen 3/16

How far will the outboard be set back from the back of the transom. The lowest part of the jet leading edge of the intake should not be immersed much below the bottom of the hull or the rest of the intake and pump profile will
spray up copious amounts of water around the motor.
So the two options to minimize this are
1) when the engine is mounted on the bracket, you will have to extend the trailing edge of the hull bottom up to the leading edge of the intake
2) modify the transom, incorporate an engine well, it can be quite narrow so as not to lose much interior space, the modified well can provide strength to hang the engine off. I do not have the specs here but you need to know the height of the
transom where the motor attaches to the bottom of the hull. Often it is not a standard 20 inch.

What will happen if you just offset a bracket and do not extend the hull to meet the intake of the jet?

The jet intake will be free running. As mentioned above, the leading edge of the intake needs to be just below the water. If it gets more fully immersed the water will spray up and increase drag. If the bow drops and the intake moves up too high,
the pump will ventilate. If the bow comes up, the spray will be impressive
So it is not as easy as just hanging the new motor on the back.

Assuming that your 1/8 inch transom thickness is correct.
If it were my project.
I would get the specs from the motor. Ie transom height. I expect that the transom will have to be cut down somewhat where the engine will bolt on.
Design a motor well, ( you can easily google outboard jet boat manufactures to see what they do or visit a dealer who has such a set up. Expect it to be maybe 12 inches wide, maybe slightly wider at the engine location.
The motor well and wider cross section will run the width of the boat and tie into the gunwales. This will add strength to the upper motor mount location.
Below that you have the side stringers coming down the boat hull. We are not sure where the lower mounting holes for the OB are, but I would incorporate another structural piece across the transom to pick up the side trays as well.

Actually, if it was my task, I would remove the rear transom completely. Build a new one out of 1/4 inch, incorporate the motor well. tie the 1/4 inch into the gunwales and side trays. Run a structural piece across the transom, attached to the side and the trays.


After having another look at the picture, I would expect that you will have to cut the transom down at least in the engine mount location, to permit the motor to be tilted up.
Below a link, Page 3, to show the leading edge of the intake to the bottom of the hull, or a hull extension
http://outboardjets.com/download/jet_kit_instructions/BC.pdf


Or better yet, Google " outboard jet mounting height" images to get some ideas
This link from the jet manufacturer, go to the installation section, I think the 14 inch wide tunnel might be a little narrow Installation http://outboardjets.com/installation/

 

The OP needs to offer pics of the outside as well. He probably wants to bracket because the transom is not one piece, etc.

 

I was just discussing a Merc Outboard jet with a boat manufacturers representative and he said that the factory supplies a transom with a 27 inch height. Note this is a hull with a Delta pad at the back, Ie a flat section ahead of the pump inlet.
As you will have to deal with removing the existing intake, it would be easy to install the flat. There is a chance then that your existing upper transom cross support may accept the mounting of an OB jet with minor modifications. Note
you still need to support the lower mounting bolts and provide proper stiffeners for the transom. You may be able to omit a motor well. Motor wells are normally necessary if the motor height is for a short shaft or even long shaft motor so when you stop, the stern doesn't take a gulp of water. With a 27 inch transom, this should not be an issue.

 

@Barry I loved your detailed reply above - however I get the impression that Ballfeng is planning on fitting a standard Honda 150 hp O/B motor, which I presume comes with a propeller rather than a jet?

 

I did the same conversion to mine about 20 years ago.

Strengthening the transom was easy. I added a 1/4" plate to the outside that was a little larger than the mounting surface of the outboard. I added a length of square tubing inside near the top to bolt the motor through. I used a 3"×3" plate on the inside of the lower motor mounting bolts. I also added two angle braces at about 45 degrees on the inside of the transom.

Use spacers in the square tubing so you don't crush it.

I had the old intake welded shut, and bolted a circle of aluminum over where the pump exited the hull.

My transom was 25" inches, so no modification to the height was required. But cutting it to the correct height if needed isn't difficult. No wood was used anywhere.

Most jet boats that size with outboards don't have a motor well, but you can go either way. I didn't add one.

With a bracket the transom height won't make a difference. You can look at other brackets to get an idea of how to build it strong enough, or buy one.

I don't live where I really need a jet anymore, so I may convert to a prop. I'll use a 25" outboard so I won't need to modify anything.

 

Ha!!! As it was a jet, I assumed incorrectly that he bought an outboard jet.
Glad you caught my error