formula1

I have on old 1985 Hydrasport 1800cc with an old Yamaha 150
2-stroke that is our mess around and go fishing boat. I just got the opportunity to upgrade it to a newer Yamaha 115 4-stroke with a 20" shaft. The 150 2-stroke is a 25" shaft and is mounted on a fixed jack plate that raises the motor some, but it sits way to low below the transom (which is notched). So far, the 4-stroke is mounted so that the cavitation plate is approx 2" above keel.

My ultimate questions are:
1. How does the notched transom & jack plate (which moves the motor at least 6" further back from the transom) play a role in the positioning of the motor?

2. Does the cavitation have to be even with the keel exactly? What will happen to the performance of this boat if the motor is mounted too high?

As always, thanks for your replies!

mrv8outboard

if the engine is mounted too high you will loose water pressure and steering. general starting point is the anti ventilation plate level with the bottom of the boat. performance one inch above. this is not set in stone. trial an error will find the sweet spot. props will also have an effect on engine height. make sure you have a good water pressure gauge and hydraulic steering is a must.

formula1

Thanks MRV8Outboard!

But, how does the Jack Plate (which moves the outboard 6" from the back of the boat) and the notched transom affect performance and the mounting location of the outboard?

MOBILEMERCMAN

Formula, The step and the jack plate will allow the motor to be higher. Velocity 22 would put 20" motors on a 25"transom. If the keel lines up with the center of your water intakes its worth a try. You must have a water pressure gauge and a good prop. Basic aluminum will slip too much. If you have about 15- 20 lbs of pressure you should be ok. It will be possible to over trim and lose pressure so pay attention. Performance wise will depend on the prop. The Height should let it run pretty well.
Jim

formula1

The keel actually lines up with the top of the water intake. I'll give it a try. It does have an aluminum prop however. I'd like to take the stainless one off of the 150 2-stroke, but I'm sure it's probably too much prop for this setup...

MOBILEMERCMAN

formula1, If your keel is even with the top of the inlets, you'll actually end up with a near stock height. Your aluminum may work OK.A high rake stainless will hold better. The water will probably rise to hit the bottom of the cav plate with 12" of total set back. Don't forget the pressure gauge.
Jim

formula1

Thanks for the info guys! The shop water tested the boat as it is and they said it ran great! Apparently, it zipped on plane and topped over 40 MPH! No cavitation issues or anything!