What is wrong with this prop height?

[Mark42]

Today I mounted a 2003 Johnson 90 hp on a 1985 Bayliner. I did a bit of research and decided to mount the motor so that the cavitation plate was 1 inch higher than the bottom of the hull. When the motor was hanging from the hoist and marking the holes to drill, something did not seem right.

This is how it ended up. The cavitation plate is almost 1 inch higher, but not along its full length. I think when the motor is trimmed out a little to its normal running trim, it will be 1 full inch higher.

But look what the top of the tranom looks like!!! There is two full inches of space between the transom cap the the bottom of the transom mounts!

So I pull out the service manual and check, the 90 hp only came in 20" modles in 2002-2003. This measures 22 inches from top of transom mount to cavatation plate. That would be a "special" according to BRP, but no special 22.5" models were made those years. So this is a 20"?

I checked the model number, and it was obliterated by the idiot who did the prep for the paint job (its not stamped, it is a sticker!). But knowing that there were only 20" models made in the V4 configuration, I'm guessing this is the normal mid section for this motor.

If I lowered the motor to have the cavitation plate even with the bottom of the hull, it would only be 1 inch higher than the transom cap.

When I drilled the holes, I made sure that the motor can be lowered on the transom bracket if I want to.

I'm going to test it like this, but I am surprised it sits so high above the transom.

BTW, the transom measures 19.5" height.

Any comments?

 

[Haffiman]

Re: What is wrong with this prop height?

That bottom is not a clean 'V', but has a pad.
You may during testing discover you are too high, but depends on conditions you are testing in.
You may have to go down just to flush+ .

 

[EddiePetty]

Re: What is wrong with this prop height?

...if it were mine:

First I would get a longer straight edge in order to pick up more of the plane of the hull. Either tape, block or get a helper to hold the straight edge in place. Lay a level on the straight edge that is beside the lower unit, jack the trailer until the level reads, well, level. Shift the level over to the ventilation plate and trim the engine until level is reached. Now you can measure down to the straight edge to determine the actual height above the hull. Minor adjustment will be required once waterborne.

 

[steelespike]

Re: What is wrong with this prop height?

To the best of my knowledge a 20" motors doesn't measure 20" to the antivent (cavitation) plate.Generally longer. Additional length to accomadate various hull configurations easier.I bet if you measure similar motors they will be the same.
And if you measure 3 different 20" transoms you will get 3 different demensions.

 

[Haffiman]

Re: What is wrong with this prop height?

All outboards have a leg-length designed to fit a 15/20/25" std transom.
However a 15/20/25" transom rarely measures 15/20/25".
The reason for this they use a std transom with a given transom angle and bottom angle as reference.
Make a 'pad' like here and the transom may be shorter, make a deep 'V' and the transom will be higher. The same may happen if the transom angle is changed.

 

[Mark42]

Re: What is wrong with this prop height?

Thanks for all the good info, guys. One thing that threw me off is the old Force motor is really 20 inches from inside transom clamp to cavitation plate. The Force also rested right on the transom. So imagine my surprise when the Johnson was different. Also required removing some of the decoratve/sealing trim from the transom cap to allow the transom bracket to bolt flush to the transom.

I guess the proof is in the pudding, as they say. Will see how it performs when it hits the water.

 

['96 Charger]

Re: What is wrong with this prop height?

Good luck. Looks like a nice motor. What was wrong with the old Chrysler?

 

[Dhadley]

Re: What is wrong with this prop height?

The idea of a hull with a pad and a stepped transom is so you can run the motor higher. Most pad bottom stepped hulls will measue closer to 25" but you still run a 20" motor. Some "20 inch" motors will be slightly longer. A "20 inch" OMC V8 will measure 23 inches. Most "20 inch" transoms measure somewhere around 20 3/4".

You will probably be close with your mounting but may need a better design prop.

Note - when we measure the X dimension we make sure the motor is trimmed so the AV plate is parallel to the pad or step.

 

[Mark42]

Re: What is wrong with this prop height?

Good luck. Looks like a nice motor. What was wrong with the old Chrysler?

The motor is getting old, and it doesn't have tilt/trim. Decided to go with a power tilt/trim motor that will have full parts support for a few more years.

The idea of a hull with a pad and a stepped transom is so you can run the motor higher. Most pad bottom stepped hulls will measue closer to 25" but you still run a 20" motor. Some "20 inch" motors will be slightly longer. A "20 inch" OMC V8 will measure 23 inches. Most "20 inch" transoms measure somewhere around 20 3/4".

You will probably be close with your mounting but may need a better design prop.

Note - when we measure the X dimension we make sure the motor is trimmed so the AV plate is parallel to the pad or step.

The motor swings up and out, so bringing it to parallel with the pad will raise it some. Hopefully I'll get some time to day to get back to the motor after doing yard work I've been neglecting.