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Lowering the motor

K

keithm

Contributing Member
I've had an issue with this setup since I put this motor on this boat. 1979 baja. 18ft ski boat.
1976 johnson 85.
problem is when the boat comes on plane it feels like is loosing traction. After doing some reading on this, I did some measurements. The top of the transom to the bottom of the hull is 23". The top of the mount to the ventilation plate is 21". When moving thru the water the plate is right on the water.
could it be the prop is blowing out. I do not have a hydrofoil on it. Would that help? I'm looking to avoid the expense of a jack plate if possible.

Thanks
keith
 
so is the ideal set up to be even between the aeriation plate and the bottom of the hull? cutting the transom hmm that would be an option on this boat as long as i dont have to redrill holes, do they make a simple basic jack plate? ive seen a lot of very nice high tech plates because of the age of this boat im trying to keep expense down.
i appreciate the help on this.
 
In past years... going pretty far back, unless one was deep into racing, the usual ideal position of the cavitation plate was between 1/2" to 3/4" beneath the keel at the transom.

Above that and there was the risk of cavitation... below 1" and there was excessive drag and the possibility of the ole rooster tail!
 
Does it seem to loose traction when it's getting up on plane, or while it's on plane? I run a similar boat (16 Sidewinder) with a 150. My motor is on a 6 inch jackplate , with the anti-cavitation plate about 3 1/2 inches above the keel Also, what prop is on the motor?
 
Seems to loose it just as it coming up to plane, the prop is a 13.250 dia 17 pitch. I went out and did some closer investigation and found that the bottom of the plate is 1/4 above the keel at the transom. I have a 12.75 dia 21 pitch prop. I'd like to try it but I'm trying to get the height right first.

btw the engine turns 5500 wot with the 13.25 if I went to the 12.75. Would be a 800 rpm decrease would that still be in the rpm range for that engine
 

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Yes it is hooked down at the center. My biggest concern is when it's coming out of the hole just as it breaks onto a plane it feels like it losses traction. It comes back after everything settles for a moment. I'll play with the trim and weight and let you know. I'm not really looking for speed. Just trying to get it dialed in.
 

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Its just cavatation so no need to lower or cut transom. You need a quality prop with large diameter 13-1/2 to 13-3/4 and about a 14 pitch
 
I assume decreasing the pitch with a largest diameter prop keeps the load about the same on the motor so as not to exceed 5500?
 
I-Boats says 5000. Do not run a 14" pitch prop, you'll over rev the motor IMO. You've got power trim on that motor.. are you trimming the motor all the way in when you are taking off? I run my motors so that the anti-cav plate is clear of the water at WOT. When I was running a 140 hp Evinrude on an 18' Charger SK, I needed a hydrofoil just to get the boat up on plane with a 26" pitch prop.
 
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