Excessive prop slip

[khyatt444]

1991 Stratos 295 Pro
1991 Johnson 150 no jack plate
Raker 14.5x22 @ 5600 - 44 mph gps
Renegade Bass 13.5x25 @ 5200 - 44 mph gps

Thats approx 30% slip which seems high. Seems like it should be running mid to upper 50's. Any suggestions?

 

[steelespike]

Re: Excessive prop slip

Assuming about 1500 lbs dry weight I figure a 24" prop would produce about 60 at about 5500.
Perhaps a setup problem.Are you running a foil on the motor or trim tabs?
Is the motor as high as possible on the transom?

 

[khyatt444]

Re: Excessive prop slip

Assuming about 1500 lbs dry weight I figure a 24" prop would produce about 60 at about 5500.
Perhaps a setup problem.Are you running a foil on the motor or trim tabs?
Is the motor as high as possible on the transom?

Yes. 1450 lbs dry hull weight. No foil or trim tabs. Pretty sure anti-cavitation plate is mounted level with bottom of hull but will take a measurement this afternoon. Holeshot with the Raker is really good and not that bad with the Renegade. Just seems as if it doesnt want to get on the pad.

 

[steelespike]

Re: Excessive prop slip

You might want to verify the tach is accurate. sure seems right except for the speed.
Is the bottom clean and smooth?
Is the bottom straight? No hollows or bulges especially near the transom?
Does the boat respond to up trim? Any thru hull fittings that might disturb the water near the prop.
Are you sure the motor is getting full up trim.
If all else fails try lowering the motor one hole.

 

[khyatt444]

Re: Excessive prop slip

Bottom is clean and straight. All thru hull fittings are on the step near transom abo e the pad. Only response to more trim is increased RPM and a high rooster tail. No increase in speed. Had time to measure today and anti-cavitation plate is 2.5" above the pad and prop shaft is 6" below pad.

 

[Texasmark]

Re: Excessive prop slip

Jack your engine up a notch at a time till you get what you want or start having blow out problems in turns without changing trim (all that much). You already have built in setback. I forget the numbers for inches up vs inches back, but you probably need to raise your engine some more. If you could get a shot of your mid section at WOT/best trim, you could see just where your AV plate was with respect to the water coming out from under the boat. You ought to be able to see the AV plate above that water. All you want out of the engine is thrust, you don't want the AV plate acting like a shovel and determining your pitch attitude. Let the hull do the work for which it was intended. If you get it right, the trim will not make all that much difference when on the pad. Just trim her out till she blows out (rpms up speed down), tuck her back in till it stops. Can you hear your engine rpms AND your prop slapping the water...humming?

Mark

 

[khyatt444]

Re: Excessive prop slip

There is 12" of built in setback from pad to transom. I only have 1 hole available to raise motor 3/4".

 

[Texasmark]

Re: Excessive prop slip

I don't know what the ratio is. Open a new thread in the boating section, first forum, and ask the question. What is the ratio of setback to engine height. I am remembering something like 3"/inch. For 12" that would mean 4" from and extension of the bottom of the boat to the antivent plate with engine vertical with respect to the pad.

If you will think about it, you only want as much engine in the water as is necessary to reduce drag. You want your antivent plate above the water going by it so that it doesn't act like a shovel and mess up your boats pitch (bow up angle) on the pad.

Since the boat runs the pad at a nose high attitude, that causes anything past the end of the pad to be deeper in the water so you have to jack up the engine to get the things mentioned above.

If you only have one hole left and that doesn't do the job you may need a setback plate to get more height.

Mark

 

[khyatt444]

Re: Excessive prop slip

It's been raining all day so I didn't get a chance to run it today. Will try tomorrow so that I can see where the anticavitation plate is at WOT. I will post results and await more suggestions.

 

[Texasmark]

Re: Excessive prop slip

It's been raining all day so I didn't get a chance to run it today. Will try tomorrow so that I can see where the anticavitation plate is at WOT. I will post results and await more suggestions.

Be careful.

 

[khyatt444]

Re: Excessive prop slip

Finally got a chance to run it today.
Raker 14.5 x 22
5400 rpm - 51 mph gps - 16% slip
Anti cavitation plate is out of the water while running.
Can trim higher but no speed increase. Just have a really high rooster tail with increased rpm and torque steer. First run of the day solo engine reved to 5800 but i didnt get a gps speed. Posted results are with passenger and boat was more balanced. Plan to try a 24 raker next time out and will double check for play in thro5le cable. Anybody got any other suggestions?

 

[jestor68]

Re: Excessive prop slip

How many hard hours on that 21 year old motor?

 

[khyatt444]

Re: Excessive prop slip

How many hard hours on that 21 year old motor?

I would guess that it has less than 50 hrs since rebuild. Compression is 92-96 on all cylinders.

 

[steelespike]

Re: Excessive prop slip

Rule of thumb is 100 psi minimum. On a still fresh rebuild I would expect more. You might try a different gauge.

 

[Texasmark]

Re: Excessive prop slip

Rule of thumb is 100 psi minimum. On a still fresh rebuild I would expect more. You might try a different gauge.

Agree compression is low. Merc manual says if below 120 to expect problems even though the cylinders may be within 10 or so % of each other.

editing: However, in retrospect, if the rings haven't seated it could be low! Maybe you want to run it awhile longer before you pass judgement.

Mark