Moderated Discussion Areas
ContinuousWave: Whaler Performance
|Author||Topic: Prop slip|
posted 03-10-2003 10:46 AM ET (US)
I have an 1984 18 OR with a 91 200 hp Evinrude turning a 21 pitch prop.
It is running 39 miles an hour at 5500 rpm.
According to the prop calc. this is 35% slip.
What causes this and how can I correct it.
posted 03-10-2003 12:15 PM ET (US)
What kind of prop do you have? Is it damaged? You should be getting 20 more mph with that set-up. Try a cupped stainless steel prop.
posted 03-10-2003 01:19 PM ET (US)
Huh? That engine redlines at 6k so even with a 17" pitch at 5500 you should be running mid-40's. I do think either your speedo, tach, or engine are not working right. What is the compression on all 6 cyls? Are you running on all 6 cyls? Are you sure it is not a 150hp with a 200 cover?
posted 03-10-2003 01:23 PM ET (US)
The prop is stainless steel, I don't know if it is cupped or not, it has a few little dings in it but nothing big.
I always thought is was a power prop, to carry heavy loads.
I have a four bladed 25 pitch prop, I will try that and see what difference it makes.
posted 03-10-2003 01:29 PM ET (US)
The motor is running fine, I think it will wind out past 6000 ( never tried ) it hits 5500 with room to go.
The motor is a 200 my family has had the boat since it was new, this is the second motor.
I had the dealer we bought it from check it throughly this last fall. and It was in good shape.
posted 03-10-2003 01:32 PM ET (US)
The speed was checked by GPS on serveal occasions.
I will have the tach cheked next time it is at the dealer.
posted 03-11-2003 02:10 AM ET (US)
I don't know of, or ever heard of a dealer selling a big time over powered boat.
I think max hp on that boat is 150 hp isn't it?....[ it's on the metal tag on the starboard side of the boat.
As bigshot said, that engine is rated for 6,000 rpms & with a 14.25 x 21 at 5,500 rpms, that boat should be doing 55 mph & at 6,000 it should be doing 60.
With a 23 pitch it should be getting close to 65 mph & with a 25 pitch she should push 70 mph.
If she's only getting 39 mph at 5,500 rpms, something is "DRASTICLY" wrong.
It sounds like your running a 15 x 16 stock aluminum prop thats been pretty banged up, or that engine is really sick.
I would think that engine should be able to twist a 14.25 x 23 or 25 pitch on that size boat.
Get back to us & tell us what size prop your twisting.
posted 03-11-2003 10:15 AM ET (US)
What is the gear ratio of your lower unit? It is impossible to assess the current performance without accurately knowing this value. Based on your numbers, it sound like your gear ratio is 1.8:1. Is that correct?
Below top speeds, it is normal for the propeller to show some slip, although 35% seems a bit high.
Some possible causes of excessive slip:
--damaged propeller blades producing poor thrust
posted 03-14-2003 11:52 PM ET (US)
Thank you Bigshot, Sal, Jimh and everyone else in this forum. I really appreciate your comments and advice. It is really great to tap this brain trust of information.
The prop is a 14 1/2 X 21.
Could it be a waterlogged hull. Boat, T top, leaning post, 1/2 tank of gas, 2 batteries, anchor, motor and trailer with spare, weigh 3400 lbs.
posted 03-17-2003 11:32 AM ET (US)
Tried a different prop put on a four bladed 13 3/4 X 25 and the performance was no better, really about the same. 5500 rpm ran 41 mph.
Could the motor be too low, it is mounted one hole up from bottom, the cavitation plate is even with the bottom of the hull.
posted 03-17-2003 01:42 PM ET (US)
A 25" and you are getting 5500? Something screwy. You should not be able to spin anything more than a 23" which is about equiv to a 21" 4 blade. Your prop is way too big. I would try another 21" 3 blade from someone. Yours might have been reworked or your tach is wrong.
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