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Author Topic:   Prop slip
dcace posted 03-10-2003 10:46 AM ET (US)   Profile for dcace   Send Email to dcace  
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.
Perry posted 03-10-2003 12:15 PM ET (US)     Profile for Perry  Send Email to Perry     
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.
Bigshot posted 03-10-2003 01:19 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
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?
dcace posted 03-10-2003 01:23 PM ET (US)     Profile for dcace  Send Email to dcace     
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.
dcace posted 03-10-2003 01:29 PM ET (US)     Profile for dcace  Send Email to dcace     
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.
dcace posted 03-10-2003 01:32 PM ET (US)     Profile for dcace  Send Email to dcace     
The speed was checked by GPS on serveal occasions.
I will have the tach cheked next time it is at the dealer.
Sal DiMercurio posted 03-11-2003 02:10 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
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.
jimh posted 03-11-2003 10:15 AM ET (US)     Profile for jimh  Send Email to jimh     
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
--damaged propeller hub causing mechanical slip of the propeller on the shaft
--propeller designed for higher speed and lighter loads, and not appropriate for your boat, motor, and load combination

With a 21-inch pitch propeller and an engine turning 5500 with a 1.83 gear reduction, you should be able to hit 50-MPH with 15-percent slip.

dcace posted 03-14-2003 11:52 PM ET (US)     Profile for dcace  Send Email to dcace     
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.
If the motor was weak, wouldn’t it have trouble hitting the upper rpm’s. It winds up to 5500 easily, and jumps right up on a plane, if you are not carful it will over rev.

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.

dcace posted 03-17-2003 11:32 AM ET (US)     Profile for dcace  Send Email to dcace     
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.
Bigshot posted 03-17-2003 01:42 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
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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