Causes of prop slip?

[mpsyamaha]

I have a prop slip of 16% and i am wondering why it would be so high?

My boat is a 21' Offshore... it has a 8' beam, fairly deep V, center console, not sure of weight but probably in the 3000lb area. The motor is a 200 ocean pro. WOT is 5700rpms at 43.5mph on gps with a Powertech 17p stainless prop (has a good bit of cup, so i entered 18p in the prop slip calculator). Ventilation plate is dead even with bottom of hull.

my last boat only had a prop slip of 9% with a crappy aluminum factory yamaha prop, so im wondering why the slip is so high on this one?

 

[seabob4]

Re: Causes of prop slip?

You've got to remember that slip is a purely theoretical concept, and the "slip calculator" equation is at best, the closest mathematical approximation the marine community could come up with to approximate prop slip.

There are so many static variables involved with the boat, not to mention the HUGE dynamic variable involved with the water...

Don't worry about, go catch fish, before they become self-lubricated...

 

[26aftcab454]

Re: Causes of prop slip?

sounds close to right to me...

 

[grego]

Re: Causes of prop slip?

prop slip??? Please inform me?

 

[Mark42]

Re: Causes of prop slip?

prop slip??? Please inform me?

A long standing problem caused by using too much prop grease.

Sorry, I couldn't resist.

 

[grego]

Re: Causes of prop slip?

oh well ,you ask a simple question?

 

[Mark42]

Re: Causes of prop slip?

oh well ,you ask a simple question?

Use the search button. This is a topic that has been discussed up, down and sideways. So there is a lot of good info on the forum about it.

 

[QC]

Re: Causes of prop slip?

(has a good bit of cup, so i entered 18p in the prop slip calculator).

Heck, call it a 16 and you'll be happier I'm with seabob. Props are voodoo, but you could buy lots of props until one of them gave you a better slip calc. Orrr, you might have a number wrong.

grego, search for prop slip here. But a quickee for you . . . If your prop was a cork screw, imagine screwing the prop into the cork in a really big bottle of wine, the distance it travels through a "solid" (the cork) in one revolution is pitch. The distance it travels through the water with a boat in front of it is something less than that. The difference is slip and is stated as a percentage. If a 20" pitch prop went 15" through water, it would have 25% slip, and mpsyamaha would be even more bummed out. If it went 18" it would be 10% slip and the yammie guy would be happier . . .

 

[trendsetter240]

Re: Causes of prop slip?

Prop slip is the difference between the theoretical distance the prop will travel in one complete revolution and the actual distance it travels.

Every prop has some amount of slip and prop slip is actually required for the prop to be working correctly.

Prop slip is usually a good indicator of the effective performance of the prop but is not the only factor to consider.

Really what you should be looking at is the blade design that works best for your boat. When you find that design, then your prop slip will be in what are considered acceptable levels (6%-15% at WOT) for a standard planing hull.

Chosing the correct prop for a specific boat involves chosing the correct blade design to transfer the best combination of forward thrust, stern lift and bow lift. For example if the blade design provides more forward thrust than lift required for your boat then you will see high prop slip.

The optimum propeller will provide enough lift to get the hull high enough out of the water to reduce drag and the rest of the energy will be transferred to forward movement.

Finding the optimum propeller is a bit of a challenge and your best bet is to search for a hwsiii form and fill that out.

Cheers.

 

[grego]

Re: Causes of prop slip?

ThANKS TRENDSETTER i did not know that it was called prop slip. being from south, we called it prop slop. Thouhgt I was learning something new!

 

[trendsetter240]

Re: Causes of prop slip?

ThANKS TRENDSETTER i did not know that it was called prop slip. being from south, we called it prop slop. Thouhgt I was learning something new!

No worries grego.I was responding to the original poster but glad you learned something. Tylenol will help with the headaches

 

[mpsyamaha]

Re: Causes of prop slip?

well i do have a snap-on digital tach, i can bring that next time and check my rpm reading.

im pretty happy with the performance with this prop, great hole shot, lots of grip in turns and high seas, cruises nice at 33mph at 4600 rpms, i just feel like it could have more top end. maybe one day ill try another prop, but im in no hurry. the slip number i came up with just bothered me. thanks for the advice guys

 

[hwsiii]

Re: Causes of prop slip?

Mps, if you will send me about 4 pictures of the anti vent plate with the plate parallel with the bottom of the boat and a straight edge from the bottom of the boat to the vent plate we will see what we can do about your setup to get you some more speed with the prop you have now.

6. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a straight edge under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height

When the boat is running in the top 20% of your speed you should be able to see the anti ventilation plate very clearly at the top of the water and have water just splashing over the top of it.

Correct Anti Ventilation Height

H

 

[mpsyamaha]

Re: Causes of prop slip?

Im taking the boat out tomorrow, ill take a few pics of the transom. im wondering if raising the engine a hole might help.

 

[hwsiii]

Re: Causes of prop slip?

Mps, You might try reading this for a better understanding of motor height, changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057[/URL



H

 

[mpsyamaha]

Re: Causes of prop slip?

i have read it! there is a lot of great information on this forum.