How much slip does your prop have ?

C

cyclops222

Commander
Joined
Mar 21, 2024
Messages
2,146
Mercury Racing has a Prop Slip calculator.
If you are unlucky with your present UNDAMAGED prop ? You could be wasting 25% of your engine power. Go to Mercury Racing site and find out.
 
M

Mc Tool

Petty Officer 1st Class
Joined
Aug 7, 2024
Messages
384
cyclops222 said:
Mercury Racing has a Prop Slip calculator.
If you are unlucky with your present UNDAMAGED prop ? You could be wasting 25% of your engine power. Go to Mercury Racing site and find out.
Click to expand...
Awww dag nabbit ! I haven't got a Mercury. 😁
 
jimmbo

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,707
Without Slip there would be no Thrust. How much is acceptable, depends on Hull Design, Weight of loaded Hull. If you drop Pitch and increase Diameter, Slip will get lower, but that likely will not result in higher Speed. On a Planing Hull, Slip will be at its greatest upon Initial Acceleration and as the Hull gets on Plane. As the Hull gains speed, properly trimmed Loadwise and Engine Trim Angle, less Hull is wetted and gains speed and Slippage Value drops, but will NEVER near 0. Most Planing Hulls run with 10 - 15% Slip which is considered Normal-Reasonable
 
tpenfield

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
18,220
It's just math, so any prop slip calculator should give you the results. :D

I made a spreadsheet that I use to calculate slippage and compare props. I should run the numbers on my current boat . . . might be scary though . . . o_O
 
J

jlh3rd

Senior Chief Petty Officer
Joined
Jul 10, 2017
Messages
672
but what difference does "slip" make if I'm hitting my proper rpm @WOT?
Any change in pitch, diameter, or blade count is either going to put me above or below my ideal rpm, right?.....
I've wondered about this for awhile.
 
P

Pmt133

Senior Chief Petty Officer
Joined
Jan 6, 2022
Messages
792
jlh3rd said:
but what difference does "slip" make if I'm hitting my proper rpm @WOT?
Any change in pitch, diameter, or blade count is either going to put me above or below my ideal rpm, right?.....
I've wondered about this for awhile.
Click to expand...
Unless its excessive slip, it really doesn't matter in the way people think it would... You can have a 23p with 25% slip that'll run near a 19p with single digits and have the same speed at any given RPM. I've tested props that behave that way and the only thing I've noticed is that they tend to be slower to plane and blow out easier. In the same breath, props with really low slip are more sensitive to blowing out too and be slow to plane in my experience.... so it works both ways lol.

And the other thing... calculations of slip based on theoretical speed to actual speed are at best a guess... between blade surface area, cupping, pitch variations... what you calculate and what it actually is may very well be completely different. There's lines of calculations that would need to be done to tell you what the actual slip would be. It's more like CFD. And I've never cared enough to learn.
 
C

cyclops222

Commander
Joined
Mar 21, 2024
Messages
2,146
Yawn
Slip is extremely important for getting all 95 % of your horsepower converted to the prop doing WOT performance from a stop to Rev Limiter top speed. Been there. Power Prop Tech support made a fantastic boat out of a 51 mph GPS boat. Problem was complete blowouts if floored at any lower speeds or turned sharply if on plane.
Prop Slip is EXTREMELY important to a safe boat.
 
Last edited:
JimS123

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
8,282
racerone said:
Similar to the ----" what oil is best " ----Threads.------There is no easy answer.
Click to expand...
Welllll.....sort of.

Low prop slip equates to full use of your HP, plus optimum speed and good fuel economy. Now, if you are a racing boat that NEEDS HIGH speed, then all bets are off.

My SeaRay / 150 Merc OB came from the factory raised 2 notches on the transom, running a base aluminum prop. Slip was 15%. RPM was in the top half of the range.

I replaced the prop with a SS Enertia prop and added a Doelfin. Prop slip dropped to 8%, I picked up 3 mph, WOT was right at max and mid range, fuel economy (where we usually run) increased about 10%.

I'm glad I found the Slip Calculator Program. It saved me from having to write one myself.
 
J

jlh3rd

Senior Chief Petty Officer
Joined
Jul 10, 2017
Messages
672
JimS123 said:
Welllll.....sort of.

Low prop slip equates to full use of your HP, plus optimum speed and good fuel economy. Now, if you are a racing boat that NEEDS HIGH speed, then all bets are off.

My SeaRay / 150 Merc OB came from the factory raised 2 notches on the transom, running a base aluminum prop. Slip was 15%. RPM was in the top half of the range.

I replaced the prop with a SS Enertia prop and added a Doelfin. Prop slip dropped to 8%, I picked up 3 mph, WOT was right at max and mid range, fuel economy (where we usually run) increased about 10%.

I'm glad I found the Slip Calculator Program. It saved me from having to write one myself.
Click to expand...
If my prop only gets me to the upper half of my rpm range, I'm changing my prop (which I did) regardless of what the slip is, I'm propping to rpm....which you did....
which, BTW, is the recommended sticky on this website. There is no mention of "slippage" as a propping tool....just say'n.
 
JimS123

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
8,282
jlh3rd said:
If my prop only gets me to the upper half of my rpm range, I'm changing my prop (which I did) regardless of what the slip is, I'm propping to rpm....which you did....
which, BTW, is the recommended sticky on this website. There is no mention of "slippage" as a propping tool....just say'n.
Click to expand...
You are absolutely right that the proper propping tool is rpm. I just found it interesting that doing it also improved the slip. Is there a correlation? I dunno.
 
P

Pmt133

Senior Chief Petty Officer
Joined
Jan 6, 2022
Messages
792
It's not a tool so much as its data to compare against. My hull likes lower slip. Others have run better in the 15-18% range (better planing and speeds despite higher slip and less prone to blow out, tested first hand by yours truly) and better resulting fuel burn. It's entirely dependent on so many more variables than "its supposed to go this fast but only goes this fast so there's my slip."

I can say that unlike jims my boat was slower at the same rpm with an enertia and needed 400 more rpm just to come on plane... but the slip numbers would look the same at wide open as rpm and speed was nearly the same....
 
tpenfield

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
18,220
If you had 100% slip, you'd be going 0 mph at your WOT RPM. If you had 0% slip, you'd be going at your theoretical maximum speed based on WOT RPM.

Slip is like prop efficiency. Less slip is better.
 
JimS123

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
8,282
Pmt133 said:
It's not a tool so much as its data to compare against. My hull likes lower slip. Others have run better in the 15-18% range (better planing and speeds despite higher slip and less prone to blow out, tested first hand by yours truly) and better resulting fuel burn. It's entirely dependent on so many more variables than "its supposed to go this fast but only goes this fast so there's my slip."

I can say that unlike jims my boat was slower at the same rpm with an enertia and needed 400 more rpm just to come on plane... but the slip numbers would look the same at wide open as rpm and speed was nearly the same....
Click to expand...
As a Professional Engineer, all this stuff interests me immensely. However, I MUST have data, and that's what I like to collect.

The data I posted above is quantifiable. Here's some more:

Hole shot - 3 seconds to plane.
The changes reduced the time to 30 mph from 9 seconds to 7 seconds.
Bow rise ZERO. THAT's not quantifiable, but it makes me grin a lot.....LOL.

Back in College I experimented a lot. I had a 26 mile course and a 6 gallon tank. My changes were measured by how much fuel I had to put in to fill the tank back up.

Boy, technology has improved a lot....
 
P

Pmt133

Senior Chief Petty Officer
Joined
Jan 6, 2022
Messages
792
JimS123 said:
As a Professional Engineer, all this stuff interests me immensely. However, I MUST have data, and that's what I like to collect.

The data I posted above is quantifiable. Here's some more:

Hole shot - 3 seconds to plane.
The changes reduced the time to 30 mph from 9 seconds to 7 seconds.
Bow rise ZERO. THAT's not quantifiable, but it makes me grin a lot.....LOL.

Back in College I experimented a lot. I had a 26 mile course and a 6 gallon tank. My changes were measured by how much fuel I had to put in to fill the tank back up.

Boy, technology has improved a lot....
Click to expand...
Exactly. I haven't fussed around with the fuel burn since it doesnt bother me enough. But RPM, speed, slip, time to plane, how much trim is needed etc are all noted in my logs of testing props over the years.

You can quantify bow rise... I have a digital angle meter I messed with over the years to measure that. Not 100% accurate but gives some consistency.
 
A

airshot

Vice Admiral
Joined
Jul 22, 2008
Messages
5,464
All this feels like a headache coming on......my Merc runs great, the old two stroke pushes my 16' side console at over 31 mph, easy on fuel, and looks showroom new. Why would I care if the prop has some slippage ? Not gonna try to fix what isn't broke !
 
JimS123

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
8,282
Pmt133 said:
Exactly. I haven't fussed around with the fuel burn since it doesnt bother me enough. But RPM, speed, slip, time to plane, how much trim is needed etc are all noted in my logs of testing props over the years.

You can quantify bow rise... I have a digital angle meter I messed with over the years to measure that. Not 100% accurate but gives some consistency.
Click to expand...
If my bow light does not go above the horizon, that's good enough for me.
 
JimS123

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
8,282
airshot said:
All this feels like a headache coming on......my Merc runs great, the old two stroke pushes my 16' side console at over 31 mph, easy on fuel, and looks showroom new. Why would I care if the prop has some slippage ? Not gonna try to fix what isn't broke !
Click to expand...
I agree that if everything meets your requirements, why bother? But my professional career required me to collect data and justify my raise, so for me it was natural to apply science to my hobbies as well.
 
P

Pmt133

Senior Chief Petty Officer
Joined
Jan 6, 2022
Messages
792
JimS123 said:
If my bow light does not go above the horizon, that's good enough for me.
Click to expand...
I stand and don't have that problem most of the time lol. But yeah. Most of my career is data driven as well. It's just the mental midgets above me... well... I'm not going to get started. :LOL:
 
You must log in or register to reply here.
Top