Re: need educated about prop slip
In simple terms, slip is (Theoretical speed minus Actual Speed) divided by Theoretical Speed.
For example, a prop with a 20" pitch would theoretically move your boat forward 20" per revolution if it had zero slip. But in the real world, it requires an angle of attack to generate thrust, therefore it will move your boat forward only about 18" per revolution. If your boat had a 1.5 gear ratio and turned 4800 RPM, it would go 61 MPH if it moved 20" forward for every revolution of the prop. But since it only moves forward 18" for every revolution of the prop, it will only go 55 MPH.
So (61 - 55) / 61 = 6/61 = 10%
The 2" difference between the theoretical advance and real advance is the slip. Since 2" is 10% of the nominal 20" pitch, we would say that this prop had 10% slip.
Prop slip will vary from 100% down to less than 10%. The slip acts like an automatic transmission. Tie your boat to the dock with the engine in gear, and your slip is 100%, just like your car idling at a redlight with the brake on.
Some boats and props can actually show a negative slip number at top speed if the prop's effective pitch (due to cupping, etc) is greater than it's theoretical pitch. The numbers you'll see on forums generally refer to the prop's slip at WOT.
The vast majority of people confuse slip and efficiency. Slip has NOTHING to do with efficiency. You can easily have a prop with high slip that is faster and more efficient than one with low slip. Slip can be easily compensated for by increasing prop pitch. For example, an efficient 23" pitch prop, with a 1.5 gear ratio at 4,800 RPM and 15% slip, will propel your boat at 59 MPH. A less efficient 21" pitch prop with 10% slip, with the same 1.5 gear ratio and same 4,800 RPM, will only propel your boat to 57 MPH.
Too little slip is just as harmful to performance as too much slip. Unfortunately, slip is a very easy parameter to calculate by people with an incomplete understanding of propellor performance, so it's frequently used to give bad advice on prop selection.
You can put numbers like those above into the Mercruiser Prop Slip Calculator here to see the effects:
http://www.mercuryracing.com/propellers/propslipcalculator.php
Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust.
To understand slip you really need to understand angle of attack and how propellors generate thrust. Mercruiser used to have a good basic explanation of how props work. Unfortunately, I think they removed it when they revised their website a year or so ago.
Here's a screenshot of the part on slip, hopefully it will be readable!
