Any propellor slipping more than about 9% is wasting fuel / energy / potential speed.

Imagine your wheels on your car spinning on loose gravel (or a wet slippery road on the first rain after summer when the roads covered in oil and detergents) when you plant your foot - but the car goes almost nowhere!

If your prop is slipping more than 9% then thats about the same effect as the spinning wheels on your car analogy!

So - how in heck can you tell if your prop is slipping and by how much?

Well it's pretty easey!

But - you do need some "info" to work with before you can start!

I'm going to do an "example" with my own boat, and the prop we just serviced in a previous thread to work out just how efficient (or inefficient) it really is!

In order to work out your own prop details you will need the following information!

1. Your top end speed at W.O.T. (Wide Open Throttle) measured accurately by GPS (in my case 29 knots)

2. Your engine RPM's at W.O.T. from your tacho. (in My case 5800rpm's)

3. Your Prop specifications (Diameter and Pitch) (in My case 13 & 3/4 inches dia x 15 inches pitch)

4. Your OB engines lower unit Gear reduction ratio. (In my case 1:2)

5. OB Manufacturers specified Max WOT Rpm. (In My case 6000 Rpm)

You will also need the following imperial measurements / coversions, whatever!

a) 12 inches to 1 foot
b) 3 feet to 1 yard
c) 1760 Yards to 1 Statute (Land) Mile
d) 5280 Feet to 1 Statute (Land) Mile
e) 1.18 Statute (land) Miles to 1 Nautical Mile (at the equator)
f) 6230.4 Feet to 1 nautical mile at the equator
g) 60 Seconds to 1 minute
h) 60 Minutes to 1 hour

This is how we do our prop slip calculation

Step 1. Calculate the theoretical WOT speed with your existing prop!.

To do this, you take...
Your actual Max WOT Rpm (5800) and divide it by the gear box reduction ratio 1:2 (5800/2) = 2900 rpm

Now multiply that figure (2900 x Your Prop specifications (pitch) in inches (15)

(2900 x 15) = 43500 inches travelled in 1 minute with zero % slip (if we assumed your prop was turning in a semi solid substance like say gelatine).

In one hour your prop would therefore travel 43500 inches x 60 minutes = 2 610 000 inches!

We know there are 12 inches in a foot so dividing 2 610 000 inches by 12 = 217500 feet travelled in one hour.

We know there are 6230.4 feet in 1 nautical mile at the equator so, if we further divide 217500 feet by 6230.4 feet in an hour - we get 34.9 knots!

Now - I told you at the beginning with my example I actually get 29 knots for 5800rpms compared to the calculated theoretical maximum with zero % slippage of 34.9 knots so what % of slip do I have?

34.9 - 29 = 5.9 knots

5.9 knots / 34.9 = 0.1643 x 100/1 = 16.43% slip!

What did I say at the beginning?

Any more than 9% slip is too much so - I'm potentially "wasting" 7.43% of my fuel in excess slippage due to the wrong prop pitch or wrong OB setup!

To put that in $ terms, a 60 Nm return offshore trip usually sets me back about 200 litres of fuel purchased at say $1.30 a litre at the local bowser (200 liters x $1.30) = $260.

If I could save that figure of 7.43% of my fuel burn with better pitch props or better OB setup, how much would that equate to in $ terms per trip?

0.0743 x $260 = $12.30/trip!

Anyway, you get the idea - we can work out how efficient our existing prop is and what savings per trip might accrue if we spent $ to 'get better figures with a different prop!

This is always good to know-BEFORE you start spending $1000's on new props etc!

So any one else care to plug in their prop figures and see how they are going efficiencey wise?

We will look further at my particular setup in another post or two on this thread and see where maybe some efficiencies could be gained as a way of developing on the knowledge base we have thus far built up!!!

Some of these factors COULD well apply equally to your boat!
Do the excercise and follow along with the further analysis we can iuse to "guesstimate" what we COULD get in the way of top end speed, or fuel savings, more range with a better prop setup!

Iwill also do some figures calculations of what % slip i get when the bottom of the hull is covered in marine growth after 4 months in the water without antifouling paint!! Be prepared to be shocked!

Cheers!

too lazy too do the above manual calculations there is always of course an easier way! ;o)

Couple links it might pay to add to your "favourites" list for future reference!! ;o)

http://continuouswave.com/cgi-bin/propcalc.pl

http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

http://www.go-fast.com/Prop_Slip_Calculator.htm