Inboard engine prop slippage

W

Wingspar

Member
exMember
Joined
Jan 1, 2000
RO Number
1979
Messages
331
My 40' semi displacement hull has a prop slippage of about 45%. Seems to me that this is a high number.

I'd be interested to know what other boats in the 40' plus, non planing hulls prop slippage are.

I'm thinking of going from a three blade 24x21 to a four blade 24x18. Savings in fuel costs should pay for them.

Dave

Faster now but still steady
 
What speed/rpm?

Seems really high. I have not run the numbers on the Tolly yet, but I think I will now. I'm sure the numbers would have to at LEAST half what you are experiencing. Makes me wonder if you are at 12 knots or so and not on plane.
 
With twin 200 HP volvos and weighing 31000 LBS, my boat doesn't have enough power to really get up on plane. With little fuel and water it gets close.

I run at 2500 RPMs with a gear ratio of 2.71/1. This gives me about eight knots. At 3800 RPMs, I go about 12 knots.

Dave

Faster now but still steady
 
My Tolly has about a 27% slip at 3200 rpm. I am running twin 270 hp crusaders with 17" nibral props (13.5" pitch).
 
I don't think 45% is far out of line for a semi-displacement hull. Full displacement can slip even more than that.

Mine is around 45% too: 1.93:1, 18 pitch, 2800 RPM, 14 mph.

Edit: From a quick google, I guess 45% seems a little bit high. FWIW, I'm dropping my pitch to 16, which probably will put my slip in the upper 30's.
 
Thanks for the response Mike but I'm looking for numbers from boats designed to operate in displacment mode.

Dave

Faster now but still steady
 
IMO once your bow starts to lift you are past displpacement mode. Calculate at 7 KTs or so. Slip can become infinite if you try to plane but cant.
 
"This gives me about eight knots. At 3800 RPMs, I go about 12 knots."

unless you have to run from a bad thunderstorm.... there is not point in going at 12 kts. you're getting 50% more speed but probably getting almost 3 times the fuel burn.

with naturals, my 53 can't plane and i never push past 9.3kts (hull speed). at 13 kts, i get 50% more speed and burn 3 times more fuel.
 
Pascal, I run at 8 knots for economy. When I'm close to my destination I reun it up to about 3200 RPM for about eight minutes and 3800 RPM for about three minutes to burn out the carbon.

Have you calculated what your slip is?????????????????

I'm just trying to find out if this is a true normal number or if a prop change is in order.

Dave

Faster now but still steady
 
Wingspar,

my boat is a planing hull so it may not be a great comparison but a few years ago, I did a very in depth prop study that you might find interesting. I did a very detailed analysis of my boats prop performance at 200 RPM intervals from idle to WOT graphing many parameters including - prop slip. I even ran the numbers in opposite directions to cancel out current/wind as well as with different prop sizes and pitch.

The part I think you will find intersting is that from about hull speed (1400 RPM) through to being fully on plane at (2400 RPM)... (in other words working from dsplacement mode, to semi displacemwent to planing), my slip values were ALL well above 40% (as high as 53% at the hump) with smaller wheels. And, the slip values only came down to 36% when larger wheels were fitted but even then, 36% was at hull speed (6.8 knots). Trying to go at 1.5 times hull speed (around 10 knots), slip was back upto 44%. I think your number is not that bad.
 
so what speed did you use for your calculation????
 
I too am surprised, did you have any bow lift?? SOG against a current???
 
Bow lift doesn't really start until 11 knots. SOG is not against a current.

I'm thinking that with that amount of slip I've got to be loosing alot of efficiency...higher fuel bills. Can't really increase the diameter but I can the number of blades with a reduction of pitch. Thus, at the same RPM it should give less slip, more thrust better fuel economy at the expense of top speed. or at least i think so.

Dave

Faster now but still steady
 
I really think our definition of lift is very different. On my 44' I get lift starting above 8KTs or so. At that point you are increasing slip and using more fuel. Try charting speed vs RPM every 50 RPM or so and look for the point where it gets wierd.

You are correct that 4 baldes are more efficient but to save the cost of props you must use your boat a whole lot.
Is there room for more diameter?
You dont have big collar zincs do you?

Another thought is that your props may be screwed up. Have you had them scanned?
 
A suggestion: You may want to pick up a copy of Dave Gerr's "Propeller Handbook" (available on Amazon) and do some of the arithmetic to figure your actual slippage. In my case, I was also kinda high, and needed new props. I bought the Michigan Wheel Dyna Jet props that have a larger blade which reduces slippage some. According to my fuel flow gauges and GPS, I picked up about 1 knot speed at 2700 rpm with a slight decrease in fuel burn. I can only conclude the props are more efficient (less slip).

Delaware Jim
 
I just checked Gerr and I find that my insincts are calibrated for a much faster boat. .4 is right on the money for the speed.
 
I agree with Ghost (and the recommendation on Gerrs book). Just as a point of clarity, "slip" is often refered to as "efficiency" but true "efficiency" is the measure of a props ability to convert (rotary power) "input" to (linear thrust) "output" . 4 blade props (our any props that have greater blade area for a given size) do generally "decrease" slip, but their efficiency is normally lower, not higher. The lower the blade count (all other factors being equal), the higher the efficiency.
 
Slippage is not inefficiency as far as the engine is concerned. It will still take the same amount of work to push the boat through the water.

Better props can help but it won't be huge numbers. The best advice, here, is to get a copy of Gerr's "Propeller Handbook" which is also abailable at Boatfix.
 
You must log in or register to reply here.
Back
Top