Prop slippage ?

[StingRay_90V4]

Can anyone help me figure out the amount of prop slippage I have ? I am more curious then anything to be honest. I have read a bit about it. But I'm a tad unsure as to how to work it out.

My engine is as some may know. A 1990 Evinrude 90hp crossflow. The current prop is a 13 1\4 x 17p prop. Unknown brand. I want to say solias. But I'm not sure. My top speed depending on the day is 44 mph at 5600 rpm or 45 mph at 5700rpm. I have no idea on the gear ratio in the lower unit. Those speeds have been checked with a gps. Surprisingly the Speedo isn't all that far off. It only reads a mile per hour fast. At some pionts it actually reads slower then the gps says. Its definitely not linear or consistent. But after 35 mph it actually isnt thatvfar off at all. Is anyone able to help me figure out the amount of slippage I have ? And is it a reasonable amount ?

 

[alldodge]

Going to need the ratio to get exact figures. You could enter a guess and see how it works up. I use bblades calculator

http://bblades.com/propeller-slip-calculator/

 

[StingRay_90V4]

Using that web site. And entering my data in. I get a 5% slippage ? Is that even possible ? I entered 1.99 for the gear ratio. From what i read some are saying i should have a 2 to 1 gear ratio. So my slippage is around 4 to 6 percent high 4's at 45mph and high 5's at 44mph ? I dont knkw ifbthat is acurate ? And if it is acurate . Are they low or high ? Or average.

 

[alldodge]

The 5% is probably accurate. My Rinker (23 foot 4800 lb) is in the 6 to 7%, but my Formula (27 ft 11K lb) is 15 to 16%

For a light boat it should be under 8%

 

[Texasmark]

www.go-fast.com BAM prop slip calculator. Enter what you have and solve for what you want to know.

Example:

Gear ratio of your engine, if 2:1 put in the number 2
RPM put in the rpm at the speed and with the trim and all at the point where you want to know the slip. Usually at WOT trimmed to the sweet spot.....best mph.

Prop pitch in inches. If a 20P prop put in 20.

Punch the max possible solution box....this is a screw in wood type thing, no slippage. Say you come out with 40 (mph)
Fill in the actual box....actual gps speed at point of measurement....say your actual speed at this point is 35mph.

Punch the prop slip box and you will get a number. If it's 15 for example, your slip is 15%.

Actual slip for your rig depends upon a lot of things, some you can correct, some in the design of the equipment. No one number is best for all rigs since all rigs are different in some fashion. The lower the number the better.

Usually the smaller the wake you make the lower your slip, like in my Avatar my slip was less than 10% due to the fact, measured by lack of much wake, not much boat was in the water so not much drag (wind resistance was low too since it had no windshield or things of he sort) so not much to counter the thrust the prop was producing so slip wasn't very high.

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

 

[preventec47]

There is no such thing as SLIPPAGE. In reality it was a term manufactured that is easier to
understand by the masses that works out. In reality the blade is a lifting body foil and it
has the best angle of attack AOA at a certain angle where the best L/D is produced ( Lift over Drag).
IF the angle gets past the best L/D AOA the drag of the prop goes up and the thrust goes down.
IF the angle is smaller than the best L/D AOA then the drag also goes up and the thrust
goes down making the overall efficiency ( defined as the percentage of rotational torque converted
to thrust ) shrink.

Now you can go back to imagining your prop slice through cheese and slipping a little bit
but please dont obsess over something that does not really exist other than to help folks
feel like they understand something they dont.

The only thing I read by the SME ( resident subject matter experts here ) that is wrong
is that he said that the smaller the slippage the better. That is not always true. It is only
true when the AOA is greater than the optimum or best L/D AOA. When it meets the
best angle it is the most efficient and then as the angle goes down further past that point
the efficiency goes down. Consider that at a certain speed that will never happen
unless your boat is being towed faster than it will travel on its own.... the engine will
at a certain speed produce zero thrust and any faster than that the prop will be producing
over negative thrust even though the engine is spinning at WOT.

 

[alldodge]

Hang in there with your AOA and I'll stick with the term and calculation

that works out

 

[Texasmark]

"There is no such thing as SLIPPAGE." Okaye....it's a "term" but a definitive one. If you twist a screw and it doesn't move according to the pitch of the threads, it's doing something besides screwing; that for which it's designed and "slipping" is a good term to me. Opinion: "Stripped" is an option, but not appropriate for threads and water since nothing is stripped.

I'm not a marine engineer nor a prop designer, but what Mr. C47 is saying is proven by performances of multi blade props vs a minimum 2 blade, among other things that I think Mr. DHadley mentioned years ago. Opinion: That's why Carl used 2 blades on his OEM engines since they were the most efficient, but only being 2 "paddles" in 360 degrees, had the highest vibration. Course there's been a lot of water under the bridge since then and we're smarter and the application varieties have soared. But comparing a 3 blade to a 5 blade on a fast boat, the proof is in the pudding....blades get in the way of each other and create drag as C47 eluded.