Prop Slippage

[Tony Athens]

I am currently involved in a potential repower project with a Southern Cross 52 that has 1987 Vintage 8V-92 TI’s in the 730-735 HP range/ 2300 Rated RPM.. 1.54:1 gears swinging 28” x 25” 4B props (maybe 26” of pitch—conflicting email numbers) .. Not sure on any cup but probably has some (it could even have a lot, don’t know) .. I am using 50,000 lbs as the weight.. The performance numbers posted are from the owner. And, more or less, they generally match the numbers from when the boat was close to new 30+ yrs ago…

So here is the ??.. Current performance data shows 28 Kts at 2200-2250 engine RPM.. Based on what I thought I have seen over the past 30 yrs or so, is it possible that this hull at WOT speed could really have such a low slip % ?
My math shows well under a 10% slip.

I’d love some input ( math) from some of our readers as to their observed WOT speed vs. their prop theoretical pitch speeds on vessels that make about 25-35Kts at full speed…

Thanks, Tony

Topic

[Steve Lewis]

Rob and Tony,

I have an accurate idea of the slippage of our Sea Ray, but only at Cruise RPM. I have not done a full RPM graph of performance. Shame on me I guess. My numbers are within reasonable error with Rob’s around 20kts.

I sat towards the back of the class. I was 6′ 4″ in the 8th grade so I got complaints that I was blocking the view of those behind me………….

I have a spread sheet that does the maths for me. I just need to fill it in for the whole range of RPM. On my To Do list for this year.

My equation is just a minor reordering of what Rob has. ((RPM/Trans Ratio)*Prop Pitch)/1056. For those wondering about the 1056 that converts “Inches per minute” to “miles per hour”

Trans Ratio = 2.5:1
Prop Pitch = 21.5
RPM = 3500 (we cruise here most of the time)
Theoretical speed = 28.5 MPH
Actual speed ~ 23 (at the beginning of the season)
slip = ~20% which is close to Rob’s 18%

[Rob Schepis]

Nobody? Nothing? Someone must have sat in the front in math class……

[Rob Schepis]

There’s many online calculators that are basically all the same and I include a link to two examples below.

Whenever possible, it’s always best to understand the math and theory before going to a calculator.

Slip is basically the loss in speed from theoretical speed to actual speed. This amount is divided by the theoretical speed to put it into a percentage format.

The prop is like a screw and the pitch is the distance the screw travels with one revolution. This is theoretical speed.

Because of drag and forces against it, in the real world the prop does not move forward in one revolution a distance equal to the pitch number. This results in actual speed.

The theoretical speed must be calculated first as follows:

(RPM x Pitch) / (Gear Ratio X 1056) = Theoretical Speed MPH

The 1056 multiplier is simply a constant factor for units conversion because RPM is in minutes, pitch is in inches, and speed is in miles and hour. So units of distance and time are converted. Therefore, for these calcs be sure to use MPH, not Knots. (1 Knot = 1.151 MPH)

So using my 2,400 cruise (posted above) as an example:

(2400 x 26) / (2.04 x 1056) = 29 MPH Theoretical Speed

Prop Slip % is calculated as follows:

(Theoretical Speed – Actual Speed) / (Theoretical Speed)

(29 – 26.1) / 29 = 10%

Online calculator examples:

http://www.mercuryracing.com/prop-slip-calculator/

http://www.heringpropellers.com/prop-slip/

[Luke Nelson]

Hi Guys,
Which is the best calculator to use to give the numbers? I’m too slow but curious of the answers.

Luke

[Rob Schepis]

2,300rpm cruise = 20 knots (22.8 mph) = 18% slip

2,400rpm cruise = 22.7 knots (26.1 mph) = 10%

2,900rpm wot = 27 knots (31 mph) = 11% slip

24×26 cupped 4-blades

cup is 35 thou at 50% out and 50 thou at 90% out … basically a “medium” cup

330B’s with 2.04:1 gear (315hp @ 2,800)

Easy to plane hull ….not too different than that in your pic…

[Author]

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