Prop Slip results

[salar600]

I tested my E-Tec 75hp with 2 old BRP Alu prop - 3 blades and these are the results I get.

Does it make sense for a 16ft fiberglass boat? Prop slip numbers seems to be very low or very good???

Note: I didn't push to WOT for the 13'' of pitch prop to keep RPM in the safe range and not over rev the motor.


TheSpd- Rl Spd- RPMs- Pitch- Gr Rtio- Shft RPM- Prp Slip
13 3/4x15
39.1 / 37.9 / 5500 / 15 / 2.00 / 2750 / 2.97

14x13
35.1 / 31.1 / 5500 / 13 / 2.00 / 2850 / 8.23

I'm suprised the 13''pitch has more slip.

 

[trendsetter240]

Re: Prop Slip results

Are you using GPS for the speed? Are you certain the tach is accurate? Are you driving with the tide/current/wind etc?

All of these things affect the slip calculation. Using your numbers for the 15p prop I get -1% slip which is impossible. So, one of the numbers must be wrong. Speed, RPM or prop pitch. We know that gear ratio is 2:1.

I also get 8.14% slip using your numbers for the 13p which is a much more reasonable number.

 

[salar600]

Re: Prop Slip results

Are you using GPS for the speed? Are you certain the tach is accurate? Are you driving with the tide/current/wind etc?

All of these things affect the slip calculation. Using your numbers for the 15p prop I get -1% slip which is impossible. So, one of the numbers must be wrong. Speed, RPM or prop pitch. We know that gear ratio is 2:1.

I also get 8.14% slip using your numbers for the 13p which is a much more reasonable number.

Yes, I used a GPS. Light wind on the lake but got the same speed on each way.

I used Excel to calculate, can't see any mistake. How can you get -1% of slip if real speed is slower than Theo speed?

Mart

 

[trendsetter240]

Re: Prop Slip results

Yes, I used a GPS. Light wind on the lake but got the same speed on each way.

I used Excel to calculate, can't see any mistake. How can you get -1% of slip if real speed is slower than Theo speed?

Mart

You're right, I made a mistake. I used the first number (39.1) instead of the second (37.9) which comes out at 2.98% slip.

2.98% is still very low especially for an aluminium prop. It could be that the tach is off a couple hundred RPM or the prop has slightly more pitch that stated.

Also different blade designs have a different effect on the attitude of the boat. Some props lift the stern, some lift the bow and some provide overall lift. The 13p might not be lifting the boat as much as the 15p and therefore there is more surface area of the hull in the water. More hull in the water = more drag and more prop slip (in most cases).

 

[salar600]

Re: Prop Slip results

Tach and motor are brand new from BRP dealer so I hope it gives accurate numbers... might be on the prop's side? They are 2 old prop so I don't have any data about there design.

The 15p is on target for RPM and has almost no slip. But just for thought, based on these numbers do you think that any prop changes could only results in worst performance/numbers. Let's compare it to a 17p, 15p-4blades or 13p-4blds.

Mart