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Tom W Clark wrote:
As a general statement, too much is made of propeller slip. More slip is not necessarily bad. If the boat is faster (or more fuel efficient, accelerates better, whatever your goal is) with a higher slip, then so be it. The proof of the pudding is in the tasting.
I agree, which is why I pointed out earlier that high-performance hulls with ventilated bottoms often slip at 15-20% at WOT, even with 4-blade props. But in this case, the amount of slip seems atypical for this hull. Granted, I have very little experience with Whalers so I'm judging typical slip by looking at a number of NEW boat tests. I realize there are shortcomings to that approach. I select the tests I use carefully. I can't tell you how many boat test results I've come across where the data is mathematically impossible. So, either the data was wrong or there was a misprint.
Kamie, I wasn't aware of your tach setup but now that I know, I'm suspect it's accurate as you said. What was the WOT data with your 150hp Mercury? And prop? It would be interesting to compare.
Edited by Ric232 on 09/06/08 - 11:29 AM
Ric
2008 130 Sport
Merc 40hp 4-stroke
Laser II 19P - 5200 @ 42MPH
Offshore 17P - 5600 @ 40MPH
Engine was a 1995 150HP Merc 2L
Boat had a T-Top, since removed but basically everything else was the same
Ok. Assuming the Merc had a 1.87 gear, the slip figures are similar between your old and new motors so nothing went awry with the addition of the new motor. I guess it's characteristic of your particular boat for the reasons we discussed previously.
The plot thickens. In that case, the old Merc motor exhibited 10-12% slip, which is more reasonable. I'm baffled as to why a motor change would cause your slip to increase to 20%. It may not really matter at the end of the day but I'm curious.
Edited by Ric232 on 09/07/08 - 7:06 AM
Ric
2008 130 Sport
Merc 40hp 4-stroke
kamie wrote:
I doubt the Tach is wrong since it it nothing more than a head unit for the engine, that would imply that the engine didn't know it's RPMs. I am running I-Command Digital gauges, not your standard analog. As for the GPS, it's possible that it could be off, but since I don't have a spare, this one will have to do. The GPS is a Garmin GPSMAP180, with an external antenna. The Navman 3100 has been calibrated, and I will check the calibration before I do more testing, because I need to fill the tank, while I have the boat pulled out for storm avoidance. It is possible the 3100 is slightly off, I know that it and the engine often disagree about fuel flow, although not by much.
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Does the Evinrude gauge provide a GPH readout? If so, what are the readouts at the various RPM? Assuming the fuel flow and RPM previously reported are correct, you've got to figure out why the fuel flow rate is low. If it is truly low, then part of the problem with the expected higher top speed is that the motor is simply not getting the quantity of fuel it needs to get to that higher top speed.
Is the GPS set to display SMPH versus NMPH? 42 NMPH is 48 SMPH. If the GPS was set to read out speed in NMPH, then the 19 Laser numbers look about right. Still wouldn't explain the low fuel consumption rate across the board however.
The Evinrude gauge will display GPH, but I did not use that gauge only the 3100FF. I can note the differences next time i am out testing props. My GPS is always set to SMPH, I can't ever do the conversion from NMPH.
I struggle with a fuel issue as the cause, especially since the identical props gave simular results on my older Merc 150HP. If we were to look at fuel delivery as an issue then I would need to swap out my fuel tank, or at least replace the fuel pickup. All hoses from the tank to the water separator and from there to the engine were replaced when I installed the new outboard. To be sure, I could eliminate the water separator filter and run a new line from the tank pickup, directly to the engine. That would not eliminate the tank fuel pickup, but it's the best I could do.
My plan is to pickup a 19P Vensura and see how that prop plays out. Ultimately speed it not really what I am after, hole shot and mid range crusing are tops on my list. I will also retest the Laser II with solid plugs and see what that does for me.
