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  Props--Push, Pull or Both??

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Author Topic:   Props--Push, Pull or Both??
davej14 posted 07-02-2004 12:04 PM ET (US)   Profile for davej14   Send Email to davej14  
Here is an interesting question for persons knowledgable in propeller dynamics. I have been experimenting with various props on my Dauntless 14 foot with 75 merc 2stroke and have observed that up to the higher midrange of RPMs, I calculate about 6% or 7% slippage. As I approach WOT, however, the slippage decreases and actually goes negative at WOT by about 2% to 3%. Here are some numbers using a Black Max 19 pitch aluminum prop to illustrate my point:

3200 RPM = 23 MPH (about 6% slippage)
4000 RPM = 30 MPH (about 6% slippage)
5200 RPM (WOT) = 41 MPH (about -2% slippage)

I am measuring speeg using a gps that accurately tracks with my car's speedometer. So what is going on here? Either my tachometer is significantly off at WOT or could it be the hydrodynamics of the prop blades at higher rpms are actually creating "lifting" forces on the front side of the blades that propel the boat faster than if they were simply pushing the boat. As an old sailor, I am aware that the forces on the downwind side of a well trimmed sail can be 7X+ greater than forces pushing the sail from the windward side. Could something similar be happening here? There are a number of posts that seem to have hull speed in excess of what a prop should produce at WOT. Before concluding that my tachometer is erroneous I would appreciate some knowledgable opinions.

Plotman posted 07-02-2004 02:49 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Much more likely that the effective pitch on your prop vaires from what is stamped on it. Remember that pitch is an average across the blade (from stem to tip), and that things like cup can make a prop act like it has more pitch than a simple measurement of the angles might.

So my guess is that you probably have a prop with more pitch than it says, and that your slip is greater at the slower speeds, and more like 5-6% at WOT.

jimh posted 07-09-2004 02:00 PM ET (US)     Profile for jimh  Send Email to jimh     
There are several sources for measurement error:

--tachometer (as you mentioned)
--GPS (not much error if WAAS or DGPS)

Also, the effect of current or wind could easily influence the speed measurements.

The tachometer could easily have some error. It would not surprise me that a boat tachometer gauge might have a ten-percent error. These days there may be inexpensive ways to accurately measure speed of rotation with some sort of a "gun" device, and see if the tachometer is accurate. A good engine shop should have a "shop tach" which has been tested and calibrated, and this could be used as a reference.

A modern GPS ought to give accurate speed measurement, but sometimes they may be affected by interference or by intentional dither on the signal. As the boat speed increases it will take more processing power to keep up with the speed calculations, and some GPS units might not be up to the task with as much accuracy as you'd like.

And it goes without saying that the GPS derived speed is in reference to the ground, not the water, so any current will affect the boat speed. A tail wind or head wind will also affect the boat speed. Running trials over the same course in opposite directions will help to cancel these effects.

If all measurements are accurate, then there is always the notion of the propeller's rated "pitch" being something of an average of the actual pitch. Modern propellers are not always designed with a constant pitch across all portions of the blade. The effect of this may be more noticeable as the boat speed increases.

The manufacturer may well know that a particular propeller is not exactly the pitch he advertises, but the way the propeller works and responds may be similar to propellers with a constant pitch of the same rating, so that is the number he uses to refer to the blade pitch.

But all things being equal, test two propellers against each other on the same boat, with the same instrumentation, on the same water, and in the same conditions, you ought to be able to get comparative measurements. You may not get absolute measurements, and thus end up with problems like negative numbers for the SLIP.

davej14 posted 07-09-2004 09:50 PM ET (US)     Profile for davej14  Send Email to davej14     
Jim,

All good points. When I determine speed I always average two opposing runs on the same track to eliminate the effects of current and windage. I know the GPS (a Garmin etrex vista with WAAS) is accurate because I use it in the car all the time and it tracks with the car speedometer within .1 mph. The two remaining variables are tachometer accuracy and effective prop pitch. I'm going to see if Mercury will respond to a question about effective prop pitch. I am wondering if anyone has experience with merc tachometer accuracy. There sure seem to be quite a few speed claims beside mine that do mot seeem to be credible.

Marlin posted 07-10-2004 09:32 PM ET (US)     Profile for Marlin  Send Email to Marlin     
I calibrated my tach last year using a video camera and a PC. I shot video of the tach, with the engine noise on the audio track. I uploaded it to my computer, then did an audio analysis (FFT) to find the principal frequency in the soundtrack. Turns out with an inline-4 4-stroke, the principal frequency component is equal to the RPM (when converted to revs per second, that is). I'd expect an inline-4 2-stroke to have a principal frequency 2x the RPM.

I'm a sick, sick man. Or, just a normal engineer.

The tach, by the way, was right on.

-Bob

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