17p prop running at 0% slip, not possible right?

[sundaypaper]

I'm thinking the answer is no way to run at 0% slip. That being said, I have a 2012 starcraft deck boat with a mercury 150hp fourstroke engine. The number below are from my gauges, I would think they would be in the ballpark being a new boat and outboard. I think the 4 blade numbers below are 0% slip. Does this mean that my tack or speed gauge are off by 10%, or maybe both? How do I check using a gps, can I use my smartphone in some way? The first 2 measurements are from the dealer, the 17p 3 blade is what I had on the boat and matches with the dealer numbers, who must have used the boat gauges, I run the 4 blade now.


SS 19P 3 blade 5600 51mph
17P 3 blade 5900 47mph
17p 4 blade 5800 49mph

 

[smitty477]

Re: 17p prop running at 0% slip, not possible right?

"Does this mean that my tack or speed gauge are off by 10%, or maybe both? How do I check using a gps, can I use my smartphone in some way?"

Yes - this statement from you is likely correct but there are other factors with props.
For example - I have Solas 4 blade SS props on 150 4 strokes and because those props have a decent cup they act more like 1" additional pitch than what they are marked. Similar to your results the original 3 blade props showed a reasonable slip but the new 4 blades calculated out to almost none (measured with GPS and also measured 1/2 mile) on our very light boat.
So each prop brand and style has a characteristic which will defy the exact math of these prop calculations - perhaps if you identify the exact props someone here can comment in their experiences with those props.

Hope this helps

 

[mpsyamaha]

Re: 17p prop running at 0% slip, not possible right?

its unlikely to get 0% slip, the lowest I have seen verified is 4-5%. Usually a boat with a fairly high power to weight ratio with a proper motor height and the perfect prop will get the lowest slip numbers of well under 10%. The easiest first step is to verify your speed with gps. There are apps for smartphones, and if you have a portable gps of some type (even one designed for cars/street navigation) it will accurately show speed. As mentioned above, you should add 1" pitch to the prop if it has heavy cupping.

 

[sundaypaper]

Re: 17p prop running at 0% slip, not possible right?

its unlikely to get 0% slip, the lowest I have seen verified is 4-5%. Usually a boat with a fairly high power to weight ratio with a proper motor height and the perfect prop will get the lowest slip numbers of well under 10%. The easiest first step is to verify your speed with gps. There are apps for smartphones, and if you have a portable gps of some type (even one designed for cars/street navigation) it will accurately show speed. As mentioned above, you should add 1" pitch to the prop if it has heavy cupping.

I'm not sure what type the SS was except it was a mercury, the 17p 3 blade is a black max, the 4 blade is a Solas Amitia, both alum. I understand that modern props have some variable pitch so your explanation makes sense to me. The gear ratio is 1.92, the boat is 2250#, the outboard is a bit less than 500#. The boat was definitely lightly loaded at the time. I will work on getting a gps measurement this weekend. In general, the numbers sound reasonable to me until I started doing math, I would have expected in the high 40s for top speed with at least the 19p, with a high WOT of 5800 I could see it happening with the 17p props also.

 

[steelespike]

Re: 17p prop running at 0% slip, not possible right?

My figures were 2% for the 19. 4% on the 3bade 17 and 4% on the 4 blade calculated at 18".
Possible? Likely there is an error or a number of small errors causing the low results.

 

[sundaypaper]

Re: 17p prop running at 0% slip, not possible right?

Thanks for everybody taking time to help my sanity, I will get some better numbers using a gps and report back if anything weird occurs