Rookie prop slip question

[bryonthefly]

First of all I would like to say hello to eveyone, you have a great site here. I traded in my good ol '87 Basstracker for an '08 Lowe FM165 with a 75hp Mercury Optimax last fall and really didnt have much time to dial in a prop set-up. Im running a Blackmax 13 by 19 prop now that gives pretty good performance. Good holeshot, 40.2 mph on GPS but blows out a little in the corners. I can trim down a little to eliminate the loosness in the corners so Im really not to concerned about that aspect. My concern is WOT rpms, only showing 5200-5300 on tach. I ran the numbers through Mercurys slip calculator and Im only showing 2-3% slip. Is this even possible? Im suspecting that my tach may be off a bit?? How do you make sure your tach is not lying? The numbers: 19p prop, 2.33 gear set, 5300 RPM, 40 MPH equals 2% slip. Thanks!

 

[Bondo]

Re: Rookie prop slip question

Is this even possible?

Ayuh,.. Probably Not.... 8% to 12% is considered Very Good....

You can test your Tach against a know good Shop tach to see if it's telling the truth...

 

[180shabah]

Re: Rookie prop slip question

Were you traveling up river or down river?

Cane make a HUGE difference in your SOG, and thus slip number.

 

[hwsiii]

Re: Rookie prop slip question

Bryon, I know nothing about the Black Max props, but there are two things that can make a prop do that. First is if it is a Progressive Pitch prop, on a progressive pitch prop the pitch on the leading edge may be 17" and the pitch on trailing edge may be a 21" or more. When they sell progressive pitch props they claim they are the average pitch of the prop, which makes it a 19". Also if it has any cup on the trailing edge that can also easily add up to 1.5" of theoretical pitch, which makes it act as a 20.5" pitch when you are running. It could have come from the factory like that or they could have brought it to the prop shop and had cup added to it. But, if you didn't make a run in the opposite direction, it just may be that you had a current pushing you to that extra speed, or your tachometer could be wrong. If neither of those possibilities are the case then these are my recommendations. What I see wrong is that you are running on the low end of your maximum RPM. It is better to be running in the top half of the manufacturers recommended power band in RPM for longevity of the motor so it is not lugging. I wish I had the answer for you, but I don't because I don't know if the prop has been changed in geometry since it left the factory. But if it hasn't I would recommend you change to a 17" in the exact same prop. This would raise your RPM to right at 5,650 RPM and is in the manufacturers WOT recommended upper power band, and better for the motor. I can tell you for sure though, you are NOT running at a 2% slip factor. Here are some numbers I ran on your boat and motor. Hope this helps.

H

 

[bryonthefly]

Re: Rookie prop slip question

Thanks for the excellent info! I do have a 17p prop ready to try out and Im going to take it by the dealership to have the tach checked out. It may take a couple weeks for the weather to cooperate but I will post the results of my further testing. Thanks again.

 

[hwsiii]

Re: Rookie prop slip question

I can't wait to hear how it works for you Bryon.

H

 

[Silvertip]

Re: Rookie prop slip question

Before spending money on props you should always work with engine height. We are not talking trim here -- it's the physical mounting of the engine. Check the position of the AV plate on the engine in relation to the bottom of the hull. A good starting point is an inch or so above the bottom of the hull. Fine tune from there. This process alone can pick up a bunch of rpm that directly relates to speed. Just don't go so high that you lose water pressure. Also remember that props from various manufacturers will all perform differently so just because you went from a 19P Merc prop to a 17P from someone else, you may experience very different results that may turn out to be negative or positive.

 

[bryonthefly]

Re: Rookie prop slip question

Before spending money on props you should always work with engine height. We are not talking trim here -- it's the physical mounting of the engine. Check the position of the AV plate on the engine in relation to the bottom of the hull. A good starting point is an inch or so above the bottom of the hull. Fine tune from there. This process alone can pick up a bunch of rpm that directly relates to speed. Just don't go so high that you lose water pressure. Also remember that props from various manufacturers will all perform differently so just because you went from a 19P Merc prop to a 17P from someone else, you may experience very different results that may turn out to be negative or positive.

Thanks, I just went out and double checked and my plate is 1-1/8" above bottom. If I dont get good results from the different props I have to try, Ill try a motor height change. I have aquired quite a selection of props for testing. I have 2 Turning Point Hustlers, 13.25x19 and 13.25x17 that I purchased right here on Iboats for an outstanding price ($87.99 each with hub!) I also now have 2 props on loan to try, a Mecury Vengeance SS 13x18 and a High Five SS 19p. Just waiting for mother nature to let me get out a do some testing.THANKS AGAIN.

 

[hwsiii]

Re: Rookie prop slip question

You are very welcome byron, and please get back with me on the WOT RPM and speed numbers, as I use these in my database to make sure I have the most up to date formulas for the numbers I give to people on changing props to find the right one.

H

 

[bryonthefly]

Re: Rookie prop slip question

Just a little update, 4-25-09
I finally got the boat out yesterday in less than ideal conditions for prop testing. The wind was swirling around the lake at 25-30 mph with gusts in excess of 40, like i said, less than ideal conditions but i had to get out since the temps were in the low 80s.
Load... two BIG guys and normal day of fishing gear with 13 gallons of fuel. First of all, the Turning Point Hustler solved all issues of breakout in the turns i had with the Mercury Black Max, period. Tight full throttle turns with no trim change.
I tested the 13.25x19 first. Excellent holeshot, boat jumped right up with no "mushy" feel from the prop vents. Top speed with this pitch was 38.7 MPH on GPS over a heavy chop. WOT rpms was 5400, I noticed that i could get more bow lift by being able to trim higher without breaking loose than the Merc prop, quite a bit higher on the trim guage.
Next I put on the 13.25x17p Hustler. Again, excellent holeshot, not any better than the 19p though. Got 38.3 MPH at 5600 RPM. Also was able to minimize wetted boat surface area with more trim angle than the Merc prop.
Conclusion, the conditions didnt allow for me to make a definate choice, Im going to keep both and do more testing on a better day in the future.
Im Leaning towards running the 19 and trying to raise the motor per Silvertips advice to gain a couple more hundred RPMs to get as close as possible to the top of the 5000-5750 range on the Optmax.
Thanks to everyone for your help, Ill post further testing results, Thanks again.

 

[hwsiii]

Re: Rookie prop slip question

Bryon I am glad everything has worked out great for you. But I do have one question, I show only a 1% slip on your 17" Hustler and yet I show an 8% slip with the 19" Hustler. Do you have any ideas why?

H

 

[bryonthefly]

Re: Rookie prop slip question

Bryon I am glad everything has worked out great for you. But I do have one question, I show only a 1% slip on your 17" Hustler and yet I show an 8% slip with the 19" Hustler. Do you have any ideas why?

H

Im guessing I caught a wind gust to propell me up to 38.3 on the 17p. All results were max speed on Garmin GPS. The conditions with the swirling winds make the data pretty useless, just wanted to give an update. I hope to give you some solid numbers for your calculations soon hwsiii. Its like comparing apples to oranges cause specs with the Merc prop were on the same lake but fairly flat conditions.

 

[hwsiii]

Re: Rookie prop slip question

No problems, and I thank you very much for the update Bryon. Riding in those kinds of winds I can understand why. And I do agree with Silvertip, he is right, anytime you can raise the motor without creating ventilation problems you are going to gain speed. LOL

H