New prop test results

McGillicuddy · #1 05-18-2009, 09:54 PM

I finally ran a new Merc Black Max 13-1/4 x 17 on the Seafari yesterday - closer but not quite there. 2 of us ran with 30 gallons of gas, 20 gallon bait tank, a pretty standard fishing gear load. Got 30 knots from the 115 hp I-6 but only 5000 rpm. I went from 4400 rpm on a 21 pitch to 5000 rpm on a 17" pitch. I was hoping for at least 200 rpm gain per inch of pitch drop... I guess I'll have to drop some more pitch. Gas was old. Would that have any effect on rpm? Also have a stingray foil which I'll shed next week for performance comparison. I'll check my cav plate height, too. I was hoping to get closer to 5500...
Any thoughts?

Bushwacker · #2 05-18-2009, 10:36 PM

Gillie - that's about what I got on my 115 with that same size prop. A 13 3/4 x 15 pitch stainless prop should get you about 5500 with cav plate about 1" above keel. I could usually run 32-33 mph, maybe 34 with a light load, and boat has bottom paint on it. Denny

fdheld34 · #3 05-19-2009, 09:06 AM

Gillie-
The stingray foil slows down the boat??? Will it hinder you from getting up on plane quicker??..also does it assist with better and easier steering (keeps boat from pulling to one side??? I was thinking of purchasing one as well. Dont know to much about props!
-Fred

McGillicuddy · #4 05-19-2009, 01:07 PM

I don't know if the stingray slows the boat down. By design, it helps the boat to get on plane faster. I've heard arguments that it will reduce top end but I don't see the relevance of 1 or 2 mph. I think the foil can be a detriment to steering, especially if making hard turns - it may grab too much. I also think it also puts a lot of drag on the leading side when trying turn into a quartering sea.

I'll take the foil off, run fresh gas and try to note the difference next weekend. Definitely need to go to 15 pitch to get the rpm up. I didn't calculate efficiency cause my fuel transducer wasn't being read by the meter (always something) . And I was hung up on that instead of just doing the math.

The only reason to have a foil is to assist you in getting on plane if you have stern squat issues. It will also help you stay on plane at slower speeds and therein lies the argument that it will help fuel efficiency. I think a four blade prop or a stern-lifting three blade prop like the Black Max and its equivalents is a better choice. And trim tabs would help a lot more in controlling planing and balance. Also, out here we fish in and around the kelp and the wing is one more thing to get you hung up on kelp. I have another foil, which I may try also. It has better speed ratings in some tests I've read.
http://www.sesport.com/5_29.asp

Anyway, looks like its back to the drawing board,more testing to follow.

eggsuckindog · #5 05-19-2009, 01:20 PM

If this is an inline Merc 115 they don't like alot of RPM, just an FYI. If you no cavatation issues raise the motor at least 1 hole, look and measure first it may take 2 - but that will give 100 to 200 and help the hole shot. If the motor is low it will drag I would think ??

McGillicuddy · #6 05-19-2009, 03:48 PM

ESD, it is an '83 inline-6, and I appreciate the the insight because this is my first Merc. Right now the bottom of my anti-cav. plate is at 1-1/4" above the keel so I think that's about right. I'm not cavitating or ventilating. Not sure what you mean about the motor not liking too much rpm...

Here's Fr. Franks earlier comment regarding this motor.

"The inline 6 "Tower of Power" Mercury motors are fantastic motors and nearly bulletproof when cared for, but there are some idiosyncrasies to worry about. Never, Never run these motors between 6200 and 6900 rpms - they'll self-destruct quickly from engine harmonics. (Believe it or not, running higher rpms than that is okay up to about 9000 rpms, as long as you run the right fuel-oil mix. Mercury Marine used to race these motors turning 8000 to 9000 rpms, making over 225 hp out of the 115/125 hp motors, and over 250 out of the inline 150 motors.) But these motors love to run at 5400 to 6000 rpms. I have owned no less than 9 inline 6 Mercury motors in my life, and still have an '86 115 in the shed.

I suggest you be very careful with the fuel/oil mixture, based upon your average rpms. If you run mostly over 4800, I suggest a 40:1 mix rather than 50:1 mix, that'll keep your top cylinder better lubed. However, if you run mostly below 4800, stay with the 50:1, or your spark plugs will need periodic cleaning."

Are you in agreement with this, or what do you consider "a lot of rpm"? Merc manual's range is 5000-5500 for the 115 hp.

I'm at 5000 rpm WOT now with an avg. fishing load. I think that a 15" pitch will get me to 5300 rpm and hopefully 5500 rpm, but I doubt the 5500, based on the fact I only increased 150 rpms per inch of decreased pitch.

Am I OK running this 17 pitch for a while or is it putting undue stress on my motor?

Fr. Frank · #7 05-19-2009, 05:06 PM

The max-power for the inline-6 is variously listed as 5000-6000 rpms. With all of my I-6 motors, I have aimed for 5800 as my target max rpm. Some of the newer I-6 (newer being a relative term) motors achieved rated hp at lower rpms with the new IC ignition systems.

Check the specs on your particular motor. Try to find a prop that gets you the maximum rpm of the maximum recommended rpm range while your boat is at max load. Max of max at max.

Note: This is NOT the most efficient setup for fuel economy.