New prop test

[ris]

Tested new 14 inch X 9 pitch today. Here is the comparison to the 13 3/4 inch X 13 pitch that came with the motor.

RPM 13 3/4 X 13
2000 8 mph
3000 10 mph
4000 no data
4500 WOT 19.5 mph

RPM 14 X 9
2000 6.5 mph
3000 9.1 mph
4000 15.5 mph
5250 WOT 21.5 mph

Both tests were 2 people, 24 gal fuel and the boat was basically empty.



The motor is the honda 60 hp with the high thrust, 2.33:1 ratio. The book says the WOT rpms should be between 5000 and 6000. I did reach this with the 14X9. The 13 3/4 only would reach 4500. My question is for those that know about this stuff is "Is the 5250 rpm high enough that it will not damage the engine running this prop"? I have read enough to know that the WOT of 4500 would eventually damage the motor.
What is the hull speed of the Mac 26X?
Would you save fuel running the 13 3/4 at 1800 rpms to reach 6.5 mph verses the 14 running at 2000 rpm to reach 6.5 mph?
Would it be safe to run the 13 3/4 prop if you stayed around 2000 rpm?

[kurz]

I have read enough to know that the WOT of 4500 would eventually damage the motor.

Whay it should damage the motor?
When you tow with the car a trailer you will not reach max speet. So what????

[Don T]

Hello,
At 10 mph 3000 rpm the 13 pitch is slipping 37%
AT 9.1 mph @ 3000 rpm the 9 pitch is slipping 17%
I'd have to say you will get better fuel efficiency with the 9 pitch.
Of course the efficiency goes up as you slow to hull speed (7.5 mph)

WOT numbers don't jive.

4500 rpm with the 13 pitch @ 19.5 mph = 18% slip
5250 rpm with 9 pitch @ 21.5 mph = 112% (not possible AFAIK unless there is current not being accounted for).

[Don T]

I have read enough to know that the WOT of 4500 would eventually damage the motor.

Whay it should damage the motor?
When you tow with the car a trailer you will not reach max speet. So what????

It's called "lugging" and causes bearing wear and crankcase bearing bore elongation.

[raycarlson]

the days of damaging your motor from under propping are long gone since the advent of computerization,timing and fuel mixture are all auto-adjusted to compensate. No damage will occur in any form, your only losing efficiency. The 13 incher will yield better fuel burn rates at any rpm below 3000.Without a multi-geared transmission like an automobile your motor is geared for max efficiency at or near only full throttle, but at below half throttle your motor makes more than enough power to turn a larger prop. At idle rpm's your 60hp would easily turn a 14X15 prop but would stall out over about 1500rpm.

[Sumner]

I know nothing about the props on these motors but if you are looking for your best prop for the 'loop' you will be spending way, way more time at hull speed or under doing the loop so from what you posted it looks like the 13 inch might be the winner.

Saying that I think I'd run about 450 miles of the loop up the ICW with one prop keeping records of mpg and then switch props and try the other for 450 miles. That will get you most of the way up the east coast. Then I'd go with the winner the rest of the trip,

Sumner

============================

1300 miles to the Bahamas and back -- 2015

The MacGregor 26-S

The Endeavour 37

Trips to Utah, Idaho, Canada, Florida

Mac-Venture Links

[ris]

Thanks for replying to my post. Don not sure what you mean by WOT numbers don't jive. Please explain in simply words. Here is a little more info on the test.
Boat has 2 people, 24 gallons of fuel. 13 3/4 X 13 prop test. Motor had 5 hrs on it, winds about 5 mph, prop stainless. 14 X 9 test. Motor had 30 hrs, winds about 17 mph but we ran test with and against waves, there was little variance in speed or rpm, prop aluminum.

[Jimmyt]

The 14x9" prop math shows you are going faster than the prop is capable of moving the boat (not meaning to put words in Don's mouth). At 5250 rpm with a 9" pitch and 2.33 ratio, I get 19.2 mph at zero slip (which worked out good cause I was out of fingers and toes at that point). Any number of issues could be responsible for it, but the numbers don't work out to an immediately plausible solution.

[ris]

Thanks Jimmyt and Don. So either my tac or my 2 gps are giving me false information. I have no interest in the speed part but if my rpms are wrong then I need to fix that. This is what you are saying isn't it, that one or the other is wrong or am I barking up the wrong tree.

[Jimmyt]

I would verify the prop pitch first (stamped on it). Then the tach and gear ratio. The odds of two gps being off by the same amount is the kind of luck that I specialize in, but rarely happens to anyone else. At any rate, you are correct - something is amok in your data.

[grady]

5250 rpm with 9 pitch @ 21.5 mph = 112% (not possible AFAIK unless there is current not being accounted for).

Unless he painted the screw blue!!!!!!!!!

[1st Sail]

On my M with 50hp Etec I run a 13.75 x 13 for general use. I keep a 14 x 11 as spare. However, I cannot run WOT with the 13.75x13 full ballast. Max rpm is 5400 and the motor will overheat. Switching to the 14 x11 full ballast max rpm is 5700-5800 rpm, ballast out rpm 5900-6000 no overheating issues as the motor is rated 6000rpm WOT. I would be very concerned about not getting to rated rpm at WOT, lugging the engine and overheating.

[ris]

Jimmyt I checked out the gear ratio and it is 2.33, the prop is stamped 14 X 9, the tac is one of those all in one gauge things with a needle and a small electronic panel. When we take it out again, I will bring up the rpms on the electronic gauge and see it it matches the needle. Thanks again for your response.

[Jimmyt]

Sure. Sounds like you've narrowed it to the tach being the prime suspect. If you can, check the calibration at a few different engine speeds. There is a neat calculator on line at http://www.mercuryracing.com/prop-slip-calculator/

Good luck with it and merry Christmas to you and yours.