Prop selection for 25 hp Mercury

[swash29006]

For the past two weeks, I have been reading posts in this forum and have learned a great deal from the posted experiences of the forum members. Thank you! This is my first post, so I hope that I follow the rules and do this correctly.

I have a 14 foot fiberglass jon boat with a wood floor (fairly heavy boat). This boat is powered by a 1986 2-cycle 25 hp Mercury. I need the forum's feedback on the best prop for this boat. Here's what I have done so far...

I just purchased an EZ Tach Plus (ES 328) and took by boat to the lake with two different props (13 degree and 14.5 degree). I ran my boat alone and then again with five 5-gallon buckets of water (approximately 200 lbs.) up front to simulate a passenger. I did this with both props. The 14.5 degree was about 1 mph faster; however, the RPM's were very different. The maximum RPM's that I could get out of the 14.5 was around 4900. The maximum RPM's that I could get from the 13 degree was around 5700. The maximum RPM range for my motor is 5400-6000. Even though the 14.5 degree was faster, the motor seemed to be under a load with this prop. My thoughts are that it would be better on my motor to use the 13 degree since it allows to motor to reach the proper maximum RPM range. Is my reasoning sound? Also, should I try an 11 degree prop since my boat is fairly heavy?

Thanks!

Steve

 

[sutor623]

Re: Prop selection for 25 hp Mercury

Hey bud, great work! Too bad not all posters ask questions this way. In order to keep that motor as healthy as possible, run the 13 prop. You fall directly in the center of your manufacturers range, which means that if you add some variables here and there you will still be in range. Stick with the 13. The 11 is bound to throw you over your RPM limit, and the 14.5 is overpropping the motor. What was your top speed with the 13P?

 

[swash29006]

Re: Prop selection for 25 hp Mercury

Alone, I was able to get 27 MPH. With the added weight up front, 24 MPH.
Thank you for your reply!

 

[sutor623]

Re: Prop selection for 25 hp Mercury

No problem man. Ease your mind and go out and do some fishin'! Thats plenty of speed for that heavy little boat.

 

[steelespike]

Re: Prop selection for 25 hp Mercury

Prop pitch is measured in inches..Turning in a solid a 13" pitch prop would move 13" forward.
In a liquid there is slip typically 6 to 12% could be slightly more or slightly less. While your speeds seem reasonable
your slip is around 29% with the 14.5 and 34% witrh the 13. I suspect the tach is not accurate or your speed isn't accurate.
More likely the tach.Is the speed by gps?
Checked props for your motor your 14.5 is almost the highest pitch available
and the 13 is on the high side.
Going another way with the calculator it indicates that at 10% slip(a guess) your rpm would be about 4300 at 28 mph.

 

[swash29006]

Re: Prop selection for 25 hp Mercury

steelespike,
Thank you for your feedback. You are correct that the speed was taken with a GPS. I assumed that the tach was correct since it read between 700-800 RPM at idle, which is correct for my motor. However, in addition to the tach readings, I could hear that the motor sounded to be under more strain with the 14.5 prop. Based on this, I still believe that the 13 degree prop is better for my motor and boat combination. I have two friends with the exact same boat. In the past, they both have run 25 hp 2 cycle Mercurys. They both tell me that their speed is usually around 28-30 MPH. As I indicated in my initial post, this is a fairly heavy boat. I have added 1/2 inch plywood in the floor covered with outdoor carpet. When you add things like 8 gallons of a gas, a trolling motor and battery, the setup really gets heavy. Your feedback has got me thinking that maybe I need to look at how the weight is distributed in the boat. Again, thanks for your feedback.
Steve

 

[swash29006]

Re: Prop selection for 25 hp Mercury

UPS just delivered the Mercury Factory Service Manual for my motor. Looking at the specs, it lists the gear ratio as 2.25:1 Using an online prop slip calculator with the following data; gear ratio 2.25, pitch 13, RPM 5700, I get a theoretical speed of 31 mph. Entering actual speed of 27 mph returns slip as 13%. Given the design and weight of my boat, this appears to be possible. Am I calculating this correctly?
Thanks,
Steve

 

[Parrish416]

Re: Prop selection for 25 hp Mercury

Too bad not all posters ask questions this way.

 

[steelespike]

Re: Prop selection for 25 hp Mercury

2.25 ratio 5700 returns 29 mph at 6% slip and as you said 13% at 27 mph. Certainly makes sense.
And 13% could be the result of a heavy boat.
I have observed that a fiberglass boat with an uncluttered bottom may run faster than a similar usually lighter aluminum boat.
My only question about the ratio is my unverified info indicates 2.25 arrived in 91.90 and older says 1.7.
Its a pain but you could varify by pulling the plugs , mark the flywheel and count
the revolutions to turn the prop one revolution.

 

[swash29006]

Re: Prop selection for 25 hp Mercury

2.25 ratio 5700 returns 29 mph at 6% slip and as you said 13% at 27 mph. Certainly makes sense.
And 13% could be the result of a heavy boat.
I have observed that a fiberglass boat with an uncluttered bottom may run faster than a similar usually lighter aluminum boat.
My only question about the ratio is my unverified info indicates 2.25 arrived in 91.90 and older says 1.7.
Its a pain but you could varify by pulling the plugs , mark the flywheel and count
the revolutions to turn the prop one revolution.

steelespike,
I removed the plugs, placed the motor in gear, turned the flywheel to a timing mark, and placed a piece of masking tape on a prop tab. It took 2.25 revolutions of the flywheel for the masking tape on the prop tab to make one revolution. Like you, I had found a unverified source on the web that indicated that the the ratio was 1.7:1, but the Mercury Service Manual indicated 2.25:1. Thanks for sharing with me how to verify this on my on. As I stated in my original post, I am learning so much from this forum and I appreciate everyone sharing their experiences and expertise.
Steve