New Pitch Recommendations For Jon Boat???

FRANKtheTAU

Cadet
Joined
Mar 2, 2009
Messages
7
I've got a 2002 Mercury 25hp 2 stroke. I had the motor on a 14/40 riveted jon and it ran like a champ. Well a few weeks ago I upgraded to a 16/48 Lowe. This is a duck boat. I'm going to have it fully loaded (about 900 lbs) pretty regularly. With that said, I've noticed something while driving it. From 3/4 throttle to wide open throttle, the motor revs out like it should, but it's like the power isn't getting to the prop. I guess it's like the prop isn't "pushing" enough water. I've adjusted the tilt pin and that didn't help. It is sitting on the third hole(in the middle). The holeshot and all is fine, but anything past 3/4 throttle its like the power isn't getting to the prop. The prop is a stock 10 3/8 x 13. Is it possible that a new prop with a different pitch could fix the problem. I'd kinda like to know for sure before I just go spending money. I'm looking at a 10 1/4 x 14.5. Thats another 1.5" in pitch. What would that do to my performance, RPM, etc.? Would a 14 pitch prop be fine? Thanks in advance for the help guys.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,291
Re: New Pitch Recommendations For Jon Boat???

I'd kinda like to know for sure before I just go spending money. I'm looking at a 10 1/4 x 14.5. Thats another 1.5" in pitch.

Ayuh,... You're going the Wrong way...
You put the motor on a Bigger boat,.. That's a Bigger load...
The motor needs a Smaller prop to build more Power...
 

FRANKtheTAU

Cadet
Joined
Mar 2, 2009
Messages
7
Re: New Pitch Recommendations For Jon Boat???

Okay. Rookie mistake. Any suggestions on what my new pitch should be?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: New Pitch Recommendations For Jon Boat???

The bigger boat should get up on the water easier.And should get up better with a load.(more surface) Assuming both boats are bare bones.I think you will be pleased when you load it up.My 18ft bare bones Crestliner voyager with an old 25hp Gale planed easily with 6 adults aboard.The old 15 footer would be lucky to get up with 4 aboard.
Where is the antivent (cavitation) plate in relation to the bottom?
You should raise the motor to a point just before venting becomes a problem.
From my limited experience jon boats sometimes run really flat when light and can be difficult to get the bow up.
You can tell a lot better where your at with a tinytach(reasonable,easy to install) and a gps for speed.You shouldn't make any prop changes without that info.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: New Pitch Recommendations For Jon Boat???

Frank, welcome to Iboats prop forum and I will try to help you as much as I can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as humanly possible, if you would prefer to not go to this much trouble there are other people who will gladly help you. My expectations are that you are willing to expend as much time and energy as I will to find a better prop for your boat.

Just copy this form to your computer and paste it into a word processing program and fill it out, then paste it into your post.

Iboats Boat and Motor Info

1. Year, make and model of boat

2. Length, width and base weight of boat, look for boat decal on back of boat

2a.What is the maximum recommended HP for your boat

3. Number of people and gallons of gas normally on boat

4. What do you use the boat for

5. Is it a Deep Vee and if so how many degrees of Deadrise

6. Year, manufacturer and model of motor

7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor

8. Manufacturer?s recommended Wide Open Throttle (WOT) range

9. Anti-ventilation Plate height above the bottom of the transom of boat if it is an outboard in inches, use a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

AntiventilationPlateStraightEdge-2.jpg


10. Is it a bass boat or does it have a pad bottom

11. Does it have a hydrafoil, dolefin or trim tabs

12. Manufacturer, model, diameter, pitch, number of blades and whether SS or aluminum props.
IMPORTANT
1.
2.
3.

13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for less than $ 50
RPM ___________ Speed (GPS)___________ No. of people ____________ Gal. Gas ________

14. Are you at sea level or a higher elevation, give us the elevation in feet __________

15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by only using the control on the console, checked compression, looked at the plugs and checked spark, is the bottom of the boat clean and barnacle free, and have you checked the Tachometer against a mechanics tach, all of the foregoing could be the reason your prop is not attaining full RPM.

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop.

17. Does the prop show any damage that you can see

18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.
1. 3000 RPM
2. 3500 RPM
3. 4000 RPM
4. 4500 RPM
5. 5000 RPM
6. 5500 RPM
7. 6000 RPM

REMEMBER, The numbers I give you will be NO better than the information you give me.​

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

You also might try reading this for a better understanding of changing props for better performance.

http://forums.iboats.com/showthread.php?t=365653"]http://forums.iboats.com/showthread.php?t=365653"]http://forums.iboats.com/showthread.php?t=365653



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