If I’m looking for more speed out of the boat and not concerned about planing, which direction would I go from a 10 pitch? Would I be looking at an 8 pitch or a 12 pitch?
Short answer is an 12 pitch would be “faster” than a 10. But we would need more info. What kind of boat, motor, what speed and rpms are you turning at wide open throttle. Are you sure it is a 10 pitch or is that the diameter? Give us all the numbers you see on the prop and the brand.
I plan on living forever, so far so good
Egret 167, Etec 130
Wood Driftboat
The motor is a 2004 Tohatsu 25hp 4 stroke carb. model. The motor had a 10 1/8" X 10 pitch aluminum prop that was pretty beat up I replaced it with the same size stainless. I thought a “whole” prop would do it good but little changed. The boat is a 1546 All Welded Alumaweld Jon boat. The boat weights 550 and the motor weights 159. I have 6 gallon gas tank and a battery and a plywood carpeted floor, all probably adds another 150. I estimate the boat runs about 20 with one person and about 14-15 with two people which seems slow to me (compared to my last rig which was similar with 25 Mercury 2 stroke). Part of my issue is I have no way to determine my rpms. Any thoughts would be appreciated.
Just noticed you had another thread about borrowing a tach. That would be your best option. If not maybe someone can chime in that has a similar setup. Lacking that you will probably need to go with trial and error.
I plan on living forever, so far so good
Egret 167, Etec 130
Wood Driftboat
If you are only concerned about top speed, yes, just increase your pitch. However, the best prop generally will get you right up to your redline RPM and not beyond it. That should get you max speed. Too little pitch and you will over-rev and will have to back off the throttle a bit. You will likely get less speed as well, cause you will have to back the throttle off. If it’s too pitchy, you’ll never get up to max RPM, and so also be slower than optimum.
Tidewater 196DC
Yamaha F115
Pungo 120I purchased a tach off ebay for 17.95 and determined my rpm’s are 4620 at wide open throttle with the 10 pitch. Manual says wide open throttle should be 5-6000. I have check under cowling and the throttle cannot be open any further. I read that 1 degree in pitch will change rpm,s about 500. Therefore to achieve 5500-5600 I need to drop down to about an 8 pitch. Any thoughts would be appreciated.
I changed the stock prop off my 25 Merc on a Carolina Skiff J16 to a 4 plade composit prop from OVERTONS, what a differance, faster and hoped out of the hole! One of the best things I have done to the skiff
you need to drop pitch size if your rpms are accurate
do you have access to a hand held gps to determine speed?
Knowing your RPMs and speed are accurate is your first step to determining what you need to do. And assuming the boat is not under powered, which yours does not seem to be under powered, what you should be targeting is getting your WOT RPMS within the Mfg suggested range. If you are running under the recomended range at WOT the motor is working too hard The math for getting you in the ballpark in determing what pitch you need is as follows:
Theo Speed rpm/gear ratiopitch60/12/5280
Slip % (Theo Speed-Act Speed)/Theo Speed*100
So for example with the Tohatsu (current year model) 25 4S has a 1.92 gear ratio. I assume they have not changed the gear ratio. But if you have the owner manual you can confirm the ratio.
Theory
Pitch Gear Ratio RPM Theo Speed
10 1.92 6000 29.6
10 1.92 5500 27.1
What you reported
Pitch: 10
Gear Ratio: 1.92
RPMs: 4620
Theo Speed: 22.8
Act Speed: 20
Slip% 12.23%
Theo Speed = 4620/1.921060/12/5280 or 22.8 mph
Slip %= (22.8-20.0)/22.8*100 or 12.23%
So based on what you reported 4620 rpms, mathematically a 10 pitch prop on that engine should go 22.8 mph with 0% slip, slip percentages of 10% and below are normal. If you are losing speed because of slip more “cup” in the prop gives it a better bite thus less slip and more speed. In your case the slip is not too bad and more cup would give you alittle speed but would lower your RPMs alittle.
So consider the math below below you pick your pitch size. Also the general rule is 1" of pitch equals 200 RPM not 500 RPM. So if you drop to an 8 pitch prop you should pick up about 400 RPMs which will get you in the manufactures range, but you will only pick up .9 mph based on theoretical calc.
Pitch Gear Ratio RPM Theo Speed
8 1.92 6000 23.7
8 1.92 5500 21.7
9 1.92 6000 26.6
9 1.92 5500 24.4
So something is strange with your setup, based on your reported speed and rpms the math is roughly correct (mean
Pursuitgang thanks a million for the calculations. I had already purchased an 8 pitch prop before I found your explanation. The 8 pitch prop is turning 5620-5680 at WOT. That puts it within range. I could have probable got away with the 9 pitch though. It’s alot better out of the hole but not much difference in speed. I will figure out a way to determine speed and post my findings.
Make sure you are running on all cylinders. A 10p should be about right for your setup. Crank the motor with the cowl off and then use a pair of well insulated spark plug wire pliers and pull one of the wires off of the spark plug. If the motor slows down or cuts off at all, then that cylinder is operating properly. If there is no change then you have problem. Put the wire back on the plug carefully and repeat the process for the remaining plug wires. If you find a problem, its probably with one of the carbs. Have them cleaned by a reputable mechanic. Just be careful because those plug wires will shock the pee out of you!!
Thanks, I’ll check it out. Anyone out there with this same motor, 25hp Tohatsu 4-stroke, 2004, that can tell me what prop pitch you are running, rpm’s and speed?