Prop selection help to get max rpm

I have a 1748 Tracker Grizzly with a 25hp 4 stroke Mercury Bigfoot engine. Now I'm aware of the gear ratio in the motor and it's original purpose to push bigger boats/heavier loads rather than what I have. That being said, I have tested the prop on it now which is the factory 10.88X11. The results were ok getting the boat going about 23mph fully loaded with 2 people on board. Here's the problem, I'm only clocking about 3000 RPM on my tach, so I'm not getting the engine's full potential. Now I'm using a Tiny Tach and verified it on other motors to make sure it's correct, and it is. So question is whats my best way to get to my optimal range of between 5500-6000 RPM? Should I decrease my diameter and pitch? Any help would be great, thanks guys.

Re: Prop selection help to get max rpm

You have a huge discrepency with the numbers and I suspect the tach is NOT correct. Why?? Because a 11P prop at 3000 rpm with a 2.42 gear set cannot produce 23 MPH with zero slip. It is mathmatically impossible. Fact is, at 6000 RPM with the exact same gears, produces 23 MPH with 11% slip leading me to believe your engine actually is running 5500-6000 rpm but the tach is showiing only 3000. Your ears should be able to distinguish between 3000 and 6000.

Re: Prop selection help to get max rpm

There is no way you would do 23 at 3000 I suspect you have a tach intended for a 2 stroke and you need to double your rpm for the 4 stroke.The 11" pitch is about in the middle of the chart
So should be in the ball park. What is the dry weight of your boat and what is your"fully loaded"
additional weight.Guessing a fully loaded 17' jon boat w a 25 would make about 23.Is the speed by gps? Generally only the pitch is adjusted if necessary.Diameter is a function of the pitch and application.

Re: Prop selection help to get max rpm

Hi guys I belive the tiny tach has settings to read two and four strokes, You have to read the instructions.
For sure it is turning 6000 double what it is indicating. Motor should indicate 900 at idle.

Re: Prop selection help to get max rpm

That could be my problem, the tiny tach has settings based on how many sparks per revolution. I have it set to 360degrees meaning once per rev, but I have 2 cylinders so maybe it has to be something different. I calculated the weight with everything, and the motor size with the type of hull and the results were just about what I'm getting based on this equation=

There is another formula for predicting top speed if ideally propped and trimmed.Prop HP=ABoat gross wght.=BBottom factor=C: 180 for V bottom, 200 for nearly flat, 225 for Flat.Take the square root of A/B and multiply by C.Example: My Montauk has 70hp, weighs about 1900lb. with max load. 70/1900 =.037. Sq. rt of .037 =.19,Nearly flat bottom factor, 200 X .19 = 38.4mph.GPS WOT reads 38.5mph

So that being said I'll be checking the tach, FYI the total weight with persons and gear, including motor batts is 1650lbs. And all speeds are checked using GPS. Thanks guys for the input I'll check it out this weekend.

Re: Prop selection help to get max rpm

Tiny Tachs sense spark on ONE cylinder. Doesn't matter if the engine has one cylinder or a hundred. Also, a four stroke fires once every other rev whereas a two stroke fires on every revolution. You have the right prop and your engine is running 6000 RPM -- not 3000.

Re: Prop selection help to get max rpm

As I understand it the induction wire goes on one plug wire so your 4 stroke one cylinder fires every other revolution a 2 stroke is every revolution.
Formulas will generally have a weak link
in many cases it is hull shape your flat/nearly flat bottom I think really helps the formula success.
other hull shapes are more vague deep V with a known degree probably easier, but the many varied soft chined hulls, rocker hulls,cathedral,tri hull, whaler or twin V types.