Correct Prop for my Motor

[Dappermuis22]

Hey Guys.

I went fishing this weekend and got a thorough testing of my boat with the motor and the prop.

Boat: Rebel 440
Motor: 75HP Mariner
Prop: 4 Blade D4 x 12.5 X 21RM
Max Speed: 62 kmh - 65 kmh
Max Revolutions: 4100 rpm - 4300 rpm

My Questions are:

1. Im not sure what pitch the prop is that im running at the moment?
2. What pitch prop would be a better option?
3. When im driving at full throttle are the Revolutions correct for the setup of Motor and Prop that im running?

Please guys i need all the help i can get cause i want to get the most and best performance out of my Boat.

All the help and info would be much appreciated.

Thanx. :roll:

[Riprap]

You are running a 21 pitch prop which basically means for every revolution of the prop, it should move the boat forward 21". Rule of thumb, for every 1" decrease in pitch, the rpm should climb about 200rpm and visa versa, so a 19 pitch prop will increase you wot by about 400 rpm.

Which model motor do you have. Your wot seems about 1 000 rpm at least less than max rating?
How's your hole shot when fully loaded?

[Dappermuis22]

If im correct its a 2006 model.

Fully loaded with me alone on the boat i get to 65kmh - 67kmh.

If i change to a 17pitch prop i know the rpms will climb but will my speed be the same? increase or decrease in speed?

And what would be the best a 3blade or a 4 blade?

[BassBaas]

Dappermuis,
Prop selection is a very big subject and every boat and setup is unique.
What I do want to add is that your motor must be able to operate withing the WOT range as prescribed by the manufacturer to get maximum fuel efficiency, extended motor life, best hole shot and overall performance. So even if your boat will not run faster with the correct diameter and pitch prop, at least you will save the motor and fuel. A motor that cannot reach WOT RPM are working too hard all the time.

All the best with your choice but once yoy find the right prop it will be worth it.

[Dappermuis22]

Would a 17Pitch 3 blade prop be a good choice then?

[BassBaas]

Dappermuis,

From what I can gather on the net, your 75 Mercury has a gear ratio of 2.31:1 this means that for every 2.31 rpm’s, the prop will rotate one time.
The recommended WOT for the 75 Mercury is 4750 – 5250 RPM and this is where you ideally want the motor to run when at WOT (Wide open throttle)

Now if we calculate what prop and speed you can do theoretically for your boat (not knowing exactly what the drag of the hull in the water is)

(pitch x RPM) / (gear ratio x 1056) = Theoretical boat speed in Miles per hour.

Since we do not know what pitch prop you are currently running, let us determine with the formula:
Speed = 65 km/h = 40,6 Miles/h
RPM = 4300
Gear ratio = 2.31
Pitch = ?

[40.6 x (2.31 x 1056)] / 4300 = 23.03

The calculator indicates that you are running a 23 pitch prop currently (remember that we did not take into account slippage) Now the general rule is that you will pick up 150-200 RPM for every inch in prop pitch that you come down and visa versa.
So you could effectively increase RPM by 600 -800 if you go down to a 19 pitch prop and then you will be running well within your WOT recommended range.

So my advice would be to go for a 19 pitch prop and not a 17

[BassBaas]

Dappermuis,
Looking at the prop measurements that you have given, you may very well be running a 21 pitch 4 blade prop and it may also be cupped which will add additional pitch.
A 4 bladed prop can run at lower RPM especially if it is cupped and therefore the difference with my calculation bu I still think a 3 blade 19 pitch would be the right choice.

[Dappermuis22]

Thanx for the info Bass

If anyone is willing to borrow me a 17 or a 19 pitch prop 3 blade or 4 blade please for to test it on my Mariner it would be much appreciated guys.

I do not want to spend a lot of money on the wrong thing.

thanx for all the help guys

[Navrik]

Hi Dappermuis22. Please update you profile to indicate where you from should anyone in your immediate area be able to assist.

[Dappermuis22]

just to give you guys feedback i was fortunate enought to get hold of a prop from a friend on the forum to test on my boat.

the prop was a stainless steel 17pitch 3 blade appollo prop.

the results: Before

Prop: 4 Blade D4 x 12.5 X 21RM
Max Speed: 62 kmh - 65 kmh
Max Revolutions: 4100 rpm - 4300 rpm

After:

Prop: 3 Blade 13.5 x 17 Apollo Stainless Steele
Max Speed:62 kmh - 65 kmh
Max Revolutions: 4900 rpm - 5200 rpm

So indeed guys the 3 blade 17 pitch prop works much better on my Motor. My revs are running exactly where they should with not affecting my speed at all and even increasing how fast my Rebel gets on the plane.

Only thing that the 3 Blade does not have what the 4 blade does is more Torque cause i felt it while driving upwind and on heavy choppy water.

So i will definitly be looking at a 17pitch 3 blade.

Thanx for everyones help

[Riprap]

Dappermuis22 - glad you are happy. Main difference with a 4 blade is that it will lift and hold the stern a bit, less blowout on cornering, slower planning speed and allow more trim up than a conventional 3 blade.