I would appreciate anyone that has experience with changing props to match a boat with using conditions to shed some light on me or tell me where I can read up on this. I have to replace a stock prop and want to make an improvement. If anyone can help it would be appreciated. Great site.

Here is a copy of what I posted on another website. It is about the pitch of a prop.

Paul

In short, the lower the pitch the faster the boat will accelerate and the slower the top end speed. Think low gear in a car. The higher the pitch, the slower the boat will accelerate and the higher the top end speed. Think high gear in a car.

The pitch of a prop is just like the pitch of a screw. A 15 prop would theoretically move a boat 15 in one revolution.

Now there is a lot of variables, slippage is one of them, motor HP and Torque in another.

Hold on. This will get long winded. Sorry :-) as if it is not already.

I have a 16' aluminum boat with a 25 HP motor. With a 13 pitch prop it will go 25 MPH if I am by myself. If I put someone else in the boat it will only do 18~20 MPH and it takes longer to get on plane. The motor also does not reach 5500 rpms. This is where it has the most HP. Check your manual for your motors operating range. Now, if I put on the 11 prop with 2 people the motor will reach it 5500 rpm range, it will go 25 MPH again and it will plane much faster.

Now the reverse. Told you this would be long.

If I have the 11 prop on the motor with just me on the boat will accelerate like a bat out of hell. Top speed is still 25 MPH but the motor is going 6000 RPMs this is beyond the recommend range. I think I need a 12 to have the best of both worlds and not have to change props depending on load.

So after all the above what do you do? I think you should find the pitch of your current prop. Does the current prop perform well with you normal load? If it does, replace it with the same pitch. If your hole shot could be better go down 1 or 2 want a little more top end go up 1 or 2. Now do not think someone can put a 16 prop on their outboard and go 50 MPH. This is where HP and Torque come in. A 100 HP may be able to turn a 16 prop and go 50 MPH but a 60HP may not be able to turn a 16 prop.

Slippage is another factor. A prop turns in a liquid, less friction. A screw turns in a nut or threaded hole or wood. There is a lot of friction there.

Sorry again for the long winded explanation.

My prop is a 23" pitch. It is on a 90 hp Mercury behind a Nitro LX 700. Slow out of the hole. Anyone know what stock is?

Usually the boat manufacture will chose the prop for the boat that will give the best all around performance. You can try calling them.

Do you know what RPM your motor is turning at WOT (Wide Open Throttle)? You will need to find this out before you change props. You do not want to under prop and over rev the motor. A 1" pitch change will change motor RPM around 200 RPM. This depends on gear box ratio.

If you want a faster hole shot drop to a 21" prop and see what that does for you.

Some mercury dealers have a prop testing program where you can try props before you buy them. You should check your local one out.

Paul

How do you measure a three blade prop? Can I just measure radius and double? I was thinking a 21" would be what I need to try. Thanks for the info. so far.

are you trimming all the way down  before  opening up on the throttle

As far as I know there is no easy way to measure the pitch of a prop. The pitch is usually stamped on the hub. You might me trying to measure the Diameter.

Paul

I'm not trying to measure the pitch of the prop, just the diameter so as to know what size prop I have. Isn't that what size is, the distance across the blades, tip to tip? I am trying to find something similar to mine on ebay just so I will have an idea what I'm looking for.

Yes, I am trimming all the way down when I start out. It starts to come out of the water but won't finish and sit down. It just seems to hang there and to get it down and going you have to start trimming. That's the reason I think I need to go down in pitch.

Can anyone tell me what the numbers on the prop mean? The last number is the pitch I know but the others I don't know.  48 77350 A45 23P is on the side of the prop and 48 77350 C1(or Cl) 23 is on the back of the prop on the hub. I have found a prop with the numbers 77348 A45 which is a 21" pitch. Can I determine from these numbers if it will work on my boat?

old goat check your messages