Prop for 25 HP Mercury

[Riverman128]

Hi Folks,

I have an Alumacraft 1542 Jon boat with a 25 HP 2 stroke Merc. Max RPM's 5000-6000. With a 15" pitch SS prop, I turn 5570 RPM's and run 29.2 mph solo with light gear. I had my 13" pitch SS prop rehubed and tried it today. I ran 6140 RPM's and 25.6 mph solo. With the decrease in pitch I expected more RPM's but also thought I would see an increase in speed. Why did my speed not increase? Thanks for any help.

Rick

 

[steelespike]

Re: Prop for 25 HP Mercury

First your speed 29.2 is pretty good have you fine tuned your setup.
Motor vertical height can give you more speed.careful trim as well.If you have no ventilation problems your probably not high enough.You can raise the motor by adding 1/4 inch sticks on top of the transom.Raise it untill venting becomes a problem.Be sure clamps have a good grip and it pumps water.Under ideal conditions you could gain a couple of mph.
Generally dropping pitch will produce less speed and more hole shot.
I bet she jumps up quick.Generally 1" pitch change results in a 200 rpm change.You gained close to 600 rpm.You may have a little more slip going on.

 

[mthieme]

Re: Prop for 25 HP Mercury

That's pretty good speed to begin with.
I question it - gps?
Anywho, that motor is good for 6000 rpm and you just exceeded that. You can take it to a prop shop and have it turned into a 14P which sounds like it would be just about perfect as far as rpms go.

 

[Riverman128]

Re: Prop for 25 HP Mercury

Thanks Guys for the replys. I have 2" I can raise the motor to bring the cavitation plate even with the bottom of the hull. I think that should put me over the 30 MPH mark. I plan to do it in steps as mentioned. With the 13" pitch prop, I'm definitely overreving and don't intend to run it that way. The hole shot is very good. I will use this prop with a load (two people & gear) in the boat. As a side note, I installed a "Tiny Tach" on the engine and am very pleased. I installed it on the tiller handle with black zip ties and ran the pickup wire to the engine through plastic wire loom. It doesn't look quite factory but you have to take a second look to notice. I don't plan to have another tiller steer outboard without the "Tiny Tach". It provides both RPM's (2 second delay) and hours operated.

Rick

 

[Riverman128]

Re: Prop for 25 HP Mercury

By the way, I forgot to mention that the speed readings were obtained by GPS.

 

[steelespike]

Re: Prop for 25 HP Mercury

I think that is the first detailed tinytach with mount info we have had.
If you can get 2 inches I bet you will be well over 30.I'll be interested to see the results.
I think a 25 is only rated for about 30 mph in ideal conditions.
light fast boat.

 

[mthieme]

Re: Prop for 25 HP Mercury

I think the 25 is only rated for about 30 mph in ideal conditions.
light fast boat.

I didn't know they rated engines for (top) speed.

 

[steelespike]

Re: Prop for 25 HP Mercury

I didn't know they rated engines for (top) speed.

They don't presently that I know of.But within reason hp is hp and if you keep in mind we are working with approximate figures you can get an idea
of how a given motor is doing from published specs.