Prop pitch (photograph)

[Pack Rat]

After reviewing manf. recommendations I purchased a spare prop (above left) 9 3/4" diameter with 9 1/2" pitch. I could not determine what the original prop was other than 9 3/4" diameter (above right). There are no stampings on the prop.

If I think of the props as a wood screw, left prop fine thread, right prop coarse thread; the left prop will move the boat through the water a shorter distance than the right prop...correct?

 

[steelespike]

Re: Prop pitch (photograph)

It does appear so.What are you trying to accomplish?

 

[Don S]

Re: Prop pitch (photograph)

With no slip
A 17" pitch prop would go 17" in one revolution through water.
A 25" pitch prop would go 25" in one revolution through water.

 

[Pack Rat]

Re: Prop pitch (photograph)

It does appear so.What are you trying to accomplish?

Slow the boat down for trolling, get on plane faster. Heres a slightly diff view of the props.

 

[steelespike]

Re: Prop pitch (photograph)

Being a smaller motor the pitch change my slow the boat enough. on larger motors its usually a lost cause.
What motor are you running it on?What boat?
You need to be aware depending on the setup the motor could over rev. Do you have a Tach?

 

[Pack Rat]

Re: Prop pitch (photograph)

What motor are you running it on? What boat?
Do you have a Tach?

2010 Mercury 25 hp 4 stroke, on a 12.5' inflatable, and no tach.

I think I figured it out with the crude drawing below. The prop on left is the new prop and if I compare the angle of how it sits on the prop (red line), the degree is not as severe as the original prop (on right). I definitely should get more RPM with the new prop which is what I wanted. I'll see what I can do to hook up a tach to verify.

 

[Johny25]

Re: Prop pitch (photograph)

That to me looks like a huge reduction in pitch? With that little boat I would think your motor is going to run higher than recommended RPM at WOT. I know when I compare my 11 pitch to my 13 pitch I can barely tell the difference but that is very noticeable! That one on the right looks to be a 13 pitch or better. And dropping that much pitch could raise your RPMs by 800 or more. I would be careful with running that low of a pitch on a light rig. Just my 2 cents : )

 

[steelespike]

Re: Prop pitch (photograph)

A tinytach is reasonable easy to install and can be used as a hand held or permanent.Also will work on any motor.Though if used on a multi cylinder may need a shielded induction wire.I think thats more than 2.

 

[Pack Rat]

Re: Prop pitch (photograph)

Yes it's a three cylinder.

The photograph is somewhat distorted showing the original prop (on right) at a much more severe angle...but it was the best I could do to show the difference. I have attached the instructions from Mercury regarding props that came with the motor. The 9 1/2 dia, 9 1/2 pitch is one of their 4 recommendations as you can see.

 

[Lyle29464]

Re: Prop pitch (photograph)

I would really cool it on the gas till you get a tack. Looks like the new prop is for a much heavier boat

 

[TorchedGT]

Re: Prop pitch (photograph)

I would really cool it on the gas till you get a tack. Looks like the new prop is for a much heavier boat

I feel like you're going against the suggestions of Mercury and those on this forum (and you know it) but are still looking for vindication.

Obviously the pitch is a dramatic reduction - no drawing needed to realize that.

The prop you bought is for a different sized boat and a different purpose - the one which was on there before was chosen for a reason. If you run that prop, your top speed will take a HUGE hit, and to get to that speed - you're going to risk over-revving your engine and damaging it. Without a tach you'll never actually KNOW how close you are to damaging your engine (your previous prop acted as a rev-limiter, and by removing that, you're asking for trouble)

This all seems like a huge gamble to end up with a slightly better trolling speed??

 

[steelespike]

Re: Prop pitch (photograph)

What trolling speed do you need?
The 9.5 isn't the end of the world but you do need to have a tach to know where the wot rpm is.
I assume it has a rev limiter but thats not a good way to estimate rpm.
That type inflatable is reasonably light but not particularly fast. the 9.5" at 6000 will do about 26
a 12.5 would do about 34. These speeds may vary do to the hull design.If you order a Tinytach be sure to tell them it is a 3 cylinder
as it may need a shielded cable.