Impact of Prop Pitch on Trolling Speed

[LDUBS]

Because I'm curious and asking dumb questions doesn't bother me.

Suppose the following:

1. Idle RPMs are correct and at idle the boat speed is 2.3 mph.
2. With the current prop the motor is running below the WOT specified RPMs.
3. Prop is replaced with a lower pitch one to raise the WOT throttle RPMs to where they belong.

I understand, I think, that idle RPMs won't change. My question is does that lower pitch prop have any impact on the boat's speed at idle?

 

[FuzzyGrub]

Because I'm curious and asking dumb questions doesn't bother me.

Suppose the following:

1. Idle RPMs are correct and at idle the boat speed is 2.3 mph.
2. With the current prop the motor is running below the WOT specified RPMs.
3. Prop is replaced with a lower pitch one to raise the WOT throttle RPMs to where they belong.

I understand, I think, that idle RPMs won't change. My question is does that lower pitch prop have any impact on the boat's speed at idle?

Your idle in-gear speed will go down a little.

You can use one of the prop slip calculators, like Mercurys. Enter all the info for your current idle speed, rpm’s, gear ratio, and pitch. It will calc slip. Change pitch and recalc the MPH.

 

[DaleH]

A lower pitched prop has less thrust all-around, hence less load on the OB... so if anything your idle RPMs will go UP, but I doubt enough to notice. And if tuned well (needle valve) then the idle speed adjustment is toot simple ...

 

[FuzzyGrub]

A lower pitched prop has less thrust all-around, hence less load on the OB... so if anything your idle RPMs will go UP, but I doubt enough to notice. And if tuned well (needle valve) then the idle speed adjustment is toot simple ...

It might, but should readjust idle speed to spec.

 

[Still Afloat]

Maybe a four blade prop? It should keep the rpm under control at wot but, allow an even smaller pitch. That’s pure speculation.. Perhaps someone that runs four blade props will chime in.

 

[thill]

The lower the pitch, the higher the THRUST, but the lower the speed.

The prop is a gear. The lower the pitch, the slower the speed per RPM, but the more pulling power it has. Think of low gear in your truck. You rev the engine, and it doesn't go fast, but it pulls hard, enough to pull up a tree stump. You can't do that in high gear.

So in a boat, the lower the pitch of the prop, the more weight it will move, but at a slower speed. So if you lower the pitch of your prop to get in the proper RPM range, your idling speed will go down a little.

Most props are variable pitch, so it may not be noticeable. If you look, props are usually curved. At the base of the blades, the pitch is rather steep, and then they tend to flatten out at the tips for the final pitch. This is a good all-around design, but again, you may not notice much difference at idle.

 

[DaleH]

You are confusing idle speed with idle RPMs … was asked about idle RPMs …

 

[FuzzyGrub]

If his idle rpms changes with a prop change, then he should re-adjust the idle rpms to spec. My self, never have seen an idle rpm change with a prop change that is usually within 1-2" of current one, but it might, if his motor calls for in-gear idle speed adjustment vs in neutral.

I suspect that his idle speed is an utmost concern for trolling speed. Myself, it hasn't been a concern, so suggest using the calculator to see what kind of change might happen.

Merc Calc

 

[Still Afloat]

Interesting. Per Merc’s calculator there’s only 4% slip on my barebones 1448 with a DF20A and 9.25 x 10 prop. It was doing 27 MPH @ WOT and a 2 MPH trolling speed with no wind when I took it out last week

 

[LDUBS]

Thanks for all the input. What I'm really wondering is if a lower pitch prop would allow me to troll slower (speed through the water). Sounds like the answer is yes.

 

[eeshaw]

If you want to troll slower get a drift sock to use. That way you can keep your current prop that works for your boat most of the time.

 

[MrGiggles]

I have no data to back this up but I suspect it won't be enough to really make a difference, but that would depend on your goals for trolling speed. Dropping from 2.3 to 2mph may be attainable but if you're trying to get below 2, that is tough with an outboard of any size, and you'll start to sacrifice performance at speed.

That said I think it is a rather lousy way to control your trolling speed when conditions on the water can be so variable, get a stiff tailwind and you're back to square one. A trolling plate or drift sock is a better solution that can be adjusted to suit.

 

[LDUBS]

Thanks everyone. Like I said at the beginning, just curious. I wouldn't change out the prop just for this reason alone.

Currently on a calm day I can go as slow as 2.3 mph. Outboard has a trolling control switch so I can adjust in 50 RPM increments between 800 & 1100 RPM.

 

[Still Afloat]

That’s a major reason why electric trolling motors are so popular. I often troll at 1.2mph. No smoke and not much noise.

 

[thill]

Because I'm curious and asking dumb questions doesn't bother me.

Suppose the following:

1. Idle RPMs are correct and at idle the boat speed is 2.3 mph.
2. With the current prop the motor is running below the WOT specified RPMs.
3. Prop is replaced with a lower pitch one to raise the WOT throttle RPMs to where they belong.

I understand, I think, that idle RPMs won't change. My question is does that lower pitch prop have any impact on the boat's speed at idle?

Thanks for all the input. What I'm really wondering is if a lower pitch prop would allow me to troll slower (speed through the water). Sounds like the answer is yes.

Sounds like you got your answer. A trolling plate works pretty well, if you need to slow further. I had one for a little while, but ended up selling it, as I didn't need it. I use my electric motor for that purpose.

Prop it for the correct WOT RPM's for the health of your motor.

 

[LDUBS]

That’s a major reason why electric trolling motors are so popular. I often troll at 1.2mph. No smoke and not much noise.

Same here -- when I want to troll really slow I use the trolling motor. There is nothing worse than the line mess that happens when a lure spins out because I was going too fast. Typically I'm trying to cover more ground so am around 2.5 mph.