9 Pitch >>> 8 Pitch

[Sea Rider]

My Tohatsu 18 HP is running at it's max factory stated wot rpm, that's 5800 with me alone on a light rib, was wondering if placing a 8 pitch prop how much more rpm will be achieving ? definetely on the over rewing side. Any ideas 300 to 500 + ?

Happy Boating

 

[eavega]

Re: 9 Pitch >>> 8 Pitch

I believe the rule is +200 RPM for every inch of pitch you drop. that is a "rule of thumb" and not a precise measurement. I've had motors that would run at the exact same RPM with 2" pitch difference. The props were from two different manufacturers (OMC and Michigan Wheel) and you could tell that the blade shape was not the same, and one weighed more than the other.

Rgds

Eric

 

[steelespike]

Re: 9 Pitch >>> 8 Pitch

Generally a smaller motor will will respond more to a prop change If you look up a 40 hp and a 18 hp I think you will find the 40 has
more choices of props.Because the responce from prop change is a little less.A 9.9 will heve even less choice.
Is your motor a 2 stroke?
Again these numbers aren't carved in stone.What are you trying to accomplish?

 

[Sea Rider]

Re: 9 Pitch >>> 8 Pitch

Hi Steelespike,

It's a 2 stroke engine, lots of OEM size pitches to play with. Was inquiring because my standard 9 pitch prop delivered with engine has a bent blade, it's impossible to be repaired here. The only inmediate option is a brand 8 pitch which will be over rewing at wot while at light load, unless you limit the wot throttle adjuster, which is easy to do. Or with inducttion tach along limit the throtttle adjuster to achieve 5800 as it it were a 9 pitch. If it's 200 rpm + not an issue, if it's + 500 would be an issue...

Happy Boating

 

[steelespike]

Re: 9 Pitch >>> 8 Pitch

Are you saying the only prop available is a non OEM at 8"? If you can get a 9" at a reasonable price order it and run the bent prop till it arrives. If your the only one running the motor just throttle back to 5800. You'll get used to it.
You don't want to over rev.

 

[Sea Rider]

Re: 9 Pitch >>> 8 Pitch

The issue is that wouldn't want to buy a OEM 8 pitch locally available to find out that's over rewing more than + 200 rpm, say 300-400 +, could be a useless & costly experiment. Usually go out with max 1 extra boater, max wot rpm drops -200 rpm, performance still ideal using a 9 pitch prop. My engine wot range is 5200-5800. Other alternative is to buy a OEM 9 pitch in USA from a Tohatsu delaer which only costs about $ 65-70 and have a good Samaritan bring it down here...

Happy Boating

 

[dazk14]

Re: 9 Pitch >>> 8 Pitch

The 8 would normally run higher than 200 more...Unless the hp just fell off the chart.

You may as well give us your gear ratio and top speed. If you're handy, you could add some cupping fairly easily.

Will iboats ship to Lima...what what would be the cheapest cost from any delivery service. ie someone hand delivered to Post office.

Edit: If you want to spend $112 at iboats, Solas makes a darling of a little cupped polished stainless, thin blade prop and get a little better mileage and performance.
You could lift the motor a little if your getting splash back, trim out more.
I think it's even got a little progressive pitch to it.
I have a Suzuki version in the shop if you want me to verify progressive pitch tomorrow.

 

[Sea Rider]

Re: 9 Pitch >>> 8 Pitch

Dazk,

Thanks, got a brand new OEM 9 pitch prop from a dealer in USA, a good samaritan brought it down here. Inquired Iboats Solas props, but flying it down here was price prohibited, this item must pass through customs add 20% +. Now running again at 5790 out out max 5800 wot factory stated for that model, that's with me alone.

Happy Boating