Another Prop Question

[Starman]

I have read the earlier post about changing the pitch on a prop to increase top speed. As many, I want to buy a second prop, and use it for the top speed prop. I am running a new Yamaha 115hp Four Stroke on an 18 foot boat. The current prop is an 13.25" with a 17" pitch. I know that increasing the pitch to much can be bad by dropping rpms to low at full throttle. My question is, can a person reduce the size of the prop say like from 13.25" to 13" and not have to increase the pitch as much? Or can you mess with the diameter of the prop at all?

[Starman]

Any help out there or is this just a dumb question.

[upnorth]

try that question at iboats.com. there are some real experts there.

[Starman]

Thanks Upnorth!

[Guest]

On older motors, or motors with a heavy work loads, they do not recommend getting to far astray with the pitch either up the scale, or down.

It will add increased stress to the motor and possibly reduce its life or increase it's probable maintenance costs.

You may be able to achieve better results with a "Jack Pate" or fine tunning your trim with less stress to the motor?

~Determining RPM's~

Now, determine your wide open throttle (WOT) RPM

For safety and efficient performance, it is critical that your engine operates within the RPM range recommended by the manufacturer. Matching the right prop for the load is the most significant factor of RPM adjustment.

Effect of Prop Pitch on RPM
A pitch change can increase or decrease the RPM's and bring RPM's into the recommended range. A 2" increase in pitch (for example, from 21" to 23") typically results in a decrease of approximately 300-400 RPM.

First, Determine Manufacturer's

Recommended RPM
Find the manufacturer's recommended RPM range in the owner's manual or ask your dealer.

Test for Maximum RPM
Using the existing propeller or a new propeller, make test runs to determine the maximum RPM and boat speed. Vary the trim angle for optimum performance.

RPM Higher Than Recommended.
If the actual WOT RPM's are above the recommended RPM range, install the next larger pitch propeller to decrease your WOT RPM. Re-test the WOT RPM.

RPM Lower Than Recommended.
If the actual WOT RPM range is below the recommended range, install the next smaller pitch propeller to increase your WOT RPM. Re-test the WOT RPM.

When you combine all these factors, you have the information you need to select the correct propeller for maximum performance, safety, and fuel efficiency.

~Usefull info links.~

www.rbbi.com

www.theoutboardwizard.bizhosting.com/

www.solasuk.com/Selection_of_x.html

www.caterpillar.com/services/shared/marine/pleasure_craft/product_support/docs /PMYART.DOC

www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=UTF-8&newwind ow=1&safe=off&q=Propeller+pitch+chart

www.boat-props.com/match/match_omc_props.html

Ed "Backwater Eddy" Carlson

Backwater Guiding "ED on the RED"

[This message has been edited by Backwater Eddy (edited 05-11-2003).]