Prop size guides?

[Orion Jim]

Are there any guides to help select a prop online or must I go to a dealer for the info?

[Boracay]

I carefully calculated my required pitch using online and printed guides. Worked out that I needed 2" less pitch, down to 14".

Took my prop to the shop, asked for 2" off the pitch. He took off 2 1/2"!

I still only get 2350rpm (of a possible 2500), up from 2050.

It may just come into the black magic/witchcraft area, or involve spending lots and lots of money.

[Celestialsailor]

Originally Posted by Jim Walsh

Are there any guides to help select a prop online or must I go to a dealer for the info?

Just call any reputable prop dealer. They will tell you what you need.

[GordMay]

The Propeller Handbook:
The Complete Reference for Choosing, Installing, and Understanding Boat Propellers
by Dave Gerr
is available for free online reading at:
The Propeller Handbook: The Complete ... - Google Book Search

Gerr’s excellent book is 176 pages long, suggesting that the subject is a little complex for a simple plug & play solution. Nonetheless most manufacturers* and many retailers provide on-line prop’ sizing guides:

ie:

Michigan Wheel: http://www.miwheel.com/PropItRightSi...dAnalysis.aspx

Sailboat propellers are estimated by calculating diameter and pitch* needed using continuous motor HP @ RPM, transmission gear ratio, boat water line length, boat displacement, and more.

Water line length and displacement determine the maximum speed of the boat and the power required to attain that speed.

The HP, motor RPM, and gear ratio are used to determine the diameter of the propeller needed to transmit the HP, and how far the boat must travel with each rpm (propeller pitch) to attain the desired speed.

Experience and judgment are needed to select the correct information and efficiency factors to enter into the propeller calculations. Each manufacturer will use efficiency factors developed for their propeller and often disagree on the propeller for a particular boat and motor combination. In many applications the most important determinants of the propeller are the motor horsepower at RPM and gear ratio of the transmission.

* Most manufacturers mark the propeller with prop diameter, pitch and shaft diameter. Some times the letter R or L for right or left hand will appear in the marking. A prop marked 12r7x1 would be a 12 inch diameter right hand 7 pitch prop for a 1” shaft. The markings will vary a bit between manufacturers, so the same prop could also be labelled 127R1.

[donradcliffe]

http://www.alberg30.org/maintenance/...r/propcalc.xls

[rgesner]

See Vicprop - Prop calculator for Displacement and semi-displacement hulls