|
*** WARNING *** This page is Under Construction !
Sections
|
|
Propeller Rake
Rake is defined as the fore or aft slant of a blade with respect to a line
perpendicular to the propeller axis of rotation.
- Forward or Negative Rake
-
Blade slant towards forward end of hub. If a blade sticks straight out of the hub,
or even perpendicular, the prop has zero rake. This helps hold the bow of the boat
down. Zero rake offers optimum stern lift for boats that squat too much when running.
- Aft or Positive Rake
-
Blade slant towards aft end of hub. This helps to trim the bow of the boat upward. The higher
the degree of rake, the greater the bow lift.
Effect of blade rake on direction of thrust
This diagram depicts how blade rake on a surface prop changes the direction of thrust. However, the same effect would apply
to a fully submerged propeller. If you're trying to resolve a popoising issue or just want to go faster, try a propeller
with more or less rake. We've noticed a significant speed increase by switching from a zero rake propeller to one with more
aft rake.
(Index)
Power loss from blade inclination
- Steeper shaft angles increase upward thrust which increases transom lift and decrease bow lift.
- Lower shaft angles reduce upward thrust and increase bow lift. More thrust is available to propel the boat.
Reference :
Basic Propeller Theory - Prop Shaft Angles (sfmbc.net)
|
|
(Index)
|
How to measure propeller rake
The Rake measurement indicates the angle of blades’ tilt towards or away from the transom. The rake
angle is measured on a line extending from the center of the hub through the center of one blade. A
propeller with 0 degree rake has blades 90 degrees perpendicular to the center of the hub. A 20 degree
rake results in a blade that is angled 70 degrees from the hub.
|
|
|
|
Propeller is mounted to a propeller shaft and levelled horizontally in a bench vise.
|
Rake is the measurement of the angle of the tilt of the blade's tip toward or away from the propeller's front hub
face. The angle is measured on a line extending from the center of the hub through the center of one
blade.
|
An inclinometer (angle finder) is used to find the rake angle. The setup presented here isn't
very accurate, but will work if you need a quick determination of rake angle when you're in the field.
|
|
|
|
Here's our new OJ 12x16p LH 3 blade Nibral propeller.
|
|
|
|
|
|
(Index)
|
The Hot Rod's propellers
|
Propeller # 1 is a 12" diameter by 15" pitch left hand rotation propeller.
(30-OCT-2010)
|
Propeller # 2 is a Menkens 11-1/2" x 15p forged steel 2-blade. We encountered severe ventilation
and vibration with this propeller.
|
The prop on the left is our old OJ 12x16p LH 3 blade Nibral propeller. It has performed flawlessly, but
we wanted to increase our cruise RPM by several hundred RPM, so we had a new OJ propeller reworked to an 11x14p.
(22-MAR-2011)
|
|
The propeller shaft was shortened by 5/8" to reduce the distance between the aft end of the strut and front face of propeller hub.
(16-JUL-2011)
|
Diameter was reduced from 12" to 11" which yields 1/2" more blade tip clearance.
|
|
(Index)
|
Calculators
-
Shaft HP VS Speed Calculator 07-FEB-2015
-
The SHP VS Speed Calculator determines optimum propeller diameter from HP and RPM using Crouch's Diameter-HP-RPM Formula.
It will calculate attainable speed and propeller sizing for up to two engines with four transmission ratios per engine.
We've included an algorithm to calculate and plot minimum, average and maximum pitch ratio curves. If the pitch ratio falls outside
these curves, the shaft speed is unsuited to the boat and must be changed using either a different reduction gear and/or an
engine of a different rated RPM.
Click the image to the right of the screen for a PDF preview.
[Read More ...]
(Index)
-
PropCalcPlus : Expanded Excel spreadsheet Propeller Calculator. 20-FEB-2013
-
Here's an expanded version of RBI's Excel spreadsheet propeller calculator. We've extended the application to
include a Theoretical Speed Table and Diameter-HP-RPM calculations.
Click the image to the right of the screen for a PDF preview.
[Read More ...]
(Index)
-
Diameter-HP-RPM Formula : Excel spreadsheet to find propeller diameter from HP and RPM
-
This Excel spreadsheet uses the Diameter-HP-RPM Formula from Dave Gerr's Propeller Handbook to
find optimum propeller diameter from HP and RPM.
Click the image to the right for a PDF preview.
[Read More ...]
(Index)
-
Propeller Diameter Notes
-
This article discusses how to check for optimum propeller diameter.
[Read More ...]
(Index)
-
Calculate Shaft Angle from Strut Drop and Transmission Output Flange Location
-
Our latest calculator obviates the need to make a detailed shaft layout diagram to determine the shaft angle and shaft entry point for a
given strut drop. Inputs required are limited to the distance from the transmission's output flange to the transom, keel to transmission
output flange centerline and strut drop. If you're stretching the hull, values may also be input to determine
the shaft angle and shaft entry points for the stretched hull.
Click the image to the right of the screen for a PDF preview.
[Read More ...]
(Index)
-
Propeller Slip Calculator (20-APR-2013)
-
Our downloadable Excel spreadsheet calculates propeller slip given MPH, RPM, gear ratio and propeller pitch.
Click the image to the right of the screen for a PDF preview.
[Read More ...]
(Index)
-
Theoretical Speed Table
- We've devloped a Theoretical Speed Table for various RPM, propeller pitch, reduction and overdrive
ratios.
Disclaimer : These are purely theoretical numbers and are meant to provide ballpark prop
sizing. Your real world results may vary ! Your engine must be in top mechanical condition to attain maximum
performance.
- Given the desired engine RPM, the table presents multiple gear ratios, propeller pitches and
resultant speeds.
- Given the desired speed, engine RPM for multiple ratios and pitches can be
determined.
Theoretical Speed Table
(Index)
-
Calculate HP required to go faster
Click the image to the right for a PDF preview, or download our Excel spreadsheet to do your calculations :
-
HP change is a function of SPEED squared.
Click the image to the right for a PDF preview, or download our Excel spreadsheet to do your calculations :
Calculate HP required to go faster
(Index)
-
More propeller calculators
- (1)
http://www.rbbi.com/folders/prop/propcalc.htm
- (2)
Planing Hull Calculator (Victoria Propeller Ltd.)
|
|
(Index)
|
Links and Refences
-
Propeller Terminology
-
http://www.miwheel.com/propellers/resources/terms/
-
Propeller 101 (Victoria Propeller Ltd.)
-
References
-
The Propeller Handbook by Dave Gerr - Google Books
|
|
(Index)
|
