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Author Topic:   Propellers With High Rake and Low Pitch

jimh posted 06-20-2009 10:49 AM ET (US)
In looking for the perfect propeller for my boat, a 1990 Boston Whaler REVENGE 22 W-T Whaler Drive powered by a single Evinrude 225-HP outboard, I have come to the belief that a propeller with high rake angles for its blades may be useful. The design of the Whaler Drive appendage seems to create a lot of aerated water for the propeller to operate in. Also, with a single engine on a Whaler Drive the engine mounting height is inherently quite high, even if mounted in the lowest position. This puts the propeller in aerated water.
It is mentioned by some propeller manufacturers that propellers with high rank angles on their blades can operate better in aerated water. For example, Turbo-Props say:
"Propellers typically operating in ventilated or disturbed water can enjoy performance benefits from having higher rake."
Cf.: http://www.turbo-props.com/cPage.php?id=393
Generally propellers with high rake angles are also propellers designed for high speeds, and accordingly they have a high pitch. In my application the propeller pitch needs to be limited to a range of 15- to 17-inches for best results.
I am interested to know what propeller styles are available in high rake and low pitch. Also, I would be interested in any comments about the suitability of such a propeller for my application.
Tom W Clark posted 06-20-2009 11:14 AM ET (US)
Jim -- In general, high rake and low pitch do not go hand in hand. It depends on how you define "high rake" but in my estimation, there are no "high rake" propellers in a 15" or 17" pitch.
The highest rake propeller models in those pitches are the Mercury VenSura/ Offshore (only available down to 17"), MIRAGEplus, Stiletto Advantage and Turbo 1.
Only the MIRAGEplus has the blade area and diameter suitable for your boat.
There are other means to reduce ventilation and improve performance in aerated water. Propellers designed for pontoon and deck boats typically have low rake, low pitch but very great amounts of cupping to improve grip.
jimh posted 06-20-2009 11:23 AM ET (US)
Here is my favorite propeller, a modified OMC SST 15. This photograph shows the rake angle of the blades:
The propeller has been worked to add cup. Overall it is the best performing propeller I have found. The rake angle appears to be 124-degrees. I will try to measure the rake on my MIRAGEplus using the same method.
Tom W Clark posted 06-20-2009 11:49 AM ET (US)
Actually, rake is typically measured as the angle between the blade and a line perpendicular to the propeller shaft.
In other words, the compliment of the supplement of the angle you have shown, or 34 degrees.
34 degrees would be considered extraordinarily high rake and I suspect some error has been introduced by measuring a photograph of the blade.
jimh posted 06-20-2009 11:51 AM ET (US)
Here is a Mercury MIRAGEplus 17-inch propeller. The angle of the blade rake appears to be slightly less than the OMC SST:
jimh posted 06-20-2009 11:55 AM ET (US)
The rake angles according to Tom's reference are
SST
124-90=34°
MIRAGE
122.5-90=32.5°
jimh posted 06-20-2009 12:01 PM ET (US)
This measured angle seems to confirm the correlation between rake angle and performance in aerated water. The SST holds on better than the MIRAGEplus. The SST appears to have a higher rake angle.
Tom W Clark posted 06-20-2009 12:04 PM ET (US)
I should mention the Mercury Enertia. This propeller model has both a lot blade rake and a lot of blade sweep.
I believe they make Enertia models down to 14" of pitch.
My own experience with the Mercury Enertia is that it grips extremely well.
jimh posted 06-20-2009 12:07 PM ET (US)
The ENERTIA is on the list of propellers to try. Now to find a ENERTIA in 15-inch or 17-inch pitch to put on the boat for a trial run!
Tom W Clark posted 06-20-2009 12:11 PM ET (US)
As a point of reference: on my boat, the 19" Enertia allowed an additional 250 RPM compared to the 19" MIRAGEplus.
Tom W Clark posted 06-20-2009 12:39 PM ET (US)
I have confirmed that the Enertia is made down to a pitch as low as 14".
They make standard rotation Enertia models in one inch increments up to 22" of pitch.
Counter rotating models are made only in the odd numbered pitches.
For Jim's boat, I recommend he try either the 14-3/4" x 16" or the 14-1/5" x 17" size.
makoman310 posted 06-20-2009 01:51 PM ET (US)
tom that is the prop that is on my 22 outragewd with a verado 250 they are going to use 15x15 3blade should be in the water today to test run with the new motor motor is bolted two holes up i posted earler this month about rigging this boat but was held up due to hydralic hoses thay were back orderd they are progaming digital throttle as i post i will let every one know how it works next prop if that dont work is 4blade 15x16
R T M posted 06-20-2009 06:29 PM ET (US)
I asked Ron Hill for his advice. He is a race prop builder, and prop guru. Hope he chimes in.
jimh posted 06-21-2009 09:41 AM ET (US)
Tom sent this nice photograph of a Mercury ENERTIA propeller. I measured the rake angle using Tom's suggested point of reference.
The ENERTIA has more rake than both the SST and the MIRAGEplus.
Peter posted 06-21-2009 11:06 AM ET (US)
I was looking at some Enertias yesterday and noticed they seem to have significant blade surface area for the under 15 inch diameter due to a swept blade design.
Tom W Clark posted 06-21-2009 12:26 PM ET (US)
Yes, the Enertia does a have a lot of blade area because of its unique design.
I sent Jim a series of photos of both a 19" Enertia and a 19" MIRAGEplus that I took a couple years ago to show how different those two propeller designs are.
Perhaps a separate thread could be created wit those photos to discuss the differences.
Len R posted 06-23-2009 02:41 PM ET (US)
You can definatly expect 200 - 250 more rpm and 2MPH with the Enertia vs. Mirage Plus....I went from a 17 Mirage to an 18 Enertia and gained both RPM and top end...Another advantage is the Enertia is available in 16, 17, 18, 19 pitch and allows you to fine tweak the performance you are looking for.
L H G posted 06-23-2009 04:51 PM ET (US)
It sounds like the Enertia is measuring up to Mercury's original introductory advertizing of more speed when compared to the Mirage performance it was meant to replace. 22" pitch and below. High pitch Mirage's are still primarily used for the big V8 sterndrives.
It has a lot of blade surface because it is designed for heavier boats with big Optimax or Verado power.

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Author	Topic: Propellers With High Rake and Low Pitch
jimh	posted 06-20-2009 10:49 AM ET (US) In looking for the perfect propeller for my boat, a 1990 Boston Whaler REVENGE 22 W-T Whaler Drive powered by a single Evinrude 225-HP outboard, I have come to the belief that a propeller with high rake angles for its blades may be useful. The design of the Whaler Drive appendage seems to create a lot of aerated water for the propeller to operate in. Also, with a single engine on a Whaler Drive the engine mounting height is inherently quite high, even if mounted in the lowest position. This puts the propeller in aerated water. It is mentioned by some propeller manufacturers that propellers with high rank angles on their blades can operate better in aerated water. For example, Turbo-Props say: "Propellers typically operating in ventilated or disturbed water can enjoy performance benefits from having higher rake." Cf.: http://www.turbo-props.com/cPage.php?id=393 Generally propellers with high rake angles are also propellers designed for high speeds, and accordingly they have a high pitch. In my application the propeller pitch needs to be limited to a range of 15- to 17-inches for best results. I am interested to know what propeller styles are available in high rake and low pitch. Also, I would be interested in any comments about the suitability of such a propeller for my application.
Tom W Clark	posted 06-20-2009 11:14 AM ET (US) Jim -- In general, high rake and low pitch do not go hand in hand. It depends on how you define "high rake" but in my estimation, there are no "high rake" propellers in a 15" or 17" pitch. The highest rake propeller models in those pitches are the Mercury VenSura/ Offshore (only available down to 17"), MIRAGEplus, Stiletto Advantage and Turbo 1. Only the MIRAGEplus has the blade area and diameter suitable for your boat. There are other means to reduce ventilation and improve performance in aerated water. Propellers designed for pontoon and deck boats typically have low rake, low pitch but very great amounts of cupping to improve grip.
jimh	posted 06-20-2009 11:23 AM ET (US) Here is my favorite propeller, a modified OMC SST 15. This photograph shows the rake angle of the blades: The propeller has been worked to add cup. Overall it is the best performing propeller I have found. The rake angle appears to be 124-degrees. I will try to measure the rake on my MIRAGEplus using the same method.
Tom W Clark	posted 06-20-2009 11:49 AM ET (US) Actually, rake is typically measured as the angle between the blade and a line perpendicular to the propeller shaft. In other words, the compliment of the supplement of the angle you have shown, or 34 degrees. 34 degrees would be considered extraordinarily high rake and I suspect some error has been introduced by measuring a photograph of the blade.
jimh	posted 06-20-2009 11:51 AM ET (US) Here is a Mercury MIRAGEplus 17-inch propeller. The angle of the blade rake appears to be slightly less than the OMC SST:
jimh	posted 06-20-2009 11:55 AM ET (US) The rake angles according to Tom's reference are SST 124-90=34° MIRAGE 122.5-90=32.5°
jimh	posted 06-20-2009 12:01 PM ET (US) This measured angle seems to confirm the correlation between rake angle and performance in aerated water. The SST holds on better than the MIRAGEplus. The SST appears to have a higher rake angle.
Tom W Clark	posted 06-20-2009 12:04 PM ET (US) I should mention the Mercury Enertia. This propeller model has both a lot blade rake and a lot of blade sweep. I believe they make Enertia models down to 14" of pitch. My own experience with the Mercury Enertia is that it grips extremely well.
jimh	posted 06-20-2009 12:07 PM ET (US) The ENERTIA is on the list of propellers to try. Now to find a ENERTIA in 15-inch or 17-inch pitch to put on the boat for a trial run!
Tom W Clark	posted 06-20-2009 12:11 PM ET (US) As a point of reference: on my boat, the 19" Enertia allowed an additional 250 RPM compared to the 19" MIRAGEplus.
Tom W Clark	posted 06-20-2009 12:39 PM ET (US) I have confirmed that the Enertia is made down to a pitch as low as 14". They make standard rotation Enertia models in one inch increments up to 22" of pitch. Counter rotating models are made only in the odd numbered pitches. For Jim's boat, I recommend he try either the 14-3/4" x 16" or the 14-1/5" x 17" size.
makoman310	posted 06-20-2009 01:51 PM ET (US) tom that is the prop that is on my 22 outragewd with a verado 250 they are going to use 15x15 3blade should be in the water today to test run with the new motor motor is bolted two holes up i posted earler this month about rigging this boat but was held up due to hydralic hoses thay were back orderd they are progaming digital throttle as i post i will let every one know how it works next prop if that dont work is 4blade 15x16
R T M	posted 06-20-2009 06:29 PM ET (US) I asked Ron Hill for his advice. He is a race prop builder, and prop guru. Hope he chimes in.
jimh	posted 06-21-2009 09:41 AM ET (US) Tom sent this nice photograph of a Mercury ENERTIA propeller. I measured the rake angle using Tom's suggested point of reference. The ENERTIA has more rake than both the SST and the MIRAGEplus.
Peter	posted 06-21-2009 11:06 AM ET (US) I was looking at some Enertias yesterday and noticed they seem to have significant blade surface area for the under 15 inch diameter due to a swept blade design.
Tom W Clark	posted 06-21-2009 12:26 PM ET (US) Yes, the Enertia does a have a lot of blade area because of its unique design. I sent Jim a series of photos of both a 19" Enertia and a 19" MIRAGEplus that I took a couple years ago to show how different those two propeller designs are. Perhaps a separate thread could be created wit those photos to discuss the differences.
Len R	posted 06-23-2009 02:41 PM ET (US) You can definatly expect 200 - 250 more rpm and 2MPH with the Enertia vs. Mirage Plus....I went from a 17 Mirage to an 18 Enertia and gained both RPM and top end...Another advantage is the Enertia is available in 16, 17, 18, 19 pitch and allows you to fine tweak the performance you are looking for.
L H G	posted 06-23-2009 04:51 PM ET (US) It sounds like the Enertia is measuring up to Mercury's original introductory advertizing of more speed when compared to the Mirage performance it was meant to replace. 22" pitch and below. High pitch Mirage's are still primarily used for the big V8 sterndrives. It has a lot of blade surface because it is designed for heavier boats with big Optimax or Verado power.