Propeller Diameter in Relation to its Pitch - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Propeller Diameter in Relation to its Pitch

search | FAQ | profile | register | author help

Author Topic:   Propeller Diameter in Relation to its Pitch

fodimi posted 03-22-2010 05:14 PM ET (US)
Calculating propeller diameter according to Gerr:
Diameter = (632.7 x SHP^0.2) / RPM^0.6
[where RPM is propeller shaft turning speed, SHP is propeller shaft horsepower, and D is propeller diameter in inches]
My engine is a SUZUKI DF 300, gear ratio 2.08:1, engine speed range 5,700 to 6,300-RPM.
[Rearranging Gerr's] formula to [solve for propeller shaft speed with a fixed horsepower of 300-HP] :
RPM = (632.7 X 300^O.2) / D^0.6
If having a [propeller of] diameter of 16-inches, according to the formula [the shaft speed appropriate for 300-HP would be]:
RPM = (632.7 X 300^0.2) /16^0.6 = 2,965.8
[With the SUZUKI gear ratio of 2.08:1, this implies] the engine RPM = 2965.8 X 2.08 = 6,169-RPM.
Suppose we change the diameter of the propeller to 15-inch; according to the formula the shaft speed would be:
RPM = (632.7 X 300^0.2) / 15^0.6 = 3,163.5
Thus the engine RPM = 3163.5 X 2.08 = 6,580.
Suppose that with a 16-inch diameter x 21.5-inch pitch propeller the engine reaches 6,169 RPM. Recall the maximum RPM the manufacturer gives is 6,300. If the diameter be reduced by 1-inch D=15, then according to the formula: engine's RPM will reach to 6,580, that is 411 RPM more than the recommended maximum.
If the propeller pitch is increased about 2-inches, then the engine's RPM will be reduced by 400.
In conclusion, if we change the 16 x 21.5 propeller to a 15 x 23.5 propeller, then the engine RPM will come back to the proper limits.
So, the increase of 400 RPM because of the decrease of the diameter of 1-inch is controlled by the 2-inch increase of the propeller's pitch.
Tom W Clark posted 03-22-2010 05:41 PM ET (US)
Given that formula, yes.
In the real world, maybe yes, maybe no.
jimh posted 03-22-2010 08:23 PM ET (US)
In your approach the gear reduction is fixed by the manufacturer. In Gerr's approach, the gear reduction is a variable which the designer selects to fit the application.
fodimi posted 03-23-2010 06:55 AM ET (US)
Is there any other formula able to calculate the result of the influence of the change of 1-inch in propeller diameter, on the engine RPM?
If not, can you calculate this influence from your experience (approximately)?.
Something like pitch of propeller: 1-inch of propeller’s pitch = 200-RPM .
Have you any example personally which will show the changes that occur when we change the propeller diameter?
And, how can we hedge these changes?
jimh posted 03-23-2010 08:25 AM ET (US)
In the calculations set out above, it is proposed that:
"If the propeller pitch is increased about 2-inches, then the engine's RPM will be reduced by 400."
I believe this is based on a "rule of thumb." Gerr mentions (in his PROPELLER HANDBOOK), there is a long standing rule of thumb with propellers that states:
Every two-inch increase in propeller pitch will decrease engine speed by 450-RPM, and vice versa.
My own experience matches the rule. See:
http://continuouswave.com/ubb/Forum4/HTML/003906.html
Note that this rule governs the engine speed. The rule must therefore include the gear reduction influence.
Tom W Clark posted 03-23-2010 10:06 AM ET (US)
In my extensive propeller testing experience, a two inch change in propeller pitch will result in an inverse 300-400 RPM change in engine speed for most large outboard motors.
It does, of course, depend on the gear ratio which in most large outboards is between 1.75:1 and 2.25:1.
Gerr's book, while including some token comments about outboard motors in the newer edition, is really focused on much larger inboard powered craft which use different gear ratios.
fodimi posted 03-23-2010 10:27 AM ET (US)
Jim-
What about prop’s diameter?
Is there any other formula able to calculate the result of the influence of the change of 1-inch in propeller diameter, on the engine RPM?
If not, can you calculate this influence from your experience (approximately)?
Have you any example personally which will show the changes that occur when we change the propeller diameter?
And, how can we hedge these changes?
jimh posted 03-23-2010 04:55 PM ET (US)
Shakespeare had a rule of thumb, too. From ROMEO AND JULIET:
--
Enter ABRAHAM and BALTHASAR
ABRAHAM
Do you bite your thumb at us, sir?
SAMPSON
I do bite my thumb, sir.
ABRAHAM
Do you bite your thumb at us, sir?
SAMPSON
[Aside to GREGORY] Is the law of our side, if I say
ay?
GREGORY
No.
SAMPSON
No, sir, I do not bite my thumb at you, sir, but I
bite my thumb, sir.
GREGORY
Do you quarrel, sir?
ABRAHAM
Quarrel sir! no, sir.
SAMPSON
If you do, sir, I am for you: I serve as good a man as you.
ABRAHAM
No better.
SAMPSON
Well, sir.
GREGORY
Say 'better:' here comes one of my master's kinsmen.
SAMPSON
Yes, better, sir.
ABRAHAM
You lie.
SAMPSON
Draw, if you be men. Gregory, remember thy swashing blow.
They fight
--
I mention this as an example of how a thumb or a rule of thumb can lead to fights or disputes.
number9 posted 03-23-2010 08:05 PM ET (US)
Looked like using the formula in earlier reply a 1" change in prop diameter may result in a 400rpm change in WOT. In the real world the smaller diameter OB prop is going to be designed differently than a larger diameter prop for the same motor or HP range.
A larger diameter prop is likely to improve low speed, reverse maneuverability, reduced on plane speed and possibly better fuel economy. Trade off likely is increased drag and reduction in WOT speed.
We should be able to leverage diameter/pitch choice for our use but just as a simple lever it's give/take.
jimh posted 03-23-2010 08:23 PM ET (US)
Gerr's forumula is:
Diameter = (632.7 x SHP^0.2) / RPM^0.6
The relationship shows that diameter increases with horsepower (SHP) , but diameter decreases with shaft rotation speed (RPM).
Gerr is giving a starting point for propeller selection based on horsepower and shaft speed. The underlying idea is that a propeller needs to be a certain diameter in order to effectively transmit the horsepower applied to it into forward thrust. As I recall--I don't have the book beside me at the moment--this formula is for typical three-blade propellers.
boatdryver posted 03-24-2010 09:57 AM ET (US)
And, if, in fine tuning changes in pitch and diameter, one winds having to change propeller manufacturers, the rules of thumb may not apply at all due to the great variability in blade design.
JimL
fodimi posted 03-24-2010 05:10 PM ET (US)
Number9-
We absolutelly agree,and in real wold, when i get something more at the high RPM,then i lose something at the low RPM.
I personally believe that there is always the possibility of not losing anything important from the decrease of prop' s diameter(because of the type of the hull),whereas on the other hand i will definitelly gain the benefits of the increase of prop's pitch.
1.Do you agree that this possibility exists,in some cases such as: for example 'hull with steps'
2.Do you generally believe that the decrease of prop's diameter(against the manufacturer's proposal)could be hamfull for the engine or the define of prop's diameter is only matter of 'gear ratio' and nothing more.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Propeller Diameter in Relation to its Pitch
fodimi	posted 03-22-2010 05:14 PM ET (US) Calculating propeller diameter according to Gerr: Diameter = (632.7 x SHP^0.2) / RPM^0.6 [where RPM is propeller shaft turning speed, SHP is propeller shaft horsepower, and D is propeller diameter in inches] My engine is a SUZUKI DF 300, gear ratio 2.08:1, engine speed range 5,700 to 6,300-RPM. [Rearranging Gerr's] formula to [solve for propeller shaft speed with a fixed horsepower of 300-HP] : RPM = (632.7 X 300^O.2) / D^0.6 If having a [propeller of] diameter of 16-inches, according to the formula [the shaft speed appropriate for 300-HP would be]: RPM = (632.7 X 300^0.2) /16^0.6 = 2,965.8 [With the SUZUKI gear ratio of 2.08:1, this implies] the engine RPM = 2965.8 X 2.08 = 6,169-RPM. Suppose we change the diameter of the propeller to 15-inch; according to the formula the shaft speed would be: RPM = (632.7 X 300^0.2) / 15^0.6 = 3,163.5 Thus the engine RPM = 3163.5 X 2.08 = 6,580. Suppose that with a 16-inch diameter x 21.5-inch pitch propeller the engine reaches 6,169 RPM. Recall the maximum RPM the manufacturer gives is 6,300. If the diameter be reduced by 1-inch D=15, then according to the formula: engine's RPM will reach to 6,580, that is 411 RPM more than the recommended maximum. If the propeller pitch is increased about 2-inches, then the engine's RPM will be reduced by 400. In conclusion, if we change the 16 x 21.5 propeller to a 15 x 23.5 propeller, then the engine RPM will come back to the proper limits. So, the increase of 400 RPM because of the decrease of the diameter of 1-inch is controlled by the 2-inch increase of the propeller's pitch.
Tom W Clark	posted 03-22-2010 05:41 PM ET (US) Given that formula, yes. In the real world, maybe yes, maybe no.
jimh	posted 03-22-2010 08:23 PM ET (US) In your approach the gear reduction is fixed by the manufacturer. In Gerr's approach, the gear reduction is a variable which the designer selects to fit the application.
fodimi	posted 03-23-2010 06:55 AM ET (US) Is there any other formula able to calculate the result of the influence of the change of 1-inch in propeller diameter, on the engine RPM? If not, can you calculate this influence from your experience (approximately)?. Something like pitch of propeller: 1-inch of propeller’s pitch = 200-RPM . Have you any example personally which will show the changes that occur when we change the propeller diameter? And, how can we hedge these changes?
jimh	posted 03-23-2010 08:25 AM ET (US) In the calculations set out above, it is proposed that: "If the propeller pitch is increased about 2-inches, then the engine's RPM will be reduced by 400." I believe this is based on a "rule of thumb." Gerr mentions (in his PROPELLER HANDBOOK), there is a long standing rule of thumb with propellers that states: Every two-inch increase in propeller pitch will decrease engine speed by 450-RPM, and vice versa. My own experience matches the rule. See: http://continuouswave.com/ubb/Forum4/HTML/003906.html Note that this rule governs the engine speed. The rule must therefore include the gear reduction influence.
Tom W Clark	posted 03-23-2010 10:06 AM ET (US) In my extensive propeller testing experience, a two inch change in propeller pitch will result in an inverse 300-400 RPM change in engine speed for most large outboard motors. It does, of course, depend on the gear ratio which in most large outboards is between 1.75:1 and 2.25:1. Gerr's book, while including some token comments about outboard motors in the newer edition, is really focused on much larger inboard powered craft which use different gear ratios.
fodimi	posted 03-23-2010 10:27 AM ET (US) Jim- What about prop’s diameter? Is there any other formula able to calculate the result of the influence of the change of 1-inch in propeller diameter, on the engine RPM? If not, can you calculate this influence from your experience (approximately)? Have you any example personally which will show the changes that occur when we change the propeller diameter? And, how can we hedge these changes?
jimh	posted 03-23-2010 04:55 PM ET (US) Shakespeare had a rule of thumb, too. From ROMEO AND JULIET: -- Enter ABRAHAM and BALTHASAR ABRAHAM Do you bite your thumb at us, sir? SAMPSON I do bite my thumb, sir. ABRAHAM Do you bite your thumb at us, sir? SAMPSON [Aside to GREGORY] Is the law of our side, if I say ay? GREGORY No. SAMPSON No, sir, I do not bite my thumb at you, sir, but I bite my thumb, sir. GREGORY Do you quarrel, sir? ABRAHAM Quarrel sir! no, sir. SAMPSON If you do, sir, I am for you: I serve as good a man as you. ABRAHAM No better. SAMPSON Well, sir. GREGORY Say 'better:' here comes one of my master's kinsmen. SAMPSON Yes, better, sir. ABRAHAM You lie. SAMPSON Draw, if you be men. Gregory, remember thy swashing blow. They fight -- I mention this as an example of how a thumb or a rule of thumb can lead to fights or disputes.
number9	posted 03-23-2010 08:05 PM ET (US) Looked like using the formula in earlier reply a 1" change in prop diameter may result in a 400rpm change in WOT. In the real world the smaller diameter OB prop is going to be designed differently than a larger diameter prop for the same motor or HP range. A larger diameter prop is likely to improve low speed, reverse maneuverability, reduced on plane speed and possibly better fuel economy. Trade off likely is increased drag and reduction in WOT speed. We should be able to leverage diameter/pitch choice for our use but just as a simple lever it's give/take.
jimh	posted 03-23-2010 08:23 PM ET (US) Gerr's forumula is: Diameter = (632.7 x SHP^0.2) / RPM^0.6 The relationship shows that diameter increases with horsepower (SHP) , but diameter decreases with shaft rotation speed (RPM). Gerr is giving a starting point for propeller selection based on horsepower and shaft speed. The underlying idea is that a propeller needs to be a certain diameter in order to effectively transmit the horsepower applied to it into forward thrust. As I recall--I don't have the book beside me at the moment--this formula is for typical three-blade propellers.
boatdryver	posted 03-24-2010 09:57 AM ET (US) And, if, in fine tuning changes in pitch and diameter, one winds having to change propeller manufacturers, the rules of thumb may not apply at all due to the great variability in blade design. JimL
fodimi	posted 03-24-2010 05:10 PM ET (US) Number9- We absolutelly agree,and in real wold, when i get something more at the high RPM,then i lose something at the low RPM. I personally believe that there is always the possibility of not losing anything important from the decrease of prop' s diameter(because of the type of the hull),whereas on the other hand i will definitelly gain the benefits of the increase of prop's pitch. 1.Do you agree that this possibility exists,in some cases such as: for example 'hull with steps' 2.Do you generally believe that the decrease of prop's diameter(against the manufacturer's proposal)could be hamfull for the engine or the define of prop's diameter is only matter of 'gear ratio' and nothing more.