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Author Topic:   Rule of Thumb

jimh posted 09-18-2005 05:53 PM ET (US)
According to Dave Gerr (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.
Here are my observations when using the same style propeller in three different pitch configurations and the engine's wide-open-throttle speed that was possible with them:
PITCH vs ENGINE SPEED
19-inch = 5,000 RPM
17-inch = 5,500 RPM
15-inch = 5,900 RPM
This should be a good test case because all of the propellers were from the same manufacturer (OMC) and of the same style (three-blade SST). The results are in very close agreement with the rule of thumb.
Teak Oil posted 09-18-2005 08:40 PM ET (US)
Did top speed remain the same in each test?
jimh posted 09-18-2005 11:23 PM ET (US)
If you are asking about the boat speed, it was different. The rule of thumb does not infer anything about the boat speed. But the rule does seem to be quite accurate at predicting the effect of pitch change on engine speed.
If you want to see all the boat speed numbers, see the results of all the propeller tests (all 11 of them) in the Reference area:
http://continuouswave.com/whaler/reference/propellerWDSingle.html
kamie posted 09-20-2005 09:49 PM ET (US)
19 Pitch = 5200 RPM Laser II ( 3 blade)
17 Pitch = 5600 RPM Vensura ( 4 blade)
Whalerific posted 09-22-2005 03:12 PM ET (US)
Kamie:
What hull/engine combination are your pitch/rpm numbers for?
jimh posted 09-22-2005 08:49 PM ET (US)
The Rule of Thumb does not consider hull types.

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Author	Topic: Rule of Thumb
jimh	posted 09-18-2005 05:53 PM ET (US) According to Dave Gerr (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. Here are my observations when using the same style propeller in three different pitch configurations and the engine's wide-open-throttle speed that was possible with them: PITCH vs ENGINE SPEED 19-inch = 5,000 RPM 17-inch = 5,500 RPM 15-inch = 5,900 RPM This should be a good test case because all of the propellers were from the same manufacturer (OMC) and of the same style (three-blade SST). The results are in very close agreement with the rule of thumb.
Teak Oil	posted 09-18-2005 08:40 PM ET (US) Did top speed remain the same in each test?
jimh	posted 09-18-2005 11:23 PM ET (US) If you are asking about the boat speed, it was different. The rule of thumb does not infer anything about the boat speed. But the rule does seem to be quite accurate at predicting the effect of pitch change on engine speed. If you want to see all the boat speed numbers, see the results of all the propeller tests (all 11 of them) in the Reference area: http://continuouswave.com/whaler/reference/propellerWDSingle.html
kamie	posted 09-20-2005 09:49 PM ET (US) 19 Pitch = 5200 RPM Laser II ( 3 blade) 17 Pitch = 5600 RPM Vensura ( 4 blade)
Whalerific	posted 09-22-2005 03:12 PM ET (US) Kamie: What hull/engine combination are your pitch/rpm numbers for?
jimh	posted 09-22-2005 08:49 PM ET (US) The Rule of Thumb does not consider hull types.