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| Propelller and fuel consumption |
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jason james calleja
Member
Username: jasoncalleja
Post Number: 8
Registered: 08-2009
| | Posted on Wednesday, October 07, 2009 - 01:55 pm: | 
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Hi all. I have my Honda bf 20 installed on a rigid hull inflatable boat of 12ft. when I run the outboard at full throttle it goes to 6000rpm in less than 20sec and it goes at 27knots. It will keep these speed all the time it will not gain any other rpm and it consumes about 5ltrs off fuel per hour at 5000rpm.
The propeller is a standard 4 blade. Do you thing that if I replace the propeller with a hi pitch I will gain in speed and fuel consumption ? |
Graham Lamb
Senior Member
Username: galamb
Post Number: 5977
Registered: 05-2007
| | Posted on Wednesday, October 07, 2009 - 02:43 pm: |
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Jason, if you moved to a 3 blade prop you would have to add 1 or 2" of pitch as to not over-rev the motor.
Every 1" drop in pitch will increase rpms by about 200 (conversely), a 1" increase will drop the rpms by 200.
Your fuel per hour sounds excellent - rule of thumb would dictate that a 20 horse should burn about 2 gph (7.66 litres).
A 3 blade prop will technically give you more top end speed than a 4 blade (simply due to less drag because it has 1 less blade) - but a 4 blade will get you out of the hole and on plane quicker which ultimately saves fuel, plus you get less engine vibration with a 4 blade than a 3.
However, to keep in in perspective we are talking about a 20 horse model here, not a 200. Any performance increase would be so minimal that you would not notice it unless you lied to yourself.
It sounds like you have your prop PERFECTLY matched to your rig and if you want MORE SPEED the only way to really achieve that is to replace the entire motor.
For every 50% increase in horsepower you will net a 15% increase in performance - so moving up to a 30 horse will give you maybe 2 more knots - fiddling with your current prop will yield substantially less.... |
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