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Author Topic:   5 blades vs 3 propeller- fuel economy

pasino posted 05-12-2007 04:46 AM ET (US)
A friend, BW owner, suggested me to install 5 blades propellers on my Revenge -22 W/T rigged with twin Yamaha F115.
He said: "if you don't mind about top speed and you are looking for fuel economy, a couple of 5 blades will allow your boat to go on plane at a minor rpm."
I answered that probably if I put 5 blades I will have to decrease the pitch, result: same rpm=less speed=more gas consumption. He answered back that the higher torque of the 5 blades will compensate the lesser pitch.
Does anybody have experimented a 5 blades propeller, better a couple of 5 blades on his boat?
deepwater posted 05-12-2007 10:18 AM ET (US)
[Recommended trying to borrow propellers for testing from a local Boston Whaler dealer.]
Tom W Clark posted 05-12-2007 11:19 AM ET (US)
Pasino,
The advice you've been given is bad on a number of levels.
First of all, there is no point in discussing propellers in overly simple terms such as number of blades. There is far more to propeller design than just the number of blades and pitch.
There are a lot of propellers out there but not many five blade props. The only five blade propeller (I know of) that will fit a Yamaha F115 is the Mercury High Five. The intermediate hub High Five is only made in pitches of 20", 22" and 24" and I do not think any of those are going to do you any good.
I also know from personal experience that the design of the High Five does NOT mean you necessarily drop pitch when switching to it from a model of propeller with fewer blades. On my own twin engine Whaler I discovered that when using the Mercury High Five propellers I need to INCREASE the pitch by a couple inches compared to numerous three blade propellers I have tested.
Maintaining a slower planing speed does not necessarily mean better fuel economy. What you want to find is a propeller that gives good boat speed at the point on your engines' RPM curve where the ratio of fuel burn to torque is relatively low. This propeller may have three or four blades.
jimh posted 05-12-2007 03:43 PM ET (US)
Several propeller manufacturers are now touting certain propellers as being designed for optimizing fuel economy at mid-range cruising speeds. The propellers which are often cited as having this feature are three-blade propellers with rather large blade surfaces.
pasino posted 05-13-2007 03:40 AM ET (US)
Guys, thanks. My friend's suggestion seemed to me a little unscientific.
Tom, where could I find the data you're talking about (ratio of fuel burn to torque). I looked for them on the internet, but couldn't find them. I only found the performance bulletins where the data are influenced by the boats and prop they used.
lordswork2007 posted 05-17-2007 09:45 PM ET (US)
Just can't make sense out of the concept that the best economy is found where the "fuel burn to torque" ratio is the lowest.
I assume the writer means full throttle torque. The highest full throttle torque occurs at or near the point of maximum volumetric efficiency VE. VE times RPM (times other constants) equals fuel burn. Lowest fuel burn to torque ratio is probably at fairly low RPM, but I cannot understand the value of this metric. Lowest fuel burn per horsepower is, in contrast, a valid metric of engine efficiency and in fact defines specific fuel consumption.
Best mpg does not necessarily coincide with best sfc, however, because the boat has varying requirements of power to drive it at different speeds.

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Author	Topic: 5 blades vs 3 propeller- fuel economy
pasino	posted 05-12-2007 04:46 AM ET (US) A friend, BW owner, suggested me to install 5 blades propellers on my Revenge -22 W/T rigged with twin Yamaha F115. He said: "if you don't mind about top speed and you are looking for fuel economy, a couple of 5 blades will allow your boat to go on plane at a minor rpm." I answered that probably if I put 5 blades I will have to decrease the pitch, result: same rpm=less speed=more gas consumption. He answered back that the higher torque of the 5 blades will compensate the lesser pitch. Does anybody have experimented a 5 blades propeller, better a couple of 5 blades on his boat?
deepwater	posted 05-12-2007 10:18 AM ET (US) [Recommended trying to borrow propellers for testing from a local Boston Whaler dealer.]
Tom W Clark	posted 05-12-2007 11:19 AM ET (US) Pasino, The advice you've been given is bad on a number of levels. First of all, there is no point in discussing propellers in overly simple terms such as number of blades. There is far more to propeller design than just the number of blades and pitch. There are a lot of propellers out there but not many five blade props. The only five blade propeller (I know of) that will fit a Yamaha F115 is the Mercury High Five. The intermediate hub High Five is only made in pitches of 20", 22" and 24" and I do not think any of those are going to do you any good. I also know from personal experience that the design of the High Five does NOT mean you necessarily drop pitch when switching to it from a model of propeller with fewer blades. On my own twin engine Whaler I discovered that when using the Mercury High Five propellers I need to INCREASE the pitch by a couple inches compared to numerous three blade propellers I have tested. Maintaining a slower planing speed does not necessarily mean better fuel economy. What you want to find is a propeller that gives good boat speed at the point on your engines' RPM curve where the ratio of fuel burn to torque is relatively low. This propeller may have three or four blades.
jimh	posted 05-12-2007 03:43 PM ET (US) Several propeller manufacturers are now touting certain propellers as being designed for optimizing fuel economy at mid-range cruising speeds. The propellers which are often cited as having this feature are three-blade propellers with rather large blade surfaces.
pasino	posted 05-13-2007 03:40 AM ET (US) Guys, thanks. My friend's suggestion seemed to me a little unscientific. Tom, where could I find the data you're talking about (ratio of fuel burn to torque). I looked for them on the internet, but couldn't find them. I only found the performance bulletins where the data are influenced by the boats and prop they used.
lordswork2007	posted 05-17-2007 09:45 PM ET (US) Just can't make sense out of the concept that the best economy is found where the "fuel burn to torque" ratio is the lowest. I assume the writer means full throttle torque. The highest full throttle torque occurs at or near the point of maximum volumetric efficiency VE. VE times RPM (times other constants) equals fuel burn. Lowest fuel burn to torque ratio is probably at fairly low RPM, but I cannot understand the value of this metric. Lowest fuel burn per horsepower is, in contrast, a valid metric of engine efficiency and in fact defines specific fuel consumption. Best mpg does not necessarily coincide with best sfc, however, because the boat has varying requirements of power to drive it at different speeds.