Propeller Efficiency Curves
03-05-2002, 02:13 PM
#21
The curve from the Mercury book does looks like a made up example to teach the concept. Lookng at a real curve with the x axis labelled would help. Just want to know which side of the peak I'm on.
Ted - I just looked at your profile. You know more than you're saying!
Don't hold out on us, man. How about some details on the Skater and how you dialed it in.
Ted - I just looked at your profile. You know more than you're saying!
Don't hold out on us, man. How about some details on the Skater and how you dialed it in.
03-05-2002, 03:49 PM
#22
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Tomcat,
I wouldn't hold out...
On the 36 Skater (Old Ocean Outboard boat) we took out about 800 pounds, got the engine heights right, the right props, got water to the engines and de-trinketized some of the goofy stuff that was on the boat.
We are not done yet...will be testing at Lake X first week in April.
Thanks again for starting this thread...very interesting topic.
Ted
I wouldn't hold out...
On the 36 Skater (Old Ocean Outboard boat) we took out about 800 pounds, got the engine heights right, the right props, got water to the engines and de-trinketized some of the goofy stuff that was on the boat.
We are not done yet...will be testing at Lake X first week in April.
Thanks again for starting this thread...very interesting topic.
Ted
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03-05-2002, 05:28 PM
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Gave it to Michael...he got it going and sold it...bought a 30 Spectre, then we sold that to buy the Skater.
That Cigarette was a great hull.
Thanks for asking.
Ted
That Cigarette was a great hull.
Thanks for asking.
Ted
03-05-2002, 06:35 PM
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03-07-2002, 07:32 PM
#28
Rambunctious - I got your email, but having trouble with ISP right now when sending out. So I'll answer here.
I looked at the equations for drag and the website you linked. Very informative. Skin drag, form (or eddy) drag and wave drag. The equations do suggest power is proportional to speed cubed, but it looks like these are all geared towards displacement hulls, and the flow of water along the hull is likely laminar. What happens with a planing hull? There isn't much wake when you're really moving so skin drag should predominate, and I'll bet it's turbulent flow.
A point to ponder. Years ago, in the Turbocharger bible written by Hugh MacInnes, there was a short section on Marine turbocharging. There was a graph showing power required vs. speed. The curve showed a distinct hump as the boat began moving in displacement mode with a rapidly increasing power requirement (cubed?). Then as the boat fell over onto plane the power required dropped a bit and started a much gentler rise (squared?). I lost my copy of this book so I can't scan the graph and post it, but you know this effect is real. Once on plane it takes less power to go faster than "plowing speed".
I looked at the equations for drag and the website you linked. Very informative. Skin drag, form (or eddy) drag and wave drag. The equations do suggest power is proportional to speed cubed, but it looks like these are all geared towards displacement hulls, and the flow of water along the hull is likely laminar. What happens with a planing hull? There isn't much wake when you're really moving so skin drag should predominate, and I'll bet it's turbulent flow.
A point to ponder. Years ago, in the Turbocharger bible written by Hugh MacInnes, there was a short section on Marine turbocharging. There was a graph showing power required vs. speed. The curve showed a distinct hump as the boat began moving in displacement mode with a rapidly increasing power requirement (cubed?). Then as the boat fell over onto plane the power required dropped a bit and started a much gentler rise (squared?). I lost my copy of this book so I can't scan the graph and post it, but you know this effect is real. Once on plane it takes less power to go faster than "plowing speed".
03-08-2002, 08:48 AM
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This is as good as anyplace for this...
I got my Poker Runs America in yesterday and i was dying to read Steve Sandler's article "Round Trip Ticket" on prop rotation.
I don't know Steve Sandler. I'm sure he is a fine guy but the article was worse than awful, it was trivial. You can sum the article up in 4 words. "You need to test."
His assumptions on loading on the props in my opinion are just plain wrong. His assumptions on portions of the props being blocked by Vee hulls is nonsense. In-water props are set back enough to be in the water, and surface piercing props couldn't possibly have the force vectors he describes as blades are out of the water. Talking about docking i put under the "who cares" category (maybe that is just me).
Conclusions were tentative...basically wishy washy. PRA could have found many experts to "get into" this topic from a performance boat point of view. They didn't.
Ted
I got my Poker Runs America in yesterday and i was dying to read Steve Sandler's article "Round Trip Ticket" on prop rotation.
I don't know Steve Sandler. I'm sure he is a fine guy but the article was worse than awful, it was trivial. You can sum the article up in 4 words. "You need to test."
His assumptions on loading on the props in my opinion are just plain wrong. His assumptions on portions of the props being blocked by Vee hulls is nonsense. In-water props are set back enough to be in the water, and surface piercing props couldn't possibly have the force vectors he describes as blades are out of the water. Talking about docking i put under the "who cares" category (maybe that is just me).
Conclusions were tentative...basically wishy washy. PRA could have found many experts to "get into" this topic from a performance boat point of view. They didn't.
Ted
03-08-2002, 09:59 AM
#30
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tomcat, I 'm going to try something here, posting a screen grab.
jsut to show the equations to others.
the point of the equations was skin drag WAS proportional to v squared, jsut like the drag through air for the hull frontal area and drag of the outdrive through the water (displacement). My explanation for the drop in force upon plaining is that the wetted area is part of the constant in front of the v squared functions, upon the transition to planing, the hull creates lift to significantly reduce the wetted are. I agree at this "zone" the pure v cubed curve doesn't fit. But, Upon planing, where the wetted area does not change significantly upon speed, and in the range we are discussing for performance, this is where the aireo drag constant x velocity^2 + the outdrive drag constant x velocy^2 + the wetted surface area constant x velocity^2 ( as stated in the attached equations) all sum to the total force. By the associative properties of algebra, the net force simplified is (Aero factor + outdrive factor + skin drag factor) X Velocty^2.
This equation simply states my understanding that skin frinction is a funtion of V^2.
Therefore the POWER, = Force x Velocity = a function of V^3 !!
I know I'm wearing you down Tomcat. come on over the the V^3 rules side
seriously, I would love to try and demonstrate these fundamentals with your program and real testing. I still reserve the right to get smarter. maybe I'm all wet (no pun)
jsut to show the equations to others.
the point of the equations was skin drag WAS proportional to v squared, jsut like the drag through air for the hull frontal area and drag of the outdrive through the water (displacement). My explanation for the drop in force upon plaining is that the wetted area is part of the constant in front of the v squared functions, upon the transition to planing, the hull creates lift to significantly reduce the wetted are. I agree at this "zone" the pure v cubed curve doesn't fit. But, Upon planing, where the wetted area does not change significantly upon speed, and in the range we are discussing for performance, this is where the aireo drag constant x velocity^2 + the outdrive drag constant x velocy^2 + the wetted surface area constant x velocity^2 ( as stated in the attached equations) all sum to the total force. By the associative properties of algebra, the net force simplified is (Aero factor + outdrive factor + skin drag factor) X Velocty^2.
This equation simply states my understanding that skin frinction is a funtion of V^2.
Therefore the POWER, = Force x Velocity = a function of V^3 !!
I know I'm wearing you down Tomcat. come on over the the V^3 rules side
seriously, I would love to try and demonstrate these fundamentals with your program and real testing. I still reserve the right to get smarter. maybe I'm all wet (no pun)
