More speed by dropping pitch?
02-28-2002, 03:33 PM
#11
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Not to bore jdnca1 with our technical combats but.... i think to some extend we are in "violent agreement". Iagree with Steve david completely that ....
"...You might find that by dropping 2 inches of forward travel (minus your slip factor) per prop RPM, you won't pick enough RPM's to make up the difference. ...."
Dean, you are right that the FORCE to move a prop through the water is a function of velocity squared. Therefor by physics, the POWER required is force x velocity or now a function of velocity cubed. i feel The increase you talk about is the additional increase to "Free spin?" the props blade projected frontal area through the water. the force to move the boat at a specific speed stays the same.
What the power is to "free spin? " a prop in moving water would be very valuable information for the optimization we are trying to find. because like you said Dean is true. it will require more power to spin a prop faster. if this is more that the change in power output of the motor due to new operating rpm, then you didn't gain anything.
If you lab finish, you are reducing the power required to "free spin ?" a prop through the water. this frees up that power to move the boat faster. by moving the boat faster to a new terminal velocity, it moves the engine rpm to a new power point, thus allowing additional speed till a new equilibrium.
for $150 each. york will lab finish and polish props. that still remains my suggestion.
If you have a lot of slip. maybe a larger prop diameter is needed. Here you balance the parasitic power requirement to swing a larger prop with the efficiency gain you will see .
my more than asked for $0.02
thanks for listening
this technical combating is fun right
what else to do when the snow is still melting.....
Rob
Rambunctions
B.S Physics
M.S Mechanical Engineering
"...You might find that by dropping 2 inches of forward travel (minus your slip factor) per prop RPM, you won't pick enough RPM's to make up the difference. ...."
Dean, you are right that the FORCE to move a prop through the water is a function of velocity squared. Therefor by physics, the POWER required is force x velocity or now a function of velocity cubed. i feel The increase you talk about is the additional increase to "Free spin?" the props blade projected frontal area through the water. the force to move the boat at a specific speed stays the same.
What the power is to "free spin? " a prop in moving water would be very valuable information for the optimization we are trying to find. because like you said Dean is true. it will require more power to spin a prop faster. if this is more that the change in power output of the motor due to new operating rpm, then you didn't gain anything.
If you lab finish, you are reducing the power required to "free spin ?" a prop through the water. this frees up that power to move the boat faster. by moving the boat faster to a new terminal velocity, it moves the engine rpm to a new power point, thus allowing additional speed till a new equilibrium.
for $150 each. york will lab finish and polish props. that still remains my suggestion.
If you have a lot of slip. maybe a larger prop diameter is needed. Here you balance the parasitic power requirement to swing a larger prop with the efficiency gain you will see .
my more than asked for $0.02
thanks for listening
this technical combating is fun right
what else to do when the snow is still melting.....Rob
Rambunctions
B.S Physics
M.S Mechanical Engineering
02-28-2002, 03:42 PM
#12
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Guys;
I appreciate all the insight, especially the scientific stuff. I'll just have to do some testing this spring with 28's - 32's on radar to see what happens. My guages are all Gaffrig and I have tested my tach with the MSD ignition tester..its dead on. My speeds have all been verified on radar (my Gaffrig reads 2 mph optimistic) Timing is through MSD Crank trigger system, (Verified @32 degrees) Butterflies open all the way. I've actually seen 109 with a 175hp shot of NOS, taking total HP close to 1100. Turned my 30 (not labbed) bravo 6700. So in that respect I think 115 natural is very optimistic to say the least. Maybe a 32 will be the ticket, I'll let you all know. thanks again for all the insight.
I appreciate all the insight, especially the scientific stuff. I'll just have to do some testing this spring with 28's - 32's on radar to see what happens. My guages are all Gaffrig and I have tested my tach with the MSD ignition tester..its dead on. My speeds have all been verified on radar (my Gaffrig reads 2 mph optimistic) Timing is through MSD Crank trigger system, (Verified @32 degrees) Butterflies open all the way. I've actually seen 109 with a 175hp shot of NOS, taking total HP close to 1100. Turned my 30 (not labbed) bravo 6700. So in that respect I think 115 natural is very optimistic to say the least. Maybe a 32 will be the ticket, I'll let you all know. thanks again for all the insight.
02-28-2002, 05:47 PM
#14
Registered
Anyone ever see an actual propeller efficiency curve for the props we all play with, Mirage, Bravo, Hydromotive? It might help us when we are trying to optimize a setup.
Prop efficiency = Thrust HP/propshaft HP
Mercury says that 80% is about the best you can get and that value varies according to slip(angle of attack). If slip is too low or too high, then the amount of propshaft HP that actually gets converted into forward thrust is reduced. From the one efficiency curve I have seen efficiency dropped to 65% in a fairly narrow range of slip(angle of attack). This would make a huge difference in speed since it is ultimately prop thrust that overcomes the drag of the hull and drive.
Prop efficiency = Thrust HP/propshaft HP
Mercury says that 80% is about the best you can get and that value varies according to slip(angle of attack). If slip is too low or too high, then the amount of propshaft HP that actually gets converted into forward thrust is reduced. From the one efficiency curve I have seen efficiency dropped to 65% in a fairly narrow range of slip(angle of attack). This would make a huge difference in speed since it is ultimately prop thrust that overcomes the drag of the hull and drive.
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