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hp vs mph

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Old 03-04-2002, 05:24 PM
  #21
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Exactally, it will measure shaft Tq with a strain guage and convert it to HP and then output it in a 0-5volt signal to a guage, computer or data logger. I am having Norgskod guages made now. Right now it is for a Stellings bravo extension box. There is no room in the stock mercruiser/bravo one setup.
I am one you thinks and agrees with TQ being the critical factor. HP is only a calculation of TQ. TQ is what is measured and what matters.
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Old 03-04-2002, 05:44 PM
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Thanks Marty! Your project has me intrigued. Where are you pullin' the readings from? Are you using old-fashioned piezoelectric strain guages or some kind of strobe set-up.

I remember from working with turbines some setups we would use photodisplacement cells to measure torque on high-speed totating shafts. Another method would use a scribe line paralell with the shaft and we would use a diode(kind of like a timing strobe) to measure when the line would pass by the receptor. Naturally, it the shaft was torqued one end of the line would pass by sooned than the other side. By measuring the time difference and knowing the rpms you could determine torsional displacement.

This all sounds really Cool! Good Luck!
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Old 03-04-2002, 05:52 PM
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Ya gotta Whipple it! Get the torque pick up all the way down the curve (I believe). One of you math guys ought to figure out the payback time considering the gas saved at cruise - considering most of our time is spent at cruise any ways. Dustin, have you done any calculations on this? This will be especially important for me as my cat needs about 75mph to really get up on air and that speed is pushing the rpms pretty high pre blowers.

p.s. Cool thread, thanks for the info guys.
 
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Old 03-04-2002, 05:54 PM
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CM? did you consider a trust load type sensor? are you trying to measure drivetrain power output net from the outdrive? or power thrust being delivered to thrust the boat. One would be output of powertrain. the other the net power after all inefficiencies.................... I guess both are useful.

Do you pick up the rpm from the motor to calculate the power?

sounds cool !!
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Old 03-04-2002, 06:16 PM
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Wow,
you guys are WAAAY to smart for a dumb designer like myself !!

Clay ,
I tried both of your formulas and they were VERY close.
My real new speed was 80.1 and the formula said 81
My estimated HP was 675 the formula said 650.

I should have paid more attention in math class !!
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Old 03-04-2002, 07:58 PM
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Default tom cat speed vs hp info

tom cat here is the info you requested to fine tune my hp to speed . boat weight 5500 with full tank of fuel, 23p mirage plus 3 blade lab finished, 1:50 bravo 1 drive , max rpm 5000 wot, expected hp 650 at crank with procharger @5psi.
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Old 03-04-2002, 08:30 PM
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26Sonic ,
With that much HP and that gear ratio you are going to be WAAAAAAY higher than 5000rpm at WOT with a 23 mirage.
You will probably need a 25 Mirage plus or a 26 Bravo 1, even with the B1 you will probably be around 5400-5500.
I'm making between 650-670hp and with a labbed B1 I'm spinning 5800rpms.
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Old 03-04-2002, 09:28 PM
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Hey cobra marty - Good for you on the dyno, good luck with it! But once you get it working and know your torque at a given RPM we are going to take a look a some prop efficiency curves (power out/power in) and pick/lab a prop to deliver more thrust at the same RPM and same torque/power. It's thrust that matters, we just have no way of measuring it directly.

Hey Rambunctious - Propeller efficiency is the last frontier! There are research labs in the world that have propeller test tanks instrumented for both propshaft torque and propeller thrust. With these two measurements you can calculate power into the prop and thrust horsepower out of the prop, therefore propeller efficiency curves. If cobra marty does add a thrust sensor/washer he will be able be able to do the same thing.
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Old 03-04-2002, 09:37 PM
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Some of you guys are too smart out there. I am using a piezoelectric strain guage and some fancy electronics with wireless connection and conversion to 0-5volt output. This will measure the torque or twist of a shaft. The housing will pickup the rpm which is crank rpm. By knowing tq and rpm the electronics will calculate hp. Current applications in top fuel measures up to 10,000 foot-lbs of TQ!! In a car application I have to pick up rpm from a different source.
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Old 03-04-2002, 09:58 PM
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26 scarab the 23p mirage plus labbed is for my current set up non pro charged . i plan to have to
use a bigger prop , not sure what size yet. the rev limit will be kept the same 5150 rpms after installing pro charger .sorry i did'nt give enough info.
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