triman
07-09-2007, 03:52 PM
I first posted this on another site but it is worth repeating here because it gives you something to read while waiting for the ramp to clear.
Propellor Efficiency
Your you-beaut new toy has just been delivered. A 6.0 metre dedicated fishing machine, powered by the absolute latest from the workshops of Yamazuki. 180 horsepower of 4-stroke grunt.
You take it down to the ramp, dunk it, ruthlessly follow the break-in instructions then decide to see just what this baby will do.
5800 rpm, 19” propeller, 31 knots!!! Goes well, doesn’t she? Well, maybe.
The gear reduction through the bottom end is, say, 2.5 to 1, so the propeller is turning at 2320 rpm.
The prop wants to travel 44,080 inches each minute, 3673 feet per minute, 220380 feet per hour, or 36 and a bit knots.
Oops. Mathematically you should be doing 36+ knots with that engine/propeller/gear combination, but you are only getting 31.
So what’s happened? Where are the missing 5 knots?
Two immediate answers, inaccuracies and slip, that’s what happened.
Let’s look at the inaccuracies.
How accurate is your speed? How was it measured? It starts to get murky here. Are you using a pitot-type speedo? Is it reading right? No, I used my all-singing, all-dancing Navrance 999WRX combi GPS/Fish-finder/Radar/Coffee maker and it’s never wrong! Um, hang on. Where did you do the speed run? In a bay, with tide and wind, or a river with current and wind. Under these uncontrolled conditions to get a good approximation to your speed you should do at least 3 runs in each direction, 180o apart, in as close to the same conditions as possible. Remember your boat is moving through the water but the GPS is measuring your speed over the ground. This might not account for 5 knots, but if you did a run in the opposite direction you might have had a speed of, say, 33 knots. Your average speed is now 32, better but still not the 36 predicted.
How accurate is your tacho? Is it a gee-whiz electronic marvel that only works to 2 significant figures? Your engine speed could have been anywhere from 5751 rpm to 5849! If you want to know the true speed potential of your rig you need a super-accurate tacho, but let’s move on and say that you were doing 5800 rpm.
Where else can we have a source of error? What is the true pitch of your propeller? Why do props only go up in 2” pitch steps? A good question well asked, which means I don’t know, but if we are kind we could say your prop pitch could be anywhere from 18 to 20 inches. Let’s compromise and call it a true 19” prop.
So now we have a calculated speed of 36 knots, average speed of 32 knots, revs of 5800, prop pitch of 19 inches and a gear ratio of 2.5 to 1,(it would be difficult to get this figure wrong!). Where are the missing 4 kts? Propellor slip!! If your propeller is not pushing you as fast as it should your propeller is slipping and this is caused by cavitation and/or ventilation. The following is from Wikipedia;
Cavitation can occur if an attempt is made to transmit too much power through the screw. At high rotating speeds or under heavy load (high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapour pressure of the water, resulting in the formation of a pocket of vapour, which can no longer effectively transfer force to the water (stretching the analogy to a screw, you might say the water thread 'strips'). This effect wastes energy, makes the propeller "noisy" as the vapour bubbles collapse, and most seriously, erodes the screw's surface due to localized shock waves against the blade surface. Cavitation can, however, be used as an advantage in design of very high performance propellers, in form of the supercavitating propeller. (See also fluid dynamics). A similar, but quite separate issue, is ventilation, which occurs when a propeller operating near the surface draws air into the blades, causing a similar loss of power and shaft vibration, but without the related potential blade surface damage caused by cavitation. Both effects can be mitigated by increasing the submerged depth of the propeller: cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream
But let’s not worry about all that technical stuff. If we accept that water is not a perfect medium for transmission of power we will always have some level of propellor speed loss and can be measured in terms of propellor efficiency or slip. These terms are connected thus; Slip = 100% - efficiency. Efficiency is basically (Observed speed/calculated speed) x 100%. So, in our case above (32/36) x 100 = 89% or a slip of 11%.
Several sources on the Internet suggest racing boats achieve 10 to 15% slip, planing ru***outs 20 to 25%, planing cruisers 30% and so on, so if you have a slip figure of 11% that’s one efficient rig you have there! Ignore the 11% because you’re in front of the crowd.
Finally what’s the maths for all this stuff?
(Rpm x pitch)/gear ratio = inches per minute.
(Inches per minute x 60)/(12 x 6080) = knots.
Propellor Efficiency
Your you-beaut new toy has just been delivered. A 6.0 metre dedicated fishing machine, powered by the absolute latest from the workshops of Yamazuki. 180 horsepower of 4-stroke grunt.
You take it down to the ramp, dunk it, ruthlessly follow the break-in instructions then decide to see just what this baby will do.
5800 rpm, 19” propeller, 31 knots!!! Goes well, doesn’t she? Well, maybe.
The gear reduction through the bottom end is, say, 2.5 to 1, so the propeller is turning at 2320 rpm.
The prop wants to travel 44,080 inches each minute, 3673 feet per minute, 220380 feet per hour, or 36 and a bit knots.
Oops. Mathematically you should be doing 36+ knots with that engine/propeller/gear combination, but you are only getting 31.
So what’s happened? Where are the missing 5 knots?
Two immediate answers, inaccuracies and slip, that’s what happened.
Let’s look at the inaccuracies.
How accurate is your speed? How was it measured? It starts to get murky here. Are you using a pitot-type speedo? Is it reading right? No, I used my all-singing, all-dancing Navrance 999WRX combi GPS/Fish-finder/Radar/Coffee maker and it’s never wrong! Um, hang on. Where did you do the speed run? In a bay, with tide and wind, or a river with current and wind. Under these uncontrolled conditions to get a good approximation to your speed you should do at least 3 runs in each direction, 180o apart, in as close to the same conditions as possible. Remember your boat is moving through the water but the GPS is measuring your speed over the ground. This might not account for 5 knots, but if you did a run in the opposite direction you might have had a speed of, say, 33 knots. Your average speed is now 32, better but still not the 36 predicted.
How accurate is your tacho? Is it a gee-whiz electronic marvel that only works to 2 significant figures? Your engine speed could have been anywhere from 5751 rpm to 5849! If you want to know the true speed potential of your rig you need a super-accurate tacho, but let’s move on and say that you were doing 5800 rpm.
Where else can we have a source of error? What is the true pitch of your propeller? Why do props only go up in 2” pitch steps? A good question well asked, which means I don’t know, but if we are kind we could say your prop pitch could be anywhere from 18 to 20 inches. Let’s compromise and call it a true 19” prop.
So now we have a calculated speed of 36 knots, average speed of 32 knots, revs of 5800, prop pitch of 19 inches and a gear ratio of 2.5 to 1,(it would be difficult to get this figure wrong!). Where are the missing 4 kts? Propellor slip!! If your propeller is not pushing you as fast as it should your propeller is slipping and this is caused by cavitation and/or ventilation. The following is from Wikipedia;
Cavitation can occur if an attempt is made to transmit too much power through the screw. At high rotating speeds or under heavy load (high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapour pressure of the water, resulting in the formation of a pocket of vapour, which can no longer effectively transfer force to the water (stretching the analogy to a screw, you might say the water thread 'strips'). This effect wastes energy, makes the propeller "noisy" as the vapour bubbles collapse, and most seriously, erodes the screw's surface due to localized shock waves against the blade surface. Cavitation can, however, be used as an advantage in design of very high performance propellers, in form of the supercavitating propeller. (See also fluid dynamics). A similar, but quite separate issue, is ventilation, which occurs when a propeller operating near the surface draws air into the blades, causing a similar loss of power and shaft vibration, but without the related potential blade surface damage caused by cavitation. Both effects can be mitigated by increasing the submerged depth of the propeller: cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream
But let’s not worry about all that technical stuff. If we accept that water is not a perfect medium for transmission of power we will always have some level of propellor speed loss and can be measured in terms of propellor efficiency or slip. These terms are connected thus; Slip = 100% - efficiency. Efficiency is basically (Observed speed/calculated speed) x 100%. So, in our case above (32/36) x 100 = 89% or a slip of 11%.
Several sources on the Internet suggest racing boats achieve 10 to 15% slip, planing ru***outs 20 to 25%, planing cruisers 30% and so on, so if you have a slip figure of 11% that’s one efficient rig you have there! Ignore the 11% because you’re in front of the crowd.
Finally what’s the maths for all this stuff?
(Rpm x pitch)/gear ratio = inches per minute.
(Inches per minute x 60)/(12 x 6080) = knots.