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Gearbox ratio and propeller pitch


IanD

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It will be MPH over ground as measured on a GPS speedometer so the calculations should be relatively easy.

and fixed rpm settings

 

run 1 speed over ground (with current) 7 mph

run 2 speed over ground (against current) 4 mph

 

7 + 4 = 11 mph /2 = 5.5 mph

 

Now if the one grey cell is working here the mean through water speed is 5.5 MPH at rev setting 1

 

Now one fly in the pie is, will the rpm increase when going with the current as the current could be reducing the load on the prop.

 

One proviso to apply to that for an accurate result, . . . both runs must be over exactly the same stretch of river and follow exactly the same course relative both banks on both runs. There can be a considerable variation in current speed across the width, and on different stretches of river, due to changes in the X-sectional profile of the river bed.

 

No change in rpm whether upstream or down, . . . boat speed through the water is unchanged, . . . unless you've got a strong up or downriver wind.

Edited by Tony Dunkley
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One proviso to apply to that for an accurate result, . . . both runs must be over exactly the same stretch of river and follow exactly the same course relative both banks on both runs. There can be a considerable variation in current speed across the width, and on different stretches of river, due to changes in the X-sectional profile of the river bed.

 

No change in rpm whether upstream or down, . . . boat speed through the water is unchanged, . . . unless you've got a strong up or downriver wind.

 

 

I agree, there are a couple of long straights where they row on the river I have in mind, so it should be possible to run centre stream both ways over the same stretch of river. As you say the wind could be the problem and upset the apple cart. If it is steady and dead ahead or astern it should cancel out, but the result may not be as accurate because of the shape of the topside is not the same forward and aft.

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One proviso to apply to that for an accurate result, . . . both runs must be over exactly the same stretch of river and follow exactly the same course relative both banks on both runs. There can be a considerable variation in current speed across the width, and on different stretches of river, due to changes in the X-sectional profile of the river bed.

 

No change in rpm whether upstream or down, . . . boat speed through the water is unchanged, . . . unless you've got a strong up or downriver wind.

 

The rpm change thing was a joke, but you can hear an small change in rpm if suddenly goes in and out of rapidly changing or whirling water, in an airplane, the rev counter react faster then the ASI or VSI (fixed pitch)

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The rpm change thing was a joke, but you can hear an small change in rpm if suddenly goes in and out of rapidly changing or whirling water, in an airplane, the rev counter react faster then the ASI or VSI (fixed pitch)

 

My mind went to looping a Pit Special and watching everything sink back down the dials :)

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the axiom with 18" x 22 deg have 16" pitch at 70% radii

 

The outflow water column from these Axiom propellers must have a very strange configuration, and also contribute a significant reduction in efficiency.

The 18" x 22 deg Axiom has, in round figures, a pitch of 4" at the blade roots and 23" at the tips, so a mean pitch over the whole length of the blades of 13.5".

Edited by Tony Dunkley
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The outflow water column from these Axiom propellers must be very strange, and also contribute a significant reduction in efficiency.

The 18" x 22 deg Axiom has, in round figures, a pitch of 4" at the blade roots and 23" at the tips, so a mean pitch over the whole length of the blades of 13.5".

 

I think most boat propellers of today have some reduction in pitch near the hub, if that goes for just the bigger ones i don't know, airplane props have about 60% pitch at r/R .15 to take into account the reduction in flow near the hub due to engine cowling/fuselage, but the propeller see big variation in flow speed and direction, so the propeller will have an average pitch to suit the local flow, then as you say it is a question to have a minimum induced drag, load profile over the blades radii, the induced drag is a result of the thrust, and we want a large thrust, so to minimise the losses the blade load profile is important.

 

An airplane propeller can be 90+ % efficient (on faster plane) the profile lift to drag ratio can be 50/1 that is 2% loss, the rest is rotating losses and tip losses + the big induced losses.

what we call slip, that really is induced speed relativ the forward speed can be as small as 10-20%, half the induced speed increase happen in the propeller disk, the other half of the increased is far behind the propeller. this delta speed increase is what we call slip stream or prop wash.

Prop wash or slip have nothing to do with efficiency numbers as some think, sometimes i see it even in writing that exempel a slip of 50% have an efficiency of 50%, So an propeller with no prop wash have 100% efficiency ??

We will not get any thrust if we don't push something backward ! but it have some merit, due to the fact that best possible efficiency is had if we take a large diameter of air/water and increase the speed a little.

Edited by Dalslandia
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The outflow water column from these Axiom propellers must have a very strange configuration, and also contribute a significant reduction in efficiency.

The 18" x 22 deg Axiom has, in round figures, a pitch of 4" at the blade roots and 23" at the tips, so a mean pitch over the whole length of the blades of 13.5".

 

Tony,

 

You have me puzzled the prop that is on my boat the angle of the blade is an apparent constant from tip to root. Now are you referring to the blade tapper and the effect that has?

 

 

ed I think Dalslandia has explained what was in my mind. On a boat it is even worse than on an aircraft, the blade is also shielded at the top

Edited by Graham.m
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Tony,

 

You have me puzzled the prop that is on my boat the angle of the blade is an apparent constant from tip to root. Now are you referring to the blade tapper and the effect that has?

 

 

ed I think Dalslandia has explained what was in my mind. On a boat it is even worse than on an aircraft, the blade is also shielded at the top

Graham, an propeller with constant pitch along the blade is said to be some 3% less efficient then one with reduction in pitch near the hub, but even so the blade angle never become constant on any radii.

 

The pitch is circumference at the local station times the angle Tan.

as I said the 18 diameter 22 deg is 16" pitch at 70% radii

70% is the normal place to measure the nominal pitch that is stamped on the hub, and might also be the station with the center of thrust, and peak of thrust along the blade.

 

in your prop case, it is Tan 22 (18 x 0.70 x Pi)

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Prop wash or slip have nothing to do with efficiency numbers as some think, sometimes i see it even in writing that exempel a slip of 50% have an efficiency of 50%, So an propeller with no prop wash have 100% efficiency ??

We will not get any thrust if we don't push something backward ! but it have some merit, due to the fact that best possible efficiency is had if we take a large diameter of air/water and increase the speed a little.

 

I've tried to explain this point about propeller 'slip' many times to different people, and with differing degrees of success.

The best example I've managed to come up to illustrate the point is that of a tug undergoing a bollard pull test. Obviously, it's an extreme example, but it does demonstrate that slip when considered in isolation is not inversely proportional to thrust, as many seem to think.

With the tug applying a maximum power pull to a fixed point or bollard on shore, the tug's propeller has zero speed of advance and therefore the the propeller is producing it's maximum thrust at the time that 'slip' is at 100%.

If we can get everybody understanding this first, we could then complicate it all a bit more by bringing in the differences between real and apparent slip, but let's leave that for later.

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The word slip is something we have to live with, just as inch, feet and HP

 

If just take my boat , with a 32 x 26 propeller, 667 rpm max at 8 kts

it have an theo pitch speed of 14 kts or forward speed is 56% of the "slip"

the hull shape and deadwood keel isn't optimal and will change when time is right.

 

we don't use the word slip when we talk about wings, it travel in the air with a angle (alpha) it lift up the air in front of the wing and send it downward after the wing.

 

The alpha isn't what it seems to be either with a camber in the airfoil. but we tend to use the chord line or flat bottom on propellers as ref line. and then adjust it with zero lift line angle.

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The word slip is something we have to live with, just as inch, feet and HP

 

If just take my boat , with a 32 x 26 propeller, 667 rpm max at 8 kts

it have an theo pitch speed of 14 kts or forward speed is 56% of the "slip"

the hull shape and deadwood keel isn't optimal and will change when time is right.

 

we don't use the word slip when we talk about wings, it travel in the air with a angle (alpha) it lift up the air in front of the wing and send it downward after the wing.

 

The alpha isn't what it seems to be either with a camber in the airfoil. but we tend to use the chord line or flat bottom on propellers as ref line. and then adjust it with zero lift line angle.

 

 

I think this is where the Axiom wanders away from the normal. I think the easy way to explain is it is a double aerofoil mounted back to back with the leading edges on the opposite edges of the blade. The idea being that astern produces the same level of thrust as ahead.

 

THis slip thing is difficult to get one's head round.

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I think this is where the Axiom wanders away from the normal. I think the easy way to explain is it is a double aerofoil mounted back to back with the leading edges on the opposite edges of the blade. The idea being that astern produces the same level of thrust as ahead.

 

THis slip thing is difficult to get one's head round.

 

That does describe the design and X-sectional shape of the Axiom blades very well, but my view is that this design feature gives equal performance in ahead and astern by degrading the ahead performance relative to the astern performance, rather than by improving astern performance. The blade X-section on most conventional propeller blades is flat on the after face and curved, like an aerofoil, on the forard face and creates 'lift' which adds to the ahead 'thrust' from the after face when the prop's rotating ahead. With the prop rotating astern this 'lift' still occurs in the same direction and subtracts from the astern 'thrust' from the forard face.

 

Slip is best thought of as just being the difference between the distance a propeller would advance in one complete revolution if driving through a solid substance, as does a bolt in a tapped hole ( 0% slip), and the distance it actually advances, against total hull resistance (air resistance, and frictional and wave making resistance), in one complete revolution driving in a water. The difference between the two distances of advance, expressed as a percentage, is the slip.

Edited by Tony Dunkley
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That does describe the design and shape of the Axiom blades very well, but my view is that this design feature gives equal performance in ahead and astern by degrading the ahead performance relative to the astern performance, rather than by improving astern performance. The blade X-section on most conventional propeller blades is flat on the after face and curved, like an aerofoil, on the forard face and creates 'lift' which adds to the ahead 'thrust' from the after face when the prop's rotating ahead. With the prop rotating astern this 'lift' still occurs in the same direction and subtracts from the astern 'thrust' from the forard face.

 

Slip is best thought of as just being the difference between the distance a propeller would advance in one complete revolution if driving through a solid substance, as does a bolt in a tapped hole ( 0% slip), and the distance it actually advances, against total hull resistance (air resistance, and frictional and wave making resistance), in one complete revolution driving in a water. The difference between the two distances of advance, expressed as a percentage, is the slip.

 

Right Tony, now that version of slip goes straight in, I hope, just to check. It is the distance lost or not achieved that one turn would have achieved in a solid medium expressed as a percentage. Now from my brain, this means the prop has left a hole behind itself having sucked the water out and a lower pressure area the depth of say one half turn of the prop. So for the next turn there is less water there as the refill is not instantaneous, thus the water moved drops. May have taken it too far.

 

I find it interesting that you think the astern betterment has decreased the forward action of the prop. From the figures I have seen and the Durham water tunnel experiments that should not be the case. However that could show up in the tests by more power being required to achieve given speeds. Thus it will be interesting to see if the revs for the various speeds are greater or less than they were with the old prop.

 

 

 

 

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Slip is not slip, to make thrust the propeller need to deplace a mass of fluid, to do that it need to first have enought pitch to overcome the forward speed,

the effictive pitch is speed V. in mph * 88 / rpm * 12 = inch

then the propeller pitch need to overcome the induced speed

and finaly it need some alpha between the incoming fluid (V + V induced) to make it happend. the alpha depends on the load, speed and blade area, and the airfoil shape and thickness.

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Thinking about power v revs v fuel input (throttle setting) v thrust from the prop and comment about the improved astern affecting the forward thrust (?speed?).

 

If I have got this right?

 

RPM of an engine is related to its load and the amount of fuel the engine is fed.

 

Thrust of the prop is related purely to the speed of rotation (assuming the power is there to turn it)

 

So to test whether there is a reduction in the forward thrust from the prop the important comparision is maybe the thrust (speed)it gives for a given throttle setting (fuel into the engine).

 

If so my throttle is marked in three places and I know the speeds that gave with the old. Not highly accurate but will give a goodish idea.

 

I could be completely off beam here

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Think the HR2M is not governed, not sure as manual is not here. But engine responds to minor changes of throttle

I think it's highly unlikely that your engine is not governed. Diesels are inherently unstable, unlike petrol: consider slightly too much fuel per injection, the rpm starts to increase but since each injection is a fixed dose of fuel that means the fuel flow increases and the rpm continues to rise, especially neutral. Ditto if there is slightly too little fuel, the engine will slow and stop.

 

Of course there are varying degrees of "hardness" (gain) of governing and perhaps yours is quite soft, but I'm sure it must have some governing.

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I think it's highly unlikely that your engine is not governed. Diesels are inherently unstable, unlike petrol: consider slightly too much fuel per injection, the rpm starts to increase but since each injection is a fixed dose of fuel that means the fuel flow increases and the rpm continues to rise, especially neutral. Ditto if there is slightly too little fuel, the engine will slow and stop.

 

Of course there are varying degrees of "hardness" (gain) of governing and perhaps yours is quite soft, but I'm sure it must have some governing.

 

I think the important point here is the ability to maintain a given output RPM for a given load at a given throttle setting. I don't see whether the engine has a control to stop over revving or under revving is relevant in this context. I think the mean fuel consumption is going to be stable. Sorry

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I think the important point here is the ability to maintain a given output RPM for a given load at a given throttle setting. I don't see whether the engine has a control to stop over revving or under revving is relevant in this context. I think the mean fuel consumption is going to be stable. Sorry

The point is that if with propellor A you set the throttle to a marked position, you will get a certain fuel flow. If you change to propellor B (which presents a different load) and set the throttle to the same marked position, whilst the rpm might be the same, the fuel flow (and engine torque) will be different.

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The point is that if with propellor A you set the throttle to a marked position, you will get a certain fuel flow. If you change to propellor B (which presents a different load) and set the throttle to the same marked position, whilst the rpm might be the same, the fuel flow (and engine torque) will be different.

 

Please go away, I have no reason to trust you or your cohorts rather every reason not to and it seems that you think you know it all about everything. I have been using and maintaining Lister diesel engines on & off for over 55 years and I do not know it all and re the HR2M I am learning its foibles.

 

Ed If fact I know very little about engines

Edited by Graham.m
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The point is that if with propellor A you set the throttle to a marked position, you will get a certain fuel flow. If you change to propellor B (which presents a different load) and set the throttle to the same marked position, whilst the rpm might be the same, the fuel flow (and engine torque) will be different.

 

well put Nick the Norman, think of it as a car, you drive up hill and you give same more throttle to come up without losing speed or needing to shift gear, after the crest you have to lift your foot so the speed don't go up, if the downhill is steep enough the speed will go up with no throttle, the RPM have been very much the same the whole time. the torque and the power have been all over the place, so power have nothing to do with RPM (hiding for incoming)

More correct, you can have any power at a specific rpm that goes up to that full throttle power curve RPM point that is published in the handbook (minus sales talk)

 

a propeller have its own DEMAND curve, that also depends little on forward speed, it takes more power to turn it a specific rpm at low speed then it does at a higher speed, (dive/climb) Matching the prop to the engine rpm is important, but sometimes the propeller efficiency is that much higher at a slightly lower rpm that it shine.

 

I miss judged the installed power ones, the resulting propeller become over propped, but take off and climb was slightly better, cruise was a lot better and even top speed higher then with previous propeller, saved a lot of fuel, all because of a better efficiency

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Think the HR2M is not governed, not sure as manual is not here. But engine responds to minor changes of throttle

 

The HR is most definitely governed, and what Nick says about fuel flow is dead right. The speed control sets the speed, not the fuel flow

 

I don't have any photos to hand unfortunately. The governor is in the gear casing between the engine and the gearbox and has a lever and rod connected to the fuel pump rack. The speed control connects to a spring that pulls on the rack

 

Richard

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