Gearbox ratio and propeller pitch

[IanD]

Given that online advice about propeller design says "don't exceed 1000rpm", why don't modern engines use a higher ratio gearbox than 2:1 and a bigger pitch prop?

 

For example, the standard choice for a Beta 43 is a 2:1 PRM150 and an 18x12 prop, which at maximum revs of 2800 will be turning at 1400rpm. Why not use a 2.8:1 PRM150 (I think this is a no-cost option) and an 18x17 prop turning at 1000rpm?

 

(yes I know a bigger diameter prop is more efficient, but then you need a deeper draught and...)

 

From what I could find the best diameter/pitch ratio for a 3-blade prop is close to 1:1 for best efficiency so you'd think this would be a better choice -- also probably with less vibration (tiller and hull) and prop noise due to lower prop speed.

 

Is there some other reason this isn't done?

 

 

[by'eck]

Could it be that modern high revving engines with feeble low down torque can't manage such large/course props even with a higher ratio gearbox?

[IanD]

Shouldn't make any difference, 1400rpm at 12" pitch should give the same boat speed as 1000rpm at 17" pitch since the same diameter water column (18") is being chucked backwards at the same speed (about 16mph), so the required power and torque from the engine should be identical.

 

However having done some more reading, the answer is probably that for slow boats which travel a lot more slowly than the prop wake speed (trawlers, narrowboats etc) the maximum prop efficiency is with pitch ratio (pitch/diameter) less than one, quoted optimum range was 0.55-0.8 -- and the middle of this (logarithmically speaking) is almost exactly 2/3, hence 12" pitch on 18" diameter -- so maybe Beta Marine do know what they're talking about...;-)

 

(a pitch ratio of ~1 is optimum for much more lightly loaded faster boats, not narrowboats)

[nicknorman]

Might be talking rubbish but is there a compromise between starting and stopping, vs efficiency at cruising speed? Perhaps a slow revving course pitch prop is best in the cruise, but does it have the same starting and stopping power as a finer pitch faster revving prop?

[IanD]

What makes the big difference is prop diameter, a bigger prop is better from all points of view -- efficiency, bollard pull, starting/stopping, noise -- because it's always better (for low-speed boats) to push a bigger diameter column of water backwards more slowly than a smaller diameter one more quickly. It's why working boats with huge props can outpull any modern small-propped boat.

 

For a given diameter what matters most for prop sizing is pitch*rpm since this pretty much sets the amount of power absorbed from the engine. Think about it, the clue is the name "screw propeller", it pushes a given diameter column of water backwards at a speed of (pitch*rpm), and it's this water that gives the thrust.

Edited October 9, 2015 by IanD

[by'eck]

Might be talking rubbish but is there a compromise between starting and stopping, vs efficiency at cruising speed? Perhaps a slow revving course pitch prop is best in the cruise, but does it have the same starting and stopping power as a finer pitch faster revving prop?

 

I would tend to favour the larger/coarser prop for stopping as well, if only because a smaller higher revving one easily accelerated by the engine, will likely suffer greater cavitation when being used to stop the boat, especially in a panic situation.

[nb Innisfree]

Pitch is secondary, a smaller prop results in less blade area so a wider blade is needed to restore that.

[Keeping Up]

We have always had a 2.8 ratio gearbox, compensating with a coarser high-efficiency prop (no room for a bigger one but the HE has a larger blade area) originally on a Perking MC42 but now on a Beta 43 and have always found it excellent. The torque question is a red herring, a modern engine has less torque but the gearbox increases the torque at the prop to compensate. On the plus side the prop is turning slowly so if you get rubbish on the prop it is easier to keep it turning at the lower speed as well as there being torque available to do so. I would definitely go for that option again, there is plenty of starting/stopping capability and it also tows a butty extremely well.

Edited October 9, 2015 by Keeping Up

[Dalslandia]

Intersting tread, as said diameter have a big effect on efficiency on slow propeller driven crafts, the faster the smaller the optimal propeller diamer will be, all other the same, Why is betas engine sold with 2.1 as standard when 2.5 or 3.0 might be better, A wild guess they sell engines to other boats also then NB's and canal boats.

 

A 18x12 will have about 16.86 degree blade angle at 70% radii

A 18x17 will .................. 23.24 degree ....

Some difference

A slower prop with same diameter will need larger blade area, by blade width or more blades.

[IanD]

More rpm or bigger pitch or bigger diameter or more blades or more blade area = more thrust/power. For props with the same diameter, number of blades, and blade area ratio, if rpm*pitch is the same then power and thrust should be the same -- so switching from a 2:1 gearbox to a 3:1 (2/3 the rpm) and increasing the pitch by 50% should give the same performance. Think how a screw works...

[nicknorman]

More rpm or bigger pitch or bigger diameter or more blades or more blade area = more thrust/power. For props with the same diameter, number of blades, and blade area ratio, if rpm*pitch is the same then power and thrust should be the same -- so switching from a 2:1 gearbox to a 3:1 (2/3 the rpm) and increasing the pitch by 50% should give the same performance. Think how a screw works...

No I disagree. That is how a screw works when screwed into a solid thing like a lump of wood, but a propellor is moving in a fluid, which can result in all sorts of strange effects like cavitation, stalling, recirculation, vortexing etc.

Edited October 10, 2015 by nicknorman

[Sea Dog]

No I disagree. That is how a screw works when screwed into a solid thing like a lump of wood, but a propellor is moving in a fluid, which can result in all sorts of strange effects like cavitation, stalling, recirculation, vortexing etc.

Hence why propellers developed beyond being a simple screw thread into what we see today.

[bizzard]

Modern aircraft propeller technology has come on a long way too, what with carbon fibre and super accurate balancing. Quite a few turboprop aircraft are now capable of cruising at speeds approaching 400mph and over with a tail wind. For example, certain Beech Super King air, twin turboprop planes and many others such as the Bombardier Dash 8 aircraft another twin turboprop small passenger airliner and indeed probably the type that Nicknorman flies backwards and forwards from Aberdeen on with Flybee.

Whilst on holiday in St Newlyn East near Newquay in Cornwall years ago I was talking to an old chap who was a young lad during WW2 and lived close to what was then RAF St Mawgan ''still in use'', and he told me how he saw an American air force 4 engined Liberator taking off, which left the ground, stalled and sadly promptly crashed straight into a cliff. The investigators discovered that the propellers variable pitch control was set in the course pitch position which is the position for cruising at its cruising altitude when the props should have been set to the fine pitch setting for take off which allows the engines to rev out to their maximum power torque. In course pitch on takeoff the engines would have been struggling big time for revs and power hence the stall and crash. The pilot may have changed to the course pitch far too early, that nobody ever found out, I don't think.

[Dalslandia]

The RPM or more correctly the speed of the airfoil that travel in the fluid is effecting the forces with the square factor, so by reducing the speed by 50% it can not simply be compensated by increasing the pitch 50%, other things need to be changed too, with the diameter fixed, pitch and blade area will change.

 

if we say that forward speed is the same, the body blockage /slowed down inflow is the same, if the propeller is more efficient, the induced flow will increase (sucked in) That is best don with a larger diameter if possible, but often it is not possible or practical, best efficiency (net. Thrust) is had by accelerating the fluid just a little, a smaller diameter make a faster "jet" then a larger diameter.

 

The pitch can be said to work best at its optimal alpha to the incoming flow. with a small angle.

so we have the forward speed, slowed down by fuselage, we have the induced speed, well we have the half induced speed increase at the propeller, and the other half increase far behind the propeller.

 

The pitch is then forward speed * reduction + half induced speed / rotation speed + the angle of attack (to make it simple)

The angle of attack is determined by the lift needed and shape of the airfoil.

A narrower blade will then have a higher lift/load and need a larger alpha (the pitch stamped on the hub will be higher)

more blades/blade area will have a lower load and alpha less pitch.

 

The "optimal" alpha will probably lead to a very small blade area and cavitating will be had.

 

So the simple answer is if we change one thing we have to change all the other things too. if we want the propeller to absorb the same power at the same engine rpm

 

More rpm or bigger pitch or bigger diameter or more blades or more blade area = more thrust/power. For props with the same diameter, number of blades, and blade area ratio, if rpm*pitch is the same then power and thrust should be the same -- so switching from a 2:1 gearbox to a 3:1 (2/3 the rpm) and increasing the pitch by 50% should give the same performance. Think how a screw works...

 

[DHutch]

I guess one of the reasons is that while most modern engines will rev well past 2000rpm, hence meaning a 2:1 ratio box would give over 1000rpm prop speed, in practice they are often powerful enough and prop'ed such they they cruse all day at 1700rpm. At least the hireboat I was on last weekend this was the case.

 

Second question is likely to be cost, a 2:1 box and an 18x18 prop is a lot cheaper than a 3:1 and a 25x28 prop!

 

 

 

Daniel

[IanD]

No I disagree. That is how a screw works when screwed into a solid thing like a lump of wood, but a propellor is moving in a fluid, which can result in all sorts of strange effects like cavitation, stalling, recirculation, vortexing etc.

These are all second-order effects, go and look at documents or online calculators for prop size. But I did make a calculation error, at 2/3 the speed you need more than 50% extra pitch because thrust goes as square of RPM, not linearly. Using the calculator here http://www.vicprop.com/displacement_size.php and putting in a 55' x 7' boat drawing 2 feet with a Beta 43, I get the following:

 

2:1 gearbox 1400rpm at prop 7.94 knots max using 3 blade 17.3" x 10.6" (pitch ratio 0.61)

2.8:1 gearbox 1000rpm at prop 7.94 knots max using 3 blade 21.2" x 14.8" (pitch ratio 0.70)

 

Since thrust at a given rpm is proportional to blade area (square of diameter) x pitch and (21.2/17.3)^2 is 1.5, you'd get similar results with a 2.8:1 gearbox and a 17.3" X 22.2" prop -- but this is a very big pitch ratio (1.3) so would have lower efficiency than the recommended one, so would need even bigger pitch to compensate, which would have lower efficiency, which...

 

In other words I've answered my own question about why a 2:1 gearbox is used -- unless you can fit a much bigger diameter prop in (which most modern boats can't, 18" is typically the limit) it's the best choice :-)

Edited October 10, 2015 by IanD

[IanD]

I guess one of the reasons is that while most modern engines will rev well past 2000rpm, hence meaning a 2:1 ratio box would give over 1000rpm prop speed, in practice they are often powerful enough and prop'ed such they they cruse all day at 1700rpm. At least the hireboat I was on last weekend this was the case.

 

Second question is likely to be cost, a 2:1 box and an 18x18 prop is a lot cheaper than a 3:1 and a 25x28 prop!

 

 

 

Daniel

Boats should be propped so that the engine reaches maximum revs in open water, in this case about 8 knots at 43bhp/2800rpm for a Beta 43. Required power drops very rapidly as speed drops, to keep a narrowboat moving at normal speeds typically takes less than 10bhp which would typically mean less than 1400rpm engine revs.

 

I don't think there's any difference in gearbox cost but there sure is in prop cost -- but unless you can fit a bigger diameter prop in (which most modern boats can't) the issue goes away anyway...

[nicknorman]

Boats should be propped so that the engine reaches maximum revs in open water, in this case about 8 knots at 43bhp/2800rpm for a Beta 43..

You seem somewhat categorical in your statements, whereas in reality what you say is only true if the owner wishes to extract the maximum power from the engine. Many canal boats are over-propped and thus require less rpm to cruise on canals. That they have a slightly reduced top speed on open water may be of no concern to the owners.

These are all second-order effects, go and look at documents or online calculators for prop size.

They may be secondary effects, but only within a smallish range of parameters. They can become dominant effects further from "normality". Your statement was categorical without any bounds, hence my disagreement with it.

[IanD]

OK I'll rephrase that -- if you want to make the best use of the engine power (which you've paid for, after all...) for situations like venturing out onto rivers, the boat should be [...].

 

As you say this doesn't matter in many cases so overpropping means lower rpm when cruising. Though to be honest, the real reason this is an advantage is the frankly rubbish noise and vibration reduction on many boats, the engine would often be happier (more efficient, better battery charging) running a couple of hundred rpm faster -- say around 1400-1500rpm instead of the 1200-1300rpm that many boats are propped for.

[Arthur Brown]

In many waterways a boat with a 24" prop would be called a plough! A modern(!) canal is a shallow waterway with no room for deep draught boats with big props. As the mud comes up something has to go and it's now usually boat draught and prop size.

[IanD]

You seem somewhat categorical in your statements, whereas in reality what you say is only true if the owner wishes to extract the maximum power from the engine. Many canal boats are over-propped and thus require less rpm to cruise on canals. That they have a slightly reduced top speed on open water may be of no concern to the owners.

 

They may be secondary effects, but only within a smallish range of parameters. They can become dominant effects further from "normality". Your statement was categorical without any bounds, hence my disagreement with it.

I'm an engineer, and always concentrate on the most important things first because they dominate the design. Assuming you're not silly enough to design/run a prop so far away from the norm that it cavitates badly or has a pitch double the diameter, all the curves for things like prop efficiency are pretty smooth -- they may have an optimum, but the performance difference isn't significant unless you go a long way away from this.

 

Maybe next time I'll prefix any statement with "except for really stupid designs"... ;-)

In many waterways a boat with a 24" prop would be called a plough! A modern(!) canal is a shallow waterway with no room for deep draught boats with big props. As the mud comes up something has to go and it's now usually boat draught and prop size.

Agreed, this is one reason modern boats don't start and stop as well as old ones with much bigger slower-turning props -- it's always better to have the biggest prop you can fit in for slow boats, pushing a lot of water slowly is better than pushing less water faster. . But since a lot of the engines started off life in tractors, maybe ploughing the canal is just returning to their roots...

Edited October 10, 2015 by IanD

[IanD]

Modern aircraft propeller technology has come on a long way too, what with carbon fibre and super accurate balancing. Quite a few turboprop aircraft are now capable of cruising at speeds approaching 400mph and over with a tail wind. For example, certain Beech Super King air, twin turboprop planes and many others such as the Bombardier Dash 8 aircraft another twin turboprop small passenger airliner and indeed probably the type that Nicknorman flies backwards and forwards from Aberdeen on with Flybee.

Whilst on holiday in St Newlyn East near Newquay in Cornwall years ago I was talking to an old chap who was a young lad during WW2 and lived close to what was then RAF St Mawgan ''still in use'', and he told me how he saw an American air force 4 engined Liberator taking off, which left the ground, stalled and sadly promptly crashed straight into a cliff. The investigators discovered that the propellers variable pitch control was set in the course pitch position which is the position for cruising at its cruising altitude when the props should have been set to the fine pitch setting for take off which allows the engines to rev out to their maximum power torque. In course pitch on takeoff the engines would have been struggling big time for revs and power hence the stall and crash. The pilot may have changed to the course pitch far too early, that nobody ever found out, I don't think.

A similar issue is what caused all the problems with the Napier Sabre H24 sleeve-valve aero engine in WW2 -- it had a lot of power and manual pitch control, and the pilots found that by keeping the pitch in coarse the aircraft cruised more quietly and smoothly at lower revs -- but the engines weren't designed to be flogged flat out at low revs all day and regularly seized up, which is not a good thing in a fighter plane. Once the pitch control was made automatic the problems went away.

[Bee]

It all gets into maths and things that normal folk don't understand but it would be much easier if there were easy to find power curves for props that showed how much power they absorbed at different revs. and how much shove they produced as well. I think most of us have engines that are way too big for 99% 0f the time.

[IanD]

According to one source, propellor power is typically proportional to RPM^2.7 -- don't know how accurate this is for low-speed boats...

 

http://continuouswave.com/whaler/reference/propellerPowerCurve.html

 

rpm(%) power(%)

100 100

90 75

80 55

70 38

60 25

50 15

40 8

30 4

Edited October 10, 2015 by IanD

[Dalslandia]

The power needed to drive the prop is an increasing concave curve, as IanD say, and at most time the engine curve is the other way convex. The difference is nessecerely to be able to increase rpm, but it also means that engine is at very light load at partial throttle, making the engine inefficient, and also the propeller is to small to be efficient at low speeds, here help the "over prop" situation. but then can't reach rated rpm and power on open water.

 

One more thing, in shallow water, the water speed is higher then the boat speed, so it should have a little higher pitch in a shallow water then in deep water.

 

With a modern high speed (relative) high power engine, a slight over prop can be a good thing on a canal boat.

 

I over estimated power on a Homebuilt airplane ones, the engine did not reach the redline, fuel consumption was down 26% at the same cruise speed over the previous propeller, take off was the same, top speed little higher, the Scottich owner was very happy.