Propellers - big vs small

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cmedsailor

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I have read (in a recent PBO issue) that if you change the propeller of a boat with something bigger (assume same number of blades and ignore drag issues, this is more a theoretical question or to makes sence say you change from a smaller fixed to a larger folding/feathering), the boat will be able to achieve maximum speed, or better performance, at lower revolutions. Just to put some figures let’s say that the new bigger propeller can achieve “better” performance than the smaller at 80% of rpm.
Two questions:
- What happens if (for any reason) push the gear stick (sorry, don’t know what is called!!!) further forward? Higher rpm but same speed? What about any adverse effect (damage) to the engine?
- What about consumption at these 80% of rpm. If with the smaller propeller consumption at 80% was X litres per hour, does this stays the same?
Thanks
 
There are three principal factors in determining the efficiency of a propellor. Diameter, pitch (how far forward it moves in one revolution) and surface area (expressed as a % of a complete circle). An engine can only produce so much power, but propeller choice can determine how effectively this is transmitted to the water. Generally speaking you want the largest diameter you can swing with a large surface and then match the pitch to the power available. This will give lots of bite and quick stopping ability, it will however produce the maximum drag. Hence two bladers and folders are frequently used on sailing vessels. If a boat is overproped the engine will not reach full revs and may well produce black smoke. If it is underproped it will rev freely but not produce optimum movement. Prop manufacturers use software to calculate the best prop for a particular boat bearing in mind hull speed, displacement, power available, gearbox ratio, max engine revs and how much drag is an issue. It aint simple and on a sailing vessel compromises have to be made. Two bladers for example generally compensate for a lack of area by having increased pitch - less drag, less bite astern.
 
Ideally you want to match the most effective speed of the propeller with the maximum efficiency point on the torque curve of the engine. So, for example, if you put a great big propeller on that limited the engine revs to 1500, beyond which it just wouldn't rev, at least without blowing out clouds of black smoke, you would not have achieved much. Consumption would increase and flexibility would decrease. Conversely, if the prop is too small the engine will run at maximum revs without reaching top speed, especially in a chop. Again, consumption will suffer.

This is the big stumbling point in the green argument that maximum vehicle speeds should be reduced. If we drove at 50 mph on the motorway with a reasonably aerodynamic car we would almost certainly consume more fuel than if we were doing 70 mph.

As with most things in life, you need to compromise /forums/images/graemlins/grin.gif
 
There is an optimum size of propeller for each engine/box/boat combination. "size" is a combination of diameter, pitch and number of blades. The aim is to achieve the maximum theoretical hull speed within the maximum rated revs. If the prop is too "big" then the engine will not achieve maximum revs, nor possibly hull speed. If you push the "stick" forward, the classic symptoms are black smoke from unburned fuel. This can cause damage. Conversely if it is too "small" the engine will reach full revs easily and may not reach hull speed.

If propped correctly then boats usually achieve a good cruising speed at about 60-70% of maximum revs when it is at its most efficient. Fuel consumption per hour is a function of power produced, so if you are using more power because the prop is too small fuel consumption will be higher. Generally speaking larger diameter propellers are more efficient for motoring, but clearly have to be flatter pitch (everything else being equal) to be the right "size". The constraint on diameter is usually clearance of the tips in relation to the hull and drag when sailing.

Propulsion efficiency is a function of the efficiency of the propeller in turning the engine power into forward motion. Generally speaking larger diameter is more efficient but blade size and shape also has an effect.

Fuel consumption in relation to distance is then more a function of hull shape, cleanliness of bottom and sea/wind state.

Generally speaking folders and featherers tend to be larger diameter than fixed. My boat has a 16 inch fixed and is "efficient". All the recommendations I have had for folders and featherers have been for 17 inch - but not convinced worth the money!
 
"If we drove at 50 mph on the motorway with a reasonably aerodynamic car we would almost certainly consume more fuel than if we were doing 70 mph."

Beg to differ. Someone wot knows about these things told me that almost every 'ordinary' car is optimised for max fuel efficiency at 56 mph (90 kph) coz that's the speed at which official fuel consumption figures are measured, and that effects tax bands, etc..
 
[ QUOTE ]

Beg to differ. Someone wot knows about these things told me that almost every 'ordinary' car is optimised for max fuel efficiency at 56 mph (90 kph) coz that's the speed at which official fuel consumption figures are measured, and that effects tax bands, etc..

[/ QUOTE ]

Have to agree with that, having done a number of long boring journeys in a Vectra with an onboard computer fuel consumption was markedly better around the 60mph mark and you could see the average drop off at 65mph plus.

However when I had an S type a while back it did seem to be more fuel efficient at continental autobahn speeds so perhaps Jaguar didn't bother with the fuel consumption figures.
 
"There is an optimum size of propeller for each engine/box/boat combination."

Just a word or two about the one component of your formula you didn't expand on: gearbox ratio.

a 1:1 ratio box will limit the amount of HP-to-torque transfer to the prop available, requiring either a smaller diameter or finer pitch. That is why my 30 footer's 35hp is only driving a 12 x 8 inch prop.
 
Yes, I did say "engine/box/boat" combination - although strictly speaking it is shaft speed that is important - calculated from engine revs/reduction ratio. So my Yanmar 1GM with a 3to1 box can swing a 15 inch prop!
 
If you base a propeller with the luff of a sail, in theory leaving everything else aside for a moment, you would want the prop diameter to be as wide as possible with thin blades.
Bit like a windfarm turbine.

We know that when you reef a jib going to windward, you often cannot get enough power to get through the waves, hence it's often nice to have a full hoist blade jib.

There have been yacht designs to accommodate swinging very large diameter props moving at low speeds.

'Logic ! The art of going wrong with confidence !'
 
Drag increases as a square on speed. So the faster you go, the more energy required, but as the law says, it's not linear.

I know of no circumstances (at sea level on a boat) where increased speed is achieved without increased energy consumption.

PWG
 
Get yourself a Bruntons Autoprop. Sorts out the pitch automatically, whether motoring or motor-sailing, and when purely sailing, it feathers, so very little drag. Best of both worlds. Expensive though. I have no axe to grind - I'm just a very satisfied customer. /forums/images/graemlins/smile.gif
 
[ QUOTE ]
I have read (in a recent PBO issue) that if you change the propeller of a boat with something bigger (assume same number of blades and ignore drag issues, this is more a theoretical question or to makes sence say you change from a smaller fixed to a larger folding/feathering), the boat will be able to achieve maximum speed, or better performance, at lower revolutions. Just to put some figures let’s say that the new bigger propeller can achieve “better” performance than the smaller at 80% of rpm.
Two questions:
- What happens if (for any reason) push the gear stick (sorry, don’t know what is called!!!) further forward? Higher rpm but same speed? What about any adverse effect (damage) to the engine?
- What about consumption at these 80% of rpm. If with the smaller propeller consumption at 80% was X litres per hour, does this stays the same?
Thanks

[/ QUOTE ]

No it doesnt work like that. The prop must be matched to the engine and boat so that the engine develops its maximum speed at the highest speed through the water the boat can develop given the power available. If you increase the diameter of the prop, then you must reduce the pitch by a corrsponding amount. i.e. a 15x12 prop will be the same as a 14x13 or a 16x11. However as you move away from the optimum pitchxdiameter ratio, so efficiency falls away, so a 17x10 would probably not be able to propel the boat as fast as the original 15x12.

Matching a propellor to an engine/hull set up is a bit of a black art. Nigel warren has produced a table of approxiamte dimesnions for a given engine power/water speed, or you can download something like the Prop Calc programme, but you will find even these 'aids' are not necessarily very helpful as they can give different results from the same data.

In principle on a displacement motor boat, the bigger the prop, the more power it can transmit, and the shaft speed can be geared down from the engine to acheive a reasonably effective level of thrust. However on our sailyboats, a big prop acts as a brake when we aren't using it, so a compromise has to be acheived - usually best done by a folding prop which reduces drag to near nil.

Put too small a prop on, and it will not load the engine properly. A bit like driving the car at high speed in low gear. Engine damage is then possible - though unlikely with most small diesels which have governors to prevent overspeeding. In that scenario you use lots of fuel, to little effect because the engine is racing but the power is not being used efficiently.

Correspondingly an oversize prop means the boat will take off at 3-4kts at tickover, but opening the throttle will not produce much increase either in engine or boat speed - a bit like driving a car in too high a gear. The engine does not develop full revs so is not producing enough power. It also makes for some very interesting handling at close quarters in the marina! /forums/images/graemlins/laugh.gif
 
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