Propellers and Fuel Consumption

[SnaxMuppet]

I have read all the posts concerning propellers and propeller sizing seems to have been "posted" to death but there has been very little discussion about the effect of propeller size on fuel consumption.

I have both the standard 17" and a 21" pitch prop for my 140hp 4-stroke outboard.

My question is this: Would I get better fuel consumption from the 21" pitch prop or the 17" pitch?

I understand the issues surrounding WOT rpm. Both props would be fine if you use the WOT rpm test... the 17" puts the WOT on the max allowed and the 21" puts it on the minimum allowed (convenient!).

Clearly, the 17" would give better acceleration and better times to planing and the 21" would give lower rpm when cruising at a particular speed (and hence quieter) but which would give me better fuel consumption?

The issues for me seem to be that with the 17" I will need a higher rpm but the engine will be more lightly loaded.

This is a tricky one for me and I would appreciate your thoughts.

 

[brianrunyard]

In theory, both will require the same power at the same cruising speed, so both will require the same quantity of fuel to produce that power.

 

[ontheplane]

Not sure that's true...

I would think that the 21" would give the best economy cruising as it's like having a higher 5th gear on a car.

It may not give the best however if the boat is heavily loaded as the engine is having to labour....

Only way to tell is to fit a fuel flow meter then measure consumption at a give speed in the same conditions with same loading etc etc then change prop and repeat...

Perhaps someone with more knowledge than me will be able to help.

 

[capnjack]

Interesting question and if water were thick like toffee and the prop had no slip the answer would be the 21" pitch would be more economical, assuming then the two props are exactly the same apart from the pitch.

In reality of course this is not so. Firstly boat engines are designed and tuned to start and then run at high revs and then be switched off, well thats the simplistic account of it. There must a little cruising power bands built in. So a lot will depend on the engine type, make etc. and what power bands it operates in. To complicate matters a little more the two props will be different diameters and have different blade size as well as probably having different cupping etc.

Therefore I don't think it is as easy as you think to just put one against the other in that way. I suppose if I had to choose it would be the 21", but you have all put forward arguments which are very sound for both props being better with different loads and speeds.

I really think as ontheplane said you would need to put a fuel flow meter on the engine.

 

[aquaholic]

I can only give the experience I found with 2 differing pitch sizes I used on my first speedboat years ago.

13" Pitch quick on the plane and topped out at around 30mph
15" Pitch slower on the plane but gave 35-37 mph
Used them both many times for the same kind of use, ski-ing messing around etc but always found the 15" pitch used more fuel than the 13" pitch. Dont ask why cos I aint got a clue

/forums/images/graemlins/confused.gif

 

[oldharry]

The theory is that the propellor that allows the engine to develop its full rated RPM in service is the most efficient and therefore gives the best fuel economy. Unfortunately there are many other variables - the loading of the boat (how many large people on board /forums/images/graemlins/smile.gif?) Running into a head wind or head sea, for example will vary the amount of power the boat needs to develop a specific speed. So a heavily loaded boat running against a rough sea would need a 'lighter' pitched prop to develop its optimum power at full throttle, while the speed will be down.

Also the design of the prop itself has a bearing on efficiency. All props 'slip' to a certain degree: this means that a 20 inch pitch prop which gives a theoretical forward movement through the water of 20 inches may have say 50% slip, meaning it only actually propels the boat 10" forward per rev. Planing props develop much higher slip figures. Slip varies with the loading due to weather weight etc, but also with the design of the prop itself, its RPM, and the power input.

The maths for calculating slip is complex, and even more so is the calculation of fuel consumption. The far easier option is to measure the actual fuel consumption with a simple fuel flow meter, or fuel computer. This is the only practical way of finding out

 

[duncan]

21"

 

[SnaxMuppet]

In a couple of weeks I'll be able to answer my own question... I am getting a new boat and it has a Lowrance LMS-520 chartplotter that has a NMEA 2000 interface. Fortunately, the new boat has a Suzuki DF115 engine that has a NMEA 2000 output for engine stats and that means I will have fuel flow on the chartplotter, along with range, tilt, rpm etc etc.

So, I will run some tests at various speeds with each prop (this time a 19" and a 21") and see what the fuel flow actually is on the same day with the same load and same conditions.

I will post when I have the results.