Why overprop or underprop?

[lionelz]

I am not an engineer by any imagination and often fail to understand the concept of basic engineering. So I have asked a number of people this question and don't get an answer that I really understand so I am hoping that you guys can help me. This is purely for my own piece of mind and at present have no intention of changing my prop's (of course that may change when I understand the implications and outcomes)

1. So what does putting a bigger prop on do? how does it affect your STW and the engine rev's? Why would you over prop? How does it affect fuel consumption?

2. What does putting a smaller prop on do? how does it affect your STW and engine rev's? why would you under prop. How does it affect fuel consumption?


Thanks in advance plain English and in very simple terms would be very appreciated

 

[Divemaster1]

People put on bigger (or heavier loaded) prop(s) in the belief that they will go faster at a given RPM. They may do so, but they also draw out more HP (burn more fuel) at that same RPM ... and if they do not reach full WOT with the "bigger" prop(s), then your engine is overloaded and you will face Big Bill at some stage.

Smaller (or lighter loaded) prop(s) will do the opposite and give less load on engine (at given RPM) ... and as a diesel engine should not over-rev, then you may not reach "best" speed. however, if you always are heavy laden (fuel, stores and people), then a "smaller" prop may be preferred...

Note you can have same size prop, but more pitch = heavier loaded as well...

 

[capnjack]

OK, so very simply a propeller size i.e. diameter/pitch/cup/rake should make a boat perform to its optimum with whatever engines it has. The decision about sizing to the engines will give the boat its optimum speed.
It's very similar to how a car works with a certain gear. Most prop manufacturers try to make props in set diameters because this works for their propellers so that we as general public only look at one area which is usually the Pitch!
Without going into greater detail.....
Too higher gearing (bigger pitch) will put a strain on the engines making them work too hard and maybe not achieve planning speed and burn out quicker.
Too low gearing (not enough pitch) will achieve planning speed very quickly and not go fast enough but will make the engines rev too high and burn out quicker.

If you want to go into further detail I'm sure there are others who will be delighted to help. I will be too, but I hope this is simplistic enough.

 

[BartW]

............. plain English and in very simple terms would be very appreciated

I can't garantee that

in addition to the info above,

so with the ideal prop you will get max engine RPM at WOT,
optimum load on your engine, and best performance of the boat.

some people choose a prop slightly smaller than that;
- on a sport boat, to get better acceleration / on the plane when towing a skier fe
- on a bigger boat, to take in to account, additional weight from fully loaded with fuel or stuff...
and or to cope with barnacle growth on bottom and underwatergear, during the season. (giving more resistance on the movement of the boat)


forget about putting a bigger prop for reducing fuel consumption on cruising speed,
that doesn't work like that, and is not good for your engine

 

[lionelz]

Ah, So as I see it there is no real advantage of doing anything to as standard boat, so I suppose the question is how do you know what size prop you should have in relation to you engine and gearing of outdrive (if that's what you are on) I just leave it to my engineer and hope he gets its right

 

[BartW]

Ah, So as I see it there is no real advantage of doing anything to as standard boat, so I suppose the question is how do you know what size prop you should have in relation to you engine and gearing of outdrive (if that's what you are on) I just leave it to my engineer and hope he gets its right

start with the prop that was "originally" with your standard boat, from there you can test if you need to test with a bigger or smaller....
or check whats on a similar boat.
you can try to borrow another prop from a dealer or a friend or neighbour to test with a different pitch.

 

[Latestarter1]

I am not an engineer by any imagination and often fail to understand the concept of basic engineering. So I have asked a number of people this question and don't get an answer that I really understand so I am hoping that you guys can help me. This is purely for my own piece of mind and at present have no intention of changing my prop's (of course that may change when I understand the implications and outcomes)

1. So what does putting a bigger prop on do? how does it affect your STW and the engine rev's? Why would you over prop? How does it affect fuel consumption?

2. What does putting a smaller prop on do? how does it affect your STW and engine rev's? why would you under prop. How does it affect fuel consumption?


Thanks in advance plain English and in very simple terms would be very appreciated

The golden rule again 'propellers move boats, engines merely respond to the load placed upon them'.

Also remember that up to certain speeds (around 35 knots) effective diameter is more efficient at than pitch in transmitting the optimum thrust.

For a mechanical diesel engine propeller should be sized to allow engine to reach maximum rated speed plus governor droop of say 10% with clean bottom and half tanks. This condition will allow engine not to be too overloaded as season progresses and hull becomes weeded. Remember that even failing to make rated engine speed by say just 100 rpm will result in significant and dangerous engine overloading as the load is not linear, it is by the propeller exponent.

Propping too light has zero detrimental mechanical effect on a diesel engine and the propeller demand being light means that engine is operating at way less than its potential power resulting in lighter fuel burn, downside is that top speed is compromised.

 

[MapisM]

Also remember that up to certain speeds (around 35 knots) effective diameter is more efficient at than pitch in transmitting the optimum thrust.

Care to elaborate further, LS? I for one don't get that.

 

[MapisM]

This is purely for my own piece of mind and at present have no intention of changing my prop's (of course that may change when I understand the implications and outcomes)

Not a lot to add to what has already been said, but beware: if you think that understanding the implications of pitch, diameter, etc. is enough to identify a prop that can improve your boat performance, think again.
Unless you have a way to try before you buy, it's likely to become an expensive exercise...

 

[Latestarter1]

Care to elaborate further, LS? I for one don't get that.

Put another way, up to about 35 knots diameter is the single most important factor in determining the amount of thrust delivered however once we reach 35 knots a large wetted surface area then creates excessive drag. Gerr 'Propeller Handbook' page 20, Effect of Diameter.

 

[BruceK]

Care to elaborate further, LS? I for one don't get that.

+1
I know turning a bigger diameter / pitch prop slower to get the same performance is more efficient, but that depends on gearing ratio's and so requires a holistic view of the drive chain and not just props.

 

[scottie]

There should be more correctly propped boat now than ever before but they will only be correct at one combination of boat speed and engine revs along with boat weight
The use of computer aided propeller design and manufacture should also increase the chances of getting it right 1st time.
You are probably correct in saying leave it to the builder or prop supplier to advise you especially the builder who can do the fine tuning by experimentation

 

[MapisM]

Put another way, up to about 35 knots diameter is the single most important factor in determining the amount of thrust delivered however once we reach 35 knots a large wetted surface area then creates excessive drag. Gerr 'Propeller Handbook' page 20, Effect of Diameter.

Understood, thanks. I perfectly see why the slower the speed, the higher the relevance of diameter.
Otoh, drawing a line at 35 knots regardless of the hull characteristics and displacement sounds like a bit of a generalization...
...but I'm not pretending to know better than Gerr, of course!

 

[Latestarter1]

+1
I know turning a bigger diameter / pitch prop slower to get the same performance is more efficient, but that depends on gearing ratio's and so requires a holistic view of the drive chain and not just props.

Yes you are quite correct, but life is very much more simple, just a matter of applying the basic rules.

When arriving t correct propeller spec I first look a tip clearance. 15% is normal, however sometimes I work with 20% to ensure that there is no rumbling.

Having established largest propeller I would like to swing one looks at tip speed at rated engine speed in order to chose the most suitable available transmission ratio. One simply graunches in sufficient pitch to absorb around 80% of CORRECT rated engine power. Simply remember that whilst you have purchased a certain horsepower engine you can never have it all.

I do it with a calculator pencil and paper, no computer required, and it is not a black art and it appears that subject has has many running scared.

Forget calling Clements Marine, I would suggest that The Propeller Handbook is recommended reading for all boat owners.

 

[scottie]

Yes you are quite correct, but life is very much more simple, just a matter of applying the basic rules.

When arriving t correct propeller spec I first look a tip clearance. 15% is normal, however sometimes I work with 20% to ensure that there is no rumbling.

Having established largest propeller I would like to swing one looks at tip speed at rated engine speed in order to chose the most suitable available transmission ratio. One simply graunches in sufficient pitch to absorb around 80% of CORRECT rated engine power. Simply remember that whilst you have purchased a certain horsepower engine you can never have it all.

I do it with a calculator pencil and paper, no computer required, and it is not a black art and it appears that subject has has many running scared.

Forget calling Clements Marine, I would suggest that The Propeller Handbook is recommended reading for all boat owners.

I suspect you you actually wear a pointy hat as well or have done it so often you know the spells without looking in the spell book!

 

[lionelz]

I suspect you you actually wear a pointy hat as well or have done it so often you know the spells without looking in the spell book!

LOL I was going to say the same thing, I did not understand a word of what was said then, I think that was down to me rather than LS. There are some bright engineering minded people on this forum, I am just not one of them :nonchalance:

 

[Trundlebug]

Just one minor thing to add to the above.

There is more of a case for overpropping displacement boats and it's quite common for this to be done.
Especially where a displacement boat has generously powered engine(s) for the normal cruising speed of the boat (which is often very close to the hull speed).
Top speed in a displacement boat is almost irrelevant because in 99.99% of circumstances it will never be used, save for an emergency.
This is because to achieve the extra one or two (maybe even three) knots of maximum speed compared to hull (crusing) speed takes enormous power; power usage goes up exponentially above hull speed, creates huge wash, uses loads of fuel but the actual speed increase is minimal.

However as the load at hull speed is significantly less, much quieter and more relaxed cruising can be obtained by a degree of overpropping.
It doesn't necessarily damage anything or cause overheated exhausts etc if done sensibly.
There still remains debate about this but some people still do it for this reason.

 

[MapisM]

There is more of a case for overpropping displacement boats and it's quite common for this to be done.
Especially where a displacement boat has generously powered engine(s) for the normal cruising speed of the boat (which is often very close to the hull speed).

100% true - that's precisely how my own boat is propped.
But that's very specific of overpowered D boats, which aren't so common after all, and I suspect that we would only confuse things further to the OP by debating that...