prop choice 2 smaller and faster

[jgrace]

For some time I've been of the opinion that I'm not getting as much speed out of my setup as I ought. So seeing jwalker's excellent explanations and roycruse's handy calculator I reached for some jotting paper. I got so far, but I'm still not sure I've got an answer.

My 60hp 4 stroke Yamaha currently gives me a max of 30mph on the gps at 5000 rpm on the tach. I think it's supposed to give maximum output at 5500rpm, so I'm hoping if I can get the extra 500rpm I'll get some extra speed as well.

My prop is 11" diam / 15" pitch and the gearing is 1.87 so I entered the current max 5000 rpm scenario into roycruse's calculator, adjusting the slip percentage to get a speed reading of 30 mph. The slip precentage turned out to be 20%. Disappointing, but within the bounds of realism. So far so good.

Changing the pitch to 17" shows an increased speed at 5000 revs. BUT 17" is a courser pitch, so surely the engine will be under more strain and therefore max revs will drop?

So logically I think I need a finer pitched prop. But entering 13" pitch gives me 29mph at 5500 (assuming the max revs climb with the finer pitch) or 26.3mph at 5000. How do I use the propcalc to pick a prop that will give me my best speed, presumably at 5500rpm? Help!

I must be missing something here, but I can't think what. Help!

[roycruse]

No - you seem to have got the jist of it - going up a pitch will only give you more speed if you don't bog your engine down and it never reaches max - or near max revs.

changing down a pitch will only increase speed if your engine is bogged down enough that the extra revs you get out way the loss in max speed due to the finer pitched prop.

there are however other variables that can be played with.

1. the amount of cupping on the prop will help reduce slip in harsh maneuvers but will lower revs when going flat out with a low load.
2. the diameter can be played with - dropping half an inch on the diameter will increase slip but increase rpm and vice versa.
3. rake of the propeller (maybe someone better than me can explain this better but as i understand it higher rake gives better bow lift and therefore should reduce friction for higher speed).
4. number of blades - increasing blades increases surface area of the propeller and decreases slip - but the increased drag will lower rpm.
5. material - a stainless prop with its thinner polished blades should both decrease slip and increase rpm.

[jgrace]

Truly a black art!

[roycruse]

by the way - you can use my calculator the other way - you dont have to keep adjusting the slip until the speed is right - type in the speed and then hit the = button under SLIP and it will back calculate the slip for you.

[roycruse]

Originally Posted by jgrace

How do I use the propcalc to pick a prop that will give me my best speed, presumably at 5500rpm? Help!

The prop calculator unfortunately does not allow for hull drag and weight of boat etc so will only ever give you a theoretical speed.

The slip percentage will never be 0 as it represents the the force that drag exerts on the prop causing it to "slip" or "spill" some of the water that it is trying to force backwards.

so the art of getting your boat to go fastest is about minimising drag (usually done by lifting as much hull out the water as you can) and hence minimising slip but balancing that against getting as steep a pitched prop as you can get whilst maintaining max revs on engine (as its only at max revs that the engine is producing its rated HP)

The problem then remains that a fast boat is not always a well handling boat and you may fined that after trimming the boat for max efficiency and speed that she ventilates or cavitates at the slightest turn and becomes impractical for everyday use - or takes so long to get on the plane that you put an extra person in the boat and its useless...

[jwalker]

Originally Posted by jgrace

I must be missing something here, but I can't think what. Help!

Nope, you've got it right. You loose speed because the pitch goes down but you gain speed because the engine revs higher. You are trying to get the best compromise to end up with the engine producing maximum horsepower. However, if the engine does not produce much difference in power between, say, 5000rpm and 5500rpm then the calculation may give you a theoretical pitch but a prop with that exact pitch may not exist. You will have to compromise on your choice. There is something else which we've not mentioned and it is to our advantage. As the boat speed increases, the prop slip reduces.

The info roycruse has given is designed to give particular characteristics to the prop or to improve it's efficiency. Some of these characteristics can be used to your benefit. You can produce an intermediate prop which more closely matches your engine/boat and gives you extra speed. The extra you are likely to gain on a 30mph boat is small.

Put all the details into the calculator and allow it to calculate the exact pitch.

Don't loose sight, in all this, of the reason for the engine manufacturer's requirement for proping the engine to achieve close to the recommended maximum revs. This is to ensure the engine does not labour at part throttle.

[jgrace]

Originally Posted by roycruse

by the way - you can use my calculator the other way - you dont have to keep adjusting the slip until the speed is right - type in the speed and then hit the = button under SLIP and it will back calculate the slip for you.

Thanks, I had missed that bit!

[jgrace]

Originally Posted by jwalker

The info roycruse has given is designed to give particular characteristics to the prop or to improve it's efficiency. Some of these characteristics can be used to your benefit. You can produce an intermediate prop which more closely matches your engine/boat and gives you extra speed. The extra you are likely to gain on a 30mph boat is small.

Put all the details into the calculator and allow it to calculate the exact pitch.

Don't loose sight, in all this, of the reason for the engine manufacturer's requirement for proping the engine to achieve close to the recommended maximum revs. This is to ensure the engine does not labour at part throttle.

Sounds like I should leave well enough alone. I'm very happy with the boat's handling at sea, just having my head turned by all the mention of faster boats on the forum!

[floater]

after all this calculations and theory I sure that you carn't beat trying diffrent props because you can calculate all you like but at the end of the day trying diffrent props ally stainless size and pitch is all going to count but it can be a very expensive experiment
so what i reckon we need is a "prop swap" service or loan a prop
from a freindly chandlers or just keep buying and selling on ebay.

any ideas


Andy

[roycruse]

Originally Posted by floater

after all this calculations and theory I sure that you carn't beat trying diffrent props because you can calculate all you like but at the end of the day trying diffrent props ally stainless size and pitch is all going to count but it can be a very expensive experiment
so what i reckon we need is a "prop swap" service or loan a prop
from a freindly chandlers or just keep buying and selling on ebay.

any ideas


Andy

Absolutely right - I would love to see a service like that - I went through all this on my last boat tried several props and ended up cleaning them all very thoroughly and sending them all back as i gained nothing on the original prop - even the postage cost me a fortune in the end.

I was amazed at the difference 1/2 inch diameter or slightly different cupping made to my boats performance.

Without a prop swap service or unlimited funds all you can do is calculate as close as you can - make some educated guesses on the other characteristics and then just live with your decision ...

GOOD LUCK