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Old 05 August 2007, 09:02   #1
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Country: UK - N Ireland
Boat name: Muzungu
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Prop Test

I have a 21" prop on a Suzuki 200hp, max revs for the engine is 6,000 rpm, I could only get the engine to 5,400 rpm so contacted the dealer. We both thought a smaller 19" prop would perform better and on the first test I thought it did as the engine could reach 6000 rpm. We measured the speed, rpm and fuel rate LPH. The conclusion was the 21" prop was faster, more economical and need less RPM even though the engine could not reach max rpm. See PDF http://www.tempoweb.com/muzungu/Prop_Performance.pdf

21 Sp 21 LPH 19 Sp 19 LPH
3000 18.5 18.5 13.5 15.5
4000 28.3 31.7 23.7 23.7
5000 37.4 51.0 32.0 39.0
5400 41.0 63.1 34.8 48.8
6000 39.0 63.4
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Old 05 August 2007, 09:34   #2
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Remember max revs are NOT the same as maximum power. Many people make this mistake - ideally you need to look at the power curves - failing that 1/2 way between the upper and lower rev limits is the place to be!!!
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Old 05 August 2007, 10:30   #3
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I can't argue with your findings but three things come to mind which you might wish to consider.

1) A change of 600rpm for 2" of pitch is not normal so it implies something else also changed; prop styles, load, wind, tide etc.
2) Proping near or at max RPM is to ensure the load on the engine is satisfacory at lower engine speeds. Proping to 5400rpm and then adding more load, possibly extra fuel and/or passengers, may take you well outside the acceptable load the engine is designed to run at.
3) A better comparison would be comparing fuel consumption at the same boat speed rather that at particular RPM. This will give a better indication of economy.
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Old 05 August 2007, 12:51   #4
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Quote:
Originally Posted by jwalker View Post
I can't argue with your findings but three things come to mind which you might wish to consider.

1) A change of 600rpm for 2" of pitch is not normal so it implies something else also changed; prop styles, load, wind, tide etc.
2) Proping near or at max RPM is to ensure the load on the engine is satisfacory at lower engine speeds. Proping to 5400rpm and then adding more load, possibly extra fuel and/or passengers, may take you well outside the acceptable load the engine is designed to run at.
3) A better comparison would be comparing fuel consumption at the same boat speed rather that at particular RPM. This will give a better indication of economy.
Now you both have me confused!

1) The test was in a lake with trim set the same, 3 up, cross wind, 15 L less fuel for the 19"
2) I thought it was better have the lower revs of 20" for the same speed fuel consumption ot the 19".
3) The chart shows fuel consumption and speed against rpm.

21" - 30 kt - 35 leters per hour 4200 rpm
19" - 30 kt - 35 leters per hour 4800 rpm

21" - 39 kt - 57 leters per hour 5200 rpm
19" - 39 kt - 63 leters per hour 6000 rpm

With the above weight the 19" could be over reved.
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Old 05 August 2007, 14:09   #5
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Quote:
Originally Posted by Bulligmore View Post
21" - 39 kt - 57 leters per hour 5200 rpm
19" - 39 kt - 63 leters per hour 6000 rpm

With the above weight the 19" could be over reved.
Those figues plus your comment indicate that something else is influencing your results. Are the props at their stated pitch?

Taking your result for the 21" prop, and guessing your gear ratio to be 1.8:1, the slip works out at about 22% which is a little high but quite possible. However, fitting 19" pitch and 6200 max rpm (because you said it would over rev) into the equation gives a slip of about 28% which is way too high and not very likely.

Now, using your figures for the 19" prop and a slip of 22% your 19" prop works out to be about 17.5" pitch. I would normally expect a prop with less pitch to have less slip in a given application.

If you'd asked me, my guess for your slip would be about 18% and using this figure your prop works out to be about 17" pitch.

However, again, if your props are not of the same pattern you are comparing apples and pears it's just the size that you are varying. Are the props of the same pattern?
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Old 05 August 2007, 18:28   #6
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I will will check the props in the morning. They are both Suzuki props.

I have another which I got from Steel Devolments which I should also try, a 20".

I did not think about the slip.

Thank you for your help.
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Old 06 August 2007, 05:32   #7
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I think I see what you mean, I am not using like for like.

3 X 16 X 21.5 R which Suzuki recomends for a DF 200
3 X 15 1/4 X 19 R which Suzuki recomends for a DF 150/175

The DF 200 full throttle operating range is 5000 - 6000

So what do you think I should try next.

Thanks
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Old 06 August 2007, 08:34   #8
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If your present fitment is the 21.5" pitch and it's giving you 5400rpm, since the WOT is 5000-6000rpm you're within range. Provided you are happy with the performance, just stick with it. The large(ish) diameter on that prop will give a bigger blade area and should help at lower speed and in difficult conditions.
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Old 06 August 2007, 08:54   #9
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Thank you for your help. I have found sites with slip calculators and it seems that the props get more efficient at the high end of the rev band.

Were did you get the 22% slip from? The gear ratio is 2.29:1

I found

3000 rpm 26%
4000 rpm 11%
5000 rpm 5%
5400 rpm (max) 2%

But I might have made a mistake.
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Old 06 August 2007, 12:46   #10
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I guessed the ratio was 1.8:1 because that's what many 2 strokes of that HP are. With 2.29:1 the figures will be different but the difference between the two props will remain.

Slip does reduce as speed increases but it is not likely to drop to the 2% you suggest.

Using 5400rpm, 2.29 ratio, 21.5"pitch and 15% slip your boat speed would be about 41mph. This is low for a 6mtr boat with 200 horse, I would expect about 48-50mph.

You mentioned 41kts in your first post and this is not far off 47mph. So, working back; 5400, 2.29, 21.5", 47 mph = a slip of 1.74. Something is still wrong, either your boat speed or engine speed or both is not accurate.

If I presume your speed is GPS speed then that's likely to be OK so were're looking at a tacho error.

I don't know your boat but if I make another presumption that is it medium weight and not a sports hull then assume a good slip fugure of, say, 12%; 2.29 ratio, 21.5" pitch, 47mph, slip 12% = engine speed of 6029rpm.

There is a good prop calculator on Ribnet. I think it was Hugh Jardon who wrote it. Do a search, it's a nice wee VB program.
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