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Old 15 July 2010, 10:49   #11
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Quote:
Originally Posted by jyasaki View Post
The rpm question is a little more straightforward (as hydrodynamics go): an increase in pitch equals greater load, which should knock the rpms down. Increase in diameter (with the same pitch) equals more load, which should knock the rpms down. There are, of course, other variables that may come into play: blade shape, prop weight, etc, but they shouldn't affect things all that much.

So the question is, why isn't that happening in this case?

jky

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Originally Posted by bjornaa View Post
we got the boat with a 10-1/2" x p13 prop
<snip>
so we went with a 10 x p16
At a guess, the 3" pitch change is being nicely offset by the half inch diameter change.
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Old 15 July 2010, 11:27   #12
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9d280

What you are saying makes sence to me, but everywhere I look all the online prop calculaters they only use the pitch to finde the speed (rpm ect.) and I guess the prop slip % that apparently it huge on a sib...

What I'm thinking is:

the more whater is forced back the bigger the forward speed, big prop big pitch must be the way to go...
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Old 15 July 2010, 16:26   #13
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Not quite like that. Prop pitch and diameter is just like bicycles.

You will not be able to rev the engine up to its optimal level, thus your engine will not render as much power.
If you use too short pitch and tiny diameter the rev limitator will stop the engine (and if it doesn't, the engine will burn).

Thus, to get as much power as you can from your engine:
high load -> short pitch short diameter
one man -> big pitch long diameter
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Old 16 July 2010, 12:54   #14
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Originally Posted by 9D280 View Post
At a guess, the 3" pitch change is being nicely offset by the half inch diameter change.
Seems a bit out of balance though.

My recollection of the rules of thumb is 200-250 rpm per inch of pitch, and 100 rpm per 1/4" of diameter.

so +3 and -3 should still change by about 300 rpm. Then again, there may be other stuff involved too... Or I may be remembering wrong.

jky
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Old 17 July 2010, 07:06   #15
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Thx a lot guys!

I will get back to this post whith updated after i try something new, have a great summer!
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Old 18 July 2010, 12:39   #16
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bigger boat, my MKIII with johnson 40, scuba gear, or just people

22-24mph

with both of my props
11 3/4 X 17
11 3/4 X 15
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Old 18 July 2010, 14:34   #17
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Well thats strange, one would think the mkIII was faster, and with your big props and high pitch... its wird that it does not go faster?

the props you have :

11 3/4 X 17
11 3/4 X 15

How did you diside them? engine manual, or what... maby they are too big and then you dont get up to the rated rpm? therfore the low speed...
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Old 18 July 2010, 19:57   #18
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My propulse four bladed prop in a MK2 Futura with a 40 hp Nissan 3 cylinder is 10.4" in diameter and adjust from 12 - 16" in pitch. Like I said before a sib has a very inefficient hull at higher speeds. Resistance or drag rises at a much higher rate than a hard hulled planing surface. A few more hp, a different prop and changes in load are going to have more effect at the low speed end. As you get up in speeds the small changes are going to have less impact on top speed. My hole shot has gotten better, but my top speed hasn't varied a huge amount. Yes, the original prop on my outboard did not hit the target wot rpm. I did change props to get the correct wot rpm.
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Old 19 July 2010, 12:10   #19
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Quote:
Originally Posted by jyasaki View Post
Seems a bit out of balance though.

My recollection of the rules of thumb is 200-250 rpm per inch of pitch, and 100 rpm per 1/4" of diameter.

so +3 and -3 should still change by about 300 rpm. Then again, there may be other stuff involved too... Or I may be remembering wrong.

jky

I agree with the pitch ballpark, as I have proved it umpteen times with differnet boats but the diameter thing is anything but linear....... (My old Yam 55 on the current boat I uppped diameter by 1/4" and went from chickening out as it passed 6500 still accelerating (old school engine with "big bang" rev limiter) to topping out nicely at 5500, both props standard ally 3 blade Yam at 14" pitch.
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Old 19 July 2010, 12:42   #20
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Have also noted that If using different brands of propellers, the results are not always logical, there is so much variation in the design between various brands.
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