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Old 29 October 2007, 14:29   #1
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Unrealistic slip

I was looking at the attached page from Suz, when I (using the merc prog)calculated the slip, it came out at -12.5%...

http://www.suzukimarine.com/boat_bui..._175_cs/df150/

23"
2.50 ratio
5500rpm
50 mph,

Now there a strong chance I made a cod of the calc,so any competents out there want to check it be my guest.

The set up on mine is;

25"
2.50 ratio
5800rpm
54 mph.
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Old 29 October 2007, 16:19   #2
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Quote:
Originally Posted by IBWET View Post
23"
2.50 ratio
5500rpm
50 mph
I get -14.78% for that one

The figures can't be true (?) 5500rpm = 132000rph @ prop = 3036000 inchs/hour = 48mph (best case)

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Originally Posted by IBWET View Post
25"
2.50 ratio
5800rpm
54 mph.
and 1.68%
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Old 29 October 2007, 19:23   #3
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I'm with DJL. If your prop and gear ratio is correct then you're either doing about 47mph or about 6700rpm. Or both of these are wrong and it's somewhere in between.
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Old 30 October 2007, 03:02   #4
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I'm with DJL. If your prop and gear ratio is correct then you're either doing about 47mph or about 6700rpm. Or both of these are wrong and it's somewhere in between.
JW,

The first set of figures are from suz's website!! Surely they have a calculator over there.

The second set is from my boat, now I suspect the ratio will be on the money, (unless the calculator in engineering was bought from the same supplier as marketing’s one.. ).

The prop could be stamped wrong, that's a possible, although I've never heard of it before (and on both engines)?

The revs, it’s a new engine with a new rev counter however it’s possible....

The speed is GPS based so will be right (also consistent).

I think the hull shape of my rib does not encourage speed as a lot of it sits in the water when it's on the plane, I would have thought this would have increased drag/slip, making the figure for my boat even more unlikely.

I'm fairly keen to know the answer to my own set up,but thought everyone would be interested in seeing if suzuki got it wrong.
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Old 30 October 2007, 07:47   #5
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Let's say your figures are correct and using a slip of about 13-18%, which I reckon is likely, then an explanation is that your prop is not at the pitch you think it is. Although, it may be correctly marked. I have no experience of what I'm about to suggest but I can't see why it's not a possibility.

Turbulence can form a barrier which can behave almost as a solid. Codprawn frequently recommends Volvo trim tabs (although, apparently, having not yet fitted his) which are plates that project down into the water parallel to the transom. They work by causing an area of turbulence at the rear of the hull which reprofiles the hull into a downward hook. The faster the boat travels, the firmer the hook.

I see no reason why a prop blade with a pronounced cup or blade curvature could not form turbulence within the hollow of the curve which would then behave as a solid and increase the effective pitch of the blade as its speed increased. It would be a neat trick if it worked because it would behave as a variable pitch prop.

Your prop would need to increase from 25" to about 28-30" to make your figures work and that doesn't seem beyond the bounds of possibility and an unreasonable increase to me.

But, of course, I could well be talking bollox as usual.
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Old 30 October 2007, 10:07   #6
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Quote:
Originally Posted by jwalker View Post
Let's say your figures are correct and using a slip of about 13-18%, which I reckon is likely, then an explanation is that your prop is not at the pitch you think it is. Although, it may be correctly marked. I have no experience of what I'm about to suggest but I can't see why it's not a possibility.

Turbulence can form a barrier which can behave almost as a solid. Codprawn frequently recommends Volvo trim tabs (although, apparently, having not yet fitted his) which are plates that project down into the water parallel to the transom. They work by causing an area of turbulence at the rear of the hull which reprofiles the hull into a downward hook. The faster the boat travels, the firmer the hook.

I see no reason why a prop blade with a pronounced cup or blade curvature could not form turbulence within the hollow of the curve which would then behave as a solid and increase the effective pitch of the blade as its speed increased. It would be a neat trick if it worked because it would behave as a variable pitch prop.

Your prop would need to increase from 25" to about 28-30" to make your figures work and that doesn't seem beyond the bounds of possibility and an unreasonable increase to me.

But, of course, I could well be talking bollox as usual.
JW if this was the case would the prop manufacurer's marketing people not be jumping up and down saying look at our amazing pseudo-variable pitch prop technology. You will still get great whole shot and 20% exta speed? Oh - and by the way it will cost an extra £100 too...
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Old 30 October 2007, 10:23   #7
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Maybe it's already happening. The 'general' wisdom of adding a cup to the trailing edge of a blade is that it increases pitch. I've never seen an explanation for this statement but it is commonly said.

I've not really believed it and felt it more akin to putting the flaps down on a plane wing thereby increasing the lift and therefore giving the impression of extra pitch. The one thing that niggled me in my argument was that flaps only work well at low speeds whereas props will a cupped edge work well at high speed. Perhaps the cup is a flap at low speeds but as the prop speeds up it begins to create turbulence and does increase the effective pitch.

In IBWET's case, a 5.5mtr boat with 200hp is likely to do about 54mph so I believe him and I'm sure if his revs were up to 6700 he'd know about that too . The gear ratio is likely to be correct and I'm sure he can read his prop markings so there has to be something else in the equation.
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Old 30 October 2007, 11:33   #8
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As far as rpm's go I had an interesting experience at the suzi dealer. they told me that the port engine df140 had been run at 6200 rpm's for 3 hours which I said was impossible because I never run over 5900 rpm's.

So we hooked up the computer to the engine and took it for a run. My tack on the boat read 5900 rpm's and the computer read 6200 rpm's So apparently the tacks supplied from suzi are not that accurate. The tack are made for suzi by faria. Apparently the digital tacks plug right into the computer so you get a accurate read.
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Old 30 October 2007, 13:06   #9
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As far as rpm's go I had an interesting experience at the suzi dealer. they told me that the port engine df140 had been run at 6200 rpm's for 3 hours which I said was impossible because I never run over 5900 rpm's.

So we hooked up the computer to the engine and took it for a run. My tack on the boat read 5900 rpm's and the computer read 6200 rpm's So apparently the tacks supplied from suzi are not that accurate. The tack are made for suzi by faria. Apparently the digital tacks plug right into the computer so you get a accurate read.
That's surprising; I never thought to consider the accuracy of the instruments. However I'm fairly sure my engine has a limiter at 6,100 and I've certainly never bounced of that. The 175 with (the same engine with variable valve timing) can rev to 6,100, I have no idea where the limiter cuts in on this engine. I think this might account for some of the discrepancy.

Hope your dealer doesn’t hold the over revving against any warranty issues
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