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Old 19 August 2009, 18:07   #21
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Not really
Still Chris, even if neither of us understand the physics, twins are rigged with a slight overprop. A quick Google will confirm this.

A Singleton should be wary of offering advice to a Twin on this matter.
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Old 19 August 2009, 18:09   #22
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Erm.....what are.....ahem.....ventilation plates?
I think he means Cavitation plates, but what would I know...

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Also, er.....does 18", 19" or 20" when that measurement applies to a prop. mean the distance it will travel forward during one full revolution
Yes.
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Old 19 August 2009, 18:22   #23
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Still Chris, even if neither of us understand the physics, twins are rigged with a slight overprop. A quick Google will confirm this.

A Singleton should be wary of offering advice to a Twin on this matter.
More to the point I dont understand your physics I am just working off the information that BRP give and that I have used when installing and setting up my 150.

The engines are designed to work best within a certain RPM range and I can see nothing in the installation manual that I used when fitting both my 115 and 150 to suggest that when running twins you should over prop the engines.
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Old 19 August 2009, 18:23   #24
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I think he means Cavitation plates, but what would I know...
Anti Cavitation plates or Anti ventilation plates. Same thing
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Old 19 August 2009, 18:30   #25
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More to the point I dont understand your physics
This is good, I have none...

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The engines are designed to work best within a certain RPM range and I can see nothing in the installation manual that I used when fitting both my 115 and 150 to suggest that when running twins you should over prop the engines.
Mine were rigged by the RIBS manufacturer in conjunction with the original buyer, a RNLI Cox'n/Instructor. What DOES your manual say about Twin Installations? (I didn't get a manual and have to rely on the internet )
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Old 19 August 2009, 18:47   #26
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This is good, I have none...

Mine were rigged by the RIBS manufacturer in conjunction with the original buyer, a RNLI Cox'n/Instructor. What DOES your manual say about Twin Installations? (I didn't get a manual and have to rely on the internet )
Loads of dealers rig these engine to run at the lower limits of the RPM range as you can get a bit more speed out of them by doing this however BRP came out with the optimum range to ensure that dealers were setting up the engines to get the best performance and ensure the best economy and engine lifespan.

I could probably get a couple of extra knots by fitting a 21" prop and pulling 5000 RPM but have actually decided to follow BRPs lead on this RPM thing.
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Old 19 August 2009, 21:14   #27
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I think he means Cavitation plates, but what would I know...



Yes.
I could at this point say: "Ah, yes of course." However; although I've heard of cavitation, even seen it mentioned in a magazine, but, I don't actually know what it is and how cavitation plates prevent it. Or do they help it?
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Old 20 August 2009, 02:37   #28
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I could at this point say: "Ah, yes of course." However; although I've heard of cavitation, even seen it mentioned in a magazine, but, I don't actually know what it is and how cavitation plates prevent it. Or do they help it?
they don't they prevent ventilation - but for some reason the misnomer anti-cavitation plate seems more popular!
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Old 20 August 2009, 06:34   #29
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they don't they prevent ventilation - but for some reason the misnomer anti-cavitation plate seems more popular!
I hang my head in the closest thing I know to shame, apologies to Chris

This seems to explain the relationship and nomenclature nicely: http://www.boatlicence.au.com/cavitation.htm

... so Anti-ventilation plate it is then, Polwart
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Old 20 August 2009, 06:37   #30
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I suppose this would be a good time to chuck in a couple of definitions:

Cavitation is seen on the back edges of the blades, and is the result of the pressure dropping so much behind the blade that the water will boil at the ambient temp it is at. (higher pressure = higher boiling point - conversely the lower the pressure the lower the BP). The boiled water forms steam bubbles, which can't stay in that state for long (milliseconds), and so the steam bubbles collapse rapidly, forming hthousandds of mini explosions. Keep it up for long and you start to erode the blade surface. Normally seen a short distance from the tip on the back of the blade.

Ventilation is simply that the prop pushes more water out the way than can get in to replace what has been chucked out. Gravity assists the stuff above the prop do drop, and so the "hole" is preferentially filled form above. There isn't a lot of water up there, so as the resulting walls of water either side fall in to fill the hole, alots of air gets into the mix & you know what happens next.


Theory of the plate is that it helps to keep the water above the prop air free by not allowing anything in form the top, so water is pulled in from the sides to fill the gap (where it is still 100% water) rather than a water / air mix air form above. Not a spectacularly teccie description, but hopefully it clears things up a bit.
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