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Old 09-06-2010, 00:38   #1
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This Way or That ?

Today I was approached by a local power boater and he was saying that my screened thruhull was backwards. I said "HUH". Whatchu talkin about. He pointed at my newly installed motor inlet and said "it's backwards".

So, I proceeded to educate him on aerodynamics.

He points at the power boat next to mine (we're on the hard) an says "see".

I said "Yeah! That's a power boat, it's different".

He goes "HUH?"

So, I had to explain to him that at low speeds (sailboats) the screen acts like an airplane wing, The water passes over the big end and smooths out creating the least resistance, as well as allowing debris/grass to pass over w/o clogging up the screen (no cavitation).

Where as, Power boats go fast causing cavitation on the back side (air going under the hull) so it needs the screen in the front to force the water thru the screen. Then I showed him some sailboats in the yard and they were all backwards (to him).

So, another fun day on the farm.
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Old 09-06-2010, 05:23   #2
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Forward facing creates a positive pressure on the intake and can overcome the impeller causing engine flooding. Best to not have one at all.

I don't think hydrodynamics comes into play.

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