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Old 01-08-2010, 02:41   #1
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Two-Bladed Gori - Pain in the . . .

Hi. On the boat I recently bought previous owner installed 2 bladed folding Gori. The motor is 18bhp 2cyl Penta. With no waves and no tide the boat reaches 4kts at reasonable rev. She reached 4.8kts SOG at full rev for a few seconds. Manouvering is terrible on reverse. I decided to put back the old 3 bladed prop, or go for Kiwi feathering. Not sure though if the old prop was chosen properly. How to calculate the prop diameter, pich, amount of blades and blade area or shape (if at all) to get the best output out of the motor? (the boat is 25ft, 5000lbs fin keel). With this Gori, we set of from Lowestoft, couldn't do 2kts sog against 1m waves. Having this Penta onboard she should cut across with no probs. Cheers

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Old 01-08-2010, 04:01   #2
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I have a Mirage 25(4400 lbs.)with a 1GM10 Yannmar(8hp.)My prop is a 13 x 9 Gori and it works fine.How does this compare to your prop dia. and pitch?
Might be that your prop is undersized.An 18 hp.engine should have enough power to get to hull speed or close to it,easily.


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Old 01-08-2010, 04:33   #3
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The boat goes out of the water in a few days, then I'll be able to measure it properly. Previous owner has no idea what that is, he probably fitted whatever he managed to snatch out of second hand market... I can live with reverse maneuvering problem, the boat is small and even with skeg I can manage in most of the situations... but more thrust would be much appreciated. Not mentioning saving the motor and decreasing fuel consumption...
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Old 01-08-2010, 10:47   #4
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Aloha Sakura,
If you do a search in the search engine beside my signature using Proper Propeller I think you'll up with discussion, book recommendations and graphs.
kind regards,
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Old 01-08-2010, 11:00   #5
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I recently replaced my 16 x 13 two blade Gori with a 15 x 13 two blade Varifold. Although top speed is the same, I went from 4.2 knots @ 2500 rpm, to 5.9 knots at the same rpm. I can motor at 5.5 knots at only 2000 rpm. What an improvement!
The only downside is I need to go from neutral to forward for a second before going to reverse to unfold the prop. If I forget, the prop usually won't unfold in reverse.
"I have never understood why it is "greed" to want to keep the money you have earned,
but not greed to want to take somebody else's money"

-Thomas Sowell
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Old 01-08-2010, 12:10   #6
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The cure for reverse non-folding as I was told is full rev for a sec in reverse gear. Some guy recommended this technique to me, and it works well. However gentle maneuvering on reverse isn't an easy thing... There is one thing that I love in folding prop though... Couple days ago I caught large, colorful, beach towel into the prop in the middle of the sea.. imagine the odds Drifted a bit with tide, caught a bit of a wind into sails - and surprisingly the towel slipped of the folded prop on its own

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