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Old 13-05-2010, 23:29   #1
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NorSea 27 Aperture - Largest Recommended Prop Size ?

I am re-powering from a Yanmar SB8 to a Yanmar 2YM15. Prop recommended by Yanmar for the 2YM15 with a 2.62 ratio without regard to boat specifications is a 16x11 two blade prop.

CDI has the following view:

"Your comment about any prop being correct on you boat is probably correct. Most older boats with an aperture have an incorrectly sized propeller. The aperture was sized for a specific motor, gear ratio and propeller. The motor becomes obsolete and the new motor is geared differently, or is a different HP and a larger propeller is needed but cannot be used because of the aperture. We estimate that 30 to 50% of the boats have that problem. Usually the solution is a three blade propeller (they are about 1 inch smaller in diameter for the same HP) However a three blade usually results in slightly slower performance under sail. (extra drag for the third propeller blade.) Hull speed is usually a calculated estimate and is a reasonable approximation. Individual boats will vary a bit. Sailing magazine in a review of the norsea 27 reported getting 6 knots under sail. I was looking through the photo files after you sent your photo and found some dimensions for a norsea aperture. The opening looks to be the same as yours. Measuring a photo is not the best way but I figure that you have about 7 ½ to 8 inches from the centerline of the shaft to the hull at the top. This allows 15% clearance for a 12 inch propeller but only about 10% for a 13 inch propeller. Which is a bit small but it would still fit. The shaft is offset from center on the aperture. It is impossible to say just what the close fitting prop might do with a well built boat as the norsea, but in a lighter built boat usually added prop noise and reduced prop efficiency results. We do have a 12 inch 10 pitch propeller that would work but the top steed would be affected. Since these are 2 blade propellers your sailing speed should be slightly improved as compared to your present tree blasé propeller. The 12 dia 10 pitch propeller is the same cost.
The only propellers that we manufacture are two blade propellers You may find a three blade might give better motoring performance but will degrade sailing performance. As usual in sailing there are few obvious choices but lots of compromises."


Since my boat is at anchor in the Brisbane River and visibility poor I cannot measure the aperture. Does anybody have experience sizing a prop based upon what they know are the dimensions of the NorSea's aperture? Any NorSea users out there who know whether the recommended Yanmar prop will fit with sufficient clearance given the aperture dimensions?


Yanmar also recommends a 16"x13 prop for the 3YM20 with 2.62 ratio....

Thank you.
svarmido
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Old 14-05-2010, 05:58   #2
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I keep my NorSea on a lift and can measure or take a picture for you as soon as I get enough water to get her back on.
I don't know the size of the aperture exactly, but I am swinging a 14-9 three blade with a 1GM10.
It's a lot of prop and I'm sure that I'm paying a small price for it under sail, but truthfully, I haven't noticed a drop off in speed. The extra torque I get in a headwind or current is really worth it for me.
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Old 14-05-2010, 06:26   #3
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Old 14-05-2010, 18:28   #4
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GordMay and knothead. Thanks for your posts. knothead, it will be most appreciated if you can measure the aperture next time your boat is out of the water. GordMay, your reply confirms that in order to extract maximum efficiency from a prop certain clearances are recommended. Supports CDI's approach, but still in order to do this right I need to know the NorSea's aperture dimensions.

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Old 16-05-2010, 08:41   #5
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I hope these help.



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Old 16-05-2010, 21:03   #6
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knothead, thanks for the contribution. I forwarded your photos with measuring tape to CDI. It will be interesting to see what kind of recommendation he provides.

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Old 17-05-2010, 22:24   #7
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knothead,

I notice the prop zinc is missing. Does it contact your rudder when installed? CDI accurately notes that your shaft protrudes out farther than mine which allows you to use a larger prop. There is about 1 to 1 1/4 inch clearance between my prop and cutlass bearing. Probably 2 1/2 to 3 inches in your photo. CDI says of your setup: "The 14x9 3 blade on the 1gm10 must have a 3:1 or higher ratio or is overloading his engine."

I am considering buying a R&D flexible coupling PYI Inc. Max-Prop PSS Shaft Seal Seaview Radar Mounts R&D which is 1.28" thick. Placing the new engine flange at the same location of the old SB8's would require pushing the shaft outward the 1.28" in order to accommodate the flexible coupler. This will enable me to use a larger diameter prop. May even place the engine back another 1/2" or so to extend the shaft out a bit more. Waiting for CDI's opinion.

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Old 18-05-2010, 06:23   #8
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Quote:
Originally Posted by svarmido View Post
knothead,

I notice the prop zinc is missing. Does it contact your rudder when installed? CDI accurately notes that your shaft protrudes out farther than mine which allows you to use a larger prop. There is about 1 to 1 1/4 inch clearance between my prop and cutlass bearing. Probably 2 1/2 to 3 inches in your photo. CDI says of your setup: "The 14x9 3 blade on the 1gm10 must have a 3:1 or higher ratio or is overloading his engine."

I am considering buying a R&D flexible coupling PYI Inc. Max-Prop PSS Shaft Seal Seaview Radar Mounts R&D which is 1.28" thick. Placing the new engine flange at the same location of the old SB8's would require pushing the shaft outward the 1.28" in order to accommodate the flexible coupler. This will enable me to use a larger diameter prop. May even place the engine back another 1/2" or so to extend the shaft out a bit more. Waiting for CDI's opinion.

svarmido
When I repowered, I added a thing called a Shaft Saver. This pushed the shaft back by an inch or so. I can still use the zinc, but since the boat is dry sailed, I just haven't bothered.
I can't remember what the gear ratio is at the moment, but I can tell you that I can achieve full rpm. 3400 in gear or neutral. I am not overloading the engine. It never smokes.
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