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Old 30-10-2008, 14:25   #1
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Need help with prop sizing

New to the forum, so please be gentle.

We designed and built our sailboat in the backyard. She is a cutter rigged heavy (about 25,000 lbs) cruiser, 38 ft hull, 42 ft overall, 32 ft LWL, full keeled, propeller in aperture. She is powered by a VW Jetta Diesel engine 1.6 that came from a car. Even though it's rated 53 Hp at 4,000 RPM, we all know that is not going to be achieved on a boat. We are assuming a max of 3,500 RPM and 45 Hp. The transmission is a Hurth with a ratio of 2.6 to 1. Shaft is 1 1/4".

Any kind souls out there that can help us figure out what would be the right fit 3 bladed propeller? I submitted the data to 6 propeller manufacturers and I got 3 different prop sizes.
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Old 30-10-2008, 14:57   #2
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sailingmonica check out david gerr's book on subject he has quitw a treatise on the subject being a designer
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Old 30-10-2008, 15:05   #3
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Thanks, Mike, I did and based on my calcs it comes to 18X12. Propeller manufacturers came up with 18X12, 18X14 and 19X12 respectively. I just wanted some reassurance before I order a prop. I have been trying to find a used 3 bladed 18X12 RH to try before buying a new one but to no avail.
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Old 30-10-2008, 15:28   #4
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Monica,

Good idea to check your tip clearance before you decide on the diameter. With a prop in an aperture you want the maximum diameter that will fit in order to get the blades into free flowing water as much as possible. You need about 10 to 15% of the prop diameter in tip clearance so for the 19 inch diameter around 3 inches tip clearance is the minimum.

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Old 30-10-2008, 15:29   #5
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Sorry, I meant to say 15 to 20%, the 3 inch measurement is still correct.

Wayne
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Old 31-10-2008, 04:32   #6
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Old 31-10-2008, 07:01   #7
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I can tell you this much about what I know, Island Packet 40 comes with a 17X14 three blade prop and 50HP engine and a Hirch Gulfstar 45 comes with a 17X16 three blade prop and 50 HP engine. Both boats weigh about 12 to 13 tons. The Island Packet is a full keel and the Hirch is a fin keel. The Island Packet runs 7 knots @3300 RPM. the Hirch runs 8 knots @ 3000 RPM. As you can see these are factory props that come on much different boats and perform not so differently. I would expect your boat to perform more like the Island Packet. I hope this was of some value.
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Old 31-10-2008, 11:36   #8
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Thanks, Wayne

I have plenty of room for the blade tip clearance, even though it's a full keel it's also long (deep?). Not an issue.

Monica
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Old 31-10-2008, 11:50   #9
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Give these Guys a call for used props

Give these guys a call they have tons of used props and other stuff. MINNEY'S YACHT SURPLUS - WE MAKE BOATING AFFORDABLE

Jack
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