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post #1 of 8 Old 01-10-2007 Thread Starter
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folding prop speed pickup

i m sure there are many qualifying factors, but i m curious how much speed one would expect to pick up on a larger sail boat (40footish) by switching from a 2 or 3 blade fixed prop to a comparable folding prop? thank you
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post #2 of 8 Old 01-10-2007
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I switched from 3 blade fixed props on a privlege 48 cat to max props feathering props. No appreciable difference in speed from freewheeling the fixed prop. The difference could only be tenths of KTS if any. That is with two props. I would imagine that at very slow speeds the difference may become greater i.e. .1KT/8.5kts vs .1kt/4kts. If you are racing, it may be worth it. The other difference is you don't hear the shaft spinning. A little gain for a few thousand dollars.
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post #3 of 8 Old 01-10-2007
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Mrkeith, I have a 40 footish with a Gori racing folding prop.

When deciding what prop, I contacted all the manufacturers and decided on Gori for several reasons.
One of the questions that were asked and answered by all was overall speed efficiency increase with a folding.

They all replied that the gain can be up to 30% t0 35% wich is around 1kt.

Then, depending at the speed you are going, it can be anywhere from 1 kt to 3 kts, for my boat.
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post #4 of 8 Old 01-10-2007 Thread Starter
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its interesting that the difference seen in a cat is so minimal- i wonder if thats just because of the lower drag in a cat by its nature. i would be putting it on a fixed keel btw
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post #5 of 8 Old 01-10-2007
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We went from a folder to a low drag 3 blade Campbell prop on our Choate 40, and estimated we lost an average .5 - .75 knots depending on conditions.

As we primarily cruised this boat it was not a great concern, and the increased thrust, smooth operation and better "brakes" more than made up for the loss of speed.

In a race boat, however, such losses would be considerable.
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post #6 of 8 Old 01-10-2007
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"i wonder if thats just because of the lower drag in a cat by its nature."

If the sail plan is able to overpower the boat, the parasitic drag would be compensated for.
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post #7 of 8 Old 01-11-2007
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Quote:
Originally Posted by mrkeith
i m sure there are many qualifying factors, but i m curious how much speed one would expect to pick up on a larger sail boat (40footish) by switching from a 2 or 3 blade fixed prop to a comparable folding prop? thank you
I have a 33 foot cruiser racer with a fin keel, spade rudder and cutless bearing strut with a typical 12 x 6 Michigan Wheel on a direct-drive Atomic 4. I was sold at a deep discount a Gori folding 11 x 8.

I noticed several interesting effects:

My prop walk was reduced, but my stopping power remained the same, once compensation for the "fold/unfold" process of gear shifting on the prop was taken into account.

I needed to add more throttle under power to get the same speed. Not an issue as I was near hull speed at just over half-throttle with the old boat, and I rarely motor with intent.

My "dead slow" speed: minimum throttle in gear, roughly 700-800 RPM...dropped from 2.5-2.7 knots as per GPS/knotmeter to 1.5-1.8 knots. This has more than compensated for the less responsive reverse, as my too-fast dead slow put me in the habit of "dead sticking" in neutral and coasting into my dock or mooring anyway. A controllable, slower speed is a boon.

My sailing speed in a given range of say, 10 knots, is up between 1/2 and 3/4 of a knot. The boat gets into the groove faster and accelerates faster, noticeably so. I have even had to rethink my sail changes (I have hank ons), because what I call my "apparent wind speed bands" have altered: because the drag has lessened, I am over-canvassed somewhat earlier than I used to be. This I can well live with, however.

My new boat is a 13 ton steel motorsailer, about as far from the old boat as you can get. However, the experience of losing this drag (and I intend to sail more than motor) has been so positive that I will likely replace the 18 x 13 three blader with a VariProp four-blader of similar size. I would have gotten an AutoProp, but would have needed a half-inch out of the rudder stock, something I won't alter.
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post #8 of 8 Old 01-11-2007 Thread Starter
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all- thank you for the informative feedback!
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