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Old 11-03-2007, 18:00   #1
Kai Nui
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Two blades or three?

The time has come to purchase the folding prop. I have done the shopping, and narrowed it down to a few quotes. Now, the one question left. Should I go with 2 blades or 3? 37' Piver Lodestar trimaran. 30HP Yanmar. I want all the speed I can get. Since the two blade is nearly half the price of the three blade, I am wondering what benefit there would be to spending the additional money.
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Old 11-03-2007, 18:04   #2
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Aloha Kai,

2. Happy to help!

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Old 11-03-2007, 19:36   #3
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Sailing speed or motoring speed?
Smooth motoring or vibrating motoring?
Small$ or big $ ?
folding, feathering or fixed?

Our old mono had a 3 blade Maxprop; smooth motoring, good sailing, but $$ to buy. Our current trimaran (Dragonfly 1000) has a 2 blade folding Gori.
It's ok, but not as good motoring performance as the Maxprop.

3 blade fixed on a sailboat, especially a multi? fuggedaboudit!

Steve B.
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Old 11-03-2007, 21:54   #4
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To answer the question simply, 2 blade.
Now a complex why.
The more blades you have, the more effort to turn the prop. This is from friction caused by the greater surface area. Two blade is far easier to turn than three, so you have greater Hp turned into propulsion than into spinning the prop.
However, because a three blade has more surface area, it also has less slip. So it is more efficient at providing propulsion, or forward movement with every turn. The problem is working out the critical change over points between which best suits a need.
Rules of thumb are, a two blade prop suits low Hp and high revving engines/shafts. A three bladed prop suits slower reving and higher Hp engines.
In your case, you have low Hp, reasonable Shaft speed and you need the lowest profile when furled to ensure the least drag, seeing as you have a very fast hull. 2 blade is IMO the best choice for you.
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Old 12-03-2007, 02:17   #5
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My new 3 blade bites the water like a Rottweiler. It throws me forward when put in gear and stops me so quick the Mrs nearly fell over the bow the first time I think that they are pretty good.
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Old 12-03-2007, 15:53   #6
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Folding, or Feathering

Two points not covered, Kai.
A) Some of the discussion has refered to feathering props, not folding. The two critters are very different. Feathering is much more efficient for powering. Folding props are generally cheaper, and much less drag, but not so good for powering. They are the choice of racing boats, but not I think of cruisers.
B) Two blades or three. I recently had a Max-Prop installed. The marina manager had originally suggested a two-blade, but when he saw the boat, said I needed three blades. The question is, what's in front of the prop. If the shaft comes out of the keel or a skeg, when one of the blades passes behind the obstruction, there will be a hammer effect. He told me, and I believe him, that three blades greatly reduces this effect. If the shaft is supported by a strut, the two blades should work.
Our boat has not gone back in the water here in Rhode Island, so I can't report on the results. Perhaps someone else has expertise/experience?
March 12, just a few more weeks, can hardly wait!
Regards, Bligh
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Old 12-03-2007, 19:02   #7
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Kai:

Don't know anything about props but I do have a spare from my J105 lying around. Might be willing to trade it for a cabin heater or something like that.
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Old 12-03-2007, 20:47   #8
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A feathering prop has a less efficient shape. It has a flatter blade with less curvature to lessen drag when feathered. A quality folding prop has a much more efficient blade shape so I would disagree that the feathering prop is more efficient for powering. I think the biggest benefit of the feathering prop is the variable pitch so it can be fine tuned. Maybe this makes up for the less efficient blade shape. With the folding prop you have to get it right the first time as to diameter and pitch. So I don't think either one has a big efficiency advantage over the other. I would go with the folding prop due to the fact that it is half the price and a simpler mechanism. You have to get a quality one though and make sure the blades have interlocking gears so they swing out together. Some cheap two blades do not have this. I like Flex-O-Fold.

The two blade Volvo folder is I used for several years did the job but was not very good in reverse(a problem with most two blade props) but when I switched to a three blade I did notice a little higher speed and much better in reverse. I think the three blade better although the Volvo was a quality piece of gear and I have nothing bad to say about it.
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Old 13-03-2007, 00:13   #9
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OK, lets keep this simple and on track to KaiNui's question folks.
Firstly, he is asking about Folding Props.
Kai, I just re-read your post and have a question. When you say "needing speed" are you talking engine driven speed or sail speed?? That part is kinda important. I am, presuming sail driven speed.
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Old 13-03-2007, 06:09   #10
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I purchased my 10 ton vessel which included a three blade MAX prop. It is a magnificent piece of equipment! My vessel should not make 7 knots due to her short water line length and her weight. When I crank up my diesel to her cruising rpm, the boat begins to plane! She will make 7.5 knots. The thrust coming from that prop is incredible.

Under sail, the prop feathers itself so that the water pushes right over her blades with ease.

Lastly, I can put that prop into reverse at 6 knots and with some thrust of the engine, she STOPS. I can't believe how well this prop works on this 32 foot tank I call a sailboat.
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Old 13-03-2007, 08:16   #11
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Since I'm the process of installing a new engine..steping up to a Yanmar 56 HP engine..I've found the experience of the remarks relating to performance invaluable..

Having been a seaman for many years...I always stay grounded when I relate to tradition...Making long passages our engines..wide open at sea speed..When approaching the sea buoy..We would test gear..and set our engines on maneuvering sped..waiting for our pilot to come aboard..to safely maneuver to a point where we are safely tied up and stop engines...This is how things are done..and how tradition on the highest levels has evolved.

I relate my sailing vessel to areas of the same tradition..My sails represent sea speed..my engine maneuvering and piloting...So I expect, and desire the maximum amount of efficiency.

If my vessel's intention was primarily racing..I would sacrifice maneuverability for performance from a two blade prop eliminating as much parasitic resistence possible.."There is always a trade off "..Since my vessel is a cruising vessel and engine purhased with intention of maximizing maneuverability..I will willingly accept the trade off of increased parasitic resistance sacrificing performance benefits..and increased maneuverability

So what I take away from this forum is the idea that those with 3 bladed Maxi folding props have been quite happy with their choice ..which the way i'm inclined to go reading all the aformentioned accounts.

If those wh have Maxi props or other alternatives from experience care to comment I'm all open ears at this point.
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Old 13-03-2007, 08:35   #12
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Tom,

I also have the 56 HP Yanmar on my 32 footer. It's a bit overpowered. That engine coupled with the max prop is a fabulous combination.

HERON
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Old 13-03-2007, 11:20   #13
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Originally Posted by Heron
Tom,

I also have the 56 HP Yanmar on my 32 footer. It's a bit overpowered. That engine coupled with the max prop is a fabulous combination.

HERON
Thank's for the re-assurance...mine is 40 loa with 36 wl..23,000 lbs...that's what I'll probably go with
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Old 13-03-2007, 15:12   #14
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I am also seriously considering a new prop. The 2-blade feathering prop currently fitted on the boat is pretty old (probably original) and quite worn, plus I think, being a racing boat, the prop was specced for maximum sailing performance rather than motoring so, as a result, motoring performance is less than stellar.

I had been thinking of putting on a 3-blade feathering prop... I would like to be able to motor comfortably at 6 knots, even with a little bit of chop and breeze, and I think that the engine is good enough for the job; its only 29hp but the boat weighs less than 7 tonnes (16000lb).

Will I really take a significant hit in sailing performance with a 3-blade feathering prop over a 2-blade?
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Old 13-03-2007, 15:52   #15
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I can't tell you the difference from a two blade vs a three blade for sailing performance. What I do know is that my 3 blade MAX virtually disappears when the engine shuts off and the sailing begins. I have "seen" the prop do its thing when I was in a small power boat going along my sailing vessel. The props keep adjusting to the lean of the vessel to reduce drag. I can't even see any real turbulence around the prop area when the boat is sailing along at 6-7 knots.

HERON
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