2 or 3-blade prop? - SailNet Community
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post #1 of 7 Old 11-27-2003 Thread Starter
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2 or 3-blade prop?

My boat is underpowered with an old Volvo Penta MD1B. I''m wondering if I could get a pit more out of it by replacing my two-bladed prop with a three-bladed one? Would it be worth the tradeoff with more underwater resistance? Expense-wise, I don''t think a folding or feathering prop is an option.
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post #2 of 7 Old 11-27-2003
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2 or 3-blade prop?

With a small engine like the MD1B, on a compartively low drag boat like Schock 30, a three bladed prop will provide no real advantage and would have a lot of potential negatives such as causing the engine to lug and carbon up and adding a lot of drag under sail. The MD1B with a two blade prop should be adequate for the boat in question.

You should be able to buy a reconditioned used two blade Martec folding prop for about the same price as a new 3 blade prop. If you are not going to buy a folding propellor, I suggest that you mark the shaft so that you can align the propellor in vertical position to minimize drag when sailing. Low sail area/low stability boats like the Schock 30 really benefit from minimizing drag anyway that you can.

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post #3 of 7 Old 12-04-2003
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2 or 3-blade prop?

A three-blade will definitely give you more bite, expecially in reverse (which your boat probably needs with that old Volvo--I had a MD2B in my boat for a long time so I know what you have). A three-blade should not bother that engine at all, but will offer a little more resistance sailing, depending on the type of boat you have,
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post #4 of 7 Old 12-04-2003
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2 or 3-blade prop?

Sizing a prop to a boat/engine combination is a science, and you would be best served getting the advice of an expert. When we converted our boat to a MaxProp the vendor PYI collected detailed specs on our boat to determine the size and number of blades for the replacement prop. Three was not necessarily simply better. I believe the ability of the prop to transform the engine power into drive is driven by its blade area, you only need three blades when a correctly size two blade won''t fit with the required clearance. As noted previously, too much prop may prevent the engine from operating at cruising speed.

First talk to a pro and see if your current prop is the right size and pitch, if not get a recommendation on what you need - with this size boat and engine, I''d be surprised if a three blade would be useful. Otherwise you may throw money down a hole for no return or worse, buy yourself some trouble.

And I gotta ask, is this a sialboat or a powerboat! Get yourself a folding/feathering prop and enjoy more zip, especially in light air. You''ll get most of the cost back in the value of the boat when you sell it...plus the feathering props like MaxProp are as effective in reverse as in forward.

Good luck.
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post #5 of 7 Old 12-24-2003
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2 or 3-blade prop?

I recently joined this group and read some on props. I just launched my 40'' trimaran after some updates. One included adding a three bladed Kiwi feathering prop which John assured me would solve my problem of reverse. Two days ago, I delivered her to the west coast of Fl. Last year in bringing her home to mid fl. we almost lost it in the locks as the two bladed fixed prop would not even slow her down and only a quick action of my wife on the line stopped the boat from smashing into the end of the lock.
Getting the boat out of the so called marina, was difficult and a test of the props ability. I had to manuever through junk boats and fit between a fine Brown 34 and overhanging trees and stumps. I had a wonderful and controlled reverse here and lots of power which I used to bulldoze back the brush with the star,d ama. After getting past this we hung up on another tree and easily manuevered in the tight channel. I could not have done this before. On the Caloosahatchee we did an easy 8 mph at 3000 rpm and 9 mph at 3400 rpm with our 18 hp. yanmar. In the locks with a tail wind, reverse stopped me so easily I had to relearn how to handle the boat. I am a convert to three bladed feathering props. Lets face it folks, most sailboats spend most of their time motoring unless out to sea, so good power is very important. Brad
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post #6 of 7 Old 12-26-2003
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2 or 3-blade prop?

Anyone have any experience with th Perfect Pitch prop from CDI (www.sailcdi.com)?
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post #7 of 7 Old 12-26-2003
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2 or 3-blade prop?

I''ve switched props and repitched them too on at least a dozen sailboats. Two had Volvos...specifically MD1 & MD2 models. A large 2 blade will do about the same as a smaller 3 blade with the right sizes. Clearance is a problem with some boats and 3 blades are necessary anyway.

I start with the "calculated" prop size and take it to a prop shop to modify diameter and/or pitch after benchmarking with sea trials. I use the mfgs engine specs for torque/rpm/hp to establish the rpms to run the engine. On small engines in big boats (such as a small MD2-16hp in a 24k lb cutter)I ran cruise rpms at max torque. This matches max hp and max rpm about as good as it gets.

Most of the production sailboats I''ve been around were very underpropped from the factory and used 2 blades. I don''t know why they put a 15hp prop on a 20hp engine but it happens enough to tell me they aren''t paying attention.

Folding props are nice and dandy but I believe the speed reductions are greatly overstated. In light air or on multihulls maybe, but most 30''sail will easily overpower the drag. Try towing a small bucket over the side that matches the prop for drag and see what it does.
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