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Old 28-09-2010, 18:54   #1
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Propeller Advice

I have fixed, 2 blade stock props on a Lagoon 380 Catamaran. I've been researching a variety of prop upgrades, but haven't been able to justify the expense, based on my Cost/Benefit review. $1,425 X 2 = $2,850 for Kiwiprop, $2,700 X 2 = $5,400 for Gori (3 blade), etc. My questions / concerns follow:

1) Will a folding prop fail to open or unfold once aged with barnacles, growth and seaweed? Seems that the exposed rotating gears in folding props would create a new maintenance nightmare.

2) Will I only increase my sailing speed 1/2 knot? That speed increase doesn't seem worth it.

3) I've heard conflicting reports regarding whether my motoring speed will increase or decrease. I would hope that going from a 2 blade to a 3 blade would increase speed, particularly where there is a strong current?

4) I've heard that the Gori props would give me a more powerful reverse, when the Kiwiprop a less powerful reverse?

5) Are the folding props, like Gori, fixable locally or do you have to send them back to Gori if damaged or bent?

6) Fixed props only have the pressed in hub and pitch to worry about. With all of the moving parts on the feathering and folding props, is there a lot of maintenance?

7) If you have folding or feathering props, did they meet your expectations or would you gladly take your money back?

8) Does it make more sense to get 3 blade for the additional torque?

Since these after market props are a costly option, I would appreciate input from experience to justify or rationalize the cost, if it can be justified.


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Old 28-09-2010, 19:15   #2
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I think you would have to consider carefully going from fixed to feathering or folding.

We went from folding to feathering (MaxProp) and it was the best upgrade we did to the boat. The folding prop had marginal forward performance and poor reverse performance.

We went from 3 1/2 knots forward to 5 1/2 knots forward just with the prop change. Reverse performance for stopping was like night and day.

The prop has never failed to feather no matter how fouled. The maintenance is to remove it once every 14-16 months when we do the haulout. It needs to be removed to replace the saildrive anode and while it is open we clean the inside and repack it with grease.

I am not sure I would spend $3,000-$6,000 to improve sailing speed slightly if I had fixed props but I would definitely replace the stock folding prop again.

Here are some shots of our prop. This is typical condition after removal of residual grease, removal of external barnacles etc. and before polishing.

(edit - PS I moved this thread to the propeller area where it might get more traffic.)
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Old 28-09-2010, 19:22   #3
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That's a lot of money for a slight gain. I am also one who apprecites simplicity and the reliability that comes with simpler designs.

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Old 28-09-2010, 19:30   #4
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Originally Posted by timbenner View Post
3) [...] I would hope that going from a 2 blade to a 3 blade would increase speed, particularly where there is a strong current?

8) Does it make more sense to get 3 blade for the additional torque?
According to Dave Gerr, an optimally sized and pitched 2-bladed prop is more efficient than 3-bladed one. The reason to go to a 3 bladed prop is then you don't have sufficient clearance for an optimal diameter prop. So the choice between 2- or 3- bladed depends on your boat and engine.

Having said that, you will certainly give up some sailing speed in light air with a 3 bladed prop.
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Old 28-09-2010, 20:17   #5
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I had a Martec

Maelstrom came to me with a Martec 2 blade folding prop. It was marginal in fwd an near usless in reverse, would simply fold back up. Occasionaly only one blade would deploy in fwd resulting in much vibration. I only had to pop it into neutral then reverse and back to fwd to fix the issue but was still annoying. I replaced it with a 3 blade fixed and feel much better about docking. If my boat made longer voyages on the ocean I may have kept the martec but where I live motoring often becomes important and that little 12 inch egg beater simply wasn't up to the task. I'm a little slower for it under sail but I don't loose any sleep over it. If I were rich I would have a feathering screw but that will be in another life I'm afraid.

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Old 29-09-2010, 05:18   #6
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We have 3 blade volvo folding props. No problem with growth folding/unfolding. A folding prop will increase your sailing speed 1/2kt per prop - that will be a full knot for your boat. I know we are a knot faster when they are folded vs. unfolded. They reverse well, but I don't know their motoring performance vs fixed.


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Old 29-09-2010, 07:18   #7
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Here at Beta Marine we have good reports from our customers on both of the prop types that have asked about.

I wont go into the Kiwi vs Gori debate but you have also raised the question of 2 vs 3 blade. In theory your max motoring speed should be the same with a 2 blade or a 3 blade propeller but how you get to your cruise speed will be very different. The 2 blade propeller will have a lot of slip and thus you will slowely gather momentum. The 3 blade propeller will seem to bite into the water and thus you will have good acceleration. This will help if you have to motor off a lee shore.

I suspect that you are correct with the about 1/2 knot speed gain under sail for the 2 fixed blade 2 blade propellers as opposed to a folding or feathering propeller.

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Old 29-09-2010, 07:48   #8
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Also consider that feathering props produce double the thrust of fixed props in reverse. Several times I was able to get off from a soft grounding by reversing the engine where I am not sure that a fixed prop would have worked. To me that is as much an advantage as improved sailing performance.

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Old 29-09-2010, 08:03   #9
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Folding props are cheap and good. They do not grow barnacles if you use them. Make sure you have space for them.

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Old 29-09-2010, 08:20   #10
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There have been considerable advancements in folding props over the last few years. To me, there are four reasons to consider them:
1. you gain a bit of speed - 0.5kt isn't much going a few miles, but if you're crossing an ocean it can be considerable
2. less chance to snag on submerged items - more hydrodynamic and less chance of snagging lines or gunk
3. no need to lock off the propeller shaft or worry about the transmission
4. better performance in reverse - since the prop can be designed to provide a better bite in reverse, you can get a bit more control (if your harbor or marina is more difficult to navigate under power, you might gain better control)

Maintenance is generally minimal and not difficult. Some folks will grease the gears during haulouts, check and clean the prop when cleaning the hull, or occasionally run the prop forward and astern to keep the operation smooth and controllable.

Are they worth the money? Can't answer that because I don't know your requirements. IMO, I'm looking at them as my prop, courtesy of the PO, isn't what I'd call in serviceable condition. If I could find a well respected one for about the cost of a fixed 3-blade, I'd probably go for it. My cruising plans call for crossing oceans.

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USCG/MCA IV/M.I./C.I. 500-ton Oceans
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