Propellors yet again..

[amit]

Hello Folks,

I have a 14 LH 14 twin blade non folding propeller.

I have a volvo MD11C, 23 hp engine.

And I have a Cape Dory 33.

SO, my questions are

1) Will a 3 blade prop work better, specifically for reverse.

2) Will inherently a 3 blade prop be a better choice, especially since the twin blade one doesnt necessarily align with the keel ( even in neutral sail)

3) Does anyone have the same engine on the same boat and have a different propeller.

I am asking this because I am going to get the boat on hard for bottom paint and can use the opportunity to switch propellers if needed.

--Thanks in advance

Amit

[moctrams]

Mine works well.

http://www.westbynorth.com/

[Bob Ohler]

Moctrams, which one do you have?

[moctrams]

I have the Campbell Sailer propeller. It really works well and the boat backs up straight.

[amit]

Motcrams,

What engine HP do you have?

Thanks
Amit

[moctrams]

I have the Beta BD722. It is 20hp fresh water cooled. Great engine.

[Paul D.]

i have the Universal 5424 with a three blade Michigan, non feathering/folding. Works fine going forward and can push us even in a chop better than some two bladed boats I've chartered. There is a tendency to back to port but that's on windless days. I'd be real curious to try a pricey feathering three blade to see the difference in power, reversing and drag under sail. But unless someone buys me one, this one works well. I would not want to move to a two blade fixed just for the sailing drag reduction. One thing you could do is mark your shaft when the blades match the keel. But still, on the 33, who's going to go crawl down the side of the port cockpit locker and line the shaft up every time you turn off the diesel!

[amit]

Thanks everyone.

Paul on my CD if I do mark the shaft for alignment, I can move the flywheel from the galley area very easily. ( I have a volvo, so flywheel is very accessible from the galley area)

But again, I really wanted to know if there is a possibility of better motion in reverse with 3 blade prop.

So far with the two blade the two complaints I have is

1) Vibration. Which I thing would certainly improve with 3 blade prop.

2) Weak pull (prop walk) to starboard in reverse.. which pretty much means reverse is dominated by windage and wind direction.. which is maybe okay most times except docking with wind not dead ahead.. and of course anchoring ( deploying the anchor)

[Ken Cave]

On our 28 CD, I installed a three bladed prop, and it really helped going forward, but in reverse,
the boat has its own mind and does what it wants to do.

If you do change over to a three bladed prop, be sure to go to a dealer that can match your
engine as well as the weight of your boat. This will give you the right pitch as well as size.

This is important as you do not want to have a prop that will overpower the engine or vice versa.

Hope this helps

Ken Cave

[Paul D.]

Amit,

I don't believe a 3 bladed prop will reduce your prop walk in reverse unless it is a special newer type of prop. I have no experience with those on our boats. I think the 3 blade would give you more umph power in reverse though, which could be good if you are trying to stop coming into a dock a little too fast. Been there. Not sure if it would reduce vibration. It might but there are other factors that contribute to that including # of engine cylinders, motor mounts, cutless bearing wear and shaft coupling alignment. I agree with matching up the prop to the motor and aperture. Dave Gerr has a good 101 on propellers in his Nature of Boats book.

If I had unlimited resources I would probably ask around about a feathering three blade prop too see how they do. Auto prop, J prop and Max prop come to mind. I think a 3 blade is good for our hull types and can better get you through a chop or headwind than a 2 blade.

[Jim Walsh]

I think a 3 blade is good for our hull types and can better get you through a chop or headwind than a 2 blade.

I agree. Use a folding or two bladed prop for the last ounce of sailing speed. Use the fixed three bladed prop for superior handling under power. Clearly, if the folding three bladed props were anywhere near reasonably priced we'd nearly all choose that option.

[Neil Gordon]

On our 28 CD, I installed a three bladed prop, and it really helped going forward, but in reverse,
the boat has its own mind and does what it wants to do.

It pretty much boils down to two forces acting on the boat:

(i) Wind will always take the bow, turning the pointy end;

(ii) The force of water on the rudder will turn the other end.

Going forward, there's lots of water forced against the rudder, and it more than offsets the force on the bow. In reverse, not so much.

The good news is that the boat will always do the same thing under the same conditions.* Plan accordingly.

*When applied to docking, the laws of physics change every season. You need to relearn and practice every spring.

[JWSutcliffe]

Vibration is going to be primarily an issue of engine/shaft alignment. A 3 bladed prop will provide superior thrust, forward or reverse, albeit with more prop walk in reverse with a full keel. All a matter of compromise. If I wanted better backing I would have a fin keel - or maybe a power boat with a bow thruster!

[Tom Keevil]

When we changed from a two blade to a three-blade Campbell Sailor, there was a major increase in thrust, but no change in reverse control. It also was much smoother, but we also replaced the motor mounts, cutless bearing, and realigned the engine at the same time, so who knows? We researched a Max Prop, and found that it would not fit the aperture of a CD 33. Saiing performance didn't change much.

[amit]

Thanks All.

Looking at everyone's opinions, almost everyone uses a 3 blade prop and maybe worth the change. I will ask around for an "equivalent" 3 blade prop that can replace the 2 blade 14 LH 14 on a 23 HP @ 2500 rpm, without stressing the engine.

Regards
Amit