Two bladed prop

[S/V Laelia]

As usual, I am going against the flow. I want to go from my 3 bladed prop to a 2 bladed prop. I am getting ready for what I hope will be circumnavigation. If a 2 bladed prop will let me sail 0.5 kt faster, it will cut almost 30 days off the time at sea. So - I have two questions:

1. Have any of you made the change from 2 blades to 3 (or 3 blades to 2 blades) and made measurements of speed under sail with each?

2. What are the specs for the prop?

Laelia is a 365 ketch with Westerbeke W40, Paragon transmission and Walter V drive with the 2:1 reduction.

I saw one post for the 2 bladed prop on a 367. Would it be the same for the 365?

[Jim Cozy]

Ralph- I have made no such measurements, but based on PHRF rating prop allowances I would guess you might get as much as .5k going to a full feathering prop but only about .15k going from 3-blade to 2-blade. I am not sure of my math but PHRF allows 6 sec. per mi. for 2-blade and 9 sec. per mi. for 3-blade over a feathering prop.   Jim

[S/V Laelia]

Jim,

Thank you. I hadn't thought of that approach to getting measurements.

0.15 kt would only cut off 8 days - maybe not worth it.

I have heard stories of feathering and folding prop failures. Being of the KISS school of though, I am somewhat reluctant to consider them. Some fail to feather/fold, some won't unfeather/unfold. Anybody have experience with them?

[swiftibis]

Ralph,
I have a 3 bladed MaxProp on my P367.  I haven't had any problems with it thus far, but I can tell you that either excessive growth or a fouled line would prevent it from feathering due to the way the blades sweep over the centerbody.  Furthermore, each blade is mounted to the hub via a beefy shaft that rides as a bushing.  I suspect a significant impact could damage the bushing surfaces causing the prop to not actuate, although you will likely have additional problems from such an impact.  The inside has a toothed spline.  A similarly bad impact would probably mess up the splines.  The bottom line is that the MaxProp is great and low drag if kept clean and not run into anything, but I think it could fail under ocean circumstances.

It's on my todo list to replace it with a 3-blade Campbell Sailor (~700$) which offers much less drag than a traditional prop while remaining quite efficient.  Several guys at my marina swear by it.  I particularly appreciate its hydrodynamic design and am more likely to believe their sales pitch based on the science I know.  The blades are smaller in area but pitched a bit more aggressively.  Their roots are very stocky and their edges are quite blunt.  This is very different from typical props.  However in my case it would likely require a new shaft as I don't have a hole in which to place a cotter pin due to the way the PO installed the MaxProp, unless I drill through a nut and the shaft which might be an option to save another 600$ or more.

But then there's the last, no-cost but get wet option that I'm surely makes too much sense and will likely be ignored.  When crossing oceans, remove your prop.  You'll save all that drag, and be less tempted to motorsail and burn 6$/gal diesel.  Certain passages will require a prop, or the water will be cold, or the weather will be too nasty to go swimming and install a prop when you will need it.  But it is a decent thought. 

Mike
S/V Tardis,
Pearson 367 Cutter Hull 26

[S/V Laelia]

Mike,

Thank you for all the details about the Max-Prop. It doesn't sound like a bullet-proof solution to me. I will check out the Campbell prop.

I have thought of the last approach too but I hate getting wet. I especially hate getting cold and wet 

[SVJourney]

We have a kiwi prop that came with Journey.  It broke almost as soon as we got it as the housing cracked and it wouldn't reverse anymore.  Fortunately, we had the original 3 bladed onboard as a spare and Kiwi has an unconditional lifetime guarentee.  They sent me new parts to repair it gratis.  The only issue was that a puller must be used to remove a Kiwi prop as the hub can easily be damaged with a hammer or mallet.  We found a diver in Guadeloupe that changed it for us.

I didn't really notice all that much reduction in speed changing to the fixed prop.  There must have been some but we were in tradewinds most times with enough to power us up anyways.