'88 M25xp - 10" or 9" Pitch for 3 Blade Folder?

[kurt]

Hello everyone,

I've queried my mechanic, potential suppliers,  and this group and now have some conflicting data points regarding pitch I'm working to sort out.

Specifically, I'm considering adding a 3 blade folder to my '88 C34 w M25xp engine (2:1 ratio I believe) and have received several recommendations for pitch - 15" is consistent for diameter.  (I do not know the pitch on my current 3 blade fixed.  It does rev to > 3000 rpm panel tach view.)  I understand from this board that the standard C34 3 blade for the XP is 9".  My mechanic recommends 9" for the 3 blade folder.   Folder vendor A recommends 10" (but 9" is ok.) Folder vendor B recommends 9".    I'm leaning towards the 3 blade 9" as the conservative choice that has little downside as far as I understand it.

Does anyone have any data suggesting what the velocity difference at ~2300 rpm would be between the 9" vs. 10" pitch of a similarly designed prop in flat water?   Would flat water provide the worse case delta?  Is this loss of velocity for given rpm the sole downside of the 9" pitch?  Should I expect to continue to get to >3000 rpm with a prop of 10" pitch on the 25xp in flat water?   In all sea conditions?   Any thoughts or advice here?   

Thanks in advance for offering any data or experiences here.

Kurt

'88 C34

[tonywright]

Kurt

The advice that I received is that folding/feathering propellors all have different ideal ratios and even blade size, depending on the design of the blades.  If you select a specific brand and size, members on the board can tell you what works for them on a C34.

My preference was to buy a feathering prop that can be adjusted with some ease, preferably in the water. The J Prop is one that allows this. (Not sure if there are any folders that allow adjustments).  I really like the sense of security that equal power in reverse gives me when docking.

Tony

[Steve Sayian]

Tony,

The Kiwi feathering prop will also allow in-the-water adjustment with a hex-wrench.

Steve

[David Sanner]

I recently picked up a 2-blade Martec Slipstream (all stainless, geared)
folding prop.  I believe the pitch is 10".  I was pushing for more but
Gary at Martec said this is what his numbers showed... for that prop
with a c34/25xp.

However his numbers for their brass 15" folding prop
worked out to 12" of pitch.  (2 blade again).

So basically I've been hearing what Tony has as well, that there are
a lot of factors in prop power not just diameter/pitch.  Two that come
to mind are blade camber and area.

The interesting thing is my 2blade-15x10 folder gets nearly another
1/2 knot over my 3 blade 15x9 sailor prop and even though it's a two
blade prop it does better in a seaway than my old 3 blade.  Neither
were overloading the engine but not undersized.

... and did I mention how nice the boat sails without dragging the
prop in the water?  All around a huge improvement... best upgrade
that I can think of...

[Craig Illman]

David - How is the prop walk with your Martec?

Craig

[David Sanner]

I'd say there's a bit less prop walk than my old fixed prop...
and a bit more powerful backing but nothing like a feathering prop. 

I haven't really played around with it enough to really know though.
Either way it very manageable, a bit of walk at idle (which I like),
and within a few seconds there's enough flow over the rudder
to go where I want to go.

The thing I was happily surprised about this prop is how it
was less effected by wind/wave than my old 3 blade... I guess
it's the more modern/efficient blade design along with more
effective blade area in two blades than three blades.
(The area of the outer third of the two blades are comparable
perhaps more than the sum of the this area on my three blade.)

[kurt]

Thanks for all of your comments.  These are helpfull to my decision making process.

I have several additional followup question for Catalina 34 owners with 25xp engine with 2:1 transmissions: for the 15 x 10" flexofold 3 blade users what is your cruise speed at ~2300 rpm and does your engine rev past 3000 rpm easily/cleanly?    9" flexofold 3 blade?  10" Gori 3 blade?  9" Gori 3 blade?  (I know the Gori's have the "overdrive" so assume that is cruise mode.)

Thanks again for any comments.

Kurt

[kurt]

Hello All,

Based upon vendor recommendation, I went with a 10" pitch for the 3 blade flexofold 15" diameter for my '88 25xp.  (I did secure their agreement to go down to 9" pitch at nominal cost if I desired to do so.)  Also learned during installation procedure that I previously had a fixed, 3 blade 15x10" Michigan Sailer prop.  I have not calibrated by tach which makes the following data suspect but I have the original alternator and the numbers seem to be in the right ball park. 

So, my previous prop at load got up to a max rpm of ~3000. 

The folder that I just added now gets up to ~2800 rpm.  Further tested it last evening evening with mainsail up, motoring directly into a 19 knot breeze with small chop, slack tide and got 2800 rpm.  May be getting a small amount of grey smoke at this rpm but not sure.  Finger across stern did not find any soot when back on the dock but only ran for a couple of minutes at max rpm.   

Any thoughts regarding these results?  I'd like to see 2900 or 3000 but am leaning to staying as is with the 10" pitch and avoiding the hastle of replacing blades.  9.5" pitch would probably be perfect!

Thanks in advance,

Kurt

[steve stoneback]

Kurt,

You mention how the prop changed your RPM but didn't' mention now, or if it changed your boat speed.  Does this prop move the boat faster, slower or no change?

Steve

[kurt]

Steve,
Yes - did pick up ~ 1/4 to 1/2 knot.  I cruise at around 2200 - 2400 btw now at ~ 6 knots.  Of course, in my harried planning forgot to document the old prop velocity by rpm so this is bump in velocity is approximate and based upon a rapidly fading recollection.
Kurt

[David Sanner]

Kurt,  it sounds like 9.5 might be perfect but powering into
19kts (apparent?) and some chop with you main up
(flogging, increasing the drag) I'm not surprised you
couldn't get max rpm. 

I'm curious how you would do on flat water in light
wind, smaller chop, and no sails.  Sounds like you would
turn nearly 3k or close enough.  Since you're not producing
much if any black smoke it sounds like you might be alright
at 10" of pitch.

[kurt]

Dave,

I have to admit to be scratching my head on this one but I got 2800 with 7 knots of wind on the beam in flat water heading back into the marina.  The same rpm as with the mainsail flogging motoring directly into 19 knots of wind with small chop.  (I did see 3000 one morning with no wind in flat water.) This similarity of rpm has me leaning towards declaring success and moving on to the next boat project!

Kurt

[Bill Asbury]

I have a max-prop on my 91 c34 with 25xp, Kurt, and love it.  Runs up to 3000-3200 under way no problem.  Going by the oem tachometer, which hasen't been calibrated, but seems okay.

Best wishes.