Do as I say, not as I do…

I must eat my words.  I jinxed it.  I should have known better.  A few posts ago I mentioned that Chris & John checked over the Max-Prop and transmission to confirm that the transmission reduction was what I thought it was and that the prop should, in fact, be set up for right hand rotation.  With that confirmation I declared myself a non-idiot.  I was wrong.

It goes like this.  Just to check one more time, after launch and after a successful start-up of the diesel Chris tested the prop by putting the engine in forward and reverse and…  the boat went forward and reverse just like it is supposed to.  What?  You didn’t think I was that bad did you?

But wait, there’s more.  On Sunday we departed Back Creek, next stop Chesapeake City in the C&D Canal, about 50 miles away.  After filling up with diesel we puttered out into the Bay and pushed the throttle up to a vigorous cruising RPM of 2800.  The boat speed on our new NKE instruments shot up to 4 knots and leveled off.  Hmmm…  We should be hitting 7.5 knots or so.  Maybe the new knotmeter needs calibration?  A quick look over the side at the water and the shore and I think it doesn’t seem like 7.5 knots, but it’s been a long time and I’m not sure I can judge our speed that well.  Check the GPS?  Aw shit, we are in fact storming along at only 4 knots.  Well, good thing Shearwater sails like a witch and on we carried as I puzzled over what went wrong.

Obviously the pitch was set wrong, but why?  We confirmed the transmission reduction was an unusual 2.14:1.  I gave this one a great deal of thought over the next few days, particularly as we motored through the C&D Canal making only 3 knots against the current and when our speed dropped to 3.75 knots with only a 10 knot headwind.  Fortunately, we sailed the vast majority of the time and then I didn’t care so much about the prop.

So fast forward a few days to our planned haulout and I grab the Max-Prop installation manual and head down below the boat to the prop to investigate and make a change to the pitch setting.  I open up the installation to the first page and there is a conversion table.  You see, most props are described by their diameter in inches and the pitch, also in inches, which refers to the number of inches forward the boat would move in one rotation of the prop, assuming no slippage.  According to our boat/engine/transmission combination we want a 16” x 12” prop.  The previous prop was 17” x 15” which made no sense for our boat and was confirmed by the fact the engine could only reach 2800 rpms instead of the typical 3400.  And right then I had my face-palm moment.  The conversion table is used to convert inches to degrees because the pitch on a Max-Prop is set in degrees and like the engineers for the Mars Lunar Lander I had made a not quite as fatal error.  Instead of looking up 12” and converting to degrees (that would be 22 degrees) I set the prop up for 12 DEGREES.  Ugh!  Degrees?  Inches?  Really?  Yes.

That's what 90 degrees looks like. Fast for sailing. Not so much for motoring.

On the bright side that explains the issue entirely.  With nearly twice the pitch angle at 22 degrees the boat will move almost twice as fast in flat water and that will get us right around the expected 7.5 knots.  10 minutes later the Max-Prop was all back together with a 22 degree pitch angle.   That should make for some much more enjoyable motoring next time.

It’s an interesting lesson, this.  All winter long I fretted over the transmission gear ratio and the prop rotation and chastised myself for not checking these two things carefully only to discover on inspection that I had gotten them both right.  I was entirely focused on the inputs to the process.  What I never questioned was whether I’d executed the procedure correctly.  As it turns out, I had the correct inputs, 12” of pitch, right-hand rotation, but I failed to actually set the prop up for that configuration.  Aw well, in the end, no harm was done, and my ego can take the beating.

Onward…