Friday, March 23, 2012

Engine Update & Prop

After a couple days of doing the math, I managed to get a plan on setting the prop.

Here's the thing. A few years back, I decided to see if by changing the pitch of JJ's Max Prop (it's a 3-blade 13" prop) I could affect the performance of the boat under power. Ha!

So being an ignoramus, I decided in my little head that the Gearbox had a 1.5:1 gear it in - but here's the thing, I reversed the ratio. Not a Reduction gear, but the other way. Problem Number one.

Number Two is the head issues with the Vetus M2.05 engine. More on that in another post.

So I had set the prop to be WAY TOO aggressive. I think I set it to be about 11-13 inches of pitch. On a small boat with a tiny engine. Huh. Not good.

So with the engine yanked out, at the diesel spa and being probed from top to bottom, the right thing is to adjust the prop and make sure it's right going forward.

Dave Gerr's Propeller Handbook from Amazon helps. It has a very useful chapter about Crouch's Method to figuring prop pitch. Did that a few times with different assumptions, and ended up with pitches in the range of 4.3 to 9.8 inches. Also found the fine "propcalc" spreadsheet which also ends up producing pitches in the 6-9 inch range. But if you look at the range of the answers, you see the differences along the lines of 50-150%. Not good.

But several places on the web mentioned that PYI, the Max Prop people, have great customer service (which in many business' case is an oxymoron), and so I just called. Bearing in mind that it took me several hours of time to find the references above, and several hours more to ponder the intricacies of the methods and then to actually Do the Math, I found my PYI exchange to be refreshing. After asking about the length of the boat (23'), the engine (10.5 hp at 3600rpm), the hull shape (displacement, deep), the Reduction Gear (key word: reduction 2:1 (not 1.5 like I've been imagining), size and 'bladed-ness' of the prop (13" and 3-blade), Dave said "7" inches


Total conversation was about 4 minutes, complete. THAT's the difference between an amateur and a professional.

Lovely word, 'seven'. So the professor and I set the prop to 7 inches of pitch (which is 16deg on a Max Prop). Was an unseasonably nice mid-March day when we did this. Now. Let's get the motor back together and see what happens. Will she "come up to cruising rpms" like never before? Yes, yes, she might.

Hope is a spring fever for a sailor.