Re-pitching 3 Blade Prop

[Les Luzar]

I hauled out last week for a bottom job and after all of the threads on rudder seepage I was so happy to inspect mine and see no problem at all. I did decide to repitch my 3 blade 15x9 prop to 15x10 at a cost of $194 at Wilmington Propeller. Prior to the repitch @ 2,500 RPM I would go 5.7 to 6 knots over ground, and 6.1 to 6.4 at 2,700 RPM. I can't wait to see if there is any improvement in performance and how this extra load will affect my engine temperature which usually runs at 175 degrees on my M25XP. Stay tuned.

I also changed my cutlass bearing because it was a slight bit sloppy at the stearn. How long do these things last on average?

Les

[Ralph Masters]

Les,
I'd say about 25 years on the cutlass bearing.

Ralph

[Joe Holmes]

Hi Les- I re-pitched last spring as you did, on a 1988 model M25XP.  Positive results all around- no noticable change in engine temperature, just goes faster.

[Les Luzar]

I can't wait to get her back in the water to "sea" the improvement....

[Ron Hill]

Guys : Back in the late 1990s I wrote an article on repitching your prop.  

I strongly recommend and still do, that you choose a prop shop that uses "Prop Scan" to get the most accurate repitching possible!!

[mainesail]

Guys please keep in mind that Universal / Westerbeke want to see you attain MAX rated RPM with a clean bottom and prop at full throttle. Over pitching can lead to lots of problems such as frequent clogging of exhaust elbows and changing your HP curve across the entire HP range and causing lugging of the engine. A little "under propped" is often better than a little "over propped" and allows you to cruise at 80% WOT cruise without climbing your bow wave...

This is from the Universal M-25XPB but the recommendation is the same on all Universal/Westerbeke engines.

M-25XPB
"Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle."

If the 15X10 allows you to reach full rated RPM, confirmed with a hand held photo tach, then you're fine...

As Ron mentioned you should only be buying props from shops that set them up using either Prop-Scan or Hale MRI machines. The differences are AMAZING, quantifiable and smooooooth....

[Ron Hill]

Guys : Mainsail has not seen the article I wrote in the Mainsheet a few years back on Prop Pitch and Boat Speed.

Anyway, one of the bottom line points I made was that Catalina turned out about 1800 C34s and over 2000 C36s.  They went from a 21hp engine to 23hp to 30 hp to 35 hp and still kept the same 3 bladed 15" X 9" prop on all of those boats!!!   Catalina finally increased the engine horse power by 14hp and still kept the same pitch?!?    Daaa!

So you figure it out!!  

[mainesail]

Ron is correct I have not seen that article. Perhaps a link????

Our 1987 C-36 was incorrectly pitched from the factory as was our 2005 C-310. Just because it came from the factory with a specific prop does NOT mean it is pitched correctly. Always test it against a photo tach and re-pitch as necessary to match what the engine manufacturer wants. Both our C-36 and C-310 were properly re-pitched. The C-310 was re-pitched at Franks expense despite his begrudging arguments that "it works"..... If "it works" means pushes the boat, then Frank was right. But if "it works" as in it maintains the factory warranty on the engine, then no, it did not work... Catalina is not the only manufacturer to ship boats with incorrect pitch props... Sometimes they get it spot on and on some boats they miss by 200 or more RPM.. One size does not always fit all but it does save money..

I recently went through this with a Campbell Sailor on our CS-36. Norm got it wrong by close to 200 RPM. Joe at Westerbeke was  adamant that the engine should be within 100RPM of max but preferably less than 50RPM or the best be spot on. Norm re-pitched at no cost.

When buying a prop always try to get a written agreement that the prop will be properly pitched for your engine manufacturers performance specs or they re-pitch at no cost to you.

I am now going through this with a brand new Flex-O-Fold which is not pitched correctly.... Unfortunately Flex-O-Fold now does business directly out of Denmark...... What a PITA!!!

[Les Luzar]

Mainsail,
You mentioned that over pitching a prop, can cause an exhaust elbow to clog. What actually causes this situation? A dock neighbor has a 96 C-34 with a Martec folding prop, and after repowering his M-35 with a new M-35, within two years, his exhaust elbow clogged! I have had my 87 C-34 for 6 years now and have never experienced a clogged exhaust elbow. It seems odd to me that his exhaust elbow would clog go quickly after the repower. Any thoughts on what could cause this?

[mainesail]

When you load the engine more than it is designed to be, for a given RPM, you "lug" the engine. Just like driving a standard shift car up a hill in the wrong gear. Lugging a diesel produces more soot, a hotter exhaust temp (the engine is working harder than it should for the given RPM, and the soot can coke the exhaust elbow. Bad fuel, water in the fuel or a dirty tank can also cause this as can improper cooling water flow and a temp too high. Also belt dust can also case issues with coking of the elbow. Many folks assume marine diesels don't need air filters, no road dust, but rubber belt dust can be as bad or worse thus when the foam element goes REPLACE IT or buy a real filter like a K&N....

This loading of the engine starts at low RPM and continues up through full throttle across the entire HP curve. You are developing less HP than the HP curve is intended to produce for a given RPM.

For example if you were supposed to develop say 17HP at 1900 RPM with a properly sized propeller you are now only developing 14HP with the wrong prop at the same RPM and a re "lugging" the motor or loading it up beyond design spec.

It is like trying to drive 25 MPH in fifth gear and is not good for the engine. This is why Westerbeke/Universal, Beta and others are so nit picky on your motor being able to develop max rated RPM with a clean bottom and prop. If you have other high load items such as engine driven refrigeration or a large alternator size your prop to accommodate those items too....  You are generally better to be a tad under pitched than over pitched.

[Les Luzar]

Mainsail,
Thanks for your explanation. This is a very interesting subject. Can this same phenomenon also clog the thermostate as well? One dock neighbor had a completely clogged thermostate and another had the completely clogged heat exchanger elbow. The buildup seemed like the same material and both were solidly clogged. Both boats have folding props as opposed to the original 3 blade prop. Perhaps this is an engine load problem causing the buildup? This gives me more knowledge on what to pay attention to going forward.

[Stu Jackson]

The thermostat is only in the freshwater coolant circuit.  Don't see how lugging the engine would cause this.  Most likely the skipper hadn't changed his antifreeze in years.

[mainesail]

The thermostat is only in the freshwater coolant circuit.  Don't see how lugging the engine would cause this.  Most likely the skipper hadn't changed his antifreeze in years.

Or he mixed antifreeze products/brands/types, which can cause sludge....

[Les Luzar]

Stu and Mainsail,
Thanks for these comments. This is interesting, and I will inquire about this for curiosity sake. I did see the build-up in the thermostate housing and it was virtually solid buildup, as with the heat exchanger elbow (on another boat). It makes sense that the cause would be different since one system is a closed coolant system and the other a saltwater cooled system. Thanks again for your thoughts on this issue.