Two-Blade vs Three-Blade Prop

[Pete the Cat]

I seem to recall that MIT research determined that the shape of the prop blades made no measureable difference in efficiency in boat making less than 20 knots or so. Not sure about specifics of study, perhaps someone can help.

[Forever Freedom]

Forever Freedom's engine is a 23 year-old Yanmar 3GM30F, which was overhauled top and bottom in 2004. Maximum rpm is 3600 and maximum continuous rpm is 3000. The original propellor was 2-blade, irreverently called "the egg whisk". It pushed the boat along adequately, but produced some ideo-syncratic handling in harbour. Some years ago we changed it for a 3-blade Brunton Autoprop. The Autoprop made a noticeable improvement to boat handling, it seems to produce extra propulsion at lower revs when motor-sailing (we seem to do a lot of that now!), and has eased close quarter manoeuvring dramatically. Nevertheless, I am now having reservations about it. Recently a boat here in Porto Lagos had to have it's engine lifted out and repaired because of water in the oil. That was all sorted but, examination of the exhaust mixer showed it to be almost completely blocked with carbon, which had to be drilled-out. The engineer who repaired the engine said that the carbon deposits were caused by running the engine at low revs for long periods of time. Since fitting the Autoprop, we can maintain cruising speed under engine at 2/3 rds of maximum continuous revs, i.e., 2000 rpm instead of 3000 rpm. The engineer recommended running the engine at a minimum of 75% of maximum continuous rpm always, and running it at maximum continuous rpm for 2 minutes in every hour. Unfortunately, with the Autoprop, we can no longer reach 3000 rpm, and at 2750 rpm we produce clouds of black smoke and a trail of soot on the water. It would seem we are significantly "over-propped" and that, for the benefit of the engine, we should re-fit the original 2-blade prop so that we can maintain higher rpm - something I never thought would be necessary. I was wondering if any engine gurus have any comments?

[PaulMarion]

Over propping that causes black smoke WILL cause premature engine problems, as well as excessive fuel wastage. As a mechanic with 50 years experience I have come accross a few examples of this. The black smoke is unburnt fuel. It can be caused by many engine problems- poor compression, incorrect injector pump timing, old fuel not burning correctly, etc. However with regards to props and underwater boat condition, there are a number of issues also that can be a real problem, when we know all's well with the motor. Obviously, as has already been spoken about, it's paramount the hull and prop itself is clean and free from barnacles, weed and slime, as all this will affect engine performance, boat speed, manouverability and fuel consumption dramatically. I had a Cloud 9 Catamaran in the 90's, lived aboard and cruised for 7 years. During that time I replaced the 2 blade fixed props with a pair of Australian designed and made "Autostream" 3 blade feathering and fully adjustable props. The differences were noticed immediately. Vibrations were reduced significantly, better cruising speed and fuel economy. The great thing about these props was that you could adjust the pitch individually for forward and reverse! Now I guess not everyone is happy to hop over the side with a 1/2' spanner and stumpy blade screwdriver to play around with prop pitch! But what a chance to get the best from your prop, ehh! I was able to set the props pitches for each motor in forward, AND set a different pitch for reverse also. As we all know, it's great to come into a marina and have your boat stop when YOU want it to! I could in fact wind up the pitch so far in reverse as to actually stall the motors! Having had this experience really gave insight as to the importance, again as has already been mentioned, in getting the correct balance of RPM, boat speed, and economy from your prop/motor combination. It is correct that if you cannot get to rev the motor, under load, to good maximum revs, you will find carbon and any build up of soot/unburnt fuel will cause premature engine repairs, particularly when the said motor is used less frequently. You know when WE get in a groove with our life that seems all a bit blase? Each day seems to "glaze" by. Then we get out and have a great day by doing something really out of the box, give the body a real shake up, and we feel refreshed, invigorated, got a life again! Well- believe it or not, that's what happens to your motor sitting on the same old revs, every time you start, and sit it at?? 2000 or whatever. When you get it warmed up and then give it a big flat chat run..it blows all the cobwebs out and presto! It can breathe better, operates cleaner and has helped unglaze the cylinder bores, get junk from behind the rings, which is the most common cause of loosing compression, and generally removes a lot of crud built up throughout your whole motor and exhuast system! Now if your prop is restricting you from giving this treatment to your only source of alternative propulsion...MMM. This is why commercial motors in any transport area last a looong time. No chance to glaze up bores, and in many cases even cool down. For me, I would look at repitching your 3 blade prop down to suit your needs. Why go back to a 2 blade with less propulsion and more vibes? Hope this hasn't been too long of a drag /post! Regards, Paul.