Propeller Question

[texwards]

Second time out in my boat, I'm a little disappointed in motoring performance. The boat immediately jumps to 4 knots but she does not exceed 4.7 knots. On top of this, at 3000 rpm there is a lot of black in the exhaust.  I'm thinking I am over propped...

Does this sound like a fair diagnosis? Has anyone experimented with different prop pitches? What prop pitch works for you?

[TopsailB]

From Keith Scott owner of The Sailboat Company:

"Engines are made to be used with specific propellers. The boat itself doesn't make much of a difference in which propeller is used. However, the right engine for a specific boat is important. If you bolt your engine to a dock and run the engine with the shaft and propeller in the water, what's going to happen? The engine would increase in speed as you increased the throttle. You would get to a point where the engine would start to put out black smoke from the exhaust indicating that all the fuel was not being burned in the engine. This is as fast as this engine will run with this propeller. Of course it goes without saying that you shouldn't run the engine faster than its recommended maximum speed. If you had a very small prop on the shaft, the engine would run too fast too soon and the engine would get to maximum speed before you saw black smoke. If you had a very big prop on the shaft, the engine speed would go up in speed until the engine started to put out black smoke indicating all the fuel is not being burned. The best prop for most boats is a prop that puts out black smoke just before reaching maximum engine speed. This may be more than most people want to know about diesel."

I'd contact Keith for an answer to your question re prop size.  http://www.sailboatrichlands.com/

[texwards]

good information - thank you!

[moonlight]

Before changing anything, is the bottom clean?

[Allure2sail]

Hi:
Which motor is in the boat? M12 (10 horsepower or the newer boats which I think have 16 horsepower motors. Perhaps it has the wrong prop. I have an extra prop that I can check the diameter and pitch for you. M12 version.
Bruce
S/V Allure
1987 27' model with an M12 engine

[texwards]

Yes, the bottom was cleaned by a diver this month - he said prop looked good. The motor is a universal m2-12.

[wes]

I have the original standard two blade 12x9 prop on my 27, with the M12 engine. If you look closely the diameter and pitch are stamped on the prop.

I comfortably cruise at 5 knots with the engine running around 2800 to 3000 rpm. Flat out maximum is around 3200 rpm, which will push me at around 6 to 6.2 knots, but at the cost of more fuel consumption and more wear and tear on the PSI shaft seal and transmission. Not to mention more noise in the cockpit.

All this is assuming flat water. A headwind or choppy seas will decrease these numbers.

There is quite a bit of info easily available via Google search on the subject of matching prop to engine. There is definitely a sweet spot. I'd do my research before increasing or decreasing the prop size or pitch.

My #1 complaint with my setup is the amount of prop walk to port in reverse. When backing, the boat pretty much won't respond to starboard rudder. It loves to turn to port. I have to adjust my docking strategy accordingly. One useful tactic is to gun the engine in reverse to gain some speed, then pop it into neutral before I start turning. No prop rotation = no prop walk.

Wes

[brackish]

Keith's statement is a little misleading or maybe he considers that all will assume he means the motor will be properly sized for the boat and all uses will be displacement load.  Prop size and pitch is very important on outboards.  I had a 9.9 hp on my 23 with a seven pitch prop. Did fine, gave me hull speed at about 60% throttle with a clean bottom. When I took that motor off and put it on a 14' planing skiff I built I kept getting overrev shutdowns.  I put a tach on it and found it was running about 6500 at WOT.  Rule of thumb is 150-200 rpm reduction for each increase in pitch size.  Put a ten pitch on and motor does 5950 at WOT which is right at the top of the power curve.  No more overrev shutdowns.  Any difference in hole shot time is not noticeable.

[moonlight]

Prop Walk is a gift from the gods
It allows you to turn basically in the length of your boat
The gunning tactic and counter rudder is TAUGHT to all our students as the effective way to steer.
As in forward, there is NO steering until water is flowing past the keel and the rudder; so some headway or sternway has to start first.
Once mastered, prop walk can be utilized to make you appear as king of the harbor...