Tuesday, February 12, 2008

What's WOT

The owner's manual for my new Tohatsu 9.8 hp outboard says that the engine's wide-open throttle range should be 5,000 to 6,000 RPM. The manual also notes that Tohatsu ships its long shaft (20-inch) and ultra long shaft (25-inch) engines with a three-blade propeller that is 8.5 inches in diameter with a 7.5-inch pitch. The standard shaft (15-inch) engine comes with a propeller 8.9 inches in diameter and a pitch of 8.3 inches. For any given RPM, the short shaft engine is equipped to move more water than the longer shaft engines.

The manual also comes with a table of available propellers listed from "light boats" to "heavy boats". The shorter pitch prop that came from my engine is one step down in the table from the standard shaft prop, in the heavy boat direction.

Since I installed a tachometer for the engine, and I could measure boat speed on my GPS and knotmeter, I had all the information I needed to determine whether that propeller I received was the right propeller.

After I had completed the 10-hour break-in period on the engine (my tachometer also incorporates an hour meter), I found that wide-open throttle (WOT) was at least 7,000 rpm in light wind conditions. (That is the maximum marking on the tachometer.) I also saw that at 6,000 rpm, Narrow Escape was doing 5.3 knots. At full throttle I could go 6 knots.

The Albin inboard engine, at full throttle (maximum rated power was 12 hp at 1500 rpm) also pushed the boat at 6 knots. It had a larger two-blade propeller, 12-inch diameter and 8-inch pitch. There was no tachometer for the Albin, so I don't know how many rpms it actually turned.

Given all of this information, I decided that the Tohatsu prop was probably too small. The engine was running beyond the manual's recommended full throttle range at full throttle. And the boat was running about .7 knots slower with the outboard than it had with the Albin, when I stayed within the recommended rpm range.

An advantage of an outboard is that it is very easy to change the prop. I bought the 8.3-inch propeller that comes on the short shaft engine for $75, including tax. It probably took as long to readjust my dock lines to position the Ericson in the proper spot after backing into my slip as it did to make the propeller switch. By the way, the boat backs down quite nicely with the outboard, steering easily with the rudder while the engine remains fixed in its centered position.

The results could not have been better. Wide-open throttle is now 6,000 rpm and 6 knots. The ratio of rpm to speed holds at partial throttle settings, too. 5,000 rpm is 5 knots, 4,000 rpm is 4 knots and 3,000 rpm is 3 knots.

The engine also sounds better. It is working harder at lower rpm and it is obviously happier doing it. Overall it is quieter. It is less buzzy. And it has a somewhat deeper sound. It is definitely more pleasant.