Even though we love to sail and running a motor is a drag, we sometimes need our auxiliary power to get from A to B. Not everyone is a diesel mechanic, boat owner, or sailing club operator. Our experience operating a club tells us that many people are not sure when they are using the engine efficiently and when they are over-working it.
Because every motor is different and they are all pushing different length & weight vessels through the water, we can’t give you a specific RPM as a rule of thumb. We can give you some tips that will make our motors run for years to come.
If you pay attention to speed instruments or use a GPS system to measure your speed, you will find that diminishing returns is the norm. Running at 50% throttle will typically get you 90% of your possible speed. The last 50% throttle adds almost no speed and works the engine extra hard. This is true for all types of engines. On our electric boats we have measured speed over water at 4.2 knots when drawing 40 Amps (half throttle). Increasing the draw to 80 Amps (full power) gets us to 4.5 knots. We’re pretty sure the extra .3 knots is not worth halving the battery life!
Remember, these are displacement hulls, not planing hulls. Once you reach hull speed, it’s possible to send more fuel to the engine, but it’s not possible to cover more ground in a shorter time. Some indicators that you are pushing the engine too hard are:
- Black smoke – this is a good indication that the engine is unable to burn all of the fuel that you are injecting. In other words, we are increasing throttle, but the engine is not strong enough to turn the prop any faster. Back off the throttle until the smoke clears.
- A following wake, perpendicular to the stern of the boat – this indicates that we have reached hull speed and the stern of the boat is sinking into the water. Try all you might, it’s not going to plane! Back off the throttle until there is only a V-shaped wake behind the boat.
If anyone is wondering why we don’t govern our engines to a healthy level or limit current on our electric motors, it’s because we understand the need to fight a stiff breeze or current while taking care of other concerns on the boat. There is also the issue of low speed maneuvering, which sometimes requires a bit of extra power.
As well-trained sailors, our goal is to never be in a hurry (actually that’s one of our main life-goals!). Putting too much stress on the motor or decreasing our battery life isn’t worth the extra half-knot. Take it easy and enjoy the scenery.