How much does your boat speed affect your fuel consumption?
Discounting all of the negatives associated with running a twin boat on one engine, and looking only at fuel consuption v boat speed, here is my $.02 worth.
I spent 38 engine hours this summer running on one engine, while we were too far from home to begin chasing a leak from the oil cooler on the port engine. In order to have that engine available for manouvering, I left chasing the leak till we wouldn't need the engine(s) for a couple of weeks.
I fixed it (cost just over $500 to fix) and on returning to my home marina, fuelled up. I know my usual consumption, which has been consistent over the last 12 years, at 4 gph, running at + or - 8.2 knots. This speed produces a smallish wave. From my vantage point out in front of the wave it is hard to give an accurate measure of the wave height, but I will guess it at 1 ft.
While running on a single engine, I maintained the usual rpm, rather than try to match usual boat speed. This resulted in a speed of 6 to 6.3 knots, and reduced the wave height by about 1/2.
The choice of speed was dictated by the engine itself, as to run at 8 knots produced copious quantities of black smoke, indicating that the single engine was propped wrong for this usage, working far too hard, so I slowed to the point that the smoke disappeared, as observed by the boat following us.
Doing the math on the fuel used over the 75 hours since the last fill, the 38 hours on one engine consumed $500 less fuel than I would normally have used in that time, maintaining the 2 gph per engine normal usage, and improving my mpg from 2.0 to 3.0.
After disclosing this saving to SWMBO, she observed that 8 knots was a better speed for us.
Discounting all of the negatives associated with running a twin boat on one engine, and looking only at fuel consuption v boat speed, here is my $.02 worth.
I spent 38 engine hours this summer running on one engine, while we were too far from home to begin chasing a leak from the oil cooler on the port engine. In order to have that engine available for manouvering, I left chasing the leak till we wouldn't need the engine(s) for a couple of weeks.
I fixed it (cost just over $500 to fix) and on returning to my home marina, fuelled up. I know my usual consumption, which has been consistent over the last 12 years, at 4 gph, running at + or - 8.2 knots. This speed produces a smallish wave. From my vantage point out in front of the wave it is hard to give an accurate measure of the wave height, but I will guess it at 1 ft.
While running on a single engine, I maintained the usual rpm, rather than try to match usual boat speed. This resulted in a speed of 6 to 6.3 knots, and reduced the wave height by about 1/2.
The choice of speed was dictated by the engine itself, as to run at 8 knots produced copious quantities of black smoke, indicating that the single engine was propped wrong for this usage, working far too hard, so I slowed to the point that the smoke disappeared, as observed by the boat following us.
Doing the math on the fuel used over the 75 hours since the last fill, the 38 hours on one engine consumed $500 less fuel than I would normally have used in that time, maintaining the 2 gph per engine normal usage, and improving my mpg from 2.0 to 3.0.
After disclosing this saving to SWMBO, she observed that 8 knots was a better speed for us.
It's not rocket science is it? We tend to over think and have a formula for just about every subject on this Forum.I believe we have a lot of folks getting tangled up in their underwear.
