What is the issue?There is a belief that going at full speed gets you there in half the time thus saving time and reducing fuel consumption. This may be time efficient but it is not an efficient use of fuel.
Why address this?Reducing fuel consumption delivers dramatic fuel savings, extends the cruising range of the vessel and reduces pollution.
How to address this?Find the optimal ‘sweet spot’ for your vessels distance over fuel consumption equation. The fuel consumed / distanced travelled ‘sweet spot’ typically resides at around two thirds of the vessels natural waterline or full power. This provides a simple target to aim for, to optimise fuel consumption.
This is however a rule-of-thumb. In practice each individual vessel is operating to a complex formula of waterline length with displacement hulls, engine power, weight, propeller type, hull type, conditions etc. Consequently there can be no absolute and fixed rule. However the two thirds rule offers a good guideline.
A specific example of this can be taken from our vessel. This was a traditional 38ft displacement yacht with extensive overhangs that presented a 25ft waterline driving a theoretical 6.5 Knots waterline speed. Becalmed in absolute flat conditions for ten days we focused on maximising our range and found the sweet spot to be 3.8 - 4 Knots or just less than two thirds of boat speed.
In terms of ‘diesel revolutions per minute’ this was at an extraordinarily low 1,400 rpm on a 36 horsepower diesel engine. However these were extreme conditions and you should expect your ideal spot to be in the range between 1,400 and 2,200 revs on a diesel engine. Petrol engines rev higher and manuals indicate optimal consumption at 3,500 – 4,500 rpm for outboards and 2,500 - 3,500 for inboards. Again if you simply aim for the two thirds of a displacements waterline speed, or full power, the optimal distance by fuel consumption is in this immediate orbit.
A key point when applying this rule to a planing vessel is that you must get it to ‘plane-off’ before turning back the power. Otherwise the vessel is termed to be sitting in ‘the hole’ before planning. This is both excessively fuel inefficient and also creates a very large wash for other vessels. With planing hulls you should plane off as soon as possible, trim the motor, and then back off the throttle to about two-thirds of full power. This should place you in the vicinity of the optimal distance / fuel ratio for your vessel and conditions.
Moderating your powered passage to a simple two thirds of boat speed / full power ‘rule-of-thumb’ is a simple way to dramatically reduce fuel costs, extend engine range and the longevity of the vessels engine service.
With thanks to:Michael Harpur, Yacht Obsession.
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