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Calculating range, the rule of thirds, reserve and towing costs
One third of the tank for the way out, one third for the way back and one third in reserve: that is the seaman's rule of thirds.
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Fuel consumption afloat is not a constant. Headwind, current, fouling and the weight aboard can shift it by double-digit percentages. That is why you plan on a third in reserve rather than counting on the last litre. If you take this rule seriously, you still have fuel in hand when a passage takes longer than planned.
If the boat is left stranded with an empty tank, that is not a hull claim. Engine damage is a loss event, running out of fuel is a breakdown, and hull cover does not pay for that. Salvage of a boat after a casualty is insured; towing a boat that is simply broken down is not automatically covered.
This is one of the points where policies differ and one worth clearing up before a long passage. Our policy wording distinguishes between salvage after an insured event and a simple tow; on longer cruises it is worth looking at the limit for salvage costs in your policy.
Frequently asked questions
Does the insurance pay if I am left stranded with an empty tank?
No. Running out of fuel is not an insured event but a breakdown. What is insured is salvage after a loss.
How much reserve should I allow?
One third of the tank capacity. On cruises with a contrary current or with no fuelling station in the cruising area, rather more.
Worked out for your type of boat
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