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Calculating range, the rule of thirds, reserve and towing costs
When calculating range, take 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. Policy wordings usually distinguish 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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