What exactly constitutes a fuel efficient boat?

Fuel Efficient Boats

Boat fuel consumption is a very tricky area. Boating ads usually mention it only in general terms such as "low fuel consumption". What does that really mean?

It is common to measure fuel consumption in gallons (or liters) per hour, but this is pretty meaning less. We don't measure automotive fuel use that way.  Nautical miles per gallon or liters make more sense. Gallons per hour likely became popular because the miles could not be accurately measured, but that is no longer the case.

But absolute fuel mileage isn't a very useful measure of fuel efficiency either since you need to know the size of the boat. The proper measure of efficiency is gallons per ton/mile. But that number is hard to come by.

Let us talk about relative fuel efficiency. In order from least to most consumption we have:

  1. A sailboat of any kind when sailing.
  2. A rowboat (food is fuel too).
  3. Low speed electric powered boat, maybe with solar or wind turbines.
  4. Sailboat when powering.
  5. Low speed power catamaran.
  6. Low speed troller yacht (double ender or transom clear of water).
  7. Low speed trawler yacht (immersed transom)
  8. Fast planing hull boat.

This is general in nature and individual examples might move up or down the list a bit.

Factors in favor of low fuel consumption include:

  1. Propulsive efficiency of the drive train.
  2. Number of engines (1 is more efficient than 2).
  3. Size of propeller (big and slow is better than small and fast).
  4. Hull shape.
  5. Weight (lighter is better)
  6. Windage (less is better)

Speed in Naval Architecture terms is relative to length and is expressed as S/L ratio or speed to length ratio.  In US units the formula is (speed in knots)/(square root of LWL in feet). LWL is the length of the waterline. An S/L calculator is here

Power requirements and fuel efficiency are generally best at a S/L ratio of about 1, which translates into 6 knots on a 36 foot waterline.  At this low speed, hull shape is very important and the best shape is something like a kayak, with fine ends and narrow beam.  Ideally the beam to length ratio should be about 10 to 1 for minimum resistance, but a 36 foot monohull with a 3.6 foot beam would fall over, and wouldn't have much usable room, so only catamaran hulls get that narrow.

For a low speed monohull such as a sailboat or troller yacht, the maximum practical speed is and SL of about 1.34, or 8 knots for our 36 foot waterline boat.  We have been able to improve this a bit with the PowerKeel hull form. We have put a lot of energy and research into improved fuel efficiency, but so far few clients seem to care.

One major problem for motor boats is range. Fuel is heavy, and a boat that uses a lot of fuel needs a large supply.  Ocean going motor boats like the Nordhavn range are very heavy and carry huge fuel supplies. They are actually very fuel efficient in gallons per ton/mile, but there are a lot of tons to move.  But realistically, in coastal cruising, it is more efficient to carry less fuel and fill up more often.

In the summer of 2008 I took my 37' sailboat Tangleberry to Petersburg Alaska from Vancouver BC, and back.  The longest stretch we went without fueling was 216 miles. We have only a 20 gallon fuel tank, and a 5 gallon jerry can reserve. We ran out of fuel within sight of the fuel dock at Shearwater and put in the reserve.  However there were at least two other places on that stretch where we could have bought fuel.

Anyway, unless you are planning to cross oceans, a range of 300 miles under power will handle most coastal voyages with ease, so why carry more fuel than that?

Accommodation has greatly increased in all types of boats in the last few decades.  Ask yourself how much space you really need, and how much "stuff" you really need to bring with you. We used to cruise on a boat with sitting headroom, no frig, hand pumped (cold) water and a two burner alcohol stove.  Now we have standing room, hot pressure water, refrigeration and a shower, as well as an excellent propane stove with oven and grill. We even have a furnace. Yet our boat is still considered small and spartan by many of our clients. To us it is large and luxurious.

We spent about $1500 on fuel for our 3 month 1800 mile trip to Alaska. We met some people in a 28' Boston Whaler with twin huge outboards and they said they budgeted $12,000 for the same trip, but they were a lot faster than us.

If you would like a fuel efficient power boat, it is hard to beat a sailboat, even if you only sail when the wind is "just right". We only sailed about 10% of the miles on the Alaska trip, but in local cruising we sail 60% of the miles we do.  I have often heard power boaters say that although they see plenty of sailboats they "never have the sails up".  Well, even motoring along, they are still using a lot less fuel than almost any true powerboat.

How much do you spend on fuel? I normally run my engine 100 hours per year, and burn about 75 gallons of fuel. This year I used much more because of going to Alaska. Anyway at my normal rate, even at $5 a gallon, I use less than $400 worth of fuel in a year.  Moorage is $6000 a year, insurance $1500, and general maintenance another $2000, so really fuel costs are not significant. 

What about depreciation? Sailboats and fuel efficient powerboats tend to hold their value better than guzzlers. When you bought that Cadillac Escalade, or that Sea-Ray, you knew it was a guzzler that would plummet in value, so QYDB.