Focus article 2

FUEL COSTS AND THE OCEAN CRUISING POWER BOAT

A primary design objective for FOCUS was to create an ocean capable power boat of simple, inexpensive construction suitable for the home builder, with accommodation primarily aimed at full time liveaboard cruising for a couple, though a family of four or 2 couples are provided for, and up to 8 people for short periods is no problem. Whilst it would have been possible to incorporate a third sleeping cabin, this would have reduced the size of the primary galley and saloon living area.

With lengthy passages envisaged, fuel economy was of great importance and a target of one mile per litre was the aim.  It was calculated that this could be achieved at 6.5 knots, and in fact that level of economy has been exceeded – with one litre per mile being maintained at the speed length ratio of 1.0 ( 7.15 knots) or a little more – up to 7.5 knots depending on load at the time.

Fuel economy being not just an issue of consumption, but also of purchase economy, it is maximised by buying as much as possible at the best price.  Factors influencing this include large fuel capacity, which is often sufficient enticement for getting a bulk tanker to deliver fuel, usually at a considerably better price than the dockside refueling facility.  Also, when departing for overseas, fuel in some countries is purchased excise free, which is a worthwhile saving if getting a good amount of it.  And last, but not least, there are still some countries where diesel can be bought very cheaply, and on a lengthy,  international cruise this could be a huge saving if the vessel can make it from one favorable refueling country to another, rather than having to refuel at the conclusion of each long passage.

This became a prime factor in the design of FOCUS.  Clearly a large fuel capacity was essential, hence a hull that could carry a large fuel weight.  This eliminated a power cat from consideration despite other attractive features, and in the final design more than 6000 litres of diesel tankage was allowed for.  This fuel weighs 5.1 tonne so empty fuel to full fuel is 3.75 inches (94mm) immersion, which is well within the load limits for this design.

To put some perspective on the fuel capacity issue, FOCUS could circumnavigate the globe with all the usual cruising diversions, on an initial full fuel load and just 4 refuels.  Much of the refueling could be spaced to include countries that do have very cheap diesel prices.  A Pacific cruise could be completed with no refueling en route. We cruised from New South Wales to New Caledonia, cruised in New Cal waters and then home again without any need to take on fuel. This was actually just with the primary tanks (3000 litres) filled before departure, and able to be purchased at the excise free price.

The fuel economy on FOCUS is delivered through a John Deere 6068, a 130 h.p. naturally aspirated 6 cyclinder diesel.  Full power is at 2400 rpm but the targeted cruise speed is delivered at 1500 to 1550 rpm, which the power curve for the engine suggests is utilising about 32 horsepower and requires 7.1 litres per hour, which equates pretty well with the measured performance.  This was the big surprise when we initially investigated the economy of low speed, easily driven displacement hulls – just how economical it is if we are willing to travel slowly.  The targeted speed range is at the upper end of what most cruising yachts are achieving anyway, so with a sailing boat background, we have not found it difficult to get used to.  The consistency with which it can be achieved over a large range of normal weather conditions makes planning a passage very predictable.

Confirming the easily driven hull shape is the performance with the 27 h.p. Yanmar 3 GM auxiliary engine. We had not been expecting too much with this back up unit which is primarily used for refrigeration and hot water services, and for charging batteries when not under way.  But for back up propulsion it is provided with a V drive transmission and a two blade folding propeller.  In flat water this engine drives the vessel as follows –

4 knots at 2000 rpm

5 knots at 2900 rpm

5.6 knots at 3600 rpm ( 27 horsepower)

Our conclusion is that powerboat voyaging is decidedly more affordable than many people realize as long as very moderate cruising speeds are targeted, and in the case of a vessel the size of FOCUS we could cruise for many years under power and still have outlaid less money than a sailing rig would have cost initially, even without factoring in all the ongoing sail and rig costs.  The ongoing costs of powerboat cruising other than the fuel should, by comparison, be much lower given that a heavy duty diesel engine such as the John Deere 6068 can be expected to give up to 25,000 hours trouble free (175,000 miles) before needing major maintenance if properly and regularly serviced.  Such ongoing costs as might be experienced are likely to only be similar to those that a yacht auxiliary might require anyway.