Fuel for thought...

Regardless of fuel costs, the state of the economy or the new tax regime, there will always be people who can afford to run a powerboat flat out every day of their god-given lives. For these cashed-up Captains Courageous, the cost of running a boat is seemingly of little consideration.

For most boaties, however, running costs are an issue. The increase in petrol prices has delivered a king hit to those who go boating on a tight budget. Paying $1-plus per litre can make filling up feel more like giving blood!

But even if fuel continues to spiral upwards in cost there are ways to cut your boat's consumption to more manageable levels. All it takes is a little forethought, some judicious seamanship, clever use of your electronics and a clear idea of what you are setting out to do.

There is nothing quite so inefficient as running around the ocean like a chook with its head cut off... Rule one: when you cast the lines make sure you have a clear idea of where you want to go. Have a Plan A, Plan B, and Plan C. Be clear-cut and decisive about your destination and you will save fuel.

Even before you set out you should have a plan in mind. Let me explain... For new boat-buyers, the plan should be factoring running costs into your prospective purchase from day one. If you consider running costs from the onset then you are less likely to find yourself in an untenable situation.

The thing for both new boat-buyers and existing skippers is to familiarise themselves with all the factors involved in running a boat. There are many variables which affect economy - some are obvious, others are less so - together they might just save you next time you're fuelling up.

POWER MANAGEMENT
There are savings to be made even before you start. That is, if you do have the right boat! Hmmm, it's a drastic step to sell your current craft and then buy an alternative, but the correct matching of powerplant and hull to intended use makes good economic sense.

You need to assess whether the boat you have in mind, or are presently using, is suitable for the role you set it.

Likewise, you may be tempted to buy a boat for a knockdown price, but beware, it's not unusual to find yourself paying out for the running costs of an old engine.

That aside, the first thing to get straight is: what do you want to do with your boat? Do you need to get offshore in a hurry? Do you want to merely potter about the estuaries?

If your boat is set for a life on the waves and you intend to head offshore or make quick coastal passages, then you're likely to want a reasonable cruise speed. Indeed, the key for economical (time and fuel) long-distance travel is achieving exactly this.

Again there are plenty of variables to consider, but in general terms each type of marine powerplant delivers its best compromise of speed and economy at different engine revs. With outboards it's around 4000rpm, petrol inboards around 3500rpm, small-to-middling high-speed diesels anywhere from 2000-3000rpm, and big thumpers 1500-2000rpm.

The trick is to match the engine, hull and running gear so that these rev-ranges yield a comfortable cruise speed. There are few occasions offshore when you can travel faster than mid-20kt for any length of time, so this is a good upper mark to aim for in the case of planing craft. On the other hand, there is no point having a fast planing rig if short-haul estuary work is all you have in mind.

Boat-owners can save a lot in fuel costs, maintenance and servicing by downsizing (both powerplants and boats) or turning to a displacement-style craft with an economical petrol or diesel powerplant.

Not surprisingly, one of the kinds of boats that ends up for sale after a few months tends to be overpowered trailerboats. Old-tech outboards (as opposed to new four-stroke and direct-injected two-stroke designs), especially those in the big horsepower league, are notorious for guzzling fuel. That's not to say that sensible use won't see them return better fuel economy figures.

Petrol inboards tend to be tarred with the same brush and while it's true that any large displacement petrol inboard motor is going to gulp fuel at top speeds, the day-to-day economy needs to looked at in light of the actual purchase price of the powerplant. If your yearly usage hours are not high, it can take a long time to recover the added expense of a supposedly more economical diesel - even with petrol at $1 per litre.

Sensible use of the throttle can see the current crop of fuel-injected petrol powerplants returning surprising economy. Cruise at optimum revs and waste no time getting your boat up to planing speed.

The fuel savings are quite astounding over a period of time if you become proficient at taking your boat to optimum planing speeds without unnecessary delay.

Obviously, a hull designed for displacement speeds will not benefit from any of the above advice. Nonetheless, the pairing of the engine and hull is extremely important in the case of displacement craft. There are no benefits in speed if an oversized engine is installed in a hull which is designed for a certain hull speed.

SPEED/LENGTH RATIO
Basically, this concerns displacement craft. The longer your displacement boat's length on the waterline the faster you can go at an economical engine rev-range.

There comes a point, however, with all displacement craft, that the extra power required to go faster is simply going to push your fuel consumption to unacceptable levels for the extra knot or two gained - the penalty for pushing those extra tons of water aside will exceed the benefits of economical cruising.

The prudent displacement craft owner will find out exactly what the optimum engine revs are for acceptable cruising speeds. It is fair to say that a boat that is fuel efficient at 9kt could double its fuel consumption at 9.5 or 10kt.

GETTING OVER THE HUMP
Planing boats are a different kettle of fish. As displacement boats (or a planing hull at displacement speed) goes faster, pressure waves created by a boat as it travels through the water create what some people call the 'hump'.

This is a natural 'barrier' of water that results from these waves. It occurs at the same speed no matter what shape the hull is. The relationship in natural waves and resistance is vital to the boat designer. And the power of the engine must be adequate, and the hull weight and shape must allow it to rise over the hump and begin to plane. Once on the plane, resistance diminishes, speed picks up and the planing hull can do what it's designed for.

To save fuel dollars, it is important to realise that further increases in power will certainly give more speed but a vastly increased fuel consumption.

Once over the plane, throttle back and there will come a point where the nose will start to rise and the speed decay. This is too slow for economical running and the power should be applied until the boat is level but not flat out. (Refer above re cruise revs.)

The savings in fuel can be quite astounding over a period of time.

A CLEAN BOTTOM
One of the more positive aspects of economical boating is to keep all underwater surfaces clean. This is one area where the boat-owner can positively help himself to increased fuel economy but rarely seems to do so. Every barnacle will add to the running costs and the percentages will increase with time.

Don't forget the rudders, shafts, props and engine brackets, either. Squeaky clean is the target here.

Good antifouling planning will also increase benefits greatly. There seems to be a definite preference for particular brands of antifoul over others, but it's difficult to prove that one brand can really be slipperier than others.

Don't forget, antifoul is only at its best when newly applied. Rather, it is better to just keep that bottom as smooth as possible. The same applies to any towed tenders, often overlooked in this exercise.

PROPELLERS AND GEARBOXES
A long, relatively narrow craft that is lightly built and powered by a smallish diesel correctly propelled can cruise somewhere around the speed/length ratio of 1.0:1.25. This would be a good start if optimum range were needed. Add to that, a clean, growth-free hull with a nicely tuned, serviced and maintained powerplant and you'll start to see dividends.

Another thing to realise is the efficiency of your propeller. Ideally, a large, slow-turning prop is more efficient than a small, fast-revving one.

Displacement boats can generally expect greater efficiency if the revs don't exceed 2000rpm, so gearbox reduction ratios come into play to increase power efficiency.

Whatever the case, there is much wasted power through propeller slippage (up to 40% in many cases). A larger propeller will give less slippage and more efficiency (to a point) at displacement speeds.

At the other end of the scale, a prop on a high-speed boat should not be too big, otherwise your motor won't reach the right revs for optimum cruising efficiency.

On displacement boats with continuous-rated diesel engines which are designed for long-range, efficient travel, the gearbox ratio can be as high as 3:1. This way, the motor can efficiently work yet keep a big prop swinging at relatively low speeds.

With lots of pitch on a big prop, plenty of thrust is generated, but it must be properly matched.

There are two factors that can really wreck the above observations and they are incorrect propeller size and pitch.

The science of matching props to engines and hulls is an inexact one. The reason is that there are many factors that can tip the balance out of line.

The proper thing to do here is to get expert help. You can't spend too much in this quest, because if you get it wrong the results can literally be quite devastating - and not just in terms of disappointing fuel consumption. One of the worst-case scenarios is with high-performance diesel craft. In the case where the hulls and engines are mismatched with oversized props, highly detrimental effects to fuel economy and engine condition can occur.

Diesel engines are designed to operate at optimal speeds. To achieve that state, the props must allow the engine to operate at full speed with the throttles fairly wide. Where the props are too large, the engines run slower and become overloaded causing increases in heat, friction, incomplete combustion and, obviously, poor fuel economy. All of these factors can cause costs to soar and engines to wear prematurely - sometimes in extreme cases the engines can fail in less than 1000 hours running.

Thus you have been warned, get proper advice when tweaking prop sizes to get better fuel economy. (Ed: look for a feature on prop tuning in an upcoming issue of TAB.) Don't be tempted to arbitrarily increase prop size with the view to decreasing engine revs. You may end up paying the hard way!

FILTERS, FILTERS, FILTERS!
Lack of regular filter changes, both air and fuel, can eat the bucks with horrible speed. To run efficiently all engines must breathe clean air and drink clean fuel.

Blocked air filters cause incorrect (rich) fuel/air mixtures and therefore less than peak power. In extreme cases the engine just won't run.

Blocked fuel filters clearly will stop an engine, but partially blocked filters can make an engine run lean enough to cause damage.

Fuel must be clean and clear to give up its real calorific values.

It is true that to replace air and fuel filters with the genuine article can cost a couple of hundred dollars but rest assured, it is for a good reason - you get what you pay for. The skimpy boat-owner who buys non-genuine filters and stretches the changeover times will be paying heavily for sure in extra fuel costs.

FUEL CONSUMPTION
Most engine manufacturers have exact fuel consumption figures available for the engines freely available. An engine works at its most efficient while at full or nearly full throttle, so for every horsepower produced more fuel is burned.

Many engines are extremely inefficient at low throttle. The new generation of direct-injected two-stroke and injected and carbed four-stroke outboards are a revelation for those not exposed to them. That doesn't change the fact that 'old-tech' outboards are notorious for inefficent low-speed running and can also oil up your plugs while wasting your dollars.

If you are going somewhere in an outboard-engined boat set a brisk pace and don't dawdle - otherwise you will waste money! A phone call to your engine supplier should yield accurate engine fuel consumption figures at all speed ranges.

In fuel economy terms, with some rare exceptions, the choice of outboard power on largish displacement craft is not ideal.

It's not within the scope of this article but it's fair to say there are many misconceptions regarding the economy or otherwise of modern outdrives.

Suffice to say, coupled to more economical, fuel-injected automotive-based petrol powerplants, and high-speed diesels, spinning larger props than most outboards, the modern sterndrive is, however, becoming a more popular choice.

PLANNING AHEAD
The combination of hull, motor and power-to-weight ratios has a major influence on the miles per gallon achieved. Many boaties will be quite amazed at the low mpl achieved in comparison to their cars, for example.

Therefore, it is important to plan ahead. Time spent at home planning trips that fall in with weather patterns pays off handsomely and so does route planning ahead.

Waypoint distances can be planned and proposed fuel consumptions calculated in advance. The private pilot of today must plan and calculate fuel consumptions plus reserves down to the last few litres - so too should the canny boatie.

It is easy to see how beating into short choppy seas and a howling headwind will send the fuel bills soaring. Currents too, seasonally will flow differently at different times and these will either reduce or add to your fuel costs accordingly. Think ahead.

Planning part of your route along sheltered, current-free waterways will add benefits to the budget as well as being more pleasant. It is also wise to phone ahead to be aware of fuel prices at particular destinations en route. If necessary, bring a few dozen litres of cheaper stuff with you. However, make sure it is stowed properly!

Given good conditions, twin-engined craft can certainly be cruised economically on one engine for certain legs of a trip (Ed: there are problems associated with this. Some gearboxes require the engine to be running to receive correct lubrication. Consult your gearbox manufacturer before attempting this.)

Excessive 'twin-throttle show-off handling' will put the savings back at risk though!

Excess weight onboard also costs money to transport. It is frightening to think some 40-footer will probably cart two or three tons around unnecessarily for maybe the next 10 years. You could buy a decent boat for the price of the fuel burned. If you don't need it - take it ashore!

BENEFITING FROM ELECTRONICS
Fuel-flow meters are a great idea. Available for most popular engines, these kits can really monitor your progress in dollars and cents. Route planning becomes a doddle and budgets can be lanned well ahead. Optimum cruising figures are easily adhered to by simply adjusting your throttlesetting to a specific fuel flow.

You will be surprised sometimes how slow the boat will seem to go against the current or tide. The temptation will be to open the throttle to go faster but just watch the meter-rate soar!

Many new and larger craft come already fitted out with these handy devices and many GPS plotters and navigation computer-based electronics have features that incorporate fuel-flow devices that will calculate optimum fuel requirements for trip planning underway.

Most common nav packages have complete sections concerning fuel planning strategies; time spent reading them correctly must put you ahead of the game.

Taking many factors into consideration, when it comes to it, the opportunities for reducing fuel bills are many. It may not seem much, a litre here or a litre there, but over periods of time the savings can be substantial. In fact, enough to pay for several dozen more tankfuls of the stuff!