Fuel Oil System

Observations we have made is that many trawler yachts on offer simply do not have enough fuel capacity for ocean crossing service. In vessels built from GRP it is in some way understandable as it is difficult, and many would argue extremely unwise, to construct integral tanks into their hull construction. The alternative, fitting individual tanks, is not only physically difficult but also results in limited and usually insufficient volume. For steel and aluminum vessels, where tanks can be integral with the hull construction, there really is no excuse for restricted fuel capacity.

Our research and experience has shown that a minimum steaming range for a trawler yacht should be 3500 miles. This gives ocean crossing ability almost anywhere in the world.

How do we establish the fuel capacity to achieve this range?

Our criteria is: The vessel must achieve a range of 3500 NM at a vessel speed made good of v/?L = 1.1 corrected by a weather allowance of 10% and arrive with 10% of fuel remaining.

For a typical 55ft trawler (LWL = 50ft) the following speeds would apply:

Speed @ v/?L = 1.10 v/?L = 7.75 knots
Less 10% Weather Allowance = 0.77 knots
Steaming Speed Made Good = 6.98 knots

Why a speed of v/?L = 1.10? This is a practical compromise below the normal cruising speed of about v/?L = 1.20. Equally v/?L = 1.0 could have been chosen but who wants to cross an ocean at 6 knots.

What is a Weather Allowance? Vessel power curves and resistance tables are always plotted for trial (calm water) conditions. When a vessel is steaming in a seaway most of these figures are meaningless because sea state and wind conditions can increase resistance by up to 50% in some circumstances. Therefore sufficient allowance must be made for this in fuel calculations. In the example given above the HP requirement for v/?L = 1.1 is 95 BHP and for the steaming S.M.G. of 6.98kns is 55 BHP.

In other words in these calculations we have assumed that the HP requirement for our S M G is 73% (95 ÷ 55) higher than the calm water predictions for that same speed. This provides a generous fuel margin because the figures imply adverse head sea conditions for the entire voyage where this is rarely the case.

Owners soon learn to monitor daily fuel consumption and adjust their steaming speed accordingly.

Having calculated the steaming speed made good (S.M.G.) it is a simple matter to calculate the necessary endurance in hours and from the engine makers published specific consumption figures (these vary between 0.33 to 0.38lbs of fuel per BHP-Hour) to calculate the necessary fuel capacity.

The following is our fuel prediction calculation for the WATSON 54.

Watson 54 Trawler Yacht
Endurance Predictions
BASED ON (1) DIESEL ENGINE SPECIFIC CONSUMPTION OF 0.35LB/HP-HOUR, (2) BUNKER CAPACITY 12 TONNE (26460 LBS) ALTERNATOR AT 50% DUTY
FULL POWER MODE CRUISE POWER MODE LONG VOYAGE MODE
MACHINE RPM DUTY BHP LBS/
HOUR
MACHINE RPM DUTY BHP LBS/
HOUR
MACHINE RPM DUTY BHP LBS/
HOUR
MAIN ENG 2100 98% MCR 245 85.75 MAIN ENG INCL SHAFT GEN 50% 1800 68% MCR 169 59.15 MAIN ENG 1700 50% MCR 125 43.75
GENSET 1800 50% 12 4.20 GENSET 1800 50% 12 4.20
TOTAL CONSUMPTION 89.95 LB/
HOUR
TOTAL CONSUMPTION 59.15 LB/
HOUR
TOTAL CONSUMPTION 47.95 LB/
HOUR
TOTAL FUEL CAPACITY 26460 LB TOTAL FUEL CAPACITY 26460 LB TOTAL FUEL CAPACITY 26460 LB
LESS ARRIVAL MARGIN 10% LESS ARRIVAL MARGIN 10% LESS ARRIVAL MARGIN 10%
USABLE FUEL CAPACITY 23814 LB USABLE FUEL CAPACITY 23814 LB USABLE FUEL CAPACITY 23814 LB
FUEL ENDURANCE 265 HOURS FUEL ENDURANCE 403 HRS FUEL ENDURANCE 496 HRS
PREDICTED MAX. SPEED 8.85 KNOTS PREDICTED MAX. SPEED 8.40 KNOTS PREDICTED MAX. SPEED 8.00 KNOTS
LESS WEATHER ALLOWANCE 10% LESS WEATHER ALLOWANCE 10% LESS WEATHER ALLOWANCE 10%
STEAMING SPEED 7.965 KNOTS STEAMING SPEED 7.56 KNOTS STEAMING SPEED 7.20 KNOTS
STEAMING RANGE 2110 NM STEAMING RANGE 3043 NM STEAMING RANGE 3575 NM

For large trawlers steaming range should rarely pose a problem because it is easier to build in tank capacities into their hulls. The following are the predictions for the WATSON 72 for example, where even in the full power mode the range is well over 3500 miles.

Watson 72 Expedition Yacht
Endurance Predictions
BASED ON DIESEL ENGINE SPECIFIC CONSUMPSION OF 0.35 LB/HP-HR, TOTAL BUNKER CAPACITY = 38 TONNES (83790 LBS)
FULL POWER MODE CRUISE POWER MODE

MACHINE

RPM DUTY BHP CONSUMPTION MACHINE RPM DUTY BHP CONSUMPTION
LB/HOUR LB/HOUR
MAIN ENG. 1800 MCR 334 116.9 MAIN ENG.

1600

66% MCR 232 81.2
GENSET 50% 19 6.65 INCL. SHAFT GEN.
TOTAL FUEL CONSUMPTION 123.5 LB/HOUR TOTAL FUEL CONSUMPTION 81.20 LB/HOUR
TOTAL FUEL CAPACITY 83790 LBS TOTAL FUEL CAPACITY 83790 LB
LESS ARRIVAL MARGIN 10% LESS ARRIVAL MARGIN 10%
USABLE FUEL CAPACITY 75411 LBS USABLE FUEL CAPACITY 75411 LBS
FUEL ENDURANCE 610 HOURS FUEL ENDURANCE 929 HOURS
PREDICTED MAX. SPEED 10.5 KNOTS PREDICTED MAX. SPEED 9.80 KNOTS
LESS WEATHER ALLOWANCE 10% LESS WEATHER ALLOWANCE 10%
STEAMING SPEED 9.45 KNOTS STEAMING SPEED 8.82 KNOTS
STEAMING RANGE 5423 NM STEAMING RANGE 7878 NM

FUEL TANK ARRANGEMENTS

Fuel tanks are usually a combination of deep tanks and double bottom tanks. They are arranged in the hull so that when all are full the trim of the vessel is correct and as the fuel is consumed trim can be maintained either by selective use of particular tanks or by transferring fuel from one to another as required.

Tank Fills

We have found that the best arrangement for filling tanks is through the use of a central fill system where all tanks are filled from one deck station with a ring main filling pipe to each tank. This reduces the risk of spills.

Tank Vents

All fuel tanks should have proper antiflash type vent heads fitted at least 30” above the deck.

Tank vent

Tank Sounding Pipes

It is a wise policy to have individual sounding pipes lead to the bottom of the tanks for manually checking the contents. Each vessel is supplied with a sounding table which shows volume for each graduated depth of each of the fuel tanks aboard.

Tank Gauging

As well as the manual sounding pipes each tank should have gauges for determining the contents while at sea. These can be a simple sight glass type or the pressure sensor type. The pressure sensor types can be arranged for local or remote reading, even to the extent of having a complete tank contents array mounted in the wheelhouse console.

Wing Tank

Last Updated (Friday, 10 September 2010 00:01)