Fuel Oil System
Tuesday, 24 August 2010 11:13 | 
Written by Peter Watson
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 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.
| ← Fuel Oil Transfer System | Bilge & Fire System → |
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Last Updated (Saturday, 29 August 2020 03:27)
