Fuel and Water Tank Capacities

Updated: Nov 17

The time has come for us to review fuel and water tank capacities on Vanguard. We were spurred by adding a unique need for Water/Glycol cooling systems required by the hybrid drive. So to kick this off, we looked at a few similar hulls to get a better feel for what the market is looking for:


1 – first off, we looked at other high L/B ratio hulls, specifically FPB78 and XPM78-01. These were both the same length but differed in their beam and internal layout, 3 and 2 cabin designs. (Design details for the Circa Marine 24M are not easy to find).

2 – then, we looked at another displacement hulls with three cabins and similar internal volume – Nordhavn 64, single-engine, and Bering 65 twin-engine. 

Explorer Yacht Tank Volumes

3 – we looked at semi-planing hulls of similar layout and internal volume. Twin-engine Fleming 65 and Ocean Reef. Table 1 below displays the results for fuel and water tank storage capacities across different yacht brands. What is immediately apparent is the installed power with FPB78 and XPM78 series being significantly lower for a similar speed profile to the two other displacement hulls of Nordhavn and Bering. Even more so for the semi-planing hulls. Conversely, FPB78 and XPM78 have significantly higher freshwater tank capacities. This capacity is partly as a ballast function (see fuel tank capacities) and party to reflect their passage-making profile where convenient marina facilities are far and long apart. 


Grey and Black Water

Grey and blackwater tank capacities were a little harder to find. Greywater is easier to deal with and, in most cases, can be discharged overboard unless in environmentally sensitive areas and some freshwater lakes. Discharge may become more restrictive as the legislation progresses, so we decided to install these tanks for all shower and sink drains. We will probably lead HVAC drains overboard. Blackwater is subject to pump-out requirements at marina facilities or defined distance offshore, typically 3 miles. We split our tank capacity 50% Grey, 50% black. Greywater can overflow to black if necessary but not the other way around. Greywater tanks will be marine-grade aluminum hull tanks; black water will be polypropylene fabricated tanks to limit corrosion. Eight hundred liters capacity for each seems in line with the other 3-cabin yachts irrespective of hull design. We will create an interlock in the Helm Station mimic diagrams to prevent unwanted discharge in restricted waters and install remotely operated valves in the system. 


Fuel Tanks

We looked at the fuel tanks. In considering these it is also necessary to consider installed power on each vessel. We estimated operational engine power as 60% of installed capacity except for the high L/B hulls where the second engine provides redundancy and is not power. At 60% power, we assume that the higher-powered hulls are not yet planing.  Tanks on Vanguard are smaller than XPM78-01 with similar hydrodynamic power needs and FPB78 with a slightly wider hull. However, the range remains about 7000 NM with 20% reserve, and we do not feel the need to increase this. The one change we would like to see is a reduction in the volume of the day tanks. Three-day capacity is ample, and we would like to remove a little weight from the stern of the hull and add a little more space in the engine room.  


Water Tanks

Then we looked at water storage tanks. Some of this capacity is designed as ballast compensation to keep trim as the fuel reduces. Vanguard will have an onboard water maker additional to any shore supply. We decided to retain the large water storage capacity, it adds redundancy should the water maker fail and we will need the ballast capacity to counterbalance the fuel tanks as they deplete.


Hybrid Drive Cooling System

Like many EV’s, our cooling will use ethylene glycol in a 50/50 mixture. That gives excellent heat transfer performance, low freeing temperatures, and corrosion protection for any metal it touches. Due to its toxicity, the imperative is to keep it isolated from any potable water tanks and systems that minimize accidental contamination risks. Water/Glycol system could cool the batteries, inverters, EV Motors, hybrid drive gearbox, and main gearbox. Engines themselves have antifreeze coolant systems that we will leave as supplied. 

XPM78-02 – Vanguard Tank Storage Plan

Space exists in the engine room bilge between and aft of the engines. Three frames will allow for a cofferdam aft of the freshwater tanks. Seawater in contact with the bottom skin area of the tank will cool the contents and (less efficiently) internal air contact. Being set low in the hull, any systems not operating will drain to the sump tank removing toxic ethylene glycol from the lines when possible. 


By eliminating seawater, we eliminate corrosion and fouling. We can also consider fitting third-party sealed plate coolers instead of traditional tube coolers with corresponding weight and size reductions. The engine cooling system can remain independent as there is a wet exhaust system installed so we cannot eliminate the seawater flow.  


I’d be a little nervous about this idea if the systems were either high-powered or continuously running. Apart from the engines, the operation is periodic, in the order of 1 or 2 hours. For all systems, the cooling requirements are pretty modest, in the order of a few kW. We still need to undertake a heat balance for the system to double-check, but I think we are good (see later Blog). The fallback is to add a parasitic seawater cooler into the system, which we would rather avoid right now. 


Conclusion for Fuel and Water Tank Capacities

So the final design has:

  1. retained most fuel and water tank capacities and their effect on trim and stability.  

  2. class-leading fuel range at >7000miles @ 10kn

  3. more potable water/ballast than you need to fill a small swimming pool

  4. ample greywater, and blackwater capacity.  

  5. a nifty, passive water/glycol cooling system suitable for high latitude operation, removing most seawater systems from the engine room. 

Items 1 through 4 came at no effort thanks to work already undertaken for hull number 1 Mobius. Item 5 is our add-on to that configuration. 

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