Expected Electric Auxilliary Motor Performance

It will be interesting to compare the performance predicted by Electric Yacht to actual. When purchasing from Electric Yacht they take hull design parameters such as dimensions, displacement, type of keel, and with that generate a brief report with expected performance with a recommended battery capacity.

Predictions from Electric Yacht

Electric Yacht recommended battery capacity was 48V 300A or 14.4 kWh and an 80% depth of discharge (DoD) is assumed. Repeated 80% DoD would quickly kill a lead acid battery bank. Li-ion (LiFePO4) cells can't be discharged below 20% and should not be regularly charged above 90-95%. Most Li-ion (LiFePO4) marine batteries contain internal battery management systems (BMS) and specify the usable capacity rather than the cell capacities. The 80% DoD assumption is therefore a bit conservative for Li-ion.

Electric Yacht provides a table of speed vs input power. Using this a graph can be produced as well as a table and graph of speed vs hours of operation and distance travelled with the recommended battery bank and 80% DoD. Some excerpts from this table are below.

Speed vs Power, Hours of Operation, and Distance for 48V, 300A (14.4 kWh) bank with 80% DoD for Mariner 36
Speed	Power	Run Time	Distance	Extended Distance (Nm)
(Knots)	(Watts)	(Hours)	(Nm)	680W Solar	2kW gen	3kW genset	6kW genset
2.3	480	24	55	85	fuel limited	fuel limited	fuel limited
2.9	960	12	35	54
3.6	1920	5.9	21	33
4.1	2880	3.8	16	25	67
4.5	3840	2.8	13	20	35	77
4.9	4800	2.3	11	16	25	39
5.5	6720	1.6	9	13	17	21	110
6.0	8160	1.2	7	11	14	17	40
6.4	10080	1.0	6	10	12	13	23

The last four column did not come from Electric Yacht. They were added to consider the impact of having 680 watts of solar panel, a 2kW portable generator, a 3kW fixed genset, or a 6kW fixed genset. If motoring for any length of time and that time includes daylight hours, then the solar panels will add charge to the batteries. A rule of thumb is for each watt of solar panel will add 4 Wh (mid-high lattitudes during sailing season) to 5 Wh (tropical lattitudes) on a sunny day. When no wind persists for days it tends to be sunny. This would add up to 3.4 kWh per day.

A portable generator or genset could be run continuously until the fuel supply ran out but at a limited speed. A 2 kW portable generator could run until the fuel ran out at 3.5 knots. With 10 gals of fuel (2 5 gal gas cans), with a consumption of ½ gal/hr, that would be 20 hours or about 70 miles. Diesel gensets are generally more fuel efficient. The Mariner 36 has a 44 gal diesel tank so run time would be considerably longer with a diesel genset.

What is Acceptable Performance?

Hull speed for this hull is estimated at 7.34 knots. It is clear from the table above (and from the more detailed table and graphs provided by Electric Yacht) that small increases in speed can require large increases in power and therefore shorter run time and distance travelled. For example, travelling at a bit under 1/3 of hull speed (2.3 knots) requires a modest amount of power and can therefore offer a 24 hour run time and 55 miles travelled. Travelling at 5.5 knots will allow only 1.6 hours of run time and 9 miles of range.

The much greater effort to move a boat near hull speed is well known and impacts range with a diesel auxilliary as well. Unlike an electric auxilliary a diesel loses efficiency at low RPM and therefore does not gain much range at much below about 4 knots given the amount of power generally recommended for diesel auxilliaries.

Whether the speed under power and range is acceptable to a give sailor depends on their style of sailing and whether they are willing to install a diesel genset creating a hybrid or sorts. Consider the following styles of sailing.

Type of Sailor: The hard core sailing purist
Description of Sailor	Acceptable Performance
The hard core sailing purist sees no purpose at all in having a motor of any kind and therefore this discussion is completely irrelevant. These sailors are very rare. The famous sailing couple Lin and Larry Pardey cruised to world for many decades, the whole time without a motor.	The hard core sailing purist would likely not add a electric motor along with bulky and expensive battery bank. Instead the space would be used for storage and the cost covering a few years of modest cruising lifestyle. They would simply use kedges and warps to get through channels and into harbors to the extent they don't just anchor way out and avoid the problem.
Type of Sailor: The more practical sailing purist
Description of Sailor	Acceptable Performance
Sailing purists pride themselves in patience. If the wind dies, they drift. Despite that, a motor does come in handy for entering and leaving a harbor and in manuvering in tight quarters though this is a relatively brief use of the motor at the very beginning or end of a passage.	If the wind dies on a passage while crossing an opposing current, the more practical purist may motor long enough to get out of the ocean current and then drift or sail slowly or motor very slowly until wind strength returns. The electric auxilliary with well sized battery bank and solar panels alone would be likely to perform adequately.
Type of Sailor: The mostly patient cruising sailor
Description of Sailor	Acceptable Performance
The mostly patient cruising sailor will sail whenever they can make reasonable progress under sail and consider a few knots in the direction of the passage to be reasonable. If the wind completely dies for an extended period then they will reluctantly motor.	A small portable generator would likely be sufficient for the rare occurance of no wind or extremely light wind for multiple days. The down side is the small generator would require carrying gasoline (aka petrol). If already carrying gasoline for a dinghy outboard, that may be acceptable. Limiting speed to 4 knots and using some battery capacity and then slowing to 3.5 knots beyond about 16 hours of motoring would likely be an acceptable limitation rather than sacrificing a significant amount of space for a genset.
Type of Sailor: The mostly impatient cruising sailor
Description of Sailor	Acceptable Performance
The mostly impatient cruising sailor will want to motor when the wind is not sufficient to support at least a few knots under hull speed. This occurs quite often. They may also motor to windward to maintain higher VMG (velocity made good).	If a goal is to acheive some average passage speed that is close to hull speed, then a diesel genset would be needed. The question would be how big a genset. A 3-5-4 kW genset itself consumes a lot of space. Battery chargers capable of 3kW or more would be needed to drive the 48V motor, most likely requiring multiple chargers in parallel. Gensets of 6kW or more take up even more space and double the number of chargers would be needed.
Type of Sailor: The stinkpot cruising sailor
Description of Sailor	Acceptable Performance
The stinkpot cruising sailor motors when wind is insufficient, motors when wind is too great, motors when sailing to windward, and plans to predominantly motor on certain passages particularly to windward.	If using an electric motor, a large genset would be needed and large set of battery chargers. As mentioned above, the space requirements are considerable.
Type of Sailor: The frantic weekend warrior
Description of Sailor	Acceptable Performance
The weekend warrior has a job to get to, possibly a very impatient spouse or guests, and a strict schedule to maintain. Most opt for a motor boat. Some opt for a sailboat but motor to such an extreme that the mast (or masts) and sails are mostly decorative. Some both sail and maintain their patience and work within their time pressures but are a small minority.	All but the few patient weekend warriors that sail at all rather than just buy a power boat would be better off keeping their diesel auxilliary. If their time pressures find then wanting to motor at close to hull speed then the 20-30HP auxilliary will be needed. For example, the MY36 comes with a nominal 28 HP but with typical prop setup produces a max of 2200 RPM and closer 20 HP.

The point of this, besides poking fun at stinkpotters with big sticks in the air, is that whether the limited battery range is sufficient depends on style of sailing with the most patient sailors finding it just fine and the most impatient sailors requiring a large genset.

Issues with Using a Generator or Genset

Nearly all generators or gensets produce AC output. Among marine products it is often either 125V 60Hz or 230V 50Hz. This will require battery chargers to convert to 48V DC. The set of marine battery chargers suitable for Li-ion batteries is small. The set that is also 48V capable is even smaller. Generally the power is limited with 1.2 kW being on the large side. If using parallel chargers, the total power draw has to be less than the generator or genset rated power. If increments of 1.2kW are available, then a 2kW generator is limited to 1.2kW, a 3kW genset is limited to 2.4kW, a 4kW genset may be able to drive 3.6 kW, and a 6kW genset would likely be better off driving 4.8kW of battery chargers.

Removing a diesel auxilliary and replacing it with an electric auxilliary plus batteries saves space. The diesel auxilliary and fuel tank takes nearly twice as much space and weighs a lot more.

Storage on a small boat is a scarce commodity. Adding a small portable generator loses some of that space but not much if fuel is in portable containers lashed on deck. if a proper gas locker is added, then more space is lost. These generators are typically capable of about 2 kW continuous power. With a 1200 to 1800 watt charger a continuous speed of 3 to 3.5 knots could be maintained.

The smallest diesel genset is just over 3 kW. Diesel fuel is easier to safely store than gasoline. The genset will be permanently mounted rather than portable. It will require raw water cooling and an exhaust system. A small genset with 2.4 kW or greater charger capacity should support closer to 4 knots.

Kubota and Fischer Panda both make enclosed gensets rated for 3.5-4 kW. Both take up quite a bit of space for a 36 foot boat, though less than a diesel auxilliary. Cooling water and exhaust connections are also needed. Both come in 230V 50Hz. Any space gained and weight lost by removing the diesel auxilliary will be more than negated installing this sort of hybrid. If the intent is to run mostly on renewable generated electric, then this could make sense.

For those on the more impatient side of the scale a 6 kW genset with 4.5 to 5 kW charger (ie: 230V 20-25A) could support about 5 knot continuous. The hopelessly impatient might need an 8 kW genset which then make keeping the existing diesel auxilliary the best option. In this case the net loss of space and gain in weight make this a questionable choice on a small boat.

On a 50 foot or larger boat this makes more sense but the entire system would have to be made larger. For a 36 foot boat gensets small enough for this to make sense may not yet exist.