Hybrid Propulsion; Stinkpotting for Raghaulers

For an overall discussion of this design go to: SHEWAC Aberration

Due to the fact that my wife became wheelchair bound recently, I am in the middle of the design and construction of a 35 foot hybrid propulsion wheelchair friendly catamaran.

Together with the boat design and construction masters of Scarano boats, I am converting a 1996 medium to high performance 35 foot sailing catamaran into a hybrid propulsion powerboat that is expected to hit the water in March of 2021.

Besides the classic catamaran issues of weight and trim management, the design effort also has introduced me to design complexities that made me realize that hybrid power catamaran operation is more akin to sailing than powerboating. I consider myself a raghauler through and through, and the similarity of sailing to hybrid propulsion is making the necessary transition to power somewhat less emotional for me.

Sailing is an activity of the mind. You are a participant in your environment, you need wind and tides to help you move. Most sailors wimp out to some extent, and fit auxiliary power for when the wind comes from the wrong direction or when the wind dies. Regardless, to a committed sailor, the auxiliary power is, at best, a necessary evil and becomes a variable in a never ending internal discussion. Do I wait for the wind to come in or the tide to change, or do I submit myself to the noise and smell of the engine? A secondary debate also interferes in the sailor’s mind when she is estimating the charge in the batteries and status of the reefers. Maybe I should start the engine, charge the batteries, cool the reefer and make a little headway before the wind comes in?

This is a running internal debate that is much less active in a powerboater’s mind. A powerboater starts the engines and runs. The whole power management process on a powerboat is dull as compared to sailboat power management.

So here I decided to build a hybrid power catamaran with the following power package:

One 15 kW diesel generator

Fuel tankage for 50 hours generator endurance

4 kW of roof top solar

28 kWh of batteries

Twin 10 kW electric outboards.

In the simplest terms, this package allows me one hour plus of full power battery alone propulsion. But if the sun shines, that period will be a little longer since I receive another 4 kWh boost each hour, and if I want to run continuously, I can start the 15 kW generator which will then deliver, let’s say, 12 kW to the twin outboards.

In theory I have 20 kW of propulsive power, but in practice that is only true for a short period of time and instead I am constrained by a complex mix of power sources that leaves me with almost a limitless amount of choices depending on trip and propulsion objectives.

I will not know what type of speed I can achieve with 20kW of electric outboard power until we splash the boat. When operating the original sailing version of the boat, I could achieve at least 5 knots with the twin 10 kW gas outboards (we easily made 10 knots under sail). But the hybrid power version of the boat will have greater displacement. On the other hand, 20kW of electric propulsion is very difficult to compare to 15kW of gasoline propulsion. I will have much better props and a better torque curve. I have been told that 10 kW electric propulsion is the practical equivalent of 20 hp gasoline propulsion. That may be true, or not. Right now I am assuming that the boat’s top speed at half load will be 7 knots, and at full displacement, for the sake of this example, I am assuming it is 6 knots.

Keep in mind that the boat is a slender hulled catamaran which inherently has little hump drag and can exceed classic hull speed without truly planing so we have a more linear drag curve. (I also have to manage hull trim, which on low power catamarans can make a big difference in drag, but let’s assume for the sake of discussion that I will operate the boat on best trim at each speed and displacement)

A true sailor uses his boat for many reasons. It may be a simple evening sail, it may be a two hour run to his favorite restaurant, it may be a weekend cruise to a favorite anchorage, or it may be a truly long distance cruise of hundreds of miles.

This boat is meant to allow all these uses in coastal fashion. It even has a sailboat’s theoretical unlimited range. Although, in sailboat terms, it would be a pretty bad sailboat with a maximum solar alone capability of probably no more than 16 miles per sunny day (4 times 6 = 24kWh, which is maybe 4 hours at 4 knots in light condition. Pretty good in the doldrums, but useless in a stiff headwind).

But let’s look at another part of the spectrum based on a question asked by my friend Ethan Wiseman. He asked me how fast the boat would run on the 15 kW generator alone.

To some extent I already answered that above. The 15 kw generator will probably deliver 12 kW to the outboards, which may get me somewhere near 5 knots. Not really exciting, but why even worry about that condition? I expect that I will only rarely encounter that situation, and that is where the sailor’s mind comes into play. A sailor always plans a voyage on custom circumstances. She can never predict with great certainty what the wind will do, and therefore her planning varies greatly from a powerboater who will watch the weather and plan for the next fuel stop at the speed he chooses.

Meanwhile on this boat, I may decide on a longer run from my home town (Red Bank) to Jersey City; about 40 miles. I know I will not have enough battery capacity to run on batteries alone. For that distance I know I will have to use the generator, but I have various options. If the weather forecast is clear skies, I can add solar to the battery capacity for the trip, depending on how long the trip will be. I can choose to start the generator right away or choose to start it later. I can choose to run fast (use a lot of power and get little solar charge time) or choose to run slow (get more range on batteries and more solar charge during the trip). To make things even more complicated, I can time my trip with the tides and run on batteries alone, then anchor to wait for the tide, while we swim and charge.

As a first stab, in the beginning of the trip I think I would start the generator, and run at a speed I consider to be optimal. Once I get closer to Jersey City I would probably shut down the generator once I know I have enough battery and solar power to make it to the berth. Even if I run out of charge, I can still restart the generator for the last mile or so. This may not be the optimal approach, and only experience and deep sailor’s thought will help me do better.

At the start of the trip I will not be able to accurately predict when I will arrive (it will depend on sunshine), but the cool part is that, undoubtedly, my average power would be substantially higher than the generator alone power. If I play my cards right, I may even come quite close to running at 20kW for the whole trip.

And, strangely, this brings me back to a design decision. With 20 kW of propulsive power, at first glance, it would make sense to install a generator that can supply 20kW to the propulsors. But that is a powerboater’s approach. Sailors do not think that way. To them diesel power is auxiliary power, not propulsive power; a necessary evil to be minimized at all cost. Moreover, as both a designer and a sailor I get a quadruple delight, because less power is less cost and less weight, and less weight is more speed (or reduced carbon load) and with careful planning I may still arrive as fast as with a bigger generator. In other words, to install a 25 kW generator would just turn me into a stinkpotter and I like being a raghauler even in sort of a stinkpot.