Big news on the engine front

Big news on the engine front

Date: Monday, January 21, 2019. 1154Z
Location: 53.1631° N, 9.0760° W — Aughinish
WX: 205° (SSW) at 20 knots. Cloudy. Cold

So, I might have teased in my previous posting that I had some news on the engine front. I will keep you in suspense no more…

I’ve sold it!

It’s gone, gone, gone. Freed from a peaceful existence in my garage, soon to be powering someone elses boat.

Right, first some more back-story. As I mentioned, the original owner had sealed up the cockpit well, and cut a large hole in the cockpit sole. He then installed a Yanmar 1GM10 single-cylinder diesel engine in the boat.

An inboard engine is a real luxury. OK, this is a sailboat we’re talking about, and she sails better than she motors. But, it’s hard to sail up to a mooring buoy. I’ve done it, yes. But when you’re threading your boat through a fleet of expensive yachts, an engine makes life just that bit easier. I wouldn’t like to pick up a marina berth under sail, although doing that in a 24 foot boat like the Achilles wouldn’t be as daunting, say, as a forty-something foot Hallberg Rassy.

Becalmed? Fire up the engine, and motor the rest of the way. Navigate into tight harbours, anchorages, and marinas. Know that you always have Plan B if you can’t sail the boat for any reason. But the price is high. I’d say too high, especially in the case of a smaller boat. Engines are trouble. They’re noisy, troublesome, prone to failure, require dangerous chemicals on board, etc.

When I started working on the TODO list for Into The Mystic, I culled the “nice to have” jobs in favour of just the most essential jobs needed to prepare her for the water. Amazingly, most of those jobs seemed to relate to the engine. As the boat has been out of the water for over a decade, I had to lift out the engine and have it serviced/rebuilt. The old engine mounts disappeared in a cloud of rust. The Morse gear was jammed - Henry gave me his spare. The fuel tank leaked - I bought a new one. The alternator didn’t work - I got it rebuilt. The starter was jammed - I got that rewound. The list goes on, unfortunately.

I was diligently working through that list, when I discovered something much more serious. When the original owner cut a hole in the cockpit sole, the fibreglass and wood repairs to create a new “engine hatch” were substandard. Water had gotten into the woodwork and it was rotten. In fact, water ingress into the ply sandwich meant that a lot of the ply inner core was also rotten. I started ripping out rotten wood and fibreglass wherever I found it. Interestingly, the original fibreglass work from when the boat was built, was still immaculate. The stuff I pulled out had been added to host the diesel engine.

Anyway, I found myself frustrated by the engine and it’s neediness. I can’t say I like the smell of diesel, and the two things which assault your nose when you first set foot on a boat, in Ireland at least, are the smell of stale diesel and damp. Not exactly conducive to the dream of wandering down to the seas again. Outboards are no better. Petrol is more flammable than diesel, so keeping it out of the bilge is very important. Engines ara a useful necessity, but that doesn’t mean we have to like the experience.

Anyway, back to Into the Mystic. I was reading up on electric sailboats, and discovered My Electric Boats by Charles Mathys. In addition to that, I found a lot of YouTube videos on the subject. Many people have ditched the “iron donkey” for a quieter, more efficient electric motor. Of course, it’s not entirely trivial and it helps a lot that the Achilles is only 24 feet. Trying to push a bigger boat into a seaway would require too much motive power. The boat had (yes, it’s gone!) a 1GM10 Yanmar engine. This produces a whopping 9HP of output power.

Yanmar 1GM10 Power Curve The diagram shows the power output for the engine as a function of RPMs. Given that it takes 746 watts of power to produce one horsepower, it’s reasonable to think that you would need a 6.7kW electric motor to replace the Yanmar. But look a bit closer at that power curve. The full power output isn’t delivered until 3,600 RPMs which is particularly fast for a diesel. Normally we would run the engine between 1,800 and 2,500 RPMs. That gives us a typical power consumption of between 800 and 2,200 watts (2.2kW). Even if we allow for a motor efficiency of 80%, we’re still under 3kW to replace a 9 horsepower engine. However, it’s nice to have that extra “oomph” at times, so choosing a 3.5kW motor which can produce 5kW in a pinch, is not a bad way to go.

Now, looking at that a bit deeper (and I’m happy to go a lot deeper if there’s interest), a 5kW motor would require a staggering 400 amps at 12 volts (really you’re talking 12.5 volts for a battery at full charge). We tend to think of amperage like that as a function of the maximum load of the cable. Looking at the AWG tables, we can see that the fusing current for #8 AWG wire is 472 amps. But this isn’t the real story. Imagine that there is a metre between the battery and the motor. That’s a pretty short battery cable, but however… The current has to go from the battery to the motor and back to the battery, so it’s a two metre cable length. Our chosen wire has a resistance of 2.061mOhms per metre. That gives us a resistance of 0.0041 ohms. That’s pretty small, all-told. But Ohms law tells us that the voltage drop (V) in the cables will be IR (current multiplied by resistance). For our chosen installation, we’re losing 1.64 volts. So our nominal 12.5 volts at the battery, has now dropped to 10.86 volts. Bad and all as that is, look at the power consumed by the cable, for a minute. The I^2R losses in the cable will be a whopping 656 watts! A good rule of thumb is that the voltage drop in the cable should be 3% or less. In this case, that’s a voltage drop of 0.375V. For that current, we’d need a resistance of 0.9375mOhms. Or, 0.46875mOhms/metre (for our idealized installation). The best fit would then be AWG #1 wire with a resistance per metre of 0.4066mOhms. But even at that, our I^2R losses woule be 150 watts which is unacceptable. It’s not just the fact that 150 watts of the power we should be using to drive the propellor is not available to the motor. The battery cables are going to radiate 150 watts of heat, and that’s just not good.

Yes, we can get thicker wire, and in practice, no-one would ever go as small as AWG #1, choosing something like AWG 4/0 wire from the battery to the controller and motor. But a simpler alternative is to increase the voltage by a factor of four, and as a result, decrease the current demand by the same factor. One hundred amps (approximately 50 volts driving a 5kW motor) isn’t exactly childs-play, but the power loss is the current squared so anything we do to reduce the current, will have huge benefits. For the sake of argument, let’s stick with the impractical AWG #8 wire for comparison. Our resistance of 0.0041 Ohms gives us a voltage drop of 0.41 volts. In a 48 volt system, that’s less than 1% so it’s in tolerance. It’s still 41 watts of power (I^2R) consumed by the cable, so AWG #8 still isn’t a good choice, but it’s far, far better than it was. AWG #1 wire with a resistance of 0.8132 Ohms across our two metre cable run, has a voltage drop of 0.081 volts. The cables will consume 8 watts of power, which is far, far better than the alternatives. Again, one would normally use 3/0 or 4/0 wire, but it’s good to compare.

From an electronics perspective, 48 volts is below the “pain threshold” in that it is very unlikely to kill you, and isn’t governed by the kinds of safety concerns as 110V or 220V. At the same time, 100 amps, while a lot of current, is not unmanageable.

So, having decided to go with a 3.5kW (continuous) motor, running at 48 volts, I now need to look at the battery situation. The smart money would go with some form of Lithium Ion cell, but I’m not ready to do that, for cost and safety issues. Charging Lithium Ion batteries is a challenge. Just ask Samsung. Lead acid batteries are heavier, clunkier technology but they are far, far cheaper. They are easier to charge, and more forgiving. Four of them will give me an official voltage of 48 volts. In reality, that voltage can go to 57.6 volts while charging, and as low as 42 volts when fully discharged. The down-side of this is I either have to add yet another battery for the “domestic” systems, or else some DC/DC convertors to bring the 48 volts down to the more practical 12 volts of the various on-board systems. I’m favouring the latter. It also means installing a solar system which can charge a 48 volt battery bank rather than the more common 12 volt system.

Looking at that last paragraph, many old sea salts would roll their eyes and shake their heads at the required complexity involved in all of this. It is a well-known adage that salt water and electronics do not see eye to eye. This has always been my own trepidation with the switch to electric motors. Using a three-phase brushless DC (BLDC) motor means that there are no brushes, no commutator rings, nor any of the normal baggage associated with electric motors. People have run BLDC motors under water, with no short-term effects. Obviously, rust will move in on the motor eventually, but it is nice to know that the motor will keep running, even when submerged. Removing the old DC motor brushes also removes sparks and debris from the engine compartment. In fact, your average BLDC motor is the definition of simplicity. The controller is another story…

But rolling one’s eyes skyward at the thought of bringing a complex system like a three-phase, BLDC controller into a salty environment like a boat, does not take into consideration the reality of the modern-day diesel engine. Once upon a time, the operation of a diesel engine was governed by a complex set of pumps, injectors, governors, and regulators. In the drive for better and better efficiency, all modern diesel engines include an electronic control unit (ECU) sometimes known as an engine management unit (or EMU). While it isn’t switching the types of currents involved in a BLDC motor, it is controlling a lot of the engine operations, and if it fails, the engine will be a large lump of useless iron. If you have an older engine (such as the 1GM10), you are safe, for now. But the trend is to embed more and more complex electronic and computer systems into the average diesel engine, so the argument that a motor controller is needlessly bringing complex electronics into a saltwater environment, doesn’t really hold water (pardon the bad pun). Even the argument that if your diesel engine breaks down in the middle of nowhere, it can be fixed by a local mechanic is becoming less and less reliable. More often than not, only the dealer has access to the complex instrumentation to diagnose and repair an EMU fault code. Given that a lead acid battery is almost prehistoric in terms of its technology, and that a BLDC motor has little to break down, carrying a spare motor controller can mean a much better reliability than a diesel engine. You also avoid worries about contaminated fuel, water in the diesel tank, blocked injectors, air in the fuel line, clogged seawater inlet, damaged impeller, damaged hoses, exhaust issues, oil changes, oil leaks, etc, etc, etc. You have a relatively simple motor, prehistoric batteries, and yes, a complex little box of electronic wonders, which can fail. So bring a spare!

The other argument regarding electric motors on boats has to do with the length of time you can run the motor. This is a much more valid complaint. It is no secret that the energy capacity of a 20 litre tank of diesel is far, far greater than even the most efficient of Lithium Ion cells of a similar size/weight. The Yanmar consumes between 0.5 and 1 litre of diesel per hour. Even at a good clip, you can motor for twenty hours without issue. At full power, the BLDC is consuming 100 amps. Given four batteries with 100Ah capacity, the best you can hope for is an hour of motoring. Even at that, the amp-hour rating for a battery is quoted based on either a 10 hour or 20 hour drain. Effectively, this means that your average 100Ah battery will deliver a solid 10 amps of current for ten hours. But it’s not a linear equation. As you increase the current demand, the battery capacity diminishes. So a 100Ah battery won’t deliver 100 amps for an hour. It might deliver that current for 45 minutes, at best. What’s more, this requires fully discharging the battery which is never a good idea. Lead acid batteries, even absorbed glass matte (AGM) batteries do not like to discharge fully. On the positive side, at the beginning of this post, I mentioned that I tended to motor at an engine speed of 1,800 RPMs, when the engine was delivering less than a kilowatt of power. Discharging the battery bank at a current of 20 amps still won’t deliver the advertised 100Ah, but I should be able to run the engine for four hours, without any issues. I plan on purchasing a 2kW petrol generator as a backup plan, as I anticipate that the only way of recharging the batteries will be by solar panels, and this after all, is Ireland.

Having said all of that, I am quite keen to get the boat back into the water, and the Trello list lends itself to moving TODO items from the Backlog to the Deferred Until Next Year list, so my thoughts right now are that I will mount an outboard bracket onto the stern, and use the four stroke 4HP outboard engine for the time being. That way, I can continue to research the subject and also the electric motor doesn’t just replace the Yanmar on the critical path.

If you have any thoughts about my move to the world of electric motors, leave a comment below. Likewise, if you have any questions, I’ll answer them if I can. Here’s to getting the boat into the water sooner, rather than later!

Dermot Tynan's Picture

About Dermot Tynan

Part-time sailor, full-time procrastinator. Software Engineer, Writer, Film-maker. Interested in all things cloud, sailing, autonomous systems and robotic sailboats.

Galway, Ireland