Jimmy Cornell goes Electric, with a Cat

[Adelie]

Originally Posted by smj

How can one sail with a 32” prop? That’s like towing a large drogue. We are talking sailboats, and in this thread catamarans?

It’s an RC model aircraft prop so the blades are very skinny which would have very little drag if he left it in the water when the motor wasn’t running. I’m pretty sure he pulls it out when not motoring.

[CatNewBee]

Went from Cartagena to Linton Bay, had to motor the 260nm to the wind. Run 3 days / 2 nights on one engine Yanmar 40hp, burned 240l of diesel at 2500rpm and 5...6kn approx. 4l/h (one tank empty plus 40l reserve in jerry cans).

Would be totally impossible with electric propulsion. Even with diesel a challenge.

[seandepagnier]

Originally Posted by smj

How can one sail with a 32” prop? That’s like towing a large drogue. We are talking sailboats, and in this thread catamarans?

It is an outboard so it is not in the water unless it is being used. If you leave the propeller in the water when sailing it must fold or something especially at that size, but much better just remove it completely.

I also built a tow generator it cost me $35. I can't wait to try it out, designed to produce 1, 2, 3 amps at 6, 7, 8 knots with just a few pounds drag, then change the impeller for higher speeds. Trying to find a design that flexes so it can work over a wider range of speeds.

Originally Posted by CatNewBee

Went from Cartagena to Linton Bay, had to motor the 260nm to the wind.

For sure false, as no one has to make passages.

burned 240l of diesel

Would be totally impossible with electric propulsion

It would be possible to have made the trip just by sailing.

[catsketcher]

Sean, do you have a drop prop, like a longtail? That is really big. I am envious. BTW thanks for the PyPilot. My son is putting one together for our cat.

As for kW being kW, there is more to it. A story a friend told me about motoring into the wind comes to mind.

He had a 32ft cruising cat, a nice sailing cat and the anchorage he was in got hit by a vicious thunderstorm with wind off the charts. Everyone dragged and was trying to stay safe by motoring into the huge winds. My mate's setup was really interesting, he had a diesel with a variable pitch hydraulic gearbox. In light winds he could get the diesel chugging and then change the vanes to make the prop spin fast for motorsailing. It would sip fuel as he sped along.

In the storm he varied the hydraulic vanes to allow the prop to much slower, he geared it down like we do when driving on a hill. He could then maintain heading as the diesel could get up to its proper rev range and the prop wasn't being too much of a load on the engine. Other boats struggled with the wind whilst my mate with his small diesel was okay steadily plodding away.

Diesels, even for all their torque, have low torque at low revs, they stall. Whereas an electric motor will have huge torque at low revs. As props work best at a certain ratio of slippage you can set your power and slippage and the props will work efficiently at much lower power.

Electric cars do not need gears whereas diesels do. My diesel car would not be able to put out anything like full power if I put it in 4th gear at low revs and tell it to go up a hill. Which is equivalent to motoring into a strong headwind. An electric car could output close to full power at all revs. I would need a larger diesel than I have if I had no gears.

I need Dave Gerr or some prop guru to tell me more but when we are doing 3 knots into huge headwinds, the gearing of the prop should be different from when we are in a calm. An electric motor does not need gearing ever, allowing large power to be put into a prop at low speeds. Therefore you may not need anywhere near the same size engine (in kW) to motor into a large squall with an electric motor.

[fourlyons]

Originally Posted by CatNewBee

Went from Cartagena to Linton Bay, had to motor the 260nm to the wind. Run 3 days / 2 nights on one engine Yanmar 40hp, burned 240l of diesel at 2500rpm and 5...6kn approx. 4l/h (one tank empty plus 40l reserve in jerry cans).

Would be totally impossible with electric propulsion. Even with diesel a challenge.

You are right, electric wouldn't cut it there. However, I sailed from the San Blas to Cartagena in December '96, in a 25' cutter. So there is that option. [emoji6]

[catsketcher]

Originally Posted by CatNewBee

Went from Cartagena to Linton Bay, had to motor the 260nm to the wind. Run 3 days / 2 nights on one engine Yanmar 40hp, burned 240l of diesel at 2500rpm and 5...6kn approx. 4l/h (one tank empty plus 40l reserve in jerry cans).

Would be totally impossible with electric propulsion. Even with diesel a challenge.

Gday Cat

We had a similar scenario occur a couple of times when cruising the coast. Our daggerboard cat tacked nicely into the wind whilst other, not so weatherly boats, motored all day. If the wind was straight down the course we usually arrived after 50 miles or so at the same time. If it freed a just a little we would get there first. I much prefer the motion of sailing with my little staysail and reefed main to motoring into waves.

My friends who sailed the Red Sea on the their daggerboard cats told me similar stories, 50-70 miles in daylight between marsas into the wind, every day for weeks. It can be done.

So electric intrigues me, because I always sail if there is wind, no matter the direction. My boat is lovely to sail and it makes me happy. But it would be less good for someone with a less weatherly boat, or someone on a time schedule.

[JustMurph]

Originally Posted by Adelie

The Lagoon 421 is a charter condo meant to remain in the marina when those kinds of winds are coming from where you want to go.
Electric propulsion is not meant to remediate the shortcomings of the boa, so, as you say diesel is the way to go for a boat like that.

2 knots headway into 20 knots headwind sounds drastically underpowered to me. That means you can't move in 30.

I totally agree that there wouldn't have been a problem if the 421 could have sailed to windward in those conditions. On my boat I would have been sailing and would have been doing it with a far better VMG.

However, that doesn't change the fact that reducing the power to half, will have an effect on the boats capability.

Originally Posted by seandepagnier

The thing is, you don't need any HP to begin with. In these "emergency" situations you should generally be able to use the sails. This always worked for me I am not sure what the hesitancy is about this.

I applaud you for being able to sail your self out of every situation. That is excellent. That's not the norm. In the situation mentioned above, dragging anchor in a storm on a lee shore, the first thing I'm doing is starting my motors, not raising my sails.

Originally Posted by catsketcher

As for kW being kW, there is more to it. A story a friend told me about motoring into the wind comes to mind.

He had a 32ft cruising cat, a nice sailing cat and the anchorage he was in got hit by a vicious thunderstorm with wind off the charts. Everyone dragged and was trying to stay safe by motoring into the huge winds. My mate's setup was really interesting, he had a diesel with a variable pitch hydraulic gearbox. In light winds he could get the diesel chugging and then change the vanes to make the prop spin fast for motorsailing. It would sip fuel as he sped along.

That's quite as smart system, especially in the motor sailing situation, where you want a high prop speed, but the power required to drive it is relatively low. You can bring the motor back down to more efficient revs for the power its producing.

Originally Posted by catsketcher

In the storm he varied the hydraulic vanes to allow the prop to much slower, he geared it down like we do when driving on a hill. He could then maintain heading as the diesel could get up to its proper rev range and the prop wasn't being too much of a load on the engine. Other boats struggled with the wind whilst my mate with his small diesel was okay steadily plodding away.

Diesels, even for all their torque, have low torque at low revs, they stall. Whereas an electric motor will have huge torque at low revs. As props work best at a certain ratio of slippage you can set your power and slippage and the props will work efficiently at much lower power.

Electric cars do not need gears whereas diesels do. My diesel car would not be able to put out anything like full power if I put it in 4th gear at low revs and tell it to go up a hill. Which is equivalent to motoring into a strong headwind. An electric car could output close to full power at all revs. I would need a larger diesel than I have if I had no gears.

This isn't really correct. Yes, the electric produces a lot more torque at very low revs, that's true. The thing is, the diesel in your sail boat doesn't need to. Unlike a car, you have a prop which allows slip through the water, letting the engine rev to where it does make more power. Flat chat at 10 knots my engines will rev to 3300rpm. Stationary, they'll rev to about 2700 rpm (IIRC), which is beyond the torque peak and would be pretty close to peak power.

This is different for power boats with big diameter and pitch props that have a lot of grip, eg my mates ski boat which will only rev to ~2500rpm getting out of the hole.

[wingssail]

Originally Posted by JustMurph

You must have a very small main!

BigB said he has 4.3kw of solar. at 10-15 watts per sq foot that is 360 sq foot, roughly.

I have a 43' racing boat with a huge main. It is 317 sq ft.

BTW yesteday I was onboard a new PJ 42 catamaran. The flat hull sides were roughly 6' by 40', rectangular, so that is 240sq ft, plus the portion of the other hull which would be exposed to the wind, and not even counting the cabin, I reckon that this boat would present more area to the wind as drag than the area of it's sails, not counting 1200kw of solar array.

Originally Posted by JustMurph

As Adelie mentioned above, larger props are more efficient. E.g. if you over prop a diesel, you will slightly reduce top end but will gain a decent chunk of efficiency.

Regarding the efficiency of electric setups, they are generally specified for a lower top speed/peak power than diesels typically would be, thus the power/speed duty point that the prop is specified for is quite close to your cruising speed (and hence more efficient). On the other hand diesels will be cruising at 50% or less of their rated power, so the props are also further from their specified duty.

Actually, even if the prop is specified to absorb the full power of the diesel it will still be completly efficient at half throttle (yet not as efficient as a larger diameter propeller, of course) The efficiency of our diesel (miles motored per gallon of fuel) is much more efficent at slower speeds.


Ahh, you're way off the mark there mate. The standard diesel setups in most boats are sized for "just enough power to get out of harms way in an emergency".

I think most standard diesel set-ups are sized to deliver reasonable cruising speed under power and it has nothing to do with getting out of harm's way, however sizing for the one objective usually meets the needs of the other.



Most are not overpowered by any means. Many are already border line underpowered. Pulling even more power out of them with electric setup will absolutely come with compromise. Whether you can live with that is up to the individual and their use case. Day sailer, give me some little electrics. 100%. Blue water cruiser, there's no way I'm reducing HP.

Many "blue water cruisers" I've seen seem to be way overpowered. I've often seen 45' boats with 80 or 100 hp engines. yet my 43' boat is fine with a 34hp engine and I can motor at 5 knots into 30 knots of wind. But in thirty knots of wind, even on the nose, I'd rather be sailing, it is easier, quieter, better for the boat and the drive train, and more effective.

E.g. A few years back I was sailing on a Lagoon 421 and we had to get directly upwind in 30 knots and short sharp chop. Seeing as that thing couldn't have sailed to windward to save its life, we chose to motor. With the pair of 30s pegged at WOT we barely hit 4 knots. If you believe the marketing and electric horses are bigger than diesel horses, a pair of 10kw electric motors should be "functionally equivalent".

Who here honestly believes that that Lagoon 421 with 20kw of electric propulsive power instead of 44kw from the diesels, would have still been doing 4 knots, let alone moving at all?

Big catamarans present a lot of windage and motoring into a breeze, with or without chop, is going to take some power. My friend's FP is standard with twin 30hp Volvos. Seems marginal. He opted for 50hp engines.

[JustMurph]

Let's talk about power and torque.

Torque doesn't do any work, power is what does work (i.e. actually makes something move). Power is calculated with the following formula:

Power (kw) = Torque (Nm) * rpm/9.5488

So, if you have a flat (constant) torque curve, power will increase with rpm.

Electric motors typically have a very flat torque curve to ~2/3 of the way up the rev range, at which point it will drop away. Diesels have a torque curve shaped a like a hump, gradually increasing, peaking somewhere in the middle then gradually dropping away.

The thing is, the diesel torque curve is actually pretty flat. I found the curves for a Beta 30. It makes 55lb.ft of torque at 1000 rpm, a peak of 66lb.ft at 2500rpm and back down to about 58lb.ft at 3500rpm. The torque deficit at low rpm to an electric is pretty small.

[JustMurph]

Wingsail: With the modern panels that BB is using, he'll get 4.3 kw out of 22m^2 (235sf).
The outputs available now are pretty damn impressive.

Agreed on your first point. What I should have said was that if the props on a diesel were sized for 50% of peak output (and lower revs), then they would be more efficient at that point. And yes, some boats are way overpowered. Maybe they would get away with smaller electrics just fine.

[jmh2002]

Originally Posted by CatNewBee

Went from Cartagena to Linton Bay, had to motor the 260nm to the wind. Run 3 days / 2 nights on one engine Yanmar 40hp, burned 240l of diesel at 2500rpm and 5...6kn approx. 4l/h (one tank empty plus 40l reserve in jerry cans).

You might be able to provide some insight in this other thread here too:

Motoring in Lagoon 380
https://www.cruisersforum.com/forums...ml#post3501351

[catsketcher]

Originally Posted by JustMurph

This isn't really correct. Yes, the electric produces a lot more torque at very low revs, that's true. The thing is, the diesel in your sail boat doesn't need to. Unlike a car, you have a prop which allows slip through the water, letting the engine rev to where it does make more power. Flat chat at 10 knots my engines will rev to 3300rpm. Stationary, they'll rev to about 2700 rpm (IIRC), which is beyond the torque peak and would be pretty close to peak power.

This is different for power boats with big diameter and pitch props that have a lot of grip, eg my mates ski boat which will only rev to ~2500rpm getting out of the hole.

I get the idea of slippage but props will have their efficiency range too. I know little about this but my mate's ability to gear down his prop, reduce prop rpm but increase torque seems to be more efficient at high load. I get that cars have no slippage and so any insights from them are suspicious, but my mate's experience would suggest that a highly loaded prop with lots of slippage is less effective than a prop doing what it is designed to do.

His engine was pretty small, a single 15hp for a 32ft cat. So maybe that is one issue too. For smaller motors, maybe there is a problem with loading that larger motors don't get, allowing the large motor to get to its torque band and good cooling. Lots of modern boats have larger diesels than they did when I was a kid. But the smaller diesels were adequate almost all the time, especially for motoring in most conditions and calms.

Also, I don't know that the test when stationary is a similar state to that under a large headwind load. An engine has to output lots of power just to keep you stationary in a big wind, to get you to move forward it must then push much harder. So you could find that the best way to test this is to find out how fast you go backwards in 30 knots, get towed at the same speed backwards and then test your motors ability to get up to full power. I feel this is a more representative simulation than tied to the dock. Motors do seem to more commonly overheat in pressure situations in high winds, suggesting the motors are struggling.

Of course real word data is needed here, but I can still see a few ways that could make the electric motors better for our slow boats in terms of efficiency than ICE. So I am not ready to reject the idea that one could reduce the power required when changing to electric.

[wingssail]

Originally Posted by CatNewBee

Went from Cartagena to Linton Bay, had to motor the 260nm to the wind. Run 3 days / 2 nights on one engine Yanmar 40hp, burned 240l of diesel at 2500rpm and 5...6kn approx. 4l/h (one tank empty plus 40l reserve in jerry cans).

Would be totally impossible with electric propulsion. Even with diesel a challenge.

Yes, but what about sailing?

[BigBeakie]

Originally Posted by JustMurph

Wingsail: With the modern panels that BB is using, he'll get 4.3 kw out of 22m^2 (235sf).
The outputs available now are pretty damn impressive.

Agreed on your first point. What I should have said was that if the props on a diesel were sized for 50% of peak output (and lower revs), then they would be more efficient at that point. And yes, some boats are way overpowered. Maybe they would get away with smaller electrics just fine.

For clarity & accuracy, we have 12 x 325w panels plus 2 x 270w panels = 4, 440w ( 4.44kW) nominal.

The 325w panels are 2 m2 (21.8 ft2) each, the 270's are 1.2 m2 ( 13.1ft2) each which totals 27.4 m2 or 298 ft2.

The way the internal cell connectors are arranged, the best configuration for us by far is series connected arrays of 3 or 2 panels/array, and where there is mast/boom shading they are orientated side by side ( long edge) athwartships.

Regarding wind performance of EP, OV modeled that for us using an experienced NA in this type of analysis and using our designs square meter windage calculations. We looked at 15 & 30 knot direct headwind, with associated coastal sea state estimates, and at what wind speed we'd be going backwards.

Into 30 knots was good, into somewhere between 50 knots and 55 knots, not good. Our cat has fairly high windage and we over estimated it to be conservative.

Will let you know how it works out.....

There is a charter company in Mauritius with 2 identical cats, one with 32HP diesels and one with SD10 Oceanvolts. I emailed back and forth with them and the electrics were better against wind and sea state, was their evaluation. The comment was the diesels slowed down when pushing through short swell, the SD10 drew more power when going over/through the swell but maintained speed better. The torque would explain that.

[seandepagnier]

Originally Posted by JustMurph

In the situation mentioned above, dragging anchor in a storm on a lee shore, the first thing I'm doing is starting my motors, not raising my sails.

This happened to me a bunch of times, and the correct thing to do is raise the mainsail with the 3rd reef in. This gives you just enough power to slowly make headway upwind. If needed you can add a small headsail, even a storm jib. I don't know why you think you would start an engine first: this is "poor seamanship" but more importantly a bunch of boats get lost this way every year because the engine won't start.

In fact, the sails should always be raised before raising anchor or in many cases also before untying dock lines.

As for electric propulsion, it really complements sail. It should generally not be used for more than a few minutes at most in more than in 5 (or 3) knots of wind. Which ensures it uses minimal (solar) power and can get you out of wind shadows quickly. The rest of the time the sails can give good speed.

Even in narrow locations, such as under bridges and canals with lots of traffic, still always using the sails, the electric motor can give more confidence just in case a bunch of powerboats get in the way. This is the biggest reason for electric propulsion. For example: I have no problems navigating anywhere with a sculling oar and have never had a collision, but I accept that some people have boats too big for them to manually power, so in their case they could use electric in the way I use a sculling oar and gain the same level of control but without risk of draining even small sized batteries and recharging from solar

My sculling oar is equal to about 100 watts of efficient electric propulsion (70-80%), or 1hp of engine. To give you an idea how inefficient engines generally really are of 15-30% thermal, times 20-25% as well as stuffing box and other friction the overall efficiency from fuel is < 10% usually < 5%. Going from solar power to fuel also is < 1% efficiency generally.

I can scull against 15 knots of wind on 27ft mono, or 10 knots of wind 33ft trimaran, though this is generally only a few hundred meters in a harbor where tacking, or raising the sails is more work than sculling.

Speaking of kw of power is really the wrong units. The motors in most cruising boats should be in watts (< 1kw) for the most part at how much power they should be consuming if running more than a few minutes.

Once again, the only argument for diesel engine is motoring against strong winds for extended duration. There is no reason to do this, it doesn't help anyone and it's bad for everyone else in the world to waste the energy doing this. Why not save the fuel for future generations to do essential activities like food and medical delivery? Use it to make synthetic products etc, but when people speak of just wasting it because they are unwilling to raise a sail, this is a real big issue and problem.

It makes cruisers in general look like a bunch of spoiled and selfish fools born with silver spoons (most of them from privileged backgrounds and so are the least deserving of comforts) and little to no consideration for the long-term effects or of the future. With 25% of coral reefs gone, and on track to lose 90% or more, every difference matters. Raising a sail instead of motoring is low-hanging fruit and should be exploited. Other measures are more difficult, in the near future (and already) people are making far greater sacrifices.