Jimmy Cornell goes Electric, with a Cat

[goboatingnow]

My own views are this

Electric motors are far better and more efficient then any 19th century “ bag of bolts “. The issue and will remain the extraordinary energy density of petrol and diesel

It will be several decades before we see widespread electric propulsion on sail boats Solar will need to 2x more efficient then now and the price of any system will have to be roughly comparable. Generation will need to have at least 50 % headroom to be acceptable widely

If that happens there will be a transition if not then other technologies may or may not step into the breach

The Diesel engine appeared at the turn of the 20th century , yet steam locomotives remained in use nearly 70 years later

Diesels won’t be disappearing any time soon ( as an owner of a Nissan Leaf )irrespective of what “ those on the know” may claim.

It’s certainly not something in the approx 20 years left for me on this planet that I’ll have to worry about.

[JustMurph]

Originally Posted by seandepagnier

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.

If I have my main stowed away with its bag zipped up.....it's going to take me a few minutes at a minimum to get it up and get the reef in. It's a big boat with a heavy hydranet main. Do you really think I'm better off letting the boat drag towards the lee shore for 3 minutes more, or have my engines started within 10 seconds? I know which I would do. Once that boat is held on the engines, then I can think about raising sails.

For the record, I don't like motoring. That's why I bought a cat that sails well in all conditions. I rarely go out in conditions where I'll need to motor.

Originally Posted by BigBeakie

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.

That's pretty good if one 20kw motor can push you into 30 knots. Mind you, 20 kw is a pretty decent size for an electric. If I followed the rule of thumb I'd have 2x10kw on my boat, which is only a little smaller than yours.

I'm thinking the secret sauce here is all in the prop selection. Regarding your props, was it a coincidence that the ideal selection for the diesel and the electric were the same, or did you compromise on one or both to have the same props?

[catsketcher]

Originally Posted by BigBeakie

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.

Thanks for that. That is a great comparison, 32 hp is about 23 kW. So the guys who said you need only half as much power were correct. I would love to know more about the why.

Maybe a hybrid setup would be the best for motoring and economy.

Cheers

[JustMurph]

Originally Posted by catsketcher

Thanks for that. That is a great comparison, 32 hp is about 23 kW. So the guys who said you need only half as much power were correct. I would love to know more about the why.

Maybe a hybrid setup would be the best for motoring and economy.

Cheers

If that really is the case, then maybe we've just had engine sizing, prop selection and gearing wrong the whole time. If you took a 10kw diesel, geared it to run at the same prop rpm as the OV SD10 and used the same prop, the performance would be the same.

[goboatingnow]

Torque curve for small diesel is very different to an electric motor and the delivered shaft horsepower is considerably lower then the equivalent electric motor.

Diesel engines effectively loose power as they load up ( as they slow down ) whereas electric motors simply draw more current

[JustMurph]

The torque curve, as I showed before is actually quite similar. The beta 30 produces 85% of its peak torque at 1000 rpm. That's pretty flat. It's also not relevant, because the prop slips, allowing the motor to rev.

Do you want to explain why the delivered shaft horse power is considerably lower for a diesel? There will be around 5% loss through the gearbox according to Yanmar's 3YM30 literature. That's not a lot.

[goboatingnow]

Well here’s the curve



Peak torque occurring around 2800 torque falling away as revs diminish meaning a loaded up prop will affect torque.

A typical electric versus diesel torque is more like

Thirdly shaft horsepower taking into account engine takeoffs like alternators at full load and sea water pumps can mean over 10 % reduction or more.

Furthermore temperature derating adversely affect diesel engine power.

[JustMurph]

The scaling on that graph is completely wrong. That's typical disingenuous electric marketing.

Having 15% less torque at low rpm won't matter, because at WOT the prop slip will allow the engine to rev to somewhere around peak torque.

Yes, you're right, having full load on alternators etc could reduce the power available. So in my 10kw vs 10kw, the electric would win. So, let me restate it, if you had a 12kw diesel and geared it to run at the same revs as a 10kw electric, with the same props, it would perform much the same.

[goboatingnow]

Originally Posted by JustMurph

The scaling on that graph is completely wrong. That's typical disingenuous electric marketing.

Having 15% less torque at low rpm won't matter, because at WOT the prop slip will allow the engine to rev to somewhere around peak torque.

Yes, you're right, having full load on alternators etc could reduce the power available. So in my 10kw vs 10kw, the electric would win. So, let me restate it, if you had a 12kw diesel and geared it to run at the same revs as a 10kw electric, with the same props, it would perform much the same.

But diesels have less torque at WOT.

[JustMurph]

Torque at the motor is irrelevant if they're making the same power and geared to the same prop shaft rpm. Torque at the prop shaft will be identical.

[goboatingnow]

Originally Posted by JustMurph

Torque at the motor is irrelevant if they're making the same power and geared to the same prop shaft rpm. Torque at the prop shaft will be identical.

This is not in dispute.

But diesels are not run at WOT , and their torque curves decline with reducing speed whereas an electric motor does not

Hence a 30Hp diesel is actually nothing of the sort , because derated for heat , a non linear torque curve and driving other mechanical ancillaries at typical cruising revs , it may be as little as a 15 hip engine

This is why electric advocates can claim as they do that a smaller electric motor hp ,for hp is sufficient for normal operation

[catsketcher]

When I read the Ocean volt site, it seemed pretty clear that they will provide an electric motor that will not do the same top speed as your current motor. 1 or 1.5 knots slower is suggested.

I would be happy with that. My cat can do 10 knots when light with a clean bum, but I only motor like that for 20 seconds every 2 years or so to give the engines a bit of a rev out. I need the grunt at different times.

I was out sailing today and realised that I want grunt usually when stationary - to reverse in a hurry, or to get going quickly to get steerage way. The third reason is to punch through chop and a big wind. I never care for my top speed. I only motor at 7 knots. But I have certainly used full throttle, many times, but always when slowed.

So this means I almost never get to use my full power. At low speeds the props are way to coarse pitch and a lot of torque is used top spin the propwash. The angle of attack of a prop blade should be around 5 -10 degrees but when I plant the throttle to get the boat going, the AOA would be much greater and the power used inefficiently by the prop. Same deal when plugging to windward in a heavy breeze. I spin up the motors to 70-80% and chug into the chop, doing about 6-7 knots, prop pitch too coarse again and more inefficient.

A coarse pitch at low speeds uses a lot of energy to turn the propwash, not using a highest proportion of torque and power into generating thrust forwards. A finer pitch prop has its lift angled towards the bow of the boat more than a coarse one, for us slow motorers, fine pitch should be desired.

So if I could motor, with far more low down grunt, I could max my boat out at 7.5-8 knots happily. Then, because I don't worry about over revving the engine when flat out at 10 knots, I can fine the pitch of the props. This means I have less AOA when the boat is stationary and more efficient props. On top of that, with electric, I get torque at much lower rpm and so don't have to wind the engines out, and cavitate the props, to get the boat moving.

I would like most of the torque to be producing a forward component from the prop, not mostly rotating propwash as will happen more readily with my coarse pitch ICE props.

With the price of lithiums coming down so steeply, I am now seriously looking into the idea of twin electrics powered by the batteries and having a small genset for long term motoring. Sounds like the most efficient setup to be had. By the time my motors need replacing in a few years it may be the way to go.

cheers

Phil

[Dave_S]

To propel the boat at a certain speed you need to be moving a weight of water in the opposite direction, this does not change from ICE to EP. A variable pitch prop would give you excellent advantages on both forms. If you want to cruise at 6-7.5kn you will use a high % of your electrical energy store whereas if you do the same with ICE you will not. EP seems to be ideal/best solution if you use it very sparingly, perfectly suited to day/weekend use with the occasional longer run where running a generator some days is ok. The better the boat sails the more suitable it is and vice versa.

The EP having instant power at 1rpm is of no benifit if you have a variable pitch prop.

Electric motors aren't magic, if you manufacture an electric motor to have maximum power at 1rpm, it will not have maximum power at 2rpm and above. I assume it will also have a predefined sweet spot for regeneration.

[donradcliffe]

If you want more thrust at slow speed, simply reprop your current diesels. As the life of a diesel is 40 years or 10,000 hours, it's going to be a long time before they need replacement. Replacing a working diesel with a motor, battery, and diesel generator makes no sense at all.

[Heath68]

Originally Posted by fxykty

With only a 1KW of solar, a wind generator that needs 20 knots to really produce, and 700Ah @12V battery, with all the usual consumers and an electric galley with two people living aboard, we do not replace what we consume on most days when it’s cloudy or we’re sailing north (in the Southern Hemisphere). We run a generator every second or third day for a couple of hours.

I figure another 1KW of solar will take care of the daily deficit most of the year, but I can’t imagine how much more generation we’d need if we also had electric propulsion. Not only dealing with house loads (which is hard enough), but then double or triple that to account for propulsion. Not every day of course, but sometimes several hours of motoring, or a half hour of emergency power, is needed. Well, unless your Sean or the Pardeys, but those are outliers to most live aboard cruisers we’ve met.

A hydro generator would be nice to take care of passage loads, but since we have a boat that can attain high speeds and accelerates (and decelerates!) quickly, a variable pitch prop is necessary and for a Watt & Sea system that means their racing version - prohibitively expensive.

Regeneration using the propulsion system such as OV has great promise, but if it doesn’t work when a boat is going up and down in ocean seas than what use is that? Though apparently Watt & Sea do work in lumpy conditions - what’s the difference?

And of course, if you park up somewhere remote for a while then hydro generation doesn’t help with house loads. JC was planning a high proportion of sailing, unlike most live aboard cruisers, but then Sailing Uma showed that you need frequent shore power or a generator to provide enough house and propulsion power.

Not a lot of solutions yet that don’t cost a whole heap of money, as opposed to energy dense diesel.

That's really interesting for us as we're just having 900w to our 1.1kw and we have the same bank of batteries too. I hope it'll be OK. I have also added a 5kw Fisher Panda for when the Commander demands we put the aircon con. I hope that won't be too often.
Heath
S/v Sawasdeekat