Sail boat engine choice - electric or diesel

[valhalla360]

Originally Posted by seandepagnier

This is true, but why do boats with diesel power not do these? My theory is because they can already do hull speed for prolonged periods with very inefficient drive systems so they don't bother optimizing prop efficiency, instead prefer the other advantages of smaller propellers. People who use diesel power obviously don't care about efficiency very much, since the engines themselves aren't very efficient.

Boats do use counter rotating props, kort nozzels, etc... where it makes sense. At slower steady displacement speeds, there is little efficiency to gain so no they rarely use them on displacement sailboats (variable pitch does sometimes get used but now you have issues with a complex bit of machinery permanently immersed on a corrosive high growth environment and a much higher up front cost). Where you do see giant props (relative to boat size) is tug boats. In this case they act a lot like the deep gearing in a tractor. They won't win a lot of races but they can generate very high static forces (static force is different from HP).

You could in theory jump up to a 3ft diameter prop and gain a little bit of benefit...but the cost of the prop in that size range is upwards of $7,000 vs a 10 inch fixed blade prop at around $400. Considering the cost of the engine and the amount of diesel burnt, it just doesn't make sense.

Putting the prop on an "elevator"...if your hull is otherwise conducive (we had a boat where the entire outboard did just that), that can work but still awkward with a 3ft diameter prop. On a boat where it's not conducive, it creates all kinds of problems.

Hull shape is usually the reason that limits prop size not some misguided hatred for efficiency. If you start distorting the hull shape to fit a massive prop, it creates more drag than the small efficiency gain from a larger prop.

Diesel engines are quite efficient. Particularly when you consider the losses converting the fuel source for a power plant to electricity, then loses transmitting it to the marina, then losses converting it to battery storage, then losses converting it back to electricity, and then finally back to mechanical power at the prop. Run the numbers and diesel doesn't look so bad (it does vary depending on the fuel source of the power plant). There just isn't a lot to gain in terms of efficiency with a boat.

But if efficiency is truly important, you need to do a comparison apples to apples and a diesel of roughly the same HP can turn a similar size prop....so in terms of diesel vs electric, talking about the prop is just diverting from the discussion.

[seandepagnier]

Thanks for the reply

Originally Posted by valhalla360

Boats do use counter rotating props, kort nozzels, etc... where it makes sense. At slower steady displacement speeds, there is little efficiency to gain so no they rarely use them on displacement sailboats (variable pitch

kort nozzels have the greatest efficiency gains at low speeds. counter rotating propellers give 5-7% efficiency increase at all speeds.

does sometimes get used but now you have issues with a complex bit of machinery permanently immersed on a corrosive high growth environment and a much higher up front cost). Where you do see giant props (relative to boat size) is tug boats. In this case they act a lot like the deep gearing in a tractor. They won't win a lot of races but they can generate very high static forces (static force is different from HP).

You could in theory jump up to a 3ft diameter prop and gain a little bit of benefit...but

every doubling in diameter gives 40% more thrust for the same power. This is not a "little bit" but after doubling twice (quadrupling diameter) you get a huge improvement. Larger propellers are always more efficient below 35knots on displacement boats. It is a physical fact that is largely ignored by cruising boats where the design clearly isn't to optimize efficiency.

the cost of the prop in that size range is upwards of $7,000 vs a 10 inch fixed blade prop at around $400. Considering the cost of the engine and the amount of diesel burnt, it just doesn't make sense.

The shape of the prop is completely different with increased size. The propeller can be made from carbon fiber or fiberglass. My 32 inch carbon prop cost $60. So a fiberglass propeller of 5ft diameter would cost about the same as a bronze propeller of 16 inches.

Putting the prop on an "elevator"...if your hull is otherwise conducive (we had a boat where the entire outboard did just that), that can work but still awkward with a 3ft diameter prop. On a boat where it's not conducive, it creates all kinds of problems.

This can work on any boat. I don't think it is awkward and you never have to worry about a line wrapping around the prop.

Hull shape is usually the reason that limits prop size not some misguided hatred for efficiency. If you start distorting the hull shape to fit a massive prop, it creates more drag than the small efficiency gain from a larger prop.

it is best to keep the prop far away from the hull. a typical inboard is just a poor compromise all around.

Diesel engines are quite efficient.

Most of the energy goes to heating water. This is not "quite efficient".
[quote]

Particularly when you consider the losses converting the fuel source for a power plant to electricity, then loses transmitting it to the marina, then losses converting it to battery storage, then losses converting it back to electricity, and then finally back to mechanical power at the prop. Run the numbers and diesel doesn't look so bad (it does vary depending on the fuel source of the power plant).

[quote]
refining oil into diesel is around 40% efficient. It creates more emissions at the refinery than the fuel eventually will. There is significant energy use in extraction and transport of fuel. The electrical losses you mentioned are significantly less even if the electricity is produced from oil.



I only generate electricity from solar panels.

There just isn't a lot to gain in terms of efficiency with a boat.

trimarans are a lot more efficient than monohulls, and a huge gain in terms of efficiency. In the Philippines all the boats are trimarans, and they go very fast with relatively small engines.

But if efficiency is truly important, you need to do a comparison apples to apples and a diesel of roughly the same HP can turn a similar size prop....so in terms of diesel vs electric, talking about the prop is just diverting from the discussion.

For a solar electric boat, efficiency is key. It is a relevant topic to discuss the right drive system when 99% of boats do not and seems to be ignored by both the diesel an electric sides.


The discussion of range has completely different numbers if the propulsion is efficient. I am using 120 watts to go 2 knots on a 33ft boat. This is not really anywhere near the other numbers listed for electric conversion. Everyone else is using far more power.


I use a high-aspect large diameter carbon fiber propeller: this should be the norm not exception.



There is not much room to improve electric motors or controller efficiency. batteries are already very efficient in round trip.


The areas where significant future improvements are possible:
1) battery capacity and cost

2) solar conversion efficiency and cost
3) drive efficiency

#3 is the one most easily possible for me to investigate and seems to be mostly overlooked.



It turns out, cars could also get significantly higher mileage with larger wheels. If they used 4ft diameter wheels they could get better than twice the fuel economy at lower speeds. This is obvious because bicycles already do this. So why not?

1) efficiency is not a key concern
2) they would have to go around turns much slower (unless the wheels can gimbal on 2 axes and the body of the car can adjust as well)

3) people don't want to change


It is #3 that is most infuriating and the same issue with propulsion efficiency in boats.

[malbert73]

Sean- Cool trimaran- what happened to Alexandra?

As for me I am at the stage in life with kids and work and light winds on Chesapeake such that a diesel is the only way for me to go. But I hope for this to be my last diesel - and next will be electric because batteries will have advanced and time will be more flexible.

[skipmac]

Originally Posted by seandepagnier

I am also an electrical engineer and have built hundreds of autopilots for cruising boats. I have built my own MPPT that today harvested more power into the battery than the panel was rated.

I think your math is wrong.

I got 320 watt hour lithium battery for $54 (free shipping) and bms for $12. This ends up being $660 for a 3.2kwh battery (and weight only 16kg) for those who want larger capacity. The prices are lower than ever.

Actually you just proved my math. 3.2 kwh battery bank on moderate sized cruising sailboat motoring even in a dead calm would last maybe half an hour. To motor for just 3-4 hours you would need ten times the battery capacity so almost $7,000 and 160 kg, assuming your weight spec is correct though it sounds a bit light to me.

Then there would be the issue of how to recharge a large battery bank. As I said, a large, wide cat might have enough space but certainly no monohull.

[newhaul]

Originally Posted by seandepagnier

the climate change "card" is not bs, but stating so is denying science and certainly invalidates many of your other false assumptions.

Actually the true science is very against MMGW .

You really should do your research .

Now natural climate varability is a real .

PM me and I can pass on several papers on the subject for you .
Let's not ruin this thread - Agreed, back on topic please :-) Pete 7 Moderator

[newhaul]

Originally Posted by seandepagnier

I got 320 watt hour lithium battery for $54 (free shipping) and bms for $12. This ends up being $660 for a 3.2kwh battery (and weight only 16kg) for those who want larger capacity. The prices are lower than ever.


.

More information needed .

320 watt hour at what voltage? Are you saying at 13.6 volts or 25ah
Or are you at the cell level 3.6 volt at 320 or 88 ah?

I would love to see what happens when you hit lvd or hvd with that 12 dollar bms.

Now what is the final voltage of your proposed bank?

Are the cells plastic or alluminum cased?

[TrentePieds]

Isn't it wonderful how this discussion has gone all over the shop with very little regard for what the OP said he wanted to do?

Th OP is Danish, as I used to be, and he sails in Danish waters in a very light boat designed by a Swede - Pelle Pettersen of Star-boat Olympic fame, no less - for RACING in the Swedish Skerries.

There is (generally) little current in Danish waters. The winds are prevailingly westerlies in the sailing season with obvious implications for waters where the "hidey holes" are predominantly disposed along coasts that run north to south. The highest hill in the country is something like 1500 feet ASL, and is, IIR, some 60 miles from the nearest shore. So forget about topographical worries like the ones we have in the Salish Sea.

From the medieval market and harbour town of Køge to the charming little yacht harbour of Vallensbæk Strand (suburb of Copenhagen) is a whole 10 NM - count 'em : TEN NM!! From there you go around the corner of Copenhagen's truck garden on the Island of Amager and - blow me down - 11 NM further on, I say again: ELEVEN NM, you tuck into the ancient fishing harbour of Dragør.

CarstenD is absolutely right: He doesn't need more range than a coupla hours. And it's a rare day in Danish inland waters in the sailing season that you don't get a nice mannerly westerly of 10, 12 of 15 knots.

I don't know what current regulations are, since I've been away for over sixty years, but just checking the charts I would have NO hesitation coming into the three harbours I mentioned under sail alone. Particularly not in so nimble a boat as Carsten's. Provided, of course, that it were innocent of roller furling and other such :-). It's tiller steered by birth, so that's half the battle won!

I have no idea where in Denmark Carsten might live, but let me assure you that ALL OVER the country, the distances twixt hidey holes are of the magnitude of the ones I have given.

Dockhead will bear me out. He's sailed those waters, although in a much bigger boat.

So there, Carsten :-) Let's us Danes stick together :-)!!

TrentePieds

[carstendenmark]

Climate changes are happening all the time, even before humans existed, believe it or not. Can we leave it out of this thread, please ?

@ TrentePieds :
Sorry that you left Denmark, I hope that it was worth it.
I lived for 6 years in Winterthur, Switzerland. I loved the skiing, the motorbikr rides in the Alps ! The swiss couldn't believe that I wanted to go back to Denmark. I said " sorry, no open waters in Switzerland".
Now I live in Langø (the former island of Langø ("Long Island")), a small fishing village with a very good marina, in the western part of Lolland.
Currents in the Big Belt just outside, yes, but well calculated maps tells you when and where.
There's a not-so-long narrow channel from the marina to the open waters (2m+ depth), so I need a motor, for sure. No tacking in that channel !

@ seandepagnier :
Much respect for the efficiency of a prop system put on the transom. But deplacement boat owners wants a prop that stays submerged, even in rough weather with high waves.
That means an inboard engine.

The bancas in the Phil's is not efficient in rough weather, because the bamboo outrigger system drags heavily whenever hit by a rough wave (and there are plenty).
The efficiency comes because of the narrow shaped hull. The outriggers are just for keeping the hull upright.

If I put a larger prop on my boat (with inboard), would you suggest me to use a submarine prop, a tugboat prop or an airplane prop style ? I can 3D print any shape.
Could you please provide information of your higher efficient mppt ?
Every Watt counts when sucking up energy from the sun !

@ Dockhead :
You're right, in a NEED situation, I would like my yacht to be the rabbit out of a hole.
At hull speed, in waves.
Electric would be fine for this because of the high torque. This makes me thinking of designing my system with an even larger motor (20KW v. 10KW) with a significant larger and (foldable) prop.

[TrentePieds]

Oh c'mon, Carsten :-)! If you keep the boat in Langø, getting out into Langelandsbæltet has to be a piecacake. And the reach - on most days - from there down to the south end of Langeland is all skittles and beer. From there to Bagenkop is all uphill, but still the whole distance from coming clear of Albuen to Bagenkop is how much? 15NM?

Now when I wuz a lad, we sailed Juniorbåde (not the little racing dinghy jobbie, but the Scowegian kind, the 3/4 scale version of the folkebåd, out of Odense Sejlklub just north of Næsby. This musta bin before motors were even invented. Beating out through Odense Kanal - mebbe 200 yards wide - the 4 NM to Klintebjerg, thence out through the shallows of the fjord to Nymindegap - now THAT teaches boat handling ;-0)!

TP

[valhalla360]

Originally Posted by seandepagnier

every doubling in diameter gives 40% more thrust for the same power. This is not a "little bit" but after doubling twice (quadrupling diameter) you get a huge improvement.

There's your problem. Thrust and power are different things.

Put a 40hp tractor up against a 400hp sports car:
- If you hook a rope between them and have a tow off, the tractor will easily win.
- If you measure MPG going down the freeway, the sports car will easily win.

The first is a measure of thrust.
The second is a measure of efficiency.

Of course, it's all a moot point in terms of comparing diesel to electric as you can apply any size prop you like to either and for a similar HP it will generate the same thrust and propeller efficiency.

[Dockhead]

Originally Posted by carstendenmark

. . . @ Dockhead :
You're right, in a NEED situation, I would like my yacht to be the rabbit out of a hole.
At hull speed, in waves.
Electric would be fine for this because of the high torque. This makes me thinking of designing my system with an even larger motor (20K W v. 10KW) with a significant larger and (foldable) prop.

I like it. You started off ahead of most of the electric conversion folks by at least imagining an electric motor of equal HP to the old diesel. The tendency of electric propulsion folks to use massively undersized motors baffles me -- why? There are no magic horsepower. When you need power, you REALLY need it. And larger electric motors are even less of an efficiency hit than larger diesels.



Large motor and the largest prop you can fit is all good. Bigger prop will more than make up for the slight efficiency hit of a larger electric motor.


I think you were the one who said it above -- even if we can agree that a huge disadvantage of electric is the energy storage issue, we can also agree that a significant advantage of electric propulsion over diesel from a safety point of view is its simplicity and instant availability. A big electric motor with a big prop, for getting out of some hairy situation -- yes, I would take that.

[Pete7]

Originally Posted by carstendenmark

This makes me thinking of designing my system with an even larger motor (20KW v. 10KW) with a significant larger and (foldable) prop.

What just don't. You have a slippery small yacht with I am guessing a deep keel. With good sails and an empty boat you can probably go to windward very well and far better than many of the yachts listed on here. Plus you sail in ideal waters, with little tide, tiny current of 1/4 knot and some shelter from the N Sea.

Don't wreck it with a huffing great engine, prop and battery bank, keep it light and upright, then it will be fast Buy a spinnaker instead, be much quicker and fun to sail.

https://sailboatdata.com/sailboat/maxi-87

[Dockhead]

Originally Posted by valhalla360

There's your problem. Thrust and power are different things.

Put a 40hp tractor up against a 400hp sports car:
- If you hook a rope between them and have a tow off, the tractor will easily win.
- If you measure MPG going down the freeway, the sports car will easily win.

The first is a measure of thrust.
The second is a measure of efficiency.

Of course, it's all a moot point in terms of comparing diesel to electric as you can apply any size prop you like to either and for a similar HP it will generate the same thrust and propeller efficiency.

Exactly so!

[Dockhead]

Originally Posted by Pete7

What just don't. You have a slippery small yacht with I am guessing a deep keel. With good sails and an empty boat you can probably go to windward very well and far better than many of the yachts listed on here. Plus you sail in ideal waters, with little tide, tiny current of 1/4 knot and some shelter from the N Sea.

Don't wreck it with a huffing great engine, prop and battery bank, keep it light and upright, then it will be fast Buy a spinnaker instead, be much quicker and fun to sail.

You're confusing diesel and electric motors. What is the difference in weight between a 20kW and 10kW electric motor? A 20kW brushless propulsion motor typically weighs 30kg or so, maximum 40kg. Won't move the needle, even on a light boat like the OP's.

[Pete7]

Originally Posted by Dockhead

You're confusing diesel and electric motors. What is the difference in weight between a 20kW and 10kW electric motor? A 20kW brushless propulsion motor typically weighs 30kg or so, maximum 40kg. Won't move the needle, even on a light boat like the OP's.

Well, yes but I was thinking of the total combined weight, bigger bank and prop etc. Having just got rid of the heavy old Volvo, shame to put the weight back in. That Maxi is a big dinghy, you could sail it straight into most harbours.

Perfect boat for doing that marina sailing YT video you linked to last week or so