Electric vs Diesel

[gosstyla]

Originally Posted by ad_astra

This is a big concern I have about my system, but there is a fairly simple solution: keep a spare motor controller in aluminum foil/the oven. It's probably the one spare part I should have anyway, and putting it in a lightning-proof location would allow for rapid recovery from the lightning strike......

A diesel/electric cat in the Ft. Myers area, at dock, suffered a near miss strike last month. All electric and electronic equipment was damaged. Not only the electric motor controllers but both gensets, HVC electric distribution panel, etc.

I don't know if the spare motor controller they had in the onboard Faraday cage was damaged, but regardless it will take more than replacing a controller to get those motors running again.

[skipmac]

Originally Posted by gosstyla

Interesting - electric HP is about 60% more powerful than diesel HP.

Must be that electric uses bigger horses.

Don't you know that diesels measure HP with ponies and electric motors use thoroughbred race horses?

The difference is not how many or what kind of HP, the difference is from the parasitic loads on a diesel engine. The diesel is using HP to drive an alternator, two water pumps, transmission plus the extra friction associated with each. Typical electric motor is a direct drive to the prop shaft so all the HP in the motor pretty much goes straight into propulsion.

So if you have a 65 HP diesel you may only have 75-80% of that available at the shaft to drive the boat. Replacing the diesel with an electric motor you can get away with fewer HP to get the same driving power.

Plus there are differences in the power curve between diesel and electric which allows less HP in the electric to give you more go.

[handmer]

Originally Posted by gosstyla

Interesting - electric HP is about 60% more powerful than diesel HP.

Must be that electric uses bigger horses.

That is an equivalent power. Not the best way to state it as it easily confuses people.

Ideally we need to be talking about thrust.
A faster spinning diesel motor loses significant power in the drive train. Then as little as 50% of that power actually gets turned into usable thrust. The rest is lost.

A an electric motor can be set up to directly drive the shaft, eliminating drive train loss. Since it can delivery full torque and power at a much lower RPM it can drive a much more efficient propeller. Much more of the eletric motors HP is converted to thrust.

The general rule I have read and found fairly applicable in my experiments is that for a good setup an electric motor needs half the HP to provide the same thrust as a diesel setup.

Originally Posted by gosstyla

it will take more than replacing a controller to get those motors running again.

For the majority of setups it's as simple as connecting your motors directly to the battery bank. Variable control is limited but all you need is two plugs on your main power lines. Or 2 minutes with a pair of pliers.

However I do agree that if I had an electric boat I would be putting serious consideration into lightning protection.
Or at least a way to step down the voltage and channel it into my battery (super capacitor) bank

[ad_astra]

Originally Posted by gosstyla

A diesel/electric cat in the Ft. Myers area, at dock, suffered a near miss strike last month. All electric and electronic equipment was damaged. Not only the electric motor controllers but both gensets, HVC electric distribution panel, etc.

I don't know if the spare motor controller they had in the onboard Faraday cage was damaged, but regardless it will take more than replacing a controller to get those motors running again.

So if a lightning strike knocks out the genset, how can it also not knock out a diesel engine, given that the genset is practically the same thing?

If a diesel engine can be started with the starter motor, the electrical system is intact enough to run an electric engine off of it with a backup controller or perhaps just a variable resistor....

So I think that diesel and electrically propelled boats are equally screwed by a lightning strike....

[brightnsunny]

For those worrying about range on an electric motor, I did manage to come down the river system from Chicago, UP the Ohio and Tennessee to the Tenn-Tom Waterway, and on down to the Gulf with an electric motor. Sometimes it is a case of wait for weather. Sailors should never be in a hurry anyway.

Now that I am on the Gulf I reduced the number of batteries I carry from 8 to 6 (golf cart batteries). This still gives me up to 8 hours run time at about 3 knts. Maybe more than I really want, but I'm used to it now. Usually I run the motor so little that my Honda 1000 is only on to charge my house battery and tops off the propulsion battery bank as an aside. My wish now is for more solar panels so I could surf the internet all day without the generator!

[gosstyla]

Originally Posted by skipmac

...So if you have a 65 HP diesel you may only have 75-80% of that available at the shaft to drive the boat. Replacing the diesel with an electric motor you can get away with fewer HP to get the same driving power.

Plus there are differences in the power curve between diesel and electric which allows less HP in the electric to give you more go.

Interesting. Looking at the specs for the Yanmar 4JH3-DTE it shows the Max out at Crankshaft as 121hp. It shows the Max power at the prop as about 119hp.

[gosstyla]

Originally Posted by handmer

...The general rule I have read and found fairly applicable in my experiments is that for a good setup an electric motor needs half the HP to provide the same thrust as a diesel setup. ...

Well your experiments produced much different results than mine.

The 2 33hp electric motors in my 47' cat had little, if any, better performance than the 44hp common rail, electronic controlled diesel powering my gensets would have provided if they and been used for direct propulsion.

[gosstyla]

Originally Posted by ad_astra

...So I think that diesel and electrically propelled boats are equally screwed by a lightning strike....

That is at least true for common rail and probably other electronically controlled diesel.

Older simpler diesels are not as sensitive, e.g. if they were running at the time of the lightning strike they might keep running - but there would a good chance after they stopped you wouldn't get them started again.

[Eleven]

Come on Fella's, the Mod's are listening.
HP is a fixed amount.
Typical diesel installations drive through a gearbox, lay shaft in bearings, to a prop that is sized for shaft revs and a target boat speed.
Typical Electric installations the prop is selected for the wattage chosen, which is electrical HP (torque actually) then bolted to a motor that will deliver that power at the right revs.
For a single speed diesel generator driving the electric motor this will deliver more push than the normal prop on a normal diesel installation.
If you gear down the diesel driven prop you lose some horse power to get a bit more push from the prop. If you don't gear it down you lose push at the prop because it's spinning too fast.
My drive leg, a double 90 arrangement, has a 1.6:1 ratio built into it. It costs HP but that's already lost by having the engine well above the water line. But I get a big prop. It sure does push but can run out of revs when motor sailing, so it's on a system to haul it out of the water. You can do this so easily with an electric drive, not thro the hull but slung over the back, hinged to lift clear when sailing, reducing drag, growth on the prop and lobster rope/net snagging.
Search Sillette ffor details, but it's not the best way now. A prop fitted direct to the submersible (water cooled) electric motor and mounted on a retractible system is the right way.
Battery will get you out of the Marina, a genny will take you a long way. Sailing is even better, especially without the handbrake on (prop being dragged).

[sigmasailor]

Originally Posted by gosstyla

Interesting - electric HP is about 60% more powerful than diesel HP.

Must be that electric uses bigger horses.

Strange isn't it? It can be explained by better efficiencies all along the line (click); mainly at the prop due to lower rpm. I was on Green eMotion last week and saw her cruising at 8.2 knots (on GPS, no currents) at full throttle. I myself found it hard to believe but did become a firm 'believer'; after having witnessed it in person.

[skipmac]

Originally Posted by gosstyla

Interesting. Looking at the specs for the Yanmar 4JH3-DTE it shows the Max out at Crankshaft as 121hp. It shows the Max power at the prop as about 119hp.

I think the operative word there is "Max out" at the prop. Engine makers are famous for eliminating all the real world loads from an engine to enhance their specifications.

If you can show me a real engine installed in a boat with normal accessories installed producing 121 HP at the crank that will give 119 HP at the prop I will buy a case of your favorite beverage.

Look at the data and specs on marine alternators. Depending on the size just the alternator can take 5-6 HP to drive. With no friction losses, no water pumps or any other accessories you are already over 2 HP loss.

[skipmac]

Originally Posted by gosstyla

Well your experiments produced much different results than mine.

The 2 33hp electric motors in my 47' cat had little, if any, better performance than the 44hp common rail, electronic controlled diesel powering my gensets would have provided if they and been used for direct propulsion.

Did you have an identical boat with a 44 HP diesel to compare to the 33 HP electric motors? If not, how do you know what performance that engine "would have provided" other than a guess?

Regardless, I think the point has been that slightly lower HP in an electric motor would give the same results as a slightly higher HP diesel. So 33HP electric might be a good equivalent to (not necessarily better than) a 44 HP diesel.

[DotDun]

Originally Posted by ad_astra

So if a lightning strike knocks out the genset, how can it also not knock out a diesel engine, given that the genset is practically the same thing?

Not necessarily. My drive engines require zero electricity to run after started.

Originally Posted by ad_astra

If a diesel engine can be started with the starter motor, the electrical system is intact enough to run an electric engine off of it with a backup controller or perhaps just a variable resistor....

A big difference in the amount of power required to start a diesel vs. run an electric motor for extend time. I would venture a guess that batteries and starter motors are some the more rugged parts during a lightning strike.

Originally Posted by ad_astra

So I think that diesel and electrically propelled boats are equally screwed by a lightning strike....

If I can start my drive engines, they'll run until out of fuel with no electricity. And the manufacturer offers a crank start kit as an option. Not that I would want to do that, even with compression releases, I'll bet that is a task.

[gosstyla]

Originally Posted by skipmac

....Regardless, I think the point has been that slightly lower HP in an electric motor would give the same results as a slightly higher HP diesel. So 33HP electric might be a good equivalent to (not necessarily better than) a 44 HP diesel.

The point is the electric motor needs electricity to run - the power required to make that electricity can used as effectively, or more so, to drive the props directly.

[gosstyla]

Originally Posted by Eleven

...For a single speed diesel generator driving the electric motor this will deliver more push than the normal prop on a normal diesel installation.

My experience contradicts that statement.

My experience was that if the diesel motor from the generator was used to drive props directly it could do so at least as effectively as using it in a generator to make electricity with which to run an electric motor to drive props.