Switched to Electric
- Thread starter Kolibri
- Start date
... and the engine is not at full throttle at lower rpm and is not burning full fuel rate. It's not that the engine "is" producing more power at lower rpm, it is that it "can."
A number of people have mentioned that that it only takes X hp to reach hull speed and that more is wasted. Someday you will be motoring into a near gale and find that is not true. It takes x hp to reach hull speed plus y hp to push through waves and 30-knot winds, which is very likely more than double the hull speed requirement. Just sayin'.
The gap between a diesel's power curve at a given RPM and what a fixed blade pitch prop can absorb is a physical reality.
If you didn't have a gap you couldn't accelerate either
Yes, most modern sailboat diesels have more power than required to reach hull speed. There it lots extra for adverse wind/waves adding resistance.
If you didn't have a gap you couldn't accelerate either
Yes, most modern sailboat diesels have more power than required to reach hull speed. There it lots extra for adverse wind/waves adding resistance.
Just being pedantic for a moment, diesel engines do not have a throttle - that is a gas engine device. They have a governor, the lever controls the governor set point. Fuel delivery is controlled by the governor, it will provide just enough fuel to reach the set rpm (or, if overloaded, an excessive amount.... a problem electronic governors do not have).
More instructive is a fuel map for the engine, this is a 3 dimensional graph showing the relationship between rpm, torque output, and fuel input. Just grabbing a random one from the internet, this shows consumption in g/Kw-hr plotted against rpm and BMEP (which is proportional to torque). This particular one looks like an old mechanical injection engine, a common rail electronic one would have a more extensive sweet spot. In this case if you could load the prop more at 2000 rpm (with say a variable pitch) you would improve efficiency by a little (<10%).
Panoramix
Super Anarchist
Does the rated power tells you the whole story ? I have a bit of experience of underpowered boats. IME on a true sailboat that can actually sail upwind (as opposed to a motorsailer) when at a constant cruising speed you tend to not load your engine heavily for obvious consumption and noise issues so don't notice if it is reasonably underpowered. On the other hand when the engine is a bit small for the boat it really shows when you are trying to convince the boat "do something it doesn't naturally want to do" which tends to happen in tight areas. The engine will take time (may be actually it is just a second or 2 but it feels like an eternity) to take its revs. Obviously you can often plan ahead to avoid these situations but for instance with a side wind blowing you off it is nice to come in with some speed and kill it right at the end. If you try to do this on a 1980s IOR boat equipped with a 1GM and a folding prop, obviously this will not end well... solution is probably to find a less challenging spot.The gap between a diesel's power curve at a given RPM and what a fixed blade pitch prop can absorb is a physical reality.
If you didn't have a gap you couldn't accelerate either
Yes, most modern sailboat diesels have more power than required to reach hull speed. There it lots extra for adverse wind/waves adding resistance.
isn't it due to inherent diesel design rather than lack of grunt rated power ? Obviously diesels are meant to be used at a constant(ish) speed so not well suited to how auxiliary engines tend to be actually used.
A few things to think about:
- yes most people don't motor their engines at near full power or at the continuous rating. To take the 3GM30F the maximum rating (1 hr) is 27 HP @ 3600 RPM, continuous is 24 HP @ 3400 RPM. I motored around 2600 RPM a lot of the time to keep fuel consumption reasonable.
But there were times when it was at full throttle driving into an anchorage when it was blowing 40 knots. Lots of black smoke because the engine couldn't get the boat speed up to 6.5 knots where prop was pitched. Sometimes you do need that reserve, depending on how/where you sail
- taking a second to accelerate is normal. The engine has to accelerate the prop. It only feels like forever when you are drifting towards another boat. In reality the response is very fast and typically <1 second.
- no diesels don't have to be run at a constant RPM. They should be used under considerable load; about 50% of max power minimum for longest life/best cooling efficiency
- They don't "like" to accelerate quickly because the way they do that (on mechanically controlled engines) is dump lots of fuel fast into the cylinders. The black smoke is unburnt fuel as the engine tries to accelerate and catch up with the RPM you have chosen. In those few seconds, the amount of fuel provided to the engine is higher than what it can combust at the low RPMs it is at
- electric motors typically have max torque at 0 RPM. So instantaneous throttle response at low speed is very high. Great for docking a boat.
- yes most people don't motor their engines at near full power or at the continuous rating. To take the 3GM30F the maximum rating (1 hr) is 27 HP @ 3600 RPM, continuous is 24 HP @ 3400 RPM. I motored around 2600 RPM a lot of the time to keep fuel consumption reasonable.
But there were times when it was at full throttle driving into an anchorage when it was blowing 40 knots. Lots of black smoke because the engine couldn't get the boat speed up to 6.5 knots where prop was pitched. Sometimes you do need that reserve, depending on how/where you sail
- taking a second to accelerate is normal. The engine has to accelerate the prop. It only feels like forever when you are drifting towards another boat. In reality the response is very fast and typically <1 second.
- no diesels don't have to be run at a constant RPM. They should be used under considerable load; about 50% of max power minimum for longest life/best cooling efficiency
- They don't "like" to accelerate quickly because the way they do that (on mechanically controlled engines) is dump lots of fuel fast into the cylinders. The black smoke is unburnt fuel as the engine tries to accelerate and catch up with the RPM you have chosen. In those few seconds, the amount of fuel provided to the engine is higher than what it can combust at the low RPMs it is at
- electric motors typically have max torque at 0 RPM. So instantaneous throttle response at low speed is very high. Great for docking a boat.
carcrash
Super Anarchist
I agree with all the above points.
A given diesel propulsion engine connected to a prop is almost guaranteed to be far from its most efficient rpm.vs.load, and very likely never actually USES or DEVELOPS the rated power: the prop is turning far too slowly to absorb that much power, and so it does not consume the fuel and does not generate the power that is printed on the label.
That is why my motor using 4.5KW provides identical performance to a motor whose sticker says it develops 27 HP. The diesel never generated 27 HP, it actually clearly generated 4.5KW or 6hp. It was MUCH larger than needed.
I also agree with the observations where having more than enough power can be handy in certain situations. However, the prop still isn't turning anywhere near fast enough to generate the label HP in those situations either.
Hence, while KW being different from HP is fake news, what is certainly and trivially demonstrable is that a much smaller amount of electric power consumption is equivalent to a much larger rating label on a diesel.
A given diesel propulsion engine connected to a prop is almost guaranteed to be far from its most efficient rpm.vs.load, and very likely never actually USES or DEVELOPS the rated power: the prop is turning far too slowly to absorb that much power, and so it does not consume the fuel and does not generate the power that is printed on the label.
That is why my motor using 4.5KW provides identical performance to a motor whose sticker says it develops 27 HP. The diesel never generated 27 HP, it actually clearly generated 4.5KW or 6hp. It was MUCH larger than needed.
I also agree with the observations where having more than enough power can be handy in certain situations. However, the prop still isn't turning anywhere near fast enough to generate the label HP in those situations either.
Hence, while KW being different from HP is fake news, what is certainly and trivially demonstrable is that a much smaller amount of electric power consumption is equivalent to a much larger rating label on a diesel.
andykane
Member
You're describing an underpropped setup: your diesel engine was producing as much power as the prop would absorb. So yes, if you are using a prop that's too small for the engine then it will never produce the rated power. If your prop is the right size then it absolutely will - typically you size the prop to absorb the rated power at max rpm.
As an analogy, think of pedalling a bike. On the flat in a low gear it doesn't matter how strong you are, you can only turn the pedals so fast. To actually use your strength you need to increase the load by shifting up, or going uphill.
I will grant that people are likely much more comfortable running their electric motor at full power than their diesel (cause... it's loud and stuff). But power is power.
When designing the system the wind and wave resistance are added to the smooth hull resistance figure. A fixed prop is chosen in a compromise of motoring speed efficiency and limits on diameter.
Most lighter boats spin a small prop fast and save weight on all the machinery to spin a larger prop slowly. They are also pretty inefficient.
You are likely making a comparison between a poor design and a good one.
The prop can only absorb a certain level of power. It's not hard to calculate.
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floater
Super Duper Anarchist
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I spent a lot of time matching engines to fixed props on vessels where it's important to get it right.
On smaller boats especially sailboats with auxiliaries a common approach is that anything goes, anyone installing an electric propulsion system is going to do it with more diligence.
Fah Kiew Tu
Curmudgeon, First Rank
Yep. I like the idea of electric drives but this biz of a 10kW electric drive is the same as a diesel engine of 3X the (theoretical) output just because it's using magically more efficient electrons instead of carbon is pure wishful thinking, to be polite.
If a much smaller electric motor can replace a much larger diesel, the boat never needed that size diesel in the first place. So it was originally poor engineering design and nothing more, most likely driven by marketing.
Period.
I thought about this before I built my boat which is why I have a 3:1 reduction transmission and a big propellor - 22" diameter. I wanted to be able to transfer the engine power to push, not cavitate a small prop. Wouldn't matter what the power source was if you can't transfer it efficiently to thrust.
And before anyone goes to 'yes but you drag a big bucket when sailing' - it's a feathering prop so, no I don't. What I did was spend money.
FKT
This is not certainly not true and not demonstrable. You've already said you used the same prop for a 27 hp motor and a 6 hp motor. If that worked well for both, we would only need one prop for every motor on every boat.
Horsepower is a measure of work done per unit time. It does not matter if electrons, dead dinosaurs, or live horses are doing that work. What you demonstrated was you had a sick engine or poorly matched prop or both.
Z
There's a lot of different types of electric motors used all with different torque characteristics, even synchronous AC with variable frequency drive.
For a quick perusal of the state of play, have a look at the table at the bottom of this page:
https://plugboats.com/electric-inboard-boat-motors-guide-over-150-motors/
I crewed on an O 40 T-Pac waay back when they were brand new. I am certain we could motor a lot faster than 6 kts. Probably pushed 8 kts at high end.
floater
Super Duper Anarchist
on the return delivery I presume. in which case - they probably stuck a more adequate prop on for the job. I mean, if I catch the drift of this conversation - most of our diesels are oversized because our props are undersized. which leads me to..
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weightless
Super Anarchist
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I think there is some advantage to having very low rpm torque. Typically, electric motors that are used as auxiliaries can make arbitrarily slow turns. That's kinda nice. ICE aux engines usually get set-up with a minimum turns in gear that occasionally results in more boat speed than wanted. So, we bump them in and out of gear to get lower average turns. It works but isn't ideal. It also, isn't humbug enough to motivate installing fancier transmissions.
Steam Flyer
Sophisticated Yet Humble
Certainly a strong contender, but first the rating committee want to know how many 3-strokes and 6-strokes he's had.
- DSK
