Potential liveaboards

[RupertW]

Yes, I said in post #25 that energy density was a big problem at present. Diesel has about 100 times the energy density of many batteries. It has about 5-10 times the energy density of hydrogen (compressed or liquid). These are ballpark figures but enough to give an idea of relative positions.

Of course ICE isn't very efficient at extracting the energy even though we recover a small amount to heat water instead of throwing it away. That closes the gap considerably, making liquified hydrogen a possible replacement (purely on energy density, not practicality). Batteries still have a long way to go.

Assuming we solve that problem you then run into the re-charging issue already mentioned. Wind, wave and towed generation aren't really able to produce the required amount of power consistently. Solar is probably the best option to look at briefly.

Current panels are probably around 7m2 per kW. My boat has a 36kW diesel and even using only 10kW output would need a 70m2 panel, that's close to 8 x 9m and only vaguely feasible for a multi-hull.

Of course you really need a much larger panel because a 1kW panel does not produce anything close to 1kW. However, lets ignore that not so minor problem and assume we could magically capture all of the energy. It won't happen as there's a theoretical limit around 87% and current panels have a much lower theoretical limit of around 33%.

An impossibly 100% efficient panel would be around 10m2 to output 10kW. I might manage that on my boat if I could remove the mast and totally cover it with panels. Unfortunately, that only gives me power during the day and that is limited when cloudy. I'd really need about twice that area to allow for travel at night plus some for domestic use.

Not sounding good so far, especially considering that I'm assuming a level of efficiency about 3 times the theoretical limit for current panels (theoretical, not actual). It isn't even practical for a multi-hull with current technology operating at its theoretical output. We need a step change in panel and battery technology to make it anywhere near viable.

It sounds as if we'd also need to combine it with swappable battery packs to reduce the gap between usage and generation. A bit like topping up your fuel tank, just pull in to a facility and swap for a fully charged pack. It would be quicker than re-charging but a real pain unless you designed the boat with that in mind.

Your calculations make sense but only for running the motor at part-power (your 10Kw) in the sunniest times of the day - hence my calculation of assuming 6 hours of full output per 24 hours (spread out over 12-14 hours of daylight). Which makes it even harder.
I can’t ever see the swapping thing happening purely because it was the great hope for mass produced electric cars but the reality has been that batteries are so precious compared to the rest of the car that nobody would risk swapping good ones for bad ones.

 

[Mistroma]

All back of a ciggy packet calculations. I was only attempting to show that it didn't make sense even with wildly optimistic figure and that's ignoring the financial side. People often come up with comments about technology improving soon and such things becoming practical. You know it isn't promising if you work with the actual kW per m2 from the sun and still have trouble making it work and there's still little chance with things operating at theoretical limits.

Miles away in the real world I'm afraid when you need to be able to use full power for a significant period. Unless we get cold fusion soon.

 

[Laffan78]

On the graphene side - say I do a typical 120 miles at 5 knots motoring, so 24 hours - I’ve often done longer in a flat calm. So with my 45hp motor running at cruising speed thats 14Kw, so over 24 hours that’s 336 KWhr. For that I will need (using the 24/4 rough ratio to allow for daylight hours with less sun at either end) I need 56,000 W panels on the boat. I need a bigger boat.

Yeah, a 14kw diesel isn't comparable to a 14kw electric motor though is it? The diesel is probably only 20% efficient whereas a decent electric is around 90%. Meaning, even using a diesel generator to charge the batteries if you were forced to motor would provide a more efficient output.

Yes, I said in post #25 that energy density was a big problem at present. Diesel has about 100 times the energy density of many batteries. It has about 5-10 times the energy density of hydrogen (compressed or liquid). These are ballpark figures but enough to give an idea of relative positions.

Of course ICE isn't very efficient at extracting the energy even though we recover a small amount to heat water instead of throwing it away. That closes the gap considerably, making liquified hydrogen a possible replacement (purely on energy density, not practicality). Batteries still have a long way to go.

Assuming we solve that problem you then run into the re-charging issue already mentioned. Wind, wave and towed generation aren't really able to produce the required amount of power consistently. Solar is probably the best option to look at briefly.

Current panels are probably around 7m2 per kW. My boat has a 36kW diesel and even using only 10kW output would need a 70m2 panel, that's close to 8 x 9m and only vaguely feasible for a multi-hull.

Of course you really need a much larger panel because a 1kW panel does not produce anything close to 1kW. However, lets ignore that not so minor problem and assume we could magically capture all of the energy. It won't happen as there's a theoretical limit around 87% and current panels have a much lower theoretical limit of around 33%.

An impossibly 100% efficient panel would be around 10m2 to output 10kW. I might manage that on my boat if I could remove the mast and totally cover it with panels. Unfortunately, that only gives me power during the day and that is limited when cloudy. I'd really need about twice that area to allow for travel at night plus some for domestic use. It doesn't allow for running at full throttle for hours to reach shelter in an emergency.

Not sounding good so far, especially considering that I'm assuming a level of efficiency about 3 times the theoretical limit for current panels (theoretical, not actual). It isn't even practical for a multi-hull with current technology operating at its theoretical output. We need a step change in panel and battery technology to make it anywhere near viable.

It sounds as if we'd also need to combine it with swappable battery packs to reduce the gap between usage and generation. A bit like topping up your fuel tank, just pull in to a facility and swap for a fully charged pack. It would be quicker than re-charging but a real pain unless you designed the boat with that in mind.

Yeah, battery density was what I was talking about in my previous post.

 

[RupertW]

Yeah, a 14kw diesel isn't comparable to a 14kw electric motor though is it? The diesel is probably only 20% efficient whereas a decent electric is around 90%. Meaning, even using a diesel generator to charge the batteries if you were forced to motor would provide a more efficient output.



Yeah, battery density was what I was talking about in my previous post.

It is directly comparable - that’s what the power figure means. It generates 14KW of power at medium revs (even if the diesel fuel itself contains a great deal more energy than that).

I still want to know where I put my 56,000 Watt panels on my boat? If you have different calculations for the solar you think is needed to run a 45Hp engine at medium revs for 24 Hours then I’m genuinely interested as that is the main block to running electric propulsion.

Don’t get me wrong, I was an earlyish adopter of electric motors seven years ago for my tender and my Torqueedo outboard has been brilliant - all charged by solar but just pushing a 3m rib a couple of miles a day.

 

[Tranona]

There is much useful information on the subject in the long thread on Scuttlebutt about the Salona 46E, particularly the account of Ian from Nestaway boats who has converted his Sadler 29 to electric using the new regenerating pods from Epropulsion. He has run it long enough to get some real life figures. Key to me is cruising using 3kw gives 5 knots in flat water and a range of 30 miles (or 6 hours) with an E175 battery pack (175ah at 48v) that costs £3800. He has not been able to quantify regeneration yet. The total installation cost £10k not far off a diesel installation. This range would just about cope with, say a 2 day trip from Poole to the Solent and back before recharging. My benchmark requirement for coastal cruising would be 80 miles at 5 knots on a 33' boat like mine. So the gap is still huge an largely from the energy density of batteries and limited regeneration.

If you jump up to the Salona 46 though things start to look better with a claimed range (a bit vague!) of something like 80 miles at 5 knots allowing for some regeneration from the two pod drives. However the cost is £70k premium over the diesel version and the batteries and systems take up all the space where the engine, transmission and generator would normally be. Still it is a step in the right direction and if batteries could double the energy density, then it starts to become interesting. Normally most boat propulsion developments are spin offs from automotive or industrial, but apart from batterie there is little in that area transferrable to boats. Technology for steerable, regenerating pods is available and I see that as the way to go for the propulsion system just as in the cruise ship and ferry industry.