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Old 13-03-2008, 08:50   #1
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How much electric power is needed to power a 40 - 45' cat at 5 knots?

Hello,

I want to by a relatively light cat of 40 - 45' and 24' beam with fine hulls and inclined deck house to reduce wind resistance (not straight up like the Lagoon cat for example). I have 3 cats to choose from and the weight will be between 8 and 12 t. As I don't want to have fossil power on board, I want use electric power and electric motors to propulse the cat. How much power is needed to attain 5 or 6 knots on relatively but not completly calm sea and a light wind on the bow?

I know of cause that it will depend on a lot of parameters like weight, fineness of the hulls, efficiency of the batteries, engines, propellers and nobody can give me an exact figure but maybe you have an idea. But would you say there is no way to achieve that or will it be possible with an efficient propulsion system? I found a lot of figures very diverging. According to some this would be possible and according to others not... Is there a rule of thumb how much power I need per metric ton to attain that speed?

Thank you in advance,

Kai
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Old 13-03-2008, 09:08   #2
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Originally Posted by KHK007 View Post
I know of cause that it will depend on a lot of parameters like weight, fineness of the hulls, efficiency of the batteries, engines, propellers and nobody can give me an exact figure but maybe you have an idea.
While in principle I love the idea of electric power, I wonder if it's really practical for a boat this size. You are going to need the auxilliary for more than just getting in and out of the slip. What if you need to claw your way off a lee shore? You're going to need a lot more power than I suspect you could deliver with electric only.

Besides, how will you keep the batteries charged? For the kind of heavy-duty use you're talking about you would need - ironically - a good solid diesel generator. That sort of defeats the whole purpose.

Not saying it's not possible, mind you, but I think you are right when you say it depends on so many factors. You'd have to take an educated guess and then experiment.
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Old 13-03-2008, 09:15   #3
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18 hp should be more than enough
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Old 13-03-2008, 09:29   #4
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Hi,

I know that in the end I can only try to figure it out with the whole system. The most difficult decision is to decide on a cat (hull). All the other parts can be changed quite easily. I have a masters in experimental physics and crossed 3x the atlantic and would really like to try it testing different system components like a case study. But if some people who own cats with electric engines like the Lagoon 420, Fastcat 435 can give me an advice on how much power they need with which configuration (brand of batteries, genset, electric motors, propellers) it would be easier for me...

@gosstyla: On which data/experience do you base your saying that: 18 hp should be more than enough ?

Thanks,

Kai
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Old 13-03-2008, 10:08   #5
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Kai,

For displacement boats, the old figure was that it takes one SHAFT HP per ton of displacement to move a displacement boat to hull speed.

Over many years I've come to the conclusion that that's a bit too little. Maybe something like 1.5 or 2 SHP per ton is a lot more comfortable figure.

And, if you want to go more than hull speed, a little more power will get you a little more speed, up to the point where you're fighting the bow wave and then it would take a hell of a lot more power unless you're up on plane.

Bill
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Old 13-03-2008, 14:45   #6
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a usefull reference

This link will take you to an interesting site with a slightly larger catamaran using electric motors.
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Old 14-03-2008, 07:13   #7
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Interesting that the old Snowgoose 37 with it's standard 27hp is listed somewhere on these threads as doing 18kn under power. The hulls are slim, canoe sterned with virtually no wake. Like the old slipper hulls for river cruising.
Modern PRODUCTION boats just don't put the effort into performance. Fastcat should be able to give some more representative numbers with his approach to weight and performance.
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Old 14-03-2008, 08:36   #8
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Perhaps you should look bottom up as well as top down. Take a look at the various electric propulsion solutions already available and ask yourself if solution x is sufficient. Then you'd have some real world numbers to play with. I'd also look at current boats with both diesel and electric options to see the ratio of electric hp to diesel hp. Many of those numbers have been discussed on this message board. BTW 5 or 6 kts in calm seas probably does not provide you with enough power in foul weather. I suspect you should target no less than 7kts for flat calm, although you'd be out of juice in no time without a diesel generator.

Off the top of my head I can recall four electric motor solutions:
1. Leroy Somer (Lagoon 420)
2. Solomon Technologies (Lagoon 410)
3. Torqueedo
4. Glacier Bay (Robertson and Caine)


I'd also ask Gideon who makes his green motion system (African Cats).

Brett
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Old 14-03-2008, 08:55   #9
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LtBrett seems to have summed up the all electric approach. Limited battery capacity.
Fastcats have the same problem and i'm not sure about giving up sailing performance to recharge by 'dragging' generators. Diesel, either drive or generator only, seems the most efficient/safe way of carrying energy on board. Add diesel cooking and heating and use batteries as back-up power and there's a sensible solution - compromise somewhere. I read elsewhere that motoring for three days in storm coonditions is normal. Does that mean a weeks fuel (1 l/hr *24* 7 = 170 litres).
And how does this package cost out against solar panels / wind generators. Diesel is always available (until it's used up). Not for me until the revolution in battery and solar panels the electric drive of diesel genny has improved remarkably with new tech'y available now. Add the panels when costs come down.
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Old 14-03-2008, 09:00   #10
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Hi,

@LtBrett: I looked also bottom up already:

1 + 2) The Lagoon cats are to heavy and not aero- and hydrodynaic enough. They had to increase the power of their genset from 11 to 17 kW. 11 KW would be feasable with a cat of 15 * 8 m but not 17 under real conditions.

3) I have contact to Torqueedo but their motors are not strong enough as main engines. But I want to use a dinghy with a motor from Torqueedo.

4) I might check further Glacier Bay (Robertson and Caine).

And the gobal performance depends on so many factors that it might be easier and cheaper if I test different systems (of engines, charge controllers, solar panel brands) with a light and aero- and hydrodynamic cat than doing too many simulations which are in the end not correct. I know from the swiss sun21 group that it's possible to do what I want to do, it just will take more or less time and as I studied experimental physics it would be fun for me to test different systems and do a study on the subject...

I should be able to do 7 knots over a short period to get out of danderous situations but not for hours.

I asked Gideon who found the project interesting, but they couldn't deliver before the end of 2011...

Kai
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Old 14-03-2008, 09:05   #11
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So buy a cheap used cat to evaluate your simulations. 100kUS for an old Prout. Diesel on board already but good clean hull if you stay away from the broad berth Elite's. There is a serious future in the technology but I think you are ahead of the technology at the moment. Even a hobie play cat with electric outboards will help you to polish your simulations.
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Old 14-03-2008, 09:09   #12
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It make clear what I want to do:

I want to do a circumnavigation using only solar energy with solar panels and electric motors. I thought at first I could find find a shipyard who could build me a cat exactly to my needs. But the problems are that:

1) I'm having difficulties to find sponsors for my vessel as I can't finance it completly myself. To find sponsors for a lot of services is easy but the main sponsor for the vessel isn't that easy...

2) Serious cat shipyards have in general waiting lists of 1, 2 up to 3 years and it would cost a lot more than buying a used cat suitable for my project though it might not be as optimised as a new on build completly to my requirements. I hope to be able to achieve between 5 to 6 knots under normal conditions. There is a swiss shipyard which build the suncat21 which went from Spain to NYC last year but they build normally solar cats for lakes and I want to use a light oceangoing cat!

So, I'm looking at the moment for used sailing cats between 12 and 15 m Length and a beam of 7 to 8 m. Though weight is not the only criteria, standard motor cats are in general much to heavy.

Covering the surface of such a cat with high efficiency solar panels I should be able to get enough power to go at 60 - 70 % of the hull speed of 8 to 9 knots.

Normally, I'm a sailor and have crossed the atlantic 3 times in both directions and I know what I'm in for concerning weather, sea conditions. But I want to do this time something new and different!

If you know of any other cat with LWL 12 to 15 m and beam 7 to 8 m with a global weight of 6, 7 metric tons max and fine hulls to go easier through waves, let me know.

A nice weekend from Paris,

Kai
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Old 14-03-2008, 09:26   #13
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I was reading this tread with interest, trying to understand why a cruiser would do it, but now I understand:

Is it more of a project? Something fun to try?

Do you envision having a support boat for when it gets cloudy or rainy and you run out of "solar only" power?

Will it be a sail boat, or a power cat?
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Old 14-03-2008, 09:39   #14
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It is a project which combines beeing at sea and trying something new which will be fun too because I studied experimental physics with an emphasis on semicondutor technologies used also to produce solar panels.

No, I don't want to have a support boat. If it's getting cloudy or rainy I will use lithium battery blocks and if I'm running out of juice, I'll have to wait for sunshine. Many sailing vessels wait for wind on ocean crossings if there in none because they don't have enough diesel to do all under engine...

It will be based on a sailing cat but without sails. Power cats are in general too heavy with respect ot the quantity of energy at my disposal.

Kai
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Old 14-03-2008, 09:43   #15
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I say again, the old Snowgoose has very clean slender lines, 37' o/all, narrow total beam, aero clean up top too.
Use your sponsors money to upgrade the interior and add windows etc. Respect for your ideals but we all have to compromise.
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