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arn0 07-31-2013 04:18 AM

efficient electric river cat made of 2 eight rowing hulls
Dear builders,

First sorry if this looks like another fantasy, several time discussed...Could not find it through the search function.

I am considering building an electric river boat:

- electric, efficient, and very small wave for the environnement
- about 15m to 20m long and 4.4m large to offer some confortable surface for a family and still pass the locks on the french canals
- with very low draft (approx 30 cm) to enjoy every corners of the nature
- max speed approx 20 km / h and cruising at approx 12 km / h to be safe among the bigger boats and be able to go upstream on some rivers.

One option I am considering is to build it from 2 very efficient 8+ rowing boats, 17m long, 100kg each, which can go at 20 km / h with 8 people rowing (full displacement i guess).

I assume a 5kW electric motor might easily bring the power of 8 people rowing. Maybe a 2nd one (2 x 5 kw) for extra power if needed.

Buoyancy of 1 of those big hulls is approx 1000 kg, attaching 2 of them, adding the structure, engine and 20kwh of Lifeypo4 battery (200kg), solar cells, 100l water tank, etc. could leave 800 kg of usefull load if carefully designed

Tricks I see:

- cost of those carbon hulls (new are at 20,000 euro each. there are some used eight carbon rowboats at 8000 euros, one difficulty would be to source 2 of the same models)

- the limited buoyancy for a cruising river boat of that size make it a challenge to bring all the equipments and fournitures, surely at extra costs. Indeed the boat would weight around 1,200 kg instead of 10,000 kg for a barge that long.

I am missing some physics ? Would you know a similar project ?

Thanks for your feedbacks,


Mr Efficiency 07-31-2013 04:49 AM

Carbon fibre is simply overkill for something like this, the rowing shells are designed to be ultra-light to gain that little increment of extra speed, that doesn't matter so much for you, 1% slower for a rowing 8 is a game changer, for you it doesn't matter.

arn0 07-31-2013 06:19 AM

Thanks Mr Efficiency,

I totally agree on the 1%! But it seems to be a wide range of eight rowing boats, and I suppose they have much in difference... I agree I dont need the top racing ones.

On the carbon side, having the two 17m hulls for only 200 kg in total is interesting - considering the limited buoyancy - if it is available and affordable.

Would you recommend much appropriate hulls ?

gonzo 07-31-2013 06:22 AM
You can get shells without rigging that are not competitive for little or nothing. Sometimes they are giving them away.

FatBear 08-20-2013 05:50 PM

An 8+ is a very fragile hull. It should be very easy to find damaged ones for cheap. But you'd be nuts to use them to support a power cat. There's a reason they call these "shells". Think "egg shell". I have seen the forward 6 feet of one disintegrate during a broadside collision with another 8+ at considerably less than racing speed. Even if you used the basic boat and added lots of structural strengthening and additional layers of fiberglass and filled it with foam, you would still have a boat with just a couple of inches of freeboard.

upchurchmr 08-20-2013 06:36 PM

What will you be carrying that you need 17m of space?
Do you just want the image or do you have a purpose for all of this?

If you want efficiency, why not just build some short outriggers on one hull?
That way you won't need to search for compatible hulls.

FatBear 08-20-2013 07:42 PM

I don't think it is space he's looking for, just a really long, narrow, highly efficient hull. Or two of them, actually. I understand that he is on the French canal system so I think his idea is to make something roughly the size of a barge, but a fraction the displacement and with such low drag that he can power it with a very efficient electric motor. Do I have that right arn0?

5Kw is about 6HP which is a lot less power than the usual 120HP+ diesel engines in the barges. That would be amazingly efficient, but I don't think an 8+ racing shell hull is nearly strong enough. The inrush current when locking upstream would probably tear it apart.

Munter 08-20-2013 10:35 PM

The length would be quite inconvenient in most handling situations. I think there must be a better design solution with reduced LOA.

El_Guero 08-20-2013 10:52 PM

Instead of fixing the problems, why don't you use a true catamaran design, or even a pontoon boat?

FatBear 08-21-2013 12:46 PM


Originally Posted by Munter (Post 646798)
The length would be quite inconvenient in most handling situations.

Long and narrow is the norm on European canals. The width of Freycinet guage locks prevents boats from becoming wider and the height of many of the bridges prevents them from becoming higher, so if you want more living space you go longer. Many people buy former commercial barges and remodel them to pleasure craft - many of them incredibly well done and very comfortable. But they are heavy and use engines of 120 HP or more.

I do think that a lightweight boat of barge dimensions but without the draft will have a lot of problems with windage, though. In fact many barges converted to pleasure use are ballasted way down for just that reason. And I really think using 8+ rowing shells is asking for structural disaster.

Leo Lazauskas 08-23-2013 08:23 AM


Originally Posted by gonzo (Post 642744)
You can get shells without rigging that are not competitive for little or nothing. Sometimes they are giving them away.

I agree. Some old wooden shells might be tougher and free.
I doubt that a wooden shell would be torn as easily as this Empacher.

FatBear 08-23-2013 12:23 PM

Good example photo. The rigger of the one boat just opened the other like a tin can. And those riggers are rounded, not sharp.

The older wooden ones are certainly tougher, but still not up to the task. Plus most of them have been neglected so long, due to their non-competitive weight, that the refurbishment would be more hassle than just building something new.

FatBear 08-23-2013 12:33 PM

Speaking of building something new, why not build a pontoon boat? Or has this already been mentioned? They are very conventional and well-understood, at least here in the US. Two or three long aluminum tubes, cross-tied with stringers, each with the bow end tapered up to form a pointy "stem". You could do the same treatment to the stern end, too, if you think that would give you more efficiency. The larger diameter the tube, the heavier boat it will support. Build whatever you want on top of that.

For something really long you might want to weld on an aluminum T-beam to the top of the tube to reduce longitudinal flexing. That will help get the deck level up closer to the typical height of the side of a canal.

A friend built a much smaller pontoon boat out of used aluminum fuel tanks from trucks. He welded them end to end to make long tubes. But not nearly as long as you are talking about. [Edit: just a few minutes after I wrote this I saw him go by with his boat loaded with lumber for a construction project.]

GBL smallboater 08-31-2013 05:53 AM

The boats used to put down the course at Karapiro and Ruatanwha (the 2 NZ international standard Rowing venues in NZ) are made using very heavily glassed hulls produced in a mold that was used to make 8's many years ago. An outboard mounted in a rotating frame close to each end for high manuouverability. (they can go directly sideways slowly) If you could find a manufacturer that has an old/obsolete female mold you might be able to convince them to let you produce a couple of durable hulls. The freeboard is easy to extend upwards as the topsides of most rowing "shells" are generally vertical anyway. You might also need a reasonable rake on the bow to avoid harvesting too much weed. Or you could just find a really old 8 make it straight build a mold on it and go from there. . Hope this helps.Regards

GBL smallboater 08-31-2013 05:59 AM

PS. Many a rower has sunk a boat by putting a foot on the bottom of the boat to get in or out. Go Gel coat, 10oz, 10mm of chopped strand. Extend the freeboard upward.

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