Electric Propulsion on Catamarans

[deckofficer]

Here is a recap of electric outboards currently available,

Minn-Kota E-drive 2 hp $2900

Parsun (China) 5 hp $3000, they also have 4, 7, 10 hp electrics.

Ray (USA) has a 2.5, 4, 5 hp, same electric motor but 36, 48, 60 volt $5649 for 5 hp 60 volt model.

Aquawatt has a 13, 20, 22 kw models. A drawback that I don't care for is when switched on and out of gear, motor spins at 600 rpm to drive water pump impeller for cooling, so even though electric you still deal with the clunk in and out of gear for close quarters maneuvering. $8728 USD for the 13 kw model.

Torqeedo, range from 800 watts to 80 hp. $3800 for the Cruise 4.0.

Did I miss any?

[Stumble]

I will happily concede your experimental numbers are whatever you say you have measured. But I don't concede your implication.

Lets just get a little theoretical for a moment...

Equation for Volume Froude Number.
FNV= v/(g x dis^1/3)^1/2

v= velocity in feet per second (knots * 1.6889)
g= acceleration due to gravity (32.2)
dis= volume displacement in cubic feet (62.4lbs/cubic foot)

DISPLACEMENT HULLS operate at a FNV of 1.3 or less. SEMI-PLANING HULLS operate at a FNV of 1.o to 3.0. PLANING HULLS operate at a FNV of 2.3 or more.

So for your kayak
Let's assume you weigh 200lbs, and the kayak weighs 50lbs. This is 4cubic foot of water.
FNV=(5.5*1.6889)/(32.2*4^(1/3))^1/2
FNV=9.2889/(32.4*1.5873)^1/2
FNV=9.2889/51.42^1/2
FNV=9.2889/7.1714
FNV=1.2952

So at 5.5kn you are pushing a displacement hull kayak at the very edge of what displacement hulls can operate at. At this speed the kayak is pushing up the very steep resistance curve at the tail end of the curve.

On the other hand lets look at the hulls for the electric cat at 6kn. (Note: this was done without consideration for two hulls because it gets very complicated very fast due to the higher D/L ratio). But since taking advantage of this reduces the Froude number for the cat even further it isn't necessary for this example.

FNV= v/(g x dis^1/3)^1/2
FNV=9.2889/(32.2 * 105.977^(1/3)^1/2
FNV=9.2889/(32.2* 4.731)^1/2
FNV=9.2889/152.35^1/2
FNV=9.2889/12.3417
FNV=.7526

So unlike the the almost planing kayak the catamaran is operating at far below the peaking resistance curve for displacement boats. Meaning a reduction is speed won't reduce power demands at close to the same rate as the same decrease for the kayak.

To try and compare the power demands between these hulls is pretty stupid since they are so different from each other, but assuming it's what you want to do try comparing the power used at the same Froude numbers, not absolute speed. At a FNV of .5 the kayak is at 2.123kn (Catamaran 4.32) while at a FNV of .8 the kayak is doing 3.3969kn (catamaran is 5.8460kn).

So just go run your kayak at 2.15kn and measure the power draw, then again at 3.4kn and measure the power draw. Ideally the catamaran would use a proportional amount of power. In reality of course it isn't close to this simple, which is what NA are for, but it should give a range.

[deckofficer]

Greg,

You would think your law school has a better curriculum in hull dynamics than the maritime academy I attended. You have ignored length to beam ratios. When a commercial ship is lengthen and the beam remains the same, the displacement goes up but so does the speed for the same power.

Be sure to research block coefficient (Cb) for your comeback.

[mrm]

Originally Posted by nimblemotors

It is the 40Kw of battery that is the killer expense for anyone thinking about EP at minimum $50,000 in raw cells, then add in engineering etc.

Assuming 3.2V average cell voltage on discharge:

40 [KWh] / 3.2 [V] = 12500 [Ah]

adjusting for 70% available discharge range:

12500 [Ah] / 0.7 =~ 17800 [Ah]

rounding up, gives 18 batteries @ 1000 Ah each, which costs $18,000 at Balqon rates. Roughly 1/3rd of $50k, but still no small money.

[Stumble]

Originally Posted by deckofficer

Greg,

You would think your law school has a better curriculum in hull dynamics than the maritime academy I attended. You have ignored length to beam ratios. When a commercial ship is lengthen and the beam remains the same, the displacement goes up but so does the speed for the same power.

I didn't ignore it, I was trying to create a situation most favorable to your ideas. The L/B on the catamaran is probably in the range of 10:1, while the kayak is 1:6. So you are pushing a much less efficient shape on the kayak. If you notice I also ignored the fact that the catamaran has two hulls instead of one. Which also would work to prove my point.

But where you went couldn't have been that good of a school if you still think extrapolating the power curve of a 30' catamaran from two points on a 12' kayak is a reasonable approach.

[deckofficer]

Greg,


I'm getting a bit tired of being trolled. A rule of thumb is that for a double of speed requires an 8 fold increase in power. That is all you need to know to compute energy usage. If you drop speed from 6 kt to 3 kt, you will consume 1/8 the energy. On the SeaLand container ship Developer, if I reduce speed from 24 kt to 20 kt my 24 hour fuel burn would go from 210 tons to 110 tons.

[deckofficer]

Originally Posted by mrm

Assuming 3.2V average cell voltage on discharge:

40 [KWh] / 3.2 [V] = 12500 [Ah]

adjusting for 70% available discharge range:

12500 [Ah] / 0.7 =~ 17800 [Ah]

rounding up, gives 18 batteries @ 1000 Ah each, which costs $18,000 at Balqon rates. Roughly 1/3rd of $50k, but still no small money.

I used the price of $556 per 700 ahr cell. 700 X 3.25 = 2275 whr X .8 (rec. 80% DOD) = 1820. 40000/1820 = 22 cells X $556 = $12,232, but I didn't calculate for usable whr rather just the size of the bank quoted, so 40,000/2,275 = 18 cells, or $10,008.

Considering the current political climate in the Middle East, fuel prices will go up. By purchasing lithium batteries just look at it as pre-paying your fuel bill at a much lower rate for the cycle life of the cells.

[nimblemotors]

One can't really use fire-sale clearance prices for an analysis.
As a one-off DIY you can sometimes do things very cheap.
One can buy a wrecked Leaf in some cases for $1,000, and get a used 24kwh pack, although that secret it out, not that cheap anymore!
It does still add weight even if free, compared to liquid fuel.

[zerompg]

Buy your leaf now, drive it for 3 years, if your working a 3-4 yr plan and use the 24kw battery in a boat that needs new motors. FYI, I have about 84% capacity left in mine after 2 1/2 years.

I think the OceanVolt 15 kw translates to about 20 hp. That should work on a 40 ft cat, right?

It's not just trying to find a cheaper solution, its more a decrease in maintenance, complexity and problems. Who knows in 3-4 years I'd bet there are even cheaper 15kw-20kw motors that would be excellent replacements.

[tomfl]

Originally Posted by deckofficer

Greg,

You would think your law school has a better curriculum in hull dynamics than the maritime academy I attended. You have ignored length to beam ratios. When a commercial ship is lengthen and the beam remains the same, the displacement goes up but so does the speed for the same power.

Be sure to research block coefficient (Cb) for your comeback.

My law school did not have any hull dynamics courses. But I do play a sail boat guy on the internet and had an interesting exchange with Ian Farrier about hull design. I was discussing the load caring ability of my Seawind compared to an F39. Ian pointed out that the transoms of both boats (my Seawind do to after market stern extensions) were sitting at least a few inches above the water surface.

While I do play a boat guy on the internet I also understand that someone like Ian is the true master of hull design. Bottom line being, as we all know, some cats are dogs; and some guys like Ian can design a hull that will outperform a less well designed hull with an inferior engine.

I am still looking at upgrading my electrical system and the stumbling block is batteries. On my boat there is limited space for batteries not to mention the weight. I would love to replace my 9.9 Yahamas with Torquedos but really don't have the space or load carrying ability for the batteries.

[deckofficer]

Originally Posted by tomfl

My law school did not have any hull dynamics courses. But I do play a sail boat guy on the internet and had an interesting exchange with Ian Farrier about hull design. I was discussing the load caring ability of my Seawind compared to an F39. Ian pointed out that the transoms of both boats (my Seawind do to after market stern extensions) were sitting at least a few inches above the water surface.

While I do play a boat guy on the internet I also understand that someone like Ian is the true master of hull design. Bottom line being, as we all know, some cats are dogs; and some guys like Ian can design a hull that will outperform a less well designed hull with an inferior engine.

I am still looking at upgrading my electrical system and the stumbling block is batteries. On my boat there is limited space for batteries not to mention the weight. I would love to replace my 9.9 Yahamas with Torquedos but really don't have the space or load carrying ability for the batteries.

Even taking into account the 110 lb weight savings of the Cruise 4.0s over the Yamaha 9.9s? Plus the weight savings of 6.08 lbs per gallon of gasoline?

(16) 200 ahr LiFePO4 cells weigh 232 lbs. This would be a 10.4 kwhr bank and your Seawind should use less than 300 whr per mile, so that works out to 35 miles. Would that be enough?

[roetter]

There should be plenty of Tesla electric packs - 85kWh - available in a few years. They are warranted for 8 years though. May have to wait a while.

[Stumble]

Originally Posted by deckofficer

Greg,


I'm getting a bit tired of being trolled. A rule of thumb is that for a double of speed requires an 8 fold increase in power. That is all you need to know to compute energy usage. If you drop speed from 6 kt to 3 kt, you will consume 1/8 the energy. On the SeaLand container ship Developer, if I reduce speed from 24 kt to 20 kt my 24 hour fuel burn would go from 210 tons to 110 tons.

Clearly we will never agree. I prefer to work in reality, you just discount anything that doesn't agree with your pre-conceived notions. So I will stop trying to get you to see reality after this post.

The V^3 rule which you are using is only true when comparing very similar boats at very low speeds. Well below the point at which wave resistance is a factor. You are grossly favoring the idea that slowing down reduces power demands across the entire speed range, which just isn't the case.

At the same time you are greatly overestimating the output power of solar panels.

Frankly you are a fanatic, who has made up your mind up about what will work and refuse to justify your position. Throughout this discussion every time I run the numbers to show that you are wrong, instead of responding where I might have gotten the numbers are wrong you make personal attacks. Math doesn't care if I went to law school, or am a NA.

The reality is that the best Catamaran designers I know and have talked to don't consider electric catamarans to be realistic with today's technology. Feel free to build your boat and prove us all wrong.

I will leave you with this quote from Richard Woods "I still think solar powered electric motor cruising boats smacks too much of perpetual motion to be credible."

[deckofficer]

I'm glad to see you will troll elsewhere.

[Mr B]

The Tesla company Has Released all its Patents to the General Public,

It would have some Marvellous Technology, Previously unavailable, That will give other people ideas on improvements to Electric Propulsion,

The Advances over the next few years will be Tremendous,

It will also remove our reliance on Oil and Coal Fired Generating Plants,

I was looking into electric motors to power my steel cat, The Super Magnet Motors they were building and using, at the time, Were very very small compared to a conventional electric motor and pumped out the same HP,
As an exercise, My Cat is my own design, I was trying to run just one engine, and electric motors, Swinging a 28 inch Diameter Prop,

I simply gave it away, as I would have two equal sized motors for balance, and why add two extra electric motors to the gear train,

But those tiny little electric motors stuck in my head, An electric motor a foot in Diameter pumping out 25 HP. Boggles the brain what is actually out there and currently available at the moment,
I am not going into it, you can search for your selves its all on the internet, and freely available to all,

But I am very interested in a petrol engine in my boat, inboard, and running on Gas from a HHO Tank, which is filled with salt water and works like a battery, giving off hydrogen gas to run the motor, Very Very Safe, and would take the same space, but not the weight of three or four 8D sized lead acid batterys,
Maintenance on the Saltwater Batterys X 2, Its a Cat, One for each hull, Change the salt water every six months or so, Total cost for six months running, Zip, Zilch, Nothing, Free, No cost what ever,
This is a very controversial subject, Thats why I wont go further with it,

When I was a Kid, What is a Television set, Heaven, was the only thing above the clouds, Dick Tracy, the detective in comic books had a video wrist watch,
Fiction in those days,
Reality now,
And I am not that old either,
These super Magnets they are developing now, Will Have incredible uses in a few years, as they get smaller and more powerfull electric motors,