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Please Help Me Build An Electric Boat Drive!

05/21/2012 2:11 PM

Hi all,

I am in the planning stages of building and designing an aluminum boat. I am planning on using an electric drive of some sort. According to my math it should weigh about 800 lbs with no load or propulsion system. This is an offshore boat w/ a small cabin. My goal for the propulsion system is speed. I am a novice but here is what I plan on using: 7.5HP 3phase AC motor, 7.5HP VFD (Speed control,forward, reverse), a 6.3 KW peak diesel generator to power it all(7.5HP*746W=5595) plus boat electronics, drive shaft and 14" prop, and rudder of sorts.($1800 for all-new and used) I was not thinking of using any gears and going straight from motor to drive shaft. I know about hull speed *theory* and this boat will not plane as it is deep draft for stability BUT think cat/trimaran slicing thru water. (Low wetted surface area and great prismatic coef)

Is this feasible? Or are there other ways of doing this easier, DC maybe (golf cart motor)? Generator-pass thru battery charger-DC motor.

Thanks so much for any help or new designs you all can help me with!

Lectric

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#1

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 2:47 PM

You might pick up some info here...

http://www.propulsionmarine.com/electric

Something like this?

http://www.andamanboatyard.com/picnic-21/

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#2
In reply to #1

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 3:09 PM

Thank you! Yes! Like that... Those are some very nice systems...let me add that money is an option.... or limited.. which is why I'm trying to design one and build it myself.(or with you guys help) I can buy the AC system listed above for about $1800.00. Will it work? I dunno...how easily can I build one of those systems listed on the website?

Thanks Solar!

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#3

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 5:25 PM

Have you considered the losses incurred by the system you describe?

Why not just drive the prop with a more conventional mechanical transmission?

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#4
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Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 6:57 PM

Well I imagine it's fuel costs...I would run this like a Chevy Volt, using electricity primarily with the diesel generator as a backup....It would be a lot cheaper to charge these batteries dockside or at home, then using diesel fuel all the time....my take

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#5
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Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 7:26 PM

Well,

I'm all for it if it's the way to go. I'm not sure 7.5 HP will push it very fast.

800 pounds is not light.

OP: How far out is "offshore"?

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#6
In reply to #5

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 8:04 PM

Thanks for the replies. I plan on going about 30 miles offshore in this boat and I want to use electric motors for torque. They produce more torque at a lower RPM.

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#7
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Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 8:24 PM

Will you have batteries or run the drive motors directly from the generator to save weight?

I think your generator is too small in either case.

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#8
In reply to #7

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 8:43 PM

I was planning on running AC straight to the AC motor. I am open to suggestions for DC motors and exactly how much horsepower will I need to push an 800lb non-planing boat 20mph. I recently saw a DC output generator which could possibly power a DC motor or charge a battery bank. Thanks

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#9
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Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 11:22 PM

Well to start off with it will no longer be a 800 pound boat once the propulsion system is in it. Most likely its going to be a 1400+ pound boat.

From there going 20 MPH on water is probably going to take 50+ HP!

If it was me and I was going for most the most efficient drive system with the least about of weight and volume I would be looking at hydrostatic drive using a variable displacement piston pump and motor combination.

It would have an overall mechanical efficiency in the mid to upper 90% range plus the widest range of speed control and power transfer with the least overall mass and volume of components. Plus there is no high voltage high current electrical gear to deal with in a wet environment.

But thats just me.

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#10
In reply to #8

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 11:30 PM

Well you lost me there, using a generator to drive an electric motor makes no sense...AC or DC....A diesel to drive the boat, or a diesel generator to charge a battery pack which in turn powers an electric motor, would make sense to save fuel, but would be very costly....So if you're on a limited budget, get an outboard....800lb boat is only what 14 ft? 20hp...

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#14
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 12:59 AM

I want to use electric motors for torque. They produce more torque at a lower RPM. I want to start out running by generator then slowly build up a battery bank/inverter system.

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#25
In reply to #6

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 3:55 AM

Torque is what the propeller presents to the motive unit, not the other way round!

Be aware that a propeller that is cavitating produces no more torque than a propeller that is operating just below the cavitation point. Once it is cavitating, the power must be reduced to avoid damage to the impeller and wastage of fuel.

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#26
In reply to #6

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 9:05 AM

Depends on the pitch of the prop.

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#11

Re: Please Help Me Build An Electric Boat Drive!

05/21/2012 11:38 PM

Probably worth a looksee - http://www.agnimotors.com

Used by http://www.electricboatsthailand.com/

Inventor interviewed at

http://www.dailymotion.com/video/x7oq1p_cedric-lynch-and-agni-motors-the-le_tech

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#12

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 12:19 AM

It is difficult to help you without knowing what kind of boat you want. Speaking of "not planing" and 20 Mph is only possible with a very long boat or with lots of power for your drive. When not planing, you have a water displacement ship or "verdranger schiff". This kind of ship needs to spread the water at the bow. This goes pretty well up to the hull speed but above this speed, the boat is building up a hill of water up front (la moustache) and you need power to overcome it and you are also lifting up the boat's weight "uphill". If you don't want a "planer" or "glider" an economic water line will start at 45 feet plus.

If the boat stays in the water (salt) make also sure that foresee anodic protection, because close to other boat with metal hulls it will turn into a sieve overnight.

We used aluminium and magnesium electrodes on cables all around our 70 Ft schooner.

Also disconnect the battery from the mass when not in use.

My whaler (14 feet) did 50 Mph with a 25 Hp Johnson and used 3 Gln/h, but in planing mode. There is NO way to get your 20 Mph and your power plan with a short boat without planing.

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#13
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 12:34 AM

Heard a great quote the other day - 'a couple of months in the lab can save you a couple of hours in the library'

Sounds like you want to 'remake the field'.

Don't hold your breath, but good luck. I recommend going with a proven solution, unless time & money are no problem.

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#15
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 1:02 AM

I have been raised in a maritime lab with test tank. I have put my comment off topic in case it bothers you. Just try to help.

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#16
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 1:14 AM

Hi DVM,

My comment was only to suggest that OP not try to reinvent the wheel. Not at all re your comments, no need to make them off topic!

Regards, Stuart.

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#17
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 1:47 AM

Thank you. I wasn't able to connect your answer to my post. Did you reply to it or to the OP? English is only a "borrowed" language for me and sometimes I do not get to fully understand the "between the lines" meanings. I tried hydraulic drives, electric drives and the most economic version was a chain drive without gearbox and a screw of Archimedes in a semi tunnel for buoyancy. But only good for speeds up to 7-8 Mph.

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#18
In reply to #16

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 1:49 AM

You made your reply to him rather than OP....You need to click on reply button on the post you are addressing....FYI

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#19
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 2:00 AM

I am designing a tri-hull with very sharp hull entry points. It is 10 feet long and has a modest cabin / bunks. Not your average boat... there is a school of thought that says there is no such thing as "hull speed" but rather speed/drag is determined by 'wetted surface area' and prismatic coeficient.

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#21
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 2:27 AM

Your post is about the electric drive.

I am not one of the school of thoughts you mention. If you design your boat so that it produces no waves, it has no hull and when you produce waves way longer than a referred waterline, you are planing.

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#31
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 7:47 PM

there is a school of thought that says there is no such thing as "hull speed" but rather speed/drag is determined by 'wetted surface area' and prismatic coeficient.


You won't find support for this "school of thought" among naval architects. Every hull has a theoretical hull speed, which can be exceeded or not depending upon hull form, displacement length ratio, etc. Only when hulls are very long and slender (knifing hulls rather than planing or standard displacement) (length/beam ratio of 10:1 and greater) does high prismatic coefficient have a beneficial effect. In most small boats, low prismatic coefficients are more efficient, because the shapes that make for low prismatic (pointy ends, full in the middle) also lead to lower surface area.


With 6.7hp (5000 watts), a Hobie 16 (minus the sailing rig but with addition of propulsion system) can do 9.5 knots. The Hobie hulls and crossbars are about 175lb, and I imagine this one has 150 lb of batteries, 170 lb of driver, 100 lb of other structure, and 75 lb in the electric outboard. 670 total.


To get 2000 lbs up to the same speed on the Hobie hulls, would require about 3 times the hp: 20 hp. Doubling that speed on Hobie hulls would require about 80 hp (because the Hobie hulls in that speed range increase resistance with roughly the square of speed. (Typical displacement monohulls are increasing resistance with the cube of speed in this speed range... thus it is nearly impossible to get them over their hull speed).


However, 2000 lb on 10 feet of waterline would be much less efficient that the Hobie hulls. A trimaran has much greater surface area that a monohull of the same displacement. Calculate the displacement-length ratio of your central hull, and you will find it to be unable to plane, and your slenderness ratio will not enable it to knife either. So you can expect to be in the standard displacement mode, where no reasonable amount of hp will get you up to 20 mph.

Do some observation before you buy aluminium. Notice that highly efficient non-planing hulls are very long and slender. The epitome in this direction is a rowing single, which is apt to be 50 lbs or so and 20 feet long, with a waterline beam of perhaps 12 inches. These can be pushed right past their theoretical hull speed, (using only a couple hundred watts or power) because of slenderness.


Short and fat is not the way to go, and with 10 feet of waterline and 2000 lb of displacement you cannot help but be extraordinarily short and fat.
An 8'x 4' foot hydroplane (with 150 lb driver) could do about 20 with 7.5 kW power, if you kept the boat light.

The diesel would be far more efficient driving the prop directly. The potential low speed torque advantage of an electric motor is of little-to-no use in boats, which tend to operate at a cruise speed that requires a particular hp. Getting that hp to the prop simply requires gearing: You can have a 4000 rpm 10 hp engine driving through a 2:1 gear box, or a 2000 rpm 10 hp electric motor driving at 1:1, and the prop cannot tell the difference. In a sailboat, there can be huge advantages multihulls that compensate for the increase in wetted area. Stability without great weight is the principal advantage, because in a sailboat stability determines power. In power boats, trihulls have fallen out of favor, because they really offer few advantages, and are less efficient that the alternatives.

If you've drawn up this boat, it would be interesting to see a drawing.

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#20

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 2:03 AM

Let's start from the beginning so we can all get on the same page....What is the length and hull design of the boat you want to build? What is the thickness gauge of the aluminum? What is the approximate dimensions of the cabin? How many people and total weight of gear estimate?

Maybe something like this...only a little shorter

http://tadroberts.ca/services/small-boats/displacement/pocketcruiser16

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#22
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 2:35 AM

Very similar Solar! 1/8 inch aluminum construction. 10' x 14' trimaran off sorts. Cabin above waterline and loaded out I want to be at 2000#. Boat 800, propulsion 500, 2 adults +gear 500, and 200# fuel. Again not your average boat.

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#23
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 3:13 AM

So far from you guys posts I have found my 7.5hp electric motor powered by a super silent generator isn't nearly enough to get me to 20mph no matter how much low RPM torque electric motors have. I guess its back to the drawing board.

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#24
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 3:33 AM

Either that or the local yacht chandlers to purchase a 7.5hp outboard motor weighing, oh, say 35kg tops.

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#27
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 10:38 AM
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#28

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 4:01 PM

This is interesting....land sea snow, 21 knots on the water...what's not to love?

"The new Archimedes™ Heli-RIB introduced at the February 2010 Miami Boat show by SkämtaMarin, of Göteborg, Sweden, revolutionizes the yacht tender. Unlike conventional rigid or inflatable tenders whose use is limited to operation on the water, the Archimedes operates with equal ease in water, across beaches, adjacent grassy areas and through muddy or swampy areas. For those planning a Northwest Passage attempt, it can even cross ice or snow.

The inboard engine supplies power to DriveBlades turning on a pair of bouyant cylinders called drive tubes that rotate in opposite directions. Baffle-type fenders protect the passengers from contact with the DriveBlades and spray.

The Archimedes Heli-RIB (helical drive rigid inflatable boat) can navigate areas previously prohibited to all except air cushion vehicles, but without suffering their load limitations, high noise levels and poor fuel efficiency. Not only does the Archimedes carry a greater load than comparable inflatable yacht tenders, it can carry this load to shore, and straight up the beach!

Amazingly, when fully loaded, Archimedes draws just four inches and exhibits a top speed of 21 knots. Take away the water and it still operates at speeds up to eight knots across marsh, sand, grass, and other relatively soft surfaces. Having no propeller and powered by a diesel motor, Archimedes is both safe and reliable in environments where other boats fear to tread.Viewed from almost any angle, the Heli-RIB looks like a conventional twin tube RIB. The Hel-RIB sports a lightweight carbon fiber hull with twin side baffles. The key to its ability to operate over virtually any surface lies in the design of the twin cylindrical flotation/propulsion tubes that are partially enclosed within side baffles or fenders that keep spray from passengers and the passengers from contacting the drive screw. I discovered that each of its almost full-length cylinders is made of a durable Kevlar reinforced membrane with a molded-in helical channel that spirals around the length of the tube. This channel provides the attachment point for the unique DriveBlade™ that gives the Archimedes its extraordinary capabilities. (Right and left tubes carry opposing helical channels). Each drive tube is attached at its forward and aft ends to an articulated suspension arm and bearing. The internal surface of each baffle has multiple low-friction Delrin rollers, each supported on liquid-filled shock absorbing dampers. The drive tubes are rotated in opposite directions by the air-cooled, silenced, diesel motor at the aft end of the center passenger/cargo compartment.I was impressed by the creation of a computer-controlled dual continuously-variable transmission to rotate the drive tubes in opposite directions (the port tube rotates clockwise, while the starboard tube rotates counterclockwise). All control is via a joystick, which directs the computer to rotate the drive tubes at different speeds for turning, and at slow speeds can even reverse one tube to allow the craft to spin in its own length, or crab directly sidewards.Heli-RIB marks the first time that a Helmholtz resonator active noise canceling system has been used in a marine propulsion application. By phase-controlling air flows, almost all intake and cooling air noise from the engine is cancelled in a manner similar to that used in high-quality headphones. Mechanical noise is also damped because the engine is in a double shell casing filled with inert fire-retardant oil, which is mounted on liquid dampers similar to those used inside the baffles.The entire drive system package weighs just 73 kgs, and may be fully detached from the Archimedes without tools in less than 10 minutes. This permits a spare engine to be carried aboard the mothership. An optional rechargeable electric drive system may be used in place of the diesel system to provide silent, limited duration operation for fishing or military use where silence is particularly valuable.According to company founder, Pavl Skämt, the concept for the Heli-RIB originated during his time working for Saab, the now-defunct Swedish auto company ditched in late 2009 by General Motors."I was working on a new design for an electric auto having both a continuously-variable transmission and a computer-controlled stability system," Paul told me. "We knew that being able to individually drive the wheels according to load and desired direction would allow better control, but being in Sweden, driving on ice is always an important consideration. That is when we stumbled upon the history of the helical drive system from the 1920s in the United States. One look at it, and we knew that it was a boater's dream, and so I started my company immediately. Working with Volvo Penta for the engine design, and with Anco Corporation in the US for the DriveBlade, we finished our prototype in just eight months. We are very happy to offer it to the world now."The Archimedes Heli-RIB is available for special order in lengths from 4 meters up to 7.7 meters, with load capacities up to 2,200 kg. Log on to SkamtMarin.com for more information or to order."

add on kit...

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#29
In reply to #28

Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 4:05 PM

If tou really want to get crazy:Fordson Snow Machine - 1929 Concept - YouTube

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#30
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 7:34 PM
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#32
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Re: Please Help Me Build An Electric Boat Drive!

05/22/2012 11:38 PM

Now there's a concept... If I can't top hull speed in my displacement boat...I may as well stick with the 7.5hp motor etc

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