Designing hulls for efficient slow motoring

[JonathanCole]

Originally Posted by mackid068

I suggested wave energy a while ago, what about a diesel powered...or electric OR solar boat with turbines on the underside to generate power from pushing slowly through the water.

It wouldn't make much sense under power because any turbine would introduce an amount of drag greater than the energy that you could generate. But for sail boats there may well be sufficient excess power for such a generator to make sense. It might also be worthwhile to think about an underwater generator for when a boat is anchored in a current.

In order to generate in water the turbine has to capture a large volume of water at a low speed or a small volume with high velocity. If all you can do is trickle charge your batteries, a small wind turbine or a few photovoltaic panels would be more cost effective and easier to maintain.

[BillyDoc]

Hi Jon,

Two things:

You shouldn't need to use gears to change prop direction, you can do this with your controller if it is properly designed.

Also, your prop and motor can be used to charge your batteries if there is a (water) current sufficient to turn them. If you were sailing with a good wind the odds are very good that the vessel speed would be limited by hull length and other similar factors, not the amount of available energy from the sails. Which is to say, you would be able to charge with effectively "no speed loss" due to the extra drag the charger causes.

This is, however, not as simple as it might seem. One of the rules we have to live with is called Lenz's 2nd Law, which to be very brief says that if we actually draw a current from something like a generator, then the "back electro-motive force" equivalent to that work will act like a brake on the generator. Any kid who has had a generator that rubbed against the tire for the light on his bicycle has experienced this, you switch on the light and the extra drag darn near throws you over the handlebars. (So, OK, I was a wimpy kid.)

All of which means that if you were to, say, just hook an alternator up to the prop shaft and connect the integral regulator to your battery --- it won't work. This is because the regulator (being quite dumb) is assuming that there is sufficient torque available to turn that alternator under all conditions, so it will effectively try to charge the battery at the maximum available rate. This will, in turn, bring your prop shaft to a halt and nothing will get done!

The answer is, of course, to build a "smart" controller that can sense both (electric) current and rotational speed and adjust the current output and thus the load to optimize charging for the particular conditions.

You know a little about electronics, so take a look at Zilog's little microprocessor, the Z8 Encore! XP. You can get a development kit from DigiKey.com (normally $50 but currently $10) that includes all the development software (C and assembly), the micro and accessories. Couple that with a few IGBT transistors and you should be in business!

Well, OK, after a few weeks to learn how.

Bill

[cyclops]

Why are we trying to guess on effecient hull shapes? Mother Natures laws of evoloution have created the best shapes. Copy any of them for any operating speed range desired. They do run on most small power sources very efficiently. Not too many are short and wide. Give a dowel of wood and a piece of 2 X 4 a push. Which goes further?---------------Whales, Manatees and similar shapes are for long distance travel. Few sealife have 2 or 3 bodies connected for any 1 animal. Either Mother is wrong, or we are.

[JonathanCole]

Originally Posted by BillyDoc

Hi Jon,

Two things:

You shouldn't need to use gears to change prop direction, you can do this with your controller if it is properly designed.

Also, your prop and motor can be used to charge your batteries if there is a (water) current sufficient to turn them. If you were sailing with a good wind the odds are very good that the vessel speed would be limited by hull length and other similar factors, not the amount of available energy from the sails. Which is to say, you would be able to charge with effectively "no speed loss" due to the extra drag the charger causes.



Bill

I didn't mean for changing prop direction, but rather axial direction (vertical motor shaft - horizontal prop shaft)

On that second part, I was talking about motor boats using current generators. Although you are also correct that by using a smart pulse width modulator/chopper charge circuit controlled by a microprocessor that you could trickle charge with little loss. But that implies a separate turbine for charging which is an expense and in a corrosive environment. Other technologies might give you more bang for the buck over time.

[JonathanCole]

Originally Posted by cyclops

Whales, Manatees and similar shapes are for long distance travel. Few sealife have 2 or 3 bodies connected for any 1 animal. Either Mother is wrong, or we are.

And you might call them submarines! Has anyone done any research on soft hulls versus hard hulls?

[mackid068]

Hurrah for wind power and good going with the solar power idea. However, you mention that: "Typical 30-50 foot catamaran PV powered electric boats are demonstrating a range of 30-60 miles a day on sunny days. Not too bad for a leisurely cruise. On the inland waterways on the European continent where you can tie up for little or no money in most places, this range would allow for a nice slow paced journey, say from Holland to the south of France which would be very economical and pleasant. You might disprove the maxim that a boat is a hole in the water that you throw money into!" How do you think that such boats would perform on a body of saltwater, such as the Long Island Sound versus, say, the Caribbean?

[cyclops]

A boat probably could be built today to cruise at 5-7 mph. But by the time we had it at a marketable configuration with style, USCG and NMMA safety, it would not be close to this post.-----------------Dugout canoe and 8 pairs of loin cloths are all that is needed.

[marshmat]

Power transmission from electric to prop:
Some of the double-ender car ferries around here have diesel-electric serial hybrid drives, much like locomotives. The diesels spin generators. The props are mounted on pods much like outboard legs that protrude from the bottom of the hull and can azimuth through 360-degrees around the shaft. The electric motor sits with its shaft vertically above the drive leg and coupled directly to it. It's not the most efficent setup though because of the drag from the large leg, but a 120-car ferry with this rig is more manoeuverable than my 15-foot outboard. For best efficiency, though, go for brushless DC motor with direct-drive shaft; no transmission or gears.

Solar boats:
Solar race cars (typ. 26% eff. gallium cells) get up to 170 watts per square metre. With standard commercial cells, figure 100 W/m3 or 10 watts per square foot. On a boat, with HUGE available area (esp. on cats/tris), it's much easier to get a lot of power from the cells than it is on a car. Factor in the cost of fuel, engine repairs, etc. over the life of the boat and solar-electric starts to look more attractive (especially with new polymer and quantum-dot panels coming out for less $$$). I'd definitely consider it for my next boat.

Hull efficiency:
Wave-making is a real limiting factor with regards to speed. A nuclear sub can outrun any surface ship of the same length by far when underwater, but when surfaced can only just keep up. Long, slender, low wetted area, CP around 0.5, the idea is to get the hull to move with as little energy as possible.

So my picks, for highest possible efficiency while still being somewhatpractical? Catamaran, l/b ratio around 10:1, maybe 12-16m LOA. Solar arrays everywhere. DC brushless motors with solid-state controllers and direct drive to large, shallow props. Small diesel generator and a few retractable windmills for backup. May or may not have a mast to hoist a sail in favourable winds, but probably not to keep bridge clearance down.

[BillyDoc]

Hi Jon,

I think that Marshmat has pretty much nailed it. I would like to add, though, that you do not need a separate turbine for charging; the same brushless DC motor you are using to drive the boat can become the charging generator when free-wheeling in a current.

Brushless DC motors use permanent magnets for their rotors, and that is an absolute set up for using the stator for a generator. The problem is that unlike in an alternator (for example) where the magnetic field of the rotor is controlled to control output (and thus back-emf) the brushless DC motor will be producing maximum current for the input torque at all times because now the field is from permanent magnets. So if you were to, say, put some diodes on the output and connect this directly to the batteries you would probably just bring your free-wheeling prop to a stop. Which is where that microprocessor comes in again. You can PWM the output of the motor/generator directly into the battery and effectively control the current output of the motor/generator that way. The inertia of the motor will get you through the "on" phases if they are short enough, and a series of sharp pulses is the best thing possible for charging a battery. This is the foundation idea behind the various devices for removing sulfides from battery plates. The battery, of course, makes a fine integrator. If your microprocessor has an A/D input capability you can read the battery voltage and implement the usual charging stages as well, again by modulating pulse width.

You have me thinking quite a bit about those European canals now. Does anybody know what kind of bridge clearance there is? Sixty five feet would be nice! There's a big pond in between Europe and me right now. I need masts!

Bill

[JonathanCole]

Originally Posted by mackid068

How do you think that such boats would perform on a body of saltwater, such as the Long Island Sound versus, say, the Caribbean?

Clearly the more sun, the more distance you can travel in a purely solar boat. However, with a combination of windchargers, and chargers that can be plugged into the grid when available, you still get a pretty reliable day-cruise. Since most people are using boats as a weekend pasttime, a fuel-free, quiet, low maintenance, solar-electric boat looks pretty attractive. The concept I am working on is to have a boat that actually follows the sun/weather. North in the summer, south in the winter. I am trying to develop a practical onboard lifestyle. I like living on the water.

By the way, having lived for 15 years on an independent solar-electric energy system of my own design and construction, that had no power outages for the whole time, another fact is that these systems are very easy on electrical and elkectronic equipment because of the lack of transient voltage spikes that are common to the electric grid. Lightbulbs lasted the whole 15 years. Never burned out any electronic device. In other words, this is really clean power. Which is good for keeping other kinds of boat maintenance costs low.

[JonathanCole]

Originally Posted by BillyDoc

Hi Jon,

You have me thinking quite a bit about those European canals now. Does anybody know what kind of bridge clearance there is? Sixty five feet would be nice! There's a big pond in between Europe and me right now. I need masts!

Bill

Hi Bill,

The book, The European Waterways - A User's Guide, is available from Amazon.com

You'd be surprised at how attractive it is for non-Europeans to cruise this continental grid of highly developed waterways.

Jon

[JonathanCole]

Marshmat wrote: Power transmission from electric to prop:
Some of the double-ender car ferries around here have diesel-electric serial hybrid drives, much like locomotives. The diesels spin generators. The props are mounted on pods much like outboard legs that protrude from the bottom of the hull and can azimuth through 360-degrees around the shaft. The electric motor sits with its shaft vertically above the drive leg and coupled directly to it. It's not the most efficent setup though because of the drag from the large leg, but a 120-car ferry with this rig is more manoeuverable than my 15-foot outboard.

I had such a drive system in mind for precise maneuverability. The concept I am working on is electric outboards that are mounted on an inboard cylindrical transom that can rotate. This keeps the motor above water although it is not quite as efficient as direct drive. (Although, there are direct electric drives that are meant for submersion even in salt water.) This rotating transom could be mounted aft of the center and with 180 degree rotation plus instant reverse, you should have great maneuverability. Anyone have any thoughts on this idea?

Hull efficiency:
Wave-making is a real limiting factor with regards to speed. A nuclear sub can outrun any surface ship of the same length by far when underwater, but when surfaced can only just keep up. Long, slender, low wetted area, CP around 0.5, the idea is to get the hull to move with as little energy as possible.

Still do not have too much comment from anyone on the submerged hull/ not too-small waterplane area, with ballast tanks idea, which would have the advantage of adjustable draft, and super low wavemaking.

[mackid068]

Is submerged legal? (Totally submerged, probably not in small channels)

[BillyDoc]

Hi Jon,

If you have your prop shaft comming up a sleave (tube) which is a loose fit inside a fixed tube through the hull, and with the vertical to horizontal gears down below and the rotatable tube long enough so that the motor and the top of the fixed tube are above the water line, then the shaft/prop can be rotated to change the direction of thrust with the motor kept stationary. Or, you could arrange things so that the shaft/prop/and motor are all moveable vertically to tuck the whole thing up into a recess when not in use. A fairly loose gland of some sort between the fixed hull tube and the motor shaft/sleave should keep the sea out, and the actual drive shaft inside it's own tube can be flooded with gear oil for lubrication. If the hight of the gear oil inside this tube is significantly higher than the surrounding sea level, then the pressure should be from the inside out (even with the lower density of the oil) and any leakage past an oil seal at the prop will be from the oil to the sea, thus preventing corrosion. See the illustration below. I've left out a lot of details here, like for instance it would be a good idea to prevent the motor stator from rotating, but I hope you get the idea.

Bill

[Skippy]

Jonathan Cole: Still do not have too much comment from anyone on the submerged hull/ not too-small waterplane area, with ballast tanks idea, which would have the advantage of adjustable draft, and super low wavemaking.

A trimaran with deep narrow hulls has a small waterplane without the pressurized tanks, and has room for solar panels. I like a mostly submerged hull for motoring, in fact I was suggesting that for the human-powered boat in an earlier thread. Maybe you could even combine the two ideas, with some kind of T-shaped geometry. But you do have to watch out for roll stability, since you're putting flotation down very low.