Ideas for a Fast Efficient Electric Hull

I've been following several different threads involving efficient hull design to optimize either pedal power or electric motors.

Your various comments and participation have always impressed me.

I'm no engineer [in fact, many of the discussions take me several readings to even vaguely follow], so don't be afraid of talking 'stupid' to me. I have a fairly good grasp of boating concepts from practical experience. Higher principles of theory and hydrodynamics and electronics are beyond my background and education.

So, if you have some time, I'd like to 'pick your brains' a bit... collectively...

I'd like to see if its possible to achieve a 12 to 15 knot speed for a 35 mile race (56km). Using good hull design, small hobby motors, and 24 to 36 volt battery banks, on a lightweight slender stabilized monohull, would it be possible to exceed hull speed, by a factor of two, without resorting to planing?

I'd be a 200 pound payload (90kg), yet I think the boat could come in at under (or close to) a total loaded displacement of 450 pounds (200kg). Even lighter, if LiFePO batteries could be used, possibly 300 lbs (135kg).

Could a Mars or Turnigy motor be located in each ama/sponson, driving flexible shafts? What would the best prop sizes be, and what RPM would be the most efficient? Would a carefully foiled driveleg or skeg be better suited to manage the power and control the boat at speed?

If overall length was kept under a maximum of 32 feet (9.75 m), what would the optimal hull look like? How wide would the stabilizers be spaced? I envision a sit-in version of Rick's marvelous V14, or Greg's 24-hour record setting boat, stretched longer and slightly wider.

I'm considering the option of using Tom Hill's Ultralight BoatBuilding techniques, for a hybrid Ply and Dacron hull.

And I'd be interested in exploring a weird concept of obtaining some lift, at speed, from the support arms. Perhaps a Dacron covered airfoil wing shape between the hull and the sponsons... Like a rigid-wing hang glider attached to the aft hull, with a stabilizing sponson under each tip. If it were possible to generate more lift than drag, at speed, then the hull would operate lighter in the water, gaining efficiency.

Hoping to hear lots of ideas as you all give this some thought towards what the optimal design might entail.

Any help or guidance or suggestions would be greatly appreciated. I'm wide open this summer to get this project started!

Many thanks, in advance,

David

 

David,

You'll get plenty of help here since you have clearly defined design parameters and your primary goal is efficiency with few concessions. I think I've probably driven a few members of this forum nuts with my readiness to compromise for the sake of cost and simplicity!

Be sure to listen to Rick, you'll save yourself plenty of time and mental anguish.

I'm curious, is the 32ft limit imposed by the race or by transportation/storage requirements? Assuming it is beneficial to have a hull >32ft (which it may or may not be), you could always make the main hull fold or break down into parts.

 

David,

I think you're on the right lines, although I'd probably question the Mars motor because of its weight.

Rick is probably the best person to advise on the hull, particularly as your application seems near-ideal for a faux tri design, I think.

To keep the weight down I'd opt for a single propulsion system fitted to the main hull. At the speed/length you're looking at you're into a semi-displacement design, I think, although I know that long, thin hulls can seem to defy the normal rules a bit (I watched Consuta, a 50ft long, narrow beam, steam driven umpires launch a couple of weekends ago, her turn of speed was impressive - see here: http://www.consuta.org.uk/default.html).

You'll need to squeeze every bit of efficiency you can from the whole system (hull, batteries, motor, drive system and propeller). An under-run outrunner brushless motor (or motors) will almost certainly give you the best trade-off between efficiency, power and weight. Once we've got an idea of the power needed I can probably help out with some possible motor choices.

Sounds like an interesting project!

Jeremy

 

Just a thought, since the power requirement will be in the kW range anyway, would it be possible to construct an extremely light planing hull that would plane with 1-2 kW, with the pilot and the batteries onboard? If it's possible, it should be easy to beat any displacement hulls in that race, at least if there's not too much wind and waves.

 

Are you stuck on those brands of motors, or are you just going for huge?

There are several other options I know of, although I have direct experience with none of them.

Plettenburg goes up to 15 kW for an aircraft motor, Predator 37, at about 2 kg according to their specs. No doubt that's peak not continuous. http://www.plettenberg-motoren.com/UK/Motoren/aumo_ueb.htm It has a funky mounting system designed specifically for aircraft props though and they have a disclaimer "not for human propulsion" and threaten to void your warranty. I think it's just for cooling, so that one would definitely be if you don't mind making a separate cooling system.

Another one is Astro, who makes extremely efficient motors for modeling and industry. Astro 3210 for example. http://www.astroflight.com/index.php?main_page=index&cPath=9 Some of these are efficient enough to produce your power range and have no cooling vents, meaning not enough wasted power to heat the motor that much. Bring money.

Also, if you're not adverse to winding your own there is a motor design and construction forum with some really good starter info at http://www.rcgroups.com and some references to manufacturers of components for winding your own.

 

I have attached the hull shape parameters for the lowest drag hull for 200kg at 7m/s.

It is probably shorter than you expect. This is due to the low displacement and the design being high enough that viscous drag is significant.

The beam is too small to sit in. Increasing the beam to 400mm or so should not have a huge impact on power. This is something to test for sensitivity.

If you let me know what you think the miniumum beam should be I can check the impact.

The hull is close to round section but will not add much drag to square it up for hard chine. Do you intend to build from flat pack?

 

I like the idea of transforming sponson's supports into wings. Considering the fact that it will work very close to the water surface, a wing can be designed which will use the the ground effect to obtain a pretty high L/D ratio.
At 15 kts (7.7 m/s) the dynamic air pressure is 36 Pa. With a correctly designed WIG sponsons, you can obtain several kilos of weight reduction at max speed. There will be an aerodynamic drag penalty for this, which will than have to be confronted with the reduction in the hydrodynamic drag due to smaller hydrodynamic load. It will require a several iterations until an optimum is (hopefully) reached between the hull and the WIG. Let me know if you will need some help with that part of the task.
Nice project, I will follow it with interest.

 

At 15 kts, the lift due to WIG could be as high as 300 N, and the drag as low as 10 N, after a first preliminary rough calc. The numbers could be even better, if an adequate airfoil and wing surface and shape can be used.

So, in order to explore the effect of WIG sponson supports, it would be necessary to make a map of several hullforms in a range of displacements starting at, say, 150 kg and increasing it by 5-10 kilos at each step. In that way it would be possible to evaluate iteratively the reciprocal effect of various WIG planforms and hulls/amas, until a convergence at minimum total drag is reached.

The question now is: is the total (hull + amas) beam limited, and to what value?

 

mental_boy,

---- "I'm curious, is the 32ft limit imposed
---- by the race or by transportation/storage
---- requirements?"

No, actually there's no limits... Thirty two feet just seemed to be a practical size (four 2X8 sheets with three butt seams). Certainly, if we go longer, I'd build it to come apart into sections for transport on a trailer. But, I think it would be simpler and lighter and easier if it could stay as a single hull.

David

 

Tapio:
I like the idea of a small planing hull -- but would it be as efficient for a given power supply, or as fast? And would it have other practical uses after the race, fishing or day cruising at slower speeds, for instance?


kroberts:
Not stuck -- completely open to suggestions....
I'm not experienced enough with electronics to even think of wiring my own. Might make a great July 4th fireworks party spectacle if I were to try it...


Rick:
I missed the attachment... As to beam, I was thinking of 2 feet (60cm) as an initial target, for comfort. A minimum of 18 inches (46cm) will allow me to fit in, and 21 inches (53cm) would be a happy width for prolonged sitting.

I was planning to build one of two ways. The fall-back method would be from plywood, stitch and glue. The alternate I'd like to explore, is a combination of the Geodesic frame covered with Dacron (pioneered by Platt Monfort [sorry, I mistakenly mentioned Tom Hill and Ultralight BoatBuilding, earlier... right idea, wrong concept]).

I think the hull bottom would be best if done in either plywood or polyester and epoxy over Dacron for strength and stiffness.

 

daiquiri:
Even though the math and science is beyond my realm, I can't help but think that 6 to 8 feet of fabric covered wings on either side ought to be able to generate at least 100 pounds of lifting force.

I picture the leading edge, in the center (behind my head) being some 24 inches above water level. The arms would angle downward, towards the amas and end up some 10 inches high. The trailing edge, at least in my mind's eye would be sloped towards the water several degrees, and would more or less be parallel to the water surface, clearing it by maybe from 12 inches at the main hull to perhaps just 6 or 8 inches at the ama.

Have you seen the incredible SWIFT rigid hang glider? Check out some of the vids on YouTube...


YouTube -- Swift Rigid Hang Glider and Here

 

Tuck and Lazauskas published a paper in 1996 on rowing shell optimization which could be relevant. The resistance curves in it include the applicable range of Froude numbers from 0.5 to 0.7
http://www.cyberiad.net/library/rowing/misbond/misbond.htm

 

Papawoodie,

Wiring a motor is more akin to braiding or weaving than it is to any sort of technical something. You count the number of turns, make sure they go the right direction and pack it in as tight as you can. The first person I saw doing it was my dad, with a high school education. He finds motors in the dump, picks them up for the price of iron and re-winds them.

There are instructions for free on the net, and lots of technical info if you buy a stator from someplace like gobrushless.com. Who will, by the way, help you figure out exactly what you want and sell you parts for it. Again, not any cheaper than buying a brand new motor but you can get exactly what you want.

Sorry I guess you probably aren't the motor building kind of guy. Just saying it's an option and there are lots of guys on another forum who would be glad to help. I started doing it just to figure out how it all works. This is definitely not high tech at the motor winding level.

Another option is to go to the rcgroups.com forum and into the motor design and construction section, and say what your project is and what existing motor would be best for it. Have real numbers for them to use, as many as you can get including an estimate here for what sort of prop you would need, what RPM it should have and how much the boat should weigh.

 

Battery mass is likely to be significant parameter. At 12 knots the course would take slightly over 2 1/2 hours to cover and at 15 knots slightly over 2 hours. Add in 1/2 hour margin and the batteries would need to provide the estimated power for 2 1/2 to 3 hours.

What would be achievable propellor efficiency?
What would be a reasonable motor efficiency including any shaft speed reduction gear?
What would a reasonable estimate of battery energy density based on a discharge period of 150 to 180 minutes?

Pick a resistance and speed. Multiply those to get the work done by propellor. Divide by the propellor efficiency for the "shaft" power from the motor. Divide by the motor efficiency for the electrical power. Multiply by the appropriate time for the speed for the electrical energy. Divide the electrical energy by the battery ENERGY density for the battery mass. Use consistent units.

Estimate the motor and reduction gear mass based on the shaft power.

Estimate the combined hull, battery, wiring, motor/reduction gear, shaft, propellor, etc mass.

Is a hull feasible with the required displacement and resistance?

Iterate.

Could be fun!

 

300N = 67lb lift (9.8N = 1kg, 1kg = 2.2lb)

You can get as much lift as you want, but you are limited by the weight and the drag of the wings/foils. There will be some sort of break even point that will depend on the materials you choose, the desired durability of the wings, the hull shape, how fast you're going, whether or not you like the idea of flying while going into a headwind, etc.

Adding the wings produces lift, but also adds weight, so you need to look at the net lift. The net lift reduces drag on the hull, but at some point I imagine the drag from the wings will be more than the drag saved on the hull. At high speeds minimum drag is probably achieved by hovering above the water, at very low speeds the wings may provide no benefit at all.

Anyway, sounds like daquiri has experience doing this kind of optimization, so you'll have to feed him more information (Starting with constraints on beam).

You might also want to look at the race rules more carefully to make sure you're within them.

An extreme example of a light, efficient wing that was designed for 18mph, which I believe is below the stall speed of a hang glider:

http://en.wikipedia.org/wiki/MIT_Daedalus

Also, an interesting video about a human powered helicopter I think has some relevance:

http://www.youtube.com/watch?v=caHCbuh_Yyc