Ideas for a Fast Efficient Electric Hull

There's no race rules or constraints of any kind -- except that the boat be electrically powered. Someone used 'kite-assist' in a race, but was not disqualified (didn't win either). The race is more a promotional kind of event to develop interest in electric boating. It's my idea that more interest would be generated if the boats weren't all so sedate and graceful. Not that I don't admire and enjoy a nice quiet slow ride, but perceptible speed adds an element of thrill and purpose. Can it be done without expensive technology? Can we do it with readily available parts and materials? Can we design an element of home-built affordability? And, perhaps most importantly, can we create a boat that might be enjoyable outside the realm of a race? Can we come up with something that just might also be fun to play with?

So, that sums up any and all constraints... The rest is up to whatever we can imagine and make realistically practical.

Several comments have suggested using a single motor rather that two. That's OK with me (and possibly cheaper, as well), but I seem to remember reading awhile back that two motors yields some greater speed potential. Perhaps it was merely due to the contra-rotational force of opposing props.

Then again, maybe my memory isn't so accurate...

 

My high school didn't offer physics (which to this day, I still find hard to believe...), so let me know if I'm running circles in wrong directions...

It would seem to me that if 50 pounds (26kg) of NET lift were achieved, at speed, then we've essentially reduced weight by 12.5% -- assuming a fully loaded displacement of 400 pounds (180kg).

That seems worth pursuing, if it is realistic.

My concern is whether the creation of that lift costs more in power supply that the benefits we might derive from it.

Would it take any appreciable amount of energy, at speed, to push those wings through the air?
Would the lift 'gained' allow the use of less energy, once speed is reached?

 

Daquiri said the drag was 10N at 7.7 m/s. I believe 10N * 7.7 m/s = 77w (had a few beers, so forgive me if I'm wrong). 746 watts is 1 HP, so you need ~1/10 HP for a reduction of 67 lb in net weight on the hull.

So the question is how much power do you save with that weight reduction? Obviously It needs to be greater than 1/10 HP....

 

Rick:

Did I miss the attachments? Were they part of your post...? Is it my computer or internet connection?

Would you send them again?

Eager to see what you've come up with.

Thanks,

David

 

I had to laugh when I logged on and saw your posting because I was going to log on and ask a question that is very related. So let me chime in with what my idea was.

Two sails are faster than one big one because the second sail helps to increase the total lift of the first one. As I was thinking of this it made wonder, at least for a cruising sailboat (possibly racing... I'm new and learning), why not create this same dual sail lift principal down below toward the bow and possibly mid section of the hull.

Here is what I pictured, again I'm thinking cruiser, but you'll get the idea. Picture in your mind a normal cruiser beam width... that is where my outer hull wing would be when it is extended... and now design the hull to be slightly more narrow in actuality. So the hull wing is brought inward more to hug the hull tightly on then windward side but the other side the wing is flared outward beyond the hull a bit with some water space in between to create the double hull lift at the bow. Just like the air passing in between two sails.

In essence I was picturing an asymetrical airplane wing on the hull (not as efficient, but closer to that).

When flaring out the wing, I just couldn't figure out if it would be better to have 2 "wings" on each side... the first wing, really just fills out the narrow part of the hull to make it more like a normal cruiser hull width (for that half) and then the second wing is flared out even more to act as the true second sail to create the increased lift.

If you had something like this, wouldn't you be able to sail more directly upwind with less angle of attack because you are have increased your lift?

Hopefully this doesn't sound stupid, like I said, I'm learning.

 

I may have been a bit quick on the mouse click. I was in a hurry as my paid work has to take priority this week.

Rick

 

The WIG concept is a reasonable one, but having done some design work on one a few years ago I have to say that the pitch control conundrums can make for some interesting times.

Remember that wing lift will depend on air speed, not speed through the water. Also remember that wing lift is proportional to the square of airspeed. This means that if you are travelling at 15mph into a 5mph wind you will have 20mph airspeed (15mph + 5mph). If you design for lift = 50% hull AUW at 20mph airspeed, then when you turn downwind and the airspeed drops to 10mph (15mph - 5mph) the lift will drop to only about 12.5% hull AUW. The same effect will be felt at angles to the wind, depending on the wind vector in the direction of travel.

Quite apart from the effect on motor power requirement this will have (from the wide variation in hull wetted area from the effective change in displacement) it will give you significant pitch control problems. WIGs are normally designed to auto-stabilise in pitch to some degree. They do this by changing the wing angle of attack in response to changes in airspeed or flying height (in effect, they sense the increase in effective lift coefficient with decreasing height). With the hull still in the water and subject to wave action this may well be tricky to get to work well.

Jeremy

PS: I used to design aircraft and have a seaplane rating on my licence, so have a pretty good feel for some of the challenges here!

 

Constraining the width to a minimum of 460mm takes power up to 1537W on the 200kg hull at 7m/s.

A hard chine hull suitable for flat pack would require 1469W at 7m/s. This would require that you sit above the waterline.

From the previous thread the unconstrained 200kg hull at 7m/s required 1450W.

If you are likely to operate in waves any more than 600mm high you would probably want it self-bailing as it will pass though shorter waves not over them unless you make a severe flare and this will slow you down in waves. So self-bailing and unsinkable would be desirable and this would mean sitting above the waterline. You could have a small footwell below the waterline. The battery box would be on the bottom but designed not to flood.

Windage will also be a factor so you need to consider aerodynamics.

I have attached the shape parameters for both of these hulls.

Let me know if you want to work on more shapes and constraints.

15kts with this weight is getting to be the crossover point for planing. But the ideal planing hull would be about 2m long and 900mm wide. The flat bottom on the slender hull will give better dynamiic lift than the curved bottom so the michlet values for the flat section are likely higher tha reality while the curved section is likely close or a little low.

My feeling is that if you went to a flat bottom at 460mm beam it would be inclined to pound a little in any waves.

If you have Delftship it will be easier to look at the hull shapes in that format but it also takes me a bit of time to input the shape that Michlet produces. There is a bit of fiddling involved to get something more than just underwater bit.

Rick

 

Hi Jeremy,

I've been looking forward to your feedback. I respect your background and the wealth of knowledge you bring to this picnic.

So, do you think the wing idea won't fly? Is it going to be too much trouble for too little benefit?

So far, I'm not devoted or invested in any particular concept or model or design. If trying to 'steal' some free lift is going to cost a fortune (in time, construction, or materials) without an obvious and worthwhile return (in efficiency, speed, or power), than I would be better focused on other aspects.

The idea, though -- that of sailing 'sideways' -- seems compelling... This boat, nonetheless, might not be the most suitable venue for exploiting it.

Do I recall that you are using Universal Joints in your driveshaft? Have you determined their losses to be minimal?

Would using such joints make sense in setting up for a flexible shaft -- at the transition between the motor in the hull and the the in-water flexible sections of the shaft?

If I went with 2 motors to drive 2 props (which we haven't determined yet), I can see sending a rigid tube out either side towards the amas. U-Joints within the floats would enable the spring steel to be coupled in. The resulting angle would likely be from 135 to 150 degrees, roughly.

Do you think the U-Joints are effective enough to be considered, or is a simple straight direct drive shaft clearly more efficient?

I've been wondering, at this level of power and speed, if a driveleg would have any advantages.
I understand (basically) that there would be a drag penalty imposed. Might there be any weight savings (small streamlined minimal skeg versus long bendy steel shafts) or other improvements between one system or the other?

David

 

How did you arrive at this estimate of displacement? How much confidence do you have in it?

What is the assumed battery mass? How much energy would be available from those batteries?

 

David,

Thanks for the kind words, but I only really know about a limited subset of all the elements in your project.

The WIG idea does have some merit, but should probably be fairly well down on the list of design priorities, as I'm not at all sure that the benefits will warrant the extra weight and cost. If you can make the ama supports as wings, with little or no weight or cost penalty, then it'd be worth doing, even if the overall performance gain is only a few percent. I've a feeling that the success of your project will be dependent on the sum of lots of small improvements, rather than a single winning design element.

You will get best propulsive efficiency from the prop if its axis is aligned with the direction of travel, but if the prop shaft angle is small, then the losses are modest. Rick's flexible shaft system may not work reliably at the likely power level of your boat, so you have the choice of either putting the whole drive system in the water (adds complexity and some drag, but might aid motor efficiency from better cooling) or using something to turn the shaft through an angle to better align with the direction of travel.

As far as I can tell, from measurements on my low power system, the losses in the universal joint, even when running at an included angle of 100 degrees, are negligible, certainly less than the losses in the supporting bearings. My view is that the UJ losses are much lower than those in a gearbox. I believe that the losses in the UJ will be lower if the included angle is greater, as they must be proportional to the deflection each joint makes during each revolution. The trade-off for your application will be whether the additional UJ losses from running a more vertically-inclined drive leg (like mine) would be less than the additional hydrodynamic loses from running a longer shaft, perhaps with a UJ doing the final alignment. Reliability may be the deciding factor, as to get best efficiency you need to keep the drive as thin as possible. The UJs need to be substantially derated if you run them at the sort of angle I am, which would mean, for your higher power application, using a much bigger diameter joint.

My gut feeling is that a moderately inclined shaft, fitted inside a streamlined housing, with a small UJ fitted at the prop end to straighten the drive out, might be the best compromise. I'm running my UJ in a light oil bath, both to keep it lubricated and to provide a static pressure balance at the shaft seal. I've run the shaft unloaded with and without oil and not measured any extra loss from it.

Jeremy

 

What type of u-joint are you using for an included angle of 100 degrees? That's considerably beyond where I've seen joints used for transmitting power.

 

The details are over on my "Efficient electric boat" thread, but they are standard DIN 808 type double universal joints. They will run quite happily at 90 deg included angle, but their torque rating is significantly reduced. The torque derating factors (applied to nominal rated torque) with deflection angle are:

>5 deg per joint (10 deg per pair) = 1.25
10 deg per joint (20 deg per pair) = 1.00
20 deg per joint (40 deg per pair) = 0.65
30 deg per joint (60 deg per pair) = 0.45
40 deg per joint (80 deg per pair) = 0.30
45 deg per joint (90 deg per pair) = 0.25

The consequence of this derating is that I'm using a joint that has a nominal rating that is a little over 4 times the maximum torque I'm running it at.

Jeremy

 

A 24V LiFePO4 20Ah battery weighs 4.9kg (10.8lbs).

Using Rick's estimate of the 5kWh required, indicates at least 10 batteries would be needed, weighing about 50kg (108lbs).

I, regrettably, add my own 190 lbs (86kg).

That's not leaving much for the boat...

Dacron skinned boats are extremely lightweight, however. I would guess that a 21 to 24 footer (7-8m) could be built in the range of 2 pounds per foot (~3kg/m).

That brings us close to 350 pounds. Admittedly, there might still be a need for some miscellaneous wiring, controls, and a motor or two...

So, realistically, 350 pounds would be a tight budget, but achievable.

Hope this helps...

Thanks!

David

 

I might point out that there is a wig-enthusiast page and forum.

http://www.se-technology.com/wig/index.php

I might also point out that there are very few actual full scale WIGs you can see and touch. Evidently controlling them is not as easy as it might seem. I've seen and touched several, but according to the folks on the WIG forum the ones I saw were not good WIGs.

From what I've read on WIG theory, if you made your craft specifically to be a WIG then it might work, but I doubt you have the power to take off. From what I understand, the drag from water is a very bad thing, and the faster you go the more it sucks you down.

Personally I think that's a dead end as far as any real benefit from partial lift wings. Note that I'm not an engineer, so that's an unqualified opinion.

What I might submit instead is to make a surface effect ship. Well, surface effect boat, given the scale of things here. Not quite a hovercraft, it would have hard side walls and a flexible skirt front and back. Pressurize the air under the craft and you essentially have lubricated most of the hull with air to get rid of the drag.

Bob Windt from Universal Hovercraft has made a proof of concept all-electric hovercraft with some car batteries and a pair of cheap model aircraft motors. He's also the designer of the WIGs I saw. He might be a valuable contact, but he doesn't do the Internet. You can contact http://www.hovercraft.com for his contact info. For that matter anyone there can probably help you with what you need to know.

Disclaimer: I know those guys personally. I go hovering with them every now and then, I visit their shop. I am not and never have been financially affiliated with them, but I'm a customer of theirs.