Efficient electric boat

Thanks very much for the kind words, Denny, much appreciated. Thanks also for the links to the Strider, it's uncannily similar to the hull I've pretty much settled on. I took a trip over to Swallow Boats on Monday, to look at the Winsome hull and discuss electric propulsion with them. We took the opportunity to do a brief sea trial with the right angle drive system.

As you rightly say, the prop is a poor match and tends to load the motor down too much, even with the mods. This reduced efficiency a fair bit, I'm sure. Even so, we managed to get a useful feel for the concept, and learned a fair bit. The boat used was a fairly big double ended sailing day boat, about 15ft LOA, 5ft beam, with a loaded displacement on the day of around 570lbs, so not really representative of the lower resistance, lighter weight, boat I have in mind.

Even so, we managed to get the boat to around 4kts on about 400 to 450 watts of input power to the system, adding more power, up to around 700 watts, didn't seem to make any real difference, as expected if we were close to hull speed. She would trickle along at a couple of knots on about 200 watts. I was using a higher voltage (36V nominal) battery pack (because I already had it in a sealed box) so was not using more than about 1/4 throttle. This also meant that the phase current peaks would have been very high, as the controller would have been running short pulse widths to get the motor voltage down, not good for efficiency.

The most significant problem, apart from the poor prop match, was the gearbox noise. The cheap gearbox I'm using really does make a lot of noise, so if I persist with the right angle leg design I need to switch to something like one of the gearboxes that Rick is using. I have looked briefly at the planetary gearboxes, but was put off the idea after finding out that this is the solution that Torqeedo use in their outboards. They use an RC motor fitted with one of these planetary reduction gearboxes and it makes a lot of noise. At high power levels the Torqeedo makes a really irritating whine, and that's with the gearbox under the water.

I was kindly given a link to the Canadian Disappearing Propeller Boats, with their lift up prop shaft (see www.dippy.ca for details). A modern version of this system, with a sealed electric drive unit, using an RC motor and toothed belt reduction, would seem to be an option worth exploring. I think the unit could be made as a modular bolt (or bond) in unit, fitting to a cut out on the bottom of the hull. To this end I'm currently building a belt drive unit and assembling another prototype for testing. I plan on testing it on a range of small boats with a simple transom mount bracket, so it operates rather like a mud motor, with the shaft sticking out the stern. I'm hoping that the toothed belt drive will be quieter and smoother than the gearbox.

I've also asked Swallow Boats to quote for a lightweight plywood Winsome type hull, so am now getting close to being committed to the final design, after months of researching all sorts of options.

Jeremy

 

Jeremy
As you know my preference is for the curved shaft but it is not all that transportable.

For what you are trying to do I suggest you look at the smallest Mitrpak box. These operate happily underwater and they come with 2:1 reduction ratio in the box as well. See attached.

I am setting up my new Turnigy motor for another test and have selected a new toothed belt to replace the big one I initially used. It has a 5mm pitch and 10mm wide belt. It is just about the ideal size for the power level. A 3:1 reduction is quite compact.

So with the toothed belt and the Mitrpak box you get overall 6:1 reduction. It would allow you to run a 12X12 model plane prop at about 600rpm with motor speed of 3600rpm. This gets prop efficiency of 82%.

I have operated these little boxes (of another brand) underwater for long periods. Greg K has also used one on his ocean boat and Mitrpak has made him a twin box totally sealed set for the next boat. One of the boxes will operate underwater. Not really any different to a normal outboard box.

The little boxes get overall efficiency around 97% once the seals wear in.

Rick W

 

Torqeedo

In case you'd like to see what the market place in electric outboards is coming up with I have attached a video link from Navagear

http://www.youtube.com/watch?v=0utXxauZOso

There are some interesting comments beneath the video which add some balance to the review!

 

I'm familiar with the Torqeedo, hence my reference above to the noise from the gearbox it uses. I really couldn't live with that high pitched whine for a river cruiser, it would drive me to distraction. Although it's more sophisticated than the crude and simple motors like the Minkota, at least they are pretty quiet. The other issue I have with the Torqeedo is the cost, I really find it hard to see why it costs so much more than every other electric outboard, particularly when the core components are so cheap.

I've already downloaded those data sheets on the Mitrpak gearboxes, Rick, thanks to a link you provided in another post. I think they may be an option if I stick with the outboard leg type drive, but I'd like to play around a bit more with the idea of a lifting shaft drive first, just to see how that looks.

I've just finished making a 4:1 HTD belt reduction drive that I think should be OK with my test prop, although I'm sure I will end up changing it during the optimisation process. I've opted for a 15mm wide 3mm tooth pitch, because I found it was more flexible and seemed to have less frictional loss than the 5mm HTD belt I tried previously. It should be OK for the motor torque, provided I set the current limit at a sensible level on the controller. I'm currently working towards a system similar to that on the old Disappearing Propeller Boats, as I think it might prove to be a fairly straightforward way to retrofit electric propulsion to pretty much any small rowed or paddled craft.

I've spent an hour or so this afternoon ordering up stuff, like a stainless prop shaft tube and a few other bits. It's staggering how quickly the costs add up!

Jeremy

 

Jeremy,

Pardon my ignorance - I'm a boat guy, not an electronics guy.

With your 36v pack you must have been putting 11a into the controller. Would the efficiency have been better with a 12v pack putting 33a into the controller? Wouldn't that make the pulses 3x as wide? What about I^2R losses being 9 times as much with triple the current? What are phase current peaks anyway?

I agree with you that near silence is an important goal of any electric system. It is often assumed that any electric system will be silent and the people selling them certainly want to foster that assumption but in reality any that use gears tend to make a good bit of noise. Even some BLDC motors have an annoying whine at certain speeds. (Resonance in the rotor driven by just the right (wrong?) pulse frequency? Phase of moon?) It is truly magical to glide along the water with no perceptable drive noise and well worth the effort to get there.


Rick,

1) Are those Mitrpak right angle drives silent under load?

2) Your use of APC props is interesting because they are very cheap and narrow enough to be enclosed in a fairing when not in use. Do you have data showing the slip percentage at various speeds? Have your tried them at higher power, say 750 watts?


All,

My goal is to create an auxiliary drive system for a 15' to 20' sailing dingy that can be mostly hidden within the rudder. 750w would be enough power. An APC 2 blade prop could be enclosed in a fairing along the trailing edge of the rudder when sailing to reduce drag. Adding 3 or 4 pounds to the rudder weight would be OK. Powering with 12v is a plus as that voltage is required for lights, VHF radio, GPS, etc. 24v would be doable if 60a is too much for cost effective motors or controllers.

As a data point, one 12v gp 31 (C20 = 105 ah) battery weighs 65 lbs and can be located so as to help trim the boat fore and aft and lower the CG a bit. In an Oughtred Ness Yawl (19'2" X 5'3", ~16' LWL; probably 600-700 lbs with crew) this battery and a 750w motor would give 16 statute miles range at 3 kts (15a) cruising or WOT - 5 miles range at 4.7 kts (60a).

Your comments are very much appreciated.

 

Check this out if you haven't allready: http://re-e-power.com/

 

No need for apologies for not knowing this stuff, Denny, I'm going up a similar steep learning curve of trying to understand boating terminology and science!

Pulse width modulation for motor speed control does some slightly counter intuitive things, particularly at low speeds. There's nothing really special about three phase brushless motors in this regard, except the added confusion factor of having motor phase current pulses as well as the PWM pulses. The PWM rate is always a fair bit faster than the phase switching, so there will be quite a few PWM pulses in each phase pulse (plus the phase pulses are usually trapezoidal or sinusoidal, rather than rectangular).

What happens at low rpm, high torque demand and part throttle is that the peak current flowing in the motor wires and windings will be much higher than the average current being drawn from the battery. This is because the motor winding current is primarily limited by the back EMF; at low rpm the back EMF will be low, so the peak current ends up being limited by the combination of motor winding resistance, controller resistance, battery internal resistance, wiring resistance and the winding inductance. It's quite possible to have relatively low current being drawn from the battery, perhaps only 10 amps or so, yet have motor phase current peaks of around 70 amps (which is what my controller is currently set to as a limit).

You're right, these high phase current peaks do increase the I²R losses in the controller FETs and the motor wiring. It doesn't have an appreciable effect on the battery side though, as the current is much lower. Lowering the battery voltage would reduce the motor side losses, but would increase the battery side losses, so it's a bit of a compromise. There's no doubt that I'm running at too high a voltage for the present set up, just because I happen to have a spare 36V battery. The final version, with a bigger reduction ratio, will probably end up running at around 24V, I think.

To try and reduce the noise as much as possible I'm looking at fitting the motor and reduction drive into a thick, alloy, sealed box. The motor dissipates most of it's waste heat via conduction from the stator to the motor base plate, so is should still stay cool. The motor resonance (or whatever) whine is odd, because it's very dependent on the controller. My motor is noticeably quieter since fitting the Hall sensors, when compared to running it on a cheap RC brushless controllers. I've no idea quite why this is, but suspect it might be related to the controller finding it difficult to get decent sync from the motor back EMF, as fitting sensors seems to fix it.

I had some thoughts on building a small propulsion unit into a rudder, but didn't think of the neat idea to enclose the prop in a fairing when sailing, that sounds really good. I went as far as buying a cheap drill flexible drive to experiment with, and found that it worked remarkably well. The losses were surprisingly small, as long as the curve radius was modest. I've been told that you can buy a very heavy duty flexible drive inner that is made from stainless steel. If this were fitted into a PTFE tube, fitted into a gentle curve routed into the rudder (or perhaps moulded in to a composite rudder) then I think it would be possible to build a reasonable good system. The motor and reduction drive could fit clear of the water up on top of the rudder stock.

It's probably worth significantly de-rating the RC model brushless motors for a couple of reasons. Firstly, the manufacturers often over-state the motor power quite significantly. Secondly, these motors are intended for relatively short duration operation, so they need to be derated for continuous operation. There is also another advantage in using a motor that is, on paper, much more powerful than needed. A motor with a high power rating will usually have a lower winding resistance, which will tend to increase efficiency a bit when it's run at modest power levels, as long as the motor is allowed to spin up into it's most efficient operating region.

Jeremy

 

Controller

Jeremy,

Some time ago you mentioned $22 controllers from China.

They would handle 60 volts.

If you recall this, can you give me more info on them?

Are you using one?

Thanks, Tom

 

Hi Tom,

These controllers come from a chap called Keywin Ge in China, they are often colloquially referred to as "Infineon" controllers, as they use an Infineon microcontroller. Keywin can be contacted at ecrazyman (at) gmail (dot) com

The small $22 controllers are able to handle 48V at 15 amps as they stand, but can easily be modified (by changing the FETs) to handle up to 60V at around 40 to 50 amps. You can buy them from Keywins eBay store, here: http://stores.shop.ebay.com/HuaQiang-North-Road__W0QQ_armrsZ1 The specific controller that I'm using is a modified version of this one: http://cgi.ebay.com/48V-350W-brushl...id=p4634.c0.m14.l1262&_trkparms=|293:1|294:30

Keywin can also supply much more powerful brushless controllers, or just bare circuit boards for those who want to experiment. They all need a motor fitted with Hall sensors as they stand, but Keywin can also supply small add-on circuit boards that allow use with motors without sensors. I've just acquired some of these sensorless add-on boards to experiment with, I'll report back on how they work in due course.

Hope this helps.

Jeremy

 

The gearboxes use precision spiral bevel gears. They are not noisy.

I do not use APC props. Most of my props are home made from stainless steel flatbar. The cheap plastic ones I use are from Hobbycity. The most expensive ones of these was USD8. Generally the pitch on the model aircraft props is not aggressive enough to suit my application. Bolly make a 15X26 boat prop that is OK for what I am doing.

I can get props milled to my design. Like this one:
http://www.boatdesign.net/gallery/showphoto.php/photo/14407/size/big/cat//ppuser/18624
They are quite expensive compared with model plane stuff. Bolly will also make props to a specified design.

The rating of the prop is not really the power it can absorb but the amount of thrust it needs to produce for that power. All the hulls I play with are easily driven so have low drag. Thrust is therefore low. Blades around 3mm thick and higher will easily withstand the loads I apply. The model aircraft props are quite strong. The Bolly props are CF so incredibly stiff and strong for their weight.

Rick W

 

J.H.

Without seeing the shape of your hull and location of the prop, i would be very very careful using a figure of 70%. Unless your prop is in undisturbed "free flow" and horizontal, you wont get that figure. Hence without some pic's all i would say is be more pessimistic, upper limit for most "normal" installations is around 55%.

 

Denny
My personal view on the prop in rudder arrangement is that it would be an annoying compromise for sailing. Particularly if you are racing.

Your application contrasts with what I am doing. I think your power estimates are high but I would need to look at the hull to be sure. If you take your 750W at the battery and assume the prop is taking 500W then your best prop will be something like 16X8 per attached. So relatively low pitch to diameter. You should be able to find an APC prop that has the right proportions.

The 170N thrust is quite high. I expect any of the Bolly props up around that size would handle the load:
http://www.bolly.com.au/models/glasstwo.html
They are quite expensive compared with APC. I expect the Bolly to be stiffer and stronger than the APC but it would certainly be worth a go.

The 170N would also be borderline axial thrust on a small Mitrpak box. It will handle it but life will be reduced. Unlikely to be a problem for casual use.

If you have some pictures of your boat or plans I can determine the calm water power requirement. 750W is a huge amount of power for a sailing dinghy to get 4kts if properly applied. This is the sort of power an Olympic rower can produce in a burst. I think one of these guys could get better than 4kts in a sailing dinghy.

Rick W

 

AH,

Thanks for the advice. I'd already spotted that boat props tend to be quite poor in terms of design optimisation, at least compared to the aircraft world that I'm more familiar with. It seems that prop efficiency, at least for small, low power, boats, just isn't really a priority with designers for some reason.

For example, most small boat props turn too fast, are too small in diameter, often have too many blades and have too great a wetted area. I've no idea why this is, as it's quite clear that a slow turning, high aspect ratio prop, with just a couple of blades, will give a significant efficiency improvement at low hull speeds.

Like Rick, in his search for maximum efficiency for his HPB propulsion systems, I'm trying hard to produce a "clean" unit, hydrodynamically. I can't rival the lack of inflow disturbance that Rick has with his thin, flexible, drive shaft, but I'm hoping that I can get a compromise between absolute efficiency and practicality for easy operation that comes close.

The final design will almost certainly use a larger diameter, narrow aspect ratio prop than my present test one. I'll probably end up making a prop, as I'd like to have a degree of sweep on the leading edges to help shed weed. I won't do this until I've got more hard information on the behaviour of the rest of the drive system though, plus it would be nice to get some accurate hull resistance data too. Hopefully I should have this soon, which will be another step closer.

Whilst I accept that a conventional small boat prop might only run at 55% in practice, if this is all I can get I shall be extremely disappointed. I believe that Rick and the other HPB gurus are running with much higher prop efficiency than 55%, and whilst I can't hope to match their figures I think I can probably get fairly close.

Jeremy

 

Jeremey

"..For example, most small boat props turn too fast, are too small in diameter, often have too many blades and have too great a wetted area. I've no idea why this is, as it's quite clear that a slow turning, high aspect ratio prop, with just a couple of blades, will give a significant efficiency improvement at low hull speeds..."

In a word..poor design.

However having said that, the number of blades relates to the BAR and hence the thrust loading and also the cavitation of the prop, speed, cheap engine, no gear box perhaps etc etc...so it is not always such a clear cut answer. Naval architecture is all about compromises. Designing one small 'thing' is not however, it is about getting the best out of that "thing".

This does not happen when designing boats, there are always far too many conflicting issues, often diametrically opposite to each other. It is all about finding the right "balance" (or compromise) that suits you and your requirements!

Rick obtains his higher efficiencies purely because of the near undisturbed flow and perpendicular flow into the prop. It is that simple. Real boats, in general, are very rarely like this. His is for one single application or one single design variable..which makes such 'compromises' significantly easier.

Soon as a hull is near the prop, upstream, the conditions for such efficiency dramatically changes.


".. It seems that prop efficiency, at least for small, low power, boats, just isn't really a priority with designers for some reason..."

Without knowing what compromises have been made in order to get the boat that far, we shall never know. Cost, time, geometry etc etc, all play a part. As I've said, it is one large compromise...either you're prepared to accept it or not. Nothing is perfect or ideal when design a boat. Making it work with many conflicting requirements...that is the fun and the challenge.

Good luck.

PS...have you any dwg's/layout of your 'design'..?

 

I've been doing some reading up on the differences between a prop operating in air (with which I've got a fair bit of experience) and in water. With aircraft, we usually try pretty hard to keep inflow to the prop as clean as possible, partly for reasons of efficiency, but mainly because of noise issues. "Dirty" air into a prop can make an aircraft objectionably noisy, hence the reason for lots of pusher designs being louder than their tractor prop counterparts.

The odd thing with boat/prop interactions (from my perspective) is the effect that proximity to the hull has. I've only just realised that, due to the much greater viscosity of water, the boundary layer is much thicker, meaning that hull proximity actually slows the inflow velocity down.

I cant help but notice that a lot of boat propulsion systems don't seem to pay much attention to proper fairing of the underwater parts. Someone earlier gave a link to a sealed electric drive pod that looked to be a dreadful shape, basically a cylinder with hemispherical end caps. Goodness only knows what that does for efficiency.

I'm used to working in an environment where form drag reduction is vital for good performance, so some "normal" small boat underwater design practice looks a bit poor to me. I accept that boat and propulsion system design is a big set of compromises, just as aircraft design is, so I suppose the fix for a lot of boat designs is just to have a massive excess of power by fitting a large engine.

The hull I will be using will be very similar to the Swallow Boats "Winsome", as shown here: http://www.swallowboats.co.uk/content/view/94/30/ but with a lighter hull made from ply, rather than composite. I aim to fit a canopy over the cockpit, rather like some of the Edwardian river launches that were popular here on our big rivers. The top of the canopy will be clad in lightweight solar cells, to provide a partial power source for the motor and allow battery charging when the boat is tied up. I will try and convert some of the drive unit preliminary design sketches to pictures and post them later today.

My set of compromises is pretty heavily biased towards the minimum power needed to power a small inland waters craft, so I'm forgoing some of the things that others might consider essential. I'm hoping that I can still make a practical and enjoyable craft though, as one aim is to try and stimulate interest in largely solar powered small boats.

Jeremy