Efficient electric boat

Rick, if you can access the archives of the power assist list (HPV) way back to the late '90s?, there is some good info there on belt vs. chain efficiency. As I recall, an engineer on the list found essentially no difference in efficiency between cog belt and chain for reasonable sprocket sizes if everything is tensioned properly.

One of my e-scooters back then employed what I considered a very nifty reduction system using an idler pulley with cog belts. One principle advantage of an idler is that the tension of the belt can be maintained at specs regardless of changes in temp, humidity, wear, etc. Another is that when the belt does break, it is a matter of seconds to thread a replacement. Also thinner, somewhat under spec belts which have greater flexibility can be used to achieve large reductions in a single stage. Smaller cog drive sprockets can be used without slipping or increased wear issues, by positioning the idler in such a way that you get better wrap around on the cog teeth.

Hope this helps.

Porta

 

Porta
That is very useful information.

I have already concluded that I should have an idler pulley or other means of fine adjustment for the tension. I have that on my pedal drives.

I have a feeling that part of the problem with the small belt failure was some initial overtensioning. Then again I was working it quite hard.

Rick W

 

Rick,

I use a toothed belt drive on my launch. MARS BLDC and Sevcon Millipak PMAC controller; Link 10 battery monitor.

Running just the motor (no belt) full throttle takes 2.7 amps at 48v; 130 watts.

With the belt installed turning the prop shaft supported by an MRC four bolt ball bearing flange inboard, an old fashioned flax stuffing box and a rubber cutlass bearing at the prop end running dry takes 6.4 amps. So the "overhead" load before producing any useful work is 300 watts.

A dripless ceramic face seal stuffing box may have less friction (for us$ 300) but it is hard to see how you could do much better than this in a full size boat.

I've not tried a chain drive but I understand they are very noisy at speed.

 

Denny
I have a couple of the Mars PMSM motors. I intend to use one for the propulsion.

I would be interested in the belt size you are using and the arrangement.

At this stage I intend to engineer a curved shaft. It will exit the hull above the waterline so I will not need a conventional stuffing box. Some sort of shaft tube with a lip seal should do OK to stop any splashing. At rest the shaft exit will be 100 to 150mm above the water surface.

I have already made a little outboard with one of the Mars motors but I did not like the drag from it. I used a 2:1 ratio box under water. Its overall efficiency was as good as the little motor though.

The little lipo battery I have will enable me to test the mars motor and curved shaft on one of the pedal boats at the sort of power level it will work at on the full scale boat. I should be able to get it all nicely proven before I even start building the boat. That is still a year or more down the track.

I hope to get no load losses at cruise speed to less than 50W. It is a ballance between efficiency at low power level and robustness at high power level.

One more question - how is the durability of your belt drive?

Rick

 

I can attest that you can do double duty, and a fine job at it!, if thats any consulation.

 

Thanks Oyster! (Erster?)

Rick, I'm using a 15mm wide 5 pitch belt made by Goodyear. It never slips and shows no discernable wear. 44 tooth drive sprocket , 90 tooth driven on a 3/4" shaft. No idler. Center to center distance 9.5". This belt size was the recommendation of the Gates design program mentioned above.

The motor attachment bolts ride in slots in the mounting plate. Belt tension is whatever I get pushing the motor sideways with one hand then tightening a bolt with the other - not very much.

 

One thing to clarify on the idler pulley. It was spring loaded and adjustable so that tension was maintained at a fairly constant set level. Yet, one could pull the idler back to remove a worn belt without tools and quickly thread in another. I remember keeping a couple of spare belts taped to the inside of the fork so the scooter wheel would not have to be removed for belt replacement-came in handy once.

Kevlar was being considered for cog belts many years ago. I have not kept up to know if kevlar is available today at a reasonable price.

Porta

 

An idler pulley sounds like a good way to get a better wrap around the small pulley, and allow reduced belt tension (which seems to be a good thing, as it reduces power absorbed by the belt). The idler would need to be fixed though, rather than spring loaded, otherwise the option of being able to go astern would be prevented.

I've dry run the new drive now, and it seems to work well. I need to take some measurements to see what the losses are, then move on to designing the propeller. I'm going to look at two options; a high aspect ratio folding prop, and a skewed/swept blade fixed prop that I hope will have adequate efficiency and be good at preventing weed entanglement.

The Winsome prop is an interesting design, I wish I'd taken some pictures of it when I was over at Swallow Boats. It has a fair degree of sweepback on the leading edge, but a trailing edge that is near perpendicular to the prop shaft. The result is a very deep root section, tapering to a fine tip. Although it purportedly works well under pedal power, and is pretty good at shedding weed, it looks to have far more wetted area than I would have thought optimum. It's 12" in diameter, with a pitch of around 15 to 18" I believe (I can't recall exactly), and spins at around 300rpm at 4mph. The design does make for a strong prop, due to the big root. It's moulded from glass fibre, so probably needs the big root to be stiff enough.

I'd like to see if I can combine a swept/skewed blade design with a narrow aspect ratio, using carbon fibre to get enough stiffness. I realise that it will be sub-optimal in terms of absolute efficiency, but the trade-off between efficiency and ease of use might be worth it Right now, I don't have a feel for the possible efficiency loss from swept blades, does anyone here have any ideas?

Jeremy

 

Propeller

Torqeedo must have put a good deal of thought into designing their prop - which you can buy off the shelf for about US$ 80.



It's 12d x 10p. I think is has a 10mm bore with a notch for a shear pin. Made from some high tech plastic.

 

Thanks very much for that, Denny. Quite why I didn't think to look for a commercially available prop before spending a couple of hours trying to work out how to build one from scratch is a mystery.........

I'll go take a look at the Torqueedo parts and see if they have a prop that will do the job. At first sight, a 12 x 10 seems a bit too fine a pitch, I'd need to spin it at around 550rpm and then might only get about 69% efficiency. It might be worth a try though, especially as it's available off-the-shelf.

Jeremy

 

Rick,
If this may interest you, go to cat2fold.com, a trailerable cat . But for live aboard, you will have to find adaptation. I am designing my catamaran with a specific program, 37 ft wide 60 ft long will reduce to 15 ft wide x 32 ft long for marinas and locks.
Mike

 

Jeremy
I have a procedure for fabricating props from stainless steel flatbar. If you are interested send me an email:
rickwill@bigpond.net.au
I design them for this method of manufacture and they work close to what can be done with a precisely milled prop. It depends on the effort put into shaping the blades. But the difference between what can be done if you spend 1 hour versus 8 hours is around 1% in efficiency.

Rick W

 

Mike
I have seen your cat images before. I know Fanie was looking closely at your folding arrangement about a year ago.

I am reasonably happy with the faux-tri idea but I am yet to get into any construction detail.

I could not find anything on the detail of your hulls. What material are you using and what weight are you achieving.

By the way there are currently two other threads looking at similar ideas. You already have all the details.
http://www.boatdesign.net/forums/multihulls/modular-cruising-catamaran-28446.html
http://www.boatdesign.net/forums/powerboats/power-trimarans-28511.html
The latter is on tris but it covers the gamut.

Rick W

 

Rick,

Thanks very much for the offer, re: prop manufacture. I think I may already have looked at the method you use, perhaps on one of the other HPV sites? IIRC it consists of cutting a strip of stainless to shape, twisting it to the required shape, then grinding it to the right aerofoil profile.

I've been looking more closely at prop matching tonight, and come to realise that it's a bit different to the way I'd match a prop, reduction drive and engine on an aircraft. Because of regulatory requirements, I'd be obliged to match an aircraft prop to maximum full throttle power. Essentially, I'd pick a diameter and pitch that would absorb the full power of the engine at full throttle, straight and level, at maximum continuous engine rpm.

Having looked at prop power absorption curves, my motor power characteristics and the probable hull resistance curve, I've come to the conclusion that picking a prop diameter, rpm and pitch for this application needs a different approach. I believe that I need to optimise the cruise condition for best efficiency, and accept that this will mean not using the full power potential of the motor (not a problem, as it's over-rated).

I've also been thinking about the prop drive system. A swivelling drive leg has some significant advantages, particularly with regard to removing the need for a rudder and possibly reducing the appendage drag by not having a skeg. The snag is finding an easy way to take the drive down the leg, with good efficiency, relatively low noise, and at an affordable price.

One option would be the neat little Mitrpak gearboxes that Rick has highlighted. These would still need some work to fit into a drive leg, though, plus, at $200 plus shipping and taxes, would add a lot to the price. Looking at some of the other HPB drive solutions, it occurred to me that a simple 1:1 ratio chain drive, enclosed in an oil bath, might be an option worth looking at. I've been looking at using 04B 6mm pitch chain, which looks like it might be a good solution. I could use relatively small, 10 or 12 tooth sprockets both at the bottom and top, to give a slim leg design.

Manufacture of such a leg looks straightforward, just a pair of identical turned shaft/bearing/sprocket housings, coupled together with a length of rectangular tube. The tube could be faired to a streamlined shape to reduce drag, just like I've done with the cheap drill gearbox.

Coupled to the belt drive reduction unit I've already built, I think this looks like an option worth experimenting with, unless anyone can see any obvious snags that I've missed.

Jeremy

 

Jeremy
The prop manufacture you have seen is correct. I approximate the best pitch curve by two angles of twist. There are a few traps but if you know what props look like then they are easily overcome. The prop is designed or optimised for the desired operating point. On my boats it is normally 3 to 3.5m/s. The main constraint to ballance against efficiency is to have the blades strong enough to take the maximum thrust.

If you are considering a twisted chain drive then the best I have seen is actually not a chain. It uses a twisted belt. Nauticraft use them in their little boats so they stand up well. I have tried twisted chains and do not particularly like them. The belts feel nicer and are a little more compact. A twisted chain has around 97% efficiency at the power level you are considering if it is not in oil. I expect putting it in oil will reduce that quite a lot.

When you look at the time and effort that goes into these things the gearbox works out to be good value. I still have most of the boxes I have purchased and reuse them for different things. I have destroyed two small boxes that I worked well above their rating.

The main advantage of the gearbox is the fact that you can make a very narrow fairing over the vertical drive shaft so you save energy with less water disturbance.

As you know I favour the curved shaft. It offers advantages in most aspects and no particular draw backs. Just that it is not commonly seen.

Rick W