Trolling Motor(s) for Primary Propulsion of 15' AL 'skiff'

[cahudson42]

About a year ago I rigged up a cheap 'speed coil'-type 34lb 12V MK Endura to a PWM supply. It got a lot of use this Summer on a 12' jonBoat on Lake George, NY (provided it wasn't too rough). Ref: http://www.cfnet.net/tm/

Over the Winter I plan to build V2.0 - using a surplus wheelchair drive that has 2 24V 50A 'continuous' supplies and a joystick differential throttle.

My question is: Should I use 2 motors, or 4 motors?

I can buy 4 12V 34lb Enduras for about the same price as 2 12V 55lb Enduras - $400+ either way.

The wheelchair supply has a nominal 24V input, but a max of 33V. If I run it at 30V, I can put two of the smaller motors in series - and have a pair each on each side - that with 24V and 34A should give me a theoretical 68lb each side, 136lb total. I should also be able to - by raising the V to perhaps 30V (15V to each motor) - creep up to the 50A supply rating - a theoretical 1 HP max each into each motor.

Or I can use just 2 of the 55lb Enduras - one on each side, and by pushing up against the 50A supply limit or a little over - get 55lb x 2 - or maybe 110lb total.

Which would you do - and why?

With the four motors I'd have 4 shafts in the water and four motors causing drag - but probably 'more total prop in the water' than with just the 2 55's.

The reverse with just 2 motors? Less drag? But also less thrust.

Thinking of Howard Hughes 'Spruce Goose' - I'm leaning toward the 4 small motor approach.

What would you do?

Thanks!
Chris

[daiquiri]

With the four motors I'd have 4 shafts in the water and four motors causing drag - but probably 'more total prop in the water' than with just the 2 55's.
The reverse with just 2 motors? Less drag? But also less thrust.
Thinking of Howard Hughes 'Spruce Goose' - I'm leaning toward the 4 small motor approach.
What would you do?
Thanks!
Chris

When it comes to adding up power from different engines, the rule 1+1=2 is no more valid. :)

With two 55 lbs motors you should obtain something around 5 kts.
If I were you, I would go with that combination because at 5 kts you will be running at the steep part of your boat's resistance curve and adding more engine power will increment your boat's speed by maybe a knot (and maybe less than that). Apart the fact that handling 4 motors is a bit more tricky than handling just 2. ;)

The only argument I can find in favor of 4 engines is the redundancy, especialy considering the uncertainity regarding the battery's endurance after few years of use. But you can resolve it with 2 spare batteries charged and kept in the boat as a reserve power.

Hughes' Spruce Goose is not really to take as an example. From the operative point of view, it was a real failure. It was very underpowered and has probably managed to fly only thanks to the ground effect. Come on, you can do better than that. :D

[thudpucker]

I've had an idea like your's for some years now.
You might already have improved on my idea though.

I was only going to use one large Amp Trolling motor.
I would run it with a joy stick, four directions. Your Wheel chair joy stick is good because it's setting relays which take the pressure off (less chance of buring the coils in the Rheostat) the Rheostats in the Joy stick.
But... with the relays you only get one speed! Once set, that's all the current you can provide.
So you have to figure out how to make the stick a variable speed.

Instead of all those heavy batteries....go one step further and design a circuit that will sense the Voltage needs of your Batteries (two small Auto batteries) and when they fall below 10.5V, your sense circuit will pull up a start relay on the small gasoline engine, which is connected to a small 12V Alternator from a Foreign Car or Motorcycle, which will charge the batteries until it sense's they are up and turns off.

[srimes]

why even 2 motors? How fast are you trying to go, and how far? On a small light boat 2 12v motors may not even be noticible faster than 1, unless you overvolt and/or change the prop. I've heard and model airplane props can work well and are cheap, and when you find a good one you can make it in metal.

Start with 1 motor and see what you can get out of it, and spend you money on batteries. Energy storage is why we don't use electric boats, not motor power. If you can afford it lithium's awsome.

[cahudson42]

Thanks daiquiri, thudpucker, and srimes for your thoughts.

And I see I was deficient in enough detail trying to explain what I am trying to do.

My ultimate objective is an electric launch, perhaps about 18', similar in shape to the electric ELCO launches - but built entirely of welded aluminum. But since my welding experience is limited, I will start with a simpler project - something like the 12' skiff from Trident Welding/Metal Boat Kits http://www.metalboatkits.com/12_foot_dinghy/dinghy.htm or perhaps an AL version of the small catamaran at: http://www.jemwatercraft.com/pontoon.php

In all cases I will be happy getting 'semi displacement hull speed' - theoretically in knots 1.34 X squareroot of water line length. Say for a 16' boat - approximately 6 mph/ 10 kph. I believe - from looking at various hullshapes in Delftship - I should be able to have a hull that makes 6 mph with between 100 and 200lb thrust trolling motor power.

I wish to use 2 or more motors because I would like to use a four-quadrent PWM controller, setting up the motors for 'skid steer' - sometimes called Tank Steering or Bobcat steering - so I can dispense with mechanical steering.

I plan to use the Enduras because of experience using one, for simplicity, and for low cost. To me it is hard to beat a 400W, completely sealed, submersible direct drive unit with prop for only $100 (Endura 34). In fact, I don't see why I can't even consider treating the motors as 'replaceable service items' - if I get 200 hours out of one running 800W (1 hp) - its still only 50-cents/hr - or $2/hr for 4 of 'em.

The controller I will try to use first is a surplus Penny + Giles from an old wheelchair (mfg approx 1996). If that doesn't cut it, I am looking at a new Kelley PM24201 or similar.

All boats will be used on Lake George, NY. This is a 32-mile long glacial lake approx 1 to 2 miles wide throughout. Cruising range should be a minimum of 10 miles, preferably 20 miles (2 hours/4 hours) (An earlier prototype 12' segmented cartop jonboat with 1 34lb Endura, single quadrent PWM scooter-type controller and 2 WALMART Marine 12V 105AH batteries in series, has a range appearing to be at least 25 miles - drawing 10A from the batteries @ 2.5 mph and using a 3-blade Kipawa propellor).

Hope the details might help with further thoughts on 2 or 4 motors, or? All greatly appreciated!

Regards,
Chris

[daiquiri]

Well, a semi-displacement hull speed would be a bit higher than what you wrote - it would be something between 1.4 and 2.0 per sqrt(LWL).
1.34sqrt(LWL) would be a conventionaly assumed maximum displacement hull speed.

I have enclosed a resistance curve calculated from the low-speed performance measurements of a 16', 1200 lbs bassboat-type hull.
So, for a boat of similar displacement, it tells you that with 100 lbs thrust (R/Displ = 100/1200 = 0.083) it would run at somewhere around
V=1.38*sqrt(LWL) = 5.5 kts.
With 200 lbs of thrust (R/Displ = 0.17) the speed would increase to
V=1.48*sqrt(LWL) = 5.9 kts.
A doubled thrust would yield only 0.4-0.5 kts increase in speed for that boat. The numbers will be somewhat different for your boat (for obvious reasons) but the general conclusions are pretty much the same.
So you decide if it would be convenient for you to spend double money for that result. ;)

By the way - when you say 34 lb Endura, what does "34 lbs" really tells you? The prop thrust decreases with boat speed (keeping the rpms constant) so, what is the speed at which the prop gives 34 lbs of thrust? Two different series of engines will have a different curve of thrust vs. speed. That's why you can't simply sum the power of different engines to obtain the total. So if you decide to use more than one engine, see that they are all of the same type and rated power.

[cahudson42]

daiquiri wrote:

"By the way - when you say 34 lb Endura, what does "34 lbs" really tells you? The prop thrust decreases with boat speed (keeping the rpms constant) so, what is the speed at which the prop gives 34 lbs of thrust?"

Perhaps It would have been clearer if I stated '34 lbs' as '34A into the motor.'. Nominally, 34A @12V into the motor is supposed to develop 34lb thrust. But with a 24V PWM controller - if drag is low - I can adjust the voltage upward (and the RPMs) until I hit 34A. On something very light, it might end up 15V into the motor @ 34A.

Nominally 12V X 34A into motor - 408W

But also possibly with a 'low pitch' prop/lighter boat, say 15V X 34A - or 510W

Both having the same heat loss - since that is a function of armature current.

So with 24V PWM I can adjust the V/RPM upward until I hit the 34A current limit - as long as the brushes can take it, the windings don't fly apart, and the magnets demagnetize.

And for short periods, maybe longer, likely run the nominal 400W $100 Endura at 70-100% overload (say 1 hp - 745W input).

At least that is my understanding. So far I have not 'run to destruction' one of the 34lb $100 Enduras to see what they can actually take.

[srimes]

I like what you did with the scooter controller. I'll probably get around to following your lead there with my endura 34.

For the speeds you're talking about I would use 2 vs. 4 motors (well, I'd use 1). And I'd use the cheaper motors and overpower them if necessary, but I don't think you'll have to. Just 1 motor can handle the power to push a small boat to the hull speed.

Tank steering is cool and all, and if that's what you want then go for it. Again, not what I would pick, but you didn't ask about that.

Something to consider about reliability w/ 2 motors instead of 1: Will the boat be useable if 1 of the motors goes out? If not, then you have cut your reliability in half by using 2 motors over one! Just something to consider.

As daiquiri points out, it isn't worth it going above displacement speed if you want any range. So if you're building a boat give it a displacement hullform.

[cahudson42]

I will start by using two of the smaller Endura 34's with the Penny + Giles P8 controller.

Initial 'garbage can tank test' of this controller with one Endura 34 and stock prop shows shows that it current limits at 50A with about 15V into the motor. 1 HP in.

The P8 4Q controller w/joystick seems to give the possibility of boat tank steering w/o modification - exactly as it did for the original wheelchair. My P8 is not happy unless two motors are connected, so I used one of the original wheelchair motors along with the Endura for this initial testing. Joystick full forward - both motors rev out. Joystick full sideways - one motor full out, the other actually reverses. Full backwards, both motors reversed.

All I need now is a second Endura and a boat (my Jon Boat got stolen...:>( )

After seeing the pics and photos of the Rick Willoughby catamarans, I'm thinking of something like that instead of the 15' skiff. Maybe 14" irrigation pipes each 16' long? At half-submerged, I think 14" will give about 32 lb flotation per foot - or a total of about 1000 lb for 32' total. Plenty for batteries, several people at a low cruise, fishing, or swim platform.

Any guesstimates of the hull speed I might expect with the 2 HP in total? Or say 1 HP at cruise?? All thoughts, suggestions, appreciated!

Has anyone already used the P8 in this way?

Incidently, the P8 can sometimes be found on eBay - search 'battlebot'. Usually about $70 to $90 including shipping, which I don't think is that bad for a 4Q controller with two 24V 50A channels and joystick.

11/13/2008 - a P8 has just popped up on eBay: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=140281680648&ru=http%3A%2F%2Fsearch.ebay.com%3A80%2Fsearch%2Fsearch.dll%3Ffrom%3DR40%26_trksid%3Dm37%26satitle%3D140281680648%26category0%3D%26fvi%3D1

Item 140281680648 if the above link does not work.


Thanks for your comments and suggestions!

Chris

[ben2go]

I'd just invest the money in a Briggs N Stratton electric outboard and forget inventing.You'll consume way more amperage with 4 small motors than with one motor and a matched prop.
Tiller model
http://www.briggsandstratton.com/engine_power/details.aspx?pid=172
Remote model
http://www.briggsandstratton.com/engine_power/details.aspx?pid=171

[cahudson42]

Thanks ben2go.. Yes, the Etek powered 3 HP Briggs seems a nice package - as do the Torqeedos. But both are out of my price range at about $2000. Anyone can do a lot of experimenting for a lot less. Look again at the MK Endura 34 - its apparently a 1 HP (seems to run fine at 50A, 15V) permanent magnet, direct drive (no bevel gears) completely submersible, water cooled unit with prop - and its $109. Add $100 for a surplus controller, and another $109 for a second Endura - and its 2 HP for about $300.

Plus I'm retired and might otherwise be glued to the boob-tube if I didn't fiddle around with stuff like this :)

At this point I'm 2/3 there - just need the second Endura. Finishing up the wiring harness and a remote mount for the joystick (to keep the power wiring short), 'despringing' the joystick, and adding a deadman switch/lanyard. Then ready to go - just need to decide what boat to put it on - skiff, cat, or what...

[messabout]

Cahudson: You project is most interesting. I am smitteen with the quiet pleasure provided by my tiny Minnkota 24 pound troller. It is sometimes clamped to a 16 foot flat bottomed skiff of my own contrivance. Max speed is 3.8 Knots. Total weight 360#, guts,feathers, and all. I smile a lot when using this combination.

The devil made me scrounge up an electric wheelchair a while back. It is an Invacare Mark IV (if that means any thing). It is all there ready to roll if batteries are replaced. You may have the whole thing, for free, if the components will be of use. I am in Lakeland, just down the road from you. I must protect myself from myself and one of the ways of doing so is to get rid of the temptation caused by the wheel chair/boat propulsion parts.

If interested, e-mail at RSTinstruments@AOL.com

[srimes]

Any updates?

Also, what would happen if you ran the 24v controller with only 12 volts? I'd like the option of only running 1 battery to save weight, even if it's slow, or 2 to go fast and farther.

[ben2go]

12 volts couldn't supply enough power and either kill the battery really quick or damage the 24 volt controller.Plus it would put off major heat and possibly melt or catch fire.

[portacruise]

http://www.boatdesign.net/forums/boat-design/batteries-new-battery-technologies-21869-19.html

Pg 19 post 279-283 may be of use here.

Porta



Any updates?

Also, what would happen if you ran the 24v controller with only 12 volts? I'd like the option of only running 1 battery to save weight, even if it's slow, or 2 to go fast and farther.

[cahudson42]

Hi everyone! Took me a while to get back to this, but I finally got my jonboat operational with the P+G P8 wheelchair controller and twin Enduras. Some info at:

http://www.cfnet.net/tm/v2.htm

And messabout - sorry I didn't get back to see your offer! The P8 I used came off an Invacare. If you have the thing still, put at least the cables and connectors on eBay. While the p8 (if you have it) needs to be modified so it will run without the brake solenoids, the cables/connectors are hard to find.

Comments, questions, suggestions all appreciated!

Chris

[srimes]

cool. So how well does it work?

[cahudson42]

Hi srimes,

Well, I think the wheelchair controller and skid-steer is working great with the two Endura 34's.

But the jonboat is an efficiency disaster. Sort of 'pushing a brick lengthwise thru the water'.

I really need a transomless/swallowtail/canoe-style efficient, light, long, displacement design alternative.

Ultimately I still want to weld up an ELCO-style cruiser out of AL. But I'm not likely to get to it this season.

Perhaps I can throw together some kind of light Catamaran - with canoes, plastic cheap kayaks, AL culvert tubes, whatever - for the time being. Or just keep the jonboat cruising at 2.4 - which is OK - after the sun goes down.

As it is, I get maybe 2.6 mph at 20A into each motor. It will peak at 3.4 mph or so before the 30A breaker in the 24V supply line trips. This with the standard 2-blade 'flexible' Endura props - probably efficiency disasters as well.

As always, suggestions, comments, ideas always appreciated!
Chris

[portacruise]

You have proven that joystick control can work, if that was your goal.

Is there any advantages to simpler steering is the question.

The power draw for the speed you are getting is way too high if you're looking for efficiency. Could be other things causing this besides your jonboat. Even a flying brick should not take that much power for the slow speeds you have indicated.

If you don't have deep cycle batteries, they won't last long. Even the dual purpose marine type.

Weedless props are very inefficient, you might try google of shorty evans which sells older non weedless- if they are still in business.

Your wiring losses might be too high, need jumper cable type stuff for the 20+ amps you are drawing.

Just some thoughts off the top of my head, hope it helps.


Porta

Hi srimes,

Well, I think the wheelchair controller and skid-steer is working great with the two Endura 34's.

But the jonboat is an efficiency disaster. Sort of 'pushing a brick lengthwise thru the water'.

I really need a transomless/swallowtail/canoe-style efficient, light, long, displacement design alternative.

Ultimately I still want to weld up an ELCO-style cruiser out of AL. But I'm not likely to get to it this season.

Perhaps I can throw together some kind of light Catamaran - with canoes, plastic cheap kayaks, AL culvert tubes, whatever - for the time being. Or just keep the jonboat cruising at 2.4 - which is OK - after the sun goes down.

As it is, I get maybe 2.6 mph at 20A into each motor. It will peak at 3.4 mph or so before the 30A breaker in the 24V supply line trips. This with the standard 2-blade 'flexible' Endura props - probably efficiency disasters as well.

As always, suggestions, comments, ideas always appreciated!
Chris

[cahudson42]

Thanks Porta,

The P+G P8 wheelchair 4Q/joystick 50A 24V 'battlebot' package is pretty cheap on eBay - at least it used to be - so I thought trying a joystick might beat squirming around with the tiller on the Endura.

For 'evening cruising' on calm water it works conveniently. But if its rough, my bouncing around transmits to the joystick - not so good:(

As you point out - efficiency is terrible. But the batteries I have seem up to it. They are deep cycle full marine - not 'dual purpose' marine. While they are WALMART, they are manufactured by Johnson Products. I rarely discharge them more than 30% - and even after 2 years now (2007 date code) they seem fine. I charge them with two 3-stage B + D 12V/12A chargers

Yes, 2 of 'em at 60 lbs each is heavy. But I never really could see getting LiPO, AGM or whatever that might weigh less - as I am 210 lbs, the AL jonboat perhaps 100 - 150 lbs, the motors another 30 lbs - so as a percentage displacement difference, expensive but lightweight batteries didn't seem worth it to consider.

With a single MK 34 I used an aftermarket 3-blade Kipawa prop. To keep the twin arrangement balanced, I switched back to stock to match the one on the new second motor.

I'm going to put the Kipawa back on one - run it 'unbalanced' - and then try to get the power going into each motor in a 'straight ahead' joystick condition. That should tell me if getting a second Kipawa is worth it.

I've only put a few hours on this arrangement - the P8 replaced a single channel electric bike Curtis 1505. (w/5k pot controller) But the P8 does seem to run hot to me - even at only 20A/channel. We shall see.

Next I'm toying with running the 2 motors in series - back on the 1505, or perhaps a 36V controller (maybe a Kelly). Should reduce wiring currents and perhaps switching losses (run at a higher % of V in.) - but of course I'll need identical props. Then its back to a rudder/tiller arrangement - something out of plywood with hinges for a pivot etc.

Further thoughts always appreciated!
Chris

[srimes]

why not try playing around with the trim to see if it can easily be more efficient? I like how you put the batteries in the front to balance it out. I'd try:
a) sitting in the middle w/ the batteries still in front for nose-heavy trim, and
b) try running the boat backwards like that.

for a more efficient boat cheap it's hard to beat building on with plywood. Stitch-and-glue or even screw-and-glue with chine logs is quick and cheap. Fun too.

[cahudson42]

Thanks srimes,

Sitting in the middle definitely helps. With the transom now up more out of the water, drag is definitely down - even though the bow is almost submerged. (even small chop crashes over - so its not practical).

I might go back to looking at plywood - maybe again for a catamaran prototype etc. I did look at stitch' in-glue - a number of the Glen-L designs, but here in Lake George, NY the shore is often very rocky or bottom covered with good size boulders. Since I need to beach my little electric often, I pretty much have settled on welded aluminum - not the usual 1/8 - .125 - but 1/10 - .100 - which is enough I believe, and is of course lighter. But this won't happen this season.

I'm going to try the propeller tests next - when the thunderstorms abate..

[portacruise]

When I've tinkered with motors in series years ago, I got erratic performance. This was in another application, not with troll motors though. The power delivered by each motor varies from second to second depending on what is happening loadwise with the other motor is what seemed to occur. Almost act like 2 variable resistors in series. If one momentarily draws more power, then the other is left with less. When I ran at no load and stalled the spinning shaft of one motor, then the other motor would speed up in response.

Hope this helps.

Porta



Thanks Porta,

The P+G P8 wheelchair 4Q/joystick 50A 24V 'battlebot' package is pretty cheap on eBay - at least it used to be - so I thought trying a joystick might beat squirming around with the tiller on the Endura.

For 'evening cruising' on calm water it works conveniently. But if its rough, my bouncing around transmits to the joystick - not so good:(

As you point out - efficiency is terrible. But the batteries I have seem up to it. They are deep cycle full marine - not 'dual purpose' marine. While they are WALMART, they are manufactured by Johnson Products. I rarely discharge them more than 30% - and even after 2 years now (2007 date code) they seem fine. I charge them with two 3-stage B + D 12V/12A chargers

Yes, 2 of 'em at 60 lbs each is heavy. But I never really could see getting LiPO, AGM or whatever that might weigh less - as I am 210 lbs, the AL jonboat perhaps 100 - 150 lbs, the motors another 30 lbs - so as a percentage displacement difference, expensive but lightweight batteries didn't seem worth it to consider.

With a single MK 34 I used an aftermarket 3-blade Kipawa prop. To keep the twin arrangement balanced, I switched back to stock to match the one on the new second motor.

I'm going to put the Kipawa back on one - run it 'unbalanced' - and then try to get the power going into each motor in a 'straight ahead' joystick condition. That should tell me if getting a second Kipawa is worth it.

I've only put a few hours on this arrangement - the P8 replaced a single channel electric bike Curtis 1505. (w/5k pot controller) But the P8 does seem to run hot to me - even at only 20A/channel. We shall see.

Next I'm toying with running the 2 motors in series - back on the 1505, or perhaps a 36V controller (maybe a Kelly). Should reduce wiring currents and perhaps switching losses (run at a higher % of V in.) - but of course I'll need identical props. Then its back to a rudder/tiller arrangement - something out of plywood with hinges for a pivot etc.

Further thoughts always appreciated!
Chris

[cahudson42]

Thanks Porta,

Fortunately, Lake George has little millfoil or other weeds that might stall one of the two motors. But if I see that series is workable, what you mention suggests that I probably should put a breaker in series as well. I've got a couple extra 50A, and one of them should at least protect the unstalled motor - just in case.

Chris

[cahudson42]

The rain finally stopped and this morning was clear and calm. Got to compare the aftermarket Kipawa 314 propeller with the stock MK 'weedless'.

At least for this setup, I won't be spending money for a second Kipawa.

With the Kipawa on it, at 40A into it, the MK 34 showed 10.0 Vin - and jonboat speed was 2.7 - 2.8 mph. Putting back the stock prop, at 40A into it, Vin was 10.75 - speed 3.0 mph.

So the Kipawa is going the wrong direction - more current, less voltage, less watts, more losses. (But it does make sense, since the Kipawa was designed for the stock 12V MK - where its never going to actually see 12V because of the wire/switch losses - and will deliver more than the stock prop at a given MVin.)

At 30A/motor, 9Vin, with stock props its about the same 2.8 - 2.7mph as it was with the Kipawa. But the P8 wheelchair controller - with 24Vin - is now delivering very short power pulses (to 'average' 9V) with the same 20kHz switching losses. Seems likely one reason why it runs so hot and battery drain seems higher than it should be..

I am going to rig up some cables for series running the motors. Makes me wonder if the 'most efficient setup' for the p8 might actually be 4 motors - 2 series pairs. Ugh.

Comments, thoughts always appreciated.
Chris