PocketShip and electric power

Builder's Forum
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Dear all,

My name is Edmunds, I'm from Latvia (in Europe), I'm 37 and have been sailing since I was 7 or so, some of it competitive at decent international level. At some point some years ago life changed and I stopped sailing and a few years ago I moved away from the sea and major waterways, inland. A change I have been super happy with, partly because it comes with a proper workshop, but sailing has been missing. I have been considering a boat morred where I used to sail, but clearly, I would use it only so often and mooring and maintaining a boat you have to spend 3 hours to get to by car is expensive and inefficient in many other ways.

Somehow, I had missed the boats below about 22ft and the possibility of attach [to the car] - drive some distance [an enjoyable trip, maybe] - put up and float the boat in less than 30 minutes - sail - do the before backwards sequence, but now, when I have discovered it, it feels just the thing I need.

Also, I love the idea of building a boat again, as the first boat we sailed with my two younger brother was actually one we built in our bedroom when I was about 13 years old. Our father did help in the end when the project became impossible to hide :). The construction method was basically stich-and-glue, just we did not know about stiching at the time and plywood was expensive, so we built the boat on a frame of scrap lumber with keel screwed to bedroom floor, deck up first and cardboard instead of plywood. And a heavey (too heavy) coat of glass matt and cloath and poliamyde resin on top. My mother and the dog evacuated themselves for a week to the countryside during the sanding phase and some furniture never looked like before when we were done.

With all the history out of the way, I have an idea I would like to get some opinions/input on for PocketShip modification.

I hate petrol engines. I do. They take all the joy of silence and fresh air out of sailing. With that said, where I sail, a current of 4 knots and no wind on a summer evening is not an impossible or unseen combination. Been there, done that. I do understand, an outboard, either electric or diesel is an option. But having to put that on, protect from theft, take off is not something I look forward to. And it does ruin the looks.

So, here is an idea. How about embedding an electric engine into the keel with a folding prop? A bit like you see on old school boats with long keels, but not just the propeller rod coming through it, but actual opening for the enclosure of electric motor to sit in. And imagine, practical issues, like water tight enclosures and entries into the hull are not an issue or, rather, are taken care off. Would there be anything strictly against this kind of solution?

 

Thank you for your input,

Edmunds

2

I share your distaste for gasloline outboards, which is why I seriously considered a similar modification to the Nimble Arctic 25 that I owned for 17 years.  Back then, I found two significant hurdles.  The first was battery technology (very heavy with limited capacity).  The second was that there really wasn't any marine electric motor on the market with any kind of track record.  At the time, it seemed like a manufacturer would come on the market only to go out of business a year or two later.

These days, I beleve that the battery issue is much better with the new generation Lithium batteries.  They pack a lot of energy in not much weight, but are expensive.  Assuming that you can find a good motor, I have not doubt that you could build an inboard electric motor into a Pocketship.  It would be a challenging project to do it right and there would be some technical risk.

A much easier approach would be to buy a Torqeedo outboard.  They are very light and could easily be hidden in the cabin when not needed.  They are quite expensive.  I plan on buying the 1003 in a couple of months for use on the 15' skiff that I am building.

One other issue, is that regardless of the type of motor that you put on a Pocketship, it will be struggle to make much headway motoring (or sailing) into a 4 kt current.  Unless the Pocketship will plane (which I doubt), you won't see more than about 5 kts through the water regardless of how big the motor is.      

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I am building a PocketShip now and agree with the idea of an electric motor. I guess anything is possible, but with the center board slot and other bulkheads, you would have to do some engineering to get anything to work. The keel is hollow in some areas toward the stern, but there is lead ballast that may get in the way. Also the rear of the keel is only about 3/4" thick. Would that be wide enough for a prop drive shaft?  But it you are set on it, I would get with John Harris to discuss and get his views. Do you have any links to what you are thinking about installing? 

I agree with Mark that the Torqeedo Outboard may be a simpler option. I am still trying to figure out what type of Outboard to put on my PocketShip. I love the idea of not having to deal with fuels, but I am not 100% sold. I still read mixed reviews. I also have the same concerns with currents and would the Torqeedo have enough power. I typically sail rivers and tidal areas. 

Mark, I would be interested to hear about your experience or research with the Torqeedo?

4

   Thank you for your replies. Is there a simple way to upload a sketch I have drawn on a napkin? I have a JPEG file, but forum requires the file to be stored elswhere online, right? How do you normally do it around here?

Thanks,

Edmunds

 

5

Ok, I did crack the image problem. Hopefully.

I had an even simpler arrangement than finding space inside the boat and routing the drive shaft. I have attached a little sketch that hopefully makes some sense :). Imagine you cut the motor part off a typical electric outboard. The kind many lake fishermen use. Then you take the motor enclosure with motor inside and the propeller and build it into the keel. Since the boat would be on the shore most of the time and the motor is 'low maintenance' electric, there is little problem I don't have access to it while on water. The only challenges I see would be structural if any - don't have the plans yet and the manual just arrived and making space for the propeller. Either in the rudder, the keel or both.

When it comes to the batteries, LiFe could be an option as these do not fire themselves up, but I would not consider LiPo. However, a good conventional GEL battery of, say 110Ah would give me a couple of hours of run time that would normally be enough to get home safely with ~1hp electric. Or so it seems from the specifications I have studied.

Making 1 knot against that tide is an achivement for many boats, believe me :). The tide depends on the powerplants on the river and 4 knots is kind of extreme case, of course. More often it is about 1-2 knots that you have to fight. And there are boats and conditions that just does not work for this, I know. But I want to take the precautions I can.

Thank you all for your input,

Edmunds

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   There is an "in mind" missing in the end of the first sentence. Sorry for the typos, my writing is lagging the thinking :).

Edmunds

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Keep us posted! There is another Pockeship forum for builders if you are not aware.  Its www.pocketship.net . Another good resource for you. 

You can also check out my blog for some pictures of the keel area being glued up. May give you some more ideas. http://ronspocketshipbuild.blogspot.com/

Thanks

Ron

   

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Turns out one would not need to destroy a perfectly working motor - you can buy just the underwater unit. If I understand everything correctly, this is capable of 2HP output.

Electronics is one of the things I do for living, so controller is not a problem, I would like to build my own. Now the problem is a folding propeller of the right parameters and that I have forgotten how to work out to even start looking for something to buy. Will need a refresher on that theory.

Ron, thanks for the heads up. I'm in a stage of planning the build. It is unlikely I can do anything larger than collecting useful items, thinking/dreaming and building some not so big parts until this time next year due to substantial project pipeline. This is probably good, because I'm forced to do my research and preparations properly. My natural aspiration would be to jump right at it, but it does not always give the best results :).

I have applied for the other forum, but I have not been approved by administrators yet.

Edmunds

 

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Edmunds,

The solution used for Sam Devlin's Nancy's China DC might give you food for thought.

Regards,

Dick

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Dick, thank you, this is it! Exactly what I was thinking. Now it only remains to figure out if building the motor into the rudder is the best possible idea and how badly do I need the folding prop. They claim 4 knots with 14 hours of running time, but do not specify the batteries. Anyway, it is a good 'proof of concept', I'm confident this is a good engine power solution worth pursuing then.

Edmunds

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Edmunds,

I mentioned, to John Harris, the possibility of this solution being feasible for Autumn Leaves and he responded:

"Sure, that's an elegant solution.  I don't know if there's a way to
lock the propeller when the motor's off; that would reduce drag
compared to allowing the prop to spin. (Drag of a spinning propeller
is said to be equivalent to a flat disc the same diameter as the
propeller.)"

I'd be interested in whether the motor might be used to charge the battery when under sail. Autumn Leaves doesn't have a lot of convenient horizontal space for solar panels. Locking or folding/feathering the prop to reduce drag is another possibility if charging isn't feasible. I do not have the requisite electronics knowledge and would be interested in your opinion.

From a hydrodynamics perspective, I wonder if rge stock Autumn Leaves' rudder is deep enough for the propeller. PocketShip's appears to be.

Cheers,

Dick

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Dear Dick,

A spinning motor produces electricity. This cannot be stopped :). The only question is, what you do with it. If you have a smart controller that detects the current is now flowing in another direction, it could be fed into a respective charger circuity to charge the battery while under sail. Such controller would, sure, be possible to build and would cost very little in parts. Design time would be another storry, of course.

To judge the feasibility, it would be best to take one such motor and trail it on something moving at the speed you expect to be sailing and measure the voltage on the terminals of the spinning motor. Another option is to just assume, the motor will be 50% less efficient as a generator than it is as a motor. Then, if the motor draws 250 watts to move your boat at 3 knots on the water, you can expect 125 watts of power generated at the same 3 knots. If 50% is any good value for this kind of thing I don't know, but probably one could find some formulas on the Internet as I have seen 'micro power' generators that are basically the same thing dropped into a creek.

I would also like to factor in the drag of the spinning propeller. I.e. I might be ready to produce electricity and loose 1% of my speed, but loosing 20% would sound not worth it. Sounds well worth an experiment. I might actually be able to do it this summer on my brother's boat.

Cheers,

Edmunds

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Edmunds,

Thanks for the info. I'd like to keep up with the results of your thinking and experiments. I'm at least a year away from building Autumn Leaves, but I will be researching options as time permits. My first step will be getting more specifics on the Devlin's Nancy's China DC installation.

If you don't mind sharing your contact info, it can be done privately through the CLC Builders Club. Just click the link for a direct connection where we could share email addresses.

Cheers,

Dick

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   Wouln't a generator with a prop spin faster than a propeller in the water slowing you down? Why not an electric motor mounted to a lower unit of a old out board motor, saw a video of one using a cordless drill too,HAHA.

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Edmunds,

               As I re-read what follows, I am afraid that I may have come across a bit too negatively.  The beauty of building your own boat is that you are free to do whatever you want, and I strongly encourage you to follow your dreams when it comes to your build.  That being said, I want to share what I have learned with respect to this subject, so here it is:

I have a significant technical background (MS in Aeronautical Engineering) and this is a subject that I spent a lot of time researching when I tried to design an electric propulsion system for my Nimble.  What you propose is certainly viable but there are a number of technical hurdles to overcome.  My thoughts in random order follow.

  1. Ideally, the motor would be located in the keel just forward of the rudder.  This will obviously require some redesign of the keel and possibly the rudder also.  If it were me, I would make sure that the motor is not too difficult to remove for maintenance purposes because everything that can break eventually will.

  2. I am fundamentally opposed to the idea of integrating the motor into the rudder.  The rudder is not designed to handle those kinds of loads and would likely fail unless there was significant strength added.  This would also result in a less efficient rudder when sailing.

  3. The Minn Kota motor pod that you referenced is listed as “Salt Water Resistant.”  Reading through its specs, I see nothing that says that it is designed for constant emersion.

  4. The motor pod lists a static thrust of 101#, which is pretty good.  The problem is that the prop is designed for trolling (slow speed) so the thrust will drop dramatically as the boat starts moving.  You will need to find a prop designed for higher speeds (~5kts) which may not exist on the market.

  5. The motor pod is 36V which means that you will need three gel cell batteries.  Total cost here in the US for three group 24 is about $1000 and total weight about 160#.  If it were me, I would design the battery storage so that it replaces some of the ballast and does not impact the boat’s trim.  If you went with wet cell batteries it would be a bit cheaper but they require constant maintenance and there is always the worry about acid spills during a knock down.

  6. A battery system like above will require a robust charging system with a multi-battery/multi-stage capability.  It is unlikely that you can fit enough solar onto a boat as small as a Pocketship so you are talking a shore power system.  Both will add weight and considerable expense.  Designing the wire layout will be non-trivial and you will want to follow ABYC guidelines.

  7. Torqeedo does offer a motor pod but the smallest is equivalent to a 5hp gas motor and is probably too big for your purposes.  It is also VERY expensive.

  8. As I said before, if you are committed to an electric auxiliary, the Torqeedo outboards look pretty good but are not cheap.  I am building a Goat Island Skiff and plan to buy the 1003C.  That motor only weighs about 36# with integral battery so it can be easily removed.  It will charge from any 12v (car) or wall outlet.  It has thrust equivalent to a 3hp gas motor and will run at full throttle for one hour or at half throttle for 6 hours.  Pushing my light weight GIS, that probably gives my ~5 miles at full throttle and ~20 miles at half throttle.  Hopefully I am done with the build by fall and ca give you hard numbers.    

  

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Another exemple,

Zachary Wiest's skerry:

http://skerry.knowpatience.com/category/auxpropulsion/

Ruud

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Dear Mark,

Thank you for your thorough reply. Understanding of aeronautics is not my strongest side. However, I do remember learning a great deal about how a propeller would be calculated depending on the weight of the boar, power available, RPMs of the motor and expected moving speed through water while I was in college and wanted to build my own motor boat. The project never happened, but I must have some books somewhere.

I have done some further research of what parts with what specs are available and also done some thinking, so I will try to comment on your reply as best I can for general information and interest of visitors of this forum.

1. There is no doubt the motor should be possible to repair. While this would not be the easiest from build perspective, it would be best if the motor could be taken out entirely with all of its body. How to build that is beyond the scope of this discussion, as I do not have the motor nor the boat :).

2. I would not do the rudder project either. While I'm confident in the strength of the rudder - it is subject to loads way beyond the small motor can develop during normal sailing on a lifting keel boat like PocketShip, I think. Mind you, this is not mathematically based, but 15 years of competition sailing on micro class and braking at least four rudders in various conditions and because various reasons. My prime worry would be hitting the motor/propeller. Rudders do get hit in the back by other boats, piers and other moving or chained down subjects. Also, I'm not sure, how comfortable the steering would be. I imagine it could be compared to firefighters handling the hose :). The only interest of the rudder-based project was evidence the power was reasonable for this kind of boat.

3. My plan for the boat is a few days of immersion in one go. I live where there is no water to sail or at least nothing that would take more than an afternoon to cross several times. My sudden (somehow) excitement about pocket cruisers comes from the realisation I can afford to keep a boat for a few days sailing trip down somewhere where there is enough water and most of the trip would not be getting the boat in and out of the water and ready to sail/trail.

4. The prop problem I understand. I have also mentioned before, I would like to fold the prop. I cannot really find anything that would even be close to what is required to start doing the math if that could fit. Making a mould and casting my own propeller would be time consuming, but not the end of the world. Folding - not so sure about, but looking into.

5. I have looked into the battery solution as obviously, it can easily be a deal breaker. My conclusion is, it would be possible to arrange a substantial battery bank under the cabin floor. One would have to use batteries of approx 42Ah (12V) per unit to fit them, but adding 6 such units seems very well possible and even more if one would use two 'compartments' on both sides of the centreboard box. They could be connected for required voltage, preferably 12 or 24, not 36 volts as 36 can be dangerous to humans in certain conditions and 'on a boat' sounds like asking for trouble. I hope, I could get away with between 55 and 86 lbs of thrust, not 101 for lower power consumption, but this has to be calculated and tested. A suitable battery pack for these smaller motors would cost me about 600USD delivered AGM GEL, which is not too bad.

6. Sure, charging and wiring in general would be some challenge if compared to single battery installation and solar is just for the fun of it, not actual recharge in where I live. However, the electronics part is my cup of tee and I'm looking forward to building/learning what's new as much as to building the boat itself.

7. Have not looked at it specificially, but I would like to keep the cost down as much as is realistically possible. I have inquired several China based shops that seem to be the OEM producers for European/US brands, but do advertise selling a single unit if need be. The price, of course, is very different. After all, it is a simple electric motor with some better bearings and gaskets for watertightness.

8. My resistance to hanging a motor on the transom is somewhat irrational. I do understand that is the simple way forward and if all else fails and the motor is what stands between navigation and further fiddling with the boat, I will probably give in too. For now, I want a beautiful transom with some nice wood grain and no clutter. And a challenge, which would be to come up with efficeint and clean propulsion at reasonable cost.

 

 

 

I value your input a lot, even if that would be critical, but it did not sound so critical at all this time :).

 

Thank you once again,

 

Edmunds