Tidal Generation Ideas

[thunderhead]

With the help of information and stories on this site, it looks like I'm finally bringing my wife around to living self-sufficiently, and we're arranging to go look at an island in an estuary which might suit us.  It's cheap because it's an island, and won't get mains electricity and water.  There is an abandoned house, believed to date from the mid-19th century, so there is probably water somewhere - but it will never have mains electricity.

At the moment we use maybe 25kWh a day of electricity, without heating.  We use washable nappies (diapers) for our young son, and that takes quite a bit of power.  By the time we've included heating for an energy-efficient restoration, we're looking at maybe 40kWh per day in winter.  We also need to pump water from an (assumed) borehole, and my microbiologist wife will insist we run UV sterilisers alongside the pump.  The island has been used by livestock, so she's probably right.

The weather averages about six hours sunshine a day in July, and about two hours a day in January, with most months somewhere in between.  PV isn't going to do us much good.  The wind averages about 12 knots in the nearest town, but "our" island is nearer the coast, so windpower is a possibility.  The maps don't show any streams, so no hydro.

But ... it is in a narrow inlet to a bay of about a square mile of tidal water, and my guess is that there is a tidal race of 4 to 6 knots for maybe 2 hours four times a day.

Now as I understand it, the power from a Sandia-Savonius is given by about 0.16 x area x density x the cube of speed.  That isn't much in four knots - 2m/s - except that density is 1000kg/m3!  That gives 1.3kW per square metre of profile in the flow.  That's a lot.

So I'm picturing a pair of S-Savonius rotors in stainless steel, sticking out of the bottom of a jetty, each maybe 1.2m long and 0.5m in radius, each connected to a "brake-disc" generator with maybe 80 magnets and 120 coils in a big disc.  Each one should generate about 3kW at something approximating mains voltage, but should be rated for 12kW to take account of spring tides.

So, the questions:-

1. Anybody got experience of tidal systems they want to share?  I've gone through back postings, but I've found very, very little.  Especially, has anyone actually got one working in temperate lattitudes?
   
2. can the brake-disc generator be scaled to this size?  I'm not worried about "cogging", because an S-Savonius in water will generate immense torque, but are there any other issues?
   
3. I have in mind mains voltage because I'm thinking of using immersion heaters and oil-filled radiators as the "dump loads", especially in winter, but there's also a waterproofing problem, as the stator is likely to be soaked in seawater during winter storms.  Never mind spray, most likely the generator will be briefly but repeatedly immersed by waves!  Does potting the stator provide that level of weatherproofing?
   
4. I want two generators so I can take one out of the water in the summer for maintenance and de-fouling.  It would be nice to have one rather than two inputs to the battery charger, to reduce the complexity of the dump load system.  If both generators are connected together (in correct phase!) will they self-synchronise as the flow starts?
   
5. How much storage would be good?  I have in mind 72v arranged as 36-0-36 and a homebrew modular invertor using 16 microwave transformers to produce about 12kW.  Assuming 2kW average load during the day, the batteries need to supply 8kWh during the four hours while the tide is turning.  A simple 3kW linear stepdown (three more microwave transformers, some diodes, and three opto-triacs) can charge them when there is power.  I have 20 x 100Ah of 12v deep-cycle AGMs in the back yard, but I want to use some of them for an EV project.  How many should I hold back - or should I order NiFe's from China and use only 120Ah or so, since they can be run flat without damage?
   
6. We're planning on visiting in the next few weeks to see what we need to plan and check, and speak to the local planning office while we are there.  Is there anything else I've forgotten?
   
   

[wdyasq]

Check this out:

http://tinyurl.com/7ss3w

It appears to me to be tailor made for your application.  

Depending on the rainfall amount I'd think a catchment system may be more practical than a bore hole.  I'd spend time and money either insulating the old house or building a new, properly insulated house to reduce heating requirements.

You 'power-lags' are only a few hours - 4 times a day.  Your battery bank can be sized to this advantage.

'speak to the local planning office while we are there' - isn't that nice - you are moving to an area where the government has very little change for interaction with services but still feel the need to exercise control.

Where will this take place - not actual island, but an area?

Ron

[thunderhead]

It appears to me to be tailor made for your application.

If I could buy one, I'd agree 100%.

I think it would be easier for me with my limited metal-bashing skills to make a Sandia Savonius than a Gorlov, and the S-Savonius is better characterised, at least in the public domain. The Gorlov is more efficient, but with all that energy, who needs efficient? :-)

Other than that, the principle is virtually identical.  The "Exim" system by Seapower was what got me convinced it could work, at least in theory.  If I'm going to generate power from the sea, I don't want the generator submerged.  Putting on a diving suit to go fool with electrical machines doesn't come in the part of my book headed "clever". :-(

Depending on the rainfall amount I'd think a catchment system may be more practical than a bore hole.

Boreholes are what the locals use.  The bedrock is impervious granite, and water collects in fissures in the granite.  Rainfall is 30 to 50 inches a year.  Yes, that's a lot, but the prevailing wind comes straight off thousands of miles of ocean.  As an asthmatic, that sounds good too.  :-)

You 'power-lags' are only a few hours - 4 times a day.  Your battery bank can be sized to this advantage.

That's what I was thinking, but I wondered how AGMs would hold up given that kind of four-times-daily beating.  Hence the question about how much over capacity to use, or if I should just go with the nickel-iron cells and treat my battery problems as "solved".

'speak to the local planning office while we are there' - isn't that nice - you are moving to an area where the government has very little change for interaction with services but still feel the need to exercise control.

It's the same the world over, I'm afraid.  Although the site is remote, it still has neighbours, and they'd be justifiably annoyed if we were to build some huge dayglo-orange tower.

I don't know the local law in detail, but there is always some squeeze on new construction.

Where will this take place - not actual island, but an area?

Between 50 and 60 lattitude, and with an ocean to the West.  Other than that, I'm loath to say just yet, since the way I see it my only competition for buying the place is someone else who can solve the energy supply problem.  That person is likely to be reading this list, and given a description of the bay and a state, there are about seven candidates, only one of which is for sale.

If we buy it, or if we decide it's just not practical, then I'll say where.  With lots of pictures.  Beautiful pictures.  Promise.

[ghurd]

I would avoid AGM's or NiFe's for something like this.

AGMs have the known problems... Can't fill the water, cost...

NiFes at say 9V are not going to run the inverter or CFLs or stereo anyway.

If you expect more than a 12knot ~ 14MPH average wind, it sounds to me like a couple windmills are the way to go.

Even something simple to get started, like a 'garbo-gen', ECM, or F&P, maybe with 4' Jerry Blades.  Even 500 watts average is a lot of power, and beats the heck out of buying and moving fuel, and hearing a motor running for a light bulb.

I mean while you get the big system going.

Number 5 has me lost, but home brew inverters rarely turn out good.

It would be interesting to see how a 3 bucket or S Savonius does under water.

G-

[Nando]

I hope you got a lot of money, besides the cost of the "island".

The structure for the Tidal wave generator, specially HOME MADE is highly problematic if you do not have the mechanical and submerged equipment installation experience; also the behavior of the area during storms and how deep those effects are.

I am a Design Electronic Engineer and your comments about the DC/AC converter worries me because your writing which clearly indicates what your electronic design capabilities are.

You are suggesting that:

>A simple 3kW linear stepdown (three more microwave transformers, some diodes, and three opto-triacs) can charge them when there is power.

is a simple job, I wish it were -- starting with the transformers that have a very high voltage secondary.

Also, you are not realizing the time needed to design and build such "device", including the size of it and the input voltage levels and currents for 12 KW "16 microwave transformers converter".

I am not trying to discourage you, but power design has been my job for years and I know the time needed to just prototype smaller units, and I have made modular 500+ KW units.

Better use all 10 AGM batteries for your converter, it would be much easier

WELL DAY DREAMING IS SOMETIMES HEALTHY !!!

Regards

Nando

[rotornuts]

Thunderhead, I have no experience with tidal power but I lived on a lake in the interior of British Columbia for several years and with my stepfather who was a retired excavation contractor we built several breakwaters and retaining walls that where set just below water level during low water in late winter before spring runoff.

My comment to you is to be very aware of the destructive power of water. Even on a lake like the Kootenay(2-4 KM wide 75 KM long and up to 275 m deep) which is a relatively small body of water the force exerted on a structure during a storm is tremendous and for your application I envision concrete and heavy steel in it's construction.

Given your limited metal bashing skills I think wdyasq's suggestion of a gorlov style may work really well. Previously wdyasq had suggested a straight blade gorlov for a Vawt and that's the idea I'd look at for the tidal generator because I can't help but think a savonius type would be destroyed in a storm unless it was incredibly strong because of it's high cross sectional solidity.

A three blade gorlov could be constructed with two large metal disks and three pieces of flat iron requiring custom cutting of the disks and a total of 9 welds to complete the rotor. Adding a large diameter center shaft and welding it in place would require two more hole cuts and two more welds. Really that seems simplest to me even thaough it may require some work be farmed out to a suitable shop. Adding a simple but likely not so efficient profile could be done with a cutting torch and a grinder, alot of work but a fairly simple procedure if you use something like 1" x 4 or 5" bar stock.

Your budget will dictate how you set it in the water but if a suitable flat location presents itself during low tide perhaps a concrete concentrator style structure that holds the alternator above high tide and large swells would work as it is claimed that the gorlovs work well in a concentrated flow.

Anyway, I'm sure your already savy to the fact but water will destroy anything if you give it the chance so put on your over-engineering cap for this one and I'd be skeptical about the survivability of a floating stucture.

Your alternate for a concrete concentrator would perhaps be a metal frame that you could pin down below the water level using heavy wood planks bolted to the frame for the concentrator walls. The structure could be bolted together on site then submerged.

Just my rambling thoughts

Mike

[thunderhead]

I hope you got a lot of money, besides the cost of the "island".

Some.  I'm planning to build three residences and a workshop.

The structure for the Tidal wave generator, specially HOME MADE is highly problematic if you do not have the mechanical and submerged equipment installation experience; also the behavior of the area during storms and how deep those effects are.

The hope for the Savonius design is that the only submerged part is the rotor.  Think of a conventional Savonius windmill but turned upside down and dipped in water.  Just making the rest (very) splashproof is what I'm hoping will do.  It's in an estuary which is very sheltered from wave action, so I'm hoping to avoid the worst of the surf.  I don't want energy from waves, I want energy from tides.

Installation consists of lifting a few planks on the jetty and dropping the mill into the water, then securing the generator to the floor.

But you're right, I don't have experience of sticking things in the sea for months on end, which is why I'm asking. :-)

I am a Design Electronic Engineer and your comments about the DC/AC converter worries me because your writing which clearly indicates what your electronic design capabilities are.

My BSc is titled "Electronics and Computer Science".  I mainly work in software these days, but that's because I followed the money.  So I oversimplified.  It wasn't what I was concentrating on, to be honest.

You are suggesting that:

>A simple 3kW linear stepdown (three more microwave transformers, some diodes, and three opto-triacs) can charge them when there is power.

is a simple job, I wish it were -- starting with the transformers that have a very high voltage secondary.

Starting by removing the high voltage secondary and replacing it with an 100v 15A secondary.  Microwave transformers provide cheap (free!) laminated cores and mains isolating hardware which can be rewound for up to 1500VA.  It's hard for a hobbyist to find any transformer of this power, even new, other than in microwave ovens.  But nobody would use one without rewinding the secondary, unless they wanted to build something inspired by Tesla. :-)

Rewinding the secondary is a bit tedious but it's hardly rocket science.  Three of them, one on each phase, should do the trick.  Of course, since we're no longer oversimplifying, the rectification needs to be completely isolated if they're not going to generate unexpectedly high voltages.

The triacs provide current control to limit the voltage, using optoisolators.  That way the transformer circuit can be kept efficient, since the transformer is only powered when it is used.  50Hz is a low frequency for a switched mode system, but it can be done - think of how a light dimmer works.

My existing homebrew switched-mode battery charger is based on rewound transformers and it charges my AGMs at up to 20A.  I built it about 10 years ago, from a circuit I sketched out myself, and it's doing fine, thanks.  Took me three days to get working, and it's worked without a hitch for years of continuous use.  I've never opened the case since - I'm not exactly sure I can remember the exact layout.  The 200VA shop-bought invertor I bolted to the front has died, (and I haven't got around to fixing it,) which is a neat example of homebrew versus shopbought.

Also, you are not realizing the time needed to design and build such "device", including the size of it and the input voltage levels and currents for 12 KW "16 microwave transformers converter".

I have in mind 60v to 100v input, and if they are delivering 12kW then they have to manage 750W each.  So that'll want 12A or 13A each.  I've already built switched linear invertors that deliver more than this, and consider that to be a weekend's work.  The only thing I haven't built is the control systems to switch in and out 16 of the things, and keep them accorate, since I like to use mains operated clocks.

My mains wiring includes rewiring the distribution board on our house to use RCCBs, since I have an 18-month-old son, who like most 18-month-olds likes chewing wires and sticking fingers in sockets.  At least with a homebrew invertor I'd be able to turn it off, instead of having to pull the company fuse from a live supply. :-)

And, as someone else has pointed out, NiFe batteries, should I use them, almost require a homebrew invertor.  Somehow I expect the batteries to be less reliable than the invertor, so that's where I'd like to concentrate.  Those batteries will see maybe 1400 cycles a year.  From what I do know of batteries, that worries me.

I am not trying to discourage you, but power design has been my job for years and I know the time needed to just prototype smaller units, and I have made modular 500+ KW units.

Well, most of my electronic design experience is in telecoms, so power electronics is something I've mainly done as a hobby.  The powers I normally send into a transmission line you'd probably dismiss as noise.  :-)

But it has been my experience is that getting a prototype working is not hard - the hard bit is getting from the prototype that works to a production line that has a decent yield and a decent cost.  My secret of successful homebrew design is simple circuits and massive overspecification, which of course doesn't work too well for production, since you end up paying $20 each for power transformers when $5 ones could probably be made to do.

The other nice thing about modular design is that it's modular - building sixteen is significantly easier than designing sixteen, because if you can get one working, you can probably get all sixteen working.

Anyway, this is a bit off topic.  Let's assume I can build linear battery chargers, since I have one in the next room that has been running for 10 years; and that if my invertor experiences turn out duff, I can buy one.  I'm not discouraged about that stuff.

The key questions are about the stuff I don't know: working with the sea; scaling the "brake disc" generator; weatherproofing the "brake disc" generator; synchronising the generators; and battery wear.

[ghurd]

Back to #2.

I believe it can be scaled as big or small as needed, and there should not be any cogging with that type of design until current flows but it is minimized with multiple phases.

G-

[Ungrounded Lightning Rod]

Since you have a lot of tidal flow past your site a tidal velocity turbine may make sense.  But there's another alternative.

What's the wave action like?  And can you run a floating pier out from the shore a ways?

Wave action carries a LOT of energy.  And if you can run a floating pier with joints in it to let the waves flex it, you can make a wave motor and collect the energy from waves coming in (or out) along the direction of the pier.  Or you can make a wide pier - ideally about as wide as the length of the typical large waves - or three or more parallel piers spaced abouth half a wavelength apart for typical wave lengths - to capture the energy of waves moving at right angles to the piers.

I'd put a hydraulic cylinder at each joint - as if it were a shock absorber (which it IS!) - and connect them with a "bridge rectifier" of one-way valves to pump fluid from a "negative (pressure) bus" to a "positive (pressure) bus".  At the shore I'd run the resulting fluid flow through a hydraulic motor to drive a genny.

You can get a HYSTERICAL amount of mechanical/hydraulic power conversion in a tiny hydraulic motor and cylinders of reasonable size.  (It's really impressive to look at a hydraulic press "power supply" in a manufacturing plant - with maybe a 20 to 50 horse electric motor - the size of a large pig to a small cow - driving a hydraulic pump smaller than a pot pie.  Or take a look at the one on the front of the engine on a backhoe, frontloader, or other piece of construction equipment.)

Cylinders, motors, and both pipe and flexible hydraulic tubing with long lifetimes exposed to harsh environments are pretty stock.  Don't know whether you can get one-way valves or would have to have someone machine 'em for you.  And you'll have to be carefull not to dump they hydraulic fluid into the estuary if something fails.  (Think "power steering fluid".)  But it seems to me a flexible floating pier with "shock absorbers" connected to a genny might be easier to get working right than an undersea rotor.

[thunderhead]

The site has very little waves in the normal course of things.  This is a good thing if I'm going to put structures in the sea, of course.  But it means no wave power.

[thunderhead]

Number five is really about the battery capacity and a deep cycle four times a day.  AGMs are cheap since I paid about $22 each for 20 12v/100Ah units.  But if they only last 1500 cycles, they'll only last a year.

The thing I like about NiFe is that they don't wear, but I know that means homebrew invertors, which are an ... interesting ... problem.

Wind is a possibility, but I like the idea of a "wind" that blows four times a day come Hell or, err, high water.

[thunderhead]

Given your limited metal bashing skills I think wdyasq's suggestion of a gorlov style may work really well. Previously wdyasq had suggested a straight blade gorlov for a Vawt and that's the idea I'd look at for the tidal generator because I can't help but think a savonius type would be destroyed in a storm unless it was incredibly strong because of it's high cross sectional solidity.

That is a fine point.  I hadn't thought of that difference.  I guess I need to get local builders to construct a jetty, and a local blacksmith to make a couple of Gorlovs in stainless steel.  If the jetty can stand the forces of a moored boat, I guess it should be able to stand a Gorlov of similar cross-sectional area.

Especially as the Gorlov will be smaller, since it will be more efficient.

I'm busy reading about Gorlovs used for exactly this application, and looking at the engineering they're using.  The more I see, the more I like it.

Thanks for that. :-)

[kitno455]

to properly support a mill in a high-density medium like water, you are going to need a support bearing at both ends of the shaft. that puts one of them under saltwater. if you leave it in the water during a storm, something is going to fail, most likely the genny's mounts on the jetty, or the entire end of the jetty.

it does not matter what type of genny it is. it works by slowing the moving fluid. the more efficient it is, the more fluid it will slow (up to the betz limit, then the situation reverses). could you mount a sheet of plywood 59% of the cross-section of the genny in place, and keep it there during a storm?

if you can build mountings strong enough for that, then the choice of which genny to build is really more about construction techniques than power output. i would give yourself an easy way to get the thing out of the water.

hmm, what if a single 8 inch schedule80 iron pipe stuck out from the shore, where it pivoted on a large footing. the other end would be a bit of a barge, made of 10 55 gal oil drums, or a couple heavy walled 250 gal fuel tanks. leading from the barge out along the shore in both directions could be a couple lengths of steel rope. on the barge is a traditional paddle wheel, or a savonious. you could use hydraulic motor or drive shaft inside the iron pipe to keep genny at shore. thats part of the reason to use paddle wheel, has horizontal shaft. it will rise and fall with tide, and can easily be brought to shore for repair, lengthen one steel rope, shorten other with crank.

this arrangement also has an advantage, you could build the barge end of pipe to swivel. if genny jams or turns too slow, the barge will actually nose down in the water, lifting genny out. you would have to have some 'curved' tips on your floation, to keep the nose under from flipping you, like a couple drums turned up at the end.

keep us posted, this is a neat idea...

allan

(Mech E that followed money to programming too

[Nando]

The only thing I could say : GOOD LUCK you are going to need it in several areas of your BIG "single" manpower enterprise.

I have seen too many efforts like that, coming to me trying to salvage what they have done.

The Ferro Nickel, or as well called NiFe, Batteries are quite forgiven for under or over charge conditions.

Make sure that the electrolyte does not INTERACT with the oxygen ( it is KOH + Lithium)that do not have affinity with oxygen.

The charger may need to go to 1.65 Volts per cell, Eagle Pitcher were the best, though I think that they are not in production any longer, so just the ones coming from Asia.

You could attain several decades of life with the Ferro-Nickel.

A good DC/AC converter does not need such large number of "Microwave transformers".

Just one to convert from DC low voltage to DC Bipolar High Voltage for the AC converter

There is an excellent example for a 5 KW DC/AC converter, here you have the link to download it:

http://www.energychallenge.org / 2001Reports / tamu.pdf

I separated the URL link for the "/" to appear properly due to Yahoo "criptic" writing RULES, so remove the spaces between the " / " for the LINK to work properly.

Your DREAM is big and has a difficult road in front of you.

Good Luck

Nando

[Ungrounded Lightning Rod]

One downside to a Savonius is that the tip speed under load is about 0.8 times the "wind" speed.  Your wind speed is already very low.  With a Savonius design you'll get a huge amount of torque but pretty low RPM - and the bigger the rotor, the slower the rotation.

You can run a genny at any speed.  But it's the CHANGE of flux that generates.  So the faster the magnets move past the coils, the more power you get from a given set of magnets and coils - by a square law.  Thus a slower rotation means you buy more magnet to get a given amount of power and for a given rotation speed.  Also:  the larger the radius to the rotor-stator magnetic sheer area the more power you get.  (Square law, assuming the current is limited by I2R losses:  One factor of radius for voltage vs. speed, another for increased count of magnets and coils, which maps to either increased voltage or current depending on winding and interconnection.)

With a horizontal axis machine you're somewhat limited as to genny radius because it blocks the "wind".  They get away with a small genny because horizontal axis machines run at tip speeds of maybe 5 to 7 times the "wind" speed, rather than 4/5ths of it.  And there are just such gennies already:  They look like a trolling motor with a different curve on the prop.  Downside to making one yourself is that the genny and all the bearings end up underwater unless you use a gearbox.

With a vertical axis machine you have no such limit:  Your genny COULD be wider than the rotor if you want.  You can also stack more than one stator if you want to build a BIG machine.  Then you end up with just one bearing underwater (though the other and the genny are still submerged intermittently.)  Sizing the genny at the width of the rotor has the advantage that you can mount multiple machines side-by-side.

Really big machines wind machines don't try to scale up the genny in proportion and retain direct drive.  Instead they use a gearbox to speed up the shaft, letting the genny size and speed be driven by other factors.  We avoid them in the small machines because we CAN (the shaft speed is adequate for direct drive designs), and because gearboxes increase the expense, difficulty, maintainence requirements, and number of failure mechanisms.

You might consider building a horizontal axis machine using an inboard/outboard powerhead for the transmission, bearings, and prop support.  That would give you a gearbox, seals, and housing rated for such service.  Another way to do horizontal axis is with a long shaft at a slight angle supported by cutlass bearings, so the shaft comes out of the water at your genny site.  Downside to that is the need for support and protection at a distance from the pier.   Upside is the underwater bearing doesn't have much sideforce on it - most of the load is thrust which you can take on the above-water bearing.

For either a horizontal or vertical axis machine you can increase the water speed by placing it in a structure that acts as a venturi (much as the narrow inlet of the bay already does, but giving you another step up).  You're in a good position to do that because your flow is in a well-defined direction, so you don't need a strcuture that can be aimed.  (Underwater concrete walls come to mind.)  One downside is that if you speed it up too much the fish can't avoid the rotor blades.  (That's also a downside to horizontal axis machines even without a speed booster.)

I think your best bet might be the vertical axis machines with a large-radius alternator.  But I thought I'd mention the other ideas just for thinking points.

[thunderhead]

One downside to a Savonius is that the tip speed under load is about 0.8 times the "wind" speed.  Your wind speed is already very low.  With a Savonius design you'll get a huge amount of torque but pretty low RPM - and the bigger the rotor, the slower the rotation.

You can run a genny at any speed.  But it's the CHANGE of flux that generates.  So the faster the magnets move past the coils, the more power you get from a given set of magnets and coils - by a square law.

I had figured that much, which was why I was considering maybe 80 magnets: the S-Savonius study gives load curves for different speeds and windspeeds, and I was expecting 80 or more magnets to be needed to get decent AC frequencies when the whole thing is going to be going at 50RPM to 100RPM.

But the Gorlov sounds like a better bet, since it has a TSR of over-unity.

You might consider building a horizontal axis machine using an inboard/outboard powerhead for the transmission, bearings, and prop support.  That would give you a gearbox, seals, and housing rated for such service.

The scavenged outboard is a good idea.  The only downside is, I suspect, that it'd need to be put on a floating structure (maybe an old boat?) to keep it in the water as the tide changes.  The spring tides run to over 12 feet, and the tide always runs over half that.  I doubt I will find an outboard with a twelve foot shaft.

[rotornuts]

You know thunderhead, it would be very helpfull to see the location so one could see what local materials may be available and site suitability etc. I had another though about the jetty(breakwater) but all the ones we built for mooring boats in were log cribs tied with messenger cable and pinned with rebar then filled with rock. It was the best use of local materials to keep cost down, it is self supporting, and the best part is that the log crib could be built a couple layers high on dry ground then slid into the water at low water so that the base is always below the low water mark. the only problem is that it requires suitable logs and large stone(12" and up).

Mike

[kitno455]

i've been thinking about this. scrap the gorlov and the savonious. use a horizontal shaft undershot paddlewheel on a barge- heres why:

traditional vawts, drag or lift, have to contend with the fact that half of the blade rotation moves in oppositition to the power path. you are always dragging some dead weight around with you. in fact, think about a savonious with two flat plates instead of curved plates. will it turn? heck no. its a wind vane. it is purely a function of the difference in shape of the concave v/s convex sides that make the power.

imagine if you could make the dead weight half disappear entirely, rather than just stream lining it. we cant do this in the air, but you can, just lift the dead half out of the water, by making the shaft horizontal. put it on a simple barge made of oil drums, use two large steel pipes or a single pipe and two cables to triangulate the thing out into the water. big moorings on the shore are a heck of alot cheaper than a jetty...

allan

[Ungrounded Lightning Rod]

(Note:  Nautical terminology:  The "dock" is the space in the water where the boat is, while the long thing you walk on to get there (and ususally what you tie the boat to) is a "pier".)

A floating pier is a tethered raft that goes up and down with the tide, so the water level at the pier is always the same (and your boat doesn't end up hanging from the dock lines if the tide goes out or sunk if it comes in  B-) ).  Mount your "outboard" on a floating pier and you don't have an issue with shaft length, except to get it down to where the current is decent, rather than slowed by friction with the pier floats.

There are long-shaft outboards for use as auxiliary engines on sailboats, where the prop has to be a lot lower than the head due to the depth of the hull, the height of the transom, and the significant tilt to the boat when under way (you don't want that to expose the prop).

Check out how they're built at any marina.  (Essentially: floats - foam, 55 galon drums, logs, etc. - with a wooden or synthetic decking structure over them, flexible joints between the structures, pilings through holes in the structures, and four rollers around each hole bearing against the four sides of each piling to let the pier ride up and down without appreciable friction if the current is pulling the pier to one side.)

Of course you could always put your device on a raft and tether it to darn near any anchoring structure - such as buckets of concrete with a ring in them for the anchor rode.  Put out three or more anchors (I'd use four to have one redundant in case of drag or line failure) and it will stay pretty much in one place rather than moving around a lot when the tide changes.

[thunderhead]

I have to say that my first reaction to the construction of this sort of thing was to get the locals to do it, since they'll know the local materials and the local conditions.

There are a few construction jobs I wanted to "contract out" - roofing is the obvious example, since I don't like heights and I have no scaffolding skills - and the jetty (or pier) was part of that.

I also wanted two or three slipways - at least one on the island and one on the foreshore - so I could also use them to drive an amphibious kitcar to and from the house.  Once again, that sounds like work to contract out.