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  #31  
Old 06-27-2009, 10:22 AM
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Jeremy
A small point - It is the higher density of the water that contributes to the higher drag of appendages in water. The Cd will not be much different. From memory a NACA0025 has a Cd around 0.015 in the in-line position but you can easily check it with JavaFoil.

With the motor not moving the boat through the water it is not doing any real work other than the stirring losses in the tank - these will be significant as you note but static tests do not mean much in arriving at actual power consumed when operating.

You can apply JavaProp to boat propellers by setting the Options to suit the water. It will give a rough guide for the static thrust versus power and will be very accurate for the actual operating conditions. I used JavaProp to estimate the efficiciency of your prop. You can also use it to determine the best prop. I have my own optimising method for this but it is primarily aimed at tweaking over what JavaProp does.

Some think that boat props are inherently different to airplane propellers but it is simply that there are more compromises required for boat props that result in less efficient forms. With lower power applications one is not so constrained.

If you want to go through JavaProp relevant to your design I can give you pointers. You would find significant performance improvement by going to a model plane prop. You would require something that cost a few dollars for your application so if it was damaged it could be inexpensively replaced although I think you would find their toughness would survive decent hits.

I am on the road again tomorrow so may or may not look at this site.

Rick W
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  #32  
Old 06-27-2009, 10:56 AM
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Jeremy Harris Jeremy Harris is offline
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Rick,

Although density differences account for the major change for form, or profile, drag, between air and water, viscosity has a major impact on the hull form. For example, when I'm designing a light aircraft (like this one: http://www.realityaircraft.com/escapade/index.php) I don't need to pay much attention to wetted area, as the viscosity of air is so low, that, at typical operating speeds, viscous drag is negligible; form drag dominates.

From what I've read so far on boat hull design, minimising wetted area for any given displacement is the most effective way of reducing drag for speeds below the point where wave-making becomes significant, notwithstanding the importance of a good shape to ensure that the normal rules regarding incompressible flow are adhered to.

I agree, the static test drive leg is not that useful, but it did serve to boost my confidence a fair bit. Had it drawn massive amounts of power when turning in a dustbin full of water I would have been a bit disappointed.

I've used Javaprop a fair bit too, although I've found that it takes some profile tweaking to get the blade loading correct when using it to simulate the performance of a commercially available aircraft prop. I have a couple of other simple prop tools, that use blade element theory to derive estimates of prop performance, both of which tend to work better out-of-the-box than Javaprop for ready made aircraft props. The snag is, neither of them allow the constants to be changed to allow predictions of in-water performance. I did do some rough comparisons, changing the Javaprop constants on the back page to allow for water, but for the moment will stick with the tough alloy prop.

I agree, a model aircraft prop may well be a better way to go, so I've made provision on the drive leg to allow me to switch if need be. The stainless shaft is 12mm in diameter, with a thread cut on the end. At the moment it is fitted with a bush to allow the big alloy prop to fit, but it can easily be made to fit a big model prop if I decide to go down that route later.

I've been playing with modelling a beamy open canoe with Freeship and was surprised to find that it is more stable than my design with a transom. There's obviously a lot more to this boat design lark than I thought! My first cut KMT for a similar length, similar displacement, but 1.04m beam, canoe hull is about 0.62m, so a little better than the other hull design. I already have a model of this canoe hull, using the same tortured ply construction method, so I may try and test this comparatively with the other hull model next time I can sneak into the bathroom for a "trials" session.

Jeremy
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  #33  
Old 06-27-2009, 11:03 AM
Ad Hoc Ad Hoc is offline
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Jeremy

You also need to be careful with what viscosity figure you use, since they need to be related to the temp of the water and salinity that you expect. The value given by RW is at 11.5c and roughly 3.5%.

KMt is not the measure of a vessels stability, the GM and the GZ are the measures. In terms of rolling period this is directly proportional to the inverse root of GM.

The distance between the VCG and KMt this is the GM.
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  #34  
Old 06-27-2009, 11:46 AM
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Jeremy Harris Jeremy Harris is offline
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Thanks, I'd spotted the big changes in viscosity with temperature and 11.5 deg C is pretty close to the average water temperature in UK inland waters, I suspect, although I need to adjust for fresh water, perhaps. If it gets warmer in summer, then that'll be a welcome boost in performance/reduction in power requirement.

I appreciate that KMT isn't a direct measure of stability, but I'm making the (perhaps bold) assumption that the vertical C of G will always be around the same place for this type of boat, so if KMT increases, so does the stability margin (or whatever the correct maritime terminology is, I'm struggling here with the different terms used for similar things between the aero and maritime world!).

Thanks,

Jeremy
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  #35  
Old 06-27-2009, 12:22 PM
Ad Hoc Ad Hoc is offline
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Jermey
It's about 10.1c in FW.

Your assumptions are valid, re KMt. However it is not just the upright GM, one must investigate the variation at angles of heel...in other words, the restoring moment when heeled at various angles. For example, having a wider beam and shallower draft, will increase you upright KMt, but the deck edge will immerse much quicker when heeled, as a crude example. This may cause immediate flooding, not a good idea!..or simply cover it.
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  #36  
Old 06-28-2009, 04:04 AM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by Jeremy Harris View Post
......
From what I've read so far on boat hull design, minimising wetted area for any given displacement is the most effective way of reducing drag for speeds below the point where wave-making becomes significant, notwithstanding the importance of a good shape to ensure that the normal rules regarding incompressible flow are adhered to.

....
Jeremy
Jeremy
You will not need to worry about the compressibility of water for your areas of interest.

I expect that the main factors affecting shape that you have to juggle are stability, wetted surface and wave making. Probably in that order. If you want I can provide you the lowest drag monohull solution for your design speed and displacement with an initial stability constraint. Let me know.

Rick W
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  #37  
Old 06-28-2009, 06:48 AM
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Jeremy Harris Jeremy Harris is offline
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Thanks Rick, pretty much as I'd guessed. My background is aeronautical engineering, so I've got a pretty good feel for where compressibility effects come in to play. Even for aircraft, compressibility (at least as far as the airframe is concerned) is something that only rears it's head in the high sub-sonic region.

One thing I'm learning is that, at least as far as Freeship output is concerned, the variation in hull resistance at the low speeds I'm looking at is quite small for quite big variations in hull shape. I've also been looking at the way initial stability can be markedly increased by flattening the bottom of the hull, albeit with some changes in other areas. Overall, I'm now reasonably sure I can get a hull shape that will be stable enough, will have an adequately low resistance figure at 4mph, will be acceptable aesthetically and have enough deck area for a solar array. Quite by chance, the hull shape now looks similar to this pedal boat: http://www.swallowboats.co.uk/content/view/94/104/, although slightly shorter and a tiny bit narrower. Winsome is reputed to be fairly easy to board and not too tender, so I'm hopeful that my hull shape should be similar. I may contact Swallow Boats and ask if they would consider selling a bare hull, as that might be an even better (if far more expensive) way of getting this project in the water quickly.

The question now is whether or not I can build it using the simple tortured ply technique that I would prefer to use. I've started to build another plywood model to find out, as it's the only fairly easy way to see if ply can be persuaded to take up the complex curves.

Jeremy

PS: There's an interesting solar powered electric trimaran design here: http://www.youtube.com/watch?v=7F-ZQ...eature=related
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  #38  
Old 06-28-2009, 07:12 AM
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Jeremy
That pedal boat should have adequate stability looking at its proportions. It is a monster though at 55kg. Well beyond a mere mortal to cartop. I have cartopped a 40kg catamaran alone but it takes some manual handling organisation. I target less than 20kg for easy cartopping. If you are a trained weight lifter it might be a different matter.

I personally believe you will not better the faux-tri. It will be far supperior from a drag and stability perspective while the aesthetics is simply a matter for your creative genius.

From a building perspective I prefer flat pack sandwich panel. These are fast bulds and the faux-tri is ideal for this method. A careful build for a faux-tri might get weight around 30kg or even a little lower. My V12 is 40kg but it was a very confused build. Also much heavier than needed. It was my first sandwich effort.

By the way you should be careful relying on the drag data provided in Freeship. It is based on series of hull that have certain form limits. It is hopeless for narrow hulls but your present hull is quite wide so should be reasonable. I rely on Michlet because I have had the opportunity to test it over a variety of hulls - up around twenty hulls now.

Now that you have your drive leg running I bet you would easily find kayak and dinghy owners would love to see it tested on their boat. Just a matter of making a gunwale or transom mounting bracket - offset thrust is not a big deal. Only problem will be meeting the demand if you let others see what you have made and how it performs.

Rick W
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  #39  
Old 06-28-2009, 09:18 AM
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Jeremy Harris Jeremy Harris is offline
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I agree, 55kg is way more than I would want, but the hull seems a nice shape. I'm hoping that they might be persuaded to make a lightweight bare hull only, so that I could fit it out. My upper limit is around 30kg, and even that's based on using an aid to get the boat on to the car rack, either one of the neat sliding racks like this one: http://www.kari-tek.co.uk/ELRRIntroMulti.html or just a simple pair of wheels that can be fitted at one end , like this idea: http://www.songofthepaddle.co.uk/for...hlight=trolley.

Interestingly, I've already had an offer from a canoeist who lives nearby to test the drive leg. I think I'll look at making up an over-the-gunwale clamp so that we can go out and try it. That way I should be able to get some reasonable power vs speed measurements. I think you're right about being inundated with offers, it seems that there are quite a few people in the canoeing world who would quite like the idea of quiet and effective electric propulsion. Those that have tried trolling motors have quickly come to realise that they are not optimised for the relatively high hull speed, low thrust requirements of a canoe shaped hull (with the possible exception of the Torqeedo).

I think you may well be right about the tri being the best solution, but I'm going to persevere a bit more with the idea of making a nice (to my eyes!) curved hull first.

Thanks for the tip about the Freeship resistance predictions. Before discovering that program a few days ago, I had made up a spreadsheet, comparing lots of measurements from on line sources (mainly canoes) and was using the calculation method that John Winters published on a canoeing/kayaking web site. Unsurprisingly, the results were similar to Freeship, but that's almost certainly because Freeship seems to use the John Winters method as well.

Jeremy
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  #40  
Old 06-28-2009, 09:42 AM
yellow cat yellow cat is offline
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Hi Jeremy
I have been reading this electrical motorization possibilities exchange.
This may not yet be practical but i wonder if !
I see that generally, if a product is widely available in the market, the safety and the costs are mostly refined.
Hence, friction being the primary concern in any boat design, i think it would be interesting evaluating the "at first" small hovercraft possibilities. Could a lithium leaf blower (or few) reduce a hull friction so that an electric water prop. would be fast. I am designing a Catamaran and considering the "in between the hulls" hovercraft very light kite material possibilities. One the Coast Guard concerns on hovercraft are buoyancy and fire . With the 2 very tracking, sleek hulls, all energy is possible in the right direction. If one pushes on paddle wheel with air and at the same time fills the "bag", the 2 dagger boards preventing from sideways movement, i think it is going places.
How to get air ? As mentionned above, leaf blower(s) and a steamer using a solar parabolica. The leaf blower(s) mainly for control. Parabolica Steamer for hot water, water maker, water purifyer, cooking, etc
One of the greatest advantages with electrical motors beside low rpm tork, is the possibility to raise the prop (or wheel) out of the water at will ... i dont need to elaborate why ...
Although greatly favorable to electrical propulsion, i am concerned with "on a large" or even on a not so large scale magnetic health hazards. Proximity ...
Photvoltaic DIY on a Catamaran makes sence $$$ as of yet ... combined with lithium batt. (for power uniformity) the future looks brighter. We are not even talking about maintenance yet ... H fuel cell being the major clean alternative...
Weight and $$ being large factors in the design, i see more fiberglass reinforcing bars in the market. Primarily used for bridges, i am gathering infos on possibilities with epoxy and okoome. Especially for bottom tensile strength. Could they solve the nuts and bolts corrosion at the same time ?
When i was a teenager, i checked concrete (ferrociment) boat building. Perhaps some of its techniques could benefit the lighter designs.
I have wondered if part or the whole boat could be a battery on its own ...
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  #41  
Old 06-28-2009, 10:29 AM
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Jeremy Harris Jeremy Harris is offline
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Using an air cushion would significantly decrease the resistance due to viscous drag, but it introduces some other problems for a small, low speed boat, like this. The first is the amount of power needed to maintain an air cushion. This depends on the weight of the craft, the available air cushion area and most critically, the circumferential gap where air leaks out around the skirt. A two person air cushion vehicle typically needs around 3 to 4 hp just to maintain the air cushion, which is a great deal of power for a battery system to sustain for any peroiod of time, and beyond the capability of a practical solar power system on this size of craft.

The power for the air cushion can be reduced if sidewalls are used, like a catamaran hull with an air cushion between the hulls and containing skirts only at the front and rear. This stops air leaking out from the sides, albeit with the penalty of some drag from the two hulls.

Another problem with low speed air cushion vehicles is that they displace water just like a displacement boat hull. Because the best shape for lowest air cushion power is one that is blunt at the ends, they need almost as much propulsion power at low speeds as an equivalent displacement boat.

Air cushion vehicles are only really at an advantage over conventional boat hulls (on water) at high speeds, where their propulsion power requirements are comparable to, or slightly lower than, planing craft.

I'm a scientist and am not aware of any proven health issues associated with magnetic fields. The magnetic fields in diagnostic body scanners are many times greater than those around a small electric motor, even one using neodymium magnets like the one I'm using.

Composite construction can be light and strong, but often simple lightweight plywood will be as strong and light at a fraction of the cost. Plywood is an underrated material for boat building, in my opinion, as it's combination of strength, stiffness, weight and cost is hard to beat.

Jeremy
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  #42  
Old 06-28-2009, 05:54 PM
Tiny Turnip Tiny Turnip is online now
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Jeremy, for what its worth, when I first read your first post in this thread, my first thought was of the Swallow boats Winsome. Seems to fit the aesthetic and ethos of your proposal.
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  #43  
Old 06-28-2009, 06:06 PM
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TT
Are you back into boating yet?

Rick W
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  #44  
Old 06-28-2009, 06:33 PM
Tiny Turnip Tiny Turnip is online now
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Hi Rick - yes, - life has been getting in the way with a new more demanding job since September, ontop of single parenthood and my church conversion project! I'm just fettling Fangle to take up to the isle of Gigha, off the west coast of Kintyre, Scotland for a month from mid July. Google Earth has recently improved the resolution considerably for the area. There are lots of little bays on the island inaccessible from land, and I have a couple of lobster pots, so I'm hoping the platform design will come into its own for fishing, potting and picnicking. My cycling buddy will be there too, so I'm hoping there may be an opportunity or two to embarass some sailing boats, along with some photo opps.

I've been getting quite excited by the discussion in this thread and others about the potential for these motors and cells though. Jeremy, I'm down your way a few times a year, I'd be very interested to have a peep at your boat in due course...
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  #45  
Old 06-29-2009, 02:27 AM
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Jeremy Harris Jeremy Harris is offline
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I've emailed Swallow Boats with an enquiry about the supply of a lightweight hull, so will wait and see. My guess is that it will be very expensive, based on the price of the finished pedal boat version, which may price it out of my reach.

Interesting that you thought of this hull shape, TT, my guess is that there is a sort of common evolutionary pattern to boat design, with only a limited number of hull shape options for any given requirement. The hull shape requirement for a pedal powered craft is always going to be close to that for other low power displacement craft, I think.

The other hull shapes I've been looking at have been efficient rowing craft, like gigs and the light skiffs and wherries that used to abound on rivers like the Thames. How one of these might look half-decked with solar cells is another matter, though.

They key thing for me is keeping the power consumption down to a point where a modest array of solar cells can provide a top-up charge capability during cruising and fully charge the battery pack during times that the boat is moored. I'm estimating that normal leisure use will be around 4 or 5 hours motor use per day, with around 10 to 12 hours of solar charge time. If I can fit enough solar cells to cover 50% of the propulsion requirement then I should end up with a completely solar powered design. This probably means being able to fit a 50 watt (realistic output) array, which means about 1m² of deck area being covered. Using small, thin, cells to tile the deck is probably the way to make best use of available space and keep the boat looking OK.

Jeremy
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