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  #91  
Old 07-24-2009, 03:32 AM
Guest625101138 Guest625101138 is offline
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Originally Posted by Jeremy Harris View Post
.......BTW, I used 0.000014607 mē/S as a value of v for air, 0.000001307 mē/S as the value of v for water.


Jeremy
Jeremy
I agree with your viscosity values. Many actually think water is more viscous than air.

Rick
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  #92  
Old 07-24-2009, 03:50 AM
Guest625101138 Guest625101138 is offline
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Originally Posted by Jeremy Harris View Post
Rick,

It looks like you've been using one or other of the Virginia Tech Java applets to calculate boundary layer thickness. These are OK for a flat plate, but both boats and aircraft fuselage aft sections are convergent, which makes the flat plate model lose accuracy. Obviously static pressure reduces as you run aft under a boat hull with upswept lines, so this will tend to increase the thickness of the boundary layer. ............
Jeremy
The little applet has pressure compensation but it has no relevance to the hulls I am using and the location of the prop. It will be similar to your hull at your speeds of interest. Your whole objective is to have next to no pressure drag.

Your stated objective of getting the prop into undisturbed flow will require you to offset by around 120mm from the hull. This is far more than necessary. You will find 10mm is more realistic. I can watch the development of the turbulent layer down the side of my hull on a calm day and it is not very thick - nothing like 120mm.

I place my props with sufficient clearance to avoid then clashing with the hull as they flex the strut in a turn. When I used a rigid strut I cleared by 10 to 20mm.

Saving 100mm in draft is worthwhile even on a small boat like this as it enables normal operation close to shore. Being able to lift the prop and still operate is a decided advantage as well.

Rick W
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  #93  
Old 07-24-2009, 03:56 AM
Guest625101138 Guest625101138 is offline
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Originally Posted by Jeremy Harris View Post
..........
I believe that it should be fairly easy to make a composite folding two blade prop. It'd be interesting to know how well such a device might shed weed, or whether it would spin out OK to go astern. I have a big (1100mm) three blade folding prop for a motor glider sat in my study. It's made of carbon fibre, with foam cores, and each blade only weighs about 250g (see picture below). Using this basic design (scaled down for use in water) I think it should be easy to make a nice, strong, prop. This configuration has the advantage that the carbon rovings can be wrapped around the bearing tube for the folding pivot, making the blade root attachment very strong.

......
Jeremy
Jeremy
From a reversing point of view you rely on centrifugal force deploying the blades. If this is low relative to the thrust then they will tend to fold backwards. I expect you would get some reverse but it may not be effective. My first attempt will be stainless. I know I can get these to deploy.

Rick W
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  #94  
Old 07-24-2009, 05:00 AM
Ad Hoc Ad Hoc is offline
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Jeremey

"...I think we're all coming at this concept with similar interests, but differing backgrounds, which means we'll each put a different weight on certain aspects. As an aircraft designer, I tend to want to build very light, optimise propulsive efficiency as far as practical and go for as clean a hydrodynamic solution as fits with the need for stability and practicality..."

I like the colour of your jip!!

I am a naval architect and i design commercial high speed vessels. There are only 3 things that are important about the design of high speed vessels:
1) weight
2) weight
and yup
3) weight.

Get your weight wrong...and it doesn't matter what programs or assumptions have been made about efficiencies etc...all out the window. Which is why i implore you, for your SOR, to look at the weights in very great detail.

As for the hydrodynamics, no real magic there, once the basic SOR has been decided. Hull design is not the black art many try to maintain or claim to be. Hull shape actually has very little influence in the whole scheme of things, it is the length displacement ratio that one needs to concentrate on. Hence the weight being important.

Trying to get better juice out of your power unit is great and worth doing, but weight is the biggest killer and will eat up any of your 2 or 5% gain in power or whatever etc very quickly indeed. So money spent on getting the best or latest power units, wiring, etc etc needs to be put into perspective.
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  #95  
Old 07-24-2009, 05:42 AM
Guest625101138 Guest625101138 is offline
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Originally Posted by MCDenny View Post
.......

Rick,

The dingy I'm envisioning is the Ness Yawl by Iain Oughtred, a light double ended lug rigged beach boat. A lines plan of a very similar boat is attached. LWL = 15.3 feet or 4.66m; wl beam is 4.33' or 1.32m. 650# all up weight with crew.

My estimate of power required is from Gerr's "Propeller Handbook"
Speed in kts=10.665 x square root(LWL in feet) / third root(weight in lbs / shaft hp).

This is an empirical formula fitted to "average" boats with "average" props. It does a very good job of predicting the speed vs power of my 25' electric launch. This is a long skinny 2000# boat with a normal 3 blade 11d x 12p bronze prop (see it at my website below).

Back to the Ness Yawl, the formula says 180w needed for 2.9 kts; 750w gives 4.7 kts. This assumes 90% electrical efficiency (too high, I know) and "average" prop eficiency. I would guess the actual output power thrusting the hull forward would be perhaps half these levels.

My idea is that a narrow 2 blade prop could be fully hidden within a fairing on the trailing edge of the rudder when sailing. Imagine the after 20% of the rudder is removable - when taken off it exposes the prop. Installing and removing the fairing would be much easier than the current state-of-the-art solution: mounting and stowing a small (ugly!) outboard motor. Lets just dismiss rowing


.......
Denny
I drew up the Arctic Tern per attached. The bare hull drag is very low under 4kts. From that point it rises very rapidly:

4kts - 42N
5kts - 138N
6kts - 254N

The hull has a wetted surface of 3.7sq.m. It will have very clean flow as it is a canoe stern and quite flat.

The drag figures listed make no allowance for windage, the drag on the case and drag on the rudder. Making significant allowance for these, a nice round number is 70N at 2.1m/s (say 4kts) for the prop design. You can do the calculations for drag an the various bits once you have details but I would be surprised if they exceed the allowance made in calm conditions. In wind and waves they will rise quite a lot.

I have attached the prop design data as well. Of course with the drag being a lot lower than working from an inflated power estimate you can get the prop into more efficient operation.

The power level calculated is easily achievable with the small PMSM motors that Jeremy and I have discussed here.

Rick
Attached Thumbnails
Efficient electric boat-arctic_tern_linesplan.png  Efficient electric boat-picture-38.png  
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  #96  
Old 07-24-2009, 08:25 AM
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Jeremy Harris Jeremy Harris is offline
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Quote:
Originally Posted by Ad Hoc View Post
Jeremey
............ Get your weight wrong...and it doesn't matter what programs or assumptions have been made about efficiencies etc...all out the window. Which is why i implore you, for your SOR, to look at the weights in very great detail.

As for the hydrodynamics, no real magic there, once the basic SOR has been decided. Hull design is not the black art many try to maintain or claim to be. Hull shape actually has very little influence in the whole scheme of things, it is the length displacement ratio that one needs to concentrate on. Hence the weight being important.

Trying to get better juice out of your power unit is great and worth doing, but weight is the biggest killer and will eat up any of your 2 or 5% gain in power or whatever etc very quickly indeed. So money spent on getting the best or latest power units, wiring, etc etc needs to be put into perspective.
I wholeheartedly agree. If anything, weight is more important in light aircraft design, as the vicious circle of more weight, creating more induced drag, which needs a bigger (heavier!) engine quickly makes this very problematic.

My reason for optimising the propulsion system is really to do with my desire to run as much as possible on solar power. I really need to keep the electrical power consumption down, so that the percentage power provided by the solar cells is a high as possible. The area I have for these is limited, perhaps around 2mē.

At the moment, around 65 to 70% of the displacement is crew weight, so the potential for making a worthwhile reduction in the hull and machinery is modest. I think a good diet might be a better way to go...............

Jeremy
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  #97  
Old 07-24-2009, 08:50 AM
yellow cat yellow cat is offline
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Jeremy,
Have you considered an adaptation of the bion x system. Google bionx and you will see what it can do. I am considering it for assistance to my motorization "hybrid" solar, sails, human, wind & hydro (tide and river currents) (squirl cage or prop), simply said, a bicycle (montain bike) "manual transmission" asssisting and increasing speed and upwind performance.
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  #98  
Old 07-24-2009, 11:28 AM
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Jeremy Harris Jeremy Harris is offline
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I'm familiar with the Bionx system, as I've already built my own electric recumbent bike (see picture below). The Bionx tries to provide electric pedal assistance, but I'm building a pure electric boat, with no pedals.

I did look at converting an electric bike hub motor to drive a big prop, but they tend to be heavy and don't really turn fast enough. Most struggle to do more than 200 rpm, so would need a pretty big prop to work well.

Jeremy
Attached Thumbnails
Efficient electric boat-bent-side-view.jpg  
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  #99  
Old 07-24-2009, 01:49 PM
MCDenny MCDenny is offline
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Rick,

Thanks so much for investing your time in the Freeship model to assess speed vs drag. I hope you are more adept than me at Freeship - that would have taken me several hours.

Good to hear your confirmation that decent speed is achievable with such modest power. I suspected it as my 25' 2000# launch will go 4.3 kts with 500 watts from the battery which would be around a half hp at the prop shaft.
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www.wolfEboats.com
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  #100  
Old 07-24-2009, 02:11 PM
yellow cat yellow cat is offline
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What if instead of using a prop, one would use a wheel like pedal boats, the rpm would be less and the shallow depth potential interesting. This is an interesting bike, i see those now and then, they look very confortable especially for men ... the std bikes discourage me ... i tend to use roller skates, for a hockey player it is almost as fast as a bike but more dangerous. Of course this is for slow speed . I was looking at DIY solar photovoltaic panels , accordeon panels for folding, but i had started investigating if the new film thin photo. panels, the company was in Lowel Mass. I have not had time to go further on this, but considering that a 37 ft wide and 60 ft long cat has potential for large areas (not including the inflated pull back raft 30 x 30 ft roof shade). DIY panels are the only viable option for now. With the new led lighting and the new Macs cpus , it looks like my energy demands will be less , hence i look at the economics (pumping back for $ into a USA and at the Quebec elct. power systems) using wind, solar and water. Tidal currents at the spot where we are going to be are 3 to 6 knts (so regular and predictable ...) .
The wheel also came up when i looked at an anchorage wind gust preventer system ... still working on the design being that i try to use the same aparatus for many uses. My wife having many uses potential she will be allowed ... she says she knows many uses for me also ...
Mike
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  #101  
Old 07-24-2009, 06:51 PM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by MCDenny View Post
Rick,

Thanks so much for investing your time in the Freeship model to assess speed vs drag. I hope you are more adept than me at Freeship - that would have taken me several hours.

Good to hear your confirmation that decent speed is achievable with such modest power. I suspected it as my 25' 2000# launch will go 4.3 kts with 500 watts from the battery which would be around a half hp at the prop shaft.
Denny
It took about 30 minutes. Viewing the linesplan was the hard part. It is not very clear.

Not sure if you have seen the latest evolution of my solar-wind hull. I call it a faux-tri as it is a long slender hull underwater, a trimaran on the waterplane to give stability and a reasonably beamy monohull above the water. This is the easiest driven hull I can come up with that has fair accommodation and will do 8kts with less than 1kW.

Rick W
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  #102  
Old 07-25-2009, 03:14 AM
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Jeremy Harris Jeremy Harris is offline
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Rick,

I like the idea of your hull design. I'm sure you already know this, but back in the 90's, the research agency I work for built a similar layout as a research ship. It was the brainchild of my director at that time, who spent a lot of effort persuading the powers that be to invest in a technology demonstrator. Here's a link to a short article describing it: http://www.royalnavy.mod.uk/triton/*...3/noRedirect/1

Jeremy
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  #103  
Old 07-25-2009, 03:24 AM
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Jeremy
There are quite a few images of that vessel around the forum.

I get the implication from use of the word "but" that it had some problems? Do you care to give details?

Rick W
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  #104  
Old 07-25-2009, 05:55 AM
Crag Cay Crag Cay is offline
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Quote:
I get the implication from use of the word "but" that it had some problems? Do you care to give details?
They realised there was no future for the concept when two Sea Lords were heard spluttering into their pink gins: "It may well be better, but it's just not right!"
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  #105  
Old 07-25-2009, 06:22 AM
Ad Hoc Ad Hoc is offline
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The only 'problem' were the paxman's!
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