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  #76  
Old 08-19-2008, 07:15 PM
BG_Geno BG_Geno is offline
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Rick--

I sent both hulls at about the spacing I was thinking...if you only needed one sorry, I can resend.

Also that rudder configuration was just a "doodle" as I was learning this program at the time.

The hulls are very simplified, again I was just learning this software and have zero idea what makes for a good hull design. I did it by eye for what looked about right.
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  #77  
Old 08-19-2008, 07:23 PM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by BG_Geno View Post
Rick-

Going to try to attach the file here.

The engines total about 325 lbs and I hope to have the boat weigh in at about 125 pounds. As we would occasionally bring a dog and supplies for a day trip figure 525 pounds.

As for the engines. Mid 40's who has spent too much time in this chair on the computer the last few years. Thus the decision to build the boat this winter. I would rather build it for engines in good shape then get said engines to that point then for weak engines.
BG
I will do some numbers. To avoid me going to a lot of trouble with units I will stick with metric as most of my design tools are set up for metric. I am old enough to think in both sets of units but metric is so much simpler mainly because of the way things work out in the natural world. For example the gravitational constant is 10 to within 2% and 1t of fresh water is 1Cu.m.

So design condition is displacement of 250kg and design power is 250W (both nice round numbers). I will see what optimum cat hulls look like. If they are longer than 6m I will limit them to 6m.

I can also provide an easy-to-build hard chine if you want to get started.

May not be till tonight as I have paid work to do right now.

Rick W.
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  #78  
Old 08-19-2008, 08:06 PM
clmanges clmanges is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Curtis
You get the right answer but you have glossed over a couple of points. You have assumed the same length for both boats. This is not the case for the optimum hulls.

If you use hulls of identical form but scaled for half volume then the area difference is only 26%. But drag has viscous and wave components. Once you optimise for the minimum combined drag the catamaran might be 20ft long while the monohull will be 24ft long.

Point is, arriving at an optimum for both cases is a complex task and it just happens that the power difference is around 40% for the same speed.

Your analysis gives a first approximation and as it happens it is very close to the final result but the hulls of the two optimum boats will have quite different form.

I wanted to make it clear that I did not arrive at my answer using simplifying assumptions that could be easily challenged. It is arrived at using Godzilla with in excess of 30,000 iterations per boat.

Rick W.
Rick,
This whole exercise was to help BG get clear on how surface area changes with shape, so I used the same length to compare apple to apples, so to speak. He seemed to be having a problem with that.

My intention was not to make anyone think that the design process was that simple; it isn't. If I had to design anything more complex than a box, I'd be in trouble. It never occurred to me to bring up the differences in final shape and size between the two alternatives, but you're right.

Curtis
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  #79  
Old 08-19-2008, 08:41 PM
BG_Geno BG_Geno is offline
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Yeah, what you saw Rick was a really bright guy trying to speak loudly and very slowly to a dumb guy lol.

Metric is certainly fine as there is no reason I cant just convert over and waste as little of your time as possible.

As for the hard chine...not sure exactly what that is as this is my first boat build. That said, I am a pretty capable builder so that portion of this process doesn't need dumbing down for me, at least as much as the math does =)

I have begun the transition over to CNC manufacturing for as much of my shop as I can so I can actually do some pretty neat (and accurate) stuff. My 3 axis CNC router can cut sheet stock for ribs/bulkheads to +/- .005" accuracy. I also have access to a CNC laser with an 18" x 24" bed. I even built myself a 3 axis solder paste dispenser with pick and place capabilities. Next up is a 3 axis hot wire foam cutter with an 8' x 2' x 2' cutting (or so) envelope that should come in handy for building foam forms with complex shapes very accurately. Like pedal boat hulls =)
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  #80  
Old 08-19-2008, 08:54 PM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by BG_Geno View Post
Yeah, what you saw Rick was a really bright guy trying to speak loudly and very slowly to a dumb guy lol.

Metric is certainly fine as there is no reason I cant just convert over and waste as little of your time as possible.

As for the hard chine...not sure exactly what that is as this is my first boat build. That said, I am a pretty capable builder so that portion of this process doesn't need dumbing down for me, at least as much as the math does =)

I have begun the transition over to CNC manufacturing for as much of my shop as I can so I can actually do some pretty neat (and accurate) stuff. My 3 axis CNC router can cut sheet stock for ribs/bulkheads to +/- .005" accuracy. I also have access to a CNC laser with an 18" x 24" bed. I even built myself a 3 axis solder paste dispenser with pick and place capabilities. Next up is a 3 axis hot wire foam cutter with an 8' x 2' x 2' cutting (or so) envelope that should come in handy for building foam forms with complex shapes very accurately. Like pedal boat hulls =)
A hard chine allows each hull to be constructed from four pieces. These are held in place while the seams are joined. Normally there are internal bulkheads at load points.

This is the simple method of construction for ply or aluminium sheet and I have only just started using it for flat-pack composite. You can get an incredibly light and strong hull.

The foam plug results in a lot of waste. If you leave the foam inside the hull then it also adds unneeded weight.

I normally do a hard chine comparison just to see what benefit I get from all the extra effort of a rounded chine.

Using flat-pack method results in much less fairing. I have formed the layup on smooth neoprene sheet placed on solid chipboard sheets and I can get a mirror smooth finish in the layup.

If you are not familiar with this method I suggest you have a test.

Rick W.
started three long the boat to be buil
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  #81  
Old 08-19-2008, 10:16 PM
Guest625101138 Guest625101138 is offline
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BG
I have attached the optimum 6m hull shape in igs form. Its design draft is 120mm.

I have also attached a paddlewheel speed calculator. You can change the value in the 4 yellow cells. You can see you need a large wheel to get efficiency. This tends to be what turns most off them. They do not need to be heavy.

You could make a very nice pedal system using swing arms, links and cams rather than messy chains. These work well with two riders because you can have the strokes offset by 90 degrees of wheel rotation.

I have not done the hard chine version as I wanted to get your impression first about desirability of a simple build. If you can CNC foam and get it at a good price then this can be made up as a male plug. This also works well as you only need one plug for the two hulls.

I could make good hulls in a weekend for this boat using aluminium sheet. They would not look elegant but would be fast to build, light and similar performance to something that takes much longer. So it gets down to what you want.

The good thing about it is that I can give reasonable prediction of performance in absolute and relative terms. This is what saves time in the long run. You do not need to build it to know what it will do.

Fiddling with the hull shape a little will not make much difference. Overall width is best at 1.5m or there abouts. Ideally a little wider to get a wider wheel between the hulls.

Rick
Attached Files
File Type: igs BG_Opt.igs (60.1 KB, 152 views)
File Type: xls BG_Paddle_Wheel_Drive.xls (83.5 KB, 200 views)
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  #82  
Old 08-20-2008, 12:07 AM
BG_Geno BG_Geno is offline
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Rick,

I am likely doing something wrong, but I am unable to open the .igs file.

I tried just left clicking and opening it with several programs, then saving it first and then opening it. It saves as a text file (notepad). Tried several .igs viewers also. Rapid fire, cimitry, edrawing viewer etc. Tried importing it also, no luck.

Can you please rar or zip it first,then attaching it?

Very sorry for the trouble but I tried everything I can think of before asking.
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  #83  
Old 08-20-2008, 12:45 AM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by BG_Geno View Post
Rick,

I am likely doing something wrong, but I am unable to open the .igs file.

I tried just left clicking and opening it with several programs, then saving it first and then opening it. It saves as a text file (notepad). Tried several .igs viewers also. Rapid fire, cimitry, edrawing viewer etc. Tried importing it also, no luck.

Can you please rar or zip it first,then attaching it?

Very sorry for the trouble but I tried everything I can think of before asking.
Try these.

I can open with an independent reader but it says it has a non-fatal error. Probably something to with units.

If these do not work I will send a coordinate file.

If I sent an image you could draw it in 5 minutes. It is a very simple shape.

Rick
Attached Files
File Type: dxf BG_Opt.dxf (314.1 KB, 180 views)
File Type: igs BG_Opt.igs (60.1 KB, 120 views)
File Type: zip BG_Opt.zip (14.2 KB, 49 views)
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  #84  
Old 08-20-2008, 01:39 AM
BG_Geno BG_Geno is offline
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That did the trick, thanks!
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  #85  
Old 08-20-2008, 05:25 AM
alexlebrit alexlebrit is offline
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Quote:
Originally Posted by Village_Idiot View Post
Taken a step further, could one create a pedal-powered water pump above the waterline (for drafting purposes), shooting the water out of a pipe at the stern to create forward thrust? Again, I know jet drives are inefficient in motorized craft, but can they be designed to operate efficiently in the PPB?

I'll stop rambling for now...
I've wondered about this too, although I'd site it below the waterline. In fact i've wondered if it couldn't be as simple as a ducted prop but with the prop on a slant in the duct so to speak.Fine mesh over the entrance would prevent weed getting inside although you'd still have to scrape it off every now and then. Mind you I have no idea of the efficiency of such a design.

Second though on odd propuslion ideas, what about leg powered rowing? Well kind of. Looking at people rowing I've noticed many rotate the oar when it's out of the water so it just glides above the surface horizontally. Surely something similar could be built, perhaps in a tunnel under the boat so that the "oar" is powered by the forward and back motion of a Rick style pedal system, and it feathers as it goes forwards then opens up as it pushes the water back? Obviously the longer a stroke you can get the better, but pulleys and cables should be able to increase this.

Does that make any sense?
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  #86  
Old 08-20-2008, 06:55 AM
Guest625101138 Guest625101138 is offline
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The attached cat has the same drag as the rounded chine hull version. You can see how easily it could be made from flat panels.

I have not been able to get a sensible hull from what you sent me. It seems to have wings on each end. It is as if there are guide planes for drawing it in place.

Rick W.
Attached Files
File Type: dxf BG_Dev.dxf (277.7 KB, 135 views)
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  #87  
Old 08-20-2008, 09:56 AM
BG_Geno BG_Geno is offline
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Solid Works uses sketches on a plane that are then extruded--the sketch may b showing up. I am having similar problems with yours. I can open them but they are surfaces not solids, and none of the surfaces, edges, or points are selectable so I cant close it off to make a solid. That means none of the analytics will run. I cant even dimension or measure.

I will upload some images of the hulls and some down strut ideas I have been working on a little later today.

On the water jet drive, if I had to guess, just based on the low weight and high horsepower on jet skis...not very efficient.
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  #88  
Old 08-20-2008, 10:16 AM
clmanges clmanges is offline
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I've thought about a muscle-powered jet drive, but then I thought that there must be a good reason I haven't seen it done already. My guess is that, since these things have to run at very high RPM's, the needed gearing would be too heavy and complex, and your arms/legs would not be able to spin it up quickly or easily. It has another drawback: it depends on moving a small volume of water at high velocity, which (I've heard) is harder than moving a large volume of water at low velocity. Finally, you'd have to ensure that the intake was always submerged, or you could wind up sucking air, probably just when you most needed the thrust (Murphy's law).
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  #89  
Old 08-20-2008, 10:17 AM
BG_Geno BG_Geno is offline
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Rick-

Is there another supported format in your software? There are at least 10 other file types Solid Works will save to. I send a lot of production drawings for work out in step or para solid formats for example.

Also, when I mentioned cutting precision foam hull cores I meant more as a male plug for making molds. What is great is that on the simple shapes we use for hulls you can make a second plug a few mm smaller to use to compress your layup into the female. Not that I would likely do that on something this big however. On the carbon fiber down struts and frame I might though. Make your female molds, cut plugs, lay your carbon fiber up on those plugs, put the whole kit into the females and bolt it down. I have tanks big enough to either pressure cook or draw a vacuum on the component parts. Then those would be bonded together. Lots of very interesting possibilities.
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  #90  
Old 08-20-2008, 11:33 AM
alexlebrit alexlebrit is offline
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Quote:
Originally Posted by clmanges View Post
I've thought about a muscle-powered jet drive, but then I thought that there must be a good reason I haven't seen it done already. My guess is that, since these things have to run at very high RPM's, the needed gearing would be too heavy and complex, and your arms/legs would not be able to spin it up quickly or easily. It has another drawback: it depends on moving a small volume of water at high velocity, which (I've heard) is harder than moving a large volume of water at low velocity. Finally, you'd have to ensure that the intake was always submerged, or you could wind up sucking air, probably just when you most needed the thrust (Murphy's law).
Rather than a jet-drive perhaps the thing to think of is a ducted prop with the exit directed to give thrust.

I've wondered about swinging a large thin bladed prop (standard PPB style) quite slowly in a duct. If the prop was angled almost horizontal and you had the intake delivering water above the prop with the exit below and back would that work? Would it be drastically less efficient than a standard PPB prop configuration? Because of course the big advantage is that it doesn't dangle out the bottom and get snagged on things. And even if you lose a bit of out and out speed, if you have to spend a lot less time fishing weed out then your average speed might be the same.
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