Pedal Boat Design

I should have added in the above that roll is likely to have less impact on performance than a change in longitudinal trim.

 

Yeah, I sure like the heavy duty version would be 100% Tinhorn approved lol.

I went the bicycle wheel route (great minds and all =) with CF and got it down to 16.5 pounds each wheel, or 33 pounds for both. I could add lightening holes to the outer flats between paddles and likely drop 3 pounds.

I know that sounds very heavy compared to a high performance bicycle wheel, but thats not really a fair comparison. To make it fair, make the bicycle wheel 36" in Diameter, and 28" wide =)

I likely still am thicker then needed on my CF layups for this design, but trial and error will find that. Building the wheel is as follows:

The 12 paddle sections are laid up on a mold or form and vacuum bagged for nice light smooth surfaces. Then the 12 sections are bonded together to make the paddle ring.

12 spoke sections (6 to a side) are laid up on a form and vacuum bagged. After curing they are put on a jig and the triangular section is removed (laser or router). Then 6 spoke sections are bonded together for each side.

The spoke sections are bonded to the paddle ring.

The axle and hub assembly (machined aluminum, anodized) is bonded into the spoke center holes. Threaded axle inserts are threaded into the hub on each end. The 4 bolt chain ring is bolted on. The axles ride in block mounted igus G300 1/2" ID x 1" polymer bearings on each end.

I used the COSMOS analysis wizard in Solid Works and the blades will take 200 pounds of pressure. I can't imagine they would ever be subjected to that. Will be pretty straight forward and simple to build also.

I posted a bunch of images for those interested.

 

Oops - so he does!

That's impressive as hell despite the lack of cast iron components. I'm thinking that lightening holes will reduce both weight and resistance from water striking that surface between the paddles.

 

I think your full rim wheel will be a water lifting device. It is rare to get dead calm conditions so the wheel will be subjected to varying water level. Any water entering the inside of the rim will be carried upwards with the rotation. It will not contribute anything to forward momentum as it has to be accelerated to boat speed as it is being lifted. It will therefore increase drag. I would be going for paddles mounted off a spider rather than a rim.

I also see merit in the ability to trim the angle of the blades radially. I feel there is likely to be an optimum that offset slightly to the radial line. As stated earlier maybe a trial wheel to get things sorted out.

The hulls are easily defined to a high order of accuracy. The physics have been nailed.

If you were using a prop I could give you exact design for gearbox and propeller that would work within 1% of design power.

The physics for a paddlewheel is not well defined. Development stalled 100 years ago. You can work out all aspects but it takes a lot of effort. It is usually faster to build one and test variations. The engineering often follows the practice. Once you have a good engineering model you can optimise faster. My model of a wheel is quite crude. You need to go into velocity vectors and plate geometry to improve the order of accuracy. One thing for sure is that I can see downsides with a rim and not much upside from a performance perspective.

You are giving a lot away by having two wheels. Think of the situation where there is a heavy rider (you) delivering power through one half-wheel. You have all that boat plus your own weight and gear and you have only half the blade area to work with. You can check with the spreadsheet. The drag will hardly change from the full load case. Blade area will be halved and power down from 250W to 150W. You also have the other wheel in the water that will be working in reverse churning chains and cranks through a very inefficient connection via water.

 

Another point on the rim is that you could be carving two trenches in the water as well. These will come as a huge cost in extra drag.

Think about holding the two rims fixed at the side of a boat doing 6mph. Any time they are immersed they have viscous drag and wave drag. Now let them rotate freely. The viscous drag goes away but you are left with the wave drag.

If you can guarantee the rim will never contact the water then it does not pose a problem but as drawn they look like they will. Remember there will be waves from the hull even in calm conditions.

I would only make the blades high enough to gain full depth at design waterline. If there are waves they will not lift more water. They will spin a bit easier in wave troughs but that is the problem with paddlewheels. They do not tolerate rough water very well.

 

BG,
I was shocked when I saw this. All I can think is that you may be trading ease of construction for lightness.

Mmm, I don't know . . . I'm not up on CF construction methods. I did just take a quick look, though, and all the top-line bicycle wheels that use CF only have it in the rims, and sometimes in the hubs, and the spokes are still steel. These wheels weigh about two pounds each, and that's for a competition-grade mountain wheel that will survive terrific abuse.

You can do a lot better. Really, study how bicycle wheels are made. I know you won't find spokes in the length you need, but you could likely make your own with the machinery you've got.

 

Rick-- Processing your comments, but I would likely open up the flat areas of the rim with lightening holes. I could also make the blades 4" instead of 3" (with only 2.5" submerged.

Curtis--

Again, think about the numbers. 26" in diameter and less then an inch wide and it weighs 2 pounds and is made by a factory with autoclaves and 50 ton presses etc. My half wheel is 36" in diameter and as wide as 16 bicycle wheels. 16 x 2 lbs = 32 lbs. I have to do a lot more and require more surface and am half that. Both my 14" wheels together will come in at 30-33 pounds as currently drawn. Thats the width of 32 bicycle wheels or 64 lbs....besides, those uber 2 pound wheels also start at about 2K lol.

As a frame of reference, your wheel with 7 blades at only 1/16" (1.5mm) and NO structure, just a CF center hub weighs over 20 pounds...Your a seriously inventive guy who thinks outside the box so ponder it a bit. I will draw up any interesting ideas you want to articulate and we can weigh them in SW and see what they weigh. I freely admit though that going into this I though the wheel would be the easy part. Then when I got to it I was sort of shocked at how fast they get heavy.

 

BG,
Another thing to watch out for is the way you've got the axles installed in the hubs. You may need to use a left-hand thread on one end to keep the insert from backing out.

 

we cross posted Curtis...AGAIN lol.

 

I am actually a BIG believer in old school cotter pins =)

 

I looked at how much roll you could expect with beamwise offset loading. At 180kg total displacement the CoB moves 235mm off centre with 2 degrees of roll. I expect this would roughly align with the shift in CoG with one rider. It is not a huge amount of roll.

With a single 14" wheel the efficiency will drop to around 50%. You could expect about 7.5kph at an energetic cadence of 87rpm. This is getting into the well-trained category.

With side-by-side seating you can be elbow to elbow. With fore-aft seating each rider needs about 5ft of space. You can reduce this by placing the cranks under the seat in front but this then gets close to upright seating. The riders are now sitting higher and the boat is less stable. You would need to lay it out properly to determine the longitudinal trim but I think it will be quite significant. Like I said hull drag usually respond worse to trim than roll.

 

If you are sticking with the 0.9m wheels then you should gear them up a bit. Maybe a ratio around 1:1.3.

Rick W

 

Well, you've got a big-time number-cruncher there, and I don't . . . still, I think your cylindrical rim is superfluous, or should be if you take a different design approach. The quick sketch is a cross-section. The rim spans the radial dimension of the floats, and there is no cylindrical rim at all. You probably should have some sort of diagonal bracing between the rims; I think wire in tension would work well. Depending how the flat rim is done, it could make it a lot easier for you to play with a lead angle for the floats. And, of course, the flat rim would be pierced for weight reduction. I think this method could lighten the assembly quite a bit and still be plenty strong.

CF construction is great, but is isn't necessarily the best solution to all problems.

Curtis

 

Oh, and I just solved another problem -- here's how you can make your rim to be assembled with off-the-shelf bicycle spokes.
You also see here that the floats are attached to the rim with a flange. If you slot one of the mounting holes, it will allow lead adjustment -- just remember to tighten the fasteners well once you've done the adjustment.

 

To take the place of your cylindrical rim, you'll want to mold a lengthwise rib or two into your CF paddle floats as needed for stiffness.