Pedal Boat Design

1/3 scale it is then. I figure I can follow Curtis's lead and use up some of this balsa I have had around forever. A bit of stitch and glue with a few layers of light glass to seal it up. We have a machine shop in Florida we use for precision work in aluminum. They can handle the cam channel and maybe the UHMW three point followers if I can't get a good enough surface on them.

Hot wired foam and carbon fiber for the blades.

Actually sounds like a fun project, time permitting.

Are you still thinking 70-75 RPM for the wheels? at 1/3 scale the wheel is about 2.75" in diameter so it should not take much of a motor to get that. Likely will be more of an issue finding one that will go slow enough. Would the RPM scale down to 1/3 as well?

 

BG,
Don't forget to make the weight to proper scale, too; anything will work as ballast as long as the model has the waterline to scale.

Sounds like fun. I can hardly wait to see this.

 

Aim to run it up to at least 100rpm. This gets it into a more realistic regime for both the wheel and the boat.

You can buy aluminium foil sections but the ones I know of are not the right size:
http://www.mtmscientific.com/airfoil.html
The whole thing does not need to be an exact scale because the loads and stresses do not scale linearly. As long as you can get it to work smoothly it is not hard to calculate conditions at the scale level. This will need to be done so performance can be aligned with the calculation.

What I am getting at is that the foils do not need to be exactly to scale or the wheel to exact proportions. Is a more a matter of what is easiest to make with parts that may be available. I expect aluminium foils would be good enough in scale. If you have access to milling then it could be easier to do some rough milling and then hand burnish foils. As long as the shape is known I can determine how it should perform. They could be just thin plates with rounded nose and sharp tail. Not ideal but will still work.

If you go to the effort of making the model boat you could also use it for paddlewheel tests. It is handy to have a nice size test platform and can be fun.

It is unusual to get something completely new exactly right first time. Expect to do some modifications and wheel tuning.

Rick

 

Better to tune on an inexpensive model for sure.

I also think I just got one of Curtis's light bulb moments. If you will recall, I mentioned that I was having a hard time with FREEship and calculating the hydrostatics of the outriggers because they were shaped like a shorter and narrower version of the main hull, but split down the center, with each resulting half becoming an outrigger float. Because FREEship uses only half the hull for it's numbers...I could not figure out how to do the outriggers.

It just occurred to me that maybe draw my half hull (the main hull) and put the outrigger in place and FREEship will likely just duplicate it as well. I will see how that goes. If I still can't get it to work I will just cart the drawings off to you lol.

 

Those extruded airfoils are just the right size for the full size boat. Shame they are so heavy because a 3/32" layer of CF and we are right on the money for size. Six of them at 20" each is 8 pounds of aluminum though, and they would add little stiffness compared to the CF.

 

BG
You do not need to worry about the outrigger buoyancy. They should play no role in the buoyancy. They are simply there to stop the boat from tipping over.

Ideally the outriggers have some adjustment so they can be fine tuned.

With your boat it may need to have settings for one or two people.

My outriggers are set up to fold in and I transport like this. If you wait till the very end of this video you can see the operation:
http://www.youtube.com/watch?v=ckWqIgmVM4Y
You can also see on both the yellow and black boats how the outriggers sit during operation. The aim is to avoid them adding drag by just simming now and then.

The lower you sit the better the outriggers work because you reduce the tendency for the boat to flop. If you sit high then this lifts the centre of gravity and the required righting moment increases.

Once at speed you can stabilise the boat with the rudder but not with my dipping rudders. They are too slow to apply. But even a tiny rudder under the hull will provide substantial stabilising force. I do not bother with this because the outriggers offer very little drag anyhow and I avoid the need to concentrate on keeping outriggers ballanced above the water.

Rick

 

Rick,

The problem I am having is getting the boat deep enough to make the wheel side tucked into the boat so there is ZERO resultant drag. In order to do that I have to either slope the bottom towards each end a good bit, or make the beam much narrower. If I make a 6" beam at the waterline and draw enough depth then the boat would be quite tippy I am pretty sure.

So the plan is to use the outriggers as long thin displacement hulls to stabilize the boat and at the same time fare the outer wheel sides so the sides don't contribute any drag.

I also thought about the type of hull you used on several of your earlier boats that was mostly submerged as this gets me the depth I need for the wheels.

Making the outriggers fold is not needed for this boat, and would make the mechanics needed to drive the outer wheel sides problematic.

How important is the hull curvature bellow the waterline? While looking at your very trick collapsible outriggers on the black boat in the video I noticed the black boats hull looks slab sided and mostly flat on the bottom--like the corners/edges where the bottom meets the sides had a slight radius and that was about it. Like the image bellow somewhat.

 

BG
For the general look I would go deep enough with the hull to get the wheels covered. Lets say 8". Make a nice taper bow to stern. Not the optimum but will make it easier to turn. See what width you get. I can then check the drag. If it is a lot worse than the optimum hull then take the wheels below the keel line. Fine tuning the design might mean the wheels get smaller diameter.

The width of the hull does not matter that much because it will need to be something like 14" wide before you start to get useful stability. I am reasonably confident the wheels can be set up to self stabilise. Something to test on the model.

The outboard supports for the wheels can be a streamline foil 8" long by 8" deep with a curved bottom to match the wheel. Buoyancy will be close to neutral. The stabilising float can be mounted above this. (I forgot about the need to support the wheels outboard.)

Rick

 

This is the crude plan form in the images. I left off the cross member that attaches the outriggers to the main hull and transfers power out there too to make it less confusing. Water line would be at about the top of the wheels--meaning the above the water line area might need to be taller.

They might be longer and thinner too of course.

Having the hull flush on one side and the outrigger on the other means the wheel ends that have the mechanics are not in the water creating drag.

 

The outboard wheel enclosure would best to be a foil below the longer outrigger. Keep the outrigger length the same as you have but bring most of the outrigger keel above the waterline. You really want to minimise the wetted surface where you can.

I like to have reserve buoyancy on the main hull about twice the total displacement. In your case this would be excessive as it is for two people but as drawn it is likely to be not enough. It is common to set more buoyancy in the bow of the hull. This will shed water away from the low seats when you go through waves. So keep the seats low but raise the dech at the bow a few inches higher.

Rick

 

By the way it looks purposeful. If you get the drive to work well it will be a great boat. I might make my own wheels and try them out.

Rick

 

Rick--

It "seems" logical that the wheel and any portion of the sides (the outer wheel sides) that are under water would provide displacement also right? So that area has to be calculated into the total displacement numbers as it would affect the waterline or draft...right?

I am thinking this is the case because they would provide a self correcting or self leveling moment to the boat.

Also, is placing the wheels dead center fore / aft the best choice? I know adding the rudders to those outer down strut / fins for the wheels would help, but I don't see them having much effect if they are amidships. The further aft they are the better they work right? Would moving the wheels aft say 2-3 feet be worth it to make those rudders work?

G

 

Also...

Odd idea, but what if we left the wheels centered, but made each half independent for power? I am not talking splitting them between the engines. Both engines still drive both wheels, but to make a turn you changed the right or left wheel to a gear that cut its RPM in half for example. It seems like that because the wheels ARE centrally mounted that one outputting twice the power of the other would be quite the "lever" in turning the boat.

You could also have three gears and the least gear would be a more gradual difference in RPM change for the course correction type turn and the biggest RPM change would be more for sharper turns about buoys or landing etc. Could even cut all power to one wheel so you get all power output still (to one wheel) and a little drag from the dead wheel acting like a compass point.

G

 

At least one of the old-timers claimed sidewheels are optimally placed at the 5/8 point.

(See? I'm still paying attention!)

 

BG,

Every submerged part of the boat has displacement, but not all of them will have positive bouyancy. If your drive components would normally sink, they don't count towards flotation.

Interesting thought. I wonder if someone makes a little controllable differential gear? I'm thinking your gearing concept might become more complex than you'd like.