Building an Aligator

[vtsteam]

(04-27-2015, 01:40 PM)f350ca Wrote: Need to do some actual engineering design work now, and would like some input on paddle size.
My thought is paddle size would be based on the boat sitting stationary and applying steam pressure to the engine to get it moving. Too much paddle area and you won't move the paddle through the water, too small and you won't get enough thrust to move the boat at a reasonable acceleration.
The dia of the paddles will be 8 feet. That gets the shaft at deck level and the bottom of the paddles even with the bottom of the boat.
Rotational speed would be set to give hull speed plus a certain amount for slip.      How much slip?
Once the speed is determined I can calculate the torque available at the paddle shaft based on the bore and stoke of the steam engine and boiler pressure.
So for a given torque and a 4 foot arm how do I determine how much area on the blades can be moved through still water at startup?

Any thoughts would be appreciated, there's not much in the way of engineering formulas out there on the net for paddle wheelers, wonder why?

Greg, I've thought about this problem and wanted to answer, but I can't in all honesty. Apologies..... so much depends on the particulars of this hull, displacement, the engine, etc.

What I can tell you is that this part "Too much paddle area and you won't move the paddle through the water." isn't true. The paddle will always move through the water if the boat is statiionary, unless the engine stalls.

How much thrust that is generating for the hull is variable, depending on area, depth, "slip", cavitation (if too fast), etc. But it will move. If you're tied up to the dock and the motor runs you'll get 100% slip, and cavitation too, likely. But the blade will move, unless something breaks or the engine stalls. Same for any boat tied up and running.

But abstract theory isn't really what you want (and I can't provide). Ballpark, is what you want, and for that, I'd shoot for the blades to be running at about twice hull speed at the engine top speed. So go by your engine spec, then calculate no slip on a 4 foot radius to yield 2 times hul speed. That will give you your reduction needed (if you are going to have reduction.) Double hull speed is, by reversal, what ballpark was for radial turbines, and this is basically the same thing if you think upside down, which I generally do.

If you don't have sufficient power to run the blades at double hull speed while actually moving at hull speed, your top speed will be less than hull speed. It will settle at whatever available effective horsepower you have at the wheel.

Also if you don't have a reduction to give twice hull speed, the boat will still move, but the efficiency will be reduced, so top speed will be less than it could be. Unless of course you have so much power in whatever engine she'll have below decks, that efficiency doesn't matter -- which is the modern trend in small craft.

My guess is no, you don't. Which is good!

Keep in mind this is a piece of free advice -- per your sig. and you know what to do with it.

Good luck!

--Steve