06-08-2015, 11:15 PM
Thanks Steve. Your gems of wisdom are pretty much what I've come up with too, so it must be true wisdom, or at least a good shot in the dark.
The guys Im building this with have a LOT more experience with steam than I do. They figured about 200 rpm would be a good speed for the engine at cruise. We'll have 20 feet at the water line so hull speed should be 6 knots or 607 ft/min. I somewhere ran across a 20% slip factor for paddles, not sure if that number is good your double speed probably is closer. Anyway at 20% figured 29 rpm on the paddles. We found an old gear set out of some farm equipment with a 32 inch dia bull gear with 94 teeth and an 11 tooth pinion. About 8 1/2 to 1 so 250 rpm at hull speed. If the slip number is low then it will just run faster and use more steam so more wood to fire, All seams to balance out.
Came to the same conclusion about too much paddle area, except you can't really stall a steam engine in the same sense as you would a gas engine. It will develop full torque at no rpm if the piston is at mid stroke. The engine is 5 inch bore and 5 inch stroke. We plan to keep the pressure down at 80 psi, (its an old boiler) still gives 327 foot pounds of torque at mid stroke. With the 8.5:1 reduction we'll have 2800 foot pounds torque at the paddle shaft or 700 thrust at the blades.
So think getting it moving won't be a problem, using the tried and true seat of the pants thats sounds about right theory.
So used that same philosophy and determined the most efficient paddle size. Think the boat will draw a little over a foot of water so decided the paddles should be 12 inches deep. The hulls 8 feet wide, we can get a permit to move 12 feet so clearance to the hull and the skirt around the paddles gave an exact paddle width of 16.000 inches. Decision made order the material.
Actually thanks Steve, your thoughts seam to be pretty much along with what I've come up with so as they say brilliant minds think alike or was that fools seldom differ.
The guys Im building this with have a LOT more experience with steam than I do. They figured about 200 rpm would be a good speed for the engine at cruise. We'll have 20 feet at the water line so hull speed should be 6 knots or 607 ft/min. I somewhere ran across a 20% slip factor for paddles, not sure if that number is good your double speed probably is closer. Anyway at 20% figured 29 rpm on the paddles. We found an old gear set out of some farm equipment with a 32 inch dia bull gear with 94 teeth and an 11 tooth pinion. About 8 1/2 to 1 so 250 rpm at hull speed. If the slip number is low then it will just run faster and use more steam so more wood to fire, All seams to balance out.
Came to the same conclusion about too much paddle area, except you can't really stall a steam engine in the same sense as you would a gas engine. It will develop full torque at no rpm if the piston is at mid stroke. The engine is 5 inch bore and 5 inch stroke. We plan to keep the pressure down at 80 psi, (its an old boiler) still gives 327 foot pounds of torque at mid stroke. With the 8.5:1 reduction we'll have 2800 foot pounds torque at the paddle shaft or 700 thrust at the blades.
So think getting it moving won't be a problem, using the tried and true seat of the pants thats sounds about right theory.
So used that same philosophy and determined the most efficient paddle size. Think the boat will draw a little over a foot of water so decided the paddles should be 12 inches deep. The hulls 8 feet wide, we can get a permit to move 12 feet so clearance to the hull and the skirt around the paddles gave an exact paddle width of 16.000 inches. Decision made order the material.
Actually thanks Steve, your thoughts seam to be pretty much along with what I've come up with so as they say brilliant minds think alike or was that fools seldom differ.
Free advice is worth exactly what you payed for it.
Greg
Greg