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| stuffing box and prop placement in a ... |
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Michael Marmet
| | Posted on Thursday, March 27, 2003 - 08:19 pm: | 
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Hello-
looking for some advice on where to place the stuffing box and prop shaft on a 25ft long at the water line, flat bottom, double ended dory made from steel. The boat was never equipped for a motor- but I am looking to convert it. It's displacement is about 1000lbs with the water line 4-5inches up from the bottom on the sides. Originally I had planned on a shaft going out the rear bow, but it sits so high in the water- looks like I need to go through the floor maybe a foot or two before the stern. In addition- being the boat is entirely steel- I was wondering about some sort of motor mounts to soften the engine vibration(using a 2 cylinder 2cycle from the early 1920's- still looking for a forward/reverse transmission). Your advice is greatly appreciated- thanks! |
miro
| | Posted on Friday, March 28, 2003 - 05:03 pm: |
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You've got a good geometry challenge here. The daimeter of your prop will likely dictate most of your answers. Most likely you'l want a 3 blade prop and to keep the ting submerged , you'll have to set up the engine and shaft on an incline - that's where the geometry comes in.
As for motor mounts - the usually practice, as I undertand it, is that the engines are hard bolted to the engine bed and the engine bed is made up of beams that are as beefy as you can make them - preferably white oak with crossbeams tightly bolted. You want to transfer as much of the energy of the engine to the prop, not to vibrations to the hull.
Alignment of the engine and shaft are also crucial top preventing vibration.
ANother source of vibration is that many props have blades that are not uniformly pitched - one balde is different from the others. Get 'em all piched the same and it becomes much smoother.
Seems to me that many of our pals in Oz probably have some views and experinces - how 'bout it , boys?
miro |
Michael Marmet
| | Posted on Friday, March 28, 2003 - 09:40 pm: |
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Miro-
thanks for the thoughts- the St Lawrence spec sheets called for a 14x18 prop with the "XC" 6-8hp engine, which is what I will work around for prop size. My first launch was an easy set up as all the motor mounts were there- this one is a better challenge- the "dory" is all steel versus wood- I will have to fabricate a motor mount- which is where the geometry comes in- looking over many early power dory designs, I think I have a good idea as to visual placement- how far under the water surface should the prop go as a rule of thumb? Should I end the prop under the hull or let it dangle out off the stern several inches from the rudder? speaking of rudders- this is another unit of fabrication- is a square/retangle design better than the arched fin? is there a rudder face square inch rule for the length of hull, weight of boat or HP of engine? So many questions this messing about in boats sure is fun! There must be some good math behind prop drive shaft angle efficiency, but maybe it's just good old naval architecture experimentation. Any and all thougts help- !
Michael |
miro
| | Posted on Sunday, March 30, 2003 - 10:19 pm: |
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I think the basic idea is to keep the prop submerged and to minimize te incline angle. I assume that you do not have a deadwood on the keel. There is some loss of efficiency due to the inclne angle, but heck, this ain't a work boat, so a couple of pennies worth of fuel won't bother you. Assuming that you have a support strunt and bearlng in front of the prop, you shold have no problems. As for the rudder, big is better than small. Have a look at the ruddeers on steam launches, ad you get the idea.
Actually there is a very good set of books ( I think Andrew has them) which is from the early 1960's , which are a collection of article, about steam launches and I think there were a couple of articles, which described converting small lifeboats to steam power.
Nice thing about rubbers, is that they are easy to make smaller - not so easy to make bigger. |
donwhite
| | Posted on Monday, March 31, 2003 - 07:19 am: |
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Miro
I've been reading OME a couple years and enjoy it much. I'm from Louisville, Ky, which owns and runs the Belle of Louisville (formerly Avalon) as a tourist attarction.
It's a stern wheel steam paddle boat. The planking for the paddles seems to be abuot 20 ft across (wide) and 18 inches deep and 2 inches thick. I suppose 16 sets to the wheel.
Thinking back to my 1950s USAF career, the F86 had
"dive brakes" in the aft portion of the fuselage, which were extended to slow the aircraft. I noticed the braking panels were like swiss cheese,
full of holes. I asked a pilot why and he said it
allowed excess air to escape through the panels and thereby avoided creating eddys of air that would disturb the progress of the plane through the air.
My question: Would it not add to the efficiency of the paddle wheel if one was to "swiss cheese" the individual planks, with say, holes of 2 inch diameter, spaced far enough apart so as to not adversely effect the structural integrity of the planking? It seems to me a lot of horsepower is used in "sweeping" the water aside as the paddle progresses through the water, whereas, if there were escape provisions, the paddle might actually deliver more "Ummph" or power into the water?
I'd appreciate your comments regarding this adaption of "dive brakes" to paddle wheel boats.
Don W |
miro
| | Posted on Tuesday, April 01, 2003 - 02:14 pm: |
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Seems to me that the dive brakes are dealing with a compressible fluid (air) while the paddle wheels are dealing with an uncompressible fluid(water). The dive brakes also should not create vortices that might prevent the tail control surfaces from working properly.
The trick with the paddle wheels is to size them to the amount of power available to turn the wheel. I'm sure there is some sort of optimum combination of surface area, number of planks and speed of rotation. Too bad the guys who knew about this are enjoying that river cruise through eternity. I wonder what is the equivalent slip to a regular propeller, of a paddle wheel? Regular marine props are about 25% .
I'd have to check but I recall that the steamboat book collection that I referred to, had a number of articles about paddle wheel boats and I think some of them were quite technical i.e. described the engine hp, the speed of rotation, the surface area of planks etc. Can't lay my hands on it right now - the books are at the still-snowed-in summer place that I have.
My final thought is that paddle wheeled boats were used in very shallow waters where props would simply not survive - so they were a compromise right from the get-go, but did the work intended, however inefficiently.
miro |
David Stott
| | Posted on Wednesday, April 02, 2003 - 12:43 am: |
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Here in Australia we have a river port on the Murray River in Victoria called Echuca. Here they operate the largest steam powered paddle steamer fleet in the world. This includes the oldest wooden hull steamer the "Adelaide" built in 1826 and still in regular use.
They employ shipwrights who may be able to answer your query about optimum paddle wheel size. Try www.portofechuca.org.au |
Iolanthe, San Francisco
Visitor
| | Posted on Wednesday, September 29, 2010 - 02:13 pm: |
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You might consider a common design seen in Wooden Boat: A conventional prop is placed in a raised half-tunnel at least 25% forward of the stern so that it is protected from the bottom and fully submerged at all times. A plus is that the shrouding effect reduces tip loss. The motor is forward of the raised hump of the tunnel, so it is near the design C.G. of the boat. |
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