All boats, even jet pump driven boats have some form of propeller- something to propel the boat by moving water away from the boat.
Each of the types of propeller has some type of drive shaft to couple it to a marine reduction gear or directly to the engine’s flywheel and crankshaft output. This is of course how the engine’s torque is transmitted to the propeller to move the boat.
Inboard boats like Double Eagle have a long shaft that reaches from the after end of the engine at a downward angle, through the hull in a specially installed tube or pipe, to the propeller just under the stern.
An outboard engine’s prop shaft is only a few inches long maybe a foot total in a higher horsepower engine. All shafts have bearings to support the shaft and to keep it in a line while turning at high speeds. Outboards and inboard outboards have all the bearings, shaft and reverse gear in one small streamlined housing that is underwater, but sealed and filled with oil.
That is a great design and solves lots of problems for a boat builder, since lining up the shafts and the bearings, is done in a machine shop setting, the bearings are lubricated with oil and all of the stresses are in a small volume so they’re not stretched out for 10’ or something like they are in a shaft driven inboard design boat.
Inboard shaft boats have shaft bearings too, but they’re lubricated with water. In fact the bearing are not only lubricated by water instead of oil, they’re formed of water too. Inboard shaft driven boats have shafts which are much more slender in comparison to nice short outboard or inboard outboard drives.
The thicker a shaft is in proportion to its length the stiffer it will remain in use. So an outboard lower end drive shaft that is 12” long and ¾” in diameter has a ratio of about 0.06 while a 1-1/2” diameter shaft that is 6’ long has a ratio of 0.020less than a third of the ratio of a shaft ½ as large in diameter.
What this means is that inboard shafts have to be supported not only at both ends but sometimes, depending on the diameter to length they have to supported in the middle too. If we were considering the design of supports for an 8’ long shaft then planning and placing the bearings to keep the shaft in line means there will have to be a method to place at least three welded in “bearing holders”.
That work is one of the main purposes of this series of posts about the Double Eagle. I built some 30’+ forward engine, inboard and shaft boats, of welded aluminum, that were originally designed by Ed Monk, in the late 1960’s, I believe. I’ll provide some rough sketches of the original plans, from an old and not too facile a memory, and then show some ideas about how to improve on those original plans.
If you are interested in this type of information presented in a broader scope, please let me suggest that you consider Dave Gerr’s books
http://www.gerrmarine.com/dave.html . Clearly the guru of boat design and building authors whose incredible knowledge is coupled with the ability to make the complex understandable.
I think I have all his books, and have made them central to my gift giving for years to my family and boat building friends. I can’t recommend his work enough. Our discussion is about a design and building method from the boat builder’s point of view so it is more detailed- but for a very much more limited group of readers.
I will not try to rework any of the
http://www.amazon.com/boat-mechanical-s ... pd_sim_b_2 details here. This book should be on the book shelf of all builders in my opinion, so what I will attempt to do is stand on this information, then build a few ideas that are results of my welded aluminum experience as they apply to the Double Eagle.
Cutlass bearings versus ball bearings or roller bearings, need to be mentioned so we can introduce the idea of tolerance and shaft alignment. Outboards have sealed oil fill chambers to hold he shaft bearings and these roller or ball bearings are mechanically superior to water cutlass bearings; the oil lubricated metal bearings last longer, work at higher speeds more reliably and are easier to plan for and install partly because the tolerances or clearance between the parts is much smaller than in a water cutlass bearing used to align longer inboard engine shaft driven boats.
A ball, thrust cone, or needle roller bearing relies on metal spheres, cones or cylinders rolling on (touching) precision machined cylinders, “half pipes” and cones all bathed in oils that reduce friction and allow the metals to touch without disintegrating.
Water lubricated cutlass bearings, on the other hand, rely on a film of water inside the bearing; to form a hard but liquid surface against the shaft while it turns at high speeds. Anyone who wants to review this idea- that water gets harder the faster you hit it, or harder the faster you travel through it can read the Hull Shape discussion we’ve held on another thread.
http://aluminumalloyboats.com/viewtopic.php?f=4&t=4364
Metal to metal bearings incorporate harder material than water so they have much smaller surface areas able to hold bigger loads than water can hold. Marine cutlass bearings are larger in area to allow a turning shaft to be supported in the bearing by a layer of water that is ‘hardened’ by the extreme speed of the surface of the shaft moving the water inside the tolerance of the bearing sleeve to shaft cylinder.
Lets recall a Mythbusters episode where they fired a Barrett 50 rifle into a swimming pool and the bullet didn’t penetrate at all, instead it disintegrated almost at the surface of the water. What this illustrates, and I’d like to point out here, is that if any water is impacted at super high speeds “water is harder” than at low speeds.
The circumference of a 1.5” diameter prop shaft is (pi * D = 3.14159*1.5”=4.7” around. If the shaft is rotating at 300 rpm for example, that is 300 * 4.7” = 1,410”/min= 117 feet per minute. That’s enough to get water resistance up- ‘hardening the water’= because the surface speed is fast and at 600 rpm the speed of the shaft’s surface impact on water is 235 ft/min.
What happens inside a cutlass (popular misspelling of the original cutless) bearing is; as the shaft speed increases the water forms a fairly uniform hardening sleeve that actually keeps the shaft from touching the bearing liner, and centers the shaft in the bearing’s bore.
What happens if the bearing bore is 2” and the shaft is 1-1/2”? Nothing. The tolerance of shaft OD to bearing ID has to be close.
http://www.deepblueyachtsupply.com/mari ... s-bearings here the tolerances can be explored and the many types of products reviewed.
http://www.naval-technology.com/contrac ... s/duramax/ Also a good read on the same material.
So where are we? Water lubricated bearings work by ‘hydroplaning’ the shaft inside a sleeve that allows water free access to the shaft sides as they spin in a close fitting cylinder around the marine prop shaft. The shaft’s surface speed induces a high resistance to movement by the water forming a ‘hard water’ cushion around the turning shaft, so that metal shaft can turn with little or no wear on the shaft AND the bearing liner. This is why he name is cut-less. A water film generally does not cut the bearing or the shaft.
Cheers,
Kevin Morin
Kenai, AK
Admittedly an esoteric topic in that most all welded boats on the forum have legs bolted on, so anyone building keel and shaft boats are likely to be the minority.
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