Shaft tubes

Explain to me how these shaft tubes work. If I could get that kind of fuel improvement I might try them to.

I'm not sure if they're what he is referring to, but a couple of years ago at IBEX I saw a set of shaft tubes that were sealed from water and ran oiled ball or roller bearings on the shaft. I thought they were a great idea and I could see those kind of savings available with higher shaft speeds as drag is a geometric component of speed. At very low shaft speeds, the difference between a water lubricated rubber cutless bearing and an oil lubricated steel ball or roller bearing will not be that great, but as speeds increase the drag difference on that rubber bearing will increase by the cube (as will the steel bearing) so that small difference at clutch ahead is a great difference at full throttle. A 10% reduction in shaft drag at cruise power settings I could believe.

Knowing just enough about this to be dangerous, my theory is that encAsing the shafts in tubes eliminates the magnus effect of a cylindar spinning through water. Huh?
Here's the explanation from Wiki:
"The Magnus effect is the phenomenon whereby a spinning object flying in a fluid creates a whirlpool of fluid around itself, and experiences a force perpendicular to the line of motion. The overall behaviour is similar to that around an aerofoil with a circulation which is generated by the mechanical rotation, rather than by aerofoil action."

I learned a bit about this from David Marlow. Marlow Yachts encases the shafts in logs or Velocijet skegs (depending n size of yacht), eliminating the magnus effect and protecting the shafts from things that go bump underwater.

Missed again, Henning. The highest friction on a water lubricated journal bearing is at breakaway, when the shaft begins to roll. As speed increases the coefficient of friction drops rapidly and is lowest at its design velocity. Friction slowly rises beyond that optimal speed.

Get thyself to a library and look up tribology. Leave the cubist theories where they belong.

In any event, the load increase on a shaft running on a water lubricated journal bearing will be less at high rpm than an oil lubricated ball or roller bearing. Friction in a bearing is related to the viscosity of the lubricant and water is less than 1 percent as viscous as 10W lube oil at "room" temperature so a shaft running in oil is going to absorb more power due to fluid drag than the water lubricated shaft will. This doesn't address the reduction in the coefficient of friction obtained by the property of a water lubricated plastic or rubber bearing to distort under load and allow the shaft to seek its best position, even if it is worn out of round.

We won't even go into the high friction caused by the multiple lip seals required to keep the oil in and the water out, the expense and complication of cooling the lube oil, and the absolute certainty of water contamination which leads to the destruction of ball or roller bearings. There is a reason why we use water lubricated polymer bearings on boats

The shaft spins the water around it- and thus the propeller does not dig into water that is sitting still- it bites into water that is already turning with the prop. Imagine a wood screw turning into a small piece of wood that was not secured- the force of driving the screw into the wood will start the wood spinning- and that means the screw does not drive itself into the wood, but instead spends it's energy turning the wood. You do not want the water turning- thus the shaft tubes. They are not spinning. Of course since they are larger than the shaft, they have a greater surface area, and this increases the drag. But they also allow the prop to bite into still water which gives that prop less slip. The jury is out as to if they are an improvement- if they were nearly all larger high speed vessels would have them, and that is certainly not the case.

Must be why counter rotating props don't work and Grim wheels are just a fairy tale.

Great idea though, put the shaft in a large diameter tube in front of the prop. That will certainly counter the microscopic layer of spinning water that hits the aft bearing housing, struts, and blade roots at an angle that increases with boat speed.

Hoosh -

If I remember correctly, shaft tubes and

Hatchet head rudders and

Infinitely tuned transom wedges were kind

Of a hallmark of late eighties/early nineties sportfishers -

The need for speed.

Not speaking against, just seems like they lost out

Vs really good hull design and weight control.

jsi

I have done three shaft tube installations of late and while I can't quite validate the 10% claims, 5% has been closer to our "reality". The longer term benefits of tubes are an often overlooked but more important consideration in my opinion. "Naked" shafts will accumulate growth between haulouts and the scale buildup on a spinning shaft acts like a blender to aerate the water ahead of the wheel. A tube, being painted with antifouling, doesn't have this problem. We have tried many metal antifouling coatings, with limited success, but the tubes are holding their paint better than any of them have so far.

Another consideration, which I've seen overlooked far too often, is intermediate bearing replacement. Most shaft tubes are installed "permanently" and require cutting and shaft removal to replace the cutlass bearings. This makes no sense whatsoever to me. A local machine shop has split the tubes for us and machined in collars along the tube to allow it to be removed with a few set screws in less than ten minutes. A far more maintenance-friendly design!
Bill

Grim wheels go behind the prop on ship, are they widely used? Are we talking about ships here or traditional shaft drive vessels which are looking for greater speed/ fuel savings in the 40-80 foot range that are the typical choices for shaft tubes? Counter rotating props are not a option found on traditional shaft drive vessels of which this discussion is about, and if there were, the rear prop bites into water not sitting still, but water that is moving in the opposite direction as it's turning- thus giving it a better bite.

How so? That's what this whole thread is about, so please elaborate and enlighten us.

Do you have a study you are referencing in regards to the microscopic layer claim? My post was based on a discussion with two of Viking Yachts New Gretna's engineers while I worked for Viking Yachts, and why they did not use shaft tubes. Your ship knowledge does not help further this discussion and only muddys the water with irrelevant information.

A prop is a prop is a prop.

Physics is physics, small boats or big boats, they all work the same. My background doesn't change the physics despite it including most examples of things of all sizes that float or submerge and it does not alter the topic that was under discussion.

Large ships don't typically use long exposed shafts except for some naval vessels which, by the way, don't hide them inside tubes or support them with ball or roller bearings either.

Very little of surface piercing drive technology is used on bulk carriers, and a traditional shaft drive vessels in the 40-100 foot range that are planing vessels do not "work the same" as ships. Physics is physics, but a nuclear physicist's knowledge does not qualify him to be an astrophysicist, a geophysicist or a quantum physicist.

Shaft tubes are used because some think they give some type of advantage over a naked shaft. Please give us your ideas of why they are better or worse- because little else furthers the conversation.

After reading your last post, ... nah, never mind. Have a good weekend.

Do you mean Contra Rotating props, as in two props on the same axis spinning in opposite directions?

I know it's difficult for you, Henning, but since you have an internet connection maybe one of your crew will look up the term for you.

Excellent decision.

Back to the regularly scheduled program: I'd pass on the shaft tubes. The majority of higher speed (over 30 knot cruise) vessels in your class do not have them, and the results have not been proven to the majority of builders. Have you tried SeaSlide? If you use your boat often at higher speeds then it could give you the savings you are looking for. Do you have prop speed on the shafts, props and rudders? Also keeping a clean bottom is important in fuel used at all speeds.

Or Speed Rails....
They seem to work quite well with no real negatives. I had a boat with them a couple years ago. The only issue I had was that they interfered with the straps in the travelift. You had to block around them.
I am thinking about putting them on my Bertram 46.6. Lift, speed, reduced wetted surface, reduced spray, fuel economy. There is a lot to like about them.
Maybe an alternative to the shaft tubes.

Hi,

Marmot, I wouldn't lose too much sleep over something that would fit athwartships on many of the vessels we have worked on.

Bamboo, Are you trying to say that small boat manufacturers like the one you work on have a lead on some special materials that have different physical properties and characteristics to those same materials used elsewhere?

Here is what a quick Googleing came up with for

Contra Rotating Props:

http://en.wikipedia.org/wiki/Contra-rotating

Counter Rotating Props: 1/2 way down the page, these things were even used on a popular discussion on here: Roamers. There is a lot of other good prop info on this page as well.

http://www.boatfix.com/how/props.html

Grim Wheels ( Marine Use not SUV's)

http://www.roblightbody.com/liners/qe-2/1987_Refit/qe2 grim wheels.jpg

Fully enclosed Oil Bath Stern Tubes are used on a number of larger yachts under the guise of reducing noise and vibration.

Trust me, I won't and since as many readers here know I get paid to mess around with little boats as well, I don't know why he brought big ships into the discussion. He must have a reading comprehension problem or something.

Like I wrote, a prop is a prop is a prop. Magnus effect is practically nil when the flow is parallel to the shaft, and whether counter rotating or axial props rotating the same direction ala the Schottel SPP units, the physics are the same and the props work just fine. Propeller size is irrelevant. I guess the guy has never heard of a model basin.

It is stunning that anyone would think that a propeller has to work in "still water" and screws itself forward, or that the the thin layer of entrained water around a rotating shaft will somehow screw up the inflow to an open propeller. I guess he doesn't know there is usually a strut and bearing a few inches ahead of the prop on conventional installations. To tell the truth, it looks like he doesn't know how a propeller works.

For sure that comment about how a 40 foot boat has nothing in common with larger ships is bizarre and very telling. I guess he has never seen the running gear on an Arleigh Burke or Fletcher class destroyer or other high speed naval vessel. As far as the water is concerned they pass through it just like a big sportsfish.