Sea Torque

Ah, a repeat of the old shaft in a tube discussion from 4 or 5 years ago.

I'm curious why they keep going on about the Magnus effect when there is virtually no cross flow on a vessel propeller shaft. If there was enough cross flow to matter it seems like the added drag from that phenomenon is the least of the captain's problems.

There is still viscous drag inside the annulas and the added weight has to be carried around which costs fuel. It just seems like an expensive answer to a problem that doesn't really exist for many boats.

Well said Marmot. That stuff has been tested long ago by the commercial and the pleasure shipping world. Not worse the effort.

Thats what we call, jewellery on the nightie .

I think the real benefits have to do with noise, vibration, sealing and maybe a longer life for the shaft cutlass bearings.
Have never been a fan of exposed shafts.... oil bath deals just are better.

For the speeds shafts on boat spin and the movement of angular transverse flow over then the effects are minimal. However, in certain applications spinning shafts can have huge forces generated... such a Voith's Turbo Fins used on tugs... and the various spinning shaft stabilizers.

You still need bearings in the oil, usually white metal ones in my experience, I started reading about this system but when I got to rubber mounted and thrust bearing in the same section I gave up.

Akerboom make great oil filled stern tubes a pair of which have carried me around the world.

Most large ships use(d) an oil lubricated stern tube but they are very short and the shaft does not normally extend beyond the stern frame. I say "used" as water lubricated tubes and bearings are more and more common because of pollution risks.

Oil filled tubes are a pain, they leak oil out and water in. Several ships I sailed on had a small centrifuge dedicated to removing water from stern tube oil.

It seems ironic that as the professional world moves away from this sort of thing, there are some who continue to flog the idea to yacht owners.

I would never advise that someone retrofits one. It's just another system that could create lots of problems and headaches. Traditional water cooled cutlass bearings last plenty long enough when everything is right (shaft angle, alignment etc). Even dripless packing glands can be a problem a lot of times. I just had to have a boat hauled because the dripless (Tides) froze on the shafts because someone before me never rotated the shafts (bumped the engines in and out of gear) for months and months. They had to drop the shafts and change them. Traditional packing with GFO packing in it, hardly drips, lasts a very very long time, and is easily servicable from inside the boat.

The "lost lubrication" principle is a thing of the past as far as shaft bearings are concerned. Pollution issues and the better technology with water lubricated stern tubes rule out these systems for new commercial ships and larger yachts (Marpol).

We had this "lost lubrication" principle on earlier military jet engines. A pain in the neck. One hour of flight, one quart of lub oil (per engine)! On long flights with several intermediate stops, you were carrying more oil cans than luggage .

But those concealed external shafts are not worse the investment. The Magnus effect on a prop shaft is rather neglectable and costs, higher weight and maintenance issues will never pay back. And as long as the free spinning shaft has no bends or imbalance, noise should be no problem.

Those Magnus effect tube type stabilizers work pretty well for low speed crafts and when on the hook. But their diameter, efective length and RPM is different from a prop shaft. For a medium sized planning yacht, the electrical Magnus effect stab is a nice and much cheeper alternative than a big Gyro Stab or even more a fin stabilizer (resistance). But only, if retracted into a recess in the hull during planning speeds, which means no stabilization during cruise.

If you take a look at KIBO on the hard, that is what I would call a long free spinning shaft. But the way I know A&R, it will work pretty well.

BTW, if you take a look at the keel above the SB prop, you will see the opening, where A&R normally places the pumpjet type stern thrusters on their larger hulls.

Water lubed is the modern solution to oil filled. But in general the yachting community is not as fast to update as the shipping industry. Why is yachts are basically private recreation and shipping is highly regulated money and sense BUSINESS.

Please no posts about 'commercial yachts'... that's really all a tax and liability dodge for what is commonly called an "owner who shall not be named" or more properly a "principle" or "controlling director" who owns nothing but has all the rights of control. I love this stuff... but my better half considers this all just elaborate schemes she does not want to hear about or be involved with in any way... probably as she thinks she will end up paying the "piper" or more properly the "tax authority," and if even if floats now... at some point it will eventually sink. Of course God is on her side and she is always proven right in the end, and I will die first... So sad... how true.

Back to business... Ever since my first rubber band powered bath tub motor boat I have not been a fan of exposed shafts and cutlass bearings... maybe I bent too many of them... even when I went to battery power and could move out to the pool. They were what went wrong... bent and vibrated, bent and stopped, leaked... so those early formative days prejudiced me forever!

I think the Fortjes drives are the ticket for yachts or the zeus system for higher speeds... if you live with mechanical engine to prop propulsion. But now-a-days I am thinking the rim drives like particularly the Voith Inline thruster or Propulsor are the way to go. But I am an electrical engineer by training... so I prefer electrical to mechanical. But the rest of the world is not there yet. As I get older the motor boat is appealing over sail and I am back to beginning again to semi-seriously looking. In my cursing area the winds are usually pretty light in the season so motor power is looking to make more sense. What I think would be neat is to buy a pre-existing boat and convert it to the diesel electric drive... and the rim drive. But that is crazy some tell me but I can at least think about it and not get into trouble.