Shaft Drive

[majorm]

After reading over info on the IPS and Zeus systems again I was curious how much research was done on a enclosed shaft and dual props on one shaft. I tried a search but never really found anything. The props wouldn't be counter rotating but would give more bite? However would this additional bite add to much drag or engine load to be worth it?

The other side of my question is a encased shaft. How would a fixed tube or hydro dynamically shaped casing over the rotating shaft that was tied to the strut affect efficiency? Would the work be worth the possible results? I tried to find a good video showing the shaft moving under power but came up with nothing again. Depending on the shape and size of the casing I wonder how much it would effect the rolling effect on a deep vee hull form.

So what are your thoughts on this?

 

[gonzo]

It is called a kurt nozzle and gives more torque at low speed. Because of the extra drag, the top speed is less.

 

[Frosty]

It is called a kurt nozzle and gives more torque at low speed. Because of the extra drag, the top speed is less.

An engine makes torque, the Kurtz makes thrust not torque.

 

[SamSam]

A Kurt nozzle is a shroud around one prop. Majorm is talking two props on one shaft with no shroud. Two props that rotate the same way, not counter-rotating.

 

[SamSam]

They are both interesting ideas. I would think the second prop could have more pitch than the first, as the water it receives is already accelerated by the first prop.

I don't understand the 'rolling effect' you are talking about. Are you referring to some sort of reaction to shaft torque or to waves?

 

[powerabout]

are you guys trying to say Kort after a few beers?

Rice nozzles are the go now Kort so last centruy

 

[Tigawave]

Tubes around the shaft have been used quite a bit and the efficiency gained has been measured on an oil filled system running on roller bearings as around 5-6%.

The problem with the rotating shaft is the magnus effect and also that it delivers a rotating stream of water to the prop, this gives rise to cavitation on one side of the strut as the water near the surface of the shaft hits the strut support at an angle.

The magnus effect is the same as that which bends the flight of a spinning ball, you get low pressure on one side, in theory this will try to bend the shaft to one side which can't help.

Many sport fishing boats have tried tubes and there are two systems on the market with oil filled tubes that run from the strut to the through hull.

 

[johneck]

Dual props rotating the same direction doesn't seems to be a very good way to proceed. It would be a better solution to add more blades or more blade area and design the propeller properly. The reason for contra-rotation is to try to recover the rotational energy from the wake of the upstream prop.
There have been studies done on the effects of efficiency on open or enclosed shafts and the results have not been conclusive. The shaft is operating bathed in a fluid in both cases. The effect of the propeller inflow due to the rotation of the shaft is small and the effect of have a larger diameter tube might make the axial portion of the wake worse, depending on shaft inclination, rotation rate, vessel speed, etc.

 

[Tigawave]

There have been studies done on the effects of efficiency on open or enclosed shafts and the results have not been conclusive. The shaft is operating bathed in a fluid in both cases. The effect of the propeller inflow due to the rotation of the shaft is small and the effect of have a larger diameter tube might make the axial portion of the wake worse, depending on shaft inclination, rotation rate, vessel speed, etc.

The results I've seen are quite conclusive. The boat was run with open shafts and engine loads were measured along with a video which picked up cavitation noise at a range of speeds in flat water conditions. The shafts were then changed for enclosed shafts, so same boat, same props, same engines the engine loads were less in the range 4-6% there was a noticeable lack of noise from the shaft system. Boat was a 64'sportfish boat with twin 2000hp MTU-M91's

Fuel consumption dropped along with the engine load figures. The only other difference was with the shaft tubes they were running the shafts in oil on roller bearings and taking thrust on a roller bearing rather than the engine mounts the shafts are likely to have experienced less flex from thrust effects but otherwise it wa sa fair comparison.

True the shaft is bathed in water but the surface layer will start to rotate as it travels past and along the shaft so there is not a fore and aft flow which the prop struts are optimised for. This seems worse in tunnels where I've seen asymetric cavitation marks on struts which would agree with flow across the strut at leading and trailing edges. There is a lateral force exerted on each shaft as a result of the rotation.

 

[michael pierzga]

I dont know about the hydrodynamic resistance of tube housed oil bath shafting. The P bracket and tube diameter are quite large. I do know that I can spin my propeller like a fan blade...very low friction.

http://[url=http://postimage.org/image/4rzrmo7lj/full/][/URL]

 

[Tigawave]

The bearing friction at rest is a bit of a red herring as the rollers in oil will be a lot less than a dry rubber bearing, but once they are operating hydrodynamicaly the friction on a water film is likely to be less than oil with rolling element bearings.

 

[michael pierzga]

I dont know. I was told that an oil bath shaft delivers the most shaft horsepower . Some few percent greater than a cutlass. Perhaps other factors are at work.

The main purpose of the oil bath shaft is sound deadening. No thrust on the engine mounts. After sound deadening the beauty is low maintenance. Additionally with less exposed shafting metal ,anodes last longer. That shaft pictured has 14,000 rotational hours and is twenty years old. It has never been serviced...only oil changes. A cutlass will be replaced every couple thousand hours , the stuffing box will be serviced , the shaft pulled and inspected for wear and the engine flex coupling inspected. .

 

[thudpucker]

I came along to boats after the old fishing fleets of the Pacific NW were cycling down.
It was in the late 40's.
I pulled a lot of hulks, scrapped a lot of weird stuff and enjoyed the trip.

Several boats had 'drive shaft' from the Eng/Reversing gear, back to a prop. Lot's of those old wood boats were powerd by Auto Engines and ETC.
All had Univirsel joints at one or both ends. Some had a "Vibration dampener" made of old Tire Treads at one end.
I did see one with a big Husky Steel strap over both ends of the Drive shaft.
In case the U-joint let go, you didn't want the Shaft chasing the crew around the working deck.

All that stuff Rusted to death. The benefit of it was 'out of sight' and 'down in the bilges' below a deck, and the boats were wood and had salt water in the Bilges all the time.

One boat, about 32' (a Bristol Bay boat) had the little Six banger way up front, and all the fishing stuff in the working decks. Great design for working and catching lots of Salmon.

The Prop, skeg, Rudder and shaft could be lifted up into a Slot and Cavity in the transom. In case of Beaching or working with a Net getting up under the boat.
I thought that one was about the best of all worlds for Commercial Fishing.

The Rust just killed it and the wood was soft, and all the fasteners were 'nail sick' so the "Restoration" of that one was out of the question.

Maybe some of those old ideas tickled your fancy.