Prop Shaft Systems.

[marshmat]

Hi Daniel,

Belt drive is actually fairly common in electric motor launches. I've heard of it being tried with engines, usually by backyard DIYers, although I'm not aware of any commercially produced boat with such a setup.

Coaxial counter-rotation brings some other issues with it, mechanical complexity being perhaps the big killer. (Take a look at the parts drawing of a Bravo 3 or Volvo DP and you'll get an idea of just how bad this problem is.) Coaxial props also have the potential to introduce a nasty resonant vibration issue (you get a pulse every time two blades pass each other), hence why they generally have a different number of blades on each of the two props. And you'd never do this on a long or inclined shaft- although theoretically possible, it would require more engineering time than you'd need to either straighten the shaft or coax a bit more efficiency out of the off-axis prop.

 

[Guest625101138]

I've finally got time to properly read and research the comment on here.


Moving on from that comment, would twin counter rotating props help?
- Im trying to understand the forces that would act blade on blade, and the affects of having it at an angle to the flow, as it would be on an inclinded shaft.
- If you had two props (typical each with two blades i guess) counter rotating?

Daniel
Before you move on from this comment you should sit down and draw the velocity vectors for a blade at various angles of rotation on a horizontal shaft with the blade set at AoA at 3 degrees at say 75% radial position. Just doing this will help you understand propellers. Now incline the shaft at say 5 degrees and redraw the vectors in each location.

Until you do this exercise you do not understand the problem as you have not even tried to determine the forces you need to handle. You will probably find your lecturer will be intrigued with this and may help frame the solution. Particularly if you suggest to him that he could use a much thinner shaft to solve the vibration problem providing it is nicely curved such that the prop shaft is horizontal at the prop.

Ideally the prop is larger diameter than 12" - maybe 16". A sailing boat tends to have a larger diameter prop because the draft does not constrain the diameter. For any given prop the thrust from a given power level is a function of diameter:
Thrust = (Power/(4/pi/9/rho)^0.5 x D)^2/3
This is the ideal case excluding prop losses.

Rick W

 

[marshmat]

Hi Rick,

I know this discussion sort of went dead there.... but I was giving some more thought to the flexible shaft idea on the train home yesterday. (For some unknown reason, I had put the work that actually needed to be done in the suitcase....).

Anyway, I was looking at a sailing catamaran that, in order to have a suitably sized prop at a zero-degree angle, would have put the ~40hp engines almost amidships with six-metre rigid shafts. So I thought of your model earlier in this thread and wondered if it might be possible to use a flexible shaft about two metres long, bending maybe eight or ten degrees over its length. That would allow a V-gear and second shaft to get the engine up in an out-of-the-way place where it would be much more accessible.

The question thus became, what materials are suitable for this kind of shaft, and how does one go about supporting it where it passes through the deadwood without wearing out the cutless bearing? You'd need something springy, fatigue-proof, corrosion resistant, relatively easy to work, and galvanically compatible with the rest of the boat... and does it even have to be metal?

 

[Guest625101138]

Matt
Engineering the system for a high power application could prove challenging. You would probably end up with a shaft much smaller in diameter than you would normally be comfortable with. For example I know Mark Drela has tested a 3mm shaft that is about 3m long for transmitting the sort of power generated in a pedal power boat. The water dampens any vibration as it does with my 1/4" shaft.

My view on the best system would be to determine the diameter required to transmit the rated torque with a safety factor of say 3. It will likely be a lot smaller than you expect. The flexibility will mean it is quite tolerant of shock loads. Work on a high quality stainless steel with a yield of say 1000MPa. (I have not been able to readily locate these materials in the sizes I am using so normally use painted spring steel. Spring steel would have limited life in salt water. If you have a shaft material you like already then use that material for calculations and see what it looks like.)

Once you have the diameter you can determine the minimum bending radius that will give infinite life. I can show you the calculation for this.

I currently use stainless steel or glass ball bearings in my shaft strut. These bearing actually take the bulk of the thrust load so I do not force the shaft into a tighter radius under thrust. So the strut is transferring the thrust to the hull.

The through hull bearing does not need to be flexible but needs to set up to maintain the right curve. Again I would probably consider a ball bearing with shaft seal. An ordinary gland would be OK but you need to be careful not to score the shaft such that it creates a stress raiser.

The output of the "V" drive also needs to be set up to match the curve of the shaft and will have a bending moment applied to set the curve.

The fact that you are not producing vibrations means you can use a much lighter shaft.

The first step is to determine the rated shaft torque and a shaft material so the diameter can be determined. If you do this I can see if the minimum radius will allow a viable set up.

One of the things I have thought about is using the flexible shaft as a means of lifting the prop out of the water to reduce drag. The strut needs to lift vertically till the prop clears the water. The allowable stress under this condition could be much higher because the frequency is much lower than what it sees in normal running.

Rick W

 

[marshmat]

Hi Rick,

Interesting idea about integrating a thrust bearing where the cutless bearing would ordinarily be. If I'm not mistaken, the possibility of a shaft buckling under the axial thrust load is the main reason why they usually have to be thick and rigid. Eliminate the axial load and you're left with pure torsion, much easier to handle in a shaft small enough to be slightly flexible....

I will get back to you on this in about a week. (The two most brutal exams of my academic career to date are coming up... fast.) The project in question is quite a few years off still- I have a Rideau canal cruiser in the 8-9 metre class on the drawing board first (and hopefully on the shop floor, as soon as I have a shop).

Cheers,

 

[Guest625101138]

Matt
I intend to play around with this idea for something like 5kW at 1000rpm but like you my time is tight right now. I will post details when I have some results.

Some of these things are significant departure from current approach so they are bound to have plenty of unforeseen challenges. My experience at the low power level has been extremely encouraging.

Rick

 

[MikeJohns]

Prop shaft angle is an issue with speedboats , I'd be careful extrapolating this to displacement boats (not ships), the flow field around the hull is not very predicatable at the best of times, and once in a seaway with the boat heaving yawing and pitching and its anybodys guess. You can ususally feel the prop shake as the hull is pushed around by waves, then it settles down again as the flow becomes more orderd.

In powered tuft tests the tufts are generally aligned locally with the prop shaft in the region of the prop even at extreme angles, then the real question is what happens to the pressure field from the action of the inclined prop and how does it react with that of the hull.

CFD is not all that usefull for these sorts of predictions yet ..unfortuantely.

 

[Frosty]

A flexible shaft transmitting torque will "mangle" this can be common on car prop shafts that will throw the gearbox all over the place when high torque is transmitted.

Take a straight piece of rope and twist it, it will eventually buckle into a knot, That is what the shaft is trying to do.

 

[Guest625101138]

A flexible shaft transmitting torque will "mangle" this can be common on car prop shafts that will throw the gearbox all over the place when high torque is transmitted.

Take a straight piece of rope and twist it, it will eventually buckle into a knot, That is what the shaft is trying to do.

I use 1/4" shaft from 4 to 6ft long transmitting up to 1kW and Mark Drela thinks I am using overkill. He has used 3mm shafts for the same power level that are up around 10ft long. The thrust is carried by the shaft as well.

The water provides a dampening column and the shafts are hard to damage because they simple flex out of the way. I run mine over river logs and sand bars without causing damage.

This is just a different approach that works quite well. I am not sure a piece of rope will work but maybe it will once in the water. I would not dismiss it until I had a closer look. Pushing props are self stabilising and have very large stabilising force. When they are forced to operate at an angle to flow they vibrate badly particularly if only two bladed.

Most people are skeptics until they see what is possible.

Rick W

 

[Frosty]

1KW ????

I think we are talking different stuff here I was talking mangle and I have spent many hours trying to cure it, however I was talking considerably more than 1 KW.

I did'nt suggest you use a piece of rope Rick.

 

[daiquiri]

Rick, I was thinking a lots about your flexible shafts and their possible use on commercial boats (yeah, it is growing on me )...
Have you ever tried to investigate on how they behave in heavy seas? Talking about inertial forces acting in any possible direction on the prop.
The water will dampen the oscillations, we agree on that. And it is also true that the shaft, being pre-bended upwards by the prop rotation, will enhance the rigidity of the system when the prop hub is moving upwards - though the back of the medal is that it will restitute elastic energy during the opposite motion.
But I believe (can't prove it) that there will be at least one amplitude peak (for a single excitation force) before it dampens. And since the excitation force is random, both in direction and in amplitude, there could happen a bigger-than-expected peak. Don't you think a freely travelling prop could then hit the hull or an eventual rudder support structure (assuming that rudder is placed behind the prop)?

 

[Chris Bretter]

Hi all
I am starting to build a 38 ft Wharram i would like to use a centre mounted Nissan SD22 with a 2-1 reduction box and a dropdown longshaft. All fairly simple Has anyone any ideas.Also perhaps a ducted prop with a rudder to assist low speed manouvering.You guys think out of the box.Rick you got some crazy stuff going on so cool.
Regards Chris South Africa :idea:

 

[Guest625101138]

1KW ????

I think we are talking different stuff here I was talking mangle and I have spent many hours trying to cure it, however I was talking considerably more than 1 KW.

I did'nt suggest you use a piece of rope Rick.

I realise you did not intend using rope but I was making the point that rope might actually make a workable shaft. Have you tried it? Can you prove it will not work?

I am talking 1kW at around 500rpm so around the same sort of torque you would have in a moderate displacement sailing boat using 3 to 5kW at higher rpm. Can you imagine a 1/4" shaft handling that. If you do the stress calculation you do not need a particularly large shaft to transfer a couple hundred kW when there is no concern for critical speeds.

Rick W

 

[Guest625101138]

Hi all
I am starting to build a 38 ft Wharram i would like to use a centre mounted Nissan SD22 with a 2-1 reduction box and a dropdown longshaft. All fairly simple Has anyone any ideas.Also perhaps a ducted prop with a rudder to assist low speed manouvering.You guys think out of the box.Rick you got some crazy stuff going on so cool.
Regards Chris South Africa :idea:

Chris
My interest has been chasing down losses. For the last 5 years I have been playing with pedal powered craft. I have lots of time to think when I am exercising on the water wondering how I can go faster for the same effort. I have better feel than most about what is efficient and what is not because I feel it directly in my legs.

The curved steel shaft is very good providing it is properly designed. I operate in fresh water most of the time and these days use spring steel for most shafts. The low diameter bar I use is not the best you can get but it is close to it. The yield strength is 1600MPa. It is surprising how far it can be bend before it yields. More like rubber than steel and yet surprisingly tough. I used to get worried about stress raisers with connections but now I just grind flats and sink grub screws into the flats. It is very tough but not highly resistant to corrosion. You can cut it with a hacksaw but it will dull the blade in one cut.

You can get high strength stainless but it is not easy to come by. I am going to play around with low diameter spring steel up to 9kW to see what is possible but I think you could make a practical system with a lift-up prop where the shaft just curves up out of the water.

In a few days I will post some numbers on the sort of shaft you would need to handle around the 10kW mark and what sort of bending radius is possible.

I used to bend my shafts with my big props until I worked out they needed to be aligned with flow. The attached photo is not a shaft problem but shows what can happen due to imbalanced loads on an angled shaft. The prop shown was not particularly efficient but is just one of many ideas I played with.

Life gets much better once the prop is aligned to flow.

Rick W

 

[Guest625101138]

Rick, I was thinking a lots about your flexible shafts and their possible use on commercial boats (yeah, it is growing on me )...
Have you ever tried to investigate on how they behave in heavy seas? Talking about inertial forces acting in any possible direction on the prop.
The water will dampen the oscillations, we agree on that. And it is also true that the shaft, being pre-bended upwards by the prop rotation, will enhance the rigidity of the system when the prop hub is moving upwards - though the back of the medal is that it will restitute elastic energy during the opposite motion.
But I believe (can't prove it) that there will be at least one amplitude peak (for a single excitation force) before it dampens. And since the excitation force is random, both in direction and in amplitude, there could happen a bigger-than-expected peak. Don't you think a freely travelling prop could then hit the hull or an eventual rudder support structure (assuming that rudder is placed behind the prop)?

I would never consider a completely unsupported shaft in any way practical. Greg K had his unsupported during his record breaking run but he only ever went forward.

I use a very light tension strut that is pivoted. It allows me to pull the prop up out of the water to remove weed. It also stops it from diving when I go in reverse.

The forces on the prop are powerfully aligning so it tends to track a straight course unless it is constrained. The boat can move all over the place on waves and the prop will just continue constantly aligned with the flow. So if you turn it hangs out to the side or curves in under the hull. Even with my strut I can get a situation when travelling with a beam sea and the prop down wind that it begins striking the hull. So if it is not constrained in some way then it will move all over the place.

The feature of the curved shaft is that you can use a very flexible shaft to get the prop aligned and overcome the nasty forces associated with an inclined shaft. It is far supperior to things like universal joints working under water. There is also the possibility of simply lifting the prop clear of the water to reduce drag in a sailing vessel. Even now I prefer to use the shaft strut to take the thrust load rather than adding the buckling stress to the shaft. They can handle this but it is another design consideration.

The flexible shaft is compliant so will take bumps and just bends out of the way rather than being damaged. With pedal power I have to compromise between the bending stress for the curve and low torsional rigidity for shaft wind-up that makes pedalling inefficient. With a motor the latter is not a constraint so you can build in significant torsional compliance that will give the system resilience if the prop strikes something solid. I get no damage when the prop rides over a log - at least so far.

Rick W