Prop shaft inclination

I designed my solar boat with the prop shaft inclined ~8deg to the horizontal, simply because it was convenient to fit my propeller.

Having read some good threads on here regarding vibrations due to having the prop inclined in the flow, I'm considering moving some things around - I could lengthen the prop shaft by 2 feet in order to decrease the inclined angle to ~6.5deg, but I'm wondering if this is really worth it.

We're talking about a 1" shaft, supported inside the boat and at the strut. In the 8deg case, the shaft is 4' total. In the 6.5deg case this becomes 6'. Prop will probably be a 3 blade, 12x10.

Any thoughts? My impression is that the inclination isn't too severe and the shaft is thick and short - probably not a big deal... but lets hear it from the experts.

 

Whether or not the prop is inclined in the flow also depends on the hull shape.
Anyhow, 8 degrees is quite normal and lowering the angle 1.5 deg doesn't justify using a 50% longer shaft.

 

For displacement boat inclination of 15 degrees is accaptable;
For planning boat - up to 10 degrees

 

Hello,
Any info on the boat, dimensions, motor power, etc.?

 

A flex shaft outboard would be a much more efficient configuration and much simpler in construction and maintainence, IMHO. See Rick Willoughby posts on human power boats. There is no need for concern with the shaft angle, as the shaft is completely self aligning to the horizontal. Also can take shallow water without concerns for bending and alignment or damage to the hull if the shaft strikes something solid.

Hope this helps.

Porta

 

The boat is 24', ~1000lbs. Probably about 200W shaft usually, maybe up to 800W or so peak. Definitely in the displacement regime

I've read all about Rick's curved shafts (thats where this question came from) but I don't intend to use one. This is for a remote controlled (unmanned) ocean crossing attempt so while I suspect I could make a curved shaft work, I have too many other challenges to focus on to try and sort out what I'd consider a robust solution for a curved shaft that will last months in the open ocean. Using a "standard" 1" rigid shaft opens up a huge volume of off-the-shelf hardware/seals which is what I need.

 

The hull modifies the flow anyway so it will never be parrallel to the static waterline, additionally the prop creates its own stream which depends on the disk size, the power input etc and the proximity to the hull itself that's why we do expensive tuft tests to determine flow fields.

Additionally your vessel heaves pitches and yaws in a seway which makes chasing the 'ideal' model akin to tilting at windmills.

8 degrees is quite acceptable.

But please tell us more about this project, what geographic area, material of construction, power source etc...It's certainly interesting .

 

You need to understand where the vibrations are coming from...ie hydrodynamic or mechanical. No point fixing a "problem" that ain't broke!

 

Unmanned ocean crossing?
Is this even legal?
God forbid the whale huggers find out about this!
Not to mention the fishermen, shippers, cruise liners, tugboats, and all other legitimate users.
At 24' and 10000# you can probably afford to hop on and enjoy the ride, while using your eyes and ears and possibly a radar to ensure the safety of your vessel and that of others it may encounter along the way.

 

Andrew
The greatest concern with your set up will be fouling. A piece of plastic, rope, weed or other flotsam will entangle the prop and you will not have the power to free it. You need to devise a means to prevent fouling or cleaning any fouling that occurs.

The shaft inclination is not going to cause and noticable problems at your design speed.

I would avoid any hull penetrations for a shaft as it is bound to leak. I expect you have considered a pump.

Rick

 

Honestly I think an unmanned vehicle is a plenty "legitimate" user... I'll have automatic collision-avoidance systems and a radar reflector, which is already more than I can say for your average pleasure craft. Unmanned craft haven't been a reality for very long, so the research I did into their legality was... inconclusive. (For example, consider this passage from the COLREGS defining their applicability: "The word 'vessel' includes every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water."... so you tell me, is a craft without seats or cargo space a "means of transportation"? I don't know either.)

In either case, I intend to move ahead, and I'm taking steps to ensure my project doesn't annoy others or be a hazard.

I've posted details of the project a few times, but just one more: www.lonelybot.com

As for the vibes, I'm going to not sweat it, thanks for the confidence, all.

Rick, again you have your eye on the critical issue - do I pierce the hull and add a failure mechanism (leaky shaft seal), or do I use external propulsion and add a little appendage drag? In the end I decided on piercing the hull for inboard propulsion, owing to the fact that a leaky shaft seal in an external propulsion unit would cripple my craft just as completely as if it sank entirely. I'll use a water-sensing bilge pump and include the duty cycle in telemetry - with luck a leak will start slowly and I'll have time to notice the pump running [more] and turn home or to a friendly locale.

 

What about running a longtail type motor? There would be no piercing and no seals to leak. What about using a folding prop to clear flotsam that might otherwise get entangled in a fixed prop? Some of the folding sailboat props are seamless and very streamlined in the folded position, and tangles would be washed over in the folded off position. http://www.flexofold.com/

Porta

 

Further to this. Maybe "Lonelybot" won't be very lonely for long. Lonelybot will have "company", have his eyes and ears disabled and be stolen away, and there's nothing anyone in a cubicle can do about it. Some are very naive about he real world!

A heavy boat (500lbs of batteries) like this with no commander is a menace and hazard on the ocean. Expecting other people to be vigilant and avoid collision, whilst having no ability to remain safe itself. It could do damage and result in loss of life. Why anyone would want to waste their time and effort on this for no apparent reason, while there are so many useful things to be done is a mystery.

I don't think you could insure it, and if it did do any damage, become entangled in fishing nets, runs down swimmers etc. you can be sure you would face extreme legal action or even imprisonment. What if you have a systems failure and it gets out of control? Can you rescue it?

Not the best idea really is it.

 

This is a bit dramatic, don't you think? Yeah, maybe someone will maliciously attack my project, but a) they'd have to find it, and b) the density of malicious persons in the middle of the pacific is far lower than that near any given marina... and yet thousands upon thousands of boats sleep soundly in marinas every day without dismemberment.

Furthermore, with all of the other hazards in the ocean (pleasure craft with sleeping skippers, lost and adrift cargo containers, etc), you really think a plywood boat moving at walking speed is the biggest hazard? Please. As I said, my craft will actively avoid approaching anything else in the water, the sturm und drang is premature.

And lastly, calling a robot boat a "waste of time" compared to any other other endeavors on this forum is ridiculous. I don't think any of us are saving the world with our hobby - not me with my solar robot, and not you with your wind turbine.

I am not naive, I'm just comfortable with the odds.

 

Hi

Yes, you're quite right. My comments were a bit "over the top" - well argued! Good luck with it. You never know, there may be a useful purpose for them one day.