Pedal Powered Boats

There are a number of videos on Youtube of this hovercraft. It is not the most efficient means of transport.
http://www.youtube.com/watch?v=HAQ-AdPABi8&feature=related

Rick W

 

Not the most efficient? Rick, you aussies have the understatement skills of a the brits it seems!

 

Rick,

How do you calculate the minimum bend radius and the diameter of your curved shafts.

Wouldn't a carbon fibre tube be a good solution?

Gareth

 

the hovercraft website is here.
http://steamboatwilly.org/
they refer to a fan efficiency of .61, and a propellor efficiency of .7.
from the wording, I assume these include transmission losses.
this gives an overall engine power of 326 ft lb/sec at cadence of 60 rpm. =, I think, 442 watts.
There is a LOT of information in their technical and history sections.

 

The key material properties for the shaft are material endurance limit (I work on 40% of yield strength if not given) and the elastic modulus. Ideally you want high yield strength and low to moderate elastic modulus.

The best readily available materials I have found is spring steel or machinable aluminiun. The aluminium I use has yield around 320MPa and modulus of 70GPa. Spring steel I use has yield around 1200MPa and modulus of 200GPa. The steel is only slightly better when you look at the ratios.

I do not know enough about carbon fibre to design reliably with it. It seems to have a wide range of properties with strength up to 3000MPa and modulus as high as 500GPa. I do not know how it responds to high cycle stress and what stress range is allowable. I did try some but the end connections failed. It would need enlarged ends epoxied into metal for couplings.

If I was working in salt water most of the time I would want something different to spring steel but for now it works well and I have devised a number of methods to reliable connect.

Rick W

 

Rick

What I meant was do you simply calculate the bend radius of the shaft using static beam theory and the diameter of the shaft assuming it is a straight shaft in torsion, i.e. treat both properties seperatly or have you come across a method of calulating the stress on a curved bar in torsion.

All the best

Gareth

 

Gareth
No. This would result in an approximation at the curve radius I am using. I have a constant bending moment that is induced by the end fixings not a point or distributed load as you see in a beam. My calculation gives the exact stress range assuming there is no column load. Hence my preference to take thrust at a prop strut unless it is very low.

For a pedal boat there is a compromise but it is not overly constraining. It has to do with the torsional spring constant you end up with. I like to achieve up around 20Nm/rad. If you get under 10Nm/rad you find the pedals feel jerky with my normal gear reduction of 4:1. It also causes an inefficiency because the RMS power is higher than the average power and the boat sees the average power while you legs see the RMS power. This is not a factor for a motor driven application.

My design approach is to look at the required drop of the shaft and length to work with. I then determine the curve radius and the bending stress for chosen shaft OD. If the stress range is within the endurance limit then so far so good. I then do a check on the torque safety factor and the torsional stiffness. Usually it will be the torsional stiffness that might force an increase in diameter. I then need to repeat the exercise and adjust the length if I am outside the endurance limit. Like most design it becomes iterative. With spring steel you usually have plenty of room to move.

Rick W

Rick W

 

At this point are you assuming the shaft is straight or do you have an equation for checking the torque safety factor and the torsional stiffness of a curved shaft under torsion.

Thanks

Gareth

 

Gareth, at one point I was convinced that a carbon shaft was the way to go for my application but I found it difficult to get any of the drive shaft people to work with me on the ends. I came to the point where I was just going to attempt tapers on solid stock and see what happened but didn´t have the guts. In the process, I learned that carbon is the stiffest stuff there is. When I first thought of a flexible shaft, I thought about how dang tough a solid glass poly fishing rod is but did a search and found Rick. Good luck

 

I assume the shaft is straight. I do not look at the combination of stresses.

In my application the requirement for torsional rigidity usually pushes the safety margin on torque to a high value. Another point is that the shear stress is not varying over a wide range like the bending stress. It will contribute to fatigue failure but not additively.

Rick W

 

Rick,

Thanks for that I am now up to speed !

The reason I beleive that carbon fibre may be a good alternative, is that you can alter the properties of shear modulus and elastic modulus of the tube independantly by playing with the fibre orientation.

Also it has high stiffness and very high yeild strength, and high fatigue properties, you can bend a windsurfing mast in half !

I Need to think a bit more about this.

Gareth

 

Gareth
Orienting the fibres to give good torsional stiffness and low bending stiffness would be the desirable path. The rod I tried was orientated completely opposite. It was one of the tent poles you can buy.

So I expect you could get good results with it. It is a matter of going through the development for the application.

I am self-funding and have just bought two 1270mm by 40mm tube offcuts that cost AUD500. This is for a new lightweight frame. I will post a picture when it is completed. Show you what you get for the money when it comes to well made carbon fibre materials. Expensive for unsponsored poor working man trying to save for retirement.


Rick W

 

OK, If we were to attempt to spec a carbon tube what would you be looking for.

Torsional stiffness >20Nm/rad
infinite life with 10Nm torque
minimum bend radius 2m

Do you agree, is there anything to add.

Having spoken to a tube manufacturer they say joining the ends of the tube to a stainless steel shaft with a specialist glue should allow you to sort out the end connections.

Gareth

 

Gareth, I believe it would glue better to titanium. I don´t know why but the US navy thinks it does, with composit carbon/titanium sub shafts and all.

 

The bend radius could be relaxed. You can usually achieve the drop in an acceptable distance with 5m radius.

It can be solid. Does not need to be a tube.

Diameter is important from a drag perspective. A shaft for these particular conditions would not want to be thicker than 10mm OD.

If you were going to the trouble to do this then you would want good corrosion resistance.

Rick W