Prop Guard for Pedal Powered Raft

[OldShell]

I have almost finished a six man, pedal powered raft for for a local event in July. Unfortunately the Committee would like me to put a guard around the two propellers for safety reasons. A lot of work has gone into the propellers and I am concerned that I will lose a lot of hard won thrust. If I just surround the props in a mesh cage as the committee suggests


Am I right, if so has anyone an alternative proposal, bearing in mind I have to keep in “Rafty” and do it on the cheap. One idea is a big cage, away from the fast flow. another Idea is to cut up short lengths of pipe and stack them honeycombe style fore and aft. I include a picture of: The Raft, The suggested cage, The two Props we will be using.


Any thoughts on this would be greatly appreciated!

 

[Tiny Turnip]

Hi OldShell

First, your props look good.

Second the requirement seems ridiculous; If someone is in the water in front of your props, the raft crew will obviously see and stop pedalling.

In terms of providing a guard, would a napkin ring slice of large diameter (gas mains??) pipe around the prop like a Kort nozzle be acceptable to the organisers? and feasible for you to fabricate? (or rolled out of a ribbon of thin steel? or mould a fibre glass ring around, say a large bucket? or a slice of large bucket?)

 

[BlueBell]

Wow, nice job on the props.
How much does the raft weigh?
What speed have you attained?
Six man?

Great idea TinyT!
I agree, prop guards are completely unnecessary but the Commitee has spoken.
A couple of tapered buckets with the bottoms and rims cut off would act like Kurtz nozzles and protect the props.

I hope those props are at least square. 20" x 20" I'm guessing?
RPM?

Good luck!

 

[alan craig]

I agree with the above; hopefully a ring of steel or ali. sheet would be acceptable and have less drag than all that mesh.

 

[OldShell]

Hi All, thanks for your ideas. I think the Kurtz nozzles would be acceptable, with a backup plan. I have restrung a badminton racket in the past, so with a ring of holes at the front and back of the nozzle I could string a coarse mesh front and back on the day if the nozzle was jugged insufficient. It will be big, the Props are 500 mm tip to tip but I have a lot of bike wheels left over and I might be able to do something with the rims.

Q. How much does the raft weigh? A. 650kg all up. 4 Pedalling, 1 Paddling / steering, 1 repelling borders.
Q. What Speed? A. Its theoretical but between 3 and 4 mph (1.5 m/s). I will post actual after event in July.
Q. RPM? A. 600 rpm with a cadence of 70. Hoping to get 300 Watts into each prop.

A Tapered Kurtz nozzles was mentioned. Getting techy now, but is it important that it is tapered and if so which way?

 

[Tiny Turnip]

Hi OldShell

I'm no sort of an expert, but broadly, Kort nozzles are wider at the leading edge and narrower at the trailing edge, but their design can get very techy - they usually have a foil section and behave differently at different speeds, albeit better at slow speeds, and can cause problems with turbulence/cavitation. If you used a taper, I'd not over egg it, and do some research on the profiles.

I'd be a bit wary of the drag pushing a bike wheel rim sideways through the water - if you've got the energy, and depending on how serious you are about the race, I'd definitely consider fairing the cross section with your covering material. I expect you have that in mind.

best wishes

Adrian

 

[BlueBell]

A Tapered Kurtz nozzles was mentioned. Getting techy now, but is it important that it is tapered and if so which way?

Google Kurtz nozzle and you'll see.

 

[alan craig]

I made these shrouds for 32" props (in air not water) for the same reason: safety of others. They are not really ducts, they do not influence the flow much, just a physical barrier. I used a ali. extrusion riveted to the leading edge of parallel shroud. At the size you want you could leave off the leading edge extrusion for hopefully less drag. The mounting hardware could be strips of the same ali. sheet.
Are the organisers going to require the rowers to guard their oars??

32" shrouded propellers

• alan craig
• May 7, 2018
• 3

propellers

 

[OldShell]

Thanks again all for your comments, I am working on the shrouds using two bike wheel rims per shroud and I will post the result in time. It will be difficult to get a decent profile, but I have some ideas. Mounting them is a problem and Alan Craig's solution looks well worth copying, inverted of course.

 

[PAR]

Given the power you're putting into these props and their RPMs, why the need? Even if you're peddling like mad, next to a swimmer, the blade would break or just stop on contact with a leg or arm. Additionally, why would you be peddling, particularly with enough speed/power when someone is swimming around near the business end of the drive? I see no need to burden the drive with additional drag, again given what you're putting into it. If you must, a simple, single, curved vertical arm forward of the prop, will keep arms and legs out, with a whole lot less drag than what you're envisioning.

 

[OldShell]

Perhaps I should have said more about the race and the dangers present. The Heats are a normal race, first over the line wins. The Final is second over the line wins. This makes for an entertaining finale with a lot of pushing and shoving and people in the water and hot heads. All in all I think the committee are being reasonable given the conditions.

 

[BlueBell]

PAR, the committee has spoken, nothing else matters.
I know it's ridiculous, you know it's ridiculous, any educated boater knows prop guards don't achieve their objective.
But to the layman, it's so very counter-intuitive....
And then there is the "cover-your-***" mentality...

 

[OldShell]

My team and I completed the race two days ago. We came third in our heat, we were the fasted by far but not in a straight line, we strayed into weed and had to finish the second half dragging weed chocked props using the backup paddles. The good news is the prop guards offered negligible resistance. We found that out a month ago when we did some tests in a weed free lake. That was at a speed of 3.1 mph. They are made of bike wheel rims and a leaking water but.

The props by the way are designed using JavaProp and constructed from layers of paper soaked in epoxy resin. I have written a JavaScript that takes the output from JavaProp and prints out templates for the blade layers and a former to build the layers up on. If anyone wants a copy I can forward it on.

 

[alan craig]

Thanks for getting back to complete the thread. I like the props, they seem to be arranged as tractor (pulling). Now I feel the need to make a model hull with paper and epoxy to see how it turns out.