Human powered surface drive?

[clmanges]

Rick,
I changed the design just slightly, making the ID two inches less. I thought that the weight of this thing hanging off the transom might put the back end of the boat a little lower. Anyway, OD is still 32" and ID is now 16".
I added the vectors graphically and measured the angles, and it comes out very close to your numbers: tip angle is 60 degrees, and root angle is 43. Those are measured relative to the boat's axis, and without adding the little extra you suggested. That part has me a little confused; should I not subtract the three degrees from that?

I also played with variations on boat speed and rotational speed of the wheel, and the numbers seem reasonable, showing a linear relationship (I know, that's discounting drag, and probably other stuff).

Anyway, I think I'm ready for the next step.

I know that the width of the blades -- the angular span of them -- will set the thickness of the wheel assembly, with the chosen angle as a given.

Now I want to figure out what the angular span should be, and the spacing between blades.

Since you describe this as an aerating system, it seems that the blades should span a narrow angle, to hold onto their air bubbles longer -- am I correct? I don't know.

I suspect there's some optimum way of spacing the blades apart, as well, likely affected by where each one needs to be relative to the wake of the previous blade. I can't guess that, either.

Thanks,
Curtis

[clmanges]

Rick,

and the numbers seem reasonable, showing a linear relationship

Had to give myself a dope-slap on that one, lol.

As to figuring the angular span of the blades, it occurred to me that there might be some time factor relating to how long the air bubbles will stay where you want them. If there is such, I can use it to calculate the span.

You said earlier,

You will have confused flow around the transom of the boat making real life conditions somewhat different. Would pay to set the prop as far aft as practical.

If I mount this right against the transom, the blades would be 3" away from the hull at waterline and 4" at maximum depth; those are minimum numbers, and the transom curves away from the center a little. Would that be enough?

[Guest625101138]

Originally Posted by clmanges

Rick,
Had to give myself a dope-slap on that one, lol.

As to figuring the angular span of the blades, it occurred to me that there might be some time factor relating to how long the air bubbles will stay where you want them. If there is such, I can use it to calculate the span.

You said earlier,
If I mount this right against the transom, the blades would be 3" away from the hull at waterline and 4" at maximum depth; those are minimum numbers, and the transom curves away from the center a little. Would that be enough?

Curtis
The pressure behind a blunt transom drops down quite quickly with speed. I think 3" is small but you are breaking new ground here from my perspective so just try something that is convenient. I was thinking more like 12" as a trial.

Don't bother with the angle of attack. It just means you spin a fraction faster.

The blade shape will depend somewhat on the material you use. They have to be quite strong because it is not a speed machine so a lot of force for any given power level. Surface props usually have high effective area ratio to reduce vibration - get a steady force. You should aim to keep constant amount of blade area in the water. So 4 off one 2" blades would be high pulsing. 12 or more 2" blades would give steadier thrust and reduce individual blade load. So have a look at what material you feel you can work with and do blades around that. For simple straight blades I would be looking at around 3" width and having little separation.

Rick

[clmanges]

Rick,
I've got a couple more ideas to bounce off you. The first has to do with construction of the blade assembly (first picture) -- I'm thinking that having inner and outer cylindrical rims would give me something handy to mount the ends of the blades on, with little flanges and probably pop rivets. (The drawing is not proportional, but you get the idea.) I can't think of a reason not to do it this way, and it might reduce sideways forces.

The second drawing shows the blades being arranged tangential to the waterline, as opposed to radially from the center of the wheel. Anything to be gained by this?

As to number of blades and spacing, since you suggested

For simple straight blades I would be looking at around 3" width and having little separation.

I could get up to 33 of those, with almost no spacing. I'm thinking that might be a little heavy, though.

Thanks again,
Curtis

[Guest625101138]

Curtis
Somewhat like the one at the left of the photo and bigger diameter?

The rim adds drag and I am not sure it compensates for any gain in blade efficiency for a fully submerged prop.

The main negative impact will be potential to lift water at the exit. There will be a stream of water being circulated by the outer rim.

If you use a full complement of blades you could always mount a little higher if there is more grip than needed. Grip is good as it means there is less slip but there is an optimum resulting from dragging a lot of blade area versus tolerating slip. Just too much effort to calculate. Some things are just easier to test. Learn what you can, determine a maths model and optimise second time around.

Rick W

[clmanges]

Somewhat like the one at the left of the photo and bigger diameter?

Yes, that's more like it. I presume that the rim wouldn't be too much trouble as long as it's limited to what's needed to attach the blades? As I said, the drawing isn't in proper proportion. It's just that I couldn't think of a way to fabricate the wheel without some kind of rim.

No thoughts on the tangential mounting? I got to thinking that it might lose too much to spanwise flow. I thought that maybe it would reduce sideways forces, but I'm not sure. It does have a greater effective length, but that's likely offset by some other disadvantage. There's probably a reason I've never seen any such thing in use.

[diwebb]

Hi clmanges,
for a trial what about using the largest plastic radiator fan you can find at the car dismantling yard and adjusting the speed of rotation and depth of submersion to suit??? This may give you a cheap and quick proof of concept type of approach which you could then start to develop and refine depending on the results of the initial trials.
All the best with the project.

[clmanges]

Thanks, diwebb, I'll keep that in mind.