Counter intuitive log impeller

Danny

Danny

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Joined
23 Oct 2003
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Me: St Albans. Boat: Portsmouth
www.compasscard.co.uk
Can someone explain this for me:-
We had the old problem of the log not working so I did my usual 'fix' of turning the impeller sideways and backwards, etc. to clear the weed. Didn't work. So I left it in a sideways position to see if it would clear after a while. Forgot all about it after a few minutes...
Eventually glanced at the log and saw it was reading something over TWICE our speed through the water! Scratched head a bit then eventually remembered I'd left the impeller turned sideways. Turned it back to its normal position and got the normal speed. Out of interest I turned it sideways again and, lo, the speed more than doubles again!
Can anyone explain this? It seems totally illogical to me.
 
If I dug out my old aerodynamics lab book, I could find a wind tunnel test with a flat plank mounted centrally on a spindle along the axis of the tunnel. The edge of the plank was upstream; easier with a picture!

It span like a windmill as the air speed rose, despite there being no aerofoil section. I think it was due to the fact that nothing is perfectly symmetrical, and it once it started spinning it generated lift which accelerated the spin.
 
It's probably the same reason that you sail faster at 90 degrees to the wind than when you sail downwind.
Your impeller is flat on one surface and curved on the other. When in it's normal position the flat surface is presented to the travelling stream of water and thus acts like a sail in a downwind orientation. When set at 90 degrees to the flow the water accelerates across the curved surface and creates a low pressure area on that side of the impeller (exactly like a sail) causing the impeller to rotate.

Sound plausible?
 
Interesting - I have been having problems with my impeller too. The latest incarnation is that having removed it and cleaned it and proved it worked by flicking it with my finger it still gave a zero reading on being returned to its proper place ! however when reversing out of the marina a quick glance showed that it was giving a good reading. I can only guess that there is a piece of weed clinging to the skin fitting which in forward fouls the impeller but in reverse is swept out of the way. I will have to wait for lift out to investigate.

Regards
 
I've seen similar problems - a small build up of slime or weed on the outside of the skin fitting will stop the impeller working even when clean.

When I clean the impeller I now poke a brush out through the fitting to clean it and that usually does the trick
 
When Red Jet's wake is inevitable, I invariably steer into it at right angles. Usually frees any impeller obstruction, until the barnacles get a real grip.

When you turn your log impeller sideways, it is possible that instead of working as a paddle wheel, it functions as an axial machine with the stream lines now parallel with the blades and hitting the blades at a slight angle, so the simple relationship between distance and stream velocity is lost.
 
[ QUOTE ]
Your impeller is flat on one surface and curved on the other.

[/ QUOTE ]Not so in my case. The impeller blades are flat both sides so there should be no lift if exactly sideways on. If it wasn't exactly sideways on then I would expect it to turn but slower than normal.
 
Danny,
It's actually a very similar situation - I think the effect is called 'autorotation', which works even if the flat plate is at zero incidence to the fluid flow.

Keith
 
My log reads accurately when I'm level and leaning to port, but significantly overreads when learning to starboard. Checked alignment etc so can't figure out why this happens. Something related?
 
I suppose you are sure that it was correctly aligned first and then at right angles to the flow when you thought is was.
Conceivably the paddle has been fitted at right angles and calibrated as such so that when you rotate 90degrees it actually becomes correct and hence higher reading. Otherwise I cant imagine how any rogue rotation at right angles to normal flow would give double indicated speed unless as suggested it is oscilating. olewill
 
Having previously offered you a daft explanation [excessive leeway], I'll now try a more reasoned one!

If you push a boat through water, the water is displaced. Since water can't be compressed it has to go somewhere and the only place it can go is sideways and upwards. It's like a ploughshare working in soil, only instead of furrows you get waves. Perhaps if your impellor was in a different place the effect would be less but I don't suppose you intend to go drilling 1-1/2" holes all over the bottom of your boat. So long as you are getting an accurate reading when you have the impellor facing forward it isn't a problem.

Anyway, I,ll be interested to see if the same thing happens on my boat.
 
As already said, even if the impeller is mounted clear and forward of the keel, it's unlikely that the flow over the hull and impeller is completely linear. So heeling to port or starboard will have some effect on the speed of rotation.

But I'm convinced that Danny's impeller is rotating, not oscillating, maybe he can get a camera down there? /forums/images/graemlins/smile.gif
 
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