International Canoe foils question

[cmaas]

At speeds of about nine knots and above I notice a odd low frequency wobble. Not the common "hum" but more like sailing in a powerboat's prop wash. I figure it must be coming from the rudder and or daggerboard and that it can't be fast.
The wobble can happen reaching or beating. The board and rudder do not need to be highly loaded.
The rudder section is similar to a NACA 0012. The daggerboard is an NACA 64-008. The planforms of both are basicaly straight tapers with square tips slightly rounded. They are as symetrical, untwisted and polished as I can make them by hand.

Does anyone have any thoughts about what could be causing this?
Thanks.

Chris

[gggGuest]

Do you feel anything in the tiller?

[cmaas]

Originally Posted by gggGuest

Do you feel anything in the tiller?

Just occasionaly I'll feel a wobble from the rudder transfer through the tiller extension. Otherwise I feel it through the seat.

I should mention that this is a pretty subtle thing and I can only notice it in flat(ish) water.

For all I know this could be common in fast boats and I just never noticed it before.

Tip vortices? Could that be it?

[Phil Stevo]

Chris,
Any chance there is weed on the centreboard when you sense this happenning. It will certainly cause turbulance?
Any chance of flexing of the centreboard, although a lower frequency hum would be more likely?
Movement in the case?
Otherwise maybe the daggerboard is just to thin or too sharp.
On my similar canoe, my CB is 0012, (rudder is less scientific), a tight fit, carbon on cedar, not polished and silent.
Phil S

[tspeer]

Originally Posted by cmaas

At speeds of about nine knots and above I notice a odd low frequency wobble. ...

I've felt something of the same on my F-24. I've attributed it to the rudder being in the wake of the board.

I've also felt something similar that I was able to figure out was turbulence in the water. I was sailing up the shore of Camano Island with wind with tide. The bottom drops off steeply there, so I was short-tacking up the shore to stay out of the current. As I would come away from the shore, I would start to feel turbulence from the shear layer at the edge of the current tickling the rudder and the wind would drop. That was my cue to tack back, and I hem-stitched my way all along the shore. It was amazing to actually feel what was going on with the current.

[Otter 33]

Hi Chris,
I had this problem once with two daggerboards I made for a trimaran. It would only happen when there was little load on the foils-- reaching. I tried many things and it persisted until I wet sanded with 600 grit and it went away (it had a high gloss surface before). These were also hand shaped foils... I have not had this happen to foils I have made via CNC.
I have also heard of people asymetrically v-ing the trailing edge of the foil to reduce this.
I have read that the cause is from vortexes shedding from one side of the foil, then the next side of the foil, alternatively.
Hope this helps,
Brandon
Friday Harbor

[gggGuest]

Its the low speed oscillation that's got me beat... usually foil effects are a highish pitch hum. I'm trying to work out what could resonate at that sort of speed... What sort of frequency do you think it is. The prop wash desciption is a bit vague to me. I wonder if you should try putting bits of tape or something on the foils to tripup the turbulence and change the flow... If the foil was quite different one side to the other it ought to stop a regular oscillation at a completely uneducated guess... Might help track down what is causing it. Is it continuous or short bursts? I suppose the rudder might be in a trailing vortex off the board, but I'm pretty ignorant of this stuff, Mr Speer would know more...

[cmaas]

Thank you everyone for your suggestions. It gives me some useful ideas to pursue. The most appealing of which are the weed (possible, I sail in a weedy area) and the current line turbulence that Tom noticed. No extra work required, just better obsevation on my part.

Tom, could having a daggerboard create a such a turbulent wake indicate a poorly choosen or maybe poorly shaped section or planform? Or would that be a characteristic of a "laminar flow" type section?

Phil, the board is probably the most flexible board I have ever sailed with. I wonder if that could be it?

gggGuest, the frequency is somewhere in the 2 - 4 times per second range as near as I can tell. I like your tape idea.

Brandon, I've used the angled trailing edge trick in the past to deal with humming boards. Maybe it's worth a try for this wobble. Probably what I really need is a set of your perfectly shaped CNC foils.

I guess what it all boils down to is: could this turbulence/ wobble be slowing me down? My Canoe (a DC for those of you in the know) seems quite fast. But it's also quite a different design compared to other IC's so for all I know it would be faster with different foils. Which is an option I may have to try of course. Just not quite yet.

Chris

[Paul Scott]

Chris- I had an Aquata longish (Carbon!) sailboard back in the early 80's that did kind of the same thing- a wierd subtle kick to one side with kind of a thumpish lumpy swish feel through my heels. Only on Lake Union in 2-3 K or so, inky still cold winter water. No cat's paws. Odd thing is, the Aquata had the same kind of stern planform as your DC. The Aquata had a slight v (maybe 1/4" in cross section) at the stern- the last 2 feet or so. I would rock the board from side to side slowly and watch the swirl from one straight side of the stern to the other. Sometimes I could get the feeling that way, and the stern point had to be about 1/4- 1/2 inch under the ambient level of the water for it to happen. Never happened in salt water. I can't remember or find a pic of the board, so I can't remember the exact angle of the pin, looking from the top. I thought at the time that flow was getting established on one side of the stern, and then "jumping" to the other side fast enough to cause a bit of turbulence across the v'd bottom profile (and screwing with the keg in the process). No concaves back there, by the way. Very sharp rails, but not vertical. Got a very distinctive swirl off of the outside points of the stern going away from the board once in a while, and really strange, sometimes a visible flow 45 degrees away from the outside stern edge for 3-4 inches that was outside the hull waves, but I never could correlate that stuff to the bump/swish. Although your sailing pic on the USA IC site shows the same little vertical crease of water off of the pin that the Aqauta had. Does it change sides/direction off of the pin in a wierd way? Or velocity? There was a Fatanatic longboard with really crisp rails and the same stern treatment, but I didn't sail it enough to feel anything. For what it's worth, Hans Fichtner thought that the square pin's corners made for very lively but unstable board feel. Of course, you have a rudder, which adds all sorts of complexity over a board. Are you steering the rudder when the event occurs? The bump/swish never happened when the pin was clear of the water, or when the stern was picking up the stern wave, above the ambient water level.

Paul

[Paul Scott]

Originally Posted by tspeer

I've felt something of the same on my F-24. I've attributed it to the rudder being in the wake of the board.

I've also felt something similar that I was able to figure out was turbulence in the water. I was sailing up the shore of Camano Island with wind with tide. The bottom drops off steeply there, so I was short-tacking up the shore to stay out of the current. As I would come away from the shore, I would start to feel turbulence from the shear layer at the edge of the current tickling the rudder and the wind would drop. That was my cue to tack back, and I hem-stitched my way all along the shore. It was amazing to actually feel what was going on with the current.

Tom, we draw 8 1/2 feet, forward swept keel, really high aspect fins, and we are constantly feeling the different water conditions/directions etc. under the surface. Flexing, jerking, hunting, micro course changes, speed changes, etc. No noises though. At first it kind of freaked me out, but now it's more information than anything. 'course there's also the dolphins asking our bulb what it's sign is.......

Hey Chris, critters messing with your canoe?

Paul

[Paul Scott]

By the way, the stern treatment was considered fast, but loose going fast. Could be you're feeling that.

Paul