I just completed a 18' Bluefin and find it weathercocks in side currents and wind, Since I camp and need all the room I can get I would rather not add a skeg, a rudder looks like a a lot of work with all the cables and cutting through the deck, foot pegs and all. How about a rudder that does not turn (no cables or new foot pegs) It just raises and lowers? No holes through deck, much easier install. Has anyone done this?
TIA
Phil
yes I have seen this on a plastic boat. It was a sort of clip on arrangement, could not raise/lower, it was an all or nothing. http://beach-kayak.com/en/accessoires.php - scroll down almost to the bottom, look for Costa Rica PE Skeg
Here is an option that I am considering for my SOF http://www.capefalconkayak.com/The%20modern%20skin%20on%20frame%20kayak.html - scroll down to the second last picture. You can get the channel at Lee Valley http://www.leevalley.com/en/hardware/page.aspx?p=61986&cat=3,43576,61994,61986
The other option is install a standard rudder and just secure the wires so that it does not turn. Only down side is that you cannot "dial in" the amount of skeg you need... hmmm, thinking new product opportunity ;)
Don't forget to pay attention to the loading. Weathercocking is caused by a torque formed by the difference in positions of the center of effort of your "sail" (everything that sticks up into the wind) and the boat's center of gravity. A skeg/rudder counters it using a torques caused by the difference in position between the center of gravity and the center of lateral resistance (everything moving underwater). When you're holding still, the angle your boat adopts is controlled by the torque caused by difference between the center of buoyancy and the center of gravity. When water is flowing around the boat, there's a also a torque generated by the difference between the center of lift and the center of gravity. Then there's the torque caused by distance between the paddle blade and the center of gravity. The balance between all these torques is what determines the boat's attitude.
Since the center of gravity is involved in every torque and since the center of gravity is determined by the loading of the boat, some or all of weathercocking can be cancelled by simply moving the load around in the boat. If the boat weathercocks too much, move weight backwards. If it turns away from the wind, move weight forward. Same with tracking. If it tracks too much, move weight forward. If it doesn't track enough, move weight back.
Give it a try and see if that fixes your problem. If it does, it's the cheapest and easiest solution you'll find.
Laszlo
Laszlo,
I had only used my new boat three times before I went on a camping trip. My boat was fully loaded 60+lb. of gear and 220lb. of me. I'm sure my weight was not balanced. The stern swings around with the current whether I'm moving or not. I will try and move my seat back and forth to see if that helps. Thanks for the advice.
Phil
Here is a thought experiment that may help to understand why kayaks tend to weathercock, and therefore how to control that tendency to your benefit.
Imagine two 16 foot kayaks, A and B, both with center of mass in the middle, that is, exactly at the 8 foot station, and a huge wing near the same station, resulting in a center of effort of the wind exactly at the center of the boat (the 8 foot station).
1. Suppose that A has a huge rudder mounted at the bow. You start paddling in a strong cross wind coming from starboard. Will the kayak weathercock (try to turn to starboard), leecock (try to turn to port), or be neutral?
2. Suppose that B has a huge rudder mounted at the stern. You start paddling in a strong cross wind coming from starboard. Will the kayak weathercock (try to turn to starboard), leecock (try to turn to port), or be neutral?
3. Given that kayaks A and B have identical position of the center of mass, what do you conclude from this about the significance of the position of the center of mass on weathercocking?
Phil,
In our boats my wife & I have found that moving the stash of drinking water between the front & rear hatch spaces is usually plenty. But then again, we usually don't carry around a bunch of camping gear.
Though experiment person (your name isn't showing for some reason),
If the center of mass and the center of effort coincide exactly (and stay there during the movements of the paddle), the wind will have no effect because the boats are neutrally stable. The distance between the center of mass (or center of gravity as some call it) and the center of effort is 0, so the wind torque is 0. Your huge wing has canceled out the wind.
Underwater, though, you've rigged it so that kayak A has negative dynamic stability (arrow fired backwards) and B has positive dynamic stability (arrow fired forward). Paddling from the center of the boats will cause A to try and turn backwards and B to track like a train.
The direction that A wants to swing (weather or lee) is determined by paddle stroke used, which is not stated, so it can't be predicted from the data given.
Modifying the experiment by slightly moving the center of mass forward and leaving everything else the same will result in a wind-induced torque which will try to weathercock the boats. Boat B will attempt to track straight. Boat A will still be mainly trying to turn backwards. It may or may not turn into the wind, depending on the paddle stroke and the size of the wind-induced torque. Move the center of mass farther forward and boat A will weathercock more, and be less negatively stable. Move it all the way to the front and the boat will have dynamic neutral stability plus a large wind induced torque and boat A will weathercock instead of turning backwards.
Conclusion -the center of mass relative to center of effort determines weathercocking. The center of mass relative to the other centers determines if there will be other forces to cancel out the weathercocking (or to help it along). Therefore, the position of the center of mass is significant to weathercocking.
Do I pass? :-)
Laszlo
To quote Pulp Fiction, as I always try to do, "CHECK OUT THE BIG BRAIN ON LASZLO". Great dissertation Laz and I love ya but Phil wants to know what time it is not how to build a watch..... Phil, I know you have a boat with a Greenland like stern however a rudder will get a fully loaded boat through a surf landing a lot better than a skeg will. You're a couple of hours at the most away from installing the foot braces (I'd use Smart Track) and rudder cables and you can hang a rudder on the back of a Bluefin if you want to.. Admittedly I come down on the rudder side of the great rudder vs skeg debate but that aside if you're going to be doing a lot of traveling a rudder will come in handy and you won't find yourself out of your boat in the middle of a crossing trying to shift your load around when a crosswind comes up. SEEYA Jack
Smart track it is. I’m going to Okoumefest in May. Should I wait until then, to see if they offer specials?
Sure, use the boat till then as is if you like and that will make you appreciate the rudder all the more. See you at Ofest. SEEYA Jack
Jacknlin: I understand your friendly jab at Laszlo (actually, it should have been directed at me, but it seems I managed to pull a Three Stooges stunt here and ducked at just the right time).
But actually I' rather see both kinds of responses. 'What time is it?', and 'how to build a watch'.
One response from a guy with a watch, saying what time it is right now. Quick and easy, and has a good chance of solving the immediate problem. If the questioner is in the same time zone, and is able to process the info within a few minutes, and the answer works, and he/she will never have the question again (maybe he or she is about to give up kayaking after this season)... it just may be all the info the questioner needs.
But also I like to see a second response, about how to determine the time without help from others, in the future, just in case.
To put it another way, having a fish to eat is sometimes critical, and is all that is needed.
But sometimes, knowing how to fish is the most beneficial.
It is the same with facts. Being given a rote instruction, with no understanding of why it works, is sometimes helpful. But learning how I can determine the solution, by understanding the "why", is sometimes more beneficial.
L: Sorry but I don't think your answers are right. My answers: Kayak A will weathercock, kayak B will leecock.
It's my understanding that absent other forces a clockwise force couple ALWAYS causes a clockwise angular acceleration, regardless of the position of the center of mass. Do we agree on this?
Jack,
Always trying to quote Pulp Fiction is easy and a challenge at the same time. Easy because 90% of the dialogue is the same 2 words, a challenge because of which 2 words they are. Not sure how to take your response, seeing as how Jules & Vincent blow Brett away at the end of that scene.
Mystery guy (your name didn't show up again- you Camper, by any chance?),
I agree with your statement about force couples and directions. I also know that moving the CG relative to the other centers can change the magnitude of the forces and result in a net force which eliminates weathercocking. Let's start a new thread or swap e-mail addresses to continue the discussion. That way we won't have to annoy the uninterested.
Laszlo
Chill guys, the last thing I want to do is quash the discussion. I don’t even own a 9MM so Laszlo your safe and I’m all in favor of teaching a man to fish so keep up the dialog it’s fascinating. SEEYA Jack.
Laszlo, you seem to be using "center of gravity (or mass)" as being the boat’s lateral pivot point, which I don’t believe is the case. That is the simple point that I think the "thought experiment" poster was trying to demonstrate. My understanding is that it is the immersed underwater profile that has the greatest impact on a hull’s pivot point. Shifting weight aft reduces weathercocking in a kayak because it results in the stern being more deeply immersed, and the blow less deeply immersed. Adding a large stern skeg does not appreciably change the center of gravity of a heavily loaded kayak, but it does change the lateral underwater profile, hence the pivot point, and hence the tendency to weathercock.
Your tangent on the dynamic instability caused by a skeg on the bow does highlight an important point, though. The usual discussions of steering balance focus on properly aligning the CE’s and the CLR’s, as if a boat being propelled through winds and seas is nothing more than a static teeter-totter. The truth, of course, is that all of the forces are constantly in flux, and are partially generated by the boat’s own forward motion through the water.
Grant
Looking at my kayak I can move back only about an inch before hitting coaming, is that enough to make a significant impact? Boat is 18ft long I weight 215 lb. By the waterline left on my bow (it starts about 18"back) it doesn't seem to be bow heavy. I could add some weight to the back of boat but that seems counter productive. I also paddle a SW17 which is hard chined compared to the Bluefin which the bottom is soft chined eight sections instead of four. With the SW I can lean to help correct effects of current and wind, it doesn't seem to have the same effect with Bluefin. I will be installing a rudder which will increase drag but keep me on a straight path.
Phil
Old Yeller: I think you are correct about where the disagreement lies. Is the position of the center of mass directly relevant in determining the existence and direction of the twisting force on a kayak in a crosswind? Or only indirectly, because it moves the center of effort of the water?
I think Laszlo's also right, that the 'how to build a watch' branch which I started needs to be taken offline at this point. Pls write mcamp@cinci.rr.com if you can contribute to our understanding of how kayaks work.
This is from Camper, in case my name still doesn't show up. Dismayed that my cantankerousness so easily identifies me. But not all that surprised. The Boat Wife gently reminds me of this trait whenever I forget.
Glad everybody's friends again.
Phil, unless you're exceptionally dense (in the mass per volume sense), half inch isn't really enough. That's why the wife & I move our drinking water stash between the hatches and cockpit to change the balance. I did have some success yesterday afternoon (WD12, 20 MPH+ winds, just mildly weathercocking) with moving my legs from feet on the footrests to legs crossed under me. Normally I would have moved the 2 liters of water from the front of the cockpit to the rear hatch, but what with the sprayskirt both were inaccessible. Which confirms Jack's point that sometimes a rudder is the only way to go, especially if you don't have the WD12's roomy cockpit.
Grant, you're right, absent any specification in the thought experiment I was assuming that the pivot point was at the CG.
Camper, it wasn't cantankerousness, it was your writing style. All that was missing for a positive ID was a Latin phrase borrowed from your daughter. I'll be sending you some e-mail in a bit.
Laszlo