Note: this post is out of order (it should have been dated Friday June 25th 1:45AM and have been at the start of digest 2203) because Peter and I accidentally went back channel when we really didn't mean to because I forgot to add "Paddlewise" to the "To" line when I first sent the previous post before correcting the word "sack" to "sake" and sending it on to Paddlewise. Then Peter wrote back just to me thinking I wanted to go back channel. Because Paddlewise was not listed on the to line I assumed he wanted to go back channel and wrote the following post to him. Upon realizing what happened we agreed to put this back on Paddlewise but his answer to the following post has preceded this post. To the few who may have been trying to follow this thread I'm sorry for the confusion. Wrote Peter: I don't see a "Paddlewise" on the "To" line. Are we now back channel? I'll treat it that way unless I see your post in the Paddlewise digest. See my comments inserted into your text below. Matt Broze www.marinerkayaks.com -----Original Message----- From: Peter Treby [mailto:ptreby_at_ozemail.com.au] Sent: Friday, June 25, 2004 1:45 AM To: mkayaks_at_oz.net Subject: Re: Skeg Jammers etc Hello Matt: > Picking the kayak up and balancing it determines the position of the centre > of mass of the kayak along its length. Then plonk the boat in the water, and > the centre of buoyancy is at that point. Tell me if I'm wrong.<< > You're wrong. >>I don't think so. And certainly not by the example you give which follows:-<< I believe you were wrong on the previous paragraph to the one you listed here (which was included in the block of text I was originally commenting on). The following thought experiment I wrote refers to that previous paragraph as well (and is also in agreement with the paragraph you listed above just like you said). > Put a huge amount of weight in the bow > (only) and find the kayaks center of mass by lifting it at different places > until it balances. Put the kayak in the water and now sit on that balance > point (for the sake of example, say the bow hatch). ... In order to > keep the kayak LEVEL the paddlers weight would have to be further back (far > enough back so that the CofB and the CofM lined up when the kayak is LEVEL). >>In this example you agree with my proposition above. Rather than continue to talk at odds, and pick up every wrong point, let me state this suggested method for trimming a sea kayak, without dividing your gear into two piles one twice as heavy as the other.<< I never divide or weigh the gear. Twice as much is only a goal to shoot for. I shoot for it by packing every heavy dense bag (and water) that I can into the stern and then put what won't fit there into the bow. What could be more convenient? It doesn't even matter if I overshoot my goal since all I will loose is a little top speed. A heavily loaded kayak is not as much affected by the wind so turning into a strong one is much easier than when empty and since most kayaks weather helm at trim this stern heavy trim will lessen that tendency some. >>We want to determine a convenient method of deciding if a kayak will float at trim, with its paddler and load on board. We pick up the unloaded boat, and find the balance point, which will be somewhere along the cockpit. We note that point. We locate the seat such that the paddler's CofM will be at the balance point just found. We load the boat with gear. We again pick up the boat at the previously found point, then adjust the gear until the boat again balances fore and aft. We put the boat on the water and get in. The boat then floats trim and level. You may care to disagree with this, or introduce other considerations, but it works.<< Only in kayaks that float level when empty. In those that don't you will be putting the seat in the wrong place and compounding the problem (by putting more weight behind it--not necessarily changing the out of trim angle--although it might do that too depending on the shape of the kayak and how the underwater shape changes as it sinks deeper with more weight in it). >...talking about an empty kayak and how its mass is distributed. On > its own (no paddler in it) it [swede form kayak] would likely sit in the water with a bow down > trim. In order to get a LEVEL trim the paddler will have to sit a little > further back in it than in a fish form kayak. and >You seem to be assuming that when the centers > line up the kayak will be LEVEL. >>OF COURSE I have been assuming that an unladen kayak floated on the water will be trim and level. It seems to be the case with all sea kayaks with which I am familiar.<< That is a pretty big assumption. Have you been checking this with a level? No kayak manufacturer paints a plimsoll line along the boat like a freighter. Do you know of any kayaks which are not level when they are on the water unladen? Yes, several for sure and I suspect that few if any are really absolutely level although it might not be apparent without checking with the level. Your own kayak may be so close that your method works fine for you. It wouldn't work with most of my kayaks. >>Is this anything more than a theoretical point? Can you name a kayak which requires the paddler to place his or her CofM at a point other than at the CofM of the unladen kayak to make it trim?<< Most Mariner kayaks for several. Any Swede-form kayak with more overhang at the bow than at the stern will likely be bow heavy too. A longer bow overhang will tend to help balance out a fish-form kayak though. Remember that any bow overhang is way further out on the teeter-totter with a Swede-form kayak. Many Mariners are so radically Swede-form that the physical balance point (CofM) is often very near the very front of the cockpit. Take it to extremes. Float a barely floating flat triangular piece of wood with two very long sides and one very short side in the water after finding its balance point. (if you divide each angle in half where those three lines intersect should be the balance point). Notice how much further the balance point is from the sharp end. Now build the above waterline part on such a "kayak" on this triangle. I think you would find it very difficult to not sink the bow more than the stern. When you are picking up a kayak you aren't just picking up the part where the water will be supporting it. Any weight you add in each end will effect trim as well (unless you balance how far that weight is from the fulcrum with the amount of weight added). >>In particular, can you name a swede form kayak that requires this? If you can, the distance will be so small that it will not have any practical effect on the trim method I have described above.<< Yes it will, we obviously couldn't place the seat so our belly button presses into the pointed front of the cockpit. Besides being physically impossible to do we would also just be aggravating the out of trim situation and make it much harder to get enough gear weight in the stern to correct all of our own weight we also added forward of the level trim Center of Buoyancy point. Much better to put the heavy paddler further back to balance out both the bow down trim and the uneven weight/distance multiple you've put in each end when loading. Since most kayaks don't have a seat you can easily move it is a moot point about getting the paddlers weight over the center of gravity of the kayak anyway (with a level trim). With a fixed kayak seat you need to play with the hand that the kayak designer dealt you. Your balancing method will work only if the physical balance point of the kayaks mass and the kayaks seat line up in the same place and the kayak floats level both empty and when the paddler is sitting in it at that time. That is a lot to ask. Consider, the paddler sitting in it will sink the kayak deeper in the water. What happens to trim if the bow sections are vertical above the old waterline and the stern sections are extremely flared. Answer: a bow down trim due just to the added weight being perfectly placed over the old trim CofB/CofM position. So much to consider. Rather than lifting the loaded kayak up to balance it why not just bring a small level with you and once you've gotten into the floating kayak check that the kayak is now level (or slightly stern heavy if loaded with gear or the kayak has a weather helm tendency--my recommendation). A small level is so much easier to lift than a loaded kayak and when the kayak is floating with you in it all of the confoundings that can mess up your system get accounted for as well. > You are correct that a kayak balanced when moving forward will blow at some > angle downwind (where the forces balance) if it is not moving forward. First > you are not talking of a huge difference due to different wind speeds (or > over normal paddling speed ranges) and secondly you have things backwards at > higher speeds. "Wind puts more pressure on the end of a long symmetrical object angled into the wind than one angled away." >>Hmmm. Think of this: Place a brick at 45 degrees in a wind tunnel, with the long side of the brick across the windstream. Mark one end of the brick "A", and the other "B". Look across the brick from end "A", and then again from end "B". Did the wind pressure on the brick change when you changed your viewpoint?<< Huh? Is this brick free to pivot or is it locked into place by friction or in some other way? If the wind is blowing such that it hits end "A" first that end will have more pressure on it than end "B". If the brick is like a floating kayak (free to pivot) it will end up balancing when it is completely sideways to the wind (if it is symmetrical in all respects and suspended from the balance point). What could changing your viewpoint have to do with it? This same effect is working against weathercocking or lee cocking in a kayak and as the wind gets stronger it overpowers the forces causing weather helm and eventually tends to lock the kayak into a sideways orientation (at its balance point--taking into consideration the center of wind force and the center of the hulls lateral resistance in the water at the angle it is traveling at relative to the wind). This is why it can be so hard to turn any long kayak into (or away) from an extremely strong wind. Waves compound the situation further as they also balance out a long object sideways to the waves direction. Bigger waves help some in this situation as they shield you from the winds when you are in the trough. Long waves, like swell, have less effect on the kayak because they are so much longer than the kayak hull and therefore don't combine with gravity and effect both ends of the kayak at the same time (tending to pivot it as gravity pulls it down into the troughs) >>And, we are taking a beam wind as the example here, which I think of as being the maximum weathercocking situation, and not having the kayak angled into the wind or away from it.<< The kayak turns into the wind as it weathercocks so you are soon not in a pure beam wind situation anymore. Absent any counteractions on the part of the paddler this continues until a balance is reached with the force acting more strongly on the end of the kayak pointed into the wind than on the other end. If this was not so wouldn't the kayak keep turning until it pointed directly up wind due to the weather cocking effect. The imbalance between the center of windage and the center of lateral resistance is finally resolved at some angle and that becomes a course that is easy to maintain (if no significant wave effects intrude). "Strong winds ... tend to reduce both weather helm and lee helm." >>Interesting. The worst weather helm experienced is on flat water, such as when wind blows offshore, with very short fetch over low land.<< True, see the effect of waves described above (which are not acting in this situation). >>Weather helm appears to me to worsen as the wind increases in these situations. In other higher wind situations, when the water is rough, the weather helm effect is hard to distinguish from the push and shove of waves, and wind on the bow at the crest, etc. Although all the forces on the boat are greater in higher winds, you would expect, from say a vector diagram analysis of the weather helm effect, that the weather helm effect rises too. Isn't it just that it is masked by other greater forces relatively? Cheers, PT<< Overwhelmed rather than masked would be more like it (because they are working against each other rather than in concert where the overwhelming strength of one might mask the other rather that counteract it). What kayak do you paddle? A fish-form kayak moves its center of lateral resistance further forward than a Swede-form one does as they push through the water. This leaves a longer stern (relative to the center of lateral resistance) to be acted on by the wind. Take it to the extreme by putting the paddler at the (blunt--to maintain level trim) end of each kayak and it should be obvious what is happening. Other things being equal fish-form kayaks weather helm more than Swede-form ones. When sitting still in a side wind does your kayak blow straight sideways or slightly bow or stern down wind? The reason I ask is that this may explain why our experiences differ as to what wind speeds make for the worst weather helm with the kayaks we paddle. It would seem likely that a kayak with a strong weather helm would take a higher side wind blowing (with greater pressure on the end most towards the wind) to overcome the stronger weathercocking effect. I discriminate against strongly weather helming kayaks so don't paddle them as much as those that have a much more controllable (closer to neutral) weather balance. Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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