"al.m" <al.m_at_3web.net> wrote: >What I can't figure out, is how much weight I'm lifting at the stern, when the bow is already on the bar. Should be a little less then full 58 lbs, but more than half-weight, Depends on how much of the bow is on the bar. The kayak is also not of homogenous weight--a "slice" of kayak is heavier at the cockpit than at the bow or stern. There are enough variables here to keep an engineer busy for a whole afternoon! Assuming the kayak's weight is spread evenly along its length, if the bow is on the rack exactly at the boat's 'quarter point' (1/4 the length of the kayak back from the bow), you would be lifting half the weight of the kayak, or 28 lbs., if you're lifting at the stern quarter point. If you're holding the extreme stern of the kayak, maybe your grab handle, and the bow is over the rack at the quarter point, then you'll be lifting with a force of 33.6 lbs, and the rack will be supporting 22.4 lbs. "WhiteRabbit" <whiterabbit_at_empowering.com> wrote: Just a guess but you are probably picking up about 60% of the weight. 60% of 56 lbs. is 33.6 lbs. Great guess! (33.6 lbs is still assuming the kayak is symmetrical). If it were more stern-heavy, you're probably lifting more like 65-70% of the weight. Or, you can shift the kayak forward, and lift less! shawn __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.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/ ***************************************************************************
"al.m" <al.m_at_3web.net> wrote: >> What I can't figure out, is how much weight I'm lifting at the stern, when the bow is already on the bar. Should be a little less then full 58 lbs, but more than half-weight, >> You're correct -- but it depends heavily on the angle. If you do a vector analysis of the forces, you will find out that if the kayak is perpendicular to the ground as you pick upthe stern (an unreasonable case), then you are lifting all of its weight. If the kayak is horizontal to the ground, then you are lifting with a force equal to 50% of its weight (also not the case at the ground, but the case when you are ready to slide the stern onto the rack). In between those angles (90 degrees and 0 degeees), the force decreases from 100% to 50%. If you start with the kayak at about a 30 degree angle, then the lifting force is pretty close to 60-70% of the kayak's weight. A better deal than lifting the whole thing, but not half of the whole kayak's weight To check this, put a spring scale in the system and measure the lifting force when the stern is on the ground, and watch the force needed to keep the kayak in equilibrium as you move on up. -- Dave Kruger Astoria, OR *************************************************************************** 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/ ***************************************************************************
Shawn Baker wrote: <SNIP>>>>>>Assuming the kayak's weight is spread evenly along its length, if the bow is on the rack exactly at the boat's 'quarter point' (1/4 the length of the kayak back from the bow), you would be lifting half the weight of the kayak, or 28 lbs., if you're lifting at the stern quarter point. If you're holding the extreme stern of the kayak, maybe your grab handle, and the bow is over the rack at the quarter point, then you'll be lifting with a force of 33.6 lbs, and the rack will be supporting 22.4 lbs.<<<<<<<<<<<<<< I agree with the first paragraph but not with the second. Could you explain how you arrived at your result? As I see it the paddler will only be supporting one quarter of the weight, 14 pounds. This assumes, as you did that the kayak is level and evenly distributed. If you are picking the stern up from the ground you will be lifting more than a quarter of the weight (depending on the kayaks angle). For instance, if the bow were straight up you would again be lifting all the 56 pounds from the stern. If one end was supported by the bar you would be holding half the weight. If the bar was in the middle of the kayak the bar would be supporting all the weight. With the bar one quarter along the boat you would be holding half of the half you were holding when the rack was at one end. One quarter of the weight. At first I thought the rack will be supporting the entirety of the bow quarter weight and one-half of the three quarters that is left. That calculation would leave the paddler supporting 3/8 of the weight. 56/4=14, 56-14=42, 42/2=21 pounds. Then I realized that the weight of the bow over the fulcrum of the rack would also be supporting another quarter of that kayak's weight due to the leverage. The last half of the weight would then be shared between the rack and the paddler. My original calculation would be correct if the bow was sawed completely off at the bar but not if the overhanging weight of the attached bow was levering up part of the rest of the weight. Matt Broze http://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/ ***************************************************************************
Matt Broze wrote: > Shawn Baker wrote: > > <SNIP>>>>>>Assuming the kayak's weight is spread evenly along its length, if > the > bow is on the rack exactly at the boat's 'quarter point' (1/4 the > length of the kayak back from the bow), you would be lifting half the > weight of the kayak, or 28 lbs., if you're lifting at the stern quarter > point. > > If you're holding the extreme stern of the kayak, maybe your grab > handle, and the bow is over the rack at the quarter point, then you'll > be lifting with a force of 33.6 lbs, and the rack will be supporting > 22.4 lbs.<<<<<<<<<<<<<< > > I agree with the first paragraph but not with the second. Could you explain > how you arrived at your result? As I see it the paddler will only be > supporting one quarter of the weight, 14 pounds. Close. Under the stated assumptions, i.e. level kayak with uniform weight distribution and being supported at a point at one end and another point 1/4 of the way from the other end, we have the following forces: 1) gravity effectively acting on the center of mass of the kayak pulling straight down with 56 lbs.; 2) a force, X, pushing straight up at the point 1/4 of the way from the bow; and 3) a force, Y pushing straight up at the stern. Since the kayak is motionless (no acceleration), the forces have to balance, so X + Y = 56 lbs. Also, since there is no rotation of the kayak about its center of mass, the torques from X and Y have to balance. That means that X = 2 * Y, since the force Y is pushing on an effective lever arm with twice the length compared to the force X. Together this means that 3*Y = 56 or Y = 18 2/3 lbs. [The problem with your analysis that led to 14 lbs. is that the stern half of the kayak is closer to the lifter at the stern than it is to bow support point, so he has to lift 2/3 rather than half of this portion of the kayak.] But Dave's point earlier about the angle of the boat is correct. When you start to lift the stern the boat is tilted up and this will substantially increase the amount of force that needs to be exerted by the person at the stern. *************************************************************************** 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/ ***************************************************************************
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:33:33 PDT