I may have missed an analysis, such as I briefly outline below, in earlier posts. If so, hit delete now. But it seems to me that in the pursuit of this thread we may have lost sight of what tiedowns actually can and cannot do -- or prevent. Bow and stern tiedowns have to oppose forces in three general directions (I would hate to say planes since the load direction shifts as the boat starts to move ...): 1 - side to side (let that include yaw, as well as lateral translation) 2 - forward and aft 3 - up and down (both heave and pitch) Depending on the length of your car relative to the length of the boat, the angle of tiedowns may be more or less well suited to oppose forces in any of these directions. 1 - To obtain any semblance of preventing side-to-side motion, you really need at least split tiedowns each at bow and stern, four lines, two each running from the respective ends of the boat to the respective corners of the car ... IF you can find useful attachment points there. The angles between the lines and the forces involved keep this from being an optimum solution, however. 2 - Tiedowns are not well suited on the whole to preventing forward and aft motion due to the often near right-angles between the lines (more or less vertical) and the forces on the boat (more or less horizontal). Paddlers familiar with yachts and ocean going vessels tying up to a dock, may have observed the use of "spring lines." These lines run from the bow of the vessel to a cleat or other attachment point on shore near the stern of the boat and respectively from the stern boat to a point on shore near the bow. Such a set up prevents forward and aft motion more effectively than bow and stern lines by decreasing the angle between the forces and the lines. In a roofrack tiedown application such spring lines would obviously run from the rear of the car over the roof to the bow and vice versa. That's pretty much the only way to prevent boats shooting off the top of the rack (forward or rearward). 3 - Single line front and rear tiedowns are best suited to prevent up and down movement of the boat only. General - For all bow and stern tiedown situations I would ALWAYS run two straps of some kind around the hull of the boat, one each near the bow and the stern, and NEVER rely on the grab handles, lifelines or other weaker boat fittings. Of course I then run a line forward and aft to connect these two loops to stop then slipping off the boat! My conclusion to all of the above? I dread the day on which I have to start transporting boats on a roof rack that's not securely bolted to a solid raingutter like that one on my old Volvo station wagon! That would be the day I have to start creating the spider's web of tiedowns, straps and connecting lines described above, which I may then find myself truly advocating. Until then I'll continue to rely primarily on good quality rope ties from one end of each roofrack bar to the other, hauled down tight with a truckers hitch. I will sometimes rig a bow preventer to limit the extremes of any possibly forward and aft motion (i.e., the boat might in the most extreme case somersault if I were to hit someone from behind and the forces rip off the rack, car raingutters and all, but it will not shoot through the rear window of the impacted vehicle). But then again I may have a slight advantage in my approach since I almost exclusively strap on relatively flexible folding boats, which by the tying down process become almost an integral part of the roof rack system through (elastic!) deformation and friction ... I could not vouch for hardshells under similar circumstances. And thus concludes another veiled pitch for the concept of folding boats ... :-) (Tongue now back out of cheek!) Best regards, Ralph Ralph_at_PouchBoats.com www.PouchBoats.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/ ***************************************************************************Received on Tue Apr 23 2002 - 08:10:58 PDT
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