niswanger wrote:
Jim_MI wrote:
niswanger wrote:
4" and even 3" PVC seems awful big to me, any reason why 2.5" dia PVC wouldn't work well? I plan to make bunks as well with 3 cross members about 4 feet apart.
Flexing of the PVC pipe is necessary and desirable in order to distribute the load evenly across the scupper lines of the hull. The pipe should fully conform to the rocker-bottom shape of the hull, or else the hull will deform to the shape of the pipe! I would therefore strongly recommend against using 3 cross struts, as this will prevent the PVC from fully flexing in the mid section. Ideally, you want to raise the fore and aft cross members sufficiently so that the fully flexed PVC does not "bottom out" in the mid portion. I have my PVC mounted on top of 2x6's (on edge) and the mid portion does not hit the trailer frame when travelling.
Alright, just ingesting more information as I move forward with a design. Interesting...to me it seems if you were to support a kayak of considerable size/weight/length (13'+) only on the ends it would gradually sag/deform like a banana right? So then for a PA14 and Revo13 what would be the ideal spacing for the cross members to support the bunks? I only have 8' to work with on my trailer, so in this case if I put a cross member 8' apart on the PA14 it would be ~3' in from each end (if I placed the PA14 evenly).
Would the ideal build actually have a center cross member that's offset a bit lower to allow natural contour support? If so has it been discussed on how to achieve this? The idea, I would think, would be to mimic the profile water provides (as it floats). When you say "conform to the rocker-bottom shape of the hull" is this the profile as viewed from the side of the vessel or the end (cross-section)?
Jim_MI wrote:
I have my PVC mounted on top of 2x6's (on edge) and the mid portion does not hit the trailer frame when travelling.
You mean on edge meaning the PVC rests in cut-out semi-circles and raised at least 4" (because 6" is really 5.5" and then you cut out 2" semi-circle) above the trailer frame? Wow, I couldn't imagine my kayak actually sagging 4" in the middle seems support would be needed there?
Finally, I took 2.5" and 3" PVC home to test the geometry on the hull channels (are these what you call scupper lines?) and the 2.5" fit the channels a lot better than the 3". I feel you answered this as needed larger PVC for more strength between using only 2 cross members so if I were to only use 2 cross members 2.5" might not be enough strength?
I'll get there...
As far as the rocker-bottom profile, I was referring to the vertical curve AND the slight horizontal curve of the right and left side concave parts of the hull - where the scupper holes are. Ideally, your pipe should flex both vertically and horizontally to match that oblique-banana curvature of the scupper line. Sliding a 125-lb hull onto pipe that is tightly bolted in place at both ends does not permit that very well. It may be over-engineering, but I used telescoping full-length sleeves of PVC; 3" OD sliding inside 3.5" OD. They sleeve together almost exactly but still permit sliding, allowing each bunk to grow a bit in length as the vertical and horizontal forces of the hull displace them. The 3" pipe is bolted at the front, on top a full-width 2x6 cross piece (no cutout) and the 3.5" pipe is similarly bolted at the rear. This theoretically helps distribute the load better, but honestly the "growth" between loaded and unloaded bunk length is probably only an inch or two. You can fine-tune the displacement by adding additional lengths of 2.5" PVC inside the 3" pipe to stiffen areas, but these cannot be pinned in place without defeating the telescoping design. I also used 3.5" 30-deg elbow bends at the stern end of each bunk, adding about 12" lengths of downward-facing pipe. This mod helps get the yak aligned for loading and permits easier retrieval without backing the trailer in quite as deeply as would otherwise be necessary. And finally, glue carpet strips along all pipe surfaces that contact the PA.
I keep my PA14 on the trailer in my garage, against Hobie's recommendations, but have experienced no hull deformity issues with this setup over several years of use.
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