PVC Bunk Engineering Question

I ride on some rough roads and wanted to further smooth out the ride on my HF 1195-lb folding trailer. So I just started by removing 2 of the 3 leafs on each spring, as others have recently suggested. Tire pressure is already at 15 PSI. I had originally built the front and rear bunk supports up a bit above the HF "deck" by raising them up on 2x4's set on edge, thereby forcing some bend in the middle for better fit to the hull contour. I also used a telescoping arrangement of 2-1/2 PVC inside 3" PVC, each screwed to the 2x4's at only one end, so that the bunks would grow or shrink in length to dynamically fit the hull both horizontally and vertically while moving. After all this, I realized that the outer 3" PVC is resting on top the center cross piece of the HF trailer anyway, even at rest, meaning that every road bump is being directly transmitted through to the PA hull. Before I reflexively swapped the 2x4 risers out for 2x6's in order to get the bunks up off the HF cross pieces, I thought I would review some other forum members designs first. I was surprised to find that most pics just show PVC mounted to unistrut or similar cross-pieces across the full length of the PVC, and then everything strapped down to the trailer frame.

Question 1: Doesn't this arrangement pretty much guarantee that only the center part of the hull will be supported, and that every vertical bump the trailer bed makes will be a direct hit to that part of the hull?

Question 2: Where the PVC is forced to contour to the hull rather than the trailer frame, shouldn't the PA be strapped to the PVC (rather than to the frame), in order to take advantage of the springiness of the PVC?

I thought I was over-engineering this before but find myself on the verge of making further mods to get the PVC completely off the trailer bed. Not necessary?

To answer question 1: if the boat is strapped down correctly then the boat and trailer will act as one when going over bumps, so nothing should be"hitting" the hull.

To answer question 2: even if the PVC is contoured to the hull it should still contact the trailer frame evenly over the length of the frame. If you have excessive bowing then you probably need to add some cross-bracing. If you strap the boat to the PVC and it has an excessive amount of flex you risk breaking the PVC. So, I would recommend strapping the boat to the trailer frame and not the PVC.

IMHO if you have a nice flat bed to mount the PVC to use that. My trailer is a galvanized version of the trailex (single center strut with cross braces). So, I used unistrut to make cross-bracing. Here is my setup for my Tandem Island.


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My original idea and implementation was to support the PVC at the far ends, only, so that the center could deflect and conform to the hull. Straight rigid lengths of PVC defeat the purpose that the idea was developed for.



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That's exactly what I was trying to do - kind of use the PVC as an independent suspension. Tom, do your PVC tubes support the full weight of the PA without contacting the trailer on road bumps? If so, is the PA strapped to the PVC (allowing the yak to float up and down almost independent of the trailer) or are you putting extra tension in the tubes by strapping the yak to the trailer frame?

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This video was shot in January of 2012. I think we built the trailer in 2010 or 2011. We trailered these boats a few thousand miles and the hulls remain good as new, even today.

https://www.youtube.com/watch?v=WPd5n83lY5k

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Rigid rails are the same as mounting on floor. If you use timber cross supports, particularly if there is a center support taking most of the load, they will have slight flex and can set up a seesaw action as a trailer goes over bumps. Hence rigid cross members are better.

If you have a curved rocker on the bottom of a hull you can pack the heights of crossmembers if required to maintain maximum contact with hull profile.

Keep in mind if the tubes are flexing in transit so will be the hull.

If you carry different yaks on the same rails then you may need flex to accommodate variation, vertical and horizontal, but if always same yak, then fix two outer crossrails, load yak on and set height/spacing of any intermediate supports to tailor to profile of that yaks hull chimes.

I'm not going to go into all the mechanics of the system, but no, the boat and PVC tubes don't move. The tubes stay conformed to the hull under all conditions. The trailer suspension handles the "bumps." The hull itself is never under any stress nor required to bend, indent, etc.

Having first carried my older 2010 Outback on the parallel PVC pipes and now my 2013 PA-14 for the last three years, I completely agree with Tom's analysis of the carrying system.
The trailers suspension and lower/reduced pressure tires absorb the bumps and jiggles on the road.
My single leaf spring, Hobie Trailex trailer's suspension works like it was designed to do. The photo below shows my trailer set up with the 2.9" OD parallel PVC pipes supported by the cross bars spaced about 67" apart.

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Dr.SteelheadCatcher
Warrenton, OR

Yep rigid cross supports are the go then nothing moves, except the suspension. You would be surprised just how much a timber cross section will flex, thats where you get problems. The yak will clamp the PVC still. If you only have two cross supports then most of the load will be at those points. the rest of the tube merely helps with loading.

First time I attempted it with timber, yak nearly bounced right off the trailer loosening the bolts attaching the timber to the trailer and they where 5 x 2 1/2" hardwood. Having the center span made it worse rocking back and forth.

2in flexes better... U can carpet the pvc for better shock absorption


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I am of the opinion that the suspension including tire pressure on your trailer should be responsible ride quality, and that the bunks should be used for securing and supporting your kayak on that trailer. If you need to use the bunks to augment the suspension of the trailer, then you might want to look at adjusting that suspension.

I worked very hard on increasing the weight, adjusting the leaf springs (removing 2 of 3) and finding the best tire pressure based on recommendations and trailer weight. After doing that, my trailer rides like a dream. I have over 1500 miles on it in the last 30 days. My kayak is strapped well to the frame, resting on the bunks with some lateral support provided by Rhino-Rack Saddles. I also store my kayak on my trailer, but loosen or remove the straps during storage. All trailer will bounce given the right - or wrong - conditions - but overall my setup works incredibly well - for me.

Dr.SteelHeads trailer was DESIGNED for the lighter loads of kayaks, my Harbor Freight trailer was designed to carry much heavier loads and required a lot of work and experimentation to get to the point I am today. I am hoping that the lessons I learned can be used by others right from the start. (But if you want to spend less time screwing with your trailer and more time fishing...go DR's route! I on the other hand really did enjoy the heck out of this build and learned a LOT more about trailers than I EVER wanted to know.)

Here is a picture of my trailer prior to adding a ladder rack and cargo basket (added the rack and cargo rack both to add weight and allow me to carry a second or third kayak).



Here it is with the ladder rack and cargo basket installed.

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STLKayak
2020 Pro Angler 14 360
St. Louis, Missouri
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Nice setup!

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I opted to keep my 2x4 pine cross members. These rest full-length on steel framing at the front and back of the trailer bed 8-ft apart and therefore should not flex or pinch the hull as was suggested. Due to the sag in the middle of the 3-in PVC, which under resting boat load impinges slightly on the metal trailer mid-frame cross member, I reinforced the 3" PVC by inserting full length 2-1/2" PVC inside, and then reinforced further with another 1-1/2" PVC inside of that. This arrangement still conforms to the hull shape but is a bit stiffer than the single 3" pipe. I also split a pool noodle and padded the center cross member of the trailer so that any bottoming-out of the reinforced PVC bunks at that low center point would be softened. So far, so good.

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With the Rhino-Rack saddles on mine, plus the 3" PVC I do not see any bottoming out, except when launching my kayak. If I am launching at a particularly steep ramp, the deepest V of the bow wants to strike the main Uni-Strut cross member, between the bunks when I push it into the water. The water raises the stern and forces the bow down. I am actually building a boat roller to fit in the center between the PVC bunks, that will attach directly to that Uni-Strut cross member.

I can usually avoid this by just lifting up the bow when launching on these steep ramps, but at 5am in the dark, before I have had enough coffee or nicotine, I sometimes forget...right up to the moment I hear the thunk of plastic on metal.

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STLKayak
2020 Pro Angler 14 360
St. Louis, Missouri
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STL - Yours is a thing of beauty, but the fundamental structural difference between your design and mine is the base platform. Your bunks are supported at front and back, 4 ft apart. Mine are supported at front and back, 8 ft apart. Consequently there is more bow in the middle of my PVC tubes than in yours. With a plain schedule 40 3" fibercore PVC tube, the weight of the PA14 caused the center of the tubes to rest upon the center cross member of my trailer bed, even when elevating the front and back by the width of a 2x4. That problem is what I was trying to alleviate by either stiffening the PVC (done with additional inserts, like adding leaf springs) or by further elevating the front and back on 2x6's. My goal was to spread the gravitational loading across the entire 8 ft length of the bunk, thereby minimizing any "hot spots" that could lead to hull stress.