I picked up a wenonah aurora last fall. It was an experimental build following the discontinuation of royalex. It is tuff weave but has no gel coat. It also has no foam core or ribs. It oil cans pretty severely. Would adding a foam bottom or keel of some sort fix this problem? And how?
The foam by itself will do nothing other than adding weight. Foam cores work by having to rigid skins with the foam keeping them apart but attached. With only one skin you’ve got half of the equation.
Foam ribs covered with glass would likely be a better and easier solution.
cross-rib or center rib
before the popularity of Royalex, Wenonah had two layups that were not as stiff as the cored layups used today. For whitewater they used a center-rib design with one long foam rib running down the keel line of the canoe. This rib was glassed into the hull; and shock absorbers ran from the center-rib up to the heavy duty thwarts. The other lay-up that was very popular was cross-rib. Short ribs were glassed into the hull bottom and ran from side to side. The hull did not flex under normal paddling, but would flex as the hull ran over objects in the water. I have a 1983 Spirit laid up this way and its had tons of abuse. I have reglassed two ribs that were cracked when the hull flexed upward over 4" going over a rock. And the canoe was almost 30 years old when that happened.
The center rib design would be hard to duplicate into a hull. The long foam rib would be hard to source and also the heavy duty thwarts. A cross-rib modification could be done to an existing hull since the ribs are glassed in over the hull layup. You would loose the strength and low added weight of the factory vacuum bagging, but it would eliminate your oil canning, some of which might have been intentionally engineered into your canoe to test its suitability for whitewater.
How did you get it?
How much does it weigh? I would imagine it’s fairly light without gel coat.
I had a Sundowner 17 in tuff weave w center rib from early 90’s.
Without seeing how it flexes it wouldn’t be helpful to recommend a particular reinforcement design but I see no problem doing it with cut foam and 6" wide 9 oz glass tape and epoxy. Spray paint it to protect the epoxy from uv
It is 55 lbs. Indeed the idea was to attempt to make a downriver canoe without using royalex. With two people in the canoe, it only flexes in the very center and it isn’t that much. It seems like adding some cross-ribs would work. It is clear to me how those cross ribs would allow for some flex. It is less clear to me how they would reduce the oil canning.
My other idea is to rig up one of those struts Wenonah used to make, without the center rib. I’d simply place it under the thwart. I’d only use it on lakes, and remove it for rivers. Thoughts?
I’d be inclined
To just put a rib down the center to prevent the bottom from buckling up. Maybe 8’ long X 3" wide w 6" tape over it.
The worry about putting a strut from Thwart to keel without the center rib is concentrating the load on the keel. Put something under the strut to spread the load over a larger area of the hull; maybe a piece of wood15-18" long and 4-5" wide.
Cross-ribs will stiffen the hull so the bottom won’t oil can, but the hull will flex at the bilge area under impact.
the Center-rib design does perform similarly, but it offers continuous re-inforcement from under the bow seat to just under the stern seat.
When you pass over a big obstacle (log or rock) in a cross-rib boat, the hull flexes upward between each rib, as the concentrated load passes under the hull. In a center rib hull, the whole bottom flexes upward and the load is spread bow to stern, not raising as much as the more localized load of a cross-rib.
Maybe I was unclear
No strut from thwart to keel, just a longitudinal center rib like the original
Thank you for the response. I was out of town for a while there and then got distracted.
So you think just an 8 foot strip down the middle would be sufficient? And you mentioned that the original had that design? That seems very doable.
But I want to make sure I get the bottom to the point of very little oilcanning. Would it be wise to extend the strip all the way forward and backward?
Finally, I’ll probably use Divinycell with 6 inch tape. Any recommendations on the thickness of the diviny and how many layers of tape?
The center rib construction in 80s
Wenonahs included both the center rib (about 2"x2") running about 10’ long and 3 spring loaded “pogo stick” struts from the center of the three thwarts to the rib. without the pogo sticks a center rib needs to reach narrower (stiffer) areas of the bottom and be stiff enough to transfer that stiffness to the wider center of the boat. I think from experience with ceenter rib Wenonahs in the 80s that without the pogosticks, that would require a rib quite long and tall through the center and possibly tapered to the ends. you can turn the boat upside down and push on the bottom to see where it might be stiff enough to share the load.
I have successfully stiffened a couple of royalex canoes with crossribs and because you indicate the oilcanning area is small I would lean toward adding maybe 3 ribs in the effected area. Center ribs are unhandy for kneeling if they extend under the seats.
crossribs in the center of the canoe dont interfere with feet, knees, or large center air bags.
for crossribs in royalex I used 3", 20 oz unidirectional fiberglass tape and epoxy resin at 18" on center with no foam or other core. The royalex provides maybe 3/8 inch thickness at the bottom and that was enough. The advantage of unidirectional is that all the fibers run in the direction where stiffness is needed. Im not sure it would hold up on the sides of a tall rib where some shear occurs. The resin might crack between the fibers.
In your application I would try one rib with 0 to 1/2" core centered in the soft spot first and go from there. In the early days of Kevlar, solid bottom canoes with too much flex sometimes got another layer in the affected area.
There are I think some pics or video on the Sauris River website of their flexible ribs under construction. Those might also be instructive.
Upon further review
I looked up the structural formula for deflection and found it is proportional to the cube of the span. If that center rib spans 4 feet the deflection will be almost 2 and a half imes that of a 3 foot cross rib. 64 to 27.
I have decided to install 5 cross ribs in a manner that basically looks like the original wenonah cross rib design and somewhat like the souris river canoes. This decision was made because I think I can actually accomplish the task. Center rib would be too difficult for me.
I’m sure others have asked similar questions, so if its easier to just point me in the direction of previous conversations, please do so.
My questions is, where can I get small ‘samples’ of divinycell or similar foam core products. As I’m putting in 5 ribs that will be around 2-3 inches in width, I won’t need a full or even half sheet and I’d rather not buy one.
I’ll use 6 inch fiberglass tape. Any advice/specifications that I should be thinking about? Also, how many layers will be needed?
Finally, what type of epoxy/resin.
Thanks in advance for any help.
Jam a foam slab under center thwart
This is simple, quick, inexpensive, removable, and requires no gluing or glassing.
Essentially, this is what whitewater boaters have used for decades as their center seats: foam saddles jammed under thwarts. They are used in Royalex and composite whitewater canoes. The foam will compress only slightly upon hull impacts, but will do so more than a solid strut would. Plus, if the foam slab is about three feet long and six inches wide, under-hull impacts will be spread along a wide area. Whitewater boaters with foam saddles hit a lot more rocks than a flatwater boater will, and no one worries particularly about it.
You can simply remove a jammed foam slab when portaging.
Finally, a central foam slab will add flotation to the boat, which works nicely with end flotation.
very serious dilemma.
Tampa canoe shop tried selling me a GTS, I had ordered a Titan, with a flexible layup. FLEXIBLE.
usual criminal activity.
bulkheads of 2" foam covered with glass in glued in where possible: 2-4-5 ! cardboard templates.
then if the hull moves add glued down n covered with glass foam ribs.
bringer back solid.