Thoughts on load capacity of canoe (and stress on seats)

Welcome to the new Bear Mountain Builders Forum - an interactive internet service we provide to encourage communication between canoe and kayak builders
Post Reply
User avatar
doe4rae
Posts: 103
Joined: Fri Nov 13, 2009 12:09 pm
Location: South Dakota, USA
Contact:

Thoughts on load capacity of canoe (and stress on seats)

Post by doe4rae »

Hi,
Quite a few people have started asking me if I will take them for a paddle in my new canoe when it is completed. As the question is posed I find myself sizing them up - wondering if their weight is "suitable" or if I can trust them not to do something stupid (get in it before it is in the water etc.) I have begun to worry myself with the numbers printed on my plans that state the "optimum weight capacity" which is 150 pounds to 350 pounds. Obviously 'optimum' means a little over or under this is not out of the question but I am still wondering how much the canoe can handle. What is the MAXIMUM capacity I should risk? I am 140 lb. but my husband is 220 lb.. Does this mean I have to leave the cooler and camera gear at home? I intended to reinforce the bottom of the canoe with a second layer of 6 oz glass but ruined it by cutting the material wrong. Side affect of working too late into the night. Anyway... I have 6 oz. on the inside and outside and that is it. Has anyone ever stood up in their canoe on the water and had their foot go through? How likely is that to happen? (I used to stand up in my aluminum canoe on occassion -- and obviously you have to step on the bottom to get in at least for a little bit.) As you can see I am adept at predicting disaster.
:frightened

The second thing I am wondering about is seat failure. I watched a fascinating video on different kinds of joints and what kind of force they can endure before failure, so I am pretty confident in my mortise and tenon joints on the seat frames. The link for that video is here if anyone is interested.

http://www.youtube.com/watch?v=DhLfb7m9 ... re=related

I doubt I could ever encounter anything that will put 5000 pounds of stress on my canoe seats! I think the frames will hold. However, The weight hanging off the inwales does concern me a lot. Has anyone ever ripped an inwale off their canoe when they sat on the seat?
Dawne Olson

"The human soul needs actual beauty more than bread" ~DH Lawrence
User avatar
Adamv
Posts: 109
Joined: Fri Mar 04, 2005 1:40 pm
Location: Owen Sound, ON Canada

Post by Adamv »

Hi Dawne,

I wouldn't worry too much. I've stood in my 15' Bob's Special (6 oz cloth in and out) a few times (in the water), not even thinking that it could be a problem. No problems.

Maximum weight? On the day I launched it, I tested it with myself and three other grown men in it at once. It had to have been over 700 lbs, and she still floated pretty well. I don't know what the carrying capacity is, but I'm guessing we were over it. I've also done trips with another paddler, and about 100 lbs of gear (around 450 lbs). Again, no problem. Actually, it paddles quite nicely.

Cheers,
Adam
Who travels not by water knows not the fear of God --- 17th Century Sailor
Rick
Posts: 727
Joined: Thu May 27, 2004 9:23 am
Location: Bancroft, Ontario

Post by Rick »

Dawne,

If you aren't wearing spike heels, your cedarstrip hull will be fine. Soft-soled footwear will be best.

The bolts supporting the seat will support heavier paddlers in your canoe... I tested an inwale section epoxied to a piece of plywood and the single bolt supported my entire weight without the soft pine inwale splitting off. Four bolts' worth of support will be plenty.

The bolt head should be located close to the glue joint where the inwale joins the hull, for max strength (this only applies if your inwales are much wider than the bolt head).
User avatar
doe4rae
Posts: 103
Joined: Fri Nov 13, 2009 12:09 pm
Location: South Dakota, USA
Contact:

good news

Post by doe4rae »

Well, I am very glad to hear about the 3 grown men taking a test run ... this will put my husband's mind at ease too. He has been skeptical as well if it would hold his weight!
Dawne Olson

"The human soul needs actual beauty more than bread" ~DH Lawrence
Markham_David
Posts: 47
Joined: Sat Feb 26, 2005 2:12 pm
Location: Markham, Ontario

Post by Markham_David »

Dawne, check out pages 5 and 6 of Kayakcraft for some scientific test results. It can be found at:

http://books.google.ca/books?id=KlbM8d1 ... q=&f=false

Cheers,

Dave
User avatar
doe4rae
Posts: 103
Joined: Fri Nov 13, 2009 12:09 pm
Location: South Dakota, USA
Contact:

Post by doe4rae »

Thanks for the link to the article. Those numbers are interesting but I'm still not sure I understand how they relate to the strength of the canoe. These tests were performed on a 12" square sample of cedar and glass. This quote from the link you sent is something I think I also ran across in CanoeCraft: "We found that 1/4" cedar sandwiched between layers of 6 oz. glass cloth, the proven schedule for canoes, failed when subjected to a load of 221 pounds. An additional layer of 6 oz fiberglass, as might be applied to a canoe's bottom, increased the failure point by over 100% to 450 pounds."

When I originally ran across this information I took it to mean that this was the maximum amount of weight the canoe could bear on the water! That's why I was so disappointed when I ruined my extra layer of fiberglass for the bottom of the canoe. It seemed without it I was risking breaking the canoe if I put more than one person in it.

I liked this article because it clarified the size of the test panel. In CanoeCraft it used this similar wording but I couldn't make out what was intended by "failure". This sentence alone was what concerned me about the load bearing capacity of the canoe. It seemed like it was saying that if a person of more than 221 pounds stepped into the canoe then it might break. I like the results of Adam's test of loading his canoe with 3 guys and Rick's inwale test.

If I'm to understand it correctly, the results of that test are for a panel that is not supported within the monocoque structure of the canoe and is not in the water. The reason that same square foot of 1/4" cedar between two layers of 6oz glass WON'T fail beneath 221 pounds in a cedar strip canoe is because it is on a cushion of water. Is that correct?
Dawne Olson

"The human soul needs actual beauty more than bread" ~DH Lawrence
sedges
Posts: 325
Joined: Fri Jul 09, 2004 5:37 pm
Location: georgia

Post by sedges »

That would be correct. I would not worry about strength of your hull. If you dropped a 50 lb rock in, even in the water it may result in a hole. A person moving around in the boat while afloat is not an issue.

I watched the youtube video testing joints. Do not be misled by this. The way they tested the M/T joint was not the way it would be stressed in a canoe seat. In a canoe seat the cross pieces simply hold the two rails parallel and create a place to attach the seat filler of choice. All the stress is on the rails and whatever they are hung from. The rails most commonly fail at the cross piece joint because the rail is weakened in creating the joint. If there is any grain runout at the joint, especially on the bottom face, failure is more likely.

I have not used M/T joint on my seats. I used simple lap joints with the lap being 1/3 the thickness of the rail. I have never had a seat fail, even with a 280 friend using it. I was alarmed by the amount of bow in the seat, but it held OK. Now if that person slipped a little and came down hard on the seat there is no doubt it would fail.

You made the seat and can easily make a new one if it fails. On the other hand, having a seat fail is often a huge embarrassment to the person who breaks it. If you are likely to have heavy folks in your boat make the seat stronger to start with.
User avatar
doe4rae
Posts: 103
Joined: Fri Nov 13, 2009 12:09 pm
Location: South Dakota, USA
Contact:

Post by doe4rae »

I agree, many of these tests (especially the ones in the video link I posted) are not really telling of what your particular construction will endure or is capable of. In the case of my seats, I am more concerned with the inwales breaking from the weight of the person on the seat than I am with the actual seat breaking.

Can you clarify what "grain run out" means in your description of potential for failure on the rails. (My inwales are maple)..... ok (3rd time editing this response)..... I now understand you are talking about the seat frame "rails" rather than the inwales when you mention grain run out and it makes sense that carving the mortise in it will make it weaker at that point but I still don't know what grain run out is. The seats are hard maple also and 3/4" thick X 1 1/2" wide. It seems the real issue is the size of the mortise walls on that rail though and I would say they are probably 1/4" thick by 3/4" wide.
Dawne Olson

"The human soul needs actual beauty more than bread" ~DH Lawrence
sedges
Posts: 325
Joined: Fri Jul 09, 2004 5:37 pm
Location: georgia

Post by sedges »

Maple ought to be pretty strong.

My gunnels are biggest inside about 5/8 X 5/8 and smaller on the outside 1/2 X 5/8. Screwed from outside every 6 inches, not glued. I chose to screw from the outside to preserve as much strength of the inside gunnel as possible. I have never had a gunnel fail at a seat or thwart. I use dense grain structural grade southern yellow pine with very straight grain. Nice stuff, by the way, that comes in long- to 24 feet-clear boards.

Image

The image shows grain runout. Note the top piece has grain that is almost perfectly parallel to the edges/faces of the board. In the bottom piece the grain "runs out" the top and bottom edges of the board. This is where a crack would begin when the board is under bending stress.
Tom in MN
Posts: 162
Joined: Tue Feb 22, 2005 11:42 am
Location: Eagan, MN

Post by Tom in MN »

If your seat it going to break, it will break when the person sitting in it plops down hard on it. Whenever anyone is getting into my canoes, I remind them to put both hands on the gunnels and sit down gently. Once my wife, with baby in arms, was getting into my canoe, unable to hold the gunnels she dropped down and I heard a crack. Now, this was my first canoe, I did not have the best woodworking equipment for making a mortise and tenon joint so I half lapped the joints. Poor choice on joint, maximum stress on seat with direct force of person with extra weight (baby). My wife is not that big, 140 lbs, but with baby and extra force, too much on the joint. So, expect the unexpected. Definately use M&T joinery. If your worried about it, rip a strip of hardwood about 1/2" x 1/2" and glue it on the underside of the widest part of the seat frame (bow seat would be the back seat rail) as a stiffener. This will prevent sagging and reduce stress on the joints. Keep it back away from the holes for the cane and alter the thickness if need, but most seats would be 2" stock so plenty of room for this little extra piece. I put a lot of middle seats in my canoes, so I use this method there and that really helps because of the span of the seat being in the beam area.
User avatar
doe4rae
Posts: 103
Joined: Fri Nov 13, 2009 12:09 pm
Location: South Dakota, USA
Contact:

just what I was wondering...

Post by doe4rae »

Before I checked replies this morning I was thinking to myself "I wonder if I could add an extra strip of wood under the seat rails to reinforce it?" So Thanks Tom for answering that question. My boys were holding the edges of the seat rails with my daughter sitting on the seat last night as a "test" and I wondered even with her slight weight (105!) if I saw a little bowing of the seat rail.

A picture is worth a thousand words. I can see what you mean by grain run out now. Of course my seats are already made so I am not sure if the grain runs out along the edges or not. I'll be checking that.

My inwales are 3/4 x 3/4 and I am planning to screw the outwales every 12 inches (basically centered between the scuppers) and epoxy them for the same reasons as Sedges mentioned. I thought that some screws going into the inwales from the outside might give a little extra insurance against pulling off. Maybe so few screws are not going to make any difference if the epoxied surface is going to let go.
Dawne Olson

"The human soul needs actual beauty more than bread" ~DH Lawrence
Rick
Posts: 727
Joined: Thu May 27, 2004 9:23 am
Location: Bancroft, Ontario

Post by Rick »

Dawne, maple is usually strong and flexible, and the seat should be able to take some flexing without breaking. An easy test is to take a piece of maple the same dimensions as the crossmember, place it on some blocks spaced the right distance apart and then give it some weight. Sitting, and standing on it with both feet so that the weight is distributed the same way, will show whether the crossmember can take it. There will be two crossmembers in the seat, so this is a "worst case" scenario.

Some flexing in the seat is normal and IMO it adds to the comfort of the canoe... I made a seat too strongly built once by glueing on an extra piece underneath and that felt like sitting on a block of concrete.

Maple rails (inwales and outwales) should be more than strong enough... I used soft pine for inwales with no problems even though pine is much weaker (but lighter for saving weight). Fastening the outwales to the inwales with screws won't hurt... you could add screws on either side of each drop bolt for insurance..

Grain runout, where the grain does not run exactly parallel to the dimensions of the board, can weaken it. Still, with hard maple the runout that Sedges showed would probably not weaken the seat crossmember too much, since most of the grain still runs more or less the length of the board.
User avatar
Notnats
Posts: 1
Joined: Fri Feb 05, 2010 3:46 pm
Location: Dell Rapids, SD

Post by Notnats »

Dawne
What canoe are you building, and where in SD.
User avatar
doe4rae
Posts: 103
Joined: Fri Nov 13, 2009 12:09 pm
Location: South Dakota, USA
Contact:

Hey South Dakotan...

Post by doe4rae »

.... Great to see another SD person in the forum! I've been through Dell Rapids. I live in Vermillion. This is my first boat/woodworking project and I got the plans from Bear Mountain Boat Shop -- "The Cottage Cruiser".

There is a link to an online photo album of everything I've done since mid-November here: http://picasaweb.google.com/olson.dawne ... erWinter09#
Dawne Olson

"The human soul needs actual beauty more than bread" ~DH Lawrence
Post Reply