Ash Seats - How Strong?

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BillB
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Ash Seats - How Strong?

Post by BillB » Sat Oct 22, 2005 4:58 am

I have built the trapezoidal seats in Gil Gilpatricks book using 3/4" X 1-1/2" ash. The joints are triple doweled and they are strung with plastic cane. I am quite pleased with how they turned out. Here's my problem - I have a close friend and sometimes canoing partner who is a little "beamy" (Gilpatricks term). He weighs about 300 lbs. How reliably strong are these 3/4" ash seats? In hindsight I should have probably used 7/8" ash. When I've canoed with this fellow in the past we've always used aluminum canoes so the seat strength issue has never come up before. Are these the pre-launch jitters of an overly protective builder or is there some cause for concern? Thanks for your help. Bill.

Rick
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Post by Rick » Sat Oct 22, 2005 7:31 am

Probably the only way to know beforehand would be to cut a piece of test material and subject it to the 300-pound load. Supporting a crossmember on two 2x4s spaced the correct distance then standing on it with both feet to simulate sitting load will probably tell you something. Kneeling in a canoe might be an option if sitting proves to be too stressful, for the canoe and the builder...

:frightened

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Doug
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Post by Doug » Sat Oct 22, 2005 8:53 am

Ash is about as strong a wood as you can go.
But yes, the 3/4" and 300 lbs would be a concern to me as well.

Test "before" you use it in the canoe.
A failure in the canoe would be a bad thing.

All the best,
Doug

.
"Some people hear the song in the quiet mist of a cold morning..... But for other people the song is loudest in the evening when they are sitting in front of a tent, basking in the camp fire's warmth. This is when I hear it loudest ...." BM

BillB
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Post by BillB » Sun Oct 23, 2005 6:26 am

Thanks Doug and Rick. I do have some 3/4" X 1-1/2" ash left over from making the seats. I'll test it and see what happens. Maybe the best soltion is just make my friend his own seat out of heavier ash. Bill.

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davidb54321
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Post by davidb54321 » Sun Oct 23, 2005 9:17 am

Bill,

I made my seats from 3/4" ash using Martin Steps plans. I cut the stock wider for the "legs" of the bow seat, figuring the span would be greater, with more flex. I am hoping the extra material will make the seat stronger. The stern seat fits the canoe quite snug and should be plenty strong for my weight when paddling double. I also did not contour the seats as deep as Martin suggests, to prevent weakening of the joints.
David Bartlett

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Rick
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Post by Rick » Sun Oct 23, 2005 10:42 am

Bill,
Maybe the best soltion is just make my friend his own seat out of heavier ash.


Another way could be to reinforce the underside of the crossmembers with fiberglass, if cane material isn't in the way. The fiberglass layers won't be visible from above and should strengthen the wood considerably.

When a load is applied to a crossmember, the wood fibers are under compression on the upper surface, and under tension underneath. It will likely be the tension forces that cause failure of the crossmember at some load level, and an outside skin of reinforcing material should help overcome this.

Kevlar is the ultimate way to go in dealing with this since it has far more tensile strength than fiberglass, but some amount of fiberglass reinforcement should also work... my yoke's underside was reinforced with a layer of glass, for a little insurance, and just in case.

BillB
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Post by BillB » Sun Oct 23, 2005 4:56 pm

Rick - Thanks for the suggestion to reinforce on the underside. Unfortunately the cane is tied off on the underside and would get in the way. Bill.

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Patricks Dad
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Post by Patricks Dad » Sun Oct 23, 2005 5:47 pm

Bill,

I think Rick has a good idea. I would think that you could add a layer of glaas on the bottom of the seat frame before you drill holes for the cane and then re-epoxy the holes and surrounding glass after drilling. I wouldn't think that the glass would be in the way for caning... But maybe I'm missing something.
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BillB
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Post by BillB » Sun Oct 23, 2005 7:44 pm

Patricks Dad - yes I agree it is a good idea to fiberglass the underside. But the seats are already built and caned. If I do decide to build another seat just for my firend I think I will use Ricks idea. Sounds like a good way to add strength. Thanks for your reply - Bill.

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Its Me
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Post by Its Me » Mon Oct 24, 2005 8:03 am

Well I think you need to add something. A quick calc skows that at 3/4" and 30" span your 300lb buddy will overload the seat if he puts all his weight on one cross bar. Let alone if he drops onto the seat. You would have been better off to make the rails from 1.25" stock rather than 3/4' stock. You can risk it and say "I'll just build a new seat when he breaks the old one" but that could ruin a long planned trip and a friendship.

BillB
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Post by BillB » Mon Oct 24, 2005 8:19 am

Its Me - you have confimed my fears that the 3/4" seats won't hold up. Thanks. I'll most likely build the new seat. How did you calculate that 1-1/4" stock would be needed? Bill.

Riverrat
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Post by Riverrat » Mon Oct 24, 2005 11:18 am

I built the exact smae seats. I broke one on my second trip out. They were hung from the gunnels with 6 inch bolts. I weigh 220. Funny thing though, it didn't break straight across the cross member but along its length, splitting along the holes drilled for the caning. It was the front of the seat. I did notice that there was a lot of bow in the seat as I sat, so I'm not surprised it broke. I think I should have glassed the underside of the seat frame prior to caning. I believe it would have been stronger. I was able to epoxy the crack and add a brace under the seat frame ( both seats just to be sure) and solved the broblem...I hope.
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Jeff in Farmington, MI
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Post by Jeff in Farmington, MI » Mon Oct 24, 2005 2:40 pm

These questions about the strength of the seats made me think about the strength of the attachment to the gunwales. As a first time builder who will soon mount the seats, I'd be interested to read about any failures in the seat attachment.
There have been many discussions here about how to attach the seats. Any specific things to avoid?
Has anyone experienced a failure in the area of the bolt?
I attached my gunwales with only thickened epoxy. Would it be overkill to reinforce the gunwales near the seats (perhaps with screws joining inner and outer gunwales)?

Jeff

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Its Me
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Post by Its Me » Mon Oct 24, 2005 3:00 pm

BillB wrote:Its Me - you have confimed my fears that the 3/4" seats won't hold up. Thanks. I'll most likely build the new seat. How did you calculate that 1-1/4" stock would be needed? Bill.
I just did a quick calc assuming a point load of 300 ponds on a simply supported 30" beam 1.25x.75. S = 19,200 psi. Way too much for wood. But these assumptions are too simple. The true span is likely less, the size of your friend's arse would need to be measured and its unlikely that one cross bar would take all the load.

The 1.25 depth for a new cross bar is just a guess. This will increase the load carrying ability (section Modulus for those following in their hymnals) from .1172 in3 to .3255 in3.

S = M/z

The glass on the bottom as suggested would work too.

Build a new frame, including holes and test it before varnishing and caning.

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Post by Rick » Mon Oct 24, 2005 3:45 pm

Jeff,
These questions about the strength of the seats made me think about the strength of the attachment to the gunwales.
I used thin white pine inwales when building my Huron (saving weight) and bolts to hang the seats. I didn't know whether the softwood inwale would be able to handle the load, so I tested by gluing a piece of inwale material to plywood, and suspended the bolt the same as in the canoe. The single bolt and inwale was able to carry my entire weight when I set it up in a vise arrangement. Two bolts and inwales on one crossmember when sitting on the edge should be strong enough, and four bolts would be overkill when sitting centered on the seat.

I have a 35-year-old fiberglass canoe with the original inwales made from white spruce held on by screws alone, and the bolt support on the inwale never failed the entire time. A glued joint on the inwale should be stronger than with screws only, so I'm reasonably confident the Huron with it's pine inwales will be able to support my weight as long as it's being used. A hardwood inwale would be stronger, and heavier, and I felt that the pine inwale would be good enough for it's purpose. After a year of use, still no signs of stress...

:cool

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