Understanding forces on a tin boat

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adam_mac84

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When I see people decking their tin boats. There is always conversation of structure provided by seats.

Is there consensus on what forces on the boat we are supporting with benches/bracing. Is it a compression of the sides inward from the water displacement? Or are the benches in tension to keep them from folding outward? I am no engineer. But would like to have a good working knowledge of what I am trying to support.

Does this question make sense?


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The way I see it is the sides need to be rigid because flexing causes fatigue and subsequently failure. Lot of guys that remove seats, install rod boxes or similar storage that lends rigidity to the sides.


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Some boats are designed with the seats incorporated into the structural integrity, some are designed so that the seats are just that seats and, they can be removed without any problem. For the most part, most of the time a seat can be removed without causing the boat to fail however, there may be some flexing after removing the seat. Flexing doesn't necessarily mean the boat will fail but, in the case of say a riveted boat hull it could add to loosening of rivets and such.

I removed the middle bench seat from my 1448 and the boat had good bracing incorporated into the hull, it flexes slightly but, not really noticeably, it is one of the best thing I did to the boat.
 
Removing ALL the structural supports of the topsides (sides of the boat), to the other side and/or to the floor of the skiff or the installed floor … WILL cause the boat to flex at the point about 1/3rd back from the bow.

I have seen many boats, even ~20’+ fiberglass center consoles (CCs) that were used in saltwater (hence used in offshore seas) with extensive damage in that area.

When the hull slams into a head sea at speed, the water is hard, even though the water gives away some. Without structural members to SPREAD the stress loads across the hull, it isolates at the gunnel about 1/3rd of the way back from the bow and the hull and sides WILL flex, bend and crack at those points. The hull is essentially trying to collapse on itself.

Remember too the basic construction theory behind a tin, riveted boat ... tie it all together so the loads/stresses are spread out amongst a larger area. But removing a key piece, you could be inadvertently localizing all stresses into one spot. That’s where I have seen damage firsthand on boats.
 

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I did a quick paper boat..

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notice what happens to the side walls when the boat is folded... forces similar to what could happen hitting a wake for example:


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this is why you need to add back something that will keep the sidewall straight...
 
The style/shape of the ribs and how far up the side they go can tell you a lot. On almost any older tin boat with the short, rounded ribs that also only go up a few inches on the sides the seats will be structural. Boats with square ribs that are 1.5-2" tall and extend a foot or more up the sides typically have seats that are there more for a place to house flotation foam.
 
You can always deck on top of the benches. Saves you from building a lot of frame work to support the deck, and you don't have to worry about replacing the foam that they're usually filled with.
 
Look at boats with a walk-through where there’s ‘no seat’ straight up the middle. They usually have a large ‘L’ shaped structural member connecting & tying the ribs/floor to the topsides, to prevent oil-canning.
 
When you go used boat shopping do you even consider looking at a boat that has been hacked up in this way? Me either.
 
Also keep in mind the longer the boat the more leverage you can impart on the hull, a short boat is less likely to have the same stress issues a long boat will have.
 
Good conversation. Thanks for the pearls of wisdom. I enjoyed the paper boat!!! I will likely build off the seats as a mounting point.


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Didn't do it for this reason, but I paneled over the inside of the gunnel ribs. That should provide strong box section.

Not that it matters, I don't run in any more than a 2 inch chop.
 
Unless you see some pretty extreme conditions I really don't think it is that big of an issue in most cases with smaller boats and usually in a smaller boat you should be avoiding operating in extreme conditions anyway. A longer boat say 16' and up you need to be a little more careful and make sure the boat has adequate bracing. Side storage boxes are a good addition and usually don't take much away from the floor space, plus you don't have to step over that darn bench seat all the time.
 
My experience would tell you that you would be surprised and how much rigidity the floor / seats / console add to a tin boat.
I bought this boat as a project..

DSCF0516.JPG

The floor was trash with holes and rot.

As you climbed in while sitting on trailer you could feel the gunnels give slightly..

I gutted the floor/ rod locker/ console/ Seat base/ bow plate...

20161212_114802.jpg

The gunnels would move a good inch as you climbed in and out ... I am less than 200 lbs.

I could feel the hull stiffen as I slowly replace the missing parts.... floor..

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Rod Locker/ Console/ Drivers seat......

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completed there is no gunnel movement at all...

The boat rocks on the trailer as you climb in and out... and I feel secure in two foot rollers....

Modern tin hull design is highly engineered to minimize material cost and maximize profit.

The hull is one monolithic element that your life depends on.... be safe..
 
For the next 6 years or so I will have my wife and littles in the boat. So no. No chances. I’d like to open the floor plan with a flat floor and side boxes to allow them to stand and move a bit when anchored but have a small buffer from the sides to keep them a bit.




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