Seats and Structure

[bobberboy]

I posted this on a project thread I'm working on but thought it would be of more general interest and decided to post it separately.

Almost every week someone writes in to the forum to ask about removing seats and the structural implications of doing so. I noticed on my project today some evidence of why not to take out the interior structure (this could be ribs and/or seats) without doing something to compensate. My project boat is 14'. The bench seats had long ago been removed and there was nothing tying the boat together from side-to-side from the dashboard all the way to the transom. This is nearly 10' of unsupported sides. You can see in the following three pics how the stress on the gunwales from not being supported has cracked them. The cracks are immediately aft of the dashboard which was the only structure tying the sides together. There's no question about why the gunwales cracked and why they cracked where they did. Pretty good evidence of how important the seats are in the structure of the boat.

Starboard side.



Port side



Port side close up



Without the rolled or extruded gunwales the sides of aluminum boats are not strong. It is the extrusions, rolled edges, cast corner braces, in conjunction with the seats and ribs that give the boat structure and strength. Cutting or modifying the structural members of a boat needs to be done with forethought and care.

 

[onthewater102]

Interesting leveling device you've got working in the background...very smart!

 

[MrSimon]

Taking seats out on most boats is fine .... but cutting into the gunwale like that is never a good idea!

 

[bobberboy]

MrSimon » Today, 09:33[/url]"]Taking seats out on most boats is fine .... but cutting into the gunwale like that is never a good idea!

The gunwale was not cut. It was broken because the seats were taken out.

 

[onthewater102]

Your gunwales are notched out - so they were cut, though the cracks do look to be stress related. I can't believe that the missing material would have cracked off too. The notched area compromised the strength of the gunwales by removing the heavy material running along the inside of the boat. Not saying the cracks couldn't have formed otherwise, but the notches are definitely part of the cause.

But to your point the force which caused these breaks would likely have been opposed had the seats been in place as these are breaks running from the inside of the boat to the outside, indicating a twisting of the boat sides around an axis parallel to the keel, as opposed to running from the bottom of the gunwale to the top as would be the case if the boat flexed vertically.

 

[jethro]

Yeah, exactly, what is that piece that is where the gunwale was cut? I can't even understand what it is- a spray skirt of some sort? You do not want to cut the gunwale at all, it's probably more structural than the seats, but I see your point. It would probably have cracked there even if you had the seats installed- but again, I see your point.

 

[bobberboy]

Yes, I see what you mean now. The gunwale was notched where the hood begins. I don't exactly understand the profile of the extrusion in cross-section but if you think of it as a clock face, the quarter between 12 and 3 was removed (or 9 and 12 on the other side) to accommodate attaching the hood. I was so focused on (and horrified by) the crack I didn't see the whole picture. The gunwale had to crack there as the strength of the extrusion was compromised by the notch. I still maintain that with the seats to keep the sides from spreading the crack wouldn't have happened. Given, at least in the example of this boat that there are no ribs running up the insides of the boat, the seats had to be counted on as an integral part of the structure. In a typical jon boat with ribs running up the side the seats may not be as critical to the overall structural integrity. In my case, with no ribs going up the side, it was a recipe for failure.

Here are some more pics of the area.







There are a lot of variables in the way boats are made and I don't pretend to know that much. It is my observation that welded and riveted boats may have different structures and boats intended to have open floors may have larger or heavier ribs to compensate for the lack of structure across the width. Whether you agree with me or not, I'd say think things through before you cut or remove stuff. It may be there for a reason.

 

[onthewater102]

FOAM!!! People are constantly removing foam and not replacing it with the proper amounts. 1 cubic foot for every 60# of load (including the boat's weight)

 

[MrSimon]

Wow! Someone really butchered that gunwale. The fact that only two cracks have shown up is a testament to the durability of aluminum boats.

You may want to consider fixing that properly before using it under any significant amounts of power.