Longitudinal Twist...

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AcctTwrs
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Longitudinal Twist...

#1

Post by AcctTwrs »

Is there any one factor more than another that helps prevent longitudinal twisting bow to stern torque.
My boat turns out to be 27ft. LOA and 10ft. X 4in. Beam.. I'm just worried about twisting the boat.. repeatedly.
Over time there will be welds that pop... just trying to think about that ahead of time.. build accordingly.
Any thoughts on that... Tanks..!!
welderbob
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Re: Longitudinal Twist...

#2

Post by welderbob »

I'm thinking that a well designed and built boat should not be twisting and popping welds.I know that you posted something about longitudinal stringers. To me a boat is more that just stringers. It is more like a framing system consisting of longs, transverse frames and the deck. In general I like to put longitudinal 's 12" on center. I'd rather be a little less than more. A combination of transverse frames and bulkheads no more than 48" sometimes less.Now we have most of our design work done by a navel architect.You are investing a large amount of time and money in this project, it may pay to get a little design advice I know that Kevin *(Kmorin) has a very detailed explanation of frame and plate strength. If you have already started your project a few pictures may bring some good advice.

I just surveyed a 25 year old boat about the size of yours. Although I though it was a framed a little" lite " I could only find one small crack. It was in part of a frame that should have been built a little heavier anyway. The bottom line was if the customer decides not to but the boat I would. I goes to show that a well built alloy boat should last more than a life time.

Welderbob
kmorin
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Re: Longitudinal Twist...

#3

Post by kmorin »

acctTwrs,
I'm not sure you're seeing the relative stiffness of the (welded) boat in considering (possible) torsion as something to consider?

If we made a tower assembly of 1/8" tube soldered together, the individual tubes could and would bend, or deflect. But if the same tower/tube assembly is made of 1-1/2" tube then to flex either the joints or the tubes would have to 'move'. Tube joints can't move without deflecting the wall of one tube or the other. So the joints don't move. Tubes would have to deflect if the joints don't..... but an 1-1/2" tube/pipe/"O-beam" about 20-30 diameters long isn't going to flex or deflect without literally tons of force.

I think the same is true of a welded boat. The bottom to sides (with deck) creates a single piece of metal that is a large 'box beam' bow to stern. Torsional distortion?... the sides would have to flex or distort at the sheer/gunwale to allow the idea of a longitudinal roll or twist.

If the welded boat is to 'twist' port forward to aft starboard in a quartering sea, the boat would have to so lightly built that individual components could 'move' with relation to one another by deflection. I can see this (long wise twist) in a pressed metal jon boat with a riveted frame made of 0.040" and 0.060" sheets heavily loaded running in some quartering waves, but not in a boat of 1/4" and 3/16" fully welded.

[Just like the correctly built tower assembly, the boat is a huge solid 'tuning fork'- one piece. Some of my working skiffs have been used in canneries as 20' long 'brailers'. Four lines from a dock side crane were attached to the four corners of the skiff which was full to the gunwales of salmon. This would be about 8-9,000 lb. of fish in a 20' open skiff with 34" sides and no deck. It was an emergency and the fish could not be unloaded fast enough by the normally used net bags of 800lb capacity. The skiff was used for nearly 18 hours during one huge run of fish and in each trip it was filled with fish and then lifted onto the dock after returning from midriver unloading the net boats as they returned to port. The boat is still in use, 32 years later, working as a 'yard boat' for a different fish packing house/cannery. ]

They just don't flex.

It ain't happening.

If a weld comes loose its more often from poor surface prep before welding then a point load or impact and lots of times it from trailering more than running the boat where its intended. Also as welderbob says if one or two points or welds fail its most often from the joint it self- maybe not heavy enough or not designed as well as that point could be.

maybe I don't fully understand your post/question (?) but if the boat is build (even remotely) well; longwise torsion in welded boats is not present.

If I do understand your question then I'd ask you to consider a C Channel that is 27' long, 4 deep and 10 wide with a web that is about 2' thick and legs with ('guard decks') 12" returns on them... what would it take to twist that "channel" ? Now what would it take to twist that if both ends were closed in? I don't recall seeing any torsion deflection in welded boats.

Cheers
Kevin Morin
kmorin
peterbo3
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Re: Longitudinal Twist...

#4

Post by peterbo3 »

G'Day AC,
I have no formal qualifications in either the design or construction of fully welded alloy boats but I have owned several over the years & have a lot of sea time on my own & other people's boats. :soap: :soap:
To re-inforce what Kevin has written, if they are designed right, built with the appropriate size frames & plate which have been cut accurately, then welded by a competent fabricator, there should be no problems. Things break when boats are not driven to the sea conditions, or are over loaded, over/under powered or abused due to poor slipping, launching, recovery or trailering practices. Welded alloy is inherently tough but it is not indestructible. I have attached a few pics of hull construction methods used down this way which I am sure are replicated else where in the world. These are not exhaustive & are not meant to represent the full range available to designers but they illustrate how the various components are welded together to produce an extremely strong hull.
I would not be stressing too much about hull twist in a professionally designed & built vessel. :thumbsup: :thumbsup:

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Regards,

Pete in Brisbane
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AcctTwrs
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Re: Longitudinal Twist...

#5

Post by AcctTwrs »

Peterbo3, Welderbob and Kmorin.. Thank you for the answers to my question of Twist... and all of the advise is sound
This shows me what it's going to take to ultimately strengthen stringers and floor into a single unit of framework to
make this boat as strong as I can... This is where a little overbuild will pay off.. Thank you guys ever so much..
I had to ask to remove any doubt. Three heads are better than one...
skypoke
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Re: Longitudinal Twist...

#6

Post by skypoke »

Some impressive framing shown in these pictures. I have to think that these massive frames are primarily to resist bottom and side loads. Your biggest counteracting member for long twist will be your welded floor. That, well attached to your properly spaced framing, turns the boat into a box beam....very difficult to twist. An oft repeated example of this is take an empty shoe box, no top and twist it....easy. Now put the top on and tape it to the sides...now not so easy to twist at all.

When we were building our current boat, an 8.5 meter cat, I became concerned about potential twisting also. This boat basically has no transom and is beamy,which, intuitively, led me to believe there could be a problem with twist. Despite assurances from the designer, I had to know so one day shortly before launching I put a floor jack under one aft corner with the bow blocked. I noted that when I lifted that corner 1/8", the opposite corner lifted 1/8". I quit worrying about it at that point. Eight years of hard use in the Gulf of Mexico has proven there was no need for concern.

Chuck
Texian, born and bred.
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