if boats have twist and flex is that a good thing or a bad thing ??

All ships are limber, even metal ones. It is just that FRP flexes more.

 

My little 16' tri-hull had bad stringers and the floor was broke loose from the sides of the hull so when swells and waves were encountered the floor would hover around independently of the hull which would twist and undulate with the waves. I thought it was more comfortable that way and would have left it, but it wasn't good for the structure so after awhile I had to fix it.

 

Back in the last century, the esteemed NA, Elvis Presley, successfully used twist and flex in his designs .....

 

Elvis used large "radius of gyration" in his design.

 

Need to read !!

http://www.compositesworld.com/articles/re-inventing-the-rhib-shock-mitigation

 

I think saying good or bad has to be qualified, even if you stay withing stress limits. I have observed over many skin on frame kayak builds that you can have too much flex.

I have concluded it is similar to the suspenion on a car: if you want to go fast and maintain control, you need the suspenion to respond to the road but it must be fairly stiff, if you want to go slow and have a comfortable ride, it has to be fairly soft. If you drive a race car at road speeds, you will get a rough and uncomfortable ride. If you take a heavy road car on a race track, the soft suspention will be difficult to control at high speeds.

I made a light alumnim frame folding kayak, it had a lot of flex, no failures of the aluminum tubes, but in in heavy chop it wanted to fold in the middle and made it hard to keep in a straight line. OTOH, I have paddled large "hardshell" kayaks that almost no flex, in rough conditions and it was a rough ride, the waves kept knocking the boat around and made for a very tireing ride. The typcial stiffness of a wood skin on frame kayak gives a good balance between control and confort. However, it you were to tow it behind a ski boat it would not be very controlable because of the amount of flex it would have.

I would say flex is a good think if it is matched to the conditions the hull was designed for, speed, chop, etc. Just like a car suspension. In a power boat, just like in many mondern performance cars, it might be possible to have the hull stiffen as you pick up speed, to give you control at high speed, but still a comfortable ride at lower speeds.

 

life can be interesting ,but only if you want it to be !!

The last part of your post touch's on yet another subject of the possibilities of being able to change the flex hull back to being stiff again as you say to match the conditions . that sound really interesting and I an sure its being done right now as I write is !!

 

Hey Hey... Hey Hey

Elvis also used "GOOP" to https://www.youtube.com/watch?v=xxzkUNUI0HM help him win........ https://www.youtube.com/watch?v=xxzkUNUI0HM

Jeff.

 

keep you eye on the prize !!

Twist and flex in hulls !!,
Never mind about dead has been's
What's the verdict!, yes its a good thing or no its not ??
what about panels that flex within reasonable acceptable limits ?? what ever that is !!

 

twist and flex and soft bottomed boats and how do we make these boats , all these things all go together!
no real earth shattering break throughs happening with any of this as its all pretty foreign to what we all think and are bought up to believe in as being the only truth .
the old philosophy if it moves its weak and its going to break rings true when in reality its not really necessarily true at all !!
Sure excessive amounts of twist then becomes racking and then breakage takes place but that's usually because the wrong materials were used in the first place
boats live in a fluid environment and liquid can be mirrored surface or ginormous waves that can crush and shatter most boats to bits in just seconds , but if you build so this shattering and breaking doesn't happen then after the wave has gone the boat returns to its normal shape and is quite capable of carrying on as it should .
what materials could you use to make smaller boats ( 3 to 5 mrts ) that would and could cope with these scenarios???
Anyone made anything that is seemingly indestructible?? how did you make it??? and how long did it last ?? hours? , days ?, weeks?, months? , or longer ?? why did it last as long as it did ,and what was the ultimate one thing that was its eventual down fall ??
Over the years of working with glass I have always fallen back to trusting Unidirectional glass more and more for the making most things to last and take real punishment .
smaller boats take more abuse and rough punishment and carry heavier loads for there size than any bigger boats do but we don't build bigger boats using the same materials or the same methods as we do with smaller boats and that is a really big part of why boats break and fall apart over time . do smaller boats have frames NO!!or hardly ever !! then what about bigger boats ??, why do they have frames ??

 

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Flexibility of boats is not a new concept.

 

I know it nothing new but is it a good thing and would you feel comfortable seeing the boats twist as it went through the water ??
or a bad thing and no boats should not twist at all ??

The Vikings has twist and flex and they could even change the shape of the hull as well so that something to think about as well !!

 

Rule No. 1 is it's a bad thing if if there is any flex or twist in a boat unless it would be a good thing if there was some flex and twist in a boat.

 

Frankly, how can anyone tell if flexing hulls are in general a good thing or not? What is good for you is not necessarily good for me, and vice-versa. What is desirable on a fast powerboat with a tunnel hull is not necessarily acceptable for a load-carrying displacement craft, and vice-versa.

I think you should narrow down your question and specify the vessel type you have in mind, if you want to have a meaningful discussion about this subject.

The only flexible hulls I've been on are RIBs and they have a considerable spring effect, as you know well. Depending where you are seated (on seats or on tubes, as noted in the other discussion), the spring effect can be anything from comfortable to plain dangerous. I have seldom felt a need for flexibility on slower boats (up to 35-40 kts) I've been on, and hence don't see why should these vessels adopt membrane-soft hulls like the ones described in that article about military RIBs. Hull bottom shape, boat inertia and water docility and compliance is all that is required for a soft ride at these speeds

I definitely wouldn't want a flexing monohull sailboat either. But a catamaran, that's another stuff.

If you felt a need to ask this question, then it means that you probably have other vessel types in mind, like racing powerboats, tunnel boats, or something else. So, that would be the first thing to sort out, imo - what boats are we talking about?

Cheers

 

So you cant see the point of softer riding boats and membrane skin on the outside !!
Maybe you are reading the wrong boating magazines and books !do you get Professional boat builder magazine ? recently they have had articles in there about this very subject and how damaging it is to the body of prolong pounding and banging in any kind of boat !! special seats are a big part now for inflatables and some sporty personal water craft !! so even manufactures of the ordinary run of the mill wont take long before they to start jumping on the band wagon .
Standing up in high speed boats is not the answer any more regardless how much padding you have round you ! its the shock loads that's transferred up through your feet, legs ,and back, and your neck!!. .Have you never spend a whole day out on rough water and felt the after effects ?? of a sore body from head to toes ??

The amazing thing to all this also is that people are missing is these boats are way lighter , go faster and are miles more durable that ordinary boats !!
I have proven these phenomenon myself! a few time by building , light, strong, semi flexible ,and softer riding boats with the bonus of extra speed ,much better manoeuvrability ,and much more durable !!
So ,simply by using a better glass and less of it and using it in a better way so its doing more work and all working together !!.

So after reading this last part of what I've written you probably don't have a clue what I am spelling out for you !!.