Controlled Flex

Hmm, Knut, you have not seen the entire discussion between PAR, Sasha, jonR, Portacruise and me, due to the fact that several posts have been deleted, for some good reason.

And the "Helium" joke was meant ironically, but that does´nt travel here too good.

Regards
Richard

 

Interesting to see that people still read this thread I started 6 month ago. Since then I have moved on, already tried a flexible foil but concluded that the stiffer one is better although heavier it gives a better more "crisp" feeling to it than something that bends in the gusts. Although a flexing freestanding mast and a "breathing" airfoil works very good. So my conclusion is that more flex can be allowed in air than in water. Maybe it has something to do with the uncompressable nature of water?? Or maybe it has to do with the most flexible part flex first? Or the weight?, or a combination of all the above?

But when it comes to hull design it is important that the flex is even over the entire hull even though the structure dont flex in normal situations, just to avoid breakage or fatigue. "You roll with the punches as my boxing trainer told me". Old scandinavian fishermen used to hold the stern and shake the boat to see if it was any good.

A flexible hull like on a RIB gives more comfort and control in choppy conditions just like an old Tiga windsurfer compared to my old carbon sandwich slalomboard. In extreme conditions control= speed.

 

A RIB hull is not flexible.

R I G I D infaltable is the translation.........where is the flex?

 

I' not sure this question has been properly addressed. In addition to the ancient Viking boats, there are many claims for the extra speed of a flexible boat, George Dyson's Aluminum framed Baidarkas as an example.

It's a fairly safe assumption that a boat on flat water is less likely to benefit from flexibility, so one can look at the effect of waves on a flexible boat to seek the answer. The flexing structure of the boat absorbs energy from the forward motion as waves impact the boat: the question is, whether the energy is returned like a spring, or is it absorbed as heat or vertical motion. The rigid boat will not absorb energy into its structure like a flexible boat, but energy will be lost pushing through or over waves. Which boat does best will depend on wave conditions and a whole host of design issues. It is clearly a complex issue requiring experiment under controlled conditions, but is difficult to make such radically different boats equivalent enough to ensure that the test is not comparing apples and oranges. It's almost impossible to get a firm ruling one way or the other, thus this controversy will likely go on for ever.

One can draw some conclusions. A boat with a springy frame will not retain the absorbed energy indefinitely but will return it in mechanical form, unlike a boat that flexes with a lot of friction between moving members which will absorb the energy in the form of heat and never return it as mechanical energy. So George Dyson's aluminum-framed Baidarkas may well be faster than a comparable rigid kayak, but it's by no means definite.

A poorly designed, inadequately inflated inflatable boat is probably the worst case of all, as the water hits the equivalent of a punching bag that gives nothing back in the form of mechanical energy. Even on flat water such a boat will waste energy.

Overall, IMHO the best option for performance will be a rigid boat designed for specific conditions, but the toughness of an inflatable or RIB is hard to match.

There are specific boat designs that extract energy from waves and use it for propulsion, of course.

 

There is two ways of looking at this:

1.) The toughness of an inflatable or RIB is hard to match, because it is less rigid. Therefore, flexibility allows a light but tough boat. (A rigid inflatable is still not a totally rigid noat).

2.) The use of flexibility to extract energy (Is there an exable of this?)

 

Try Googling "wave energy propulsion" - there's several types you can find on the Internet.

Inflatables and RIBs are very tough, but as far as performance is concerned, at least when the power source limited as in a sailboat or human powered boat, they do not compare with rigid boats. Some such boats do quite well, Klepper kayaks for example, but they do not appear in race sagainst rigid boats. The aluminum-framed Baidarkas designed by George Dyson have been used in competitions and have done quite well in heavy conditions, but they are not inflatables.

I think the very thing that makes inflatables so tough, which is their ability to absorb and distribute the energy from a heavy impact without harm, also slows them down by absorbing energy from forward motion. There are fast RIBS of course, but they have huge motors.

 

Ok thanks, I found some internet pages like this one: http://www.rexresearch.com/waveboat/waveboat.htm ...interesting...although looks a bit complicated. I think the aluminium framed baidarkas looks funny, I´m a bit sceptical if its a good idea to use aluminium frames, why is that better than a glass or carbon fibre kajak? Before composite material was invented, the only advantage of "skin on frame" kajaks was that they were light. Marström together with nordic kajaks have made a new surf ski that I´m a bit keen on, its in pre preg carbon and very tough and light. http://www.nordickayaks.se/doc/surfski.php

 

Concerning wave propulsion, some tests in Norway have shown that a 30m trawler could "swim" against the waves, using their energy. I am on the process to get all the data, but the figures I have in mind were impressive.

The biggest challenge is to find how to build the propulsion foil and its motion response system.

 

The wave propulsion systems that I have read about certainly do not offer much performance or reliability for their cost. I like experimenting with new ideas but that is too complex for me. Also there aren't any useful waves in my local lakes!

The skin-on-frame boats are light and surprisingly tough. However, I find I can get a very light boat using marine plywood: my current canoe build is small, only 3 m (10 ft), and very light, about 8 kg (17 lb) when finished. It is a river boat, so its design is a compromise between speed and agility. My previous canoe came out at 10 kg (22 lb), it is 3.66 m (12 ft) and very quick for its size. It's a lake boat intended for long straight runs with minimum effort, and easier to maintain a good speed than either of my plastic kayaks.

There is no comparison with carbon of course, but it is possible to get a very light boat without going hi-tech or skin-on-frame, and my boats only cost me $100 each!

I like the flexibility of design that I get with plywood. The design of the next boat I have planned is based on a 100 year old classic canoe design. I am developing a new method of construction, which may reduce weight even further. It is based on a 100 year-old system called metallic batten construction, which I am adapting for modern materials and tools. If the canoe turns out well, I may develop a kayak design from it.

 

A few years ago i worked for a Inflatebale boat company , they wanted a glass hull made for the life saving boats , These boats are used in extreme conditions of rolling and huge breaking waves and surf .
Before i started at the company a well know company had designed a laminate that was used and was surposed to be unbreakable ! the boats broke and so did the guys in the boats .
The key to making boats like you are taking about is its not flex you need but twist Two words with simular meanings but completely differant in the way they behave .
Twist is easy to do and yet the hull can be quite ridged and hold its shape very well .
A runabout hull when it comes out of the mould with nothing inside will twist! its when you start fitting floors and decks that the twist disapears . Frames will hold a hull in shape but still allow it to twist !! .Do you understand the meanings of what i have discribed between twist and flex ??
If you want twist use undirectional Glass not carbon !! Glass is a lot more forgiving than carbon !!

 

Do YOU???

 

An open boat will twist easier than one with a deck. The closer the boat gets to a closed box the more it resists twisting. I am not sure I would be comfortable with my boats twisting.