baffling question

[Juggler7823]

Re: Narrow hulls

I'm not sure of the relevance of your comment. The hull shown is really an outrider and provides balance not displacement. Not even a University boat crew could keep it upright! Your central hull is exactly the opposite. It is an extremely flattened U and is probably about 4 times as wide as it is deep.

<hr width=100% size=1>LOM

 

[extravert]

Re: Narrow hulls

Float hull shape is vitally important to maximum performance of a trimaran. When sailing at the highest speeds typical small tris can achieve (about 20 knots for Dragonflies and Corsairs) the leeward float is providing significant displacement. At these speeds there is a tremendous downwards force on the leeward float, such that it is significantly submerged compared to static conditions. As the leeward float submerges, the main hull lifts. Even though the main hull should stay at least partially in the water for safety, at this limit of performance the proportion of the main hull in the water is significantly reduced. At the highest speeds, the leeward float is doing the displacing, the main hull is almost skimming. Float shape is important!

This looks more like a V shape to me...

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[Twister_Ken]

which begs another question

Is a submerged submarine subject to the 1.3xsqrootLWL rule?

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[cameronke]

Re: which begs another question

Good point Ken

Now was it not the case in swimming that at one Olympics, the Chinese turned up and swam the breast stroke fully submerged and won everything. After that the rule was introduced that only one stroke was permitted underwater at the start?

I believe that there was less resistance when swimming submerged

but I may be wrong!

regards

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[extravert]

Re: which begs another question

No, because there is no free surface on which to create a wave.

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[cameronke]

Re: which begs another question

Then what would be the limiting factor on a submarines speed?

Torpedoes really zip along but they could hardly be considered as planing!

regards
Cameron

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[ongolo]

Re: which begs another question

The crossectioninal area of a sub or a torpedo determines how much water must be displaced or better moved out of the way of the sub/torpedo at a given speed. Then the shape plays a role, a flat frontal section will result in much higher velocities of the water moving out of the way, a pointed shape will create less turbulence, then on all sides of the hull you will ge skin friction, then behind the sub/torpedo you will have turbulences as the water has to stream in behind the moving hull and fill the area with water again.

According to bernoulli or reynolds numbers, the principle of a body moving through a media (liquid or gas in ballistics even steel) is the same, only the density changes.

For instance in a mach3 bullet a vacuum is present behind the bullet base, depending on the expected velocity the angle of the boat tail is selected to reduce the area the vacuum can act on.

As far as the comparison of a 10m long very narrow boat and a 10m long one mile wide boat is concerned, to get the identical deplacement (unfortunately overloooked) for both the narrow and wide hull, the narrow hull going down on port and coming up on starboard would also have to be one mile deep or draft and have twice as much hull surface as the wide hull.

The comparison that beam to length is meaningless when the displacement is ignored. One of the factors to describe a hull is the prismatic coefficient.

My own hull of 22 tons has for a WLL of 50ft only 12ft beam and should be excellent accordingly , but it is not. My wetted area is rather big in proprtion because of this.

Also the often quoted displacement to lengt ratio on my boat is much to good for a 22 ton steel boat, that is however in theory as there so many other factors influencing hull speed.

As far as the bow wave is concerned, you can also picture it this way. Below hull speed you have a bow wave requiring you to go uphill and you have a stern wave (the previous bow wave) that lets your stern go down hill which compensates the bow. These two compensate each other to an extent, but if you go faster and loose the stern wave, you are only trying to go uphill.

I hope my simple english is understandable to you guys.


Regards ongolo


PS, I wrote and published a 512 page book on ballistics and taken about 4000 pictures during research, many of bullets in flight (high-vel bullets in thin air behave very much like low velocity boats in "thick" water) and bullets penetrating objects and media of different densities.

PPS I did not want to post anymore, but obviously I cant help myself.

<hr width=100% size=1>1000 dream about it, 100 talk about it, 10 start and 1 completes it

 

[penfold]

This is the basic principle behind the ELAN(Extremely Long And Narrow) boats penned by Nigel Irens et al. DERA and Vosper Thornycroft are partners in the research ship Triton, which is built to Irens' rules; It is a trimaran with very slender hulls. I seem to remember the length/breadth ratio of 22:1, but I could be mistaken. The basic premise is that ELAN hulls are not bound by conventional displacement speed rules, only by yer common or garden drag. Irens had at least one ELAN boat built and I think it was fastest round britain or something. I can't remember much more; I read it in Yachts an Yachting about ten years ago.

cheers,
david

<hr width=100% size=1>OMG, Schrodinger's cat is dead!