Two hulls, which has least drag?

It also holds true for b/t=~2 aswell - according to michlet predictions.

With b/t = 1, your basically optimizing more toward a minimum wave resistance solution, because your keeping the hull beam small. Hull beam, has greatest effect on wave resistance. In this case, for the same displacement, you have a higher t, and thus a higher wetted area compared to b/t=2 - so your paying a penalty in terms of viscous resistance compared to hull that is closer to b/t = 2. This may provide an optimum solution where the hull is operating at low to medium speed around a Fn = 0 - 0.6 where wave resistance can be the greatest component of total drag.

When you design a round bilge b/t = 2 hull, you have minimum wetted area, so minimum viscous resistance, as wetted area has the greatest effect on viscous resistance. But you are also making more wave resistance than a hull which has b/t < 2. So in this case of b/t=2, you are optimizing for higher speeds where viscous resistance is the larger component of total drag.

 

Haribo

You should read my earlier post here:
http://www.boatdesign.net/forums/hy...-hulls-has-least-drag-45172-2.html#post589959

It is more about the L/D ratio than the B/T ratio.

 

Those graphs from Mollands work certainly paint a picture, however not all the peices are there...

For instance, in the results where LDR is constant and B/t was varied, at what LDR was the experiment conducted? Were further similar tests conducted at different LDR to see if the results held true across a range of LDR?

 

The as per L/D ratios presented, and noted here:
http://www.boatdesign.net/forums/hy...-hulls-has-least-drag-45172-2.html#post590093

Molland has conducted many, and presented them, as have we...it is consistent across the L/D ratios.

Understanding the L/D ratio is the key...everything else falls into place after that....

 

I found one Molland paper here -> http://eprints.soton.ac.uk/46442/1/071.pdf

Very interesting and makes perfect sense. You can see little variation across the range where L/D is constant, but large differences where L/D is varied. Some of the other results in the above paper, do show larger differences in regard to b/t particularly for hulls with lower L/D ratio, and the wave resistance at hump speed. The difference was less noticeable in the hulls with higher L/D ratio.

Its also notable, that for a hull to have a favourable L/D ratio, by physical necessity it has to be long and slender when keeping within the realms of practical hull shapes where b/t is in the range of 1.5-2.5 for minimum resistance.

Minimum resistance LDR yeilds length to beam ratios in the order of ~20-25:1 - which is of relevance when fitting a hull to purpose and the required space inside a very slender hull with a such narrow beam. Ive noticed through michlet modelling, that if you force the length to beam beyond circa 25:1 whilst maintaining the same displacement, things actually get worse in terms of resistance despite the LDR getting higher. I put this down to wetted area penalty of excessively slender hulls and is generally outside the realms of practicality anyway.

 

Ad Hoc I read all your earlier posts....in this treat

but for given length and displacment L/D ratio can not be the key (because L/D is then given....)

if B/T ratio plays no rule why not chose B/T= 0.1 or 10 ???

even in the molland figure (5.8b) between B/T=2 and B/T=1,5 is a difference abaut 5-10% of total resistance[N], this is not nothing, it shows that molat found the minimum of drag at B/T=2 maybe for his heavyer ship (L/Disp=9,5 in figure 5.8a) and not obligatory for a lighter wight sailing catamaran.....who will also sail half the day in slow winds with lower fn

it seems to be a flat water resistance, and my question is until the entery angle and B plays a biger rule for minimal pitches (min. pitch-acceleration) while sailing throu waves, how far away from B/T=2 will then lay the optimum B/T for a given L and D

 

I thought the same thing haribo, except then i realized that the l/d range they modelled, actually represents most vessels in practice, even a light weight sailing catamaran would fall within this range...

Why not use a b/t outside this range? - because you dont end up with a useful hull in terms of practicality... the hull gets too skinny to fit anything into it, or it gets so fat that it can no longer be termed a slender hull, and falls outside the boundaries of thin ship theory. Fat hulls dont work in a high speed displacement regime, they will either be resitricted by the hull speed or start to plane if enough power is available.

So the range they have modelled, is actually quite enough to represent most useful applications of a displacement catamaran, regardless of its specific purpose...

 

Why do you think thin-ship theory (or any other theory) has anything to do
with the experimental results collected by Molland et al?

 

Groper,

Thanks for posting the Molland paper.

Unless my basic calculations are messy, I noticed the S/L ratios seem quite low between: 0.2..............0.5

While for an 18 feet catamaran, S/L ratio seems to be between 1 and 5 for a speed range of 5 to 20 knts.

Any ideas if trends reported in this paper can be extrapolated ??

Cheers

EK

 

S in the paper is: seperation between catamaran demihull centerlines, with S/L 0.2....0.5 molland searches for the interference of the hulls for motor catamarans more than race sail-catamarans

a race sail-catamaran today will have S/L= 0,66......0,8,

so S/L 0.5 and a 18 feet catamaran means distance of the hullcenterlines
0.5 x 18 = 9 feet

at S/L 0.66 the bow wave of the hulls will nearly not longer touch the other hull

but if sailing in waves the pitch-motion of one hull changes the trim of the other hull.... not part of his paper

 

Sorry,
I read it to quick was sure S/L was a speed/lenght ratio.

For beach cat, I always assume flying a hull, so it's more a monohull case

Tks

EK

 

What i meant was, an extremely fat hull, with a high b/t ratio, simply would not be considered in a high speed displacement catamaran application - therefore, theres little point in attempting to model higher b/t ratios in these experiments.The aim of the experiment afterall, was to see the effect of b/t ratio, l/d ratio and s/l ratio of displacement catamrans upto a Fn=1.

I made reference to the thin ship theory as a reference to a particular domain, in which lies the context of everything we are discussing. Theres no point in talking about hypothetical ratios, if the ratio becomes so extreme its no longer relevant to a displacement catamaran, and since we are looking at speeds upto Fn=1, we can further narrow our "genre" for discussion, into a high speed displacement catamaran when referencing mollands paper.

 

Just think !!

If you have two hull and which one has the least drag??
its got to be the one on the highside out of the water

 

Fair enough, groper. I thought you might have got yourself into a bit of a
tangle there for a minute

 

IDEC, the record holder around the world, lays with B/T ~ 1 outside this range, but ok it is not realy a most represent vessel in practice....


look at the capsized hull
http://www.thedailysail.com/offshore/11/59664/0/francis-joyon-and-hurricane-irene