[brian eiland]

Originally Posted by u4ea32

......But even then, sailing multihulls only outperform sailing monohulls of similar displacement and stability once flying a hull. That's because two hulls (or three) going through the water require much more energy than one for the same displacement. More energy is required for two reasons: 1) more wetted surface, and 2) far more wave drag because the waves of the two hulls combine instead of cancel. Trying to squeeze incompressible water between two hulls takes a lot of energy!


Here is a simple experiment if you've got a row boat or kayak or canoe: get going any comfortable speed in deep water. Then, go over some shallow water, say a foot deep. You will feel quite clearly that you slow down a lot, and it takes a lot more effort to try to even approach your original speed. This is the reason that ship speed trials are ALWAYS performed in deep water.

Now, once a sailing multihull is sailing on the one hull far to leeward, speed increases dramatically. But of course, now you effectively have a very thin monohull, not a multihull!

If you really want to achieve high efficiency as promised by power cat vendors, you really want 1) light, 2) long, 3) narrow, and 4) a single hull!

I would suggest you go review some basic displacement/hull-speed theory in vessel design. You are way off base.

[brian eiland]

Originally Posted by Pelagic Dreams

very interesting....has anyone done a test comparing a cat with a mono hull of the same displacement? Pound for pound, same gross tonnage, how do the figures compare.
This is a big factor since doing my research I have really found the living "platform" of the power cat very impressive....but fuel does make the world go round.....as they say.

You might have a look at this 62' powercat belonging to a good friend of mine (not for sale). I recommended this type of vessel back over on this other subject thread....for various reasons.

http://www.yachtforums.com/forums/123583-post41.html

...and have a look at his 'custom cat dinghy'

I'll get you some real time fuel milage figures shortly...I think you will find them quite pleasing

[u4ea32]

Originally Posted by brian eiland

I would suggest you go review some basic displacement/hull-speed theory in vessel design. You are way off base.

Would be interesting to know the error. Please share.

[brian eiland]

Originally Posted by u4ea32

Would be interesting to know the error. Please share.

Wave making resistance - Wikipedia, the free encyclopedia

http://www.multihulldesigns.com/pdf/powercatslt.pdf

http://www.oossanen.nl/download/perr...orm_design.pdf

Here are just a few reference discussions that came up with goggle. I'm sure you can find many more on this very subject.

In general if you keep the slenderness ratio of those hulls above 8 to 1 your displacement (wave making) drag is seriously diminished

[Marmot]

Add this one to your reading lists:

http://eprints.utas.edu.au/6787/1/CF..._Demihulls.pdf

[u4ea32]

Brian, please directly answer: where is an error in my post?
All three of your examples clearly and directly support my post.

Originally Posted by brian eiland

...

In general if you keep the slenderness ratio of those hulls above 8 to 1 your displacement (wave making) drag is seriously diminished

Which is the same as I said.

Maybe you misread something, or I mistyped it?

[u4ea32]

Originally Posted by Marmot

Add this one to your reading lists:

http://eprints.utas.edu.au/6787/1/CF..._Demihulls.pdf

Thanks for the paper!

This is not the only study along these lines. Its an interesting concept: staggering the hulls so the wave trains somewhat cancel.

The paper does include an illustrative function showing how resistance is greatly increased when distance between hulls is small, as on typical power cats. As with the other references, this paper does support what I posted.

Please, if you see some error, let me know. I am always learning!

[brian eiland]

Originally Posted by u4ea32

But even then, sailing multihulls only outperform sailing monohulls of similar displacement and stability once flying a hull. That's because two hulls (or three) going through the water require much more energy than one for the same displacement. More energy is required for two reasons: 1) more wetted surface, and 2) far more wave drag because the waves of the two hulls combine instead of cancel. Trying to squeeze incompressible water between two hulls takes a lot of energy!

This is wrong

Originally Posted by u4ea32

Here is a simple experiment if you've got a row boat or kayak or canoe: get going any comfortable speed in deep water. Then, go over some shallow water, say a foot deep. You will feel quite clearly that you slow down a lot, and it takes a lot more effort to try to even approach your original speed. This is the reason that ship speed trials are ALWAYS performed in deep water.

Not necessarily so....I'll show you an actual contradiction to this assumed axiom ....in an upcoming posting.

Originally Posted by u4ea32

Now, once a sailing multihull is sailing on the one hull far to leeward, speed increases dramatically. But of course, now you effectively have a very thin monohull, not a multihull!

If you really want to achieve high efficiency as promised by power cat vendors, you really want 1) light, 2) long, 3) narrow, and 4) a single hull!

1)light....yes
2)long....yes
3)narrow....yes
4)a single hull....but how are you going to make a usable cruising vessel out of such a slender single hull shape....and that is the purpose of this gentleman's inquiry.

BTW we don't recommend sailing cruising multihulls up on a single hull either....nor power-cats either.

[brian eiland]

Originally Posted by brian eiland

You might have a look at this 62' powercat belonging to a good friend of mine (not for sale). I recommended this type of vessel back over on this other subject thread....for various reasons.

http://www.yachtforums.com/forums/123583-post41.html

...and have a look at his 'custom cat dinghy'

I'll get you some real time fuel milage figures shortly...I think you will find them quite pleasing

I had made reference to this 62 powercat "Hummingbyrd" before, but I just recently dug out an old article I had saved on this vessel. It was written by a gentleman Tim Askins who had been searching his next vessel to go cruising....and he was looking again at monohull trawlers that he had been comfortable owning in the past. Then he and his wife got an invite to make a trip from Fl out to the Bahamas for a nice long demonstration ride.

I made a PDF document of that old article but could not post it on this forum as it does not allow that size document. So I posted it over HERE:

I think you will find it very interesting.

[Blair]

Tennant designs have that narrow section relatively deep draft characteristics that not only offer the well known efficiencies and speed, but also offer much of the SWATH-type effect reducing roll and pitch compared with planing or more shallow draft cats.

I think it is a pity that Malcolm Tennant took 'efficiency' to such a sublime level and designed easy built/cost effective flat section superstructures - no compound curves to be seen. Some owners/builders with a leaning to more asthetic appeal baulked at this and spent some time and money on a more handsome result. Result is that Tennant cats are either quite pretty or pretty ugly! (IMHO)

His hull efficiencies are lengendry and much copied throughout the world. I have ridden in a few and I can attest to their stability and capability in rough sea states. The commercial operators love them and they are the most discerning customers of good design.

[u4ea32]

Originally Posted by u4ea32
-------
But even then, sailing multihulls only outperform sailing monohulls of similar displacement and stability once flying a hull. That's because two hulls (or three) going through the water require much more energy than one for the same displacement. More energy is required for two reasons: 1) more wetted surface, and 2) far more wave drag because the waves of the two hulls combine instead of cancel. Trying to squeeze incompressible water between two hulls takes a lot of energy!
--------

Brian Eiland replied:
--------
This is wrong
--------

Brian, I am right.

Well, that doesn't illustrate much, does it? ;-) I would rather this be a discussion based on something more than opinion.

Why do you feel the above statement is wrong?

My position is very easy to support experimentally using both power and sail vessels.

And its even easier to support theoretically, as not only do multihulls require more wetted surface for a given displacement, multihulls also include a resistance term for ama-ama wave interaction that does not exist for monohulls. Since that term (a function of hull separation) only increases resistance, needing such a term guarantees the multihull solution (for a given displacement) results in more resistance.

And since multihulls certainly require substantially more hull and deck area, for a given level of build technology, the multihull solution is certainly heavier, thereby losing even on displacement (and therefore cost).

If I am in error, please teach me, I want to learn.

[u4ea32]

[QUOTE=brian eiland;141159]This is wrong


Originally Posted by u4ea32
-------
Here is a simple experiment if you've got a row boat or kayak or canoe: get going any comfortable speed in deep water. Then, go over some shallow water, say a foot deep. You will feel quite clearly that you slow down a lot, and it takes a lot more effort to try to even approach your original speed. This is the reason that ship speed trials are ALWAYS performed in deep water.
--------

Brian replied:
--------
Not necessarily so....I'll show you an actual contradiction to this assumed axiom ....in an upcoming posting.
--------

This should be interesting, if you actually have some information that counters this body of knowledge. Its always interesting to see when "commonly held principles" don't hold (there are many examples). You have greatly piqued my interest!

[u4ea32]

Originally Posted by u4ea32
---------
Now, once a sailing multihull is sailing on the one hull far to leeward, speed increases dramatically. But of course, now you effectively have a very thin monohull, not a multihull!

If you really want to achieve high efficiency as promised by power cat vendors, you really want 1) light, 2) long, 3) narrow, and 4) a single hull!
----------

Brian responded:
--------------
1)light....yes
2)long....yes
3)narrow....yes
4)a single hull....but how are you going to make a usable cruising vessel out of such a slender single hull shape....and that is the purpose of this gentleman's inquiry.

BTW we don't recommend sailing cruising multihulls up on a single hull either....nor power-cats either.
-----------------

OK, so we mostly agree!

I have been playing with the design of such a vessel for my own, and its coming along pretty well. I can't find a driving need for needing more than very little beam. The widest accommodation I can discover is a queen sized walk around berth with floors at least 18 inches wide on each side. That's 1.5+5+1.5 feet, just 8 feet of beam. Make it 9.5 and you've got a very comfortable passageway along side that berth. Everything else is narrower than 8 feet.

Therefore, I don't think a narrow boat is any impediment to an extremely luxurious interior arrangement. Its certainly different. But so is a catamaran layout when the cat has narrow hulls. And if those cat hulls are not narrow, calling the vessel efficient is disingenuous.

Now, there is that issue of roll coupling, where a long thin vessel will tend to roll when excited by any wave energy, because its the easiest way to dissipate the energy... Several ways to fix that: many are common and straightforward (stabilizers, bilge keels, ballasted keels, ...). At least one is cheap, simple, and very efficient, so is sitting on the top of my list currently: steadying sails (with very high L/D so they work when the apparent wind is very close to being on the nose). Not hard, if you think A class catamaran rig.

The multihull has its own seakeeping issue related to bridge deck height. That issue is not inexpensive or even feasible to fix for ocean service. You just live with the bridge deck impacts and the structural catastrophic failures that occur. If you have not experienced this, I recommend you do so. Its an eye opener.

One thing I am not suggesting is to compare a 40 foot cat to a 40 foot monohull. I'm suggesting one consider a $X monohull .vs. a $X multihull. For the same $X, one can build a substantially longer monohull than multihull, at least twice as long, if not three times (material for hull+hull+wing deck instead applied to one long narrow hull). That gives the monohull the substantial performance advantage, again when comparing dollars to dollars (better than apples to apples). Therefore, its more like comparing a 100 foot long, 8 foot beam monohull to a 40 foot by 20 foot multihull. Basically the same floor area, same structural surface area, lower structural loads, so about the same (or still a little less) displacement for the mono over the cat.

Again, please let me know where I'm off base here. I'm always trying to learn. I know I am not right, but this is the best I can figure out so far.

[u4ea32]

Let's try some modeling instead of just talking.

Data: Maine Cat 47
Maine Cat: Home of the 30', 41' sailing catamarans, the new P-47 powered catamaran and bareboat Bahamas charters

LOA 47'
LWL 44.5'
Hull BWL: 3.3'
Hull LWL:BWL Ratio: 13.5:1
Lightship: 21K lbs
Full Load Displacement: 30K lbs

Proposed long thin monohull:
LOA 100'
LWL 100'
Beam 8'
Hull LWL:BWL Ratio: 12.5:1
Lightship: 21K lbs
Full Load Displacement: 30K lbs

For different speeds, lets compare Me47 sea trial data, compared to four numerical models for the 100 footer: Savitsky 2003, Gerr method B, and Gerr method A, and Groot's method. Each of the numerical methods uses a propulsion efficiency of 60% (middle of the road).

100 footer modelled using Savitsky 2003 displacement model, Me47 data from sea trials.

12 knots:
Me47: 4.5gph = 2.66 mpg
S100: 6.6 mpg or 6.4 mpg or 6.3 mpg or 3.9 mpg

10 knots:
Me47: 3 gph = 3.3 mpg
S100: 6.6 mpg or 8.75 mpg or 9.11 mpg or 6.0 mpg

8 knots:
Me47: 1.8 gph = 4.4 mpg
S100: 6.6 mpg or 10.7 mpg or 14.24 mpg or 14.1 mpg

Savitsky 2003 assumes displacement speed operations are not interesting, so it models them linearly, therefore its always 6.6 in this case.

While one might think these numbers are pretty far out there, actual tests of very low displacement:length hulls perform fairly dramatically better than these models (regressed from more typical, far heavier and wider hull forms) predict.

[brian eiland]

Originally Posted by u4ea32

Brian replied:
--------
Not necessarily so....I'll show you an actual contradiction to this assumed axiom ....in an upcoming posting.
--------

This should be interesting, if you actually have some information that counters this body of knowledge. Its always interesting to see when "commonly held principles" don't hold (there are many examples). You have greatly piqued my interest!

Quoted from that article I posted wherein both the captain/owner & the guy going for the test ride BOTH experienced this:

Shallow Water Speed:

"Entering the multi-hued, shallow water on the bank, I watched our boat speed increase 2 knots as the big cat began to 'feel the bottom'. Orrin explains the phenomenon. The hydrodynamic hull form produces a wave that provides lift when the water is twelve feet deep or less, literally pushing the boat forward. He pulled the throttle back to 1800 rpms and we maintained 18 knots. His experience shows that it is actually more economical to run the shallow waters of the ICW than to go outside due to this effect"