Power Cat vs. comparable monohull yacht

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

Which is the same as I said.

Maybe you misread something, or I mistyped it?

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!

This is wrong

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

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.

Hummingbyrd 62 Powercat

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.

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.

Originally Posted by u4ea32
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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!
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Brian Eiland replied:
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This is wrong
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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.

Originally Posted by u4ea32
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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!
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Brian responded:
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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.
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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.

Four numerical models

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.

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"

With many of Tenannt's original designs I can agree with you...not very pretty sheer lines,.....lots of straight lines.

But I've come to really appreciate the functionality of this Hummingbyrd design.

I feel that it may be possible to build a number of variations (upper structure options) upon this same basic 60-65 hull shape. It might be done in a manner and materials to a particular 'flat-panel' kit-boat from Australia that I am researching for another gentleman's sailing vessel project. All looks very interesting.

This is an interesting observation. It would be interesting to try and identify what it is about the above vessel that causes this, because this certainly goes against an enormous body of observation, theory, tow tanks, and CFD modeling. Its absolutely true that observations are suspect (ask any trial lawyer or psychologist), that theory is incomplete, that tow tanks introduce all sorts of difficult to isolate variables, and that CFD is really the science of pretty pictures. Therefore, I am not altogether surprised that unexpected observations during an experiment can and do occur.

But its not science until there is a theory, and a hypothesis that can be tested in an experiment where everything related to the hypothesis is measurable. Clearly, in the above observation, this is not done. But it could be.

Specifically, I think the observation can be explained by any one or any combination fo the following factors: 1) Current: when you squeeze a current, it speeds up, just like when you put your finger in the flow of water coming from your garden hose. If the speed was measured by GPS, current has an affect. 2) Pitot Speed Measurement: A pitot tube does not measure flow, it measures pressure that is then correlated to flow. Perhaps the location of the pitot was such that a pressure wave reflecting from the bottom locally increased pressure. 3) Water wheel speed measurement: A water wheel sensor also does not measure the movement of the boat, it measures the flow of water at the location of the sensor. Again, a constricted water pressure field certainly does cause increased pressure differentials, and so there could easily be a high pressure forward of the sensor, low pressure behind the sensor, resulting in locally increased flow.

Hence, I don't put much weight on isolated observations, especially when such observations run counter to both very well established hydrodynamic principles and to a mass of counter example observations in different vessels and locations.

Not to say this vessel in this location is not exposing something rarely seen or difficult to measure. It may well be a breakthrough. But I'd like to know what influences the breakthrough.

Oh, and one other thing: I don't believe in perpetual motion machines, so the concept of making a wave you can surf down is bogus.

... but perhaps the trim of the vessel was altered such that it achieved a more efficient running angle? Such an effect would be limited to narrow ranges of depth, speed, trim angles, vessel features such as CG, shaft angles, prop tunnels, ... . This might support a specific unexpected observation.

Sorry Pelagic Dreams, I didn't realize you were looking for such a large vessel,...and likely one that would require crew. If I had known this I wouldn't have made the suggestion I did.

That Pacific Expedition Cat certainly looks to be a very livably vessel, but its awfully big for just a couple of people. And even though she is a catamaran, note that she is almost twice the displacement of that 62 powercat Hummingbyrd I posted the references to. That means she has to support that bigger displacement on two hulls of just about the same length of Hummingbyrd.....fatter hulls of greater depth. And she is carrying twin 480hp or 600hp engines as opposed to twin 210hp ones. She will likely be cruising at half the speed of Hummingbyrd at a greater fuel consumption.

...just my observations

Split decision

I just went back thru a few of your postings you've made to the forum, and I discovered this one....appears to be your original thoughts...

Split Decision

...then I noted this posting by CaptJ

CaptJ is absolutely correct. If you have in mind operating anywhere down in that area inside that the world's second largest reef, you better have SHALLOW DRAFT. That was my emphases with my original posting to this subject thread...SHALLOW DRAFT is KING. And you better consider a hull design that protects your props, or you will be clipping those coral heads and losing your props. Then you have to find a place to haul that bigger vessel.

Now I can understand your wife not liking those wood interiors, and your dislike of those older trawler looks...both of which exist on Hummingbyrd. But I presented that vessel as 'the concept', not the actual vessel I was suggesting for you. If I re-read your original 'split decison' I come back to the same ideas I was presenting to that new buyer who had 13m to spend, but basically wanted a vessel he and his wife could operate and live-aboard on their own:
New to Yachting; 13 Million to spend

Smaller Vessels & Shallow Draft