Is it a catamaran thing, or just badly sailed?

Norman_E

Norman_E

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The other day I left Orharniye about half an hour after a large catamaran, by the time I was out in the long bay of Hisaronu Korfesi with about nine 9NM dead upwind to get to round Atabol headland the cat was at least a mile ahead of me and upwind. Now I know cruising cats are not as close winded as monohulls, but with very light wind I elected to sail well freed up, at over 60 degrees off the true wind and tacked through more than 120 degrees initially, only hardening up as the wind gradually rose from about 5 knots at the outset to about 15 knots, at which I was at about 45 degrees off true wind. To my surprise the cat quickly lost ground to me and by the time I rounded Atabol I was at least two nautical miles ahead of it, and about the same distance upwind. I do not claim great sailing skill, and all I did was keep close watch on wind speed and direction and keep the genoa tell tales flying as straight as I could.
Should I really have outpaced a cat at least as long as my boat so comprehensively on an upwind sail?
 
A cruising cat will normally sail faster than a mono of the same length on a reach but about the same on a beat or a dead run. If not sailed well it could easily be beaten to windward by a mono. In light winds the extra wetted surface of the cat will put it at a disadvantage while in stronger winds it will continue to accelerate as the mono is being slowed by wave formation. Cats have no 'hull speed'.

So - no surprise that you should be able to overtake by paying attention to your course & sail trim.
 
Which reminds me. I just read this on the web....

"Bilbo Baggins died in bed last night after an overdose of Viagra. Old Hobbits die hard."
 
[ QUOTE ]
Where are you these days?

[/ QUOTE ]Still living it up in Brazil. I don't understand why more cruising boats don't head for these waters?
 
I remember feeling very smug easily beating a large cat in about 10 knots of wind. When it increased to 15 they left us for dead.
 
I would have expected him (if trying) to make a course maybe 5 to 10 deg further off the wind than you (leeway) but to sail faster.

However, its awfully easy to overlaod a cat and the effect of weight is far more significant than on a mono. For example on my old Prout 33, filling the water tanks (60 galls) was good for between 0.5 amd 0.75 knots. Its also easy to lose a lot more through fouling since cats go faster because they are more slippery. Remove that slipperyness and the speed vanishes even to the point of not being able to tack.

But on average my old cat was 15 to 20% faster (all points of sail averaged out) than the same length mono that followed it.
 
[ QUOTE ]
A cruising cat will normally sail faster than a mono of the same length on a reach but about the same on a beat or a dead run. If not sailed well it could easily be beaten to windward by a mono. In light winds the extra wetted surface of the cat will put it at a disadvantage while in stronger winds it will continue to accelerate as the mono is being slowed by wave formation. Cats have no 'hull speed'.


[/ QUOTE ]

Agreed. It is also true that cats vary just as mono's do. My heavy old Snowgoose was generally trounced to windward this week-end in about 10-15 knots by equally sized fin keelers but still managed to see off cruisers with bilge keels and smaller boats although about 5deg further off wind than most. When the wind briefly increased to 20 knots I was able to go as close to the wind as the bilgies and go still faster but was still trounced by racy fin keelers as I would expect. Downwind however I overtook several racing boats with my big cruising chute and was thumping along at 9-10 knots in about 17-18 knots of wind.
Your point about wetted area is absolutely valid too. In light airs it's what holds us back especially one like mine dragging a centre nacelle in the water. In up to 30 knots however she comes alive and romps and I have surprised many performance monos at her ability to make good passage times across the channel in less than pleasant conditions.
I don't quite agree that they have no hull speed limitation though. It's still true that hydrodynamic resistance rises with speed. It's just that the narrower your hulls are the higher the speed limitation is.
 
On the hull speed issue I deliberately over-simplified. Fat cats with less than 8:1 length:beam ratio do dig holes and sit in them like a mono so have a hull speed limit. Catalacs and most Bill O'Brien boats are examples.

There are two components to the resistance of a hull:

(a) the frictional resistance of the skin which is proportional to the speed and wetted surface area of the hull. It increases pretty well linearly with speed

(b) wave-making resistance whenre the hull is trying to climb over its own bow wave. This is insignificant at low speeds but increases dramatically when the speed reaches 1.4 x sqrt (LWL).

Because very fine hulls make virtually no waves, the second factor doesn't come into play. The result is that as you add more power a fine multi will just keep going faster and faster. The only way a beamy boat can do so is by planing.

As an example, consider a 60 ft rowing 8 whose hull speed would be around 11 knots. Racing speed is around 20 knots with no trace of wake at all.
 
Miscalculation in my mental arithmetic. Looking again, the world record speed comes out at 11.6 knots while the 'hull speed' for 60ft is 10.8.

The point about wake stands though.
 
[ QUOTE ]
20 knots? Really? I can easily keep up with one on my pushbike along the towpath, without even trying hard.

Italy just won the M8 at the World champs in Spain in a time of 6m02s for 2000 metres. I make that 10.7 knots.

[/ QUOTE ]

I did 2000 meters in an 8 in 5min 23 seconds at the Ile de France Regional championships a while back. (Ok we did have a bit of tail wind.....)
 
Yes well I agree in principle (but not in detail) with what you say. I would only point out that your boat is built more for speed than mine which has much beamier hulls with correspondingly higher load bearing capability. More a cruising boat in fact than a performance machine. However I think we would both agree that the primary reason for choosing a cat is not speed. A cruising boat has many demands on it's capability and primarily the comfort, sea kindliness, lack of crew fatigue and sheer live aboard capability make the cat incomperable for true living aboard comfort. Frankly this is why I have one, not speed, and I don't give a flying f*** if I get overtaken to windward. What is important to me is when I arrive (usually ahead of cruising monos) I have a hot shower, nice supper, and climb into a large double bunk with a warm woman. Gone are my days of hanging over a rail at 30 degrees to the vertical, damp bunks, Fray Bentos pies and walks up the pontoon for a lukewarm shower at the yacht club.
 
There's a spectrum of reasons for buying a cat. at one end is comfort, at the other speed. Having chosen a Prout I would expect comfort to be higher on your list but I would expect the owner of a Dazcat to be prepared to rough it a bit in search of speed. I have made a few sacrifices to get more speed than I could get from any production boat and keep the weight low to help with that.

Here are a couple of formulae developed to compare performance of multihulls:

Sailing performance index = sqrt(LWL x sail area / displacement)

Motor performance index = sqrt (LWL x power / displacement)

In the second case, if you use metres, tonnes & horsepower the figure gives a pretty good estimate of top speed.

Note that these don't work well with wide-beam hulls or planing types.
 
I agree with what you say about there being many more reasons to buy a cat than speed. The majority of cruising cats are not particularly fast and while I take your point entirely, a dazcat isn't a cruising boat at all really. Having said that, no one likes to have a boat with very sluggish performance and the Prouts are not the fastest, but over maligned in my view. Having built my own I have a tall rig ( 42ft mast) and consequently carry more sail than most to compensate for the high wetted area and usually plan passages at about 6.5 knots. I usually achieve better and often surprise myself especially downwind with a kite up. Compared with other cats, including yours, that's slow perhaps, but in terms of monohull performance respectable. The point I was originally making was similar to your own. If you want something to go fast to windward and that is a major criteria, don't buy a cruising cat! There are many reasons other than that for having one though and the likes of you and I have probably chosen cats for reasons other than pure speed. You are indeed probably more interested in speed than I am which is why we don't have identical boats. All boats are a compromise and there is no such thing as a perfect boat anymore than you will ever get a perfect woman..... (bet that gets Pauline going if she reads it)

As far as your formulae go they don't of course have any more of a scientific basis than other such formulae for monos. They might work out OK for some boats but as they totally underestimate the speed I get under power (7.5-8.0 knots with 32HP) I suspect they are not entirely useful as mine is indeed a wide hulled design. If it underestimates that it will certainly underestimate others with slimmer hulls.
Having spent my working life building floaty things, and being involved with the design process too, I think the prediction of speed on a multihull is very difficult unless there is a benchmark (which there usually is). Having been involved with the build of vessels for Nigel Gee and Nigel Irens who are both pretty well respected, even the hydrodynamicists in those hallowed design teams hedge their bets and "contracted" performances are never anything but safe minimums which are usually exceeded on proving trials. Frankly it's much too complex a problem to reduce to such a simple formula. But hey ho it's as good as any "guesstimate" I suppose and I can't offer better so no sweat....
 
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