Well I must be one of the few who can actually believe his Tasar claim. I have had several 16/18/20ft cats but my last mono (20 years ago) was an early lightweight (kevlar) tasar. I can recall launching it one wild day when it needed to be held down on its trolley with its sails down. Once on the water it was amazing. Real crash 'n' burn stuff but incredible to see the rate at which the waves were being overtaken downwind. I have never heard my wife swear so much in her life! When I returned to shore I was immediately accosted by several spectators who had been impressed by what they had seen the boat do. I am not saying it could perform like this regularly, but put the right ingredients in the mix and it is surprising what can happen.

Cheshirecatman

Thats very interesting. You have raced the 18s on several occations, haven't you? From your experience, how do the 18s compare downwind? Also interesting that even the skiff sailors have a hard time believing this. As already posted, I think Bethwaite claims a top speed for the skiffs well above 30 knots.. That been said, if I'm not terribly mistaken, I do believe that the official Tornado site claimed the top speed of the Tornado to be around 30 knots as well. To me that seems a bit high. Though I did believe the skiffies would achieve speeds in excess of (at least) 25 knots..?

Now that would be a match made in heaven. The revolution combined with technical facts

This discussion relates to speed limitations of small catsailor hulls.

We all agree that wave drag is one of several components of total resistance. In very light and slender catamaran dinghys, it constitutes a small component at low speeds. However, wave drag does increase significantly when the boat travels faster than the velocity of the waves it generates.

Wouter stated that
“wave-inducing drag it is something like 10-15% of the total”.
He proposed a doubling of wave drag with increasing speed over the wave velocity (“Now, you can easily double 15% and have the beach cat hardly notice much difference”).
Do we really know this number? We do not know whether it actually increases by a factor of 5 at 20 knots http://www.boatdesign.net/forums/attachment.php?attachmentid=18865&d=1203073097

It has been proposed that wave drag is of similar magnitude as viscous drag in kayaks already at 7 knots!
http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm


I tend to believe (and agree with Wouter) that surface drag is more important than wave drag in our cat dinghys. But a major question is whether wave drag of displacement hulls constitutes one (of several) significant limiting factor in attaining high speeds.

My experience has been that it is nose diving that has stopped us from sailing faster than 20 knots. Like Wouter, I think T-foil rudders are promising (next X-mas, Santa?)


Attitude:
Wouter, I really do appreciate your contributions to this forum. Many of your posts are helpful.

But why do you always seem to respond in a condescending way?
One example among many:
Quote: “as you gave a typical tortured reasoning that is often expressed by persons who keep clinging on to Froude's law as a predictor for maximum speed”.
This was to a post that did not even mention maximum speed, and to a person who has no interest in defending Froude or any other of his time.

You tend refer to science. This is good, we look for state-of-the-art knowledge. However, a typical characteristic of almost all scientists who publish in leading international peer-reviewed journals, is that they never exhibit condescending or arrogant behavior during scientific discussions.
A respectful approach to colleagues is also very productive in developing better equipment and skills.

When it comes to hydrodynamics of hulls and foils, a lot is empirically derived knowledge rather than formulas.
We need more empirical data.

Stein

Hi Stein,



We don't know this number, because we can measure only the total drag, but we can calculate a drag break down. Wave drag and viscous drag are plotted in this two diagrams:
http://www.catsailor.com/gallery/thumbnails.php?album=lastupby&uid=230
Please note, that the speed is in m/s which is about 0.5m/s = 1kts. I selected a scale to unrealistic high speeds, just to show the behaviour at high speeds. Also note that only wave and viscous drag is shown, but no induced drag. And the displacement, pitch and heeling attitude is constant over speed in this calculation. The method used is called "michlet" and is free available.
You can see, the higher the speed, the lower the wave drag ratio. That is because after a certain speed, the wave drag is more or less constant, while the viscous (friction) drag raises and raises.


There is nothing mystical about hydrodynamics. A lot of knowledge is available for free. However your skills has to be trained to use it, it is sometimes difficult to understand.

Finally even if you can predict the drag of the boat, it is a different job to optimise a boat, which requires interdisciplinary skills. A homebuilder can still hit the right design to achieve high speeds and give boat development a new momentum, without any CFD calculation and experts. However the probability...

Your catamarans speed is limited by drag and sail power at light winds, by capsizing at strong winds and up wind courses and by pitchpoling at strong winds on the other courses. So yes, foils or ballast tanks may help on certain courses and conditions.

Cheers,

Klaus

My buddies Pontoon boat will do 35kts using a 115hp outboard. It's larger than a 18' cat and uses, what to me looks like 55gal drums with a pointy front. It's definitely not planing, and I bet it's above hull speed. I've seen pontoon boats on Lake Norman here in NC that goes a lot faster than that, people even wake-board behind them here. I guess with enough engine you can make anything move.

Yep, raced them quiet a few times now on the F18. The F18 smashed them upwind and had the edge on the downwinds with the 18 Skiffs performing pest on the downwinds in relation to the F18 when it was 10 knots or under. The Tornado always had the edge on the F18.
The Tornado class claims a top speed in excess of 33 knots. I think they are just as optimistic as Bethweight.
Once again the Tornado class ran speed trials and whilst it is surley not the quickest run on a Tornado, they would be up there. Results were 27 MPH or 23.5 knots average over the 500 foot course. Completed in 12.53 seconds.

A far cry from 33 knots, however this was an average over 500 feet. The 18s have never done these types of runs...... Perhaps because the Bethwaite claims may be exposed for what they really are (along with many of his other wild claims)..... BS.

Like I said before NO ONE KNOWS!!!!! Actually does anyone remember the actual question? <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

Clayton

The highest speed I have known the Tornado to go was at CORK '75.
A 40kn storm hit, when the the fleet got to the reaching leg.
The peak of the storm must have hit, when the leaders got to
the jibe mark. At this mark a couple boats broke their side stays,
one of them was Reg White who released the other stays, so as not
to damage the boat from the mast.
Notary was there on "Salt Water Wine", Zutec on "Twister" & Smyth.
Anyway a Canadian destroyer radared the fleet at 28kn average.
That's 33mph average, at that speed the boat was NO longer in
the water it went like a skipping stone from crest to crest 9'(3m).
It was amazing the Tornado could skip the crest, and soar over the trough.
It was sheer madness, with some poet license here, it seemed like
we zoomed over 5 crests in ten seconds, and the wind would
calm down to 2 or 3 crests in 10 seconds a la bucking bronco,
and then accelerate again, & fly over the trough.
I will never forget the thrill, not to mention wondering will
the wind ever slow up. I recall a Tornado to windward of us,
and counting it cartwheeling/rotating about it's mast 3 1/2 times, and
the crew later righted the boat, unassisted?
Anyway, I would call it somewhere between sailing/flying & oh ____.
Great wind & waves,
Chris

Did you guys see that "thing" on the front page of SA?
They got rid of hull drag by removing the hull!
http://www2.jundt.ch/blog/?page_id=35
Video of the thing sailing...

Klaus,

Thank you! I know of the Michlet code, but have not used it yet.

There is one question I wish someone could answer:

What is the reason that different sources report totally different speed-wave drag associations for seemingly similar hulls shapes?

Compare the curves you are presenting with the curves for the kayaks
http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm
- totally different relationships.
Does the sharply cut vertical stern of beach cats make the difference?

Stein

If Wouter's statement that aerodynamic drag is a major speed-limiting factor is correct, this will not be a success in terms of max speed. I think they will need to cover the tubing spaghetti with some kind of skin.
Stein

Hi Stein,



Actually, they are not different. The kayak data have a range from 0 to 7kts, which is 0 to 3.5m/s. While the scale in my diagrams ranges from 0 to 15m/s (0 to 30kts). Until 7kts the curves a quite comparable. Sorry, I don't have a zoomed view of my diagram. After 8 kts the wave drag stays more or less constant, and I would expect the same for the kayaks. There is always a risk of taking wrong conclusions if data of limited range are extrapolated, in other words: If we step beyond our knowledge.

As long as the flow is not attached at the vertical stern, a boat behaves as a slightly longer one, but with slightly reduced friction drag. If the flow 'attaches' around the stern, you get a different source of drag, called form drag. If the sterns of your cat are 'sucking', you should sit more forward to avoid this additional form drag.

Cheers,

Klaus

A bit more heresay from me.

Went out yesterday on the TS. Gusty and hard to settle the boat.
In waves it was a seriously rough ride. The faster it went over the waves though the smoother it got.
I am having trouble setting the boat so as not to lift at the reach I want.
I had one big, sustained gust headed further off the wind in less chop. The boat compressed onto both hulls and into the water, certainly NOT planing.

This was my fastest run but also by far the most scary.
I routinely sail cats in 25/30 knots and I have never been afraid of crashing. I also have a strange "slo-mo' sensation of not really fast.
I was clear yesterday that I was going faster than I would like to crash at but sadly..no planing.

A good GPS is on the way for me and yes, I am aware I cannot take those results as fact.

I did however watch the Tornado Worlds off Orakei in Auckland about ten years ago. Small rigs.
The day I watched was held in close to 30 and 35 knots.
They were absolutely traveling on the last foot of the lee hull and one of the most impressive sailing memories I have. So impressive were these TOrnados that I heard about them on my car radio and diverted to watch the spectacle spoken of. I knew then I had to sail cats.

As far as the foiler thing going fast. Why is it I am not interested in what such an oddball, PITA to set up and use sort of boat does?

Surely we all deal in a boat that has a pleasure/race capability?

And by the way. There were several people (not one group) waiting for me on the beach when I came in wanting to know how the boat cold go so fast.
One guy saying "the water coming off your boat looked like smoke" over and over. So cats can interest the public. They just have to be seen.

Klaus,

Thank you!

So for slender beachcat hulls and kayaks the wave drag vs speed relationship seems to follow a sigmoidal curve (After 8 knots wave drag stays more or less constant).
While for conventional displacement monohulls, the wave drag vs speed curve looks like there is a quadratic trend.

Is there a critical width/length ratio or critical width which determines the transition between these two very different relationships?

Or maybe the weight of the hull comes into play (the g in Froude number)?

I have seen people mention width/length ratios, but I have never seen anyone give a good reason why a certain ratio should alter the wave drag vs speed relationship dramatically.

Stein

Hi Stein,

I never did a calculation with monohulls, so I can only guess.


The first thing I would do is to make a calculation to very high speeds again, much higher than the boat would be able to go, so I can be sure to cover the whole range.
For sure the beam/length or slenderness ratio will have an influence on wave drag. But I have no idea, if it scales the curve only or changes the whole nature of the curve. For this somebody should run a lot of different hull shapes in the michlet code. However I wouldn't expect any dramitical change.
A low wavedrag non-planing monohull may be to slender to have suffient roll stability. That's maybe the reason why they go the planing configuration.

But again, I guess rather than I know for monos.

Cheers,

Klaus

I used to tow my daughter behind our Taipan 4.9 on two skies.