Reverse bows on cruising cats pros and cons

=================
Radio Controlled models have been used for years by some of the best and most innovative sailboat designers around including Alain Thebault(Hydroptere), Yves Parlier(stepped planing hull cat), Greg Ketterman( Trifoiler development), Dr. Sam Bradfield(40' Skat foiler development), Bill Burns(CBTF development), Hugh Welbourn(DSS development) and many others.

Pictures, L to R- Alain Thebault RC model of Hydroptere, Parliers test model of his Mediatis Region Aquitaine biplane stepped hull planing cat, Bradfields SKAT test model:

 

Toy models are well and good, until they build full size - it's then the real world intervenes ... and bites them on the arse.

 

==============
Toy models??!

 

Scale models are excellent for seakeeping and manoevering tests. They produce very accurate RAO's and are essential for any novel design to make sure the design meets the geographic requirements of operation ( in the SOR) . It's without this knowledge that the operator gets bitten........

 

but hardly relevant to "reverse bows on cruising cats pros and cons"...

really, is it?

Every design concept will have failures of one sort or another in the extreme, racing boats are necessarily extreme.

I wonder what would have occurred if they where Tornado shaped hulls?

 

Agreed. When, some years ago, I raced RC trimarans we had to fit T foils under the rudder to stop them nosediving. You don't need those on full size boats

Richard Woods

 

===============
Richard, that is interesting-did you sail a Mini 40? There's a big loss of pitch stability when models are scaled down-especially if they are scaled down w/o essential components like, for instance ama foils. I'm familiar with a few 2m and Mini 40 designs(48"LOA) from the 90's and while some claimed to be "scaled down" ORMA tri's they forgot the ama lifting foils that the big boats used to reduce wetted surface AND increase pitch stability. Some models used a T-foil on the rudder to some effect but they didn't work as well as the ama foil would have-I know I tried both on several small(48") model tri's.
Models work well for test platforms if you know what you can and cannot test. Multihull models lose pitch stability in spades as they are scaled down-particularly if they are scaled down from 60 footers so a 4' model of a 60' tri is way too small. I'm building a 5.5' test model of an 18' tri!
I think this part of the discussion is important because the subject is and/or can be directly related to pitch stability.
Here is what Martin Fischer says about the foils on his 40' tri (curved lifting foil + rudder t-foil on each ama) :
We are still "discovering" the boat, but we are getting better at sailing her. Jessica Rabbit has - for the size of the boat - relatively large foils and she has T-rudders. This combination of big foils with T-rudders stabilises the boat very nicely. So far we never have had any critical nose dive situation or any kind of pitch instability.
We sailed the boat once with normal rudders in about the same conditions as on the video. The difference was quite dramatic. Broad reaching the boat was much less stable. Average speed was about 2 knots lower and top speed more than 3 knots lower.

 

Yes a Mini 40 size, maybe 15 years ago.

In part the problem is that you cannot scale the water when you scale the model

A long way off topic, but still interesting/amusing

Years ago I met the "main man" who designed icebreakers (sorry I forget his name). He said that when they made model icebreakers they obviously couldn't use real ice - it was too strong. So he spent some time trying various alternatives. He ended up using his wife's short crust pastry. But of course that didn't have the right friction coefficient. So he then had to coat the pastry with an oil to make it smoother.

He also worked on plane models to check whether they would break up when crash landing on water. Again he couldn't use aluminium for the fuselage, it was too strong. So he had to develop special plastics.

When he was given the job of checking the Concorde plane he was told "don't just make a model that crashes, we want to see it fly as well" So not only did it have to break up (or not) when it hit the water, he had to make small engines and controls so it would fly, yet still be to scale in every other respect.

And then let it break. No wonder the Concorde development costs were so high!

You cannot make this stuff up! So believe me, its all true(ish)

Richard Woods of Woods Designs

ww.sailingcatamarans.com

 

If You do it right, can scale everything correctly and get valuable results. I start working with model testing in tank when I was a student, towing a model of one-tonner for resistance and using this data in self-made VPP. Then, I did by Ph.D. with towing tests again and studied non-stationary effects - added masses and added moments of inertia of hull-keel combinations, for use in sailing yacht maneuvering simulation software. Now, we are doing RC model testing...

Model testing just needs certain knowledge; of course there are limitations but it is very helpful.

 

So how do you scale water?

 

One does need to 'scale water', just need to work in range of model sizes where viscosity will not effect re results. This requires different model size for different tasks and speeds. Modeling technique is well developed and well presented in professional literature.

 

Reverse bow history

From an interview with Martin Fischer: (rest of interview is here- http://www.catsailingnews.com/2010/05/cs-interview-martin-fischer.html )

CS- Which was the 1st wave piercer you've designed
MF:Together with Peter Egner I worked on the design of the A-Catamaran “Toy”, the predecessor of the Flyer. At that time I proposed to Peter Egner to use an inverted upside-down bow. That was in 1990.

 

I also first used a test tank when a design student. I tried to find out how much drag was due to the wave interactions between catamaran hulls. That was in the tank often used by the Wolfson Unit, so a good, but not great, test tank.

However I found that tank work was an imprecise science, especially for low resistance, high speed boats. And for those who haven't been on a tank carriage it's also quite a frightening experience, as you accelerate towards a brick wall and hope the automatic braking system works.

This was the tank the Wolfson Unit used when doing their model catamaran testing in waves (discussed on these pages in the past). The videos they produced look very convincing, but only because they were taken in slow motion. At normal speed the models just bob around, as they do in most movies of ships in storms.

So I agree, you have to know what you are doing to get useful results from RC models. Never mind from test tank work

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

There is nothing imprecise.

It is simple metrology and accuracy. If you don't employ quality control, the results are useless or meaningless.

If you time a race, say round the Island, but using a watch that only measure units of time of an hour, who do you blame for many boats all clumped together as "the same time", despite crossing the line at different intervals, the race or the method of measuring time?

 

Couple of comments from the Jim Young book (still being edited):
But like Bill Couldrey said, “The trouble is you can experiment with small models but to get proper reliable data you need full size examples, real yachts - and they are expensive."

If you look at Des Townson’s beautiful sailing model yachts, they are naturally replicas of his bigger boats. Except, like all models, they have high ballast ratios and deep keels which give models an enormous amount of stability simply because of their draft. Like all small models they need it, because a particle of wind takes a fraction of time to go from one end of the model boat to the other, as compared with a full size boat. So 10 knots of breeze is like a huge gale to a model.

Oh yes; if people here haven't heard of Bill Couldrey and Des Townson, they were true and very sailing practical New Zealand yacht designer/artists plus also being superb boatbuilders (a fairly rare combination - but there are some examples around today) - they spanned the years from mid to late 20th Century.