What scales down; what doesn't?

It is important, perhaps, for aspiring rc designers and sailors to take a look at what does scale down and what doesn’t.
Some examples I’m familiar with are that a 49er hull won’t scale down very well because the viscosity of the water is such that the planing characteristics for a reeasonable weight(with enough righting moment)will not replicate in a model like it does on the full size boat. A wide light displacement full size boat hull like the one on Mari Cha would not scale down well-it would have to be much more narrow to be fast as an rc model.
I had the priviledge of working for a while with Dr. Sam Bradfield a speed sailing pioneer and designer of the full size Rave and Skat. We were working to develop an rc sailing hydrofoil that would be practical as well to research the behaviour of foils on models vs full size hydrofoils. What we found is that foils scale down exactly in planform- which is quite different than what was assumed by both of us before the extensive testing we did. If you look at model airplane wings as compared to full size wings they are generally larger. On hydrofoils we found that they scale precisely from full size to model AND the other way as well. This was news at the time and the tests he made then on the Flyer? model(that I designed) were applied to the design of the 40’ Skat.
Interestingly, while a wide planning hull doesn’t scale down too well the PLANING performance of a narrow stepped hull(like Hydraplaneur) DOES appear to scale
down. An interesting aside on Yves Parlier’s new 60’ Hydraplaneur is that it uses rudder t-foils- one of the first big cats to do so. One of the reasons for that is the behaviour of the stepped hulls: they require some form of control in pitch to prevent odd behaviour including porposing. On seaplanes the pitch control when on the step is from the elevator; on the Hydrapalneur pitch control is from the variable angle of incidence hydrofoils on the rudder. This kind of application DOES scale down.
Another area that many believe does scale down are the planforms of airfoils ,hydrofoils , wings and last but not least sails. In other words if a square top works on a full size IACC model it is likely to work on a model IACC boat IF the technical issues are solved. The technical issues like how to control the leach on such a sail with or without full battens on a model . On some boats this is solved by a gaff or bent back rotating spar that supports the leach. To date there has been some limited testing showing good results and some types of squaretop have been used in the 10 Rater Class with success.
Most concepts of movable ballast will scale down to some extent: Movable ballast on a wide stable platform like a multihull(a “robotic” crew) works very well(used on a winning mini40 in a major European Championship) while the same type of system on an rc monohull will not-generally- work too well without severely reducing sail or having some ballast underwater. An exception to this appears to be the Musto Skiff model being developed by Pat Clear in the UK though performance info is not yet available.
Canting keels ,however, work very well from a boat handling standpoint -I’ve had a great deal of experience actually sailing them-but from a racing standpoint little is known.(I’ve done extensive two boat testing with rc canting keel boats but little racing of canting keels vs other boats though that will change soon with me and the many others developing rc canting keel raceboats). The biggest problem in scaling down canting keels is scaling the the extra lateral resistance they require but based on testing they appear to scale quite well.
The verdict is stil open in most peoples minds whether or not these types of big boat technologies scale down in terms of race winning results but in terms of functionality they appear to work very well. One problem is the inability to use these technologies in most existing classes because of previously established rules.So most development work is being done in new startup classes(Formula 48,multiONE,min40 , Formula 100) or by individuals working on their own.
What does and does not scale down is an interesting topic that rc model designers have to consider every time they draw a boat and in this age of innovation in big boats is something worth paying close attention to if you’re interested in pushing the performance envelope.

edt:news flash moved to Little America’s Cup thread under “Technology”
Doug Lord
–High Technology Sailing/Racing

My dear friend lorsail!
Regarding scaling and prototypes. Using towing tanks it beacame obvious that cost reduction could be achieved if a replica scaled version could be tested in a medium with scaled properties similar to water. Years ago one tested out a fluid which handeled the problem. Unfortunatetely, the medium was toxic and the tests were abondoned. For instance, check the SSPA web-site, where the prototypes of Victory Challange yachts were tested.