Displacement vs Planning

I'm looking for information, text, videos whatever good educational information I can have about the differences of displacement sailboat hull vs planning sailboat hull. The material is intended to use for teach a group of backyard builders to develop rules in order to preserve their local native sloop class. Thanks.

 

There is no clear cut boundary where one finishes and the other starts. It has been the subject of unending debates for over a century. The term semi-displacement and semi-planing encompasses the gray area of speed in between. However, that is also a matter of heated debates.

 

Welcome to the forum.

You'd be best advised to consult with a local designer or NA, as the information you're asking for, in particular the possible rule sets that might come, needs much more than a casual conversation in an online forum. These particular subjects can become quite complex and involved across several engineering disciplines.

 

Thanks fellas! Well, I do not want to reach too deep into the technical aspects. Its more like the basics of it. Part of the controversy around here is where is the line that separate one from another in terms of how V shape or flat shape the boats should be. We want to maintain the essence of the local traditional sloop and avoid precisely the wolfs with the sheep custom. We are thinking on create rules that limit how open the V shape could be for example.

 

I don't know you're asking the right question then. If you want to know about rule writing to control shapes then there are probably better places to ask: somewhere where more serious racers hang out.

There are a number of ways rules can work to control hull shape, but a lot of it is about exactly what you are seeking to do. Planing vs non planing, to be honest, is probably the least of your worries. Things like volume distribution, fineness of bow, rocker and the like are probably going to be even more important.

I suppose the first thing to work out is exactly what you want your rule set to do - something that is often forgotten. One of the reasons the offshore racers have so much trouble with measurement rules is that they seem to want a rule that will handicap all boats equally, encourage new building and preserve investment in the existing fleet. Obviously you can't have all 3 of those together: lucky to make two!

So are you seeking to encourage new and different hull shapes, or keep the designs essentially static? Are you going to attempt to produce some kind of rating for handicapping purposes out of this rule (ouch!), or will all race level? Is what you want really something approaching a one design rule where new boats will all be essentially the same ?

 

What I think the OP is looking for is a magic bullet. Currently there are the old school versions of the sloop, with a scattering of newer designs, that though similar in aestedics and configuration, are designed to plane off, which just kills the viability of the older designs.

I don't think they need a rule so much as a rating system. Clearly some boats out preform the others. These need to be corrected on time, based on past performance and performance potential. Basically, you run a few races, typically 5 or more and averages are drawn, so a corrected time can be applied to bring them in line with the others in the fleet. Another option is a baseline run against a known design and averages are drawn up for their rating.

 

What I am seeking is discourage or more precisely, banned new hull shapes with planing capabilities. For that, I have to get boat owners to understand the differences, just that, understand them and know to identify the physical characteristics. The class is not into handicaps so box-rule will be the way. We already have boats that are being judged as planing boats. I once read in a book, don't remember what book it was, That planing boats need certain amount of sail area, weight per displacement ratio, and other stuff. I think that is possible with a box rule controlling the V shape, the beam measure, the stern measure, the ballast and sail area we can discourage people to build VO70 or TP52 style boats to race with traditional sloops.

 

Exactly that is what is happening, but handicap is not possible yet, we are not prepared for that.

 

Accessing a well designed hull form requires a good bit of understanding. SA/D, SA/WS, D/L and all the rest need to be calculated, which requires an accurate set of lines and weights, which some boats may not have available.

A box rule will control new designs, but will do nothing for current designs, again without the full set of spec's, to see if they fit. An effective box rule will be very limiting and will force even more clever design work, all of which is to circumvent the rules as best as practical. Handicaps are much easier and can be applied to existing and new designs. It's also the only good way you can have older designs remain competitive with newer ones.

 

Cmarrero, there is a great deal of expertise (or perceived expertise) in this forum. But it's really tough to get a straight answer. You have heard of reverse engineering. Try applying negative engineering. Suggest a scheme, let the posters beat you up for it. Propose another scheme, and another. The one that results in the fewest bruises is probably the best idea.

I think that rocker is the greatest limitation to planing. A curved underbody just can't generate enough lift to get a boat on top of the water. A minimum of 14 degrees of rocker at the aft waterline (a number stuck in my noggin from the ACC rule) could guarantee that a boat will not plane.

What does everybody else think?

 

It depends on how and what you measure. There hasn't been a box rule that can't be beat by creative designing.

 

I think you are very wise to try and avoid handicaps. The history of rating rules could be characterised as a succession of one abandoned failure after another. Empirical handicaps are moderately straightforward given a very large body of data and boats of readily identified classes: probably not your situation here. The key problem of empirical handicaps is that the difference between two different crews may be as much as 20% extra time per lap, enough to completely submerge differences between the speed of the boats. Measurement handicap rules are extraordinarily difficult, and best avoided.

However neither will fulfil what appears to be the prime aim here, which appears to me to preserve a historic style of craft, while permitting new boats and even new design. I have been involved with writing active box rules, but only for classes which wish to be progressive.

However this is not an original problem, and its been faced with varying degrees of success by a good number of historic classes around the world. I honestly don't think you are going to achieve very much by talking here about generalities, and you might be a bit too focussed on planing against other characteristics, I don't know.

I think it would be good to get in contact with as many class associations as you can that have faced similar problems, and see how they dealt with them. In the UK ones that occur to me are the Thames Raters and the Salcombe Yawls. In Australia the Couta boats spring to mind, and maybe the historical skiffs might have something to offer too. There will be many others.

What I'd do next, on reflection, is get out a basic description of your craft - at the moment I have no idea whether they are 6 or 60 feet long - maybe a couple of photos, and post on various forums asking for suggestions of other classes worldwide which are vaguely similar and have had reasonable success in framing rules to preserve historical character. Then you can investigate them and see what you can learn from others.

I've just, BTW, had a quick look at the Couta boat website and their rules. They might be very much on the lines you are thinking of. Take a look. http://www.coutaboat.com.au/official-class-rules-2011-2015/. I suspect what they did was to measure a bunch of existing boats and find a box they all fitted in. That's probably a good plan. If you get the lines of all the existing boats you may well be able to define a box in the same way these guys have.

 

Mmmm plenty of boats with loads of aft rocker plane fine try explaining some older INT Moths, and Nat 12's like Final Chapter......pretty sure even some of the Dubois keelboats could too and they had quite a rise aft. Planing is about a bit more than just a flat stern when it comes to sailing boats...

 

Wow, I didn't know that. That's very interesting indeed. I tried to find a photo or a diagram of Final Chapter, but I didn't come up with anything. Do you have something for us to look at?

Is it that the Nat 12 trims back on the aft segment to get a lift surface? (See photo). This would be similar or opposite to a float plane with a stepped hull.

I want to know more about this phenomenon.

 

Quite hard to find a good side elevation of a 'Chapter but the rocker is dead straight for the first 9' or more from the bow. The precursor, an experimental boat called Gryphon originally had no c/board but a bow fin and rudder only, and this was before Bill Kogh's America 3 12 meter...which used a similar sort of arrangement....

I have however found a link to a Mervyn Cook Design from some years earlier and can personally attest that these designs planed fine. Nat 12s are allowed no hollow in section aft of 1200 from bow and no steps. You can have a reverse garboard ie slightly tunnel hull though, tried with the Windfall design.

http://www.johnclaridgeboats.com/National-12