Good Morning Gentlemen (I use that last term loosely).
First, to answer Dan Sherman’s question: “Rocket Scientist” is a term that was coined when rocketry was still an experimental field (Von Braun, Goddard, etc) It has long since passed into the realm of engineering with much of the basic science now reduced to computational fluid dynamics, structural engineering and combustion modeling. So it is a term that perhaps no longer applies to those of us who make a living designing jet engines and rocket engines. But, since it has been adopted by the common culture as denoting someone with superior technical know-how, we wear it (jokingly) as a badge of honor. If you must know, by “job title” is Engineering Specialist… How dull is that?
OK, I just spent 15 minutes reading the last few days worth of posts and I am totally lost as to what you guys are arguing about! Near as I can figure there are three argument:
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What affect does hull shape and the number of hulls have on the ability of the boat to tack.
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What affect do the appendages (rudder(s) and daggerboard(s)) have on the ability of the boat to tack.
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What effect does the sail shape have on the ability of the boat to tack.
It seems obvious to me that all these peices must be working together in ways that you cannot really seperate in order for the boat to tack well.
Tacking a boat (any sort of boat) requires that you have enough momentum going into the tack to maintain speed (and therefore steerage) all the way through the tack even as the drag of the hulls, appendages and sails try to slow the boat down. I am going to ignore a common technique that we used to use in the Nacra 5.8 in extremely light wind which was backwinding the jib to “push” the boat through the final part of the tack as I think this is not a techique that is easily incorporated into RC boats…
The momentum of the boat depends on the mass and the velocity. The more speed you have before you enter a tack, the better chance you have carrying that speed through and maintaining steerage as you come out of the tack.
We all know that when you tack a boat it does not pivot about a single point, but instead follows a arcing course through the water. The degree to which the hulls must be pushed sideways through the water depends on the tightness of the turn and the beam of the boat. One of the biggest differences (in any type of boat) between the best sailors and the hacks is their ability to tack the boat well. If you “slam” the boat through the tack it is slow. If you turn too gently that is also slow. The best skippers know how to ease their boat into the tack and tighten the turn just enough to get the boat around without spending too much time but without bringing the boat to a stop. This applies to RC monohulls, multis, full sized dinghies, keelboats, multis, scows - every boat I have ever sailed. They all respond a bit differently and finding the best rate at which to tack each kind of boat takes practice.
But assuming that your tacking technique is pretty good, there are aspects of the hull design which can affect how much the boat slows down during the tack and thereby affects how much distance you loose by performing the tack. In a monohull, the amount of rocker can have a significant effect on the tacking ability. But rocker has effects on other aspects of the boat handling (pitch resistance and wavemaking) so you must comprimise all of these aspects to get the a boat that is fast when sailing a straightline course as well as when tacking. There is also a tradeoff between draft and surface area. A boat with low hull draft might skim over the surface of the water when you are tacking but the extra surface area will cause more viscous drag.
In a multihull, there are some other aspects that will effect the amount of resistance created by the hulls as you tack. The beam of the boat will have an effect since the wider the boat, the tighter the inside hull must turn and the more the inside hull will be pushed “sideways” through the water. With a trimaran, one of the amas is out of the water at all times, so the seperation between the two hulls in the water is less than a cat would have. This helps the tacking of the boat. I think this is what you were getting at Dick, right? Second is the hull shape. Since there has been some discussion on the Hobie 16, I think it is useful to take a look at those hulls as an example here. The Hobie 16 does not have daggerboards (as Dick stated). This means that the hulls mush generate the lateral resistance to keep the boat sailing in a relatively straight line. In order to help this, those hulls are designed with a lot of rocker and a sharp “V” shaped cross section such that they form a sort of long, shallow draft keel. If you try to turn this hull shraply through a tack, it creates a lot of drag because of this shape. The Nacra 5.8s that I used to race had a much shallower “U” shaped cross section with gentler rocker and less draft. Much of the lateral resistance on that boat was created by the daggerboards. This made the hulls much less resistant to turning sideways through the water and therefore had less resistance while tacking. I think this is what you were getting at earlier, right Dick? The position of the daggerboards dictated to a large degree where the hulls pivoted during the turn and therefore how much hull length forward of that point had to be pushed sideways. So the fact that you have daggerboards allows you to design a hull shape that will tack more quickly. The position of the daggerboards works together with the hull shape to dictate how much drag is created during the tack. So the hull shape needs to work together with the appendage design to effect how the boat tacks.
Rudder design can have a big effect. About 10 years ago, I was contracted by a family freind to design a new set of rudders for his A-scow. The oringinal rudders (designed in the 1920s) were about twice as long as they were deep and basically rectangular in planform. They were flat plates cut out of 3/8" sheetmetal. And they were way too small. We referred to them as the “postage stamp” rudders due to their shape and small size. They basically didn’t work. The skipper could push the tiller all the way over and the boat would barely turn. The only way to tack the boat was to have the jib man ease the jib while the skipper pushed the rudder. This created a lot of drag during the tack as this low aspect ratio “postage stamp” tried to push the boat around with massive amounds of ventalation and tip vorticity. I designed a high aspect ratio elliptical rudder with an airfoil shape cross section. The rudder had the same surface area as the postage stamp, but because of its shape was able to create much more lift when deflected while at the same time generating much less drag. Just after leaving the dock on the initial sea trials, the skipper decided to “wiggle” the rudder. The amount of control authority of the new rudders caught him off guard and his “wiggle” caused the boat to turn so violently that the crew all ended up getting thrown into the bottom of the cockpit. But the biggest change was in the tacking ability of the boat. The lower drag of the rudder allowed the boat to maintain much more speed during the tack which caused the boat to exit the tack much faster than with the old rudders. The jib did not need to be eased in order to tack which allowed the boat to carry more power into the tack. The tacking ability of the boat was greatly improved by incorporating a more efficient rudder design.
Now what about sail shape? Well the sails will have two effects. First, they will dictate how much speed you have going into the tack. Doug’s example of the H16 to F18 is perhaps not the best, but if you put a sail that was of similar shape to the F18 sail onto the H16 with the same sail area that the H16 has now, it would probably be faster which would allow it to tack easier. Secondly during the tack, the sail is luffing and creating only drag as the wind passes over it. To a very small degree, a more efficint sail planform will have lower drag and therefore help the boat maintain speed through the tack. But assuming the sail area is the same, this must be considered a very small effect compared to the drag on the hull… So unless the more efficient sailplan carries you into the tack with more speed, it will only have a small effect on tacking ability.
So, let me summarize what I believe are the most critical factors that will dictate how well a boat tacks. First, a trimaran due to the lower lateral seperation between the hulls that are in the water (center hull and ama) will have less resistance when tacking than a cat (and a monohull will have less resistance than either of these). Secondly, a boat with daggerboards will be able to hull shapes that are much easier to turn. Thirdly, the rudder shape is very important. Finally, the main influence of sail shape is to allow the boat to have more momentum when entering the tack which keeps the boat moving through the tack.
I’m not sure what I have added to this discussion. It seems like you were all saying pieces of what I just said. Perhaps by putting it all together I have helped? Maybe I have just created more controversy. I don’t know. I hope you don’t need to be a “rocket scientist” to understand these things…
Will Gorgen
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