Panel or Not to Panel?

In my rc model sails am wondering what the benefits of using paneled sails really are. Surely if they are permanently shaped when they are made they will not flip when you tack or will at least require considerable power to invert the camber.
Would flat sails perform better?
Or is this induced camber a vital aspect of sail design?

Alex

 

economy of scale

Paneled or no, there are many lines dedicated to defining the camber in depth, breadth and location and, although different situations call for different shape, I understand that starting out with some definition would be a benefit. I don't know how many lines you can reasonably put on an RC vessel but I imagine that a paneled sail and maybe some battening would save you some trouble. As far as getting the sail to "reverse", there is a trick to popping the battens but it's nothing complicated. The only time the issue ever arises is in light winds and you let the sail "fall" out and yank it back sharply and the air pressure inside the sail pops it. I usually grabbed the outhaul sheave on the end of my boom and gave it a quick yank as I tacked. I have no idea how this translates to RC though. Maybe an actuated lever could come up to the mainsheet like a flyfisherman's index finger.

 

Flat sails will go downwind as well as a cambered sail, but they will not perform well when going to windward or on a reach. You did not say whether or not the sails will be fully battened. Soft sails with leach battens have no trouble whatever in reversing camber. Full battened sails are problematic on RC boats but they are not impossible to control.

If you make your own sails you must anticipate mast bend and cut the luff curve accordingly. Good model sails usually have a higher percentage of camber in the top panels than in the lower ones. A sail of 1.5 meters in height, for example, will probably have four panels. Each of the seams will be curved somewhat in order to induce camber. A material called Trispie is the current material of choice for serious model sail making. It is a mylar like material that has a reinforcing matrix that looks something like ripstop.

The short answer to your question; Would flat sails perform better ? NO! do not even consider a flat cut sail. It will only disappoint you. If you can afford professionally made model sails then buy them. You'll be glad that you did. There are several very creditable model sail makers in England. I do not know of any in Scotland.

 

It looks like some of the other boats in the competition were using panneled sails and i think this is what let us down last year.
Am i right in saying that by reducing drag you reduce the heeling force and therefore the boat can sail more efficiently in a straighter line.

Ive been looking up on making a camber board for the model but it seems i will need to make 3 boards for the sail as the camber varies at different heights.
I think the mainsail will have leech battons and the outhaul at the boom and gaff should give even more control over the camber.

Alex

 

Alex:

A properly cambered sail will produce more drive than a flat one. Drive is what you are after here. Cambered sails will not necessarily reduce drag, they just pull harder. In fact, there may be more heeling moment with cambered sails, not less. Consider the force vectors in a sail. Roughly, the main vector points diagonally (hopefully forward). The resultants produce a forward vector and an athwartships vector. If the principal vector grows larger, then the sideways one may also be larger. Maybe perfectly formed sails would have the principal vector pointing more nearly forward and so the sideways force would not increase. That is perfect world thinking. That's the kind of stuff that sailmakers aspire to and sometimes prevaricate about.

You mention Gaff. Will your boat be gaff rigged ? Are the competitive boats also gaff rigged ? If you have the option, use a marconi style main. In all probability it will go to windward better than the gaff. Off the wind the gaff is as good or sometimes even better than the marconi. That is contingent on how well you can control gaff sag to leeward. Consider that races are largely won or lost on the windward legs. So, one is better advised to use a rig that has presumed superiority for windward work. You have not said whether you will use a foresail or jib. Conventional wisdom has it that the jib does more work per unit of area than the main does.

RC jibs are typically mounted on a club which presents many tuning opportunities. You can easily adjust jib luff tension by moving the club pivot point and you will usually use a jib leach topping lift to control leach tension. The jib is outhauled to the club and the rest is only a matter of sheeting. If you have a separate servo for the jib sheet, then trim is a matter of skipper skill. If you really want to be tricky you can use another servo to make a "twitcher". That is a device that jerks the jib sheet when you tack going downwind and you have the sails goose winged. It is easy enough to jibe the main but sometimes the jib does not go around nicely. That is when you use the twitcher. This is down wind sailing and even though upwind performance is the highest priority, you dont want to give any thing away on the running legs.

Excuse me if I have mentioned jibs and controls unnecessarily. You may already be well versed in that area. If not the information is worth consideration.

 

I am planning to have a kinda gaff rig but im unsure wether to have a gaff boom or just a stiff batton as i think the head of the sail is meant to be flatter so will not produce much lift but acts more as a kind of control for seperating the two areas of pressure perhaps?
This is kind of what i see on americas cup boats but with a straight top instead of a curve.
I will have a jib on the front which will be on a seperate jib boom. I have not really though about the controllines to the jib yet i think it will need a weight or something at the front is this to help center the boom?
I have never even sailed a boat with a jib on before in my life so excuse my lack of understanding. I think i will need some pivot point at the front like what you mentioned and also a point where it can pivot at the top of the sail.
I was then going to have a sheet which would control the amount the jib could swing out (dont realy know how to say it) this was going to be attatched to the same servo as the main control and have it so i can adjust the distance the jib travels relative to the main before each race.

I only have 2 servos for the whole boat so cant have any other stuff.

when i say less drag i mean the sail will have a higher lift to drag ratio which is what you want when sailing upwind right? As the lift vector is mainly going in the direction you want wheras drag is countering the benefit.

I have only been sailing 5 times in my entire life and that was more a case of some person telling me to pull a rope and when to do it so please forgive my ignorance in this area.

I would also like to have a 'camber inducer' (dont know if that is correct term) where the mainsail is set aft of the mast to reduce terbulent flow over the front of the sail.

Alex

 

You seem to be caught up in a bunch of high tech stuff. The latest and greatest thing in sails, at the moment, is a flat top. Those sails are used by some very fast and sophisticated boats. Of course you would be attracted to cutting edge technology.

Camber inducers are gadgets that do what is implied. These are found most frequently on windsurfer type boats. The windsurfer sails are typically fully battened and often flat topped or elliptical topped. That is all well and good but such stuff has can not be justified for an elementary RC boat.

I urge you to use a rig of ordinary design. Complicated systems, at this time in your sailing/modeling career will cause you more problems than they are worth. Fast boats and/or fast sails are not fast unless skillfully handled and sailed. There is an axiom among model boaters. It says that you should spend 40% of your time building or improving the boat and 60% of the time practicing your sailing technique. That is not an easily accepted concept because it puts the burden of accomplishment more on the person and less on the boat. If you miss one wind shift on a windward leg or at the starting line, you will be disastrously handicapped throughout the remainder of the race. Practice starting technique fervently, learn to read the water for wind condition where you boat is at the moment and the condition in the location where you'll be 60 seconds hence. Practice, practice, practice ! If you have an ordinary boat without technical gadgetry, and you are a better sailor, then you will win.

Alex, forgive me for lecturing you. I only wish to be helpful. Try to locate some good racing sailors of full sized boats, make sure that they are older guys or girls with plenty of experience. They will almost surely affirm that the best sailor wins, not the best boat. This is not to suggest that a junk boat is what you should build. It is to claim that skill will trump technology on the race course.

One last comment. Practice is not costly. Hi tech gadgetry IS costly and for a beginner, an unprofitable choice.

 

I can see what you are saying but that is where it gets more complicated it is not just me that has to sail the boat we have to be able to sail each others and the rest of my team are well very very good at sailing. I am trying to get some practice in now with my kit model boat and last years boat.

Looking at how we fared last time we were hardly laging behind the pack but it did seem our boat was holding us back .
Like most people say races are won on the windward leg where last year we had the advantage downwind this year i want to make sure we can compete going upwind as well. My friend also agrees with me where the boat was holding us back. Certainly the winners were better experienced with a better boat.

see here for results
http://www.na-me.ac.uk/symc/symc files/past comps/symc2006/2006_results.pdf

and here for pics
http://www.na-me.ac.uk/symc/symc files/past comps/symc2006/pics2006.htm

our boat was the one with the green sails. Hardly what you would call a refined sailing craft.
This one was the winner
http://www.na-me.ac.uk/symc/symc files/past comps/pics/2006/2006-05.jpg
Obviously a well refined beautiful boat with a simple design like you said.

Alex

 

Alex;
It's me again. After viewing the pictures, the following commentary from far away.....

Your boat, number 48, appears to have a swing rig. That is to say the mast pivots and the boom is fixed with respect to the mast. Good work. Many models with that configuration go very well indeed. Except for the wrinkles in the bottom panel of the sail, the whole rig looks good. A bit larger jib might be advantageous.

It is more than possible that your sail rig is not the source of lesser performance. Judging only from the picture, it appears that both ends of the boat are immersed in the water. If the transom ends drags at all you are "dead meat". With the ends immersed the boat will be slow to tack and you can not take advantage of maneuvering ability so neccessary in response to wind shifts and tactical situations. The bow immersion is the most debilitating when tacking. One is easily deluded into maximizing water line length at the expense of nimble behaviour. WL length needs to be maximized but not to the detriment of turning excellence.

Windward performance is very largely contingent on both the amount of and the placement of underwater lateral area. It is not clear how deep your daggerboard extends. It almost surely has a weighted bulb at the bottom of the board. As a guess, I think the board is too small. The efficiency of the board is a matter that needs careful attention as well. The rudder needs as much attention. Keep in mind that the rudder is also one of the elements that resists leeway.

Rig up some sort of tell tale that will indicate rudder position. Sail the boat hard on the wind and estimate the rudder position relative to the center line of the boat. If the rudder must be held more than 3 or 4 degrees to leeward, in order to maintain course, then there is either a balance problem or the board is not working well enough. Sail the boat to windward and have someone in a dinghy directly astern of the boat but at a distance behind the boat. Estimate leeway by sighting the boat against a distant fixed object. These are tests that you can make to evaluate the ability of the boat.

The streamer at the mast head is not a good enough indicator of wind direction. Make up a mast head fly that pivots efficiently. They are easy to make and will serve your pupose better. Also consider using some tell tales on the sails themselves. They do help with trimming sail, particularly for inexperienced sailors. At the same time you must have the discipline to avoid becoming transfixed by them and not watching what the other boats are doing.

 

Actually what you say now makes lots of sense. When we were testing it against the weenip class before the race it went faster ina straight line but did not respond very well at all when tacking. We thought this could be due to mast placement and too much weather helm(is that right?)

Also looking back at it the keel looks far too aft. We have made a new hull this year that shouldnt have the transom submerged as it was developed on computer for the desired displacement. The keel on no48 was as large as possible as you can see it is quite restricted height wise from deck to bulb bottom is <=30cm.

I was of the oppinion that a higher Aspect ratio keel is more efficient but i guess this does not apply as much on this scale. I guess adjusting for leeway using the rudder ads considerably more drag than just using the keel.

Alex

 

keel fin

Alex, I think I may have mentioned this to you before. According to Graham Bantock the most important dimension of a rc model keel fin is it's thickness. The fin is best with a max thickness of 6-7% of the chord. If you have to decrease the aspect ratio and/or increase the area to maintain this thickness do it. Thicker than that, according to Bantock, drag increases dramatically.

 

What method do you use to get an accurate profile for the fin is it good enough just to judge by eye this is what we did last year.

Also heres a pic of the sailplan i think we will be using this year.

 

sections

People like Bantock say the section must be done very precisely. He uses sections created by David Hollom. I have judged and shaped numerous sections by eye with good results -but that is not the right way to do it-but it can work. Mark Drela-a contributor to this forum has sections he feels will work good for your application. You might PM him. Some experts feel the section shape makes little difference; rc race results have proved beyond a shadow of a doubt that the thickness/ chord ratio of 7% or below is important.
Your rig looks ok but the peaked up version would be better.....

 

I think we have to make all the parts of the boat ourselves. I think the problem with a peaked up sail is that i will need an actuall gaff section which takes longer to make wheras on the current plan i could just use a stiff batten.

Is there any particular angle the luff of the jib should make that is most efficient i read somewhere that swept wings increase the stall angle as they pull in more air due to the diagonal compared to straight wings does the same apply to boats ? I dont really want to make the main any smaller would you say the sailplan shown has about the right ratio of jib-main.

Sorry to ask so many questions you guys are basically designing my boat for me lol i could always stick a big boatdesign.net logo on the sails

Alex

 

batten

You may have trouble with an upper batten when you tack or gybe. I've never been able to make it work well. The guy that builds the canting keel Ultimate Warrior by Wind Warrior in NZ uses such a batten-I would presume he's got it working well.