Hi all
Well just a question for all the technician…
my jib (with boom) is attached to the deck with a swiwel…now the question:
what if i change this lenght…making it longer or shorter…what will it do and why?
Thanks
Wis
http://wismerhell.esmartdesign.com/swiwel1.jpg
if it isn’t broken, don’t fix it!
Firstly I would get rid of the fishing swivel and just use 30 lb. and up line. To my experience, line can?t be beat. I do use a ball bearing from Bantock on my IOMs that also is great, but line is about the best for most any boat.
If you use line, then it becomes very adjustable with a simple bowsie arraignment. On a smaller boat, raising and lowering the jib has almost no effect either way. Yes, in extremely light air you might want to get the sails as high as possible, but moving the jib up two inches really has little advantage. But in Higher to moderate winds, a high jib will be more of a problem being that the boat will be more tender. Looks like yours is pretty high maybe. I like keeping my sails a low as possible. Again, using line will help achieve this.
Looks like a nice boat! Got more photos?
Wis,
When you say “making it shorter” do you mean moving the jib closer to the deck? I consider this to be a very beneficial move.
First, I should state, that if you do this, you need to lengthen the jib halyard by the same amount (leave the mast rake alone).
There are two benefits to lowering the jib down to the deck.
The lesser of the two effects is that you reduce the heeling moment on the boat slightly.
The more important effect is a reduction in induced drag on the sail plan due to increased “endplate effect”. I think in the past you have cited a few technical books that you have access to? If you have any editions of Marchaj’s book on aerohydrodynamics of sailing, you should look up what he has to say on the subject. He presents a graph of the theoretical induced drag of a sail as a function of the gap between the foot and the reflecting plane (water). If that gap is 10% of the mast height your drag will be 80% greater (almost double) the drag of the sail when the gap is closed. The graph is nonlinear so that as you increase the gap, the curve is asymtotic with double the drag. Thus, the drag drops very quickly as you get close to closing the gap. At a 5% gap, you have 65% more drag than a closed gap. At a 3% gap you have 50% more drag. At a 1% gap you have about 20% more drag.
One Caveat: Marchaj states that this effect is theoretical. The presence of turbulence on the deck of the boat cancels out much of the effect of closing the gap completely. But until the foot of the sail reaches that turbulent deck boundary layer, the effect has been shown to be correct in wind tunnel experiments.
If you look at most full sized racing boats, they use deck sweeping jibs (where the jib foot actually lays on the deck) to help close the gap. Many boats also try to close the gap on their mainsail as well. Take a look at how low the main booms are on an americas cup boat these days.
On an RC boat, there is a practical limit to how low you can go, but most boats are setup nowhere near that limit.
There is a competing school of thought which is that you want to raise the jib up as high as possible to take advantage of the stronger winds aloft. The strong wind gradient near the surgface of the water for our scale of boats adds some validity to this idea. But I have found with my boat that I am faster with a low jib than with a high jib.
Will Gorgen
Well guys: I luv you
That was exactly the kind of answer i was looking for[:D]
Greg: the boat is a Seawind from Kyosho, sails made by Dennis Desprois…if u want pics…got tons
Will: i didnt begin yet with Marchaj…busy with work…but in 3 weeks should be ok…thx
again thx guys, that was great
Wis
if it isn’t broken, don’t fix it!
wis
if you lower the jib to the deck , i find you will get a litlle more perfomance from your mainsail. the higher up the jib sits meens more wind the more will have to deal with . and heeling becomes a problem. you do get different wind speeds at different heights but it is not that big of a diffenence with our size of model.
if you are talking about moving the jib back towards to mast then your problem become the air spilling off the jib onto the mail. and that just does not work
how do you like sailing the seawind
cougar
Has anyone tried spreading the rig apart fore and aft? Move the jib ahead as far as possible and compensate for that by moving the mast(main)back? Theorectically I would think you could sheet the jib a hair tighter by opening up the slot and have fewer problems backwinding while maintaining good speed and pointing ability. Maybe you can’t do it in the class you are talking about but it is food for thought.
Cougar,
I did lower the jib, but i didnt sail her yet with that setup…maybe this weekend if its not snowing[:-censored]
Sailing the Seawind is a great pleasure, but i must admit in higher wind, she s a pain…she loves being a submarine (nose diving) and the rig are way too big…need to think about a storm suit…but again usually she s great…sure its not an IOM but anyway, for the price she s great (i really had a great price, Seawind + RX/TX Futaba servo and Futaba Arm servo, 4 NiMh 2100 Mah batteries with charger for…32.000 yen, sending included…about 296 USD!
Bob,
no i didnt try that yet…i should, anyway i dont race, either this class thing is nothing for me…the only opponent i have…are ducks!
thanks
Wis
if it isn’t broken, don’t fix it!
I love your cat Wis
ei ei ei
joey
Hey Bob,
Chris Staiger recently joined my class and has had some interesting thoughts on seperating the jib from the mainsail fore/aft. He feels, due to some experimentation in the Soling 50 class, Santa Barbara Class and the AC class, that seperating the jib from the mainsail does produce a noticable improvement in speed.
Everything that I know about the aerodynamics of multi-element airfoils and sails says that the opposite should be true. The jib and mainsail should not be thought of as individual sails but rather as working together. This is similar to the idea of leading edge and trailing edge flaps on airplanes. The positioning of the leading element causes the airflow to be directed onto the suction side of the main airfoil, thereby energizing the flow and prostponing the onset of stall. This allows the wing to operate at higher angles of attack and higher lift coeficients. Thus, a correctly designed multielement wing can produce much more lift than the if the elements were operating in isolation. In order to produce this effect, you have to have a closely coupled configuration with a controlled “slot” between the two sails. My experience in full sized boats is that this slot should have some fore/aft overlap. But I have not experimented much with model boats to see if the same rules hold true.
My experimental approach would involve some wind tunnel work, but you may feel that it is sufficient to experiment on the water. Do you have anyone that you can trial horse with? you might want to consider designing a mast crane that can easily allow the jib the be moved fore/aft. Some simple rake adjustments would allow you to reset the rig for balance and in a couple of hours you could try out 3 or 4 different settings to see if there is a dramatic difference. It would be interesting to see the results…
Will Gorgen
I will let the cat out of the bag. I have been separating the sails for quite some time now and it seems to really help both with pointing and speed. I was shown the technique way back in the late 80’s and it only seems to work for models-you cant really get the same scale of separation on a big boat due to rig limitations but it does work for some reason on models, my dad poo poo’d it when he heard about it and he was a very successfull big boat and model racer at that time. We set up two identical boats “Terry Allen Rovers, they were”, with identical rigs and raced around the pond one day and he couldnt keep up until he did the same thing. I have shown it to more than a few people and they are in agreement that it seemed to get the boat going a little faster with the ability to sail higher-some say it just isn’t right and go back to their comfort zone on setup which is fine. A lot of people ask me to look at their boat and sails in between races and that is one of the many adjustments I sometimes make, the control of the main and jib leach are the most critical elements to model boat performance that I check that first. Once that is set I trim in and see how that looks, then I see if it is balanced and find out how the boat feels (sluggish, spritely, ???), if I have a sluggish boat I start separating the main and jib and it usually results in a little more speed and quickness
My theory on why it works has to to with the scale of the wind-(wind can’t be scaled down and I think there is a minimum size of slot you need for optimal flow. I use the fingers of my hand as a guage three fingers from the leach of the jib to the front of the main in heavy air and 4 fingers in light I currently use this tech on my US1M and on my M. Trimming and setting sails is still an artform to some extent and I do sail with flatter more trimmed in sails than anyone I know-works for me so far but I may be completely out to lunch on this too so let me know what you think.
Bob,
Interesting, but not sure it works with all boats? My US1M also has a pretty good separation as recommended by the designer himself, Swede. I did close it up a bit and the boat seemed to behave quite a bit better. Not sure about top speed, but manners are better which as you know is major plus with the US1Ms.
My Star 45 had a larger slot then I wanted and when I brought it WAY closer, the boat sailed much better. This all may really depend on the individual boat.
I agree that a lot of the time I read Wills numbers, which I respect highly, I know that our small scale boats relate very little to full scale. RC sailboat tuning is something that really needs to be learned first hand with many, many hours of sailing. I have had very little success with placing big boat theory with our small scale.
Bob,
Chris has given me a similar story. There is not a lot of aerodynamic theory that deals with the specific size of our boats. The reynolds numbers are a lot lower than what traditional sailing deals with. And full sized boats tend to be a lot lower Reynlds numbers than the aircraft applications that most in depth analysis of lift and boundary layers were done for. So it would not take much to prove to me that there is something different going on here. It could be that thereis some significant surface turbulence that forms some sort of blockage if your slot gets too tight.
In the Fairwind class, we are faced with a bunch of builders who are poised to take advantage of this theory and a loophole in the rules that does not address this problem. It looks like we are headed down the path that the Soling 50 took which is to limit the height of the jib stay attachment to the mast. I notice that several other classes have taken a similar approach in their rules.
It would be interesting on a US one meter boat to take this theory to an absurd level. I’m thinking of a design with a mast crane that was as long as the jib boom and had a perfectly vertical jib stay (parallel to the mast). I wonder if there is a limit to how much fore/aft seperation you can have before. Maybe at some point you start going slower again. Perhaps there is an optimal seperation. If it turns out to be boundary layer blockage in the slot, then the optimal seperation would change with wind strength.
Will Gorgen
Greg/Will,
Just my experiences and theories. Just to clarify the slot I am talking about is the fore aft separation of the jib on centerline to the mast. When this is spread a little more that normal, I find I can sheet tighter and point a little higher than if the jib was closer to the mast. Also I am using a fractional rig on my one meter not the typical masthead rig you see so often, maybe this has something to do with it as well. It may also have something to do with the way I set up the sails (usually much flatter than the competition). That what makes this fun though, maybe I have gone too far and need to come back a little, I will try it next year and see.
Will, Greg,
Today I went to the pond with the jib lowered…omg it’s an other boat…more responsive and less heel…I was amazed of the difference…again thanks a lot!!
That was great!
Wis
PS i will post new pics on the website
if it isn’t broken, don’t fix it!
BobT31
You might find the following link interesting with respect to moving the jib forward.
http://www.angelfire.com/fl/modelyachts/jibs.html
Bottom of the fourth paragraph. I think Charles is talking about vintage marblehead boats, but he does seem to agree with you at least in medium to heavy wind. And he does quantify it.
Scott
Nice find. I hadn’t read or heard anything about it other than the guy who showed it to me around 1988 or so. I just keep fiddling around with it when the boat seems slow. Tuning is probably the hardest thing for most people to master. Those who can tune properly on every change of wind velocity have a huge advantage over the rest of us. I have been conversing with a couple of guys who are in the IOM class and they really confirm this. All the boats are pretty competitive the real difference comes when the velocity changes and you have to know exactly what to do to keep your boat moving well. It takes a lot of time and a lot of sailing.
wis that was great to hear. i am glad the suggestion worked. now your job is to find something that could help other people. and it is a good thing you did not have your storm. we have about 45 cm of snow here. over the last 3 days:-)
cougar
you dont want to know how MUCH snow we have here…you ll be amazed
Wis
if it isn’t broken, don’t fix it!
wis
i dont know how much snow you have , but atleast you have luquid to float you boat. we have the hard stuff here. i saw on your website the you jib is lower and that will improve the perfoamnce. but is that red boat realy a sea wind?
cougar
well…now it is impossible to sail here…all frozen and here in the village in front of my door…i got 2 meters snow…at the ski resort (15min from my home…i know i know…i am sooooooo lucky)4 -6 meters
As for the red boat…yup its a Seawind
Wis
if it isn’t broken, don’t fix it!