Jib/Boom movement

While finishing up some design parameters on my next build, a question came up I am sure has been answered before but not sure where to find the answer.

When initially setting up I’ve been told to have the main about 5* off centerline and the jib in the area of 15* off centerline. As wind speed increases, the jib should be brought closer to centerline to close the slot somewhat … bringing it in to as little as 10*. Here is my question …

Should that slot differential be maintained from fully sheeted in to fully sheeted out? On my first Footy I would initially set up as above but when fully sheeted out the main would go clear out to 90* off centerline (fully abeam) while the jib would be at about 75*.

It was my first build and no changes were made to the instructions. Fairleads were directly under the respective booms and the length from the connection at the booms to their pivot points were identical. The difference was in the winch arms … one side was ~3" (main) while the other was ~2" (jib). Obviously more sheeting was let out on the main than on the jib. Is this correct or did I blunder some where?

Thanks much!

Tom - If the distance from the fairlead to the pivot point of each sail is the same, and the distance between the sheet attachment point and the sail pivot point for each sail is the same, as you seem to indicate in your query above, then the distance from the servo’s pivot to the ends of each winch arm should be the same as well.

Some guys do have a slight mismatch of servo arm lengths, just not as much as you have. The idea being that the difference in the jib and main angles for beating would not be desirable on a wing on wing run because the jib would be out too far when the main was sheeted correctly. The amount of length differential in winch arms needs to be determined for your specific sheeting system.

Your answer confirmed what I suspected … though I have found the obvious answer is not always the correct one (which is why I ask).

Thanks Niel, and to all you others that answer questions for us newbies!


The small difference in angle is so that the jib forma a "slot"and accelerates the wind on the back of the mainsail, thus delaying the “stall” of both sails and enabling good performance when sailing close to the wind.

So the relative positions are only important when the boat is close-hauled. running downwind and broad reaching the sails work pretty independantly


Rule number 1 is that sheet fairleads should be directly beneath the boom sheet attachment points. The further away from directy beneath means that you end up having to put more and more tension in the sheet to keep the booms close hauled. Ideally the sheet to boom angle should be 90*. It is virtually impossible to achieve this at all boom angles, but setting it when close hauled is a good start.

Rule number 2 is that the vertical distance between the fairlead and the boom should be as small as possible in order to avoid unnecessary downward loads on the boom. This effect is almost irrelevant when close hauled but gets progressively greater the further the boom swings out.

The 5* and 15* close hauled main and jib angles are a good starting point - on my I0M I use the Lester Gilbert :graduate::graduate: recommended 5* and 12*. The reason for this, as hinted at by Neil and Andrew, is to achieve the magical “Slot” effect. The slot allows the jib to both direct and accelerate the air flow over the main thereby making the flow more efficient and producing more power. However, since both the jib and main have camber set into them, if the jib angle is too small then the air coming off its Leech will tend to impinge directly onto the main which in the extreme can be visible as a vertical crease up the main - not very efficient. Conversely, if the jib angle is too large then the air off the Leech will miss the main completely - again not very efficient. Lester also recommends slightly less camber in the jib to the main, like 6* jib and 8* main to go with the boom angles mentioned above. If its good enough for Lester its good enough for me :wink:

Now, getting back to your original question, because the jib has a head start over the main when it comes to letting the booms out, if the distances between the sheet attachment points on the repective booms are the same and the servo arm radii are the same for jib and main then if you set the main to swing out 90* the jib will end up at 90+(12-5) = 97* which again is innefficient whether you are gull-winged or not. You can overcome this deficiency by either having different servo arm lengths, or, more usually because it is easier to adjust,by having a slightly longer distance between the jib sheet attachment point and its pivot than you have set on the main. This usually works out at about 10% more.

There is an argument that suggests that allowing both the main and the jib to swing beyond 90* when running downwind is agood idea. This is because sails set at 90* are effectively just stalled flat plates with no actual flow over them. Letting the booms out further than 90* allows the sail Luffs to point into the wind again inducing flow and making them more efficient. This may not be a good idea with a Footy because they are quite capable of submarining as it is, and if you are using shrouds it is impossible anyway :lol:

The moral of all this of course is to use a McCormack rig on you Footy, then you don’t have to think about these problems, always assuming you design them correctly in the first place :smiley:



This is all such GREAT information, I really appreciate it.

A lot of this, I’m sure, has be discussed and rehashed in the various posts over time while broaching a lot of different topics. It would be great to ferret it all out and have a single “things we have discovered” post.

With all this free exchange of information, I look forward to each day’s new topics and posts.

Tom - Try the search feature in the banner above.