Why Jib booms

Why do we need to put a boom on our jibs? On full size boats without jib booms the sail tacks and jibes with no problem, so why do we need them on our models?


I had asked this myself some years back when I first moved from big boat into the world of small.

Most full size boats sheet the jib sheets to one side of the boat or the other and the blocks and angles of sheeting from jib to deck vary depending on wind strengths and size of jib. Unfortunately pretty much all r/c boats sheet the jib from the middle of the foredeck. Thus as jib sheet is eased, the foot and clew of the jib tends to rise, opening the slot between jib leech and mast, and allowing jib to “camber up” and this reduces ability to point.

With a jib “boom”, the sail is still sheeted from middle of foredeck, but keep in mind the forestay runs through the luff of the jib (like on a big boat) but doesn’t fasten to the deck - it fastens to the front of the jib boom. This “upward pull” on the front of the jib boom also pulls down on the clew end of the jib keeping the leech tight. The boom (in effect) becomes a lever and depending where along the jib boom is attached to the deck is how much the lever function will pull down on the trailing edge of the jib. Thus sheeting from mid-deck is merely an in/out control and the jib boom plus forestay becomes a lever to control the flatness of the sail, and the jib to mainsail slot.

A leech line (in some classes) may be added to the end of the jib boom to “pull” up the boom (countering the effect of forestay) end so the proper slot of the jib can be maintained with the shape of mainsail.

Finally most classes allow only two servos - one for rudder and one for sheeting. Thus the jib is usually sheeted at the same time as the mainsail off of a single servo arm.

Hope this explains a bit, but once you start looking, it is easy to see this explanation on an r/c boat.

Not saying it can’t be done, and on scale boats it sometimes is, but again, you need jib sheets to both sides of the deck, a method to pull in line on one side and let it out (at the same time) on the other, and finally a way to control the shape of the jib leech with some kind of deck block and track.

Cheers, Dick

I’m sure others will join in on things I forgot to include.

I understand what you are saying, I just think that a single sheet attached at the jib clew would work. I believe the jib would self tack.
I am going to settle this by adding an exit point for the jib sheet just aft of the jib leech and at mid beam. I am going to attach it to the clew and just see what happens. After all the fun is in the expermenting. I can always go back to a boom.

On real boats, of course ours are real too!, the jib sheet is pulling down on the leech of the jib to prevent the top from twisting off. Pulling down on the leech from the center of the boat will not really control the shape like using a boom. at least if you want to have a slot between the jib and main.

Let us know what happens!

What Hew said, and what Dick said above.

By all means experiement - as you say, that’s part of the fun - but in theory and in practice the jib boom is the way to go. Absent that, you would need to mimic the system we use on crewed boats, i.e. separate controls for sheeting on port and starboard and the ability to modify your sheeting position. The jib boom eliminates all of this.

Jib boom is the way to go. It keeps the shape in the jib through all point of sail, and if properly rigged, controls the twist throughout the entire range of movement.

Remember that on a full sized boat sailing down wind without a spinnaker, a wisker pole is used to push out the jib, otherwise it just collapses into a curve and loses projected area.


The jib booms on both my boats have a weight hanging off the forward end. This weight helps the boat to achieve wing on wing when going downwind.