Why have a boom on a Mainsail?

thailand69

thailand69

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I am sure their is a good reason (and no doubt many /forums/images/graemlins/confused.gif), but given that many (most?) mainsails are not attached to the boom apart from at each end (note the use of detailed nautical language /forums/images/graemlins/grin.gif).......I don't see why a boom is needed when you could attach the main sheet directly to the sail?

Go on, enlighten me as to why the many bumps on my head were worthwhile /forums/images/graemlins/grin.gif
 
Simple answer : sail shape.

Technical answer:The boom allows you to control how far apart the tack and clew are with the outhaul. It also allows attachment of a kicker. Both of which control the shape of the sail and therefore its aerodynamic properties.

Practical answer: It holds up the boom tent.

Amusing answer: It proves that your mate who claimed to have a lot of sailing experience, has in fact never been on a sailboat before. Once he climbs back out of the water that is.

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Go on, enlighten me as to why the many bumps on my head were worthwhile /forums/images/graemlins/grin.gif

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Gybing a boomless mainsail (eg traditional Bawley, Drascombe, boat with Lugsail etc) is rather interesting. Consider that the mainsheet attachment block is pretty untamable as it comes across...
 
Of course you could attach the sheet directly to the sail! Why else would we have a sheet bend?

The problem is not with running; spinnakers and gennikers do not have a boom along their foot. Yet they still set well - if properly handled.

The problem lies in getting the pull on the sheet from the correct position on the boat so that the tension on the leech and that along the foot of the sail are balanced. In most cases we would run out of boat before we reach such a position. Compare this with the position of the turning blocks for the spi's sheets: usually, right aft.

The boom provides an anchoring point for the clew of the main and, if we were to project the line of pull that would balance the tensions (as described above) towards the level of the deck, it would lie way behind the end of the boat. Attaching the mainsheet to the boom gives us virtually the same control over the main as if we had a longer deck.

Incidentally, this problem is made worse with a Bermudian rig.
 
actually where we have our berth booked in Savannah, GA, there is a boat similar to our own with a main and no boom. I have to say it looks really weird, but I guess it must work. Having said that, we have just fitted a stackpac and the tack of the main is only attached to the front of the boom and the clew to the aft, and is loose footed in between. So, what is the difference in having no boom and the mainsheet attached to the clew and the tack attached to the mast??
I need to look at the setup more when I return in the next couple of weeks.

Incidentally, plenty of mentions of them here.
 
A very successful Farrier 9AX 33' racing trimaran, out of Poole, uses a fully-battened main without a boom. The multi-part mainsheet secures directly to the main' clew. It is efficient, until one needs to reef in a seaway. The necessary gymnastics are interesting.....

By the way, a clew block that threatens to damage heads can be tamed by encasing it in several layers of the polyethelene that damaged fenders are made of. Much cheaper than split chrome leather, and easier to sew on....

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Of course you could attach the sheet directly to the sail! Why else would we have a sheet bend?

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The problem is not with running; spinnakers and gennikers do not have a boom along their foot. Yet they still set well - if properly handled.

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A spinnaker sets well on a run because it has 2 sheets and a pole to keep the clews apart. A genniker doesn't work on a run, you have to sail angles. Loose-footed sprit & gaff sails work OK-ish on a run because of the sprit/gaff, but not as well as with a boom. A bermud(i)an loose-footed mainsail would be fairly useless on a run.

Mike
 
It works on a multihull. Because the wide beam, in turn gives a wide traveler to control the angle/twist in the mainsail. A good example being the Dart 18.

The main problem on a monoull (sorry about the pun) is finding somewhere far enough aft to place the mainsheet. Years ago I broke the gooseneck on a First Class 8. Had to sail home with the sail double reefed, as the sheet angle was hopeless with a full main.
 
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A spinnaker sets well on a run because it has 2 sheets and a pole to keep the clews apart. A genniker doesn't work on a run, you have to sail angles. Loose-footed sprit & gaff sails work OK-ish on a run because of the sprit/gaff, but not as well as with a boom. A bermud(i)an loose-footed mainsail would be fairly useless on a run.

Mike

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The lower corner of the spinnaker that is fitted to the end of the pole becomes the tack and that point is normally held at a three dimensional point in space that remains relatively constant in relation to the boat. (using a combination of topping lift, downhaul and guy. Terms might not be quite correct. We have other names for them /forums/images/graemlins/wink.gif )

The other, free, corner is for the time being acting as the clew. Its position is controlled by the sheet. The other end of the pole is not attached to it, so it cannot be said 'to keep the clews apart' Therefore one cannot say that the spi pole is acting like the boom of a mainsail.

Similar geometry was applied on square riggers; the corner that was secured to one side of the ship became the tack. That is why we have 'port tack' and 'starboard tack'. It refers to where the sail was 'tacked'. As it also happens, the wind also comes from that side and so the terminology was retained

The genniker is usually set to the end of a pole when the wind is abaft of abeam. When the wind is from further aft it is quite common to move the end of the pole back and sail 'goose-winged'. This applies to less than 16 knots of wind; anything above that and one is better off using a spinnaker.

Regarding the other rigs, we are more or less in agreement. The gaff and the sprit fall off outwards helping to present a bigger area to the following wind.

You have agreed with me regarding the bermudian (or Marconi) rig - apart from the spelling /forums/images/graemlins/wink.gif

Incidentally, a very efficient rig that was used in the mediterranean is the 'Lateen'. This is similar (but not identical) to the rig on Arab dhows and other variations. Maltese corsairs were very successful because their twin-masted lateen rig allowed them to point much higher than the 'normal' boats that still had a square rig. The square rig, while convenient in the Atlantic and other open waters, was next to no use for going against the wind; it was not much use for running either because there are no trade winds in the Mediterranean. Running was not a problem for the lateen rig because the two yards were opened out, one to each side of the boat; what we would today call goosewinged.

It is, of course, loose-footed.

Now I need a drink. /forums/images/graemlins/wink.gif

If we were both sitting in the same cockpit we could easily go on discussing the various rigs, sail plans, boat design, and so on; it would be much easier! /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

Perhaps one day - who knows?
 
Cheers folks, kind of a donut question I know, but sometimes I get a question in my head where I sort of know the answer / know their <u>is</u> an answer - but.......... /forums/images/graemlins/frown.gif

I broadly understand what folk are saying (some more broadly than others /forums/images/graemlins/smile.gif)

Simple answer = it needs one. /forums/images/graemlins/grin.gif

Slighty more complex answer = sail shape

very complex answer = see earlier posts /forums/images/graemlins/tongue.gif


Now I am off to the MOBO forum to ask " that stick in the middle of the boat.......is it really needed?"
 
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Now I am off to the MOBO forum to ask " that stick in the middle of the boat.......is it really needed?"

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What stick?
The throttle? You may get lynched for asking that.
The corkscrew? You'll still get lynched but they'll also desecrate your corpse afterward. /forums/images/graemlins/grin.gif
 
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