Boom..Boom!

[David Hudson]

I had a timber boom on my Cape Cutter, ouch! I promised myself that I would acquire a carbon boom should I ever return to a light displacement/planing hull. Well, here I am with a beautiful BayRaider Expedition and I am looking.

I noticed that Matt has a carbon boom on his BayCruiser: see the joint Cruiser..Expedition video; where the latest spinnaker shapes are shown.

I have asked Matt to come back with a proposal on a carbon boom and I am talking with Steve Goacher who sails a Flying Fifteen and Ian Pinnell who sails a 5o5, both quite well. Selden have build booms for both of these Classes and I am interested to see Steve and Ian's take on the sections used.

David H

p.s. (an in thread drift) I talked with Richard Lovering, the chief designer with Hyde, last Autumn. We discussed the then Asymmetric spinnaker design and concluded that a tweak or two might be useful.

Fast forward to February of this year. It would appear that Hyde have tweaked the sail and Nick Peters; Swallow's South Coast representative; tells me the current sail does what it says on the tin. On that basis I have ordered the standard sail.

DH

[Graham W]

For those of us with gunter rig, I've always thought that the heavy wooden sprit boom on the mainsail might be necessary to help keep the sail shape, especially as we don't have a kicker.  I've been told that a sprit boom obviates the need for a kicker but not being an engineer, don't know why.

[Michael Rogers]

Doesn't the sprit boom push the clew down as well as out? It certainly did on my Storm Petrel, where I replaced the snotter arrangement with a gooseneck, and had a clew outhaul. The outer end of the boom couldn't lift more than a few inches, and a kicker would have been pointless.

[Graham W]

Does that mean that it doesn't matter how heavy (or light) the sprit boom is and therefore it could be made of carbon?

[Michael Rogers]

I'm no expert, but I would think so. What do others think?
Being knocked over the head by a CF boom would probably hurt (and not do your cranium or its contents any good). The light weight means less inertia ( - or something), so I suppose you might be less likely to go overboard.

[Julian Swindell]

The triangle of sail below the sprit boom acts as a kicking strap. The foot of the main sail effectively holds the aft end of the boom down. It isn't as powerful as a multipart kicking strap, but Bayraiders aren't about extreme sailing. The boom should also be high enough to make getting clonked on the head during a gybe very unlikely.

[Graham W]

So the luff downhaul acts as a sort of kicking strap by tensioning the sail triangle below the sprit boom?

[Julian Swindell]

Not really. The luff down haul will tension the luff and the foot of the sail could still be completely loose. The sail's foot is tensioned with the clew outhaul. This is why you can release the clew outhaul and roll the sail up to the mast in a harbour stow, with the gunter yard still up and the luff taut.

[Tony]

......At grave risk of being tedious, repetitious  – and of teaching aged matriarchs egg-related skills....
 
The sprit boom on a leg-of-mutton sail, braces the snotter against the clew and the luff tension is controlled by the down haul. The sail is not cut as a right angled triangle so this turns a flappy bit of canvas into an efficient aerofoil with the minimum of hardware. (See pictures on http://compare-a-sail.blogspot.co.uk/ if you need to.) The sprit boom is held in place by the sail itself (self-vanging) so the powerful purchase needed by Bermudian sails to hold the boom down on a run and flatten it when beating is unnecessary. Sail shape is controlled by easing or tightening the snotter, the luff downhaul or the halyard, depending on the way it's set up. A simple and, within limits, efficient sail.
The sprit can be lighter than an equivalent Bermudian boom, the forces on it being as much compressive as lateral so, on a small boat, a light-ish  carbon fibre tube, like a bit of a windsurfer mast, would be just the job. 

[Graham W]

a light-ish carbon fibre tube, like a bit of a windsurfer mast, would be just the job.

Tempting!  My wooden (Douglas Fir?) sprit boom is one of the heaviest things on my boat, perhaps second only to the mast.  It would make harbour furling a little more feasible (but not much).

I remember a recommendation from far away and long ago that the tapered end of the carbon boom should face towards the gooseneck with the larger diameter end facing aft.  Something to do with compression stresses? Also that the end plugs should be of teak, so that they don't expand when wet.

[david]

Excellent  memory  Graham. That was advise I received  a couple of years ago on this forum! Alas, I never made the boom. So, am unable to offer a result of this.

I remember a recommendation from far away and long ago that the tapered end of the carbon boom should face towards the gooseneck with the larger diameter end facing aft.  Something to do with compression stresses? Also that the end plugs should be of teak, so that they don't expand when wet.
[/quote]

[Graham W]

I also remember that because the clew and mainsheet attachment point on the sprit boom are quite far apart, there are lever-type forces that can potentially bend the boom unless it is big and strong like the wooden version.  So the carbon version would need to have a high IMCS (stiffness) score and those sort of spars are expensive.  For example, a high IMCS (>35) Point-7 windsurf mast can cost up to £500. Perhaps not so tempting after all!

[Tony]

Re: Sprit Boom
The closer the mainsheet attachment on the sprit boom is to the clew end, the less force acting on the boom, more being transferred directly to the sail. Move the sheeting point aft and you can do without the boom altogether ...perhaps using a whisker pole when on a dead run... but now you're sailing a Drascombe Lugger and have to wear a pin badge in your lapel so people can see you coming. (http://www.drascombe.co.uk/drascombe-pin-badges-i-4962.php)
On the BR20 the clew of the sail is about 50cm from the mainsheet, isn't it? Less when reefed. Imagine the force on the boom if the sheet had to be midway between mast and clew! Even 500 quids worth of high tech spar might not be enough.
The only reason for spending that kind of money on what is basically a stick is to save weight - and the higher up the mast you go the greater the justification.(Even “Four Sisters” has a Carbon yard to her balanced lugsail.) Not having access to our technology, clever sailors have, over the centuries, developed sails and rigging that works with the materials available to them - which is how the Chinese managed pretty well with rice straw mats for sails and bamboo for spars and the Polynesians could populate the Pacific.  These techniques still work well for small boat sailors (ain't that so, Michael Rogers?) and should not be ignored or derided the way they are by some  AWB sailors. A wooden boom on a BR20 is not out of place (David H) but, even for OGA members, there is every reason to use modern materials and technology when it is appropriate. I'm damn sure Mozart would have used a synth and drum machine if he'd had the chance!

An aside:
If  Churchouse Boats think it worthwhile to bring out a range of “Drascombewear” isn't it time Swallow Boats did something similar?  We already have a dandy Logo which would look good as a discrete motif on Polo Shirt or Boxer Shorts...  Perhaps a range of “His and Hers” sweatshirts? Personalised “Tilley” hats? T-shirts with witty slogans such as ...er....  “If you can read this you are not sailing a Bay Raider.”  (Printed across the back, of course.)
Photo shows the pathetic attempt at humour from the  of the yottie one of the four sisters after the Curious Incident of the Broach in the Ionian.
I don't expect it to become a best seller - even with a more elegant filling than that depicted.

[Graham W]

Tony,

I mentioned the bending carbon sprit boom because the photo below showed it happening to a BR20.  I don't remember my hunk of wood ever bending like that.  Your point about the importance of saving weight higher up is well made.  That, together with the exhorbitant cost of switching to carbon, means that I am no longer even slightly tempted to change my boom.  I never liked harbour furling anyway.....

On the subject of tasteful association clothing, don't forget the new uniform for the SBA commodore (below).

[David Hudson]

Sorry, I forgot what I was going to say.

Oh yes, the answer to boom rigidity is rather like visiting the cricket ground at Kennington.