Shelf Foot

[Don Guillette]

Cap'n Bob (Vancouver USA): Most books on sail trim contain large sections on boat and sail construction. When I was trying to figure out sail trim that really exasperated me because what I was trying to learn was how to sail the boat and I really didn't care about how it was built so when I wrote my book, The sail Trim Users Guide, it was a conscious effort to not talk about those things. that is why you did not find anything about different types of sails in my book.I don't know a lot about sail construction but when I was considering a new mainsail for my C30 I wanted one with a shelf foot. The following was my thought process.There are bolt rope and loose footed mains plus straight foot and shelf foot. There is nothing wrong with a loose footed main but I prefer the bolt rope version. For beginners to intermediates, the SHELF Cap'n Bob is referring to is a light piece of sail material at the foot of the sail that connects to the boom. How you tell if you have a shelf foot is by looking at the foot of the sail and drawing an imaginary line between the tack and the clew. If there is material hanging below the line then you have a shelf foot. If not, you have a straight foot.I am not a racer but I like the things racer like. Racers like a shelf foot because of the extra material plus they get some end plate effect. I don't know how much end plate they get but maybe it is in their heads. This extra material works well in conjunction with the outhaul because of the extra degree of fullness. The extra fullness improves performance on reaches and runs and even going to windward.With the shelf foot mainsail understanding the outhaul is very important. When you ease the outhaul the connecting material unfolds and forms a shelf and when you pull the outhaul the shelf closes and lays against the boom. I think your going to like the shelf foot main. Just spend some extra time reading the section on the outhaul.

 

[Don12364]

Shelf Foot - a Question

Don,My 376 mainsail is extremely flat in its lower quarter. I thought of trying it loose footed so I went to see the sail's designer. (He now runs a loft in England, having "escaped" from Hong Kong when the Brits gave it up).He advised that the sail would be just fine with the bolt rope not in the boom's groove but just with the slug in the boom.So I tried this but found the cloth all moved and the sail developed a great fullness up by the third batten.My question is "How do sailmakers prevent this on a dacron sail built for a loose foot"?Also, if it relies on the resin in the cloth, does this mean when the sail ages the bag will appear?

 

[Bill Roosa]

Loose footed vs shelf

Not to be a pruid Don G. but a what you describe as a shelf is actuall an edge curve. A true shelf cut sail will have a triangular piece sewn into the foot (pointy end down about 1/3 of the way from tack to the clew) to create even more curve (sailmakers call it draft) than an edge curve would and limit the draft (it causes) to the lower 1/3 of the sail and place it in the forward 1/3 of the foot.You are almost right however about loose footed sails, they are not the most common main sail for a beginner to start as a general rule. They are the most common jib sail however. I personnaly learned on a loose footed sail and thought trimming it was a lot easier than one that is attached to the boom. Guess it just depends on where you started.Donalex, The outhaul is the primary means of controling the draft of a loose footed sail. Think of a jib sheet! You will have tensions that are higher than you are normaly used to and if your boom/outhaul is not long enough (or your sail is too long in the foot) you will not ba able to "get it tight enough". You say your sail is flat already and that implies that it was designed for higher winds (or as a loose footed one!!) The sail maker will make a loose footed sail shorter in the foot (compared to one that is attached) to allow for the extra tensioning of the outhaul that is required. Seems that most folks don't want to mess with any more lines than they have to (halyard, sheet, travler) and the loosefooted sail really needs the outhaul and cunningham adjusted to work correctly. Also try your cunningham after you tension your outhaul, it will also help control the draft in the lower 1/3 of the sail. Tightening it will not decrese the draft but will move the part that is "deepest" forward and help the outhaul flatten the part near the leach. This is a great way to get your flat sail to produce more power as it will be more "airplane wing shaped". It does hurt your pointing ability due to the increased entry angle however. See now you got me talking technical. It sounds like you need to go to the book store and get a sail trim book as you clearly are ready for the "next level". I'd recommend "the sailmaker's apprentice" by Emiliano Marino.

 

[RichH]

Id disagree on using a shelf foot mainsail in a...

... loose footed mode. A shelf footed mainsail typically has a boltrope added to the foot. The boltrope is located in a 'sleeve' and the reason against flying such a sail 'loose footed' is the PRELOAD on the boltrope. "Preload" is attained by purposely shortening the boltrope (by approx 1" for every ft. of foot length; AND, you need to strain the outhaul by the amount of 'preload' to attain the designed shape at approx 15kts. of wind pressure. So, simply flying a shelf foot in a loose foot fashing will result in toooooo much draft unless you over tension the outhaul to compensate for the 'preload' in the boltrope. A sail purposely made for flying as a loose foot does not have this 'preload' as it usually has a 'tape' support system along the foot. A boltroped sail, flown as a 'loose foot' ..... is going to be 'cranky' when it comes to shape ... and because of the 'preload' will have waaaaay too much draft (and 'high' in the geometry) in most cases when attemping to fly it with little outhaul tension.

 

[Don12364]

Bill R

Bill,Thanks for your response to my question to Don G. I do know all that stuff about outhauls and cunninghams having been trained in the UK's Olympic Starboat squad for the Tallin event long ago.My mainsail bolt rope is absolutely non-extensible and also the sail has no foot curve.My question to Don G is how is a loose footed sail designed differently from a foot along a boom.Also is the cloth different.Thanks just the same.

 

[Bill Roosa]

Loose vs "attached"

A loose footed mainsail will look just like a jib in its' foot construction. I may (most likely) or may not have a edge curve. The edge curve is added to "seal" the sail to the boom and for astectic (sp) reasons. The stress reliving pannels/webbing/etc will direct the stress dirctly between the tack and clew and "bypass" the section that is below the tack-clew line. So there is not a need for a bolt rope but I have seen them installed.Rich H has it right about the "attached" sail construction.For the record I don't recommed using an attached foot sail as a loose footed one either for the same reasons Rich H notes.The sail material is the same for either given they are designed for like wind speeds.