center of effort of a gaff sail

You can get gaff-rigged sloops. Sloop doesn't mean having a triangular mainsail, it means having only one headsail (modern definition, anyway). if the questioner was comparing 3 and 4 - sided sails then the word for the former is bermudan, or marconi rig.

Splitting the gaff sail into two notional triangular sails works in the same way as that used when you subsequently combine the result with the CE of the foresails (combined if more than one) to obtain the total CE position. Any number of triangles can be combined in proportion to their areas. That leaves out of the equation of course the extent of any overlap, and the effect of airflow of one sail on another.
It's a good question whether the method is strictly accurate with a four-sided sail. That is making the assumption that all sections of a gaff sail have equal pulling power. Arguably the bit near the peak might contribute less than the same area close to the luff.

 

There are some interesting mis-understandings on this thread. Let's get a few definitions right. We may consider:

Geometric Centre - Centre of area of the whole rig.

Center of Effort - This is actually the center of pressure and is tricky to calculate.

You can take a good bet that the two centers won't coincide. Niether will thier opposite numbers on the hull. So we have two approaches... Either, use 6% to 14% lwl lead and geometric centres.... or calculate the centres of pressure and try to balance the moments to be acceptable for most points of sail.

Now the latter method is not easy, and it is a question I'm hoping to tackle for my final year project at Uni. The problem is not working out the center of pressure of the sails, that is relatively easy. The Hull is the main problem due to free surface effects (ignoring possible cavitation on the foils for the moment). It is possible to use modern CFD methods to solve a full free-surface problem, but it is time-consuming and requires some skill in that area.
Naturally, one should compare with tank-test data in order to validate the method.

So, to the origenal question, use a 3D CFD method, 3D panel method, or 3D vortex lattice method. That will give you a true (well, calculated) centre of effort (pressure). But you'll need some clever tricks to use the resultant answers.

Cheers,

Tim B.

 

Since we are correcting terminology, the modern definition of a sloop is a single masted boat with the stick located in the first third of the hull. It can carry a sprit, which may be fixed or retracting and it can carry single or multiple headsails. A cutter is similar to a sloop, but has it mast much nearer amidship, again with or without a sprit or headsail options. The farther aft stepped mast permits a better staysail arrangement, so most carry multiple headsails, though is not required for the definition of a cutter. Neither term reflects the shape of the sails, just mast location. Bermudian, leg o mutton, jib headed and Marconi are triangular shaped sails, with gaff, sprit, lug and a few others are quadrilateral shapes.

 

A little complicated to fall under the "general rule of thumb" part of the post.

"I am looking for general rule of thumb info on finding the center of effort of a gaff sail. Principles of Yacht design (Larsson & Eliasson) states that for a sloop rig the geometric center is used. Would it be safe to assume this same rule for a gaff sail"???


Steve

 

Yup ... and the "General Rule of Thumb" puts the CE ahead of CLA by 0% to 17% depending on who's thumb rules ...

 

So are we saying that the position is the length of the thumb?

Just Kidding.

I was merely pointing out that the true center of effort is more complex than rules of thumb may suggest.

Tim B.

 

Is that the modern terminology? I thought the idea of defining a sloop by its mast position went out years ago.

According to the old rule, a J/24, Melges 24, Beneteau 40.7 and just about every modern racer or cruiser/racer is a "cutter". I'm also curious about where you take the measurement....if you have a one design with the mast exactly 1/3 aft and you slide the mast back in the foot, does it go from sloop to cutter?

Surely what is actually important in terms of design, performance, construction, cost, handling, seakindliness and every other factor is the number of headsails, not whether the mast is 33.33333333333333329999999% aft or 33.33333334 aft?

Since the definition of "cutter" for example has changed often and radically, it seems perhaps unneccesary to stick to what seems to have been the CCA rule's definition of the difference between sloop and cutter. It's not very workmanlike - for example the famous Nina, always referred to as a schooner and the proud possessor of two sticks, was said by Uffa Fox to be a sloop under the 1/3 definition. That seems fairly nonsensical.

 

I agree. Mast position is matter of definition, so there are obviously going to be borderline cases. Reference to the number of headsails is unambiguous.

 

Just to be exact.....if we are going to discuss "proper" naming. A sloop requires a bowsprit/jib boom. Most modern boats are knockabouts.

 

Who's definition of 'exact' would you like us all to comply with? Who should be the arbitrator of this?

Cutter and sloop have had various meaning in the nautical world apart from their use as rig descriptions. Both related to vessels rigged in all sorts of ways. The Royal Navy has even called a type of ship's boat a cutter, the most famous of which were rigged as lugsail ketches (still used at OB Schools).

Your definition of a sloop never existed in the UK. We traditionally rigged a forestay to the stem and I can't think of a local boat here that was ever rigged like you Friendship sloops. Way to risky in stormy water.

The most generic and universally excepted definition these days of the words are that a sloop has one headsail and a cutter two. Beyond that you will semantically nit pick your way through regional variations and historical abnormalities until the cows come home.

 

Actualy that definition has come from the UK. See plate XII of Falconer's.

http://southseas.nla.gov.au/refs/falc/1450.html

In fact, Faclconer refers to a cutter as being rig as a sloop. The real difference between a sloop and a cutter is the location of the land of the lower forestay. A sloops forestay lands on the stemhead and therefore carries a jib, the cutter forestay lands on the foredeck and carries a staysail.

 

As I said, sloop meant all sorts of things in days gone by, including the example on that website from 300 hundred years ago. At that time, generally a vessel of less than 18 guns was called a sloop, even if it had three masts and was rigged as a ship. After the Napoleonic wars, the Navy adopted the term corvette to distinguish the larger sloops.

After the mid nineteenth century, the term sloop died out completely in the UK, and like the cat boat rig, is unknown amongst traditional sea going working boats in this country. The small craft rig that found favour in many areas had a forestay permanently rigged to the stem and a stowable bowsprit and headsail set forward of the forestay. They were therefore usually double headed and this popular definition of a cutter was reinforced by fashion in the first half of the 20th century when the Royal Yacht Britannia and the other similar class yachts were known as racing cutters.

The sloop rig, with the forestay rigged to a permanent bowsprit, developed on the east coast of the USA, and the fact that the smaller ones only carried one headsail, with the mast stepped correspondingly further forward, distinguished them from the British cutters. Technically, Slocum's Spray was a sloop, despite its jigger.

When Bermudan rig gained popularity, and the bowsprit less so, the term sloop became used to describe a single headsailed, fore and aft rigged, single masted vessel.

It's a evolving terminology, and there was even a time recently when the sailing magazines were promoting the term 'slutter' for boats with two foresails, but which never set both simultaneously. We cannot hold terminology to some fixed point in the past. I would suggest that in common parlance, most people understand the difference between the two rigs is the number of headsails.

Some may resent what they see as a 'dumbing down' of our nautical language, and it's probably a feeling shared by many a captain in the nineteenth century who had his three masted square rigged sloop given a poncy French name on the whim of a desk bound Sea Lord in Admiralty House. But that's life.

 

Getting closer to the original topic, the following notes are based on my own experiments. COE equals Geometric Center for a flat panel with the wind full on, i.e., angle of attack 90. COE for an aircraft-style aerofoil (which might be typically 15% thick) at a low angle of attack is typically 17% forward of GC. A sail (0% thick) is midway between these extremes so the rule of thumb range 0 to 17% sounds OK, should be furthest forward when sailing close to the wind. I don't know how this applies to hydrodynamic forces but would like to know. Angels rush in ...

 

Thyat's interesting to know. Say 8% forward of the GC is a significant shift, and very relevant when playing around with rigs on paper and cardboard cut-ots of hull profiles. An 18" forward shift on a 20 foot wide sail might totally change the relationship with the CLR.
Would a tall narrow sail (bermudan) show a greater shift than a short wide one (gaff)? That would seem to go some way to answering the original question.

 

One often forgotten factor is how much the boat heel.
I mean when we discuss "lead", how long shold the arm be between the forces from keel and sail looked from the side. This arm has to be compared to the arm of the moment who turn the boat to windward, the distance when you look at the boat from abaft, between the sail force and the resistance on the hull from the water..