As I understand it, anything above a very light wind you want a full sail and as wind builds you flatten to reduce increasing drag. Is boat speed the only way to tell if you have it right?
Lift and drag
- Thread starter Quoddy
- Start date
- Status
This is going to be a long thread.
I think the answer is there are at least several indicators but boat speed may be the ultimate test.
Other signs could be the amount of heel, ability to point, luffing of the sails the status of tell tales and I'm sure you'll hear of others.
For example take the angle of heel. If the sails are too full for the wind condition the excess power will heel the boat over more. That will slow you down.
Or, take the ability to point. If the sails are full shaped relative to the wind strength your pointing will suffer. Flatten them and drag is reduced and you can steer closer to the wind - in particular with respect to the jib.
If the front of the main is luffing its an indication that the wind angle is causing the air stream to hit the sail on the windward side. Flattening the sail can change the angle of attack and reduce or remove the luffing.
Tell tails tell you where there is flow over the sail and which direction the flow is. I think they are very useful.
In the end sails shaped to fit the wind condition translates to boat speed but many times I've had the slows and need to go the basics to get back up to speed. The basics being sheet trim, draft, draft position and driving angle.
I'm sure other opinions are forthcoming and that's what makes sailing so great.
I think the answer is there are at least several indicators but boat speed may be the ultimate test.
Other signs could be the amount of heel, ability to point, luffing of the sails the status of tell tales and I'm sure you'll hear of others.
For example take the angle of heel. If the sails are too full for the wind condition the excess power will heel the boat over more. That will slow you down.
Or, take the ability to point. If the sails are full shaped relative to the wind strength your pointing will suffer. Flatten them and drag is reduced and you can steer closer to the wind - in particular with respect to the jib.
If the front of the main is luffing its an indication that the wind angle is causing the air stream to hit the sail on the windward side. Flattening the sail can change the angle of attack and reduce or remove the luffing.
Tell tails tell you where there is flow over the sail and which direction the flow is. I think they are very useful.
In the end sails shaped to fit the wind condition translates to boat speed but many times I've had the slows and need to go the basics to get back up to speed. The basics being sheet trim, draft, draft position and driving angle.
I'm sure other opinions are forthcoming and that's what makes sailing so great.
this relates to a question I have - use of the vang. My understanding is that the main time you need to tighten down the vang is when you are running and the sheet is all the way out. Otherwise, with the sail sheeted in (as, for instance, when you are beating) the sheet itself holds the boom down. Is this right?
Beginners often tend to have their sails too tight. Flatter sails are best in higher winds. I like to have my sails as full and as loose as possible for the conditions. When beating into a stronger wind tighter and flatter. Running off in lighter wind looser and fuller. Sail shape is my main criteria. As loose and full as possible keeping a good shape and little flutter. Again in my experience beginners tend to overtighten. http://www.ipass.net/sailboat/tips.htm
I think shemandr summarized very well ...
except that when the leading edge of your main is luffing, it is because the airflow off the genny is backwinding the leading edge of the main (airflow on the leeward side). This may be unavoidable when you have a masthead rig and a larger genny, say 135 to 150. The reason for this is that the genny provides the power for a masthead rig. As the engine, the genny has priority over the main for proper sail trim, and the main will tend to luff at the leading edge when close hauled.
John, I agree with you about the vang. However, if you don't have a traveller, your mainsheet is the only means of managing the angle of attack, so in that case, the vang is necessary to manage twist, even when the boom is in board. But you are right, when you have a traveller, the mainsheet and the vang manage twist and the mainsheet is as useful for this function (as the vang) when the boom is in board of the range of the traveller.
I have read an account of a sailor who used a 'scalloping' technique to maximize speed made good on a prolonged upwind beat. The captain had established a course that utilized the best sail trim for what seemed to be the best upwind angle for their destination. One of the helmsmen, on his shift, demonstrated that velocity made good was far better by ever so slightly pinching into the wind, losing effeciency in the sail trim but utilizing momentum to gain upwind progress. By paying close attention to speed and sail trim, the steerer was ever so slightly alternating pinching and falling off again to re-gain maximum sail efficiency. The captain was upset, at first, when discovering that the helmsman was doing this, but changed his attitude when he discovered the improved progress of the boat to their upwind destination.
BTW, when you have very light winds, or zephyrs, it is better to flatten your sails to improve airflow. When the wind is too light to get around a full sail, the sail is too full!
except that when the leading edge of your main is luffing, it is because the airflow off the genny is backwinding the leading edge of the main (airflow on the leeward side). This may be unavoidable when you have a masthead rig and a larger genny, say 135 to 150. The reason for this is that the genny provides the power for a masthead rig. As the engine, the genny has priority over the main for proper sail trim, and the main will tend to luff at the leading edge when close hauled.
John, I agree with you about the vang. However, if you don't have a traveller, your mainsheet is the only means of managing the angle of attack, so in that case, the vang is necessary to manage twist, even when the boom is in board. But you are right, when you have a traveller, the mainsheet and the vang manage twist and the mainsheet is as useful for this function (as the vang) when the boom is in board of the range of the traveller.
I have read an account of a sailor who used a 'scalloping' technique to maximize speed made good on a prolonged upwind beat. The captain had established a course that utilized the best sail trim for what seemed to be the best upwind angle for their destination. One of the helmsmen, on his shift, demonstrated that velocity made good was far better by ever so slightly pinching into the wind, losing effeciency in the sail trim but utilizing momentum to gain upwind progress. By paying close attention to speed and sail trim, the steerer was ever so slightly alternating pinching and falling off again to re-gain maximum sail efficiency. The captain was upset, at first, when discovering that the helmsman was doing this, but changed his attitude when he discovered the improved progress of the boat to their upwind destination.
BTW, when you have very light winds, or zephyrs, it is better to flatten your sails to improve airflow. When the wind is too light to get around a full sail, the sail is too full!
Aerodynamics doesnt support a full (draft) sail in between very light to moderately light wind speeds. The reason - not enough energy in the wind to keep it 'attached' as a boundary layer to the leeside - invisible separation stalls where the airstream becomes 'detached' from the sail. This can be predicted/assayed by the use of 'field effect plotters', or visually with tell-tales. Tell-tales (streaming correctly to validate that the boundary layer hasnt 'broken free') AND a speedometer are the principal means to verify the correct shape w/r the fore/aft position of the point of maximum draft along the cord of the sail ..... and its 'depth of draft' (full or flat).
The same applies to the very sensitive shape of the leading edge shape of the lee side luff portion - the luff 'entry' (flat or 'rounded') ... again tell-tales are the visual indicator to denote if or not the presence of separation stalls. If the airstream doesnt have sufficient energy to 'make the turn' around the front of the luff ... then separation stall.
Again at the leech exit portion, the wind must be 'leaving' the sail with the air stream velocity on both sides (on the lees side and the windward side) of the leech at EQUAL speed! - known as Kutta condition .... tell tales streaming straight back; if not, then the Kutta condition is not met and you have a separation stall. Tell-tales for visual verification plus the speedo.
Separation stalls (broken air stream boundary layer) are invisible - the sail doesnt 'shake' and the only way to verify/maximize is with a full set of tell tales. The principal areas of separation stalls are at any place where the airstreams 'make a turn' - leading edge, at the point of maximum draft, and the leech (kutta condition). You cant get optimum speed out of a sail if there are any invisible separation stalls present anywhere on the sail .... the only way to do that is by using a FULL set of tell-tales - tales at the luff, midcord, and leech.
Sooooooo, after you validate the absence of separation stalls .... and re-shape as necessary the sail by applying tension to an 'edge' (halyard, vang, traveller, cunningham, barberhauler, etc.) while watching the tell-tales to insure 'attached flow' ......... then when all 'tell-tales' are flying correctly, THEN you adjust the amount of draft (via outhaul) while watching the speedometer and the tell-tales AT near the point of maximum draft.
So, the answer is: The shape of the sail wont optimize until the Tell-tales are flying correctly ('shaping' the sail by inducing correct tension to the 'edges' of the sail) .... then set the correct/optimum amount of draft while watching the mid-cord **telltales** AND the **speedometer**.
The 'general rule' is FLAT sails for 'speed', FULL sails for 'power'.
--------
Some parenthetical 'asides'....
1. with boltroped dacron sails, the sailmaker intentionally cuts the boltrope shorter than the luff dimension (by approx. 1 inch for every 11 ft. of luff length) - called 'luff pre-load'. This so that the boltrope (and luff) stretches out to the full design condition when the sail is used at its 'designed maximum wind range' - usually 15-18 kts.
When you raise the mainsail, you usually need to 'stretch out' (by 'extra' halyard and/or cunningham tension) this 'pre-load' on the boltrope so that the fore/aft position of maximum draft is ~correct. If you dont add this extra tension to the luff, the point of max. draft will be too far aft in the sail .... and the boat will develop 'weather helm'.
2. What you learned in USA high schools, and what you see in most 'sailing books' about how sails 'work' is totally totally wrong, and has been since the Wright Brothers discovered that it was 'otherwise'.
Aerodynamics isnt 'intuitive', sails dont have 'thickness'. What makes a sail work is ....... the air streams have 'viscosity', and the friction from the viscosity of the air contacting the sail imparts energy to the sail ... and the net airflow 'circulates around' the sail/sails !!!!
Not the only way, but in some cases, the best way. The only time it's not the best way is when you are trying to get to a destination upwind. When you are pointing you need the keel down as much as possible so generate lift towards the wind. You have to depower the rig to keep the keel down so that may actually slow the boat down, but your speed to the destination will be faster. We call that VMG and if you have a gps, it can calculate it.
But if your destination is not upwind or you are just out for sailing, then boat speed is the best indicator of good sail shape as long as that sail shape isn't hurting the sails....aka...flogging....in those cases you need to reduce sail.
Quoddy: I'm going to try to make this very simple - so I can understand it. Here's how it works through the full spectrum of wind speed. In very light to light wind speed you want your mainsail and jib to be as flat as possible. The reason is because the wind does not have enough power to get around the sail and the sails stall. You can see this by checking your telltales, they are limp. The mistake sailors make in light wind is they induce more belly in the belief that it will generate power. It doesn't.
As the wind speed starts to pick up you want to gradually induce draft depth (belly) into the sail a little at a time. What your doing is allowing the wind the ability to get around the sail and power it up. It is a balancing act - too much belly will stall the sails.
When you get up to about 10 to 12 knots you want to gradually start to flatten the sails mainly so you can control the boat. If you like sailing "on the edge" keep the belly until you are not comfortable with your ability to control the boat.
Reducing the belly or flattening the sail has nothing to do with reducing drag. It has to do with reducing power.
Lift and drag is too complicated for me. The way I sail a boat is to stay as high on the lift/drag curve that I can I can't really describe it but I know how to use it to my advantage. The settings i give up on my SAIL TRIM CHART place you at the highest point on the lift/drag curve.
As the wind speed starts to pick up you want to gradually induce draft depth (belly) into the sail a little at a time. What your doing is allowing the wind the ability to get around the sail and power it up. It is a balancing act - too much belly will stall the sails.
When you get up to about 10 to 12 knots you want to gradually start to flatten the sails mainly so you can control the boat. If you like sailing "on the edge" keep the belly until you are not comfortable with your ability to control the boat.
Reducing the belly or flattening the sail has nothing to do with reducing drag. It has to do with reducing power.
Lift and drag is too complicated for me. The way I sail a boat is to stay as high on the lift/drag curve that I can I can't really describe it but I know how to use it to my advantage. The settings i give up on my SAIL TRIM CHART place you at the highest point on the lift/drag curve.
- Status