Sailboat Design…The “WHY” of Upwind Sail Trim

In my opinion, the first commandment of upwind sailing might be: Sail to the full potential of the draft of your sails limited only by the effects of drag as experienced on your tiller or wheel.

The fascination with upwind sailing perhaps stems from the fact that it is a more precise application of scientific principles to the creative art. After all sailing is both art and science. We leave the sail loft with a new suit of sails…cut full with anticipation… a Cunningham…zipper…leach line… reefing points and any other local imaginative devices. Before your vessel leaves the yard fitted out for a new season; perhaps, your rigger set up a new mainsheet configuration…vang and traveller arrangement…maybe you have adjustable stays…Sailors will then go out on the water and learn “How” to use these controlling devices..and with practice become quite proficient in their mechanical application; but, until you know the “why” of it you’ll stay on the mechanical level and not advance to the creative level.

To become more creative you must ask yourself the question “WHY” which will allow the full realization that all these devices can be summed up in two words “Sail Control” Once you understand the scientific principles of sail control, you elevate to a new dimension of creative application. That’s what this link is all about..How to get to the “Why”

leach

To begin with…read over statements one and two…Don’t memorize them…just look them over…as we progress, refer back as needed…it will sink in the mind’s eye when your ready:

NO.1 The dynamics of sailing is a constant balancing of equal opposite forces on the sails and the underwater portion of the hull.

NO.2 The air pressure over the outside surface of the sail must travel a farther distance in the same amount of time than the air on the inside of the sail. This creates a region of lower pressure on the outside.

“Let’s get going on a mythical sail and see what’s happening”…keeping in mind of course, that conditions are always perfect on a mythical sailing. Hoist sails and come up close on a port weather tack. Our compass tells us our heading is due south at 180 degrees. Foward of the wheel on the card in the compass binnacle, there is an arrow pointing at my belly reading 000 degrees. We’re healed to the green side 090 degrees…The red side…the high side on this tack is where your sitting at 270 degrees due west. The vessel is well balanced in a 15 knot breeze…I’m applying just a touch of right rudder angle to keep her from coming up. Take a look at the sails…Although the geometry of the triangular sails is different…the curvature is similar… regard the sympathetic relationship they have…Try to visualize the streamline flow…do they look right…Look at the respective leaches…they should look like railroad tracks… Get rid of the resistance factors to the laminar flow: bumps, humps, hollows…”the crew member parked between the sails foward of the leach”…any and all resistive forces that disturb the laminar flow of your sails…needs to be trimmed out and eliminated.

Scroll back up and take a look at statement No.1 again. The equal opposite forces can be broken down into two bundles of forces: A-The Aerodynamic forces…These are focused on the geometric center of your sail plan. This bundle is known as the “Center of Effort” CE…You don’t have to understand this yet…but just remember for now that this bundle of forces wants to pull us to the green side. “That’s why we’re heeled to the green side”…The sails are not driving… the’re pulling us and they are pulling us sideways…”Planes may be lifted but more accurately…the path of least resistance for a sail boat can be better visualized as a sideways pull”. Enough about the green side for now… B- The Hydrodynamic forces are focused on your hull at the geometric center of the underwater portion of your hull. This is your Righting Force known as “The Center of Lateral Resistance” CLR. This bundle of forces want’s to pull you to the red side. So statement #1 should begin to make sense to us…If the Aero forces are pulling us to the green side… and the Hydro pulling us to the red side and they are equal and opposite…What happens?…They compromise, and we wind up going foward…If I’m still paying attention to my wheel I’m steering 180 degrees. In this example you may conclude that if it wasn’t for our red righting force…we would be pulled sideways…or more exagerated by saying we would be sailing sideways, course made good 090 degrees. But since red won’t let green have it’s way…green can only heel the vessel…The greater the Aero force (stronger wind)…the greater the pull…the greater the angle of heel.

We can’t see these forces but we surely can sense their effect. We sense the Aerodynamic force through sight…our angle of heel… more wind, fuller sails, greater heel angle. The equal opposite righting force on the vessel is realized through our sense of touch…increased weather helm…Not wanting to get ahead of ourselves…we begin to experience the necessity of more rudder angle to hold us off as the wind freshens.

What we’ve accomplished up to this point is an understanding of direction…we must now take a look at statement #2. Quite simply direction needs motion. Statement #2 suggests that when the airstreams meet the leading edge of the sail they branch off and meet up again at the exit of the leach. Since the sail is curved…the airstreams on the outside must travel faster if scientifically they must meet up again…This speed difference creates a low pressure on the outside of the curve…this is the path of least resistance which gives us our pull…when the path of least resistance is combined with the equal opposite directional forces…we wind up with this “incredibly inefficient force” empirically realized as beating toward a weather mark.

We can rightfully conclude that the greater the Aerodynamic force…the greater the equal opposite Hydrodynamic force therefore the greater the resultant force… meaning the faster we go…almost…there’s always a devil in the detail.

Getting back to our mythical sailing..our senses are telling us this. The wind has picked up…we are heeled at a greater angle and I’m using more rudder angle to keep her on 180 degrees…”Lets start applying theory”…We know that when the wind picks up and the airstrems divide at the sails leading edge…the streams on the inside are traveling with increased speed…the streams traveling across the outside curve (lee side of the sail) have to travel “even faster” this creates even lower pressure on the outside…the path of least resistance allows the vessel to pull to the green side with greater force…This means the righting force being equal and opposite must be greater…SO..remember I said there was a devil in the details…this is it…the more righting force exacted, the more rudder angle is needed. Since this affects the laminar flow of the water streams over the hull…the increased drag limits the desired effect which is the full potential of the resultant force… Not only am I experiencing excessive drag…but the question is…will my arm break before the rudder gear. Before we do anything let’s refer back to our first commandment…Sail to the full potential of the draft of your sails limited only by the effects of drag on your tiller or wheel.

Now that we have a greater understanding of these forces and how they can adversely affect our performance if left unchecked…how do we modify them? Sail Control…We flatten our sails… No doubt “You probably knew how and when…now you should know “Why”. By decreasing draft..I’m decreasing curvature…meaning the airstreams on the outside curve don’t have to flow as hard to meet their friends at the leach. Because the flow is lessened by flattening the curve on the lee side, you have raised the pressure on the outside region proximate to the sail; thereby, decreasing the aerodynamic pull on the green side… Since the pull on the green side is lessened…the pull on the red side (equal opposite) is lessened; meaning, less righting force is applied…SO… what is experienced, is an easing of the helm…Our theoretical speed has been diminished; but, by efficiently modifying aerodynamic and hydrodynamic forces through sail control, we have maximized our speed made good under prevailing conditions. Our barometer… a relaxed arm.

Before covering one other crucial area let’s get back to our mythical sailing (anything is possible with Windows 98). You remember we were sailing in a 15 knot breeze…vessel adjusted to the parameters of our first commandment of upwind sailing, only three things can happen within the confines of the argument. NO1.The wind velocity can remain the same…in this case no adjustment is required. NO.2 The wind picks up to “20 something”… Quickly realized by the senses, we first feel the force of the air on our face and our clothes…we see and feel the vessel at a greater angle of heel to the green side…we sense through touch that we need more right rudder if we’re to hold the course of 180 degrees. At this stage we should know exactly what to do within the confines of the argument. We know this…our theoretical speed has increased because the Aerodynamic forces have increased…Since the Hydrodynamic force must increase…I know that my practical speed could be improved if I eliminated the adverse effect of the hydrodynamic force on my wheel…I reason that reducing aerodynamic force to the extent of restoring helm balance is required. (If you don’t understand WHY review). NO3. The wind diminishes to 10 knots…In this case again I sense the change of reduced air velocity on my face…my heel to the green side is lessened. Although I’m employing minimal rudder to steer 180 degrees, suggesting little or no drag,…my sails look flat for prevailing conditions; so, I increase curvature keeping the sympathetic relationship between sails…Laminar flow is particulary crucial in reduced air…You have learned that as the curvature increases on the lee side of the sail, the aerodynamic effect is greater because the pressure gradient is lowered; resulting, in greater force to the green side; in addition to, greater force to the red side…the resultant effect is greater speed; provided, we have acceptable helm sensed through touch…and acceptable heel angle on the green side…sensed by sight. We trim to agreement with the definition of our commandment of upwind sailing.

The argumentative mind might raise the question…”You said there were only three possibilities… what about a wind directional change… that’s a fourth isn’t it?” A- No, that would only result in a tactical adjustment; but, doesn’t change the argument…for example if I flipped over to the starboard tack, the only thing that changes are the adjectives Red and Green….or my course… another three digit number…Confusion may lie in my insistance on steering 180…and that’s because we’re talking about helm relief through “sail control, not course change. This is the only way to explain it, or you lose the pony in the pile. Q- You make constant reference to weather helm; but, there has been no mention about the dynamic, or the effects of lee helm. A– Excellent point…we’ll get to that now…which will facilitate an understanding of the “WHY” of fore and aft trim upwind.

We’ve discussed CE and CLR and their relationships along the transverse plane as equal and opposites…but now we’ll take a look at the relationship along the lateral plane.

Let’s take the vessel we have been sailing and put the boat back on the drawing board. What you see is what the designer see’s after he has completed the design…Now erase the mast and the sails…Bow facing to the right in profile view. What you see is the product of a lot of design work on the completed hull. Notice the mark in the center of the under water portion of the vessel this is our CLR. Draw a straight vertical line from the CLR to the DWL and put a mark there. Now I have enough information by definition to discuss CLR along the lateral plane. I’ve calculated this point, in our design to be a desired 56% of the DWL. I know through my lines what the displacement is; and, have calculated my righting arms for varying angles of heel through my stability calculations. This tells me the potential of my sail plan in square feet. My next step is “where to distribute the sail area” or more precisely where do I put my now familiar CE. From experience, modified by a few erasures..I start drawing my sail plan and calculate my geometric center of the entire plan…”CE”. Draw a vertical line from this spot to the DWL and make another mark… that mark becomes the CE along the lateral plane. The line I have drawn that connects them I’ve labeled 12%…This is the “lead” relationship that I desired to achieve both in designing the hull and the sail plan. Now that you are familiar with CE and CLR viewed from the lateral plane, it’s time to analyze them. So let’s take the boat off the board and back in the water. What we are experiencing is this…As the boat begins to acquire force (AERO and HYDRO) and begins to heel, both forces move foward. CLR however moves “foward farther” than CE, overtaking the lead, and poised slightly ahead of our CE. This posture is what we are experiencing in our 15 knot breeze with full sail and balanced helm. When the wind picks up CE will move foward some more; but, CLR will move foward even further. When the relationship becomes excessive because of the increasing spread the result is excessive helm…. To relieve the effects of excessive helm…”sail control”…we flatten sail…We learned that helm balance is restored by decreasing the hydrodynamic force and its sugested method of application. Along the lateral plane, CE is retarded but CLR is “retarded even more” diminishing the adverse spread, to our our base relationship of slight rudder angle at 15 knots and a consequent reduction of excessive drag.

Now if we start to examine this relationship a little closer we can begin to understand lee helm, let’s put the boat back on the drawing board.

What if I gave the vessel a 17% lead insted of a 12% lead…this suggests that under our base sailing dynamic (15knot breeze course 180…), The CE would come foward… CLR would come foward but not far enough to overtake CE. My viewed posture is CLR slightly aft of CE…”it didn’t overtake the lead”. We experience lee helm. As many of you already know lee helm is not unusual in light conditions…because there isn’t enough wind to generate CLR past CE. Whereas 12% lead for this vessel was perfect, 17% was too much (at least in this 15 knot wind). So now you know that when CLR remains aft of CE along the lateral plane the resultant effect is lee helm. Regardless of lee helm or weather helm any angle left or right from the vertical axis produces drag… the effect reduced by sail control.

Excessive lee helm is quite serious and can be expensive to modify… not within this link. I’ve designed a vessel with a 17% lead that is exhibiting lee helm in a 15 knot breeze and I need left rudder angle to keep her at 180. First we rake the mast aft….By raking the mast aft I’m moving my CE aft, so now when the vessel is undersail perhaps the more traditional relationship of CLR foward of CE might be restored.

Now that you have an understanding of helm balance as expressed through the lateral plane…and an understanding of the corrective remedy…an astute observer might suggest that since the designed sail plan was based upon 100% foretriangle…that with his 150% Jenny the CE would be further aft and that’s correct… CE “unilaterally” moved aft will decrease the lead. maybe the 17% lead was right on. This is very much an involved design parameter and has to be discussed with the client regarding the vessels intended purpose and expected wind conditions and anticipated sail inventory…Only then can a designer start to seriously deliberate lead and helm balance. But the point is you now know why a mast may be raked aft, to reduce lee helm…The reverse fore and aft can be applied for weather helm…You now have the tools to be creative, and ways of expressing it through your use of your rigging. Once you know the “Why”… the right way to do things is in your control, tempered by the lessons learned by experience..You now have the tools of analysis for upwind work.

Conclusion

You’ve learned that the direction you want to sail upwind is a resultant force…generated as a residual effect of equal opposite excitations. Bernoulli’s Theorem explained motion and the kinetic effects of a pressure gradient. You learned to modify these forces by sail control, reducing drag on your rudder and increased effective speed through helm balance. You learned why and how to generate speed in light conditions by increasing draft… you learned the importance of a sympathetic relationship to curvature… the significance of laminar flow, and how all resistance produces drag. You’ve learned why lee helm develops in light conditions…and why a sail stalls out ie.(lack of aerodynamic flow.)…and lastly analysis of fore and aft trim… to help eliminate both weather and lee helm. You should now be able to talk intelligently to your rigger and sailmaker well grounded in theory.

Leave a Reply

Your email address will not be published. Required fields are marked *