Normal or Adverse

A discussion of Catalina sailboats will usually include two comments, they have large spacious cockpits - and they suffer weather helm.  When the C30 was designed... it was a departure from typical sailboats of its length.  It offered a large spacious aft cockpit.  That trend has been a part of Catalina as well as other designs and has been readily received and appreciated by owners who enjoy the space but does it have a cost?  Are increased weather helm issues caused by the wide aft section?  If so, what actually causes it?

From an unknown author

How does the yacht cope with being over-canvassed, particularly closehauled?

Narrow-beamed yachts are typically docile on the helm when over-pressed. They heel to large angles, start to lose speed and suffer an increase in leeway, but they remain easy enough to steer. On the other hand, wide-beamed boats, particularly those which carry their beam well aft, become very hard mouthed and difficult to control if asked to carry too much sail. They may even develop so much weather helm that it is almost impossible to stop them from tacking in a strong gust unless the mainsheet is eased.

The above describes the weather helm, which this writing is concerned.  Its well known that all sailboats will generate some degree of weather helm during heeling.  Heeling moves the lifting forces of the sails off center of the drag forces of the hull and forms a couple turning the boat to windward and will require a slight amount of rudder deflection to counter. It is not these normal amounts of weather helm that will be considered here. The issues regarding boat balance and normal weather helm are very well covered in works such as The Annapolis Book Of Seamanship, Chapman and others and are requisite study.  Rather, these comments are an attempt to understand forces induced by greater heeling angles that sometimes force a tack.

Center of Lateral Resistance (CLR)

A requisite for the discussion is clarifying one of the terms basic to boat balance theory and  lacking clarity will preclude any meaningful discussion.   A simple and perhaps traditional definition defines it as the terminology implies, as the center of the lateral resistances, the center of those forces experienced by the hull, keel and rudder that resist leeway. One illustration has a boat in a river current and lying sideways hung up on a rock at just the right point to experience balance on the currents, then that point would illustrate the CLR.  

This definition is too restrictive however in that it doesn't provide for applying all the forces involved with the hull that come into play when considering weather helm.  I like the definition offered by Chapman's 61st edition. 

The center of lateral resistance (CLR) is the equivalent position of all forces produced by the hull and its appendages. 

The importance of this broader definition will I hope become clear.  It is also important to note, that the CLR is dynamic, that if these forces are altered for any reason, that the CLR will shift.

Hull Forces

The reason for the discussion to go beyond the normal relationship of sail and hull resistant forces is because that theory doesn't explain adequately what is happening with the hull.  In fact, most traditional weather helm discussions center around sail balance and shifting the center of effort (CE) of the sailplan with little or no suggestion of dynamic forces involved with the hull.  Some discussions even state that CLR is static.  This discussion will be almost exclusively about the hull forces because that is where I think the answers to severe weather helm exist. 

One reason for this belief, is because on my C250, I can experience rounding up amounts of weather helm under jib alone with the CE as far forward as it goes and according to traditional thought... that shouldn't happen.  A CE that far forward would not allow the traditional view of sail / hull couple to do what is happening.   Without question, the forces inducing the adverse problems are related to excesses of heeling and are hull related rather than issues of the sail plan, not discounting that excesses of heeling are caused by excesses of sailplan.

To preface a study of hull forces, it is important that a student remember the variety of forces that might come into play.  The most common that comes to mind is leverage but foil lift often comes into play and especially in a hydro medium where lift forces are exponentially higher than in air. 

Bow Wave

One of the forces very often ascribed at issue is the bow wave.   Bill Holcomb in his Weather Helm  offering points to the bow wave as  possible contributor. 

As this happens, the boats forward motion forces the water to pack-up on the leeward bow and effectively try to push the bow toward the wind.  Steve Colgate describes it very much the same and provides the illustration on the right.

When a boat heels, the bow wave on the lee side becomes larger and tends to shove the bow to windward.

While this theory fits the mold of examining all the forces which may come to bear on the hull, I struggle to think it the cause of the monster weather helm we're looking for.  One reason is that if the leeward bow were being pushed to windward, it would have the effect of rotating (yawing) the boat around the boats keel or center board.  Moving the keel aft should then provide a predictable result... that the pushing forces would have an easier time of it.  The actual result however is that raking the center board aft... significantly reduces the yaw effect of heeling. 

Another reason for looking beyond the bow wave is that Catalina 250 wing keel owners discovered relief from weather helm by trimming the bow down.  A deeper bow would both increase the bow wave and  the surface on which it would push. 

Last, boats with symmetry fore and aft don't suffer major weather helm but do experience the bow wave similarly to those which suffer drastic weather helm.  The bow wave theory then might be an influence, but again isn't the monster we're looking for.

Trimming the Bow Down

It didn't take me long to realize that my boat fit the "hard mouthed" description above, which prompted gaining as much knowledge as possible about boat balance in an effort to achieve better handling.  Readings helped but didn't nail things down.  When  comments were made on the Catalina 25/250 forum from owners of wing keel 250s saying that trimming the bow down helped to reduce adverse weather helm,  my interest was piqued.  The result was opposite to what should have been expected because traditional thought says that adding aft weight and increasing the lateral resistance aft decreases weather helm.   In fact, I'd added two 70 lb six volt batteries to my aft compartment believing that doing so would decrease weather helm.

From my readings, I knew that traditional weather helm theory says that trimming the bow down moves the CLR forward increasing weather helm.  After being reassured that their observations were correct, I moved my golf cart batteries forward to the V berth and sure enough, weather helm issues abated some.  Over the next many months, I was haunted with the question of why the effect had been opposite the rules.

Again, from Steve Colgate,

Another way to change the balance of the boat is to leave the CE in one place and move the center of lateral resistance forward or aft. Since the CLR is the center of the underwater lateral plane of the boat, the only way (without a centerboard) to move it is to submerse less or more of the boat. If you depress the bow of the boat by moving crew or equipment forward, the CLR moves forward and weather helm increases. The opposite results if you depress the stern, allowing the bow to lift higher out of the water.

That the opposite is observed on the Catalina 250 is quite interesting.  Why does trimming the bow down bring relief when well established theory suggest the opposite should result?  Wishing for an explanation to this and the more general question of why a 250 and some other boats suffer adverse  amounts of weather helm with greater heeling, I followed the primary clue... that trimming the bow down changed the hull footprint.  It wasn't necessary to look very far, Chapmans and other readings referenced the transformation during heeling from symmetrical to asymmetrical footprint.

Looking back to Bill Holcomb's writing, he portrayed this in his article and provided graphics of the at rest water line profile compared to the heeled profile being at the heart of the cause. Others have proffered this concept as well. 

Again from Chapman's.

The part of the hull that is underwater becomes asymmetrical, creating turning forces that have to be counteracted.

  ..... is the asymmetrical profile the big deal?


Asymmetrical Influence

A symmetrical foil (footprint of the boats hull) will have equal lift in both directions and the boat will track straight.  When a boat heels the foil changes shape.  The leeward curvature becomes the shape of the hull chine and and the windward the bottom section.  The foil has become asymmetrical (unequal in shape)  and as water passes the hull sides, the Bernoulli theorem comes into affect which says simply that because of differing distances for a fluid to travel the two sides of a foil, a high pressure area will be displayed on the shorter side and a low pressure on the longer side.   This same principle of course is what drives the sails to provide the lift that allows sailboats to take advantage of apparent wind and sail to windward. 

There are boats which take advantage of this principle in their hull forms.  Looking at the design of the Hobie 16 or Prindle beach cats, their two hulls are asymmetrical having flat outer hull surfaces and rounded inner. These boats have no dagger boards or keel surfaces.  While flat... the lifting forces cancel each other but as they heel the leeward ama immerses deeper than the windward resulting in greater lift of the leeward and the boat is lifted windward to overcome leeway.  The lift resulting from these hull forms should not be discounted, these Cats are witches going to weather because of asymmetrical lift.  If it were not for the drag when off wind, these hull forms would remain popular.  To make a comparison, remember that these beach cats are using lack of symmetry as a tool with heeling necessary to impact a good effect while on a monohull, the forces will lift leeward instead of windward and excess heeling is undesirable.

Recent surfing took me to a UK site for Radio Control racing which, has proved the most interesting site for information and thought about hull form that I've visited.  An interesting observation was the discussion of a design with a fine bow entry, a wide aft section and a fair amount of rocker.  The result is that when heeling, the windward hull is the longer form.  This hull enjoys some windward lift and suffers no weather helm issues. Such a hull of course requires a deep bulb keel as form stability is compromised. 


The Catalina 250 Water Ballasted Center Board Design

Wide flattish bottomed hulls have seen recent use for sailboat designs.  They produce initial righting moment which is a desirable trait for a sailboat.  They provide for spacious interiors.  They are also a requirement for interior ballast designs like the C250 water ballast.  However, the asymmetry of their heeling profile is more exaggerated than a narrower rounder hull form.  Examining a Catalina 250.

• With a length to width ratio of 3:1 and a blunt entry, the hull side has a large fore to aft curvature which presents a very exaggerated leeward foil section during greater heeling angles.
• The flattish bottom with minimum rocker (fore & aft under curvature), when heeled becomes the windward foil section and presents only a slight curvature

The asymmetrical foil then of a c250 while heeling presents a much greater lifting foil than would be experienced on a narrower beam with a rounder bottom and more rocker.  The result is that when heeled the foil will present a considerable amount of undesirable lift to leeward and,

1. The increased lift to leeward must be added to the normal leeway caused by the sailplan with a resulting increase in heeling forces due to the boat tripping over its keel and rudder.  A catch 22 situation then exist.  As the boat heels more, it generates more leeward lift and more leeward lift generates more heel.
   
2. If the center of the leeward lift is unbalanced to the center of the resistance forces of keel and rudder, it will couple with the keel and result in a rotational force.  If aft of the keel, it will rotate the stern leeward and contribute to weather helm in rather drastic amounts.

Adding Center of Hull LIft to the Equation

Noted above was that the c250 displays a characteristic defying normal thought, that trimming the bow down reduces weather helm.  An explanation for that can now be proffered.  With the above described characteristics, trimming the bow down, will alter the footprint.  The apex of the foil will move forward and carry the center of the asymmetrical lift with it.  As this center moves closer to and laterally to the keel, its leeward lift force will come into better balance on the axis of rotation and dissipate as a rotational force, though it will still exist as a leeward force.

It is very easy to see how this might be true.  The 250 squats on her stern when loaded down with a heavy outboard, especially true for the wing keel version. With stern down, the asymmetrical foil shape will be enhanced aft and therefore the center of its influence will likely be aft of the keel and causing the boat to yaw around the keel.  When trimming the bow down on her lines, the foil lift center moves forward balancing better on the keel and with closer to equal forces either side of the keel, a reduction of rotational force.

It should now be clear that traditional thought of boat balance being the relationship of CE and CLR is inadequate and that a third major force (hull lift) is at play. In my opinion, I think this force is a subset of the CLR and should be thought of in relationship to the lateral resistances as long as the CLR is defined as center of all hull forces.  The formula for center of lateral resistance then would change

from :  CLR = center of lateral resistances
to : CLR = center of  lateral resistances forces +/- center of hull lift forces

The Big Picture

For a sailboat to display good manners, the various forces that she encounters need kept in balance.  We now have understanding that the lift forces of the footprint foil play a large part and that when a boat heels, it is important that center of the lift force correspond with the center of the lateral resistance so as to avoid a serious shift of the CLR when heeling at greater angles thus maintaining a well mannered boat when she heels.

This perspective also helps to understand what most sailors all ready know, that heeling excessively causes poor performance to windward for many boats.  Most sailors however ascribe this leeway to forces on the sail and that is certainly true.  However, we now  know that the leeward lift generated by heeling adds to sailplan leeway further hurting weathering performance.

 

Wrapping it up

It can now be seen mathematically why a wide aft hulled boat experiencing good trim at low heel angles can experience rounding up with greater heeling.   Also seen is why shifting asymmetrical lift forward to balance on the keel can help.    Evident as well is why a wide flat hull will be given to a lot of leeway if heeled excessively even if it doesn't go out of helm balance. 

Comparisons should be evident between the two models, one beamy, flat bottom and wide aft section and other with a narrower beam, rounder bottom and fore and aft symmetry.  The latter can usually heel without adverse affects while the former should be kept near her lines.

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