Understanding the relationship between CLP and CE in sailboat design.

[h_zwakenberg]

Originally Posted by Stephen Ditmore

Don't you mean a 3-5 deg angle of incidence with the water? If the boat is making 2-3 degrees leeway, then the rudder should only be turned 1-2 degrees off boat centerline. (Skeg or keel hung rudders might be a little different, since you might take the fixed part into account in calculating the effective angle of incidence. If the boat has a jibing centerboard or keel, a trim tab, angled bilgebords, or a tandem or CBTF setup, then the leeway angle of the hull might be nearer zero, so you might be correct in those cases.)

When you're a yacht designer and a client comes in complaining of "weather helm" it's very important to distinguish between rudder angle measured off boat centerline and issues with mechanical advantage, which can be fixed with either a change to the steering system or by adding more area on the leading edge of a balanced spade rudder.

Stephen,

the flow field around the keel causes a downwash in the streamlines aft of the keel, ie. the flow will be a bit diverted to lee. For this reason, the 3-5° I mentioned as a rough guide should be taken from the centerline. The actual angle of attack of the rudder will be a bit smaller, by the amount of the local downwash angle.

Remember: the object of this exercise is to off-load the keel somewhat and to have the rudder contribute to reducing leeway. The reduced keel's span loading reduces induced drag, hence increases L/D. Instead of letting the rudder merely keep balance at the cost of resistance, in the configuration proposed above the rudder at least produces some lift for the drag it is causing, so the overall efficiency (= L/D) increases.

bye
Hans

[Stephen Ditmore]

Well, you could be right then.... but it's more than I've seen recommended in other sources. I wouldn't want to be in the position of stalling the rudder each time I tried to turn downwind. I've had the experience of being unable to turn downwind unless I eased the mainsheet (or better, the traveller).

I also wouldn't want to be in the position of having my rig so far aft that it causes control problems downwind. Perhaps that's what the practice of raking the rig forward when going downwind is all about...?

[geoffr]

I am designing a junk rig and have my info from Haslers Junk rigs found it praqctical and full of info for do it yourself simple methods of determining lead on any yacht However not full of info on angles of heel etc.

[messabout]

Whew! Man there are some really smart dudes on this subject. Either that or some certified pedantics. I'm going to take the high road and believe that all those folks are indeed smart dudes.

Now I will display my ignorance by asking questions as follows; Does anyone pay attention to the centroid distribution of the immersed body ? I have been sort of hung up on that deal and I wonder if I am only making things unduly complicated. If one examines the section centroids at various angles of heel it seems that all sorts of variations occur. It seems to me that there is some signifigance in this and it relates more than casually to helm balance.

Take a simplistic boat....A flat bottomed skiff, say a sharpie type. Let the boat have 10 sections. When flat the section centroids are aligned of course. Heel the boat 5 degrees and the forwardmost 2 or 3 and the aftermost 2 or 3 section centroids will curve to leeward. That implies drag, but forward and aft curvatures are near equal and may tend to cancel one another. Or maybe not. If not, then some helm will be felt. Heel to 10 degrees and the curves begin to go away. Meanwhile the WL becomes longer. Depending on rocker distribution the forward or after end may be immersed or maybe both ends. If only one end gets wetter then CLR has moved. Increase heel some more and the curve begins to go the other way. That is, the ends will begin to curve to windward.

Somewhere between the two opposite curvatures there is a straight line. Does this not imply that a certain heel angle will be the most favorable ? With a flat bottomed boat the optimum heel angle also reduces wetted surface and creates a waterplane that is quite attractive (narrow, long, and straight).
In some cases, when you dont have the hull curvatures perfected, the straight line of centroids will not be parallel with the center line of the boat. The boat may be quite happy in such a case but the skipper may not be. The boat with respect to its' CL is headed up to windward. It seems to me that all this has to influence the choice of lead etc. O.K. I'll get out now

[RHough]

Originally Posted by messabout

Now I will display my ignorance by asking questions as follows; Does anyone pay attention to the centroid distribution of the immersed body ? I have been sort of hung up on that deal and I wonder if I am only making things unduly complicated. If one examines the section centroids at various angles of heel it seems that all sorts of variations occur. It seems to me that there is some signifigance in this and it relates more than casually to helm balance.

You can look at the immersed hull in waterline slices. The section will have a profile, much like a wing section. Consider the forward end of the waterline as the leading edge of a wing and the transom as the trailing edge. The centre of the profile will have a curve ( unless the water line is symmetrical). The profile will generate a moment just as a cambered wing will generate a moment.

The moment due to curve in the hull will make the hull want to turn ... some. I have not seen this moment quantified. I suspect (pure SWAG) that the turning moment due to heeled hull shape is small compared to the off centre-line forces of the keel, rudder, and sails when heeled. The driving force from the sails and the drag from the foils combine to cause a weather turning moment due to heel.

"Lead" places the geometric centre of the sail area forward of the geometric centre of the lateral hull area. Lead causes lee helm, for the boat to balance, the weather helm due to heel must balance the lee helm due to lead.

Boats that are designed to sail flat with near zero heel have very little lead. Boats that sail heeled 20-30 degrees need much more lead.

[messabout]

RHough:
Your reply duly noteed. Not sure we are talking about the same thing. You refer to waterline slices. I take that to mean plan view of waterlines as seen in horizontal lifts of models. To be sure, some reaction caused by asymetric curvature of the various waterline lifts will be in place. Conventional hulls (not a Hobie cat) will probably have more heeled curvature on the lee side. Thus, we tend to make some leeway. If the sum of the forces generated by curvature does not lie in a location that agrees with the instantaneous sail center of pressure, then we have a rotating moment. That'll make the whole lead selection process uncertain.

I am looking at the underbody from a different perspective. Draw a body plan. Draw the waterline in a heeled position. Find the center of gravity for each immersed section. Observe that the CG points will not lie atop one another. Change the heel angle and the CG points move. Lay down a plan view. Place the CG points at each section athwartships location on the plan view. Connect the dots. The line connecting the dots is not straight. Examination of such a sketch implies that a line whose ends curve to leeward will induce lee helm. Strong turning moments can be envisioned here. When the boat is heeled a lot the line of CGs curve in the opposite direction to windward. Now we have windward helm. This is a notion that is separate from the wing section phenomena. I reckon both force systems are in place, simueltaneously, along with all the other variables.

Just doodling on my scratch pad. I quickly listed 12 different variables that complicate lead selection. There may well be more than twelve of 'em. Given all that I suggest that we fix it all by sticking a little jigger sail near the transom. With profound apologies to Leonardo, to hell with complexity. Lets just trim the jigger to suit the situation and enjoy sailing.

One last swipe at the issue.....The author of those charts that list lead percentage of waterline was generalizing to a fault. I would not ignore those figures, but I would not bet the farm on them either.

Wishing one and all fair winds and neutral helm.
Gene

[RHough]

Originally Posted by messabout

I am looking at the underbody from a different perspective. Draw a body plan. Draw the waterline in a heeled position. Find the center of gravity for each immersed section. Observe that the CG points will not lie atop one another. Change the heel angle and the CG points move. Lay down a plan view. Place the CG points at each section athwartships location on the plan view. Connect the dots. The line connecting the dots is not straight. Examination of such a sketch implies that a line whose ends curve to leeward will induce lee helm. Strong turning moments can be envisioned here. When the boat is heeled a lot the line of CGs curve in the opposite direction to windward. Now we have windward helm. This is a notion that is separate from the wing section phenomena. I reckon both force systems are in place, simueltaneously, along with all the other variables.

Gene

The effect you are looking at is well known. Kayaks use heel induced steering as do sailboards. At displacement speeds the hull wants to turn away from the heel. Heel port to turn starboard, heel to leeward to turn to weather. At planing speeds the effect is reversed, heel into the turn (the same way surfboards turn).

I don't think that the turning moment from heel in displacement hulls is large. It works in Kayaks because it is the greatest turning force. As soon as you add sails and keels the turning force of the hull shape becomes small compared to the other forces.

The reality of sailing is that although it may be possible to calculate a balanced hull and sailplan under a steady state condition, steady state conditions do not exist for sailboats.

[messabout]

Rhough:
I agree that the turning phenomena is well used but cannot agree that it is well known as to cause. I turn my kayak that way to counter winds or currents, or simply to make a tight turn with less paddling effort. Most kayakers have no clue as why this works and probably dont care. Only techno freaks, like us, delve into causes.

The magnitude of the turning moment from heel is determined in large measure by the shape of the underbody. Hulls with fat ends will react powerfully from this cause. In addition, there a certain hull forms that react in one direction with a modest heel and react strongly in the opposite direction with further heel. One need not do the graphics and/or calculations to prove this. It can be experienced first hand, and some times frighteningly in various dinghys.

This thread was originally about appropriate lead for a particular boat. I have burdened the thread with my point only to suggest that there are too many variables, some of them not often considered, to assign a reliable recipe. I do agree, wholeheartedly, that steady state condition does not exist for sailboats. At least not for any measureable lenght of time.

Some of the most illustrious designers, Starling Burgess for one, is said to have balanced many of their ships by eyeball only. They usually worked too. That is not to imply that lesser mortals should not try to do so with some science in mind. On the other hand, we can shoot ourselves in the foot if we get hung up on too many obscure technicalities. My foot has a few scars from gunshot wounds.