Amount Of Heel

[Don Guillette]

This thread is for RichH, Alan, Joe from San Diego, Stu J and anyone else who can shed light on this subject for me. I'm really not into wetted surfaces, lift/drag curves etc. I just know how to make that stuff work for me. In other words, I'm a seat of the pants type sailor who happens to know a lot about sail trim and not much about anything else related to sailing but I have to explain this stuff to beginners, who I'd lose in a minute if I started talking about curves etc.

I need to know something before I head to Phoenix in a couple of months for a sail trim seminar. Here's my premise - lift comes from 3 things - the sails, the keel and the rudder (when you induce about 5 degrees from neutral). I see closehauled boats all the time heeled over so far that I can see the barnacles on the bottom of the keel and the rudder is half way out of the water and the crew is hanging over the side. My thought is they are losing lift from 2 elements (keel & rudder) and I think two MATCHED boats (the only kind of racing I really like) going head to head, the boat heeled about 15 degrees heel will win over the 25 to 35 degree heeler every time. Two or three boats in trail with a bunch of heel, which is what you see all the time, will never gain on each other because they are in the same lift situation.

When I lived in So Ca I would attend the Congressional Cup races where there were 12 matched C37 racing with top crews from all over the world. I really didn't care about the race as such. I watched the sails, the boats and crew positions very closely with high power binnoculars to see how they did it. None of these boats were ever massively heeled over. I didn't care WHY they were doing what they were doing I just wanted to see WHAT they were doing on a particular point of sail and wind condition and I made notes to myself. A lot of these notes appear in my book and SAIL TRIM CHART

So, help me out. Am I "all wet" so to speak!! How can I explain this from a scientific point of view but in plain English. Maybe it can't be done. If it can, I know you guys can do it. I spouted off my theory about two matched boats going head to head with one heeled way over and the other with small heel, at the Lake Havasu Pocket Cruisers Convention and prayed to God nobody challenged me on the subject - thank God no one did.

 

[Ted]

Don, I don't think you can generalize about the amount of heel between boats unless you state some additional facts. Some boats sail very efficiently with 25 degrees of heel while others start to loose speed after 15 degrees of heel. If all things are equal the boat sailing flatter will usually have better VMG to weather. Here's an example. Two identical boats capable of sailing efficiently with 25 degrees of heel. If one boat has switched down to a 135% headsail to maintain 15 degrees of heel while the other is still carrying a 150% and heeling at 25 degrees, the boat with the 150% would be faster. However, if both boats were using a 150% I would assume the boat sailed with less heel would have better VMG to weather because the underwater foils are working more efficiently.

 

[Joe]

Don... I don't have any hard rule about heel... I know that my C27 enjoys about 10-15 degrees under moderate conditions... All the dinghies I've had experience with needed to be sailed flat... in fact any boat with a planing hull should be kept as flat as possible through hiking out or trapezing when necessary.

Watch the speedo, the amount of weather helm and progress to the mark. If the boat is jamming and still feels balanced..... go for it. If there's a bit of a struggle with the helm when a wave passes under the boat.... start depowering.

One of the benefits of steering with a tiller is the greater sensitivity to the boat's helm and balance. Larger boats don't have this luxury, so closer attention is paid to the instruments when judging whether the amount of heel has become counter productive. That would suggest that each boat should be tested on various angles of heel, under varying conditions, to determine optimal pointing, speed and control levels. Those limits should be recorded in some kind of performance manual for that particular vessel.

 

[RichH]

With respect to the keel providing lift, the more the boat heels over the more it loses its horizontal component of lift (to weather) and correspondingly begins to gain in the vertical component (starts to lift the keel in the 'vertical' which further increases heel as a loss of heeling-moment reaction of the mass of the keel) If the boat increases speed due to its total mass now more supported by the keel's vertical force component ... but must experience a loss of lift in the horizontal direction and well as is encountering a loss of 'bite' due to the reduced angle of the heel has (less pointing ability) versus the increase of speed.

Of course every different boat hull design shape is different ... but trigonometric intuition tells me that 'any' amount of heel will reduce keel 'lift to windward' and thus 'any' heeling is ultimately 'detractive' overall, theoretically.
From watching the famous outrageously overcanvassed Chesapeake log canoes that use HUGE and equally outrageous hiking boards (so that virtually the entire large crew is 'waaaaaay' out to windward ... my intuition also favors: 'straight up' is the fastest and simultaneously provides the highest amount of lift.
Note - This ignores those boats who by their extreme shape (SCOWS, etc.) which can transfer the projected area of their 'centerboards' onto the side area (lateral resistance) of the hull at increased angles of heel ... in such boats the immersed area submerged become greater due to heeling ... but up to a finite point ... and then the hull will begin to lift 'out' of the water and begin to plane. Scows, etc. are by design optimized when at ~25 deg. of heel to get the 'lifting centerboard forces and 'to weather forces' optimized; but then again the 'bilge boards' in a scow are already permanently 'canted' about 15-20° from the vertical and about 7° angle 'off' of the centerline. Scows held all the absolute speed sailing records for almost 110 years. (I loves them scows.)

Hull heeling summary .... In general (and ignoring planing or 'trick' hulls), mathematics tells us 'straight-up' mast and straight down keel gives the optimum theoretical lift to windward. So with respect to keels the mathematical answer would be maximum lift occurs at 0 degrees of heel .... lift to windward. At 30 degrees of heel 86.6 efficiency and at 45° over only 70% of lift efficiency. .... a 'static' trigonometric analysis, etc. and ignoring any 'lifting' of the hull at higher speeds.

Mast/sail angle from vertical summary ....The same trigonometric analysis would apply to the complimentary heeling angle of the mast & sails ... and in spite of the common appearance of super-fast sailboarders holding there mast/sails at a NEGATIVE angle of heel (mast angle held TO weather). Most all of the 'boats' who broke the 50kt. record and those now attempting the 60kt. barrier all have keels/lifting bodies and sails at 'straight up'.

Thats my 'story' but I wont testify to it in court.
;-)

 

[Paul Mermelstein]

I just taught this in a Power Squadron Sail course. In plain english for new or non sailors:

True, each boat is different, so a particuluar degree of heel can't be generalized. BUT, when heeled more than optimal for any given boat, you lose speed due to several factors:

1. The sails are losing or spilling some air. Wind comes from the side as well as front, even close hauled. If the apparent is 45 degrees, you have as much wind from the beam as you do from the bow. So heeling will allow some of the wind to spill up and over vertically, not just flow horizontally to provide lift. Thus, you lose some of the power of the wind.

2. To correct for the rounding up that often accompanies excessive heal, (especially if not perfectly balanced) you must turn the rudder more. This provides an in-water brake more than lift, at this point.

3. Yes, wetted surface often increases when heeled, thus providing more friction with the water.

4. When heeled, the keel has some vertical lift and therefore acts to brake the boat. (With lift always comes drag).

So, that's my view.

Paul

 

[David in Sandusky]

Developing previous posts

"Spilling air" There are two possible effects here:

1. Reduced effective sail area presented to the wind. This reduces with the cosine of the heel angle as shown below. Notice that the loss of effective sail area increases with each 10 degrees of heel. I'm sure about this effect.

Heel Cosine Loss
0 100.0% 0.0%
10 98.5% 1.5%
20 94.0% 4.5%
30 86.6% 7.4%
40 76.6% 10.0%

2. Loss of the vortex around the sail. Lift is generated by a vortex that develops around the sail. Tilting of the sail with respect to the wind may result in disproportionate loss of the vortex off the top end of the sail, reducing the lift that drives the boat by more than the cosine. Less sure about this effect. (Edit: After further reflection, if the heeling sail is properly trimmed, its angle of attack into the wind is the same, so I would discount any vortex shedding effect.)

Spilling water. The same analysis applies to the lift from the rudder and the keel. The result of this is that the boat makes more leeway.

Rudder drag and stall. As the rudder becomes less effective as explained above, and by loss of rudder area in the water at more extreme angles of heel, more rudder angle is needed to hold the boat on course. Increased weather helm also adds to this problem. The increase in drag with increased rudder angle is significant. When the rudder finally stalls, the increase in drag is dramatic, while the loss of lift requires even more rudder, and the boat is on the way to rounding up. This link describes drag vs. angle of attack or inclination:
http://www.grc.nasa.gov/WWW/K-12/airplane/inclind.html

Hull shape. The point is already made that wetted surface increases beyond some angle of heel, increasing drag. Also, when the rail is submerged, there is a dramatic increase in turbulence, further increasing drag. This effect is specific to a specific hull design/shape.

 

[RichH]

Ill also add that when the sail is well heeled that the flow direction is no longer strictly parallel to the 'cord' but at an increasing combined angle towards 'vertical'.

So as the keel begins to 'slip' because of excessive heel angle, also too the sail increasingly 'slips' on a trigonometric and aerodynamic basis.

 

[Don Guillette]

Thanks guys for all the info you provided. If anyone questions me about this subject I don't have to try to duck the question becuase now I've got the answers.