Thinking too much about keel forces

justsomeguy · Apr 12, 2015

Think of the keel and sails as wings, flying sideways.

Both generate lift, but I doubt the keel would ever generate more than your sails, as you cannot shape your keel for optimum lift, unless the current favors your direction more than the breeze.

rpwillia · Apr 12, 2015

Book "The Physics of Sailing"

In the book they talk about the lift from the hull and the sails. They also talk about how when there is no wind, how a sailboat through it's keel can generate lift and move the boat through the water (and the air) filling up the sails and generating complementary lift off of the sails as well. Big trick to that is you need to minimize all movement because when you disturb the laminar flow it takes forever to get it back.

shemandr · Apr 12, 2015

If you sit out in the middle of the bay with no sails up your keel is not going to provide much forward movement. Both the sails and the keel provide lift. Without water moving over the keel (And Rudder) there will be no lift from it (Them). Sails run the show. When the boat starts to move the under foils start to work.

Tim M · Apr 12, 2015

The keel never has a forward force, it is the force of resistance. Here's the watermelon seed analogy: The force of sideways effort is the wind on the sails, the force of sideways resistance is the keel. Like a watermelon seed shoots forward from between your fingers from the squeezing force of your finger and thumb, your boat is propelled forward in the 'squeeze' between the effort of your rig ( the finger) and the sideways resistance of the keel ( the thumb ). I learned this analogy at one time and have used it to turn the light on for many sailing students. Cheers.
Tim

justsomeguy · Apr 12, 2015

Tim M said:
The keel never has a forward force, it is the force of resistance. -snip- Cheers.
Tim

But, but, the book!

shemandr · Apr 12, 2015

rpwillia said:
In the book they talk about the lift from the hull and the sails. They also talk about how when there is no wind, how a sailboat through it's keel can generate lift and move the boat through the water (and the air) filling up the sails and generating complementary lift off of the sails as well.

Worst of all I think I Rx'ed this book for preliminary reading for the Physics Seminar. Oh well. Yes, I have "Sailed" with no wind when the water movement over the keel was the only source of lift available. Usually current was responsible for the water movement. And once again the sails were up so it was synergistic. It wasn't like the keel was a tractor beam.

JTulls · Apr 12, 2015

Both the keel and sails are generating lift and drag -- how the vectors add together determines whether you get forward motion or if you side slip. There's always a slight side slip that allows the keel to generate the lift, that combined with the sails, moves the boat forward (more forward than sideways).

If you're interested, Navionics is sponsoring a webinar Tuesday 6:30pm PDT on the science of sailing. Registration link is here: https://attendee.gotowebinar.com/register/5701107793539392513

Joe · Apr 13, 2015

I like Tim's example. Don't forget that the hull and rudder are part of the equation also.

Jackdaw · Apr 13, 2015

The keel never has a forward force, it is the force of resistance.

Guys... No. The OPs original question was what pushes the boat FORWARD. The keel. It alone generates a force to WINDWARD. The force vector from the sail is LEEWARD. Without a keel you get pushed downwind. The better your keel, the better you go to windward.

Both the sail and the keel produce horizontal lifting forces due to pressure differentials from different water and wind speeds on their opposite surfaces. The symmetrical shaped keel acts as a airfoil due to the boats slight slip to leeward which creates an high and low pressure side.

Now comes the math; the vector sum of the lift forces from the keel (and rudder in a fixed position) and sail determines the direction of the boat.

When the boat's speed and course are constant, the net lifting force is precisely balanced by the velocity-dependent drag force on the boat as it moves through the water and air.

jibes138 · Apr 13, 2015

But jack the vector through the center of effort on the sail will not be perpendicular to the hull fore/aft centerline. This vector can be resolved into component vectors, one perpendicular to the hull and one in the forward direction. One of the components creates a heeling moment and the other a forward thrust. This is a little over simplified as the primary vector is really a 3D direction and it is a dynamics problem with constantly changing variable values. Thoughts?

Jackdaw · Apr 13, 2015

jibes138 said:
But jack the vector through the center of effort on the sail will not be perpendicular to the hull fore/aft centerline. This vector can be resolved into component vectors, one perpendicular to the hull and one in the forward direction. One of the components creates a heeling moment and the other a forward thrust. This is a little over simplified as the primary vector is really a 3D direction and it is a dynamics problem with constantly changing variable values. Thoughts?

Hey Jibes,

I didn't say or imply that the 'vector through the center of effort on the sail will be perpendicular to the hull fore/aft centerline', my diagram show that it will be roughly perpendicular to the chord of the sailplan at any given moment, which is true.

Center of effort, heeling forces are not directly part of the equation. You're right that it is all related and complex, but the 2D aspect is the actual part that move the boat forward.

A boats ability to resist heel is a function of its righting moment. This can be generated many (interrelated!) ways.

sailersteve · Apr 13, 2015

I have a Windrose by Laguna Yacht , 600lb, 6ft long cast iron swing keel. When i put the keel down more than two feet the boat slows down 2/3 its speed . ( Sure not helping lift & speed ) The front edge of the keel (was) about 2.5" wide for the first 4ft & about 5" wide the last 2ft, with nearly no pointedness . I say ( was ) because i rented cutting torches bought two used grinders and got to work for months sharpening the front edge. You could cut your hand open on it now. I collected 16lbs of specks & dust from it ( only 16lbs ) all that work.
I didn't collect all the dust because the back concrete pad & the whole sailboat is now a pretty shade of rust color that won't just wash off . I had the boat sanded and ready to paint , i got rust colored gel-coat . The boat is not painted or in the water .

Does anyone think the sailboat will be faster with the keel down more than 2ft now ?
Would like your honest opinion . Thanks.

77Ventola · Apr 13, 2015

Wow, now My head is hurting!

Everyone so far have made some valid points, I would like to share mine and also ask a question. I understand the physics of the effect of wind on my sails and how it propels my boat, but I wonder about the full effect of my keel in the equation. I sail a 23' Macgregor Venture of Newport with a swing keel, on both a small lake and also on a medium sized reservoir in Iowa (with a strong river current running through the middle of the res). Depending on the wind I always sail the reservoir in the correct direction to ensure I can get back to the marina harbor safely if I either have engine failure, or the wind dies. Since I can adjust the angle of my swing keel, can I adjust my sailing efficiency to any measurable extent? ( of course I must take safety into consideration, by lifting the keel I am reducing the effectiveness of my 700 lb keel). The ice has finally melted, and I plan on hitting the lake in a couple weeks, woohoo!

dbecker68 · Apr 13, 2015

All,
Thank Goodness there are those of you (and sailboat designers) that understand all that physics. I never have understood the science behind it, but I certainly do enjoy the art of it. I think I may be too old now to ever learn the science.

I have experienced the side-slip of a small centerboard boat, on a breezy day, with the centerboard up. It made an easy time of passing over a very shallow sandbar.

Jackdaw · Apr 13, 2015

Swing keel guys: ;^)

Going DOWNWIND
Lifting the keel will reduce the boats frontal area, and perhaps its wetted surface area. It it all likelihood will be faster.

Going UPWIND,
The deeper and more vertical your keel is, (all else being equal) the more lift it will generate. This will allow you to make better, faster progress to windward.

Reaching its more complex, you will trade off slippage for more boatspeed.

Raising your keel when sailing is a complex deal that involves lots of trade-offs, not the least of which is stability. For anyone not totally sure of the issues their their particular boat, I suggest leaving the keel 100% down 100% of the time.

sailersteve · Apr 13, 2015

Thanks Jackdaw.
Looking forward to trying out my new sharpened keel . Hoping for good speed with it down more than 2ft . I'll know in a month or two , hopefuly . Not so funny my catch 22 . Need keel in bad weather , need speed in bad weather. Can't have both .
Thanks again.

77Ventola · Apr 13, 2015

Sailersteve, My swing keel is shaped very similar to my rudder shape (which is close to an airfoil shape, it is actually slightly thinner and sharper on the back than the front). I bet that your modification will improve your boats performance some. There is a great sailing forum on yahoo called Venturenewport on yahoo which has a modification section with some good pictures of Macgregor swing keels, that might give you some additional ideas. I think you are on the right track, good luck.

Kingjim91 · Apr 13, 2015

The physics of sailing have always been interesting to me. One point is that the sails operate under the principles of aerodynamics, while the keel follows hydrodynamics.
Similar principles, but water is far more dense than air. So the balance that Jackdaw talks about is achieved with a keel surface area much smaller than the sail surface area.
In theory, it's possible to adjust the swing keel according to the amount of sail you're flying. But I don't know if that's practical, or safe.

A rounded leading edge of the keel is important to the hydrodynamic lift, so I think that altering the front edge of the keel is a mistake. There's a reason that it wasn't designed with a sharp leading edge.

albrazzi · Apr 13, 2015

Team work, the wind blows the boat over and the keel stands it back up hence the lift. Without the counteracting force of the keel the boat will blow over so no forward motion and you don't need the keel downwind, dead down anyway, you also don't need or have any lift downwind. The sail is an airfoil with its upwind shape that's what pulls you forward. Opened up off the wind past the beam roughly its not a wing anymore just a bed sheet hanging on the line. New crisp clean properly shaped sails are essential to upwind performance, downwind not so much. Still physics but this is how I understand it.