The keel is essential in determining the boat's direction, it's just out of the scope of this paper, which concentrates on how the sail generates lift. That's the specific topic a lot of textbooks get wrong.
I agree it would be cool if the paper also had a quick higher-level overview including how the forces from the hull/keel and sail interact, right now it feels like it lacks a bit of context! Thankfully this higher level is not so prone to error and I believe textbooks generally get it right, in a "spherical cows on a frictionless plane" sort of way. For example:
Do you have more information on that?
I assumed the keel was for balancing the torque that the forces on the sail generate so one doesn't capsize. In land sailing or ice yachting you don't have two medium with different densities and still achieve forward movement using a sail.
If you want to sail downwind you don't need a keel except for balance. In land sailing and ice yachting the wheels or runners serve the same purpose as a sailboats keel and provide resistance to the vessel moving sideways in a crosswind.
A boat's keel is weighted with ballast (on larger boats) for the balance reason, but it's fin shape is for the purpose of providing lateral surface area.
Even while sailing downwind, the sails are still providing "lift" (if we are calling it that here). There are opposing forces for sailing - the Center of Effort (CE) and Center of Lateral Resistance (CLR). These two need to be balanced in order for the boat to sail a proper course - of course if you are just letting the wind push you directly downwind, you could remove your keel/daggerboard, but in practice, you will want to sail on an angle to the wind - on a reach or by the lee depending on the type of boat - so as to create flow over the sails and have a greater pressure gradient than if you just let the wind push directly against the sail.
The hull of the sailboat contributes to the CLR, so it is common in dinghy racing to pull your centerboard/daggerboard up partially so as to balance these forces and minimize drag. If you watch Olympic laser sailing, you'll see while sailing downwind, they will heel the boats to windward with their daggerboards about halfway up so as to move the CE closer to being above the CLR, allowing for less rudder drag to counteract the forces and steer straight.
Every time there's anything on HN or Reddit about sailing, there's a _ton_ of misinformation and blanket statements that might apply to some types of boats, but are far from general rules. I'm happy to answer any other questions too!
I was trying not to overcomplicate the explanation, but when I said "downwind" I meant dead downwind, not broad reach. In this situation your sail plan does not need to generate lift: You can fly a symmetrical spinnaker and be propelled entirely through its drag forces, at the cost of not being able to match or exceed wind speed the way you can with a mainsail generating lift.
Do you have more information on that? I assumed the keel was for balancing the torque that the forces on the sail generate so one doesn't capsize.
I guess this is part of the reason.
As for why it isn't the full reason, think if the boat was a hovercraft or somehow otherwise had zero friction. When the wind hit the sails it would just move the vehicle in the direction of the wind (possibly turning it sideways in the process).
In land sailing or ice yachting you don't have two medium with different densities and still achieve forward movement using a sail.
In land sailing or ice sailing you can steer against a hard surface.
I'm not a specialist in fluid dynamics but this should count for something, IMO it should even get better results.
almost. a sail also deflect the force all by itself, being at an angle from the wind, so there's that - a boat can move two three times faster than the wind given the right conditions
The ice or earth still provides the same function vis-a-vis whatever part of the vehicle is touching it; it's that contact that allows for the force differential between the air & the medium to be exploited.
A center/daggerboard does not balance the capsizing moment, it contributes to it. You have aerodynamic forces towards the lee centered somewhere above the waterline, and hydrodynamic forces towards windward centered somewhere below it.
Aside from the very few boats with hydrofoils, this torque is offset by that generated by the interplay of weight and buoyancy (for iceboats, the rigidity of ice substitutes for buoyancy.)
More information? Please don't blame me for my next heretic opinions about sailing (upwind) :-)
In a land sailing, glider has potential energy (height), while the boat doesn't have any potential energy. This pot. energy makes glider moving forward (and down).
Consider glider on a airfield, and a strong wind to the nose - glider doesn't fly up and forward, it can only lift up and turn back.
But at the same time, when you have docked sailboat, and a close hauled wind (approx. 30 degrees from nose), the boat will move forward (lift) and sideways (drag). As faster it is, there will be a less drag.
Keel doesn't have to be heavy flat area or 300kg torpedo 2 m below hull, it can be just shallow longitudinal slot of meter - two.
Again, if there will be no water (neglect weight of the sailboat), just air, the sailboat will not have any tendency to move forward, otherwise this will be fantastic.
Keels provide lift upwind and in fact most displacement mode sailboats use NACA foil templates for their keels. Downwind, keels are drag and in some boats, you pull up the centerboard to reduce that drag.
>in some boats, you pull up the centerboard to reduce that drag.
Oddly enough I'm writing from such a boat right now :). Though in our case the main purpose of having it retractable is to allow navigating shallow waters (like smaller marinas).
I believe that the boats where the keel lifts up do not rely on the keel weight to stay balanced. They are also easier to capsize (regardless of whether the keel is lifted) than the ones with a fixed heavy keel that lowers the center of the mass.
Probably less, as it would go sideways rather than tip.
In fact, lifting the keel/centreboard can be a good technique for manoeuvring towards a peer; often you have to keep the bow pointing a certain direction, but want to move a little sideways.
Common usage would have you believe that all keels need a keel weight, but many catamarans have sharper structures on the bottom of their hulls that are called keels. Generally a keel is not retractable, a centreboard is.
Are these functionally distinct from normal centreboards? I'm trying to recall the term and what comes to mind is a keel that's hung like a centreboard to reduce grounding damage.
The Catalina 22 is available with a swing keel that is about 550 lbs. Those models have no other keel. By contrast, my Morgan 24 has a full-length shallow keel with 1900 lbs of ballast, plus a fully retractable centerboard that is not particularly heavy. I think the only common distinction between the terms "swing keel" and centerboard is whether it provides appreciable ballast.
In theory, a centreboard is a type of keel, when describing its purpose and action.
However, in practice, nothing that is not a fixed structural element of the hull is ever referred to as a 'keel' [edit:1]. Thus, a reference a keel can be taken to mean a structural keel.
A centreboard will only ever be a pivoting blade that rotates down from (and up into) a housing inside the hull, and such a blade will only be described as a 'centreboard', unless the meaning is otherwise clear. [edit:1 redux, 'swing keel'.]
More rarely, dinghies will have simple 'daggerboards' that slide straight down, and likewise they will only be referred to as 'daggerboards' in any circumstance where ambiguity might occurs.
Keels are usually weighted, of course, but there's no distinct word for an unweighted keel, structural or otherwise.
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1: nicwolff reports, as you might have noticed, that a (deliberately) heavy centreboard is referred to sometimes as a 'swing keel', a term with which I was unfamiliar. So, matters are less cut and dried I'd initially represented them to be. However, I think this leaves the presumption that a keel will be structural largely intact.
I feel that you are being intentionally obtuse. The article is about sail theory, and as such it is entirely reasonable to consider the sail in isolation.
I'm not sure why you were being downvoted. There's a huge amount of theory relevant to sailing any boat and no special reason one would need to consider keels above any other interaction of a sail with a boat. I do a bit of kite skiing and I'm hardly likely to complain that the article doesn't consider uphill and downwind sailing enough.
- sailboat without the keel, will not sail at all. It will just move sideways and back
- sailboat driven by a motor in a lull, with sails up and perfectly trimmed, will go slower than without the sails
- plane and air gliders comparison is always wrong, because plane is driven, and both operate in the same medium - air
Sails alone cannot drive boat forward. Only with interaction from the keel on the boundary of two medium, water and air with different density.