Jump to content

Physics of roll tacking


Recommended Posts

I’ve got one of those smart kids in my racing class who asks questions that demand good explanations. 
 

He wants to know what’s actually happening that propels a boat forward when it’s properly roll tacked. 
 

Let’s keep it simple. Single handed Laser type dinghy. Kinetics, lift, drag and the like. Reynolds numbers are a bit beyond a 15 year old. 

  • Like 1
Link to post
Share on other sites

The basic factor in propelling the boat forward is that you're "sculling" the sail thru the air. Think of it as one blade of a fan, rotating from 45deg (or however far you can heel it) on one side to 45deg or however far you can heel it) on the other. Then of course, the sail flops to the new tack, and you gain more on the rotation back to (near) level.

It's exactly the same physics as wind moving over the sail, except that you're moving the sail thru the air. In very light air, as you roll, easing the sail two inches adds a lot of power to your roll.

There is more going on... bringing the boat to a sharp heel going into the tack -without- fanning it, helps the boat turn by two forces.... one is immersing the curve of the hull, the other is putting the drive of the sail off to one side.

An exercise that's fun and helps with the 2nd part is to have then sail heeled over as far as they can, and still keep control. I like to have the kids play "Baby Ducks" which is basically follow-the-leader and I can take them on different points of sail, tack, work on right of way, etc, all close under my eye. Also at the beginner level it gives them a lot of practice in control of boat & sail at once, and learning to slow down and stop.

FB- Doug

  • Like 4
Link to post
Share on other sites

Not too sciencey... but perhaps you could speak to him on a key formula and how it occurs twice in a roll tack:

 Adding weight + Heading up = Acceleration

This effect is realized during a good tacking entry (and since there is no recovery/correction back down on the same tack, it can be taken higher than in upwind sailing while tacking). As you head up, you are effectively driving more flow over the surface of the foils and sails as you turn into the forces...  and adding weight is the accelerator that converts potential into speed, (plus the rig movement mentioned above by Steam Flyer).

He'll also benefit from this formula during a well executed tacking exit, which will always result in flattening the boat with the tiller DOWN (adding weight + heading up= acceleration,  again, to finish the tack).  So the boat must be tacked beyond close hauled to fulfill this formula. It's common for people to flatten while still bearing away, and this will result in "adding weight + bearing away" which is detrimental to acceleration. Analogous perhaps to accelerator plus handbrake. 

This concept can be emphasized in many, many areas of boat handling and speed including: starting acceleration, leeward mark rounding, downwind sailing, upwind acceleration and reset cycles, ducking, jibing, downwind upturns, by the lee pressing (in unstayed rigs) etc. 

There are a bunch of drills to groove this combination as well, if interested, to help make the formula connect to the physical actions.  It is an important cornerstone of good dinghy sailing.

Link to post
Share on other sites

You pull the sail sideways through the air and it pushes the boat forward.

why??

Newton’s laws come in handy here.

With respect to the sail sliding sideways the air is “sitting still.”

Air is pretty light stuff but making it go off in a new direction takes energy.

let’s say the sail  is a flat plane set at 45 degrees to the direction it is going as the boat is rolled 

the air will have to be “set in motion “ by the sail. 
every bit of energy that is used to push the air “backwards “ has an equal snd opposite push forward on the sail 

but...

there is sideways pushing going on as well 

every bit of sideways push on the air makes it equally more difficult to push the sail sideways 

in fact... that energy pushes the boat sideways... to leeward... you don’t want to go sideways to leeward

but the Centerboard is in water which is 1000 times heavier than the air so the boat doesn’t move sideways much 

the rudder!!!! If you put the rudder at an angle and slide it sideways through the water as part of the roll tack, the rudder dies the exact same thing as the sail 

if the sailors use their bodies to push the rudder more sideways than it is set to turn the rudder pushing on the  water adds to the forward force.

yes... you read correctly... usually, turning the rudder slows the boat as it pushes the water forward and the water which was not in motion pushes back 

 

also.... to some extent the back side of the sideways moving sail and rudder are creating a vacuum which sucks the sideways moving sail  and rudder both sideways and forward 

 

other stuff:

if you roll too violently the blades  and sail stall and create turbulence rather than organized harvestable energy.

the roll tack has to transition into the new tack. The sloppy roll tack has way more potential to slow The boat than the good roll tack has potential to speed the boat along.

ending a roll tack with flogging sails and misplaced crew destroys any good The roll tack provides.

step one is to master turning the boat through the wind and arriving at the new tacking Angie with the sails trimmed and continuing to propel the boat.

then..  the next step is to add a little rolling thrust 

In practice this might be to stay on the old high side , new low side , just a little too long for a regular tack 

Leave the sails a little looser than trimmed for the new tack and then climb to the new weather side

.

as the boat flattens to the proper heeling  angle , trim the sails and gel how the body accelerates  a bit 

as your technique improves you should be able to feel the optimum forward movement of the boat.

Anecdotal: a couple weeks ago I was out trying roll tacks on my AERO. I probably tacked fifty times and three or four of those tacks were fantastic. Most were sorta ok. Another half dozen were clumsy turns best described as roll stops. 

what I know: if I am approaching the finish line and throwing in a perfect roll tack will shoot my bow over the line just ahead of a nearby competitor, eleven times in twelve I won’t beat that other boat... 

more practice is necessary 

 

 

 

 

 


 

 

 

Link to post
Share on other sites

The one very obvious thing is when you rapidly heel the boat to weather the sails are in effect feeling a lift over the rig, so the sail will still be working until you are almost head to wind if you time your roll perfectly.

  • Like 1
Link to post
Share on other sites

You're getting good info here. Two additional points

  • A simplification of the physics is: it's an effective way of transferring your body motion - kinetics - onto the boat, and turning it into motion forward.
  • A complement: when coaches get sailors to practice roll tacks, they get them to practice tacks. Good outcomes all around.
  • Like 2
Link to post
Share on other sites

Forget the tack. It's simple.

Take a boat out in very little wind and rock and roll and it will go. It goes because you're creating apparent wind over the rig by moving it through the air.

Rocking and rolling is obviously illegal. The exception is when you tack, when you're allowed one big rock.

The rules say you can only come out of the track as fast as you go in. That rule is hard to apply and often broken.

  • Like 3
Link to post
Share on other sites

Thanks for some good information. SA is really good at providing a diverse perspective. 
 

Kinetics- moving one’s body moves the boat forward as long as you aren’t rocking or ooching. 
 

Fanning- You are getting a powerful fan stroke  on the sail which increases your wind. 
 

Pump- In some boats it’s to one’s benefit to give a short pump at the end. It’s more of a Squeeze, because you are letting the sail out on the opposite tack before you quickly trim it back to it’s typical upwind setting. Boat & condition specific. 
 

Those are all above the water. The rig &  the sailor are obvious. What’s happening underwater? Hull type(round vs flat) Does fanning the center/dagger board produce any useful forward motion like the sail? 

I’m always surprised how well & fast a Laser sails straight when it’s completely leaning over. We  play around with that to prove how far it’s possible to heel. 

Please keep going. Cheers 

 

I like to have the kids fix their rudders and try their roll racks. For the advanced ones we take away their rudders. 

This is one of the most impressive boat handling vids ever. I show this on the blowout days. 

 

  • Like 2
Link to post
Share on other sites

Two basic things are at work here:

1) Most boats have a hull design that makes them turn to weather when heeled. So, if you  heel the boat to leeward, it will head up without using the rudder, or with less rudder. Since the rudder is basically a brake, steering with the hull instead of he rudder is faster.  Then, once the boat has passed head to wind and is on the new close hauled course, heeling it the other way will stop the turn, again without using the rudder.

This is why roll tacking is legal - you're allowed to use heel to facilitate steering.

 

2) The essence of the roll tack, what makes it fast, is the burst of speed you get when you bring the boat back to flat at the end. By "fanning" the sail across the air, you push a lot of air backwards. Air goes backwards, the boat squirts forward.

You can use this idea at the start - when the gun goes off (or slightly before) roll the boat to leeward and then hike it back to flat.  Basically, half a roll tack.

 

 

 

 

  • Like 3
Link to post
Share on other sites
1 hour ago, MrSwordfish said:

 

You can use this idea at the start - when the gun goes off (or slightly before) roll the boat to leeward and then hike it back to flat.  Basically, half a roll tack.

That would be classified as rocking, which last time I looked at a rule book was illegal unless you were sailing a class where the rules specifically allow rocking, pumping and ooching..........

Link to post
Share on other sites
22 hours ago, Major Tom said:

That would be classified as rocking, which last time I looked at a rule book was illegal unless you were sailing a class where the rules specifically allow rocking, pumping and ooching..........

 

You are allowed to heel the boat to facilitate steering, so  at the start you can heel the boat to steer up to a close hauled course. 

Perfectly legal. But you only get to do it once. 

Link to post
Share on other sites
  • 2 weeks later...
9 hours ago, garland823 said:

Intuitively I always thought the centerboard/daggerboard did the majority of the work in a roll tack.

 

Yeah, big airfoils up in the thin fluid , little hydrofoils down in the dense fluid. That's sailing 101.

I don't know that the centerboard comprises a "majority" of the effect, but it's certainly a big part, which I neglected in my summary. But the sails are a big part of it too.

Both foils work together; in "normal" sailing, air and water flow over the foils and generate lift. In a roll tack, the rolling makes more fluid (water and air) flow past the foils generating more lift than would be there in the absence of rolling. Not exactly rocket science.
"

 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...