Why is it so hard for me sail downwind?

[Foswick]

Maybe that goes without saying, but sometimes I get the feeling that the "flatter is faster" argument gets abused by simplicity.

You're exactly right. It depends on what you trade off in order to flatten your boat. When heeling, if you want to flatten, you have to change ... something. You need to reduce sail, sheet out, move weight, change directions, or some combination. Some of these choices are win-win. For example, if you are in high winds having no trouble approaching hull speed, the choice to reduce sail can be a win-win. It can flatten the boat out just enough to pick up a bit of speed to reach hull speed, and also be at a more comfortable position. Or, it might be that you need to point higher, so you find that you can pinch to keep the boat flat while still making better progress toward your destination. Again, that's a win-win.

But, sometimes reducing sail area or turning to a different point of sail, just to flatten the boat out, may not improve your VMG. It might even decrease your water speed.

And, indeed, some hulls actually sail better with a slight heel.

If flatter was always faster, then you would never see world-class monohull racers heeled. And, of course, that is not the case.

 

[jssailem]

Sailing fast down wind all depends on how much sail you have up and how strong the wind is blowing. Here are several images of boats sailing down wind. They do not look to be standing still. The more sail the faster you go. Till it is to fast...

then you broach.

 

[Will Gilmore]

then you broach.

See, sometimes heeling IS slower. I'm just guessing here, but it doesn't look like that boat is maintaining her speed with that heel.

-Will

 

[Davidasailor26]

See, sometimes heeling IS slower. I'm just guessing here, but it doesn't look like that boat is maintaining her speed with that heel.

-Will

I’ll bet everything *feels* like it’s happening fast though.

 

[Daveinet]

I should correct an idea that gets tossed around regarding the affect of apparent wind on your boat speed. Yes, apparent will affect your sail trim, but it does not "add" to the speed of the wind on the sail, causing your boat to go faster. The reason you can sail faster than the wind, sailing upwind is that you can change the "gear ratio" of your sail trim. This is like change the pitch of a prop in water can make the boat go faster. Basically you have the wind pushing sideways against the keel. The angle of the keel relative to the wind makes the boat move forward. I've heard it described as like pinching a slippery marble between you finger and thumb. As you squeeze, the marble pops out at a speed much faster that you are pushing your thumb and finger together. This is the same principle as what makes you boat go forward. So its not the wind appearing to go faster across your sails, but rather the angle of attack changing the pinch ratio between your sail and the keel.

 

[Scott T-Bird]

In light winds I think sailing upwind is definitely faster than sailing downwind (unless you have a nice spinnaker to use). A few years ago, right after purchasing the boat and bringing it back from Cape Cod, Sue and I were sailing down to Atlantic City from Barnegat Inlet. We got to a point 12 miles off shore and about 3/4 of the way down the coast to A.C. on a deep broad reach (not quite deep enough for wing on wing). With the wind seeming to slack a bit, we needed to jibe and sail almost dead downwind to get to A.C. Not wanting to wallow in the waves and take forever, I turned on the engine, dropped sails and soon had a shredded belt for the water pump and I also thought maybe the impeller had failed. Stupidly not having the tools to make the repair, I pondered the options and decided to sail upwind back to Barnegat Inlet, which was at least twice the remaining distance to A.C.

We made it back to Barnegat Inlet in about 2/3 the time it took us to get to that location and we had a very nice brisk sail after all at about 45 degrees apparent the entire way. I think the wind was only 10 to 12 true and our boat speed was very pleasing, while it had been on the verge of abandonment for much of the downwind reach, given the wallowing.

 

[Daveinet]

See, sometimes heeling IS slower. I'm just guessing here, but it doesn't look like that boat is maintaining her speed with that heel.

-Will

Yes, but consider the wetted surface area. It has to be faster.

 

[Davidasailor26]

I should correct an idea that gets tossed around regarding the affect of apparent wind on your boat speed. Yes, apparent will affect your sail trim, but it does not "add" to the speed of the wind on the sail, causing your boat to go faster. The reason you can sail faster than the wind, sailing upwind is that you can change the "gear ratio" of your sail trim. This is like change the pitch of a prop in water can make the boat go faster. Basically you have the wind pushing sideways against the keel. The angle of the keel relative to the wind makes the boat move forward. I've heard it described as like pinching a slippery marble between you finger and thumb. As you squeeze, the marble pops out at a speed much faster that you are pushing your thumb and finger together. This is the same principle as what makes you boat go forward. So its not the wind appearing to go faster across your sails, but rather the angle of attack changing the pinch ratio between your sail and the keel.

That’s an interesting analogy, but I don’t think it holds true, because it only accounts for the drag presented by the sails, and not the lift that Bernoulli’s principle suggests. If the pinching analogy were completely accurate then if you were on a beam reach with proper trim you could accelerate just by trimming the sails in, to “upshift”. We can observe that’s not correct - if you overtrim your sails on a beam reach you disconnect the airflow on the leeward side of the sail, those telltales flutter, the sail stalls, and the boat slows.

The sail, except for on a run (ignoring spinnakers), is really more of a wing than part of a wedge. The sail generates lift, in a direction roughly perpendicular to its trim angle. One component of that lift vector points forward and moves the boat that way. The other component of the lift vector is sideways, and that force is countered by the keel (with some resulting leeway). As the wind moves forward you must tighten the trim angle, which actually points that lift vector more sideways, reducing the forward component, but more total lift is available because the apparent wind speed is higher. So there’s a balance between efficient use of lift on broader reaches and more total lift (being used less efficiently) at closer angles. For most boats the “sweet spot” of that balance is near a beam reach, where net forward force is maximized.

 

[Foswick]

I just want to point out, this notion of exceeding the wind speed is often misunderstood. Yes, it is absolutely possible and routinely done. But, there are some caveats here. First, not all boats are capable. High-performance and/or planing boats are much more capable. Displacement hulls and underpowered sail area make it much harder or impossible.

With a displacement hull, one must recognize that there is a very narrow window for this to occur. That is, let's say the boat has a hull speed of 6-7 kts (pretty typical). That means that, for the boat's speed to exceed wind speed, the wind must be under 6-7 kts, but high enough to allow sails to perform optimally. So, that pretty much means that winds need to be right about at 3-5 kts. Any less than 3 kts and most boats will struggle to maintain a good enough sail shape in most real-world conditions. The water needs to be rather smooth. (Lake sailors will enjoy the fact that calm winds usually coincide with calm waters. Not always the case on bigger water.)

So, with a displacement hull, when we talk about exceeding the wind speed, we are really talking about sailing just a little faster than a 3-5 kt wind. Because of the way drag works, as the water speed approaches hull speed, the forces at play are greater to overcome, making it less optimal for this phenomenon.

Also, on most boats (regardless of hull type), you are most likely to be able to exceed wind speed on a beam reach (or a slightly broad reach). The easiest way to understand this is to recognize that, on a beam reach, you are not "fighting" any headwind. The boat is free to move sideways to the wind without being translated downwind. So, all lift generated by the sail is converted into forward motion. (This is not a true statement because of what's happening under water, but is a simplified explanation. In reality, there is leeway and associated drag.)

Going upwind at a speed exceeding wind speed ... this isn't that easy on most displacement hull boats. Going a little upwind, you can enjoy the conditions of a beam reach with just a little more headwind to overcome and a slightly less-optimal wind angle. But, as you pass through a close reach, this phenomenon becomes out of ... reach ... for most displacement hulls. Close-hauled, I would be highly skeptical of any claims.

Though it is theoretically possible, in practice, any claims of a displacement monohull exceeding wind speed in anything other than ideal conditions and anything other than something very close to a beam reach ... these are probably misperceptions rather than accurate observations. (I am not talking about high-performance displacement monohulls. I am talking about most folks' boats.)

---

Only keep reading if you are a math geek ...

The way to determine if you can exceed wind speed looks something like this:

1. Calculate the apparent wind speed and apparent wind angle based on a hypothetical boat speed. I don't know a good way to represent the trig functions to calculate these in this editor, but the formulae are easy to find online. You just need the true wind speed and direction, and the boat's speed and direction.
2. Calculate lift and drag based on the coefficients of lift and drag for a given sail configuration. These formulae are pretty simple:
   L = (1/2) * p * A * Cl * V2 [V squared]
   D = (1/2) * p * A * Cd * V2
   Where:
   p = air density
   A = sail area
   Cl = the sail's coefficient of lift
   Cd = the sail's coefficient of drag
   V = apparent wind speed
3. Calculate driving force. Driving force is the component of the lift force the propels the boat forward. That formula is pretty simple:
   F = L * cos(Wa)
   Where Wa is the calculated apparent wind angle from step #1.
   Then, you can calculate the net force propelling the boat forward:
   Fnet = F - Dh - D
4. Using Newton’s second law (F=ma), the boat’s acceleration (and thus speed) can be calculated. Though, for a steady speed, Fnet must equal zero, so:
   F = Dh - D

If you make these calculations for most displacement monohulls, you find that, at best, there is a very narrow theoretical window in which the phenomenon is possible. Or there is no window at all where it is even theoretically possible. These calculations assume perfect sail trim, a perfectly clean hull with no surface friction, and perfectl flat seas. Real-world conditions pretty much close that window entirely.

 

[Davidasailor26]

I just want to point out, this notion of exceeding the wind speed is often misunderstood. Yes, it is absolutely possible and routinely done. But, there are some caveats here. First, not all boats are capable. High-performance and/or planing boats are much more capable. Displacement hulls and underpowered sail area make it much harder or impossible.

With a displacement hull, one must recognize that there is a very narrow window for this to occur. That is, let's say the boat has a hull speed of 6-7 kts (pretty typical). That means that, for the boat's speed to exceed wind speed, the wind must be under 6-7 kts, but high enough to allow sails to perform optimally. So, that pretty much means that winds need to be right about at 3-5 kts. Any less than 3 kts and most boats will struggle to maintain a good enough sail shape in most real-world conditions. The water needs to be rather smooth. (Lake sailors will enjoy the fact that calm winds usually coincide with calm waters. Not always the case on bigger water.)

So, with a displacement hull, when we talk about exceeding the wind speed, we are really talking about sailing just a little faster than a 3-5 kt wind. Because of the way drag works, as the water speed approaches hull speed, the forces at play are greater to overcome, making it less optimal for this phenomenon.

Also, on most boats (regardless of hull type), you are most likely to be able to exceed wind speed on a beam reach (or a slightly broad reach). The easiest way to understand this is to recognize that, on a beam reach, you are not "fighting" any headwind. The boat is free to move sideways to the wind without being translated downwind. So, all lift generated by the sail is converted into forward motion. (This is not a true statement because of what's happening under water, but is a simplified explanation. In reality, there is leeway and associated drag.)

Going upwind at a speed exceeding wind speed ... this isn't that easy on most displacement hull boats. Going a little upwind, you can enjoy the conditions of a beam reach with just a little more headwind to overcome and a slightly less-optimal wind angle. But, as you pass through a close reach, this phenomenon becomes out of ... reach ... for most displacement hulls. Close-hauled, I would be highly skeptical of any claims.

Though it is theoretically possible, in practice, any claims of a displacement monohull exceeding wind speed in anything other than ideal conditions and anything other than something very close to a beam reach ... these are probably misperceptions rather than accurate observations. (I am not talking about high-performance displacement monohulls. I am talking about most folks' boats.)

---

Only keep reading if you are a math geek ...

The way to determine if you can exceed wind speed looks something like this:

1. Calculate the apparent wind speed and apparent wind angle based on a hypothetical boat speed. I don't know a good way to represent the trig functions to calculate these in this editor, but the formulae are easy to find online. You just need the true wind speed and direction, and the boat's speed and direction.
2. Calculate lift and drag based on the coefficients of lift and drag for a given sail configuration. These formulae are pretty simple:
   L = (1/2) * p * A * Cl * V2 [V squared]
   D = (1/2) * p * A * Cd * V2
   Where:
   p = air density
   A = sail area
   Cl = the sail's coefficient of lift
   Cd = the sail's coefficient of drag
   V = apparent wind speed
3. Calculate driving force. Driving force is the component of the lift force the propels the boat forward. That formula is pretty simple:
   F = L * cos(Wa)
   Where Wa is the calculated apparent wind angle from step #1.
   Then, you can calculate the net force propelling the boat forward:
   Fnet = F - Dh - D
4. Using Newton’s second law (F=ma), the boat’s acceleration (and thus speed) can be calculated. Though, for a steady speed, Fnet must equal zero, so:
   F = Dh - D

If you make these calculations for most displacement monohulls, you find that, at best, there is a very narrow theoretical window in which the phenomenon is possible. Or there is no window at all where it is even theoretically possible. These calculations assume perfect sail trim, a perfectly clean hull with no surface friction, and perfectl flat seas. Real-world conditions pretty much close that window entirely.

All great points. And for those less mathematically inclined you can look at the polar diagrams for your boat that have all the math done for you. My polars say the closest I can get to wind speed is in a 4 knot wind at 100 degrees, where I should be able to go 3.95 knots.

 

[Foswick]

The only way I approach breaking the wind-speed barrier is when being towed.

 

[Tally Ho]

I usually figure if I am making better than half of the wind speed, I am doing OK !

With my new Max Prop feathered, I am now getting better than 50% of the wind speed and feel good about it!



Greg

 

[Foswick]

With my new Max Prop [...]

I've wondered about those. You're happy with yours?

 

[kappykaplan]

You're happy with yours?

I'm very happy with mine!

 

[Tally Ho]

I've wondered about those. You're happy with yours?

Very happy. Reverse thrust is really good. Feathering it gives me about 1/2 knot if speed in lighter winds. Prop is adjustable, so if you don’t love the initial setting, you can change it. I plan to adjust a tad more FWD pitch when I haul out this fall and see how it runs under power with a little more pitch.

Greg

 

[Ward H]

@Scott T-Bird and I were just talking about this a week or so back.
Our prevailing wind is from the South. We both are now in a marina in the northern part of the bay.
We were on his C320, Blue Rodeo, and enjoying working our way upwind. Full sails, 15 knots, boat was heeled over, fun!
When it was time to head back, we got set up for a long downwind ride back to the marina.
Flat boat, a couple of easy gybes, relaxing ride while we enjoyed a beer and we made it back in much less time than working upwind.
We picked a good marina to move to.

I understand Gerry Douglas, designer of my C30 and Scott's C320, designed boats to sail their best at 15 degrees heal. Always fun to test that design element.

 

[Foswick]

My boat is a bit of a pig in light winds, so the idea of gaining a 1/2 kt is pretty appealing. My main reservation is that I have read that they reduce prop walk in reverse. A nice benefit for some, but kind of a deal-breaker for me, since I really rely on prop walk. Couldn't maneuver without it.

 

[kappykaplan]

My main reservation is that I have read that they reduce prop walk in reverse. A nice benefit for some, but kind of a deal-breaker for me, since I really rely on prop walk.

I haven't noticed a significant reduction in prop walk in reverse.

 

[Davidasailor26]

Our prevailing wind is from the South. We both are now in a marina in the northern part of the bay.
We were on his C320, Blue Rodeo, and enjoying working our way upwind. Full sails, 15 knots, boat was heeled over, fun!
When it was time to head back, we got set up for a long downwind ride back to the marina.

Barnegat Bay sets up great for that. We used to be in Forked River. Our routine was to sail south in the late morning, close hauled into the light but building breeze. If we timed it right we could turn onto a run just in time for lunch and run back up the bay as the wind built. Then we’d have a couple spirited reaches at hull speed back and forth to Tices Shoal before heading in or anchoring out.