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MultiThom

The Slot...what really happens?

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"...why people sail. He mentions the paradoxical balance of the speed rush with the glow of serenity; the thrill of moving inside the body of the wind, of merging with a force that can only be felt, not seen; the historical resonance in a technology that is six thousand years old. But what Whidden seems to treasure most is knowing that every moment is a corner around which lies a new tangle of forces, a situation never encountered before, either by the sailor or possibly by anyone. For the master sailmaker it is the sense of feeling each moment flower and renew itself that makes sailing so absorbing. Every voyage is a great novel with no last page."  Quote from a Tom Whiddon (sailmaker) text-thought I'd start with this since (being winter), I'm sailing deprived.

Venturi effect has pretty much been disproven in computer models

Circulation fields seem to indicate that the pressure in the slot actually increases and flow slows-which helps the jib be more efficient and allows the whole sailplan to point higher (but makes the mainsail less efficient).  But didn't find anything about non overlapping jib-main interactions.

I did an internet search for "fallacy of the slot effect" and it turned up some interesting articles, but nothing more recent than late last century.  Since the Am Cup boats went with hard wingsails the softsail dynamics (for the rest of us) seem to have fallen out of interested investigators.

Anyone have a recent article you can point me towards?  

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Consider looking at aeroplane wing dynamics. Leading edge slots or trailing edge flaps with slot allow much higher angles of attack (reducing stall speed) in exchange for some increase in drag.

 

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17 hours ago, MultiThom said:

I found one pretty comprehensive one from 2014...http://vm2330.sgvps.net/~syrftest/images/library/20141117225747.pdf

That's actually from 1981. However, it's by Arvel Gentry, who was widely acknowledged to be a leading authority on the subject at the time.

The physics hasn't changed & probably applies equally well to any more recent designs.

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Trim the main, trim the jib gradually tighter until the boat slows down, ease the jib just a wee bit. Then enjoy sailing and forget too much analysis of " the slot effect ". Happy Sailing

 

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16 minutes ago, xonk1 said:

Trim the main, trim the jib gradually tighter until the boat slows down, ease the jib just a wee bit. Then enjoy sailing and forget too much analysis of " the slot effect ". Happy Sailing

 

Yah, if only it weren't winter.

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Think like the birds. Migrate.

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Xonk1  -  Like your simple approach a lot - would add that you might want to fine tune the trim on both sails as you accelerate but enjoying the sailing is the main thing.

Lot of good data here for winter study, Thanks.

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On ‎12‎/‎31‎/‎2018 at 2:34 PM, MultiThom said:

"...why people sail. He mentions the paradoxical balance of the speed rush with the glow of serenity; the thrill of moving inside the body of the wind, of merging with a force that can only be felt, not seen; the historical resonance in a technology that is six thousand years old. But what Whidden seems to treasure most is knowing that every moment is a corner around which lies a new tangle of forces, a situation never encountered before, either by the sailor or possibly by anyone. For the master sailmaker it is the sense of feeling each moment flower and renew itself that makes sailing so absorbing. Every voyage is a great novel with no last page."  Quote from a Tom Whiddon (sailmaker) text-thought I'd start with this since (being winter), I'm sailing deprived.

Venturi effect has pretty much been disproven in computer models

Circulation fields seem to indicate that the pressure in the slot actually increases and flow slows-which helps the jib be more efficient and allows the whole sailplan to point higher (but makes the mainsail less efficient).  But didn't find anything about non overlapping jib-main interactions.

I did an internet search for "fallacy of the slot effect" and it turned up some interesting articles, but nothing more recent than late last century.  Since the Am Cup boats went with hard wingsails the softsail dynamics (for the rest of us) seem to have fallen out of interested investigators.

Anyone have a recent article you can point me towards?  

I can assure you wing designers are very much interested in slot effects. On the C-cat the slot on the wing was very tightly controlled and tuned. In fact getting the slot right was one of the first things we learned with alpha when we went for a friendly regatta with Steve Clark in 2006. First few days, we were doing fine if not better than fine uphill but as soon as we took off downhill, Cogito was killing us with depth and speed. We went low to a certain point and then bam, it would all stall. Late one night we put the tape measure up to Steve's wing and his idlers (The device that controlled slot on the wings at the time) and lo and behold, we found we were out by say 1/2- 3/4" on some idlers. So that night we cut up pool noodle and beefed up our idlers to help close down our slot more.

Next morning, we were right in the game downhill and it was a whole new day.

Ultimately we converted to a more complex / sophisticated string system to control slot. It became kind of set and forget once the wing was assembled and sailed once. We even built a nice little calibration tool for each wing that set precisely, down to the millimeter, what each slot setting should be moving up the wing.

In short, slot is EVERYTHING on a sail plan. It's job is to energize the field at the surface of the foil and help keep flowing air stuck to the foil longer. As such you can bend more air and get more power.

Overlapping or not doesn't matter actually that much, simply think of them as two fields interacting closely together, fore and aft overlap is not a big factor, lateral distance is a much bigger deal to performance, e.g. how open your slot is. Plus don't think of it as a main and a jib, its just a sail plan, they act as one unit together when it comes to breeze.

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Slot control mockup

166628885_slotcontrol-03.thumb.jpg.6e249fe8d960fb8970a8c5d31286025c.jpg

In use on Canaan in 2010. Reduced weight further and reduced aero drag on the wing as well with no idlers. Reduced the lines to single strands of PBO.

1166368083_slotcontrol-02.jpg.d615617d1e069d06a258efbeeba2735d.jpg

downwind-001.bmp

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1 hour ago, blunted said:

downwind-001.bmp

Great info, thanks.  Converted to .jpg:

downwind-001.jpg.69c40d1c716c9d11f8be0b0e4ba78d3a.jpg

P.S.  If that tiller pivots on the aft crossbeam(?), it appears that the port linkage would move further than the starboard?  If so, 1) why? and 2) do you compensate by attaching the port linkage to a slightly longer rudder tiller?  (or slightly greater radius on the port rudder?)

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On 1/7/2019 at 2:44 PM, blunted said:

... First few days, we were doing fine if not better than fine uphill but as soon as we took off downhill, Cogito was killing us with depth and speed. We went low to a certain point and then bam, it would all stall. Late one night we put the tape measure up to Steve's wing and his idlers (The device that controlled slot on the wings at the time) and lo and behold, we found we were out by say 1/2- 3/4" on some idlers. So that night we cut up pool noodle and beefed up our idlers to help close down our slot more.

...

In short, slot is EVERYTHING on a sail plan. It's job is to energize the field at the surface of the foil and help keep flowing air stuck to the foil longer. As such you can bend more air and get more power.

Overlapping or not doesn't matter actually that much, simply think of them as two fields interacting closely together, fore and aft overlap is not a big factor, lateral distance is a much bigger deal to performance, e.g. how open your slot is. Plus don't think of it as a main and a jib, its just a sail plan, they act as one unit together when it comes to breeze.

I'm having a hard time with this related to heavier boats with soft sails (ie, mine) and going downwind with them as well as the addition of a spin.   In the Gentry article what I thought I understood was that the jib is aided by the interaction with the mainsail to be more efficient while it detracts some efficiency from the mainsail but allows the whole sailplan to point higher-which is what you want going to weather.  However, with softsails without spinnaker going downwind, we would normally barber haul the jib clew outboard to help the sailplan go faster while pointing lower (as well as release some mainsheet tension to give more camber to the mainsail).  In fact, if you keep the sailplan tight, you can't sail downwind very well since all the foils are stalled.  Add a spinnaker and the sailplan can point lower still while still going fast (in fact, you can't use a spinnaker with wind coming from very far forward).  

I think it has to do with boatspeed and apparent wind generation.  On your C class cats, (and other similar boats), your sailplan is always going upwind regardless of point of sail...that's not the case with slower boats.  ...   but I could be wrong, I know it is usually faster with asymspins to have wind in your face instead of at your back.  

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Re: rudder / tiller system.

It's a bit of an optical illusion. The tiller, the rear of it, the pushers are connected on the top and the bottom of the tiller. it makes the bottom one in that photo look like its further back compared to the other one but its not.

The total system geometry is set up in such a way however that the inboard rudder on any corner, is always turned more than the outboard rudder. The inside hull is 14' further into whatever radius you are carving out of the water, so it needs to turn faster than the outboard rudder to not become a brake.

 

485750979_Tillerbar-01.jpg.4d26f89c106a141584d5c7b159742e25.jpg

Here you can see on FYH a little closer the over / under arrangement for tiller bar.

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47 minutes ago, MultiThom said:

I'm having a hard time with this related to heavier boats with soft sails (ie, mine) and going downwind with them as well as the addition of a spin.   In the Gentry article what I thought I understood was that the jib is aided by the interaction with the mainsail to be more efficient while it detracts some efficiency from the mainsail but allows the whole sailplan to point higher-which is what you want going to weather.  However, with softsails without spinnaker going downwind, we would normally barber haul the jib clew outboard to help the sailplan go faster while pointing lower (as well as release some mainsheet tension to give more camber to the mainsail).  In fact, if you keep the sailplan tight, you can't sail downwind very well since all the foils are stalled.  Add a spinnaker and the sailplan can point lower still while still going fast (in fact, you can't use a spinnaker with wind coming from very far forward).  

I think it has to do with boatspeed and apparent wind generation.  On your C class cats, (and other similar boats), your sailplan is always going upwind regardless of point of sail...that's not the case with slower boats.  ...   but I could be wrong, I know it is usually faster with asymspins to have wind in your face instead of at your back.  

I understand your confusion.

So I'll start with the simple things. Releasing main sheet tension downhill. (For context I also sailed Tornados for quite some time, so I get the whole soft sail business). So that's a straight up trimming issue. Start with just the main, imagine the jib and spin are not there. Well all any sail "feels" is apparent wind speed and angle. So that's going to have two big inputs, what way the boat is pointing and how breezy it is. I don't think I have to explain how AWA works. So yes, if you don't "release" the main, life will suck downhill as you'll be quite stalled.

Thin flat foils are good for low angles of attack and low drag, particularly when multiplied by higher speed over the foil. I just described sailing upwind. Downwind is pretty much the opposite, in a heavy boat in particular. You want a deeper total foil (Whole sail plan, consider all as one) with more total camber. Added slots are very helpful at lower AWS's, because they help in keeping the flow stuck to the total foil. So if you don't ease off your mainsheet at all downhill, you are sailing with a very flat sail, which will tend to stall at smaller angles of attack. This is not helpful because down wind in heavy (slower) boats, the angle of attack of your total sail plan is actually quite high and a thin foil cannot tolerate that, so it stalls. If one part of the total foil is stalling, everyone else is not far behind.

There's a very simple analogy in the aircraft world. Taking off and landing, e.g. low speed, high lift requirements, maximum flaps and slats deployed, not much concern about drag, bending a huge volume of air a lot. That essentially describes life downhill for you. Conversely, cruising altitude for a jet is characterized by ten times faster airflow over foil, a desire for lowest possible drag, a thin flat foil with no slots, bending an even more huge amount of air very little. That's essentially upwind for a boat. Flat sails, less slots, less twist.

When composite foils (total sail plan of 2-3 sails) are having lower velocities of air over them, they become more sensitive to stall. Flaps (Mainsail) help to energize the boundary layer of a foil and keep flow attached (Not stalled). The optimal geometry of a slot will change with wind speed across the foils. There is a sweet spot for every combination of foils for slot geometry. bigger overlap on slots does help at comparatively low wind speed but most of the gain you get from big overlapping sails is actually simply more area being piled on and more is more in that case.

A big element that sailors face, that pilots don't, is gradient. The fact that at the surface of the water, or bottom of your sailplan, the wind is not blowing as hard as at the top of your rig. This means that the AWA and AWS at the top, middle and bottom of your rig never match and when you bear away and sail on a broad reach it gets really bad. So this is one reason why you release your main, to allow twist in your sail plan, if you don't twist it, especially downwind, either one part of your sail will be stalled or under trimmed. So if one part stalls, the stall spreads everywhere pretty quickly and then you stop sailing. So you have to release your main sheet to twist your main, move your jib cars outboard because I bet you also let the traveler down somewhat and that changed the geometry between main and jib, so you have to retune it to get the slot geometry right, otherwise you jib would end up over trimmed and it chokes off the slot. Yes, a slot can be too narrow and stop working right.

So the heavier and slower your boat, the more you will feel the effects of gradient. The more you have to twist your sailplan to keep it all trimmed correctly at different altitudes. The bigger and faster a boat is, the less it suffers these effects. You are right that boats like C-cats are essentially upwind all the time but even then, AWA is a big range. We would regularly sail at 17 degrees AWA upwind and three times that downwind. But if you're foiling, all of a sudden it only changes by a factor of about 1.5, not 3.

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“Ackerman” angle on the steering geometry to provide the two different turning radii if I recall correctly.

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In one of my more misguided design efforts we decided to try this baby out on the wing. In fact we built an entire wing around it.

I wanted another slot, so we could have n even deeper wing, with more power to slide downhill at deep angles. So we drew up the idea. We mocked up a section of foil to try out on a wing, glued it on and tried it for a day or two. 

1023683685_slotmockup-03.jpg.ad7b01013fc367bcc8ec1c481bef7db1.jpgslot-mockup-01.jpg.a12274578ea1aa0af3e156c3ad498898.jpg

So sure enough, going deep downhill, the part of the wing with this slat (Jib) installed, was able to sail deep, and handle high angles of attack without stalling. Everything above and below it would stall sooner or later, for lack of a slat.

So then we did a mockup for how the mechanics could work on the wing, so we'd have symmetric deployable slats.

1270415150_Extraslot-01.jpg.a086ec0444688a85b660ac1809d7eb12.jpg

So we got that working pretty nicely and decided to spend the winter building a whole wing that did this deployable slat thing. After hundreds of hours fucking around with strings and springs and carbon things we had a working wing.

786818110_Extraslot-02.thumb.jpg.64433473d365072068d05907de166982.jpg

Indeed this bitch could sail low, say 5-10 degrees lower than we were used to.

But it didn't work out in the end. Can anybody hazard a guess why?

 

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Slow upwind?

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33 minutes ago, blunted said:

I understand your confusion.

When composite foils (total sail plan of 2-3 sails) are having lower velocities of air over them, they become more sensitive to stall. Flaps (Mainsail) help to energize the boundary layer of a foil and keep flow attached (Not stalled). The optimal geometry of a slot will change with wind speed across the foils. There is a sweet spot for every combination of foils for slot geometry. bigger overlap on slots does help at comparatively low wind speed but most of the gain you get from big overlapping sails is actually simply more area being piled on and more is more in that case.

 

OK, I understand that wind speed over the sails is slower downwind even though most of the time boat speed is higher.  So in slower (my) boat a larger slot is better downwind which is what I've experienced.  Now, though.  Let me ask how you maximize VMG downwind.  I can make the boat go faster by pointing higher with the asymspin, but that takes me further from where I want to go.  I've "been" finding the optimum angle for my spinnaker's built in camber (which tends to be when the wind tel on the shroud) apparent wind is pointing at the mainsail clew (asymspin with 75% SMG).  But I have no idea if that is faster vmg downwind than going lower (higher).  Been dealing with this seems like forever without any quantitative or qualitative way to know for sure.   There is a point where lower stalls the mainsail (which is good sometimes since that can keep the boat upright in big breeze).  

 

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1 hour ago, blunted said:

In one of my more misguided design efforts we decided to try this baby out on the wing. In fact we built an entire wing around it.

I wanted another slot, so we could have n even deeper wing, with more power to slide downhill at deep angles. So we drew up the idea. We mocked up a section of foil to try out on a wing, glued it on and tried it for a day or two. 

1023683685_slotmockup-03.jpg.ad7b01013fc367bcc8ec1c481bef7db1.jpgslot-mockup-01.jpg.a12274578ea1aa0af3e156c3ad498898.jpg

So sure enough, going deep downhill, the part of the wing with this slat (Jib) installed, was able to sail deep, and handle high angles of attack without stalling. Everything above and below it would stall sooner or later, for lack of a slat.

So then we did a mockup for how the mechanics could work on the wing, so we'd have symmetric deployable slats.

1270415150_Extraslot-01.jpg.a086ec0444688a85b660ac1809d7eb12.jpg

So we got that working pretty nicely and decided to spend the winter building a whole wing that did this deployable slat thing. After hundreds of hours fucking around with strings and springs and carbon things we had a working wing.

786818110_Extraslot-02.thumb.jpg.64433473d365072068d05907de166982.jpg

Indeed this bitch could sail low, say 5-10 degrees lower than we were used to.

But it didn't work out in the end. Can anybody hazard a guess why?

 

Flow stayed attached at deeper angles/lower wind speeds, but the deeper angle was more than cancelled by ever slower speeds and an overall reduced VMG?

Beautiful carbon fiber work though, imo.

Autoslats are a tried and true aviation innovation. See the F-86 and A-4, for example.

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18 hours ago, mookiesurfs said:

Flow stayed attached at deeper angles/lower wind speeds, but the deeper angle was more than cancelled by ever slower speeds and an overall reduced VMG?

Beautiful carbon fiber work though, imo.

Autoslats are a tried and true aviation innovation. See the F-86 and A-4, for example.

Winner winner chicken dinner.

We did keep going about the same speed but lower, but what we should have been doing is getting up on foils, reducing drag massively and speeding up, which would ultimately deliver a better VMG, a la everyone else who was on controllable foils.

Additionally, the extra crew work was a PITA just when crew was getting even busier dealing with full blown foiling. So even gains we made in a straight line were being chewed up in corners.

Extra area, that was dealt with by making the flap, element 3, a shade smaller. We had a built in mechanical system that would not allow both slats to be deployed at once so from a measurement perspective it was less than 5% of the sail area, and we never missed it upwind.

It was a fussy bastard to maintain as well. Lots of moving parts, lots of friction, lots of alignment required.

Generally, a really cool fail, beautifully built. Rowing in the wrong direction with style.

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19 hours ago, MultiThom said:

OK, I understand that wind speed over the sails is slower downwind even though most of the time boat speed is higher.  So in slower (my) boat a larger slot is better downwind which is what I've experienced.  Now, though.  Let me ask how you maximize VMG downwind.  I can make the boat go faster by pointing higher with the asymspin, but that takes me further from where I want to go.  I've "been" finding the optimum angle for my spinnaker's built in camber (which tends to be when the wind tel on the shroud) apparent wind is pointing at the mainsail clew (asymspin with 75% SMG).  But I have no idea if that is faster vmg downwind than going lower (higher).  Been dealing with this seems like forever without any quantitative or qualitative way to know for sure.   There is a point where lower stalls the mainsail (which is good sometimes since that can keep the boat upright in big breeze).  

 

 

How to improve your VMG?

Easy, collect lots of data. You could spend a huge amount of time and money simulating your boat with a VPP etc and then let it figure out your optimal angles, but even that, done well, requires you to collect lots of real world performance data to calibrate the VPP. So in the near term its simply easier to get a Velocitek and collect data. I'd not bother with the built in VMG function as it requires you to have a very good handle on your gybe angles, etc, but rather collect a lot of data and look at your resultant polar graphs for performance. You're looking for the lowest part on the y-axis of your butterfly.(see graph below)

So here is a polar from Steve Clark, his optimal VMG as shown here is about 150-160, which his mad deep. But it gives you an idea of how to look at your data you collect and decide on good angles for you. Keeping in mind of course that it changes with wind speed. Also keep in mind you often have to start higher, speed up then gradually bear away. You find you low angle that is fast and hold that. The problem is of course if you stall, you have to heat it back up again to create more AWS, get the boat going again to full speed, then start bearing away again. For this you might want a metric shit ton of telltales (all sails) and an attentive trimmer or two who constantly speak to the helm to tell him if he's too low or about to stall.

1784225613_polar-sept142011.jpg.3b12226a8b2fbd445af8d47159ab93c9.jpgAngles-01.jpg.1a7d850a0c06a71a9f007ca86f94917e.jpg

Ultimately you can reduce your data to a table like this on the right, with your optimal angles on it. Then when you come around the top mark you have an idea of what angles you should be shooting for downhill to get in the right ballpark. you can spend a million bucks on cool electronics to help you get dialed in exactly, or you can have a cheap velocitek, work on your judgement and use the force like we did most of the time.

Ultimately to improve, for the most part, you just have to practice a lot, try different things, be prepared to fail. That's why we do a lot of straight line training, so you don't override research objectives with race objectives like tacking on a shift. The best possible way to do it is two identical boats sailing close to each other methodically working through different settings to push each other. Then collect data from both boats and compare.

What you might want to also figure out, especially for a displacement boat, is how fast can it actually go. I mean flat out hull speed. When you get to that speed its very very hard to go any faster and that's exactly when you take extra power and turn it into depth.

A displacement C-cat really doesn't want to go any faster than 22.5 knots through the water. If you do, you better be trapping off the back beam because at that speed it's just looking for a place to pitch pole, so better to go lower and not load up the boat any more.

For long slender hulls, you can use (sqrt of WL length) times 4.5. If your boat is not so svelte as a C (What is?) you might use 3.5 as a multiplier. That's your hull speed for your tri, and unless you go full MacGregor and put a 50 HP outboard on the back, that's your limit, if you're hitting that number and have power to spare, keep bearing a way until you are on the edge of stalling things. 

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So to my eye a Sea Rail 19 doesn't want to go much faster than 19.6 knots. It's possible to faster with a big A-sail as lifting the bow you can kind of achieve a semi-planning state whci is safer. What's the fastest you've ever made it go?

On that ride you kind of have two constraints to deal with. Pitch poling, or falling over side ways. I suspect that pitchpoling is the bigger issue. You can keep hiking harder, get to the back corner of the boat, make sure you load up the sail plan as hard as possible and work to fly two hulls. By the time you do all that there's more chance of going down the mine than falling over sideways, so that'll be your functional limit as to how fast you can make the boat go. If you're always on the edge of pitching it in, you need to cap your speed and start bearing away a little. you might even reduce righting moment slightly to get a decent balance.

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14 minutes ago, blunted said:

On that ride you kind of have two constraints to deal with. Pitch poling, or falling over side ways. I suspect that pitchpoling is the bigger issue. You can keep hiking harder, get to the back corner of the boat, make sure you load up the sail plan as hard as possible and work to fly two hulls. By the time you do all that there's more chance of going down the mine than falling over sideways, so that'll be your functional limit as to how fast you can make the boat go. If you're always on the edge of pitching it in, you need to cap your speed and start bearing away a little. you might even reduce righting moment slightly to get a decent balance.

This is precisely why the Pacific proa, with only 25% to 33% displacement in the ama to windward, works so well compared to the "Atlantic proa" configuration with more than 50% displacement to windward.  There are many issues and consequences related to this difference but the bottom line is that excess righting moment buries the bow before rolling the boat.  It's easy to see and your writing on these matters is crystal clear.  Mahalo.

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33 minutes ago, blunted said:

So to my eye a Sea Rail 19 doesn't want to go much faster than 19.6 knots. It's possible to faster with a big A-sail as lifting the bow you can kind of achieve a semi-planning state whci is safer. What's the fastest you've ever made it go?

On that ride you kind of have two constraints to deal with. Pitch poling, or falling over side ways. I suspect that pitchpoling is the bigger issue. You can keep hiking harder, get to the back corner of the boat, make sure you load up the sail plan as hard as possible and work to fly two hulls. By the time you do all that there's more chance of going down the mine than falling over sideways, so that'll be your functional limit as to how fast you can make the boat go. If you're always on the edge of pitching it in, you need to cap your speed and start bearing away a little. you might even reduce righting moment slightly to get a decent balance.

SR19 will go to what your guess is...perhaps a tiny bit faster to about 22.  Your comments about polars for it are good as well as the snake wake method of driving (works well both upwind and downwind)  to get slightly better performance. You are also correct about how to keep the boat upright and the value table is similar to how I drive (IE, bigger wind, point lower).   My observations have been on the boats I've owned that boatspeed is a certain percentage of windspeed on any specific point of sail until near the boat's "speed limit".  So the AWA for max VMG DDW should be close to a constant--the polar you showed above is similarly largely independent of windspeed (at least by seat of the pants sailors).  I did create a table of values for my old ride (F242) which performed similarly to the searail except the F242 is a dog in breeze between 5 and 11 kts TWS (some kind of displacement mode on the main hull (fat).   What I was hoping for was a way without instruments (or competitors alongside)  to tell the driver in real time whether or not he/she has his boat pointing correctly down the course.   

BTW, I know this doesn't have anything to do with the slot effect anymore...but I started this topic...I think I can allow it to drift, n'est-ce pas?

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You can set a waypoint far far downwind (like hundreds of miles) and then sail downwind and use the GPS VMG to experiment with trying to maximizing VMG over a couple of minutes. I think this should give you an idea of how the boat likes to be sailed in those wind conditions. On my F-27 I  accelerate the boat by sailing relatively high, then I slowly working down until speed falls off dramatically. The course just above where this happens is where I trim my sails. Then its just a matter for the helm to keep speed up by accelerating in puffs and after waves and then falling off to maximize VMG without bleeding off too much speed. This is the S course you were referring to. I would argue that on an apparent wind boat  keeping your apparent wind forward in this manner adds more than a "little bit of speed."

Learn to sail your boat by feel, don't focus too much on fancy instruments and polars that will never be accurate without $10k of sensors.

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>Learn to sail your boat by feel, don't focus too much on fancy instruments and polars that will never be accurate without $10k of sensors.

^^^^^This^^^^^

-MH

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Best racing season I ever had was after most of the instruments had crapped out - but we had sailed the boat thousands of miles by then.

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2 hours ago, Mizzmo said:

 

Learn to sail your boat by feel, don't focus too much on fancy instruments and polars that will never be accurate without $10k of sensors.

Yah, I do that mostly...but I'm bored here in N. CA in cold and rain and no wind...so puzzling about something to do with sailing--particularly since my boat is still new to me.  Also, this is better than watching TrumpTV (wife likes foxnews).  

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I did look at polars for trimarans that are in the boatspeed range of my ride.  Did some calculations and it seemed like 135 TWA gave best VMG downwind for those designs (Multi23 and SeaCart30).  Sorta makes sense...135 means pointing away from the downwind mark at 45 degrees, just like going to weather.  That's easily something I can do without instruments.  

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Not always the case,  its dependent on wind speed and boat speed, the slower the boat speed in light wind speeds. the more direct, until the wind is strong enough to take your boat out of go slow mode, and you're in the mid range winds where 45 degrees apparent wind is good ( remember though as your boat speed increases so does apparent wind direction, so initially 45 degrees heading maybe way off going directly to the mark until at about 20 knots boat speed you may well be heading almost directly for it ) and then once you get above mid teens you may well be heading almost directly effectively downwind but the wind over the boat will be almost on the nose.  

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On 1/10/2019 at 5:12 PM, MultiThom said:

SR19 will go to what your guess is...perhaps a tiny bit faster to about 22.  Your comments about polars for it are good as well as the snake wake method of driving (works well both upwind and downwind)  to get slightly better performance. You are also correct about how to keep the boat upright and the value table is similar to how I drive (IE, bigger wind, point lower).   My observations have been on the boats I've owned that boatspeed is a certain percentage of windspeed on any specific point of sail until near the boat's "speed limit".  So the AWA for max VMG DDW should be close to a constant--the polar you showed above is similarly largely independent of windspeed (at least by seat of the pants sailors).  I did create a table of values for my old ride (F242) which performed similarly to the searail except the F242 is a dog in breeze between 5 and 11 kts TWS (some kind of displacement mode on the main hull (fat).   What I was hoping for was a way without instruments (or competitors alongside)  to tell the driver in real time whether or not he/she has his boat pointing correctly down the course.   

BTW, I know this doesn't have anything to do with the slot effect anymore...but I started this topic...I think I can allow it to drift, n'est-ce pas?

I can't wait to see a GPS record from a Searail 19 at 22 kt or even at 19 !

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4 hours ago, Waynemarlow said:

Not always the case,  its dependent on wind speed and boat speed, the slower the boat speed in light wind speeds. the more direct, until the wind is strong enough to take your boat out of go slow mode, and you're in the mid range winds where 45 degrees apparent wind is good ( remember though as your boat speed increases so does apparent wind direction, so initially 45 degrees heading maybe way off going directly to the mark until at about 20 knots boat speed you may well be heading almost directly for it ) and then once you get above mid teens you may well be heading almost directly effectively downwind but the wind over the boat will be almost on the nose.  

You would think so.  But for the tri's I've owned and the polars I've seen for typical tri's I haven't owned (that is, ones with a specific sail suite of main, jib, and a single downwind sprit sail), the boat sails a constant percent of windspeed on any particular point of sail-at least with wind above 6 and below where the boat is being pressed.  So Max VMG ddw will be a constant course over ground and hence the AWA will be similarly constant through the normal sailing winds.  I probably drive too conservatively, though, since I'm mostly single handing.  In any event, sailing without instruments and by feel, it is just about the best way to start (with a new boat anyway), by driving away from the ddw mark at 45s--for me, sailing is a recreation of finding what is possible, not necessarily what is best for folks who demand perfection.

Patzefran...dunno whether or not I'll see 20s since I'm no longer racing and racing is what usually causes one to find edges of envelopes.  I did see 16 on a run this past year in 12-14 kt breeze and the boat wasn't even pressed (still sailing pretty flat with 2 on board).  Didn't seem that fast when I was sailing so I was surprised by that number on the post mortem GPS review.  Of course, GPS speed isn't necessarily accurate, but you can see it on this video --  we finally trim the spin around 3:50 into the video (I sail pretty conservative with new boat and new crew).

 

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Waynemarlow,, I reread my post and it reads like I disagree...I wasn't disagreeing (although many think I am disagreeable, but that's a different story).  In light stuff, gotta make way however you can (some conditions DDW (wing and wing) is best for me since I typically have current to deal with and hugging shore where there is less current will get you where you want to be).  With boat pressed, head down to keep her on her feet.

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I think you are missing the point that the actual wind direction over the boat is not that of the wind over the sea but a vector of both your speed & heading and wind speed & direction. You then need to compute into that your boat efficiency will change with hull speed so that there maybe huge advantages to have at say doing 10 knots rather than 8 knots.

I like to tell sailors learning the dark arts of maximising VMG that its a series of arcs to the mark, rather than fixed straight lines ie heat the boat up to say 10knots by heading up on a more broad reach and then once reaching 10knots, arcing down wind toward the mark as the apparent wind comes more around onto the nose, maintaining that 10knots by occasionally heading back onto that broad reach and then arcing down to the mark again.

Sorry, its difficult to put in words and when you ride with some skippers, they just seem to do it so naturally.

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I am not missing any points related to vector math (useta teach it).  I also agree that a skipper/helmsman who snake wakes down the course is going to get there faster than someone driving like OTTO (autohelm).   However, the base course from which one snakes is predictable for most tris in most conditions (that is, between light stuff and boat being pressed).  Those looking for that extra little boost can also use their snakes to surf down the face of some waves assuming the waves are aligned toward where you want to be.  The discussion started about how to point your boat downwind and a way for a helmsman to know he's pointing the right way to get max vmg ddw without instruments and without competitors.  SO far, no one can tell me so you just have to learn the boat.  For my previous boats and looking at posted polars for similar boats, the base course ddw should be about 135 TWA-which doesn't help without instruments...but, if the downwind mark is ddw, then pointing away from it 45 degrees ought to be pretty close for most conditions and that's where I'll start with this new to me boat next season assuming it ever warms up and we get wind and it stops raining.  Note that this is probably not the case for catamarans or foilers--in fact I looked at an F18 polar and that boat should keep true wind at 110 (AWA is from in front about 50 degrees if I remember correctly).

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12 hours ago, MultiThom said:

I am not missing any points related to vector math (useta teach it).  I also agree that a skipper/helmsman who snake wakes down the course is going to get there faster than someone driving like OTTO (autohelm).   However, the base course from which one snakes is predictable for most tris in most conditions (that is, between light stuff and boat being pressed).  Those looking for that extra little boost can also use their snakes to surf down the face of some waves assuming the waves are aligned toward where you want to be.  The discussion started about how to point your boat downwind and a way for a helmsman to know he's pointing the right way to get max vmg ddw without instruments and without competitors.  SO far, no one can tell me so you just have to learn the boat.  For my previous boats and looking at posted polars for similar boats, the base course ddw should be about 135 TWA-which doesn't help without instruments...but, if the downwind mark is ddw, then pointing away from it 45 degrees ought to be pretty close for most conditions and that's where I'll start with this new to me boat next season assuming it ever warms up and we get wind and it stops raining.  Note that this is probably not the case for catamarans or foilers--in fact I looked at an F18 polar and that boat should keep true wind at 110 (AWA is from in front about 50 degrees if I remember correctly).

MT, these are my numbers in a Weta with the flat screecher (it's not a spinnaker).

It's very hard to beat myself going ddw wing on wing. Best VMG is TWA 120-125 max. Apparent is slightly forward of the beam let's say 80. In that condition the boat is "turned on" and planing. If I try to go deeper the speed falls off a cliff. Snaking, hotting up, soaking down, surfing and looking for the low spots in the chop is an absolute must, otherwise I'd better go ddw.

Randy smyth says we should be keeping the AWA right on the beam for best VMG but I can never get there, but then I'm not a racer.

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3 hours ago, sail(plane) said:

It's very hard to beat myself going ddw wing on wing. Best VMG is TWA 120-125 max. Apparent is slightly forward of the beam let's say 80. In that condition the boat is "turned on" and planing. If I try to go deeper the speed falls off a cliff. Snaking, hotting up, soaking down, surfing and looking for the low spots in the chop is an absolute must, otherwise I'd better go ddw.

Randy smyth says we should be keeping the AWA right on the beam for best VMG but I can never get there, but then I'm not a racer.

That was our experience, as well, in my friends' Weta; loosened halyard, loosened sheet but that flat spin wanted wind from in front.  That was partly the reason I took the anti torque line out of the spin luff on my current boat, flying free, it will now rotate windward for deeper driving... And, just like  WR17 (w/ no spin), DDW wing and wing is needed to hug coast and get out of current in lighter breeze.  Gybing back and forth was more fun, but left you further from the mark instead of closer.  

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8 hours ago, sail(plane) said:

.

Randy smyth says we should be keeping the AWA right on the beam for best VMG but I can never get there, but then I'm not a racer.

Hike harder and pull the string thingy.

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On 1/14/2019 at 9:13 AM, sail(plane) said:

 

Randy smyth says we should be keeping the AWA right on the beam for best VMG but I can never get there, but then I'm not a racer.

 

19 hours ago, blunted said:

Hike harder and pull the string thingy.

But but blunted I would end up going higher on a beam reach if I do that and the issue here is bet vmg downwind? 

Don't get me wrong if a C-cat guru tells me to hike and pull thats what I'm going to do! 

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On ‎1‎/‎15‎/‎2019 at 10:44 AM, sail(plane) said:

 

But but blunted I would end up going higher on a beam reach if I do that and the issue here is bet vmg downwind? 

Don't get me wrong if a C-cat guru tells me to hike and pull thats what I'm going to do! 

I was being a shade facetious. If your boat is sticky, you will get different results than Randy's boat.

Then again, you may end up higher, you may not. The point is to heat it up until the AWA goes forward, then go down. every boat has its limits, but you don't find them if you don't try ALL the angles. That includes hiking harder than you thought sensible sometimes, just to see what happens. It's like sex, don't you want to try ALL the positions? Just to try?

 

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38 minutes ago, blunted said:

I was being a shade facetious. If your boat is sticky, you will get different results than Randy's boat.

Then again, you may end up higher, you may not. The point is to heat it up until the AWA goes forward, then go down. every boat has its limits, but you don't find them if you don't try ALL the angles. That includes hiking harder than you thought sensible sometimes, just to see what happens. It's like sex, don't you want to try ALL the positions? Just to try?

 

Randy was talking specifically about the weta, he's sailed it a lot and yeah it's sticky

Note to self: sail like you have sex... mmm dont know what will come out of that! 

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