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How smooth is smooth enough ?


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     I’ve recently read (in Tuning for Speed) that 400 grit allows that thin layer of water to cling to the hull for the best flow characteristics.  But then in Bethwaite’s book on High Performance Sailing, he uses the phrase “highly polished” and “I question whether any surface is smooth enough . . .”.    
     Wax or no wax ? Other ?
     What is the latest thinking for a dinghy ?

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I've never noticed water failing to cling to the part of a hull that's underwater, so I'm of the opinion that's a bit of advice that can be ignored. How water beads or spreads with surface tension does on a surface that isn't immersed seems pretty much irrelevant to me. When it comes to dinghies though there's a practical consideration in that you want to be able to right the ******r after you p*** it in, so a mirror finish on the daggerboard, or where you have to climb on an upturned hull may have its downsides. Its probably impossible to polish a rudder blade too much.

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57 minutes ago, xonk1 said:

Spend you time on boat handling - practice. One bad tack negates any polishing or smoothing you can do.

Spend qualitt time with lomgboard filler wetsand ans polosh to mirror.

One good shift can put you in the leascone moment, but a slow bottom drags ypu back all day. You will sail the course one way or another. How much extra time you ea t tocwaste?

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With all these discussions- The McLube Speed Polish, Wax, Sand to 5000 grit- aren't they illegal to the rules? I've gotten the idea that the original finish as delivered is what's allowed (essentially, allowing for repairs) and that anything that would enhance speed is forbidden.

For years I've seen this violated constantly but haven't seen much discussion on it.

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7 minutes ago, eastbay said:

With all these discussions- The McLube Speed Polish, Wax, Sand to 5000 grit- aren't they illegal to the rules?

Not normally, no. The RRS says this:
"53 SKIN FRICTION A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer"
What this covers is firstly a very dubious coating which IIRC (it was banned a long time ago) you sort of painted on the hull and it came off during the race, and secondly the "riblets" used on an AC boat a good while back which was a carefully patterned material that had to be glued on in panels with each aligned with the specific water flow on each part of the hull.

There are classes that prohibit refinishing, although even then ordinary polishing and sanding with a fine paper that cannot alter the shape is usually OK.

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It seems that the more you sand and polish the better you sail as your head has time to concentrate more on actual sailing the boat.  110% on wind shifts, crossings, sail trim, start position etc is better than giving back 10% on "should i have polished, cleaned or sanded more". Just say'in.

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Not in my wheelhouse, but I'll comment anyway: My feeling is that because nobody seems able to categorically state which of the common race finishes is best, then they all have equal performance ... it doesn't make a significant difference. Other things are more important than whether a mirror finish is better than XXX grit.

Second, isn't the boundary layer several millimeters thick? That is the layer of relatively stagnant wter. It is certainly not microscopic, right? If that layer is mostly moving along with the boat then ultimate smoothness seems less important, too. The friction is all in the water-to-water interfaces, not the water-to-hull interface.

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On 11/26/2021 at 1:30 PM, JimC said:

Not normally, no. The RRS says this:
"53 SKIN FRICTION A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer"
What this covers is firstly a very dubious coating which IIRC (it was banned a long time ago) you sort of painted on the hull and it came off during the race, and secondly the "riblets" used on an AC boat a good while back which was a carefully patterned material that had to be glued on in panels with each aligned with the specific water flow on each part of the hull.

There are classes that prohibit refinishing, although even then ordinary polishing and sanding with a fine paper that cannot alter the shape is usually OK.

Written to prevent the injection of 'new' substance, not the ablation of a existing substance. Boats were experimenting with injecting some substance continuously along the forefoot during a race.

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2 hours ago, fastyacht said:

If you dont do smooth you will alwaus end up in someone elses lee. Mathematical certaimty. Drag acts agaimst ypu for every milr sailef

The inches you lose to increased drag add up for the whole race. You never get them back. Furthermore, every tactical situation you get into that depends on getting your nose into a defined spot, drag will make it more likely that you'll fall short of it and lose that encounter and thus drop one more place.

Smooth is easy. People may not agree how much difference it makes, but they all agree there is no such thing as "too smooth."

But smooth is misunderstood. The first step is fairing. Many people don't do that, and any gain they may make in surface finish is lost to the humps and hollows.

For the first 1/3 of the hull and the first 1/2 of all foils, it's stupid grunt work that pays multiple dividends. And it builds a bond with your boat. Why would you NOT do it?

- DSK

 

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On 11/28/2021 at 1:19 PM, Steam Flyer said:

But smooth is misunderstood. The first step is fairing. Many people don't do that, and any gain they may make in surface finish is lost to the humps and hollows.

For the first 1/3 of the hull and the first 1/2 of all foils, it's stupid grunt work that pays multiple dividends. And it builds a bond with your boat. Why would you NOT do it?

All true once you  have basic boat tuning, are good at maneuvers/boathandling and when you have time to dedicate to it.

At least for me, it's all time on the water for maneuvers, boat handling, starts, weather/tides and tactics. Good weather, bad weather, time on the water rules.

We have just gone through a season of boat handling, and I'm shifting a bit of time/effort to boat tuning and (yes) smooth foils. Why? After a season of consistent improvement in results race to race in a mixed fleet we got lucky and bumped into a same class boat in our bay. And a top team practicing for Worlds on a brand new boat. We did a brief informal 2 boat practice/tuning run with a top team, swapped boats and repeated. Our boat was pig slow.

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20 hours ago, SunFroggy said:

Shiny stays clean. [er]

THIS ^ YES

A 400 finish might theoretically be faster, but it picks up scudge - which probably isn't.

400 is not theoretically faster. That is a discredited pop pseudoscience idea from the 1970s. Water "sticks" to it (of course it does--high suface engergy reduces the contact angle). The idea was that if water "sticks" then it is "laminar" (which is not in fact what is going on...)

Lots of people get confused by golf balls and stuff. Lots of stupid subaru owners put fake wings and fake trip turbulators on their roofs--at exactly the wrong location--and they aren't even real turbulence inducers--just draggy things.
 

People get confused about what laminar flow is, how and when it transitions, when it is valuable and when it isn't.

Add this up and you have, well, confusion.

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2 hours ago, martin 'hoff said:

All true once you  have basic boat tuning, are good at maneuvers/boathandling and when you have time to dedicate to it.

At least for me, it's all time on the water for maneuvers, boat handling, starts, weather/tides and tactics. Good weather, bad weather, time on the water rules.

We have just gone through a season of boat handling, and I'm shifting a bit of time/effort to boat tuning and (yes) smooth foils. Why? After a season of consistent improvement in results race to race in a mixed fleet we got lucky and bumped into a same class boat in our bay. And a top team practicing for Worlds on a brand new boat. We did a brief informal 2 boat practice/tuning run with a top team, swapped boats and repeated. Our boat was pig slow.

Tuning and technique get you that first 97~98% boat speed. Sound like you bumped into a great learning opportunity.

The time invested in practicing sailing technique for speed and boat handling definitely pays the biggest dividend, until you're in the front row of sailors at least.

But hell, look at the skill and attention demanded. That's difficult. Any dumbass can wetsand!

- DSK

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11 hours ago, Steam Flyer said:

Any dumbass can wetsand!

- DSK

Gonna have to call you out here.......

Good sanding/polishing, whether wet or dry, is a coulda', woulda', shoulda' thing!

The skill, back breaking effort and laborious sustained effort required means that it is much more rarely seen than it should be.

But those criteria to apply to so much in life, and especially sailing. ;)

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From what I understand, surface finishes that would actually have an improvement over the polish we normally see tend to be very tricky and expensive to manufacture.

Also, trying to gain an advantage by modifying your boat is a really good way to start some political infighting, which is a sure way to spoil the fun for everyone, you included. IMO, we're doing this for fun, let's all play fair. 

My stance is that you should make sure that your boat is faired reasonably well. For surface finish, I like to make sure that it's polished reasonably well, then I do the necessary waxing and buffing to stop oxidation of the gelcoat. This doesn't make a big difference in speed, but it does make you more photogenic on the race course. Hey, that counts for something, right?

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13 hours ago, Erwankerauzen said:

Long time ago, I was told that polishing the deck for a Tornado was more effective than the hull, for the thin film of water not to stick to the deck' surfaces as much as if not polished, so at the end polishing was about carrying less water.

cheers

The amount of weight from that thin film of water on the deck is negligible at best. Taking a piss before the start will have a bigger impact.

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5 hours ago, JM1366 said:

The amount of weight from that thin film of water on the deck is negligible at best. Taking a piss before the start will have a bigger impact.

Rodney Pattison wrote that the Flying Dutchman can carry 17 pounds of water, beaded up on it's deck. That may not be enough to effect the outcome of a race but it's more than the human bladder can hold... well, mine at least.

- DSK

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1 hour ago, Steam Flyer said:

Rodney Pattison wrote that the Flying Dutchman can carry 17 pounds of water, beaded up on it's deck. That may not be enough to effect the outcome of a race but it's more than the human bladder can hold... well, mine at least.

- DSK

19 x 6.5 x .75 = 93 ftsq. 17 lb = .27ft3.  Thats 34 mils. 2 thocks of gelcoat.

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On 12/2/2021 at 10:07 AM, Erwankerauzen said:

Long time ago, I was told that polishing the deck for a Tornado was more effective than the hull, for the thin film of water not to stick to the deck' surfaces as much as if not polished, so at the end polishing was about carrying less water.

cheers

I found that if you apply a liberal coat of McLube on deck that it helps with that load noise in the back of the boat!  

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On 11/28/2021 at 5:56 PM, El Borracho said:

Not in my wheelhouse, but I'll comment anyway: My feeling is that because nobody seems able to categorically state which of the common race finishes is best, then they all have equal performance ... it doesn't make a significant difference. Other things are more important than whether a mirror finish is better than XXX grit.

Second, isn't the boundary layer several millimeters thick? That is the layer of relatively stagnant wter. It is certainly not microscopic, right? If that layer is mostly moving along with the boat then ultimate smoothness seems less important, too. The friction is all in the water-to-water interfaces, not the water-to-hull interface.

The boundary layer thickness actually changes over the length of the boat, starting thin and then gradually increasing over the length of the boat. And it can indeed become visible with the bare eye in larger boats.

However, what really matters is the thickness of the laminar sublayer. This also exists in a turbulent boundary layer. Basically, if your surface roughness is deeply contained within the laminar sublayer it becomes irrelevant, because the turbulence cannot reach the surface. So the answer to the question how smooth is enough is when your surface roughness is well below the thickness of the laminar sublayer.

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7 hours ago, fastyacht said:

Which is why highly polished at bow is more valuable than at stern

Yes, but also because at the bow you have a laminar boundary layer that you want to keep as long as possible and roughness triggers the transition to turbulent.

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For a more practical answer I started to run some numbers. Good news is the thickness of the laminar sublayer also increases slightly along the boat. Bad news is that it becomes thinner with increasing speed, i.e. the faster your boat goes the smoother the surface needs to be.

Anyway,  I hoped there would be a relation between grit number and grain size, but found that one needs to refer to tables for this so will need some more time to finish it.

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18 minutes ago, neuronz said:

For a more practical answer I started to run some numbers.

Good luck with that. Even empirical data is elusive. Pure water flowing on infinite flat surfaces and in plain idealized pipes can be modeled with some accuracy. A sailboat hull sloshing about in a biological soup of seawater… not so much.

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6 hours ago, neuronz said:

Yes, but also because at the bow you have a laminar boundary layer that you want to keep as long as possible and roughness triggers the transition to turbulent.

Well duh. Unless of course it ia a test model. Then you want to glue rocks on up there.

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1 hour ago, El Borracho said:

Good luck with that. Even empirical data is elusive. Pure water flowing on infinite flat surfaces and in plain idealized pipes can be modeled with some accuracy. A sailboat hull sloshing about in a biological soup of seawater… not so much.

I will need to make some assumptions and simplifications that will compromise the validity of absolute values somewhat. However, I believe it will suffice for a ball park estimate and relative comparison between different flow conditions.

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On 11/26/2021 at 9:32 AM, Snipe6976 said:

     I’ve recently read (in Tuning for Speed) that 400 grit allows that thin layer of water to cling to the hull for the best flow characteristics.  But then in Bethwaite’s book on High Performance Sailing, he uses the phrase “highly polished” and “I question whether any surface is smooth enough . . .”.    
     Wax or no wax ? Other ?
     What is the latest thinking for a dinghy ?

This has been an argument among scow sailors for years. Melges is now not polishing the bottoms of their boats and some people are really happy with is and some are super mad about it. i heard both being correct and it really depends on boat to boat. Polishing the rudder and the daggerboard will always help but there isn't really much a difference between laminar and turbulent flow under the boat. 

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1 hour ago, Jack Hubbard said:

This has been an argument among scow sailors for years. Melges is now not polishing the bottoms of their boats and some people are really happy with is and some are super mad about it. i heard both being correct and it really depends on boat to boat. Polishing the rudder and the daggerboard will always help but there isn't really much a difference between laminar and turbulent flow under the boat. 

What are the winners doing?

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1 hour ago, Bill5 said:

What are the winners doing?

it's a mix really. most of the E scows have polished bottoms but i know of a few of the MC's having 400 grit on the bottom. I assume It doesn't matter much because I've seen both win nationals before so i would guess its more of a preference and depends on how you sail the boat.

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First of all on the hull nobody is sanding transversly. Swcond, what is going on with polish vs not is the friction coefficient. Npt trippomg the flow turbulent

 It will do that all by itself somewjere along the hull. 400 is already low cowfficient . And finally, the firther aft ypu go, the coarser the grit required to achieve minimim. And another finally, scows are usually sailing fast enough that the wavemaking is a lar grrr er component of resistan e.

This id why you wont notice it much.

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Another factor is cleanliness. Those who are polishing or wet sanding, no matter the grit, are very effectively cleaning the bottom somewhat regularly. Even on a trailered boat, road grit and damage is bad for frictional drag.

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