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how important to clean hull?


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At a regatta this past weekend I saw a boat dangling from the hoist while the owner buffed the hull with an electric buffer. I've never done anything like that. How important is it to clean the hull? How do you clean your boat?

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Years ago Offshore sailing school had a racing class with rock star head instructors. Four identical Impule 26 sloops with spinnaker. During the 4th week with Bill Shore the instructor, one of the boats always came in last. Bill took the helm for a practice race and came in 4th, last.

Steve Colgate had us four on-board employee instructors dive in and clean that boat's bottom. Wasn't much there, but a bit of "jelly."

That boat won the next race and placed appropriately thereafter.

Bethwaite's book has an account of a tow bar for two boats to try foils. Should be even to start the day. But one boat pulled back. It was the bottom one on a trailer. Got the road film off and all was well.

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The smaller the boat the more hull friction plays a part. 

It's about Reynolds Numbers.

At a waterline length of 1m half the drag is skin friction.  At 30m it is a very small fraction.

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At a club near mine, they have an electric hoist slipway and an electric crane (free to use) so many, but not the majority, of their 20 to 30ft keel boats are moored ashore. Quite a few owners have their own favourite way of keeping their hull clean. Some use a polish of some sort..

I'm still not sure 100% what to do, we only have a manual crane , or a vehicle pulled slipway. With only a 16ft LWL on my boat, keeping the bottom clean will be important, at the moment VC17m plus a pull out mid season  (just before our regatta week at the start of August) for a wipe down is favourite.

(Note, we are in a comparatively light fouling area, but get a lot of slime.)

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As I was there that day (double trailer, road grim, that pesky Bethwaite(s), etc etc) so my list, in order of importance

1) front 20-25% of you rudder especiall the LE

2) front 25% of the centerboard, keel, fin (what ever)

3) the rest of your fin (centerboard, keel, whatever)

4) front 25% of the hull

5) rest of the rudder

6) rest of the hull.

The really big reason is the bondary layer, at the bow and front of the foils there is no boundary layer, or it's miniscue.

As you get to the back of a 49er or 18teen at speed it ends up being about 20mm thick, and at the back of Comanche its also 20-25mm thick, you can make that thinner by using, and the best one we every found was Turtle Wax!  There was Canadian StarBright, but it had stuff in it which was really good for boundary layer supression so it was banned ecologically, everywhere but Canada.    And there was Polly-go-blue which was basically egg-whites so it's ceased to be any more.

Water is incompressible, so any deviation, even 1/2 the wave length of light (difference between gloss and matt) your asking a incompressible substance to move "more than it has to" and that generates drag.    We just cut and polish the hull, 600- 800 - 1000 - 1200 then 1500 and a polish, also important that the last 3 grades are fore-n-aft!

In the August or September SeaHorse there is a sensational photograph of what 0.1mm x 1mm disc will do to your flow.

Clean your bottom (well) guys.

                          jB

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Just pressed the send button and then remembered.

We also did a trail where one boat was finished with 80g, another with 120g.    Very significant difference.

Then we did it again seeing the difference between 800g (wet&dry) vs a polish, also a big difference.

I also remember Bora Galurai and his moth in Detroit, water was getting very close to 3.8c (water switches molecular structure about that temp) and he had to orentate the way he "effectively" scored the LE of his foils, we are talking about a scoring 5mm wide maybe, we are talking about something very fine, 800g possibly.  Had to be in the right place, very definite distance behind the LE (as a %).    Think he tried 45° and parrallel to the LE, not sure which one worked, but under the 3.8c mark, his foils would not work if he did not have the scores.      I may have got that all wrong, I was on the side of the conversation, that pesky Bethwaite (Dad) and Bora where off on another planet talking a language few uderstand.

                jB

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Years ago I raced a Lightning... a 19' centerboard sloop with a big spinnaker, a relatively heavy hard-chined boat with a powerful rig... and went thru iterations of improving my old beat-up cheap boat. When I first bought it, I worked at learning to sail it, specific tuning and trim, copying the top skippers we met.

I got brand-new crispy sails... decent jump in performance on windward legs.

Putting up the hull in my garage, I did grinding and fiberglassing to lighten and stiffen it, also put the chainplates and mast step in the correct location instead of within 1/2". Small or no improvement in results, except that I'd also rounded off the seating edges and put in control line panels, so the boat was physically easier to sail.

By now I'd been racing it a few years and was occasionally a threat to the class leaders. One of them went over my mast & rig with me, getting the tuning closer to what the sailmaker intended. Biggest improvement there was pointing, not speed, but don't sneer at pointing. Oddly, I also found that on DDW runs, I could use the vang to build a pocket in the upper main more effectively, so it did help downwind speed some too. Every boat & rig has it's peculiarities.

I had wetsanded the hull to 600 grit, and from day one was cleaning it before every regatta.

But the biggest difference was fairing and super-wetsanding (to 1500 grit) the hull. Returning to the garage, I turned the hull upside down and went over it with multiple passes of longboarding and filling. Once it was as fair as I could get it, rounded the forward chines to as round as class rule allowed, and put a nice sharp corner on the aft chines and transom, I wetsanded it (with a longboard) to 1500 grit. Then I laid on 3 coats of 2-part polyurethane auto finish, the hard stuff.

In light air, the boat was now like a rocket. I could literally take a nap and walk away from the local class leaders. At the big regattas, we were suddenly in the front row especially in light air. I'm sure there are still some Lightning sailors on the east coast who remember that goddam purple boat!

I attributed it all to my wife's crew work.

FB- Doug

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It's fascinating to read the in depth analysis of Julian B and Steamflyer on the art of fairing. Much to be learned there.

Thank you both for sharing.

I'm wet sailing a Soling, sacrilege I know but with our tidal range (5m) the Dinghy slip is too steep and crane lift is €80 a go so she stays on the marina.

We race around beer cans Tuesdays and Thursdays in a mixed fleet from J92S to Sigma 33 and J24s. Cleaning it takes us from the back of fleet when fouled to mixing it at the front when clean. At least a KT in boat speed. This is directly correlated when chugging along with the outboard.

In the spring I was tying it alongside a wall and powerwashing when the tide dropped out. Now that the water is warmer I snorkel and brush it. With no antifoul this needs to be done every two weeks before the drop off in performance is noticeable.

One thing that is apparent is the growth is markedly different on each side of the boat. I'm on the south (sunny) side of a due west marina finger and the growth on the sunny port side is much more prevalent than the starboard side. This has a significant effect in terms of speed on each tack. There is another thread going on the subject of tack speeds where everyone is focusing on rig tuning, maybe correctly in that case, but as in any case of problem analysis, eliminate the knowns and the obvious before delving deeper.

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What people fail to get is that boundary layer, so the difference between a well posiled and not even waxed hull and one that is say Anti-fouled is the amount of weight your dragging around with you.

Keep it very simple. A well detailed 49er (infact every 49er) has about 4² of surface area that is in contact with the water.

Through the 1st 1m² away because not enough time for the boundary layer to establish, but in the case of the remaining 3m², in the case of the well detailed 49er you would have a average of 5mm, so 1000kgs (per m³ of water) x 3m² x 5mm = 15kgs of water which is additional weight, inertia, that you are dragging around with you.

If the bottom is "less than satisfacory" that average could be 10-15mm thick as a average, so it becomes 30-45kgs of water.    Need to stress, this is not "weighting you down", what it is doing is dulling the acceleration down and increasing the inertia, so responding to that gust, or that wave is that much harder.

What is weighting you down is "blunt chines".     It's so important that in the 49er that Barry (the 49er ICM) check the sharpness of the chines.   29ers also.

As you start to plane (which is 90% of the time) water stops running along the boat, start squirting out sideways.  With a chine which is "sharp enough" that water seperates at the chine.   If it is a bit round, then that water is turned upwards, that turning upwards, sucks the hull down (Newton 3rd law).    More it turns, the more it drags you down.   So Steam Flyer with his sharp chine aft, makes infinite sense!

So this effect is amplified many times once you start planning, in so many ways!

I do remember in 1978, getting a laser, pretty sure the Poly-go-blue was on the top sides and the Canadian Starbright was on the hull, we taped a "imaginary" chine, so there was a very defined "release" line, so as the water squirted sideway, it travelled along the Starbright until it hit the Poly-go-blue, and the CoF [coeficent of friction] was significantly different that, that laser was substiantially faster off the wind.

Need to stress it took a couple of goes to get it right, 10-15 years later we where doing that all the time WRT the 18teens, but not with Poly-go-blue, think we did the top-side with Turtle Wax (green bottle, use it no your cars) and Canadian Starbright below the chine.

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Damm... and i thought the vertical grooves in my dagger board were good for speed... when I get around to refinishing the foils (and the dings in the paint work, Dragonfly is getting a bit tired..) I have heard differing opinions as to the finish but it sounds like a polish and then wax? Would something like PTFE spray (GT85 or such) make a difference if applied before you sail?

 

Alistair

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Interesting comments above.

Pointing ability is extremely important sailing as I do on a river (mostly), and extra degree can give you a huge gain sailing along the riverbank on the edge of luffing. Much development time will be spent on Blue Moon getting that right.

The topside of the hull had been giving me some thought, as she will sail well heeled on the wind, so sanding and or polishing the hull above the antifoul will be important. on top of that I'm assuming that a very good sand is needed if you have to repaint what had been a polished hull.

With only a 4ft beam and 6 inches draft of hull (3ft keel) a regular wipe with a sponge on a stick should keep the hull clean, which is why I was wondering whether antifoul would be needed..

 

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This is a great thread, and timely as I'm about to travel to a few regattas which is when this stuff happens for me.

The thing I've never gotten nailed down is in prepping the leading edge of the blades. Obviously it's very easy to sand along the leading edge, perpendicular to the water flow, and it's very difficult to go the other way. It's also very easy to introduce unfairness (even if it's of very minor degree) if you sand front to back along the leading edge. So what's the best technique for that?

Thanks

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7 minutes ago, D Kirkpatrick said:

This is a great thread, and timely as I'm about to travel to a few regattas which is when this stuff happens for me.

The thing I've never gotten nailed down is in prepping the leading edge of the blades. Obviously it's very easy to sand along the leading edge, perpendicular to the water flow, and it's very difficult to go the other way. It's also very easy to introduce unfairness (even if it's of very minor degree) if you sand front to back along the leading edge. So what's the best technique for that?

Thanks

1500 grit followed by lambswool with 3m polishimg compound. The direction does not matter when you get there.

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On 7/21/2021 at 2:04 AM, random. said:

The smaller the boat the more hull friction plays a part. 

It's about Reynolds Numbers.

At a waterline length of 1m half the drag is skin friction.  At 30m it is a very small fraction.

Depends on froude number. Above fn 0.3 wavemaki h dominates. Below 0.1 friction.

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15 minutes ago, fastyacht said:

1500 grit followed by lambswool with 3m polishimg compound. The direction does not matter when you get there.

Thanks. Do you go front to back with earlier grits?

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Just now, D Kirkpatrick said:

Thanks. Do you go front to back with earlier grits?

If you sand properly, it matters not. In otherwords no furrows left. Most difficult problem of surfacing by jand is patience and ovservation of grit progression.

Fairness must be achieved  otherwise exersize in futility

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Greatest thread. Have just shared with my youth coaches and squads as "Lockdown reading and prep"!!!

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This is extremely interesting and also puzzling. I will certainly clean the hull before my next big regatta. Am I correct in understanding that the front part of the hull is more important to keep clean and well faired than the aft parts? And that the foils are the most important of all? The rudder will be easy, but the board less so. I hate pulling it.  Thanks all for the great comments.

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11 hours ago, fiddlecanoe said:

This is extremely interesting and also puzzling. I will certainly clean the hull before my next big regatta. Am I correct in understanding that the front part of the hull is more important to keep clean and well faired than the aft parts? And that the foils are the most important of all? The rudder will be easy, but the board less so. I hate pulling it.  Thanks all for the great comments.

Yes and yes. Julian explained the boindary layer. That is the crux of it. Roughness entirely within the inner half of voindary layer very little loss. Roughness greater than thickness increases drag appreciably.

But the supersmooth requirement fwd has to do with the boindary layer being laminar there. Any protrusion is lossy there This includes unfairnesd.

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Some comments re up and down (LE's) and crosswise.

We learnt a huge amount from the alloy foils on 29ers, WRT span-wise-drift, it's also very common in miltary aircraft, so they use fences, but again, that's aero scenerio not hydro.

Using say 800 and up to 1000g up and down your LE's to minimise any iregularities is 100% kosher, and infact pretty important that you do it.

But into 1200 and 1500 you need to start thinking of aligning your strokes with the waterflow, LE is quite simple, just see-saw it backwards and forwards, and then short strokes for the rest of the foil.

Aligning the cutting section of the buffer head so its in the direction of the water flow is also pretty simple.

Obviously with the hull, 45° - 45° initially again to reduce longitudinal iregularities is a) simple, b) easy and c) very good.

Latter grades 1000-1200 & 1500 along the keel line, polish along the keel line.

Also to put this into perspective, Foils, getting you rudder to stick longer, and getting another 1-2% out of your fin/centerboard/keel, the fin ends up equalling possibly as much as 5% in a race, so 2mins in a 1 hour race.   Rudder, it's just about having control, and if you have the boat balanced then the rudder shoudl be carrying about 10% so its another 10-15sec in a hour.

Aft sections of the hull less gain, forward secvtion of the hull, more gain.

 

 

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Very good thread. I appreciate the insight.

How did the use of the “shark skin” film by the US 12 meter Stars & Stripes in the 1987 America’s Cup play into this discussion of mirror smooth hulls? At the the time the explanation given was the orientation of the “skin” was to reduce boundary layer thickness.

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The aligning the final wet & dry cuts with the Water Flow, is about maintaining laminar flow.

So anything that delays the onset of rolls, which are inevitable, but the delay of rolls maintains laminar flow for longer and delay’s turbulent flow, has to be a good thing.

Groves running along the line of the flow means that a roll has several variable diameters, so it’s complicated and that delays the initial roll forming, hence it works.

I have no idea about the shark’s skin.   In the real world it flexible how that translates to a ridged hull, sorry, above my pay grade.

We do know that even letting a hull flex as little as 0.5mm forward increases hull drag significantly which is why when boats go soft, they tend to go slow.

Both the 49er and 29er have “breast hooks” between the chines forward, which stops that flex, and each time we increased it (WRT the 49er) the breast hook got faster and stay competitive longer.    WRT the 29er it is the foredeck that acts like a “breast hook”.

And things like “Turtle Wax” reduce the skin friction, (again above my pay grade as to how,) which most importantly reduces the advent of rolls, which again I stress are inevitable.

You won’t stop the rolls, just think about that layer of water that is alongside the hull, say 0.25mm away, it will travel slower (because it’s slowed down by the hull) than the bit of water that is 1mm away, so at some stage you will get the rolls forming and once that happens the flow goes turbulent and you end up with a 20mm thick boundary layer which could be 5 rolls all happily churning away sucking speed out of the boat.

And yes it happens on foils also, both lift “vertically” and “horizontally”.

I need to go dig out one of dad’s very old book, with photos of the “ingress” of turbulent flow across a aero-foil.   He developed a way to test and get a visual result.    Probably late 1940’s stuff!

That may take a bit.

In the meantime, I do recall a science experiment, on YouTube, with some gel in a beaker and a churn, and at a given speed the churn can happen, (it remains Laminar) and then be un-done and all the layer return to pre churn state.     F—king extraordinary!

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Julian is pointing out what Feynman also pointed out: hydrodynamics is harder to solve than Quantum Electro Dynamics.

Delaying turbulrnt transition is Weird Science. If you look at skin frict coefficient plots over Reynolds Nimbers, the transition zone is a big band of incerainty. What Julian's suggestions are supposed to do is stay laminar to the highest possible reynolds nimber

On soaring planes some high performance ones have "laminar flow sections" on wingd, and just dust on the sirface trips them to turbulence

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When I competition "soared" your ground crew continually where moving up and down your LE, with polishing cloths, while waiting for a tow (launch).

If you went though a swam of bugs on take-off, that was the end of that fight, it destroyed the performance just like that.

Do the circuit, land and try again.

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Now bear in mind this is all about laminar flow. Such conditions are not always possible. And laminar flow sections on foils have their limitations (as I mentioned upthread).

If you want your foil to operate at greater angles of attack without stalling, you want a turbulent boundary layer.

You can't have everything.

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Years ago there was a huge discussion on one of the beach cat forums about waxing vs sanding vs washing the hulls - blades - rudders of the boat with regard and drag.

Had a few rocket surgeons engineers contributing...  in the end, I realized I was not sailing at that level where 1/100000000000000000000 of a second made a difference.

With that, I have always made it a point to keep my hulls clean and smooth as possible..  kind of hard when you're boat up one beach.

 

 

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4 hours ago, fastyacht said:

Now bear in mind this is all about laminar flow. Such conditions are not always possible. And laminar flow sections on foils have their limitations (as I mentioned upthread).

If you want your foil to operate at greater angles of attack without stalling, you want a turbulent boundary layer.

You can't have everything.

In the short term, I'll have evenings at home, and enjoy my performance at greater angles of attack. Once other facets (tuning, tactics, maneuvers...) are sorted, and the top of the fleet is calling, we'll have this (amazing!) thread bookmarked, and polish, fair and wax.

We sail a beach cat, and a big big part of the fun is the beach, which is quick to ruin your perfect finish...

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1 hour ago, martin 'hoff said:

We sail a beach cat, and a big big part of the fun is the beach, which is quick to ruin your perfect finish...

Yeuuuuppppp.... sort of defeats the whole purpose.

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