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Coefficients of friction of bottom paints


Foreverslow

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Has there been tests showing the various coefficients of friction for bottom paints?

Comparisons of hard paint, ablative paint, burnished versus just the hull etc.

Anyone run it in a test tank?

 

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It does not matter much what it's made of, what matters is the surface finish and the speed you intend to sail at. The faster you go, the finer the surface finish needs to be - there's an equation for it somewhere. Bottom line, on a 4kt SB you don't benefit from better than 100 grit, 10 kts can use 400 grit if I recall correctly.

Ablative paint looks like a ploughed field and is slow.

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Yes. At 3 kn the allowed roughness for minimum drag is 50 um, at 6 kn 25 um and at 10 kn 15 um. It's a linear function, thus 30 kn would be 5 um. Rolled antifouling with good preparation will have 50-100 um roughness, sprayed antifouling 30-40 um, slimy hull may have 200 um etc. VC thin film paints may be well below 50 um after rolling, but I haven't seen data.

P1200 sanding paper has 15 um roughness, P600 25 um, P400 35 um and P180 78 um. But that is the roughness of the paper, not how rough the sanded surface will be. The surface will be clearly smoother than the paper. I have no data about that, but I would argue that 100 grit is too rough even for 4 knots. It will also sand through most antifouling paints way too easily. I would use P240-P400 depending on the starting point and P120-P180 before painting.

You can find quite a few papers about antifouling roughness and added resistance, mainly for ships. Also you can find (very old) papers about flat plate resistance as a function of roughness. The friction resistance of the hull and appendages will be close enough to a flat plate.

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I do remember a graphite bottom paint from the 70's ... GraphSpeed?  Came and went.  Was thick and therefore great for hiding dings in boats you wanted to sell ... go fast must be good!

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You probably only need to sand the first third or half of the boat (and appendages). Boundary layer has tripped by then and roughness is in the turbulent flow. Surely studies have been done on that?

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Practical roughness of a boat that is not dry sailed would be more interesting. 

Even with a diver on a monthly schedule, we get slime in Oakland/Alameda. If reducing the surface roughness made the slime less adherent that would be good to know. 

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2 hours ago, DDW said:

You probably only need to sand the first third or half of the boat (and appendages). Boundary layer has tripped by then and roughness is in the turbulent flow. Surely studies have been done on that?

The surface roughness effect on friction is by no means limited to laminar boundary layer. 

There are two different smoothness requirements for keeping the friction drag as low as possible. First for not tripping the laminar boundary layer into turbulent one and then for keeping the friction of the turbulent boundary layer as low as possible.

The roughness needed for tripping the boundary layer sooner is typically about the same or rougher than the one that will increase friction of the turbulent regime.

For bigger boats needing antifouling the laminar boundary layer will be much shorter than 1/3 of LWL. You would be very lucky to have even 1/3 of an appendage.

Most of the roughness studies have been done for pipe flow where the length of the pipe is "infinite" and trip to turbulent flow has happened long time ago. Still roughness has a big effect on friction and pressure losses.

 

There are a lot of roughness measurements of ship hulls after painting and after use.

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The effect of surface roughness depends very much on the thickness of the boundary layer. Roughness elements within the boundary layer cause far less drag than anything sticking out of the boundary layer. Since the thickness of the boundary layer increases along the length of the hull and assuming motivation and accurateness decrease with working time, one should start fairing at the front where it has the biggest impact.

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Boundary layer thickness is much more than any roughness you like to see on the hull. Even laminar boundary layer thickness is soon more than 1 mm.

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It's polished as much as you can and a Hydrophobic nanocoat for me...

 

IMHO in real life having a clean bottom is more important than the rough/flat eternal discussion, because unlike in watertanks, in real life conditions change all the time and water is full of crap floating around.

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for example a study that looked at the efficacy of ablative and other "slippery" bottom paints in shedding slime. 

for a given roughness of a surface formulation for particular types of common algae? 

  • Does it take a minimum boat speed, e.g. powering at a certain minimum speed  per boat's length gets the nominal sailing boundary layer distance clean? 
  • An integrated boat speed * Time above a minimum threshold ?
  • More water speed than you will accomplish under power ?  
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On 4/30/2019 at 9:33 AM, Wet Spreaders said:

It does not matter much what it's made of, what matters is the surface finish and the speed you intend to sail at. The faster you go, the finer the surface finish needs to be - there's an equation for it somewhere. Bottom line, on a 4kt SB you don't benefit from better than 100 grit, 10 kts can use 400 grit if I recall correctly.

Ablative paint looks like a ploughed field and is slow.

that makes sense

Once had to tow a well prepped Olson 30 to the race course for several weeks as the motor was broke.

I lost .1-.2 of a knot off my hull speed.

 

Last week I had to pull a Catalina 30 that had not been used in 5 years and the bottom was a barnacle subdivision.

The old saying "That bottom was rougher than a night in county lockup" come to mind.

Could barely get above half hull speed and my motor was laboring the whole time.

 

You point about ablative paint is where I was headed.

How much slower is an ablative paint with a light sanding versus a hard paint or bare hull.

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"Principles of Yacht Design" Larsson et al, has a discussion on hull resistance, surface roughness, etc (pp73-81), amongst others.

Tells one  all  (I) need to know.

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