Bottom Paint and the reduction of speed

[waternwaves]

Bottom coatings.....lll

clean to dirty bottoms can show a 2 mph or more difference at our hull speeds..but properly applied surface coatings are not going to register that much difference....... here we go...... Im gonna try this with no visible math here, and at no time will my fingers touch a calculator...lol

See,,,

I told ya..........

Purple (burgandy and maroon included) has gotta be faster.......ehheheh

(yeah one of my boats is maroon and burgandy and grey let the jokes begin)

Anyway Frank.... at our speeds <20 mph, It would be difficult to get a 2 mph drop due to a coating...... unless perhaps applied with a floor mop, water ballons or grooving the suface with a old belt sander, or possibly a wiping every single brush stroke.... perpendicular to the flow of the water along the hull, with a very high solids paints that had a lot of unmixed lumps in it.......

but...... going to surface roughness of 15-1/32 (15 peaks per inch, average height RMS < 1/32 inch) Somewhat representative of bottom painted surfaces..... (note surface profiling mathemetically is subject to wide ranges even with similiar measurement tools..... )

Second problem is at a mac's regular top speed....establishing a flow model is difficult..... but lets assume no water is displaced to the sides, or forward, and all water is drag is perpendicular to the path of the boat. This would equate to maximum drag. ( very little of this layer is pushed forward of the boat)


Note, this is not an exact science, and experimental modeling in tanks shows a wide range of results,
Fluids modeling establishes at least 3 zones (where the models utilize separate equations to calculate flow, fricition and forces). For our purposes here the three zones are often described as ..

Turbulent - Velocity Defect law(farthest from the hull, first disturbed water)
Turbulent - Log law ( Closer to the hull)
Viscous Sublayer (contacting the hull)

Note...(many describe a buffer zone between viscous sublayer and Logarithmic Velocity)

The very thin layer of water nearest the hull is the Viscous sublayer ( this is where the surface roughness of the coating holds the water in place which travels with the boat. Water velocity (Distribution of velocities) is related to the shear stress and viscosity by Newtons viscosity law. Without going into a detailed model, and significant mathmatics. (dull boring computational fluid mathmatics better left for big computers anyway) the depth of this layer changes as a function of the surface roughness of the hull,

increasing the Viscous Sublayer depth (for the flow across the surface) by roughening the surface only, results in increasing the thickness for the sublayer, and increasing the drag. But functionally, this layer is only a small fraction of an inch in thickness, on the order of less than 1/10 to 1/100th of an inch, at our speeds and surface roughnesses.. 20 ft/s or so, basicly this means that at aproximatley 1 inch away from the hull the water pushed out of the way of the boat is only moving at 1/3 to 1/4 the speed of the boat anyway........

So, into the PC goes the handy dandy equations, vary the surface roughness depths and peaks per lineal inch by a factor of 10, just add water, and even if the height is quadrupled we dont get a much of a velocity/drag change, nothing near what we get throwing on a couple of square feet of barnacles only 3/8" of an inch tall.....

so, as long as the bottom is clean and relatively smooth, bottom paint is going to be a fraction of a knot less than a smooth boat at our boat speeds..

and and guess what. it matches what has been observed on mione and other boats.., basicly undetectable. An absolutely smooth hull from the factory is too sticky anyway.... you want just a thin stream of bubbles lubricating the hull anyway.....and needs to have something to reduce surface tension .
And Frank is so right on how difficult it is to determine metrics for this without two exactly similiar boats with very good instrumentation.... or even better instrumentation on a single boat....


and for those of you that cant sleep at night, I can recommend an easy to read text on Engineering fluid mechanics (lol)

from Roberson/Crowe.......

zzzzZZZZZZZZZzzzzzzzzzzzz ZZZZZZ

see its working already.....

[EZ]

I have no background in fluid mechanics, but I do keep my M in a slip and bottom painted.

With a '98 3-Cyl Evinrude, I get up to 21mph w/o ballast and 17-18 mph w/ ballast. This is with the engine screaming, which I normally don't do. Typically I run the engine at 5000RPM which yeilds a speed of 15mph w/o ballast and 12mph w/.

This is with two people on board and about 250lb of stuff. I believe the engine weighs about 250 lbs as well.

I have had the boat up to 6mph under sail, but can't say for sure if this is the ultimate top speed. I haven't gone out enough in consistant winds to get repeatable sailing performance numbers.

[Catigale]

I would be fascinated to hear from you 'flow through pipes guys' (our euphemism for Fluid engineers, sorry) on the relative effects of the hull skin resistance vs the transom turbulence.

Sticking my uneducated neck out, I see a transom effect from Catigale of at least 3 feet off the stern - if an appreciable amount of this water is being moved forward Im guessing this effect swamps the bottom effects..

(I was on Nantucket last summer and a friends hat went overboard and chased us in the transom wake varying between near the boat and 2 feet away while we grabbed a boat hook and recovered it.)

IF thats so the ultimate mod would be a stern smoothing appendage - a modest fiberglassing job (with a cut out hole for the 140 HP of course)

errr,,, can someone give this a try and let us know how that works??

[Dimitri-2000X-Tampa]

EZ, what is your engine type and HP? Also, what size prop?