Drag Reduction Using Primer? [Archive] - Scream And Fly Powerboat and High Performance Powerboating Discussion Forums

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LSaupe

01-13-2010, 05:22 AM

I am seen several threads suggesting using primer as a means of drag reduction on lower units and pads.

Can anyone elaborate as to the physics involved causing the drag reduction?

I am trying to resolve this from a fluid mechanics perspective.

Also, what type of primer wetted surface works best (as sprayed or wet sanded etc)?

Markus

01-13-2010, 06:50 AM

Primer gives you a slightly rough surface that the water will not attach to as easily as it would if the surface were smooth.

QuarterHorse

01-13-2010, 07:06 AM

Primer absorbs moisture correct? Wouldn't this be a bad idea on the hull?

XstreamVking

01-13-2010, 07:47 AM

Mark75H

01-13-2010, 07:59 AM

Primer is a well accepted speed finish on hull pads and on lowers. Works by reducing surface adhesion. Sand the primer with some 400-600 wet paper, with a random side to side pattern. Water gets trapped in the grooves and gives you a water to water contact surface. Which in theory reduces drag.....Now let's hear from the fluid dynamics experts to see if the theory is scientifically correct....

The man with the most speed records in history uses finishes no smoother than 400 grit

110+ speed records (including the 176+ outboard record) can leave no doubt, his access to guys with knowledge of fluid dynamics ... he's a manager at Boeing

transomstand

01-13-2010, 08:31 AM

Now let's hear from the fluid dynamics experts to see if the theory is scientifically correct....

The great thing about the internet is retards can reply too:D

I subscribe to the same theory as you stated. The material used to coat the bottom doesn't really matter. Texture is everything. A rougher texture creates a boundary layer of water that "sticks" to the hull, a water against water running surface creates the least friction, and is the fastest.

As for the use of primer, I suspect that racers would prefer the qualities of primer, ease of application and workability to achieve the desired texture. Racers would probably tinker with bottom profiles and repair chips or imperfections on a frequent basis, so applying a gelcoat finish would be impractible. Many primers will absorb water, as stated.

Primer is fine for racers, but not the best choice for the average hot boater. It just won't hold up well over the long term. Sanding the existing gelcoat to achieve the desired texture is the most practical method.

T-REX

01-13-2010, 08:38 AM

Markus

01-13-2010, 09:06 AM

I know I'll catch hell for this, but I use nutt'in but cheep spray can primer on geercases, and pad area...I used hi dollar epoxy primer once or twice, and it will not stay on az long az tha cheep spray can stuff...I use dollar store, and ace hardware(tha cheepo stuff)...

I5t'z amaze'in how that stuff stays on a geercase, and I reely kan't soo why, but it works...A lil test iz eazy...Git sum spray can primer on your hand, then put sum hidollar primer on your hand...the hidollar stuff will peel rite off, but the cheepo stuff will wear off, and U have oil on your hands!!!.....U figg'r!!:thumbsup:

Same here with gearcases. Have used epoxy paint and clear coat, and it got ripped right off the lower. Spray primer + spray paint stuck.

Also, we tried a paint called liquid speed which creats a rough finish on a bottom many years ago. It changed the flow of water so much that the transom-mounted water pick-up no longer picked up enough water to keep the engine from over-heating. Sanding is easier, though.

XstreamVking

01-13-2010, 09:06 AM

Probably because the good primer is an acrylic based primer, and the cheepo stuff if nothing but tinted talcum powder and oil... Cheap stuff will work w/o surface prep too..I still think the best surface is like transom stand said, sanded gellcote..

olmo40

01-13-2010, 04:56 PM

Depending on the condition of your pad ,i usually brush a bit of gelcoat on the wetted surface then sand it .I was given a product called Quikskin at a recent race meet "Guarantee Speed " havent tried it yet :rolleyes: .Couldnt agree more about the cheap spray can paint .

Ted Stryker

01-13-2010, 06:00 PM

I Heard same Bass boat Guy's say that there big touney rigs "run with nuttin' but da prop inda water":rolleyes:... I guess if You can set Your boat up to do this, You won't need any spray on Your boat :smiletest:... I haven't gooten My set-up just right enough to do that, I'm pretty satisfied if I can run with about 18" of pad wetted with a small wind ripple... For the last 4' of boat I have used just regular sandable primer with a quick prep of 400 grit just to take the shine off... If Your hull has gel-coat, give it a little work just to get rid of the gloss, and not worry about the primer... After all this, it's MUCH MORE IMPORTANT a running surface that is straight, sharp, dimesionally correct with a nice surface than to have one that not "right" and super slick...

richcraft

01-13-2010, 06:05 PM

Any ideas on a surface tension reducing finish for aluminum?

Would I polish to a mirror finish and wax it, sand it with 400 grit or prime the wetted surface?

Thanks :cheers:

Ronny Jetmore

01-13-2010, 06:46 PM

primer, but i think you need to sand and immediately put a product on first that will stop the alum from oxidizing. then prime.

i'll tell you a really cool way to protect aluminum, is to clean, sand, then brush on some neat epoxy (unthickened), then work the epoxy into the surface with wet/dry sandpaper. then let cure, remove the blush, sand lightly, then primer.

XstreamVking

01-13-2010, 06:48 PM

Maybe a brushed finish or 400 grit sandpaper would do the trick.

LSaupe

01-13-2010, 07:38 PM

Not sure if I agree with the surface adhesion and water trapping idea, but along the same lines maybe the surface becomes hydrophobic (sheds water) and traps air.

A water to water interface won't gain anything, but water to air would! The no slip boundary condition exists at the solid surface, so adhesion or surface tension won't have any effect on the water to solid interface - as is is already not moving (only moving at a short distance away from the surface in the boundary layer).

However, if air is trapped (in micro cavities / scratches / pits) between the solid surface and water; then the air can act as a go between the water and solid surface decreasing the drag.

As far as roughness goes, it will only effect drag if the roughness is greater than the viscous sublayer of a turbulent boundary layer (this layer becomes thinner with increasing speed, so faster flows will require a smoother surface and vice versa).

The very small scratches that are suggested by the cross hatching sandpaper could trap air though. Here is a foward from an interesting article about mixing a watershedding surface with micro-roughness to grab air. Very respectable reductions in drag in turbulent flows.

Not sure if these are the mechanism(s) involved with the drag reduction of primer, but it might be a clue. Basically water just rides over the bumps with the no-slip condition (hence drag) appearant only at the contact points with the solid surface.

Just my 2 cents here, and maybe that's all its worth. The best part about fluid mechanics (primarily turbulence) is that it remains one of the chief unsolved problems in physics.

As was stated at a recent fluid dynamics conference I attended, "many people feel they have solved the majority of turbulent flows, however.... none of these people agree with one another". :smiletest:

Robert J. Daniello (http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Daniello%2C+Robert+J.&possible1zone=author&maxdisp=25&smode=strresults&aqs=true), Nicholas E. Waterhouse (http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Waterhouse%2C+Nicholas+E.&possible1zone=author&maxdisp=25&smode=strresults&aqs=true), and Jonathan P. Rothstein (http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Rothstein%2C+Jonathan+P.&possible1zone=author&maxdisp=25&smode=strresults&aqs=true)
Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, 160 Governors Drive, Amherst, Massachusetts 01003, USA<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
In this paper, we demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flow drag reduction, are capable of reducing drag in the turbulent flow regime. Superhydrophobic surfaces contain micro- or nanoscale hydrophobic features which can support a shear-free air-water interface between peaks in the surface topology. Particle image velocimetry and pressure drop measurements were used to observe significant slip velocities, shear stress, and pressure drop reductions corresponding to drag reductions approaching 50%. At a given Reynolds number, drag reduction is found to increase with increasing feature size and spacing, as in laminar flows. The onset of drag reduction occurs at a critical Reynolds number where the viscous sublayer thickness approaches the scale of the superhydrophobic microfeatures and performance is seen to increase with further reduction in viscous sublayer height. These results indicate superhydrophobic surfaces may provide a significant drag reducing mechanism for marine vessels. ©2009 American Institute of Physics <o:p></o:p>

transomstand

01-13-2010, 07:46 PM

Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, 160 Governors Drive, Amherst, Massachusetts 01003, USAffice:office" /><O:p></O:p>
In this paper, we demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flow drag reduction, are capable of reducing drag in the turbulent flow regime. Superhydrophobic surfaces contain micro- or nanoscale hydrophobic features which can support a shear-free air-water interface between peaks in the surface topology. Particle image velocimetry and pressure drop measurements were used to observe significant slip velocities, shear stress, and pressure drop reductions corresponding to drag reductions approaching 50%. At a given Reynolds number, drag reduction is found to increase with increasing feature size and spacing, as in laminar flows. The onset of drag reduction occurs at a critical Reynolds number where the viscous sublayer thickness approaches the scale of the superhydrophobic microfeatures and performance is seen to increase with further reduction in viscous sublayer height. These results indicate superhydrophobic surfaces may provide a significant drag reducing mechanism for marine vessels. ©2009 American Institute of Physics <O:p></O:p>

I thought I just said that????:p:D:D

sschefer

01-13-2010, 07:49 PM

The idea behind a rough surface is somewhat similar to putting the dimples on a golf ball. The effect is similar to hyper cavitation. Look it up in Wikipedia and you'll understand how it works.

I used Interlux VC performance epoxy on my hull and it's laid down with a super fine sponge roller. The surface is smooth but slightly dimpled. I haven't got a clue if it works better or not. Most of my Tinny's don't float when I get them so I never get a chance to run them as they were to comapare.

Don't all Merc HyPo Lowers come glass beaded?

plasterer tim

01-13-2010, 08:16 PM

I first came across the roughed wetted surface area from some friends at the local rowing club, my brother was a spray painter and we went to great lengths polishing with auto type silicon based polishes, anyway the local launching ramp being alongside the rowing club he and i were hooning about one afternoon when the rowers came out to train, so we gave them the river stretch and went down river for a cruise( nothin better than bein beside a brother than at 80mph 3 feet apart)lol, when we came back helped the rowers pack their skull shells or whatever they call them away in the shed thingy, these boats get stored upside down thus revealing a sanded wetted area, as stated 400 grit, i personally didnt try it( i thought 92 on the plank and out of control was quick enough, have seen boats with a graphite coated bottom for kilo times also) not certain about success of either method but others swear by it tim

LSaupe

01-13-2010, 08:22 PM

Neat stuff. Super cavitation looks real interesting. Will have to do some reading on the topic.

Different mechanism behind the drag reduction with a gold ball though. Dimples on the gold ball are used to energize the boundary layer on the ball to help reduce the separation region behind the ball (wake). The dimples increase skin drag, but reduce wake drag, which results in a net drag reduction. Add dimples to a surface that doesn't separate and the drag increases.

But, an vapor layer around an object (which is traveling in a liquid) is quite interesting. Sounds like you might loose some directional control, but I have no background in that area.

Mark Poole ModVP

01-13-2010, 08:48 PM

The surface can be TOO ROUGH. I watched a boat builder put non skid (the aggressive stuff) on the pad and strakes. I tried to tell him that the surface was too aggressive.....too rough for the water to only touch the high spots. Well he had to try it.....it went slower!

I thought about trying "micro steps".....tiny fish scale layered steps that would reduce surface area but not cause suction.

powerabout

01-14-2010, 06:31 AM

The current swimimng suits used by olympic swimmers are either scales or shark skin style.
I think each speed requires a different finish.
( this has been tested in the yacht world but we are only talking slow of course)

XstreamVking

01-14-2010, 06:49 AM

I have seen the efforts of many bigger boat mfg to induce air onto the runnin surface. They have air vents at the waterline and the suction of moving thru the water brings the air to the bottom thru holes in the trailing edge of the steps.. Should work like running a small boat thru a slight chop. Runnin on aereated water...faster and more efficent.

gmacrae

01-16-2010, 05:23 PM

i get the reasons for using a rough surface, but surely you'd be better off using a matte paint? It should last much better