Golf ball boat - install a dimple plate?

For what its worth, I did my senior thesis at Webb Institute on this subject. At the time (early 90's) there were boogie boards and windsurfers on the market with dimples, claiming improved performance. The boogie board had tightly packed, about 3/8" diameter. The windsurfer had larger, shallow oval shaped dimples, about 1-1/2" x 1". We were able to get dimpled and undimpled versions of identical boards. We towed them for resistance in the model towing tank. We also towed at higher speed behind a powerboat. The results were the dimpled boards showed higher resistance. We were unable to find conditions where they were superior. The Clackacraft ad is the first time I have seen dimples associated with a boat. The fact that they would be put on a *drift* boat is slightly mind boggling. I am very skeptical that there would be any benefit.

 

An ol' friend of mine died this week and was looking at a video of the "Bristol Beaufighter" he flew in WWII. If you look at the video below at about 20 seconds, you'll see the nose of this fighter was dimpled. I would guess this gave some lift to the nose of this very heavy fighter.
http://www.youtube.com/watch?v=EkdF...e.com/videosearch?q=bristol beaufighter&emb=0

So, after I've sat on this for almost a year, I think my post was upside down! Dimples below the waterline would create suction - the opposite of what we'd want. But what if you put them them on the bow above the waterline?? On a fairly high speed boat, this would give a little lift right?

Also, this post never really properly addressed the idea of pimples below the waterline to break suction. My Dad went broke on a company that made duck decoys...but the "Suc-Duck" was their best seller. The one, big dimple created suction and held the duck on the water to look more real. (link below) I just seems a bunch of pimples under there would break suction easier than a step.
http://www.cabelas.com/reviews-cdn/...duct-_-8815-_-RLP-_-226994-_-description_link

 

The concavity in the bottom of the duck is for hydrostatics not hydrodynamics. It will increase the small angles righting moment by increasing the moment of volume about the CG. Much like a catamaran.

The Clackacraft dimples are a joke, otherwise known as marketing gimmick.

The point of the golf balls dimples is to create better streamlining around a poor aerodynamic shape by inducing turbulence ahead of the separation point. This is highly dependent on the Reynolds number. The golf ball happens to operate at Reynolds numbers where turbulation is an advantage. There exist other Reynold #s for which the golf ball would be better off without the dimples.

In other words, dimples are only of value when trying to maintain attached flow on an object with sufficiently severe adverse pressure gradients and/or operating at problematic Reynolds numbers.

There is also the special aspect of an object moving upon a fluid interface, which is that of fluid entrainment for possible drag reduction, where the fluid with the lower viscosity is introduced to separate the surfaces from contact from the higher viscosity fluid. This was mentioned above in the case of the viking ships and indeed all clinker built boats. This has been demonstrated in the case of the "Dragon" class which were built carvel or clinker and raced together. Neither seemed to have an advantage, implying that the effect of air entrainment about offset the resistance of the clinks.

Earlier there was mentioned the idea of holes to inject fluid into the stream. In fact, in almost all cases the holes are to suck not blow. This again is a technique for preventing flow separation, and typically with much less of a drag price than the turbulation method.

Then there is also the micro vortex generation method which is very effective at injecting energy into sluggish bottommost layers of the BL.

The dimples on the front of that airplane, if i may hazard a guess, is just a riveting artefact. That aircraft has a lot of parasitic drag, so i don't think they would have been too concerned about turbulating an area which is not prone to flow separation anyways. If you look at the very end you see in a patch of reflected light on the top of the wing how rippled the plating is. It is pretty normal when riveting thin sheets of metal. I'm no expert at that particular aircraft though, so i may be wrong on this.

 

Regarding entrained air in viking stlyle boats, i had direct experience of this last weekend. I had just sold a 1938 Rye beach boat to a Gentleman in Kent and we were delivering her across the Thames estuary to Conyer. "Billows" is a very pretty clinker built centreboard open boat. She was still a bit leaky after 3 years ashore and we kept hearing a funny fizzing noise. eventually after lifting the floorboards i found the culprit, one frame rivet near the garboard that was leaking but what was surprising was the amount of air coming in as well. This was near the stern of the boat and she was ghosting along in a force 2 with the barest ripples on the water. Wish i hadnt sold her now!

 

"You'd think someone would have brought wet exhaust to the front of the hull to ad lubrication from, for example, a filthy pair of Detroit Diesels."

This was patented long ago by a naval guy named John Paul Jones.

No not the Origonal JPJ , just the same name, maybe related.

The Patent office has plenty of schemes to make boats go faster cheaper , just most dont work.

FF

 

"Bristol Beaufighter RD 147 (PLU) represented the Mark X version of Frank Barnwell’s design, combining a "thimble" nose cone containing Air to Surface Vessel (ASV) radar "

http://glostransporthistory.visit-gloucestershire.co.uk/JetAgeRMCbristol.htm

It looks like the dimpling was done on purpose to support the Yagi type Radar used in night fighting. I would guess that a round nose would deflect incoming electromagnetic pulses, and the dimples were a compromise with the need for streamlining.

 

I'm not sure of how directly comparable the two are, but you might look at Seaplanes for an answer.

For a specific example, a Grumman HU-16 Albatross is a twin engine aircraft with a 96 ft Wing Span. Like most planes it's rivets are ground down smooth before painting, but (according to legend) an owner did a repair to the underside of his aircraft and used round headed rivets. He found that the take off speed was reduced by around 10 knots. The pilot notified Grumman of the discovery, who did a number of tests, and confirmed the finding...ever since they've left the rivets raised on the underside of their seaplanes.

Just too bad I'm a year late.

 

"The point of the golf balls dimples is to create better streamlining around a poor aerodynamic shape by inducing turbulence ahead of the separation point. This is highly dependent on the Reynolds number. The golf ball happens to operate at Reynolds numbers where turbulation is an advantage. There exist other Reynold #s for which the golf ball would be better off without the dimples."
This is how I´ve understood it - and the induced turbulence actually causes the total disturbed flow (comet tail) to be smaller. Nothing about lift. On a boat, the longer you can keep the flow laminar, the better, and it´s only going to be the front few feet. It is my take that you should get that front of your Balto-plate burnished well and then don´t bother with less than maybe 200 grit - it won´t matter because flow is non-laminar anyway. On a planing hull, more important yet, is to insure that the trailing edge is dead strait. Shark Skin, magic potions, teflon, etc. don´t do anything. Boaters everywhere swear by whatever product du jour simply because, of course, the bottom doesn´t have a beard when they first put the boat in the water, then, after a month, they swear they are slowing down because the super slick stuff is wearing off (it actually washed off almost immediately, and in any event, did not extend laminar flow and therefore was in non-laminar flow doing absolutely nothing), but are actually just dragging fur with them. America´s Cup racers and such are in a completely different league and have the money to experiment. When something actually works that applies to us, we will learn of it soon.
The float plane rivet thing is just to break suction when the plane is trying to take off - there is actually more drag.

 

Surfboards in the 80s were released with a "Speed" finish.

Rather than a gloss finish they were done in a satin or matt finish for similar reasons.

 

Yes there is a clear difference in Reynolds Number of a golf ball and a boat. A boat has about 100 times larger Reynolds number. However the main issue is, that skin friction is not very important for the golf ball, but it is very important for boats. Dimples INCREASE skin friction, but by doing so, in a very special case of a ball at suitable Reynolds number, they reduce the pressure drag (form drag) much more.

In a case of a boat pressure drag is very low or directly related to wave pattern (=residuary drag, wave making drag) or lift (planning hulls), thus there is nothing that could be reduced in a similar fashion. And the skin friction is a major part of resistance, thus you really don't want to increase that.

Actually the part of the hull where flow is still laminar does not have to be smoother than rest of the hull. A laminar flow "doesn't see" small dimples and bumps any more than a turbulent flow does. As the imperfections grow the transition from laminar to turbulent happens earlier at some point and also the skin friction of the turbulent section increases, when the surface is no longer "hydraulically smooth". This limit for the size of the dimples and bumps can actually be lower for the turbulent area than for the laminar area.

It is not a good idea to ignore the smoothness of the non-laminar area, since it is very well known how roughness adds resistance in turbulent flow (e.g. pipe flows and flow over a flat plate).

Joakim

 

A bit of a side question, I've never really thought about designing a fast boat so I've never considered laminar flow in water.

1. since water does not compress, might laminar flow be easier to achieve than in air? You wouldn't get the annoying rebound effect of air compressing. Could also be the opposite I guess.

2. it seems like dimples on a golf ball are dimples for the fact that the ball has no front, and might spin any direction. On a boat you might use either small ribs, or long gouges.

3. The idea of a some spoilers on cars is to tumble the air over the back, reducing the same large void behind the vehicle. You might infer that small turbulent eddies allow more time for surrounding fluid to fill the void. (many small eddies are better than one big eddy)
- not that it would speed you up enough to be worth the work, but maybe more from curiosities standpoint, do you think you could reduce the drag, not by dimpling the entire underside of the boat, but by watching where the turbulence starts as you move your boat through the water, and applying ridges there, or slightly before.

Of course the turbulent points on a boat change depending on water conditions, speed of boat, and of course pitching side to side...not to mention who knows what's going on along your keel.

I understand joakim's point of resistance in turbulent flow, but there is something to be said for calculated (if you can) placement of turbulence control.

 

Hi FD,

Compressibility in air is not a significant factor at low Mach numbers. At the speeds that cars, boats and most light aircraft travel, it is common to model air as being incompressible. This approximation breaks down when you start dealing with commercial airliners, 500km/h bullet trains, and other bodies travelling at significant fractions of the speed of sound.

The dimples on the golf ball serve to force a transition to turbulence, thus keeping the flow attached farther back on the ball, thus reducing the pressure drag. This works only in the very narrow range of Reynolds number in which a golf ball operates; you accept a small increase in viscous drag as a tradeoff for a substantially larger decrease in pressure drag. For streamlined bodies, or for anything outside of that small Re range, the advantages are lost.

Some of the crazy jet-skiers have been known to put longitudinal gouges in their hulls using 20-grit sandpaper. The idea here is the same as with the seaplane's rivets- the resistance is probably increased somewhat, but it tends to "un-stick" the hull from the water, making it easier to jump waves.

I very much doubt that any of this turbulence-inducing stuff would be of any benefit on a normal planing or displacement hull, that stays in constant contact with the water. Still, I'd love to see some experimental data, if anyone has an old power skiff they feel like butchering....

 

Joakim, I wasn´t suggesting ignoring smoothness in non-laminar areas - just that going all the way to 600-1000 grit and burnish, like a lot of small racing sailboats do, is probably moot. So, your belief is that a very smooth surface in, let´s say, the front third of that J21 won´t keep the flow laminar longer, thereby increasing speed? Or that the ridges of 200 grit in the non-laminar aft two-thirds are not smooth enough to stay out of said flow? My impression has been that the non-laminar flow is actually quite thick - on the order of milimeters at the minimum and therefore wouldn´t know 200 grit scritches from 2000. The only reason that I would have recommended going further than roller-stipple is for confidence, headgames against opponents, the chance of knocking off really big stuff (runs, drips), and to learn the surface of the boat. Am I lost here? Kiitos - Mark

 

In the laminar region roughness does not increase drag, but it may cause an early transition to turbulent region. The roughness size that effects transition can be estimated with k=120*nu/V (SI units), thus for water (nu=1e-6 m^2/s) and 6 kn (3 m/s) speed k=40 um.

For the turbulent region clear equations for the effect of roughness are available. The smoothness required for "hydraulically smooth" (=no effect compared to totally smooth) depends mainly on speed. For a 6 kn speed the roughness allowed is about 25 um.

These roughnesses are defined as grain sizes. You can see the grain sizes of different sandpapers from here: http://www.cs.rochester.edu/u/roche/rec.wood.misc/grit.sizes.html

The AC-boats where finished all the way with P600 (25 um grain) and models to be towed in a towing tank are finished with P300 or P400. I think P400 (grain size 35 um, BUT will result to a much smoother surface) is certainly enough for most keelboats. I don't see any sense in using a finer sandpaper for any region, since a well finished P400 sanding is already laborious and very smooth.

If your boat is much faster (say 15 kn), you might want to go for clearly finer sandpapers.

Joakim

 

You need RANDOM PROTRUSIONS on the surface, to break up the eddies before they get too large to disturb laminar flow. See my earlier post. I suspect the protrusions need to be large enough to be rather unsightly.