Q: how to reduce drag between hull and H2O
william beery
From my sailing days, we wet-sanded our hulls to allow water
molecules to attatch more easily to the hull and to keep them there.
The theory was that the water passes more easily over other water
molecules that over the hull. It was thought that waxing the hull was
definitely the wrong thing to do, as the water beaded. By this, I
mean that the water didn't want to bond to the hull, so that there was
water passing over the waxed hull. Air bubbles were also supposed to
stick to the waxed hull more easily. We used a very fine grit of
sandpaper, and always sanded in the direction of the water flow (we
even used a board to keep the most even pressure possible).
My question is this. A new shell has the brand new gelcoat
still on it (or is it an epoxy coat?) and in good condition.
Supposedly the gelcoat has a much finer surface than our 1000 grit
sandpaper can get, so it would be detrimental to sand it. But, what
about an older shell? Say, one that is about 4+ years old? Does
anyone think that it would be advantageous to sand it for the reasons
above? For that matter, is it even legal to wet-sand a shell's hull
if you want to race it?
I know that there was a thread last year on some tape (ribbed
tape?) by 3M that was supposed to do the same thing for hulls. Is
this legal too? Thanks.
William V. Beery '98
Skidmore Crew
Skidmore College
Saratoga Springs, NY 12866-1632
First Conferences
From my sailing days what we were trying to achive was a surface as
smooth as possible in order to achive this it was out with the wet
and dry paper when you finished with the paper you got out a cutting
paste and polished away with this - note this would only apply to
dry sailed (ie bosts that were hoisted out in between races dinghys
12 metres that kind of thing wet sailed boats needed antifoul so
they didn't get the polish treatment.) The whole purpose in this was
to encourage the water molecules flow to remain laminar - any
roughness would encourage turbulent flow which was regarded as a bad
thing.
Conversley, Many belive that when you left venturi self bailers down
the air they would suck in would spread out over the hull as bubbles
and reduce drag in the boundry layer - I'm not really sure if it
made all that much difference - probably not, but it felt fast
listening to the bailers slurping away. The point about riblets was
that they would reduce boundry layer drag - the problem with them is
they must be alined exactly with the flow if not they do more harm
than good.
In conculsion, I don't think there's any rule against fine sanding
your hull but I reckon you'd be better off just going for a smooth a
finish as possible - having said all that the boat speed improvment
you would get would be so small was to be useless - far better to
work on technique where there are real gains to be made.
Gidz
AK
First Conferences Tel: +44 (0)171 404 0424
85 Clerkenwell Road Fax: +44 (0)171 404 7733
London EC1R 5AR. UK
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David Pratt
a _shape_ as possible (i.e., free of bumps and lateral ridges, which
would contribute to turbulence), all you can do is keep the fine-scale
bumps (grit stuck in varnish, dried bugs from uncovered highway
transport, excess epoxy from repairs, etc.) to a minimum ... doesn't
matter whether the actual surface is varnish, gelcoat or wood, as long as
it's smooth to the touch. Forget "fishscales", long-chain polymers,
snake-oil-based "Gofast" ... they are extremely unlikely to help, and
more than a little likely to slow you down.
Oh yes ... focus attention especially on the first 2-4 m of the hull from
leading edge (water entry) ... try to delay transition from laminar flow
to turbulent flow over the shell skin. After that, the boundary layer
will be turbulent anyway, and supersmooth finish will not help further.
Paul Thomas
>tpf...@dutch.eng (Paul Thomas) wrote:
>>David, does your textbook discuss golf balls?
>Last I checked, Shells neither flew, nor spun while in motion.
For sure, but that's not the point. The dimples on a golf ball cause
turbulent flow where without them it would be (more) laminar. And the golf
ball goes further because of that. A clear case of laminar flow not
always being the best, sort of.
>coefficient of friction is lower water/water than
>water/varnish (Boat), if it is possible to hold a small amount
> of water near the surface of the boat, the hull will slide on
> this layer.
This is not true. The speed tape reduces turbulence in the boundary
layer, which in turn makes the boundary layer smaller. For rowing
shells, which are long and thin, laminar flow is what's required for
minimal drag. That's why we don't put dimples on the shell surface.
It is very misleading to envisage a `water layer' near the hull which
somehow has different properties to the rest of the water.
Paul.
-
paul....@eng.ox.ac.uk
Jeff Houser
>This is not true. The speed tape reduces turbulence in the boundary
>layer, which in turn makes the boundary layer smaller. For rowing
And how do you think the speed tape reduces the turbulent
flow?
Magic?
>It is very misleading to envisage a `water layer' near the hull which
>somehow has different properties to the rest of the water.
Tough to understand? Maybe. Misleading? No!
You seem to be denying that friction differs as substrates change.
Ever heard of longchain hydrophillic polymers?
There are products available which do exactly what you are mislead by.
Jeff H
Paul Thomas
>tpf...@dutch.eng (Paul Thomas) wrote:
>>somehow has different properties to the rest of the water.
>
>Tough to understand? Maybe. Misleading? No!
Oh yes!
>You seem to be denying that friction differs as substrates change.
>
>Ever heard of longchain hydrophillic polymers?
>There are products available which do exactly what you are mislead by.
Yes, but that is not (in my understanding) how speed tape works. There
is no chemical interaction which changes the natural physical properties
of water. The drag reducing properties come from the physical design
of the tape (ie. tiny grooves running lengthwise down the boat).
If I poured Fairy Liquid on the hull, that *would* be a longchain
hydrophillic dibber which would reduce the viscosity of the water...
Paul.
-
paul....@eng.ox.ac.uk
David Pratt
See Fig. 7.6, p. 403 and surrounding text. A racing shell is more nearly
like a long flat plate than a sphere or cylinder in crossflow. Fig. 7.6
clearly shows the lower Cd for laminar than for turbulent flow.
-- Dave Pratt
David Pratt
Please note that, for a "hydraulically smooth" surface (one which
you and I would agree is smooth to the touch), there is a laminar
sub-layer between the turbulent boundary layer and the wall. I don't
know how riblets work, they're illegal anyway (which suggests they do
work!), but I do know that there is no benefit whatever to roughening the
finish on a smooth shell surface.
Cheers, Dave Pratt
On 9 Mar 1996, Jeff Houser wrote:
> tpf...@dutch.eng (Paul Thomas) wrote:
> >In article <Pine.A32.3.91j.96030...@homer27.u.washington.edu>,
> >David Pratt <dtp...@u.washington.edu> wrote:
> >>Jason, Jason....
> >>
> >>It is not a _MYTH_ that laminar flow will give you the least drag, but
> >>quite the opposite is true: Any textbook of fluid mechanics will show
> >>you quite clearly and unequivocally that laminar flow (when you can get
> >>it) is always preferable to turbulent flow for least drag.
> >>
> >>I don't mean to rag on you about this one, but when you're wrong, you're
> >>WRONG, and I hate to see 180-degree-wrong info propagated.
> >>
> >>If you'd like, I'll cite you book, chapter and verse. Better yet, _you_
> >>pick the book, I'll show you chapter and verse!
> >
> >David, does your textbook discuss golf balls?
> >
> >Paul :-)
> >-
> >paul....@eng.ox.ac.uk
>
>
> Last I checked, Shells neither flew, nor spun while in motion.
>
> However; the roughened surface model does have some
> application here.
>
> True laminar flow is desired, but a properly prepared surface
> can decrease the resistance (surface drag) caused by the
> movement of water across the wetted area. Since the
> coefficient of friction is lower water/water than
> water/varnish (Boat), if it is possible to hold a small amount
> of water near the surface of the boat, the hull will slide on
> this layer. This is why 3m Riblet Tape was banned by the International Rowing body. It is also why surface preparations such as PolyOx are banned (Creates air/water interaction).
>
> Riblet tape is analogous to the groves on a record (for those that remember them), except that the are straight, and much
> finer. Water adheres to the riblet surface, creating a
> boundary layer for the boat to slide through. Some U.S. shells
> used this surface at the 84 Olympics to win medals.
>
> Some boat techs suggest wet sanding the hull to simulate the now banned tape, but the sanding must be exceptionally well controled. A better, and legal solution is to coat the boat with a teflon based "wax". Teflon/water is very slick.
>
>
> BTW surface drag is an important factor in longer hulls. As the hull becomes shorter for a given capacity, wave form drag is more important.
>
>
> My 2.74 CDN cents
>
> In sport,
>
> Jeff H.
>
>
>
>
David Pratt
I did some work with long-chain polymers for drag reduction on
shells many years ago (specifically Polyox, a DuPont polyethelene oxide
resin polymer), and did indeed achieve some drag reduction thereby.
The effect was called the Toms phenomena in the 60's, and no one in the
appropriate scientific/engineering literature doubts that it is/was
effective...that's why the stuff is banned (I think).
I'm not sure I've ever seen a definitive analysis of why it
worked, but it has somehting to do with modifying the turbulkence
s[pectrum. To the best oif my knowledge, no drag below laminar was ever
demostrated, only a reduction in turbulent drag belowe that if the
polymoers were not present ... by the way, in _very_ dilute suspension,
less than 1 part per million. More than that, drag went _up_!!
I have no idea how the rible tapes work ... may well be magic,
for all I know.
-- Cheers, Dave Pratt
On 10 Mar 1996, Jeff Houser wrote:
> tpf...@dutch.eng (Paul Thomas) wrote:
>
> >This is not true. The speed tape reduces turbulence in the boundary
> >layer, which in turn makes the boundary layer smaller. For rowing
>
> And how do you think the speed tape reduces the turbulent
> flow?
>
> Magic?
>
>
>
> >It is very misleading to envisage a `water layer' near the hull which
> >somehow has different properties to the rest of the water.
>
> Tough to understand? Maybe. Misleading? No!
>
> You seem to be denying that friction differs as substrates change.
>
> Ever heard of longchain hydrophillic polymers?
> There are products available which do exactly what you are mislead by.
>
> Jeff H
>
>
>
>
>
>
>
>
Dominick Layfield
> I thought that the riblets energised and sustained a turbulent boundary
> layer (yes a thinner layer) which adhered to the shell well past the
> previous laminar to turbulence point (ie towards the stern, so reducing the
> drag of the shell by delaying separation. This is how it works on Aircraft.
~~~~~~~~~~
This is a crucial point. I don't see how you can have a meaningful
discussion of drag without mentioning separation.
A related matter - why do bow-loading fours go faster than the stern-
loading variety?
XOX DOM
David Pratt
Sorry, I thought was the chat group for rowing, not golf!
CHeers, Dave Pratt
(P.S. Dimpling to reduce drag on golf balls is well-known to reduce _form_
drag, due to reduced size of wake behind the ball, at the expense of
increased _skin friction_ drag. A long, skinny shell has little wake,
and except for a healthy dose of surface wave drag, is overwhelmingly
dragged by skin friction, for which laminar flow is better, if you can
get it! Another famous example is the WWII Mustang "lamiflo" wing, where
flow was kept laminar longer and caused lower drag, hence faster
aiurplane for same power ... but this isn't an airplane chat group, either!)
BBReynolds
the Raritan....
Paul Thomas
>tpf...@dutch.eng (Paul Thomas) wrote:
>>hydrophillic dibber which would reduce the viscosity of the water...
>
It's just a brand of washing up detergent over here.
>The point is in reducing the wetted surface drag. Some liquids(soaps?) do this by reducing surface tension and molecular cohesion of=
> the water.
Sure. The British rules of racing ban this sort of thing, as well as the
speed tape. Rumour has it that one university crew once had a detergent
dispenser on the bow...
>The product I am familiar with works (I believe) by bonding a small amount of
>water to the hull,and altering cohesion. I'll send you
>a sample if you wish. (only one address thanks). This is basically how Rib
Can you send me enough to cover about 10 sqm :-)
Paul.
-
paul....@eng.ox.ac.uk
David Thompson
spending $$$ and time on perfecting the flow of water past the
hull without actually changing the design is a second more over
1500. Spend the time on an erg and gain 2 or 3!