masters and displacement of single
Richard
they don't generate as many watts per lb or kg of bodyweight as do
elite rowers. Some theory of the drag of the shell vs the length vs
the productive/power producing body weight.
I had, and still do unless corrected, believe it was strictly a
displacement issue as these aren't planing hulls.
I know Carl is the master of all this knowledge, so I put the question
out there. Are those of us of lesser wattage and greater percent body
fat under boated based on a strict weight measure?
Carl Douglas
Under-boated or over-boated? It's a terminology issue, a bit like that
between those who talk about things which can't be underestimated or
can't be overestimated. But in that case we usually know what they mean ;)
Hull drag comes from several sources. Principally you have:
1. Skin friction. Water is viscous. The molecules directly against the
hull move at boat speed while those at a distance (which increases from
nil at the bows to something you can see at the stern) are as yet
undisturbed), so there is shear across this "boundary layer", That
shearing requires an applied force which, because the boat is moving &
work is thus being done, absorbs energy & is felt as drag.
2. Form drag. It requires an input of work to make the waters part
(unless your name is Moses, I suppose?) around the boat because for the
imaginary stream-tubes of fluid to part & occupy a reduced cross-section
as they squeeze past the hull they have to accelerate. As this flow
passes to the stern the boat narrows, occupies a lesser cross-section &
the stream-tubes expand & slow down a bit, involving a partial recovery
of energy or work. The difference between the work input & returned
energy is another charge on the sculler, again measured as drag.
3. Wave drag. The water's surface is held flat by gravity, which is not
a very powerful force. If you push a boat through the water, that moves
the water around & changes its internal pressure ahead of & around the
boat. Under quite small changes in velocity & internal pressure,
because gravity is relatively weak, you get significant changes in
surface level. What goes down or up must bounce back, but a lot of the
energy diverted into moving those surface levels around is not
recoverable as those wave patterns propagate out to infinity. And there
is another important aspect to wave drag - your bow wave. A transverse
wave patter is generated as well as the v-shaped, sideways-moving wave
the ripples off the bow, & this transverse wave bounces back up near the
stern, & again at a similar distance beyond that, & so on, creating a
swell in your wake. Simply put, if that wave bounces back up under your
stern, the boat runs on a level fore-aft trim, but if the wave rises
ahead or astern of a certain point on the boat, you end up sculling a
bit uphill into the bow wave. Getting the hull length & shape right for
the speed potential of the sculler should be part of the designer's art.
The consequence of all this is that design is a compromise over many
factors. If the designer understands the compromises he/she must make,
there's a good chance of getting an effective boat. If not ....
Unfortunately, rowers understand little of this & care less - it's all
too complex for folk who are always a but tired from their training, &
too complex for those who like to spout opinion without having insight.
But don't take me there ;)
Then there's the problem of manufacturers who do not give you the range
of correctly-scaled (for power & weight) hull sizes that they may claim
to offer ....
So, while it may be appealing to suggest that an older, weaker (or
younger, weaker) sculler should have a boat either larger or smaller
than indicated for their weight, to do so would be too simplistic.
HTH?
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Jake
I thought rowing shells sank further in the water the faster they were
rowed, all other things being equal? So by way of gross simplification
a displacement hull digs a bigger hole for itself as it goes faster. I
thought I found this confirmed when I used to race coastal sculls. In
the fastest part of a racing start, or in very short flat out sprints
for a couple of strokes when the speedcoach was reading 10 or 11
knots, the bottom stays of my riggers would hit the water and throw up
splashes when in slower rowing the boat would float much higher. That
was one of the signs I looked for to see I'd got off the start well. I
may have got this wrong and it might have been an increased bow wave
hitting the riggers or something, but it always felt like the boat was
sinking further in the water in a sprint. I think this dropping in the
water at speed is more noticable an effect in a coastal scull than a
river scull due to the wider hull and shorter length, though I expect
it does occur in a river boat to a limited degree.
So on the basis of this, the faster your race time, the more bouyancy
you would need, so if anything a less powerful but equally heavy rower
would find themselves overboated, though due to the relatively small
speed range of a rowing shell it's a pretty marginal effect I would
think. I guess this is why I agree with your former view rather than
your friends'. Because the speed range is quite small and the hull a
displacement design any effect of boatspeed will be so small you can
consider static displacement as the only figure of major importance,
though I do believe the relationship of speed to displacement works
the other way round than your friend has intimated. I understand the
bulk of literature describing the performance of displacement hulls
mainly relates to relatively shorter fatter hulls such as fishing
boats, racing yachts etc. so any effects are much less pronounced in
our much finer hulls. If rowing shells had planing hulls it would all
work the other way round (once planing was achieved) and be much more
pronounced. In windsurfing, the more wind there is and the faster you
are going to go the smaller the board needs to be.
By way of a digression, In wider displacement craft, the raising of
the waterline underway is a more well known phenomenon. At the end of
WWII when the allies took posession of a lot of racing yachts from the
defeated axis powers the navy towed them across the channel to be sold
in the UK, towing them behind destroyers etc. Some of these so called
'windfall yachts' were lost due to towing too fast. If you tow a 50
foot displacement racing yacht who's hull speed is 9 or 10 knots and
have sufficient power to tow it at 25 or 30 knots it creates itself a
hole large enough to sink in, or a bow wave large enough to overwhelm
it which is what happened to a few.
ben
Hmmm...In my experience, the older you get the more water your single
will displace. It has nothing to do with the drag, length, speed or
any kind of ratio other than waste:hight.
Carl Douglas
The other rule is: The older you are, the faster you were.
C