water depth and boat speed

[Raf Wyatt]

http://www.bclocalnews.com/news/126340103.html

I came across this article but apart from vague memories of bow waves
and reflections off the bottom and my own rowing feelings don't have
the physics to explain it. Anyone else out there willing and able -
Carl, Henry???

[Carl Douglas]

Yes, boat speed can be severely limited by shallow water. There are
various effects:

1. The proximity of the bottom in shallows increases the shear gradient
in the water under the boat, which augments skin friction.

2. There will be an increased "squat" - due to the same sort of ground
effect which F1 & other race cars try to encourage by bringing the
underside of the car close to the road. The fluid pressure falls in the
fluid layer between a fast-moving object & a fixed surface, & this sucks
the boat down in the water. Large vessels experience similar effects in
shallow water, even necessitating speed reductions to prevent grounding,
while vessels passing close to a wall may be drawn onto that wall for
the same reason.

3. Every moving boat generates a wave train - the bow wave bounces up
again near the stern & you see further rebounds as transverse waves at
regular intervals further astern. The distance between wave peaks
increases as the velocity rises & it is desirable to tune length to
expected boat speed so that the first rebound is somewhere near the
stern. In shallow waters, waves get steeper & shorter (very obvious in
the case of a tsunami approaching shore), & lose energy more rapidly -
both reasons to make you go slower in shallows.

These effects can become very noticeable, which is why variations in
depth along & across rowing courses are undesirable.

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)

[William Clark]

In article <9ae33f...@mid.individual.net>,
Carl Douglas <ca...@carldouglas.co.uk> wrote:

Same reasons that pools for competitive swimming are at least 6ft deep.s

[SingleMinded]

On Aug 10, 2:49 pm, William Clark <cl...@nospam.matsceng.ohio-
state.edu> wrote:
> In article <9ae33fFa9...@mid.individual.net>,

Minus one of them of course (turns).

[Mike De Petris]

On Wednesday, August 10, 2011 2:52:55 AM UTC+2, Carl Douglas wrote:
> These effects can become very noticeable, which is why variations in
> depth along & across rowing courses are undesirable.

what depth can the limit to consider it to have no effects on rowing boats ? Is it something like swimming pools (6 feet) or more, being the boat faster and bigger? Is there a table where we can read seconds lost for each km for each boat class at race speed for different water depths?

[Carl Douglas]

On 11/08/2011 08:59, Mike De Petris wrote:
> On Wednesday, August 10, 2011 2:52:55 AM UTC+2, Carl Douglas wrote:
>> These effects can become very noticeable, which is why variations in

>> depth along& across rowing courses are undesirable.

>
> what depth can the limit to consider it to have no effects on rowing boats ? Is it something like swimming pools (6 feet) or more, being the boat faster and bigger? Is there a table where we can read seconds lost for each km for each boat class at race speed for different water depths?

As with all continuous phenomena there is no cut-off point - depth
continues to influence wave drag but with rapidly reducing effect as
depth increases.

Just to complicate life, there may be intermediate sweet points in the
depth/length/speed relationship for wave drag, just as there are in the
length/speed wave-drag relationship. And then there's the
under-explored relationship between induced drag, length & rating - it
being likely that there will be sweet spots there too.

Best get in that boat & row hard (& well), but avoid the real shallows
where course limitations allow this.

The Serpentine lake in Hyde Park, London:
http://en.wikipedia.org/wiki/Serpentine_%28lake%29
was used for rowing races for a few years, & one of the 4 lanes had a
notoriously slow section where you passed over a shallow patch. Last
weekend triathlons were held in London with the swimming in the
Serpentine - I wonder how swimmers on that side of the lake fared?

[sully]

On Aug 11, 4:24 am, Carl Douglas <c...@carldouglas.co.uk> wrote:
> On 11/08/2011 08:59, Mike De Petris wrote:
>
> > On Wednesday, August 10, 2011 2:52:55 AM UTC+2, Carl Douglas wrote:
> >> These effects can become very noticeable, which is why variations in
> >> depth along&  across rowing courses are undesirable.
>
> > what depth can the limit to consider it to have no effects on rowing boats ? Is it something like swimming pools (6 feet) or more, being the boat faster and bigger? Is there a table where we can read seconds lost for each km for each boat class at race speed for different water depths?
>
> As with all continuous phenomena there is no cut-off point - depth
> continues to influence wave drag but with rapidly reducing effect as
> depth increases.

I've yet to see the course used for the Boat Race, but isn't this one
of the major challenges
of coxing on that stretch? You can take a tighter line on a curve,
but hit shallow area that
causes more drag, slowing you more than if you'd taken deeper water
with a longer distance.

[Carl Douglas]

You'll need to talk to Alistair about steering the Boat Race. He's the
expert on that black art.

The race is run on the flood tide along a bendy course with shelving
beaches to a large extent under water. Imagine the complexity of the flows!

[Carl Douglas]

On 26/08/2011 11:03, JimBob wrote:

>> Email: c...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
>> URLs: www.carldouglas.co.uk(boats)&www.aerowing.co.uk(riggers)- Hide quoted text -
>>
>> - Show quoted text -
>
> The other consideration associated water conditions apart from depth -
> even on a straight, closed course with all lanes of equal depth - is
> any cross wind. I note cross wind as if it were a pure head/tail wind
> it would affect all crews equally, for better or worse.
>
> Even the lightest of cross-winds will disrupt the 'top layer' of the
> water. How deep is this top layer? it depends on many things but 30 -
> 70 cm is a good approximation.

You'll always get shear in surface water due to wind, just as you get
shear in wind itself. In air the shear layer will be many tens of
metres high. The shear layer in water is effectively limited only by the
bed of river or lake, the width & the resulting recirculation patterns
in the body of water (it must back-flow below or around the volumes
which are positively sheared, but how deep into the water the detectable
shear layer goes must also depend on wind speed, wind fetch & duration
of that wind.

Vespolli cited their 1991 HM8+ "D"
> shell to have a thull value of 18.41cm. The loss in blade efficiency
> in light conditions is nominal.
>
> Carl has clearly addressed wave drag and squat. The other two
> inefficiencies that I would be interested to hear your thoughts on is
> wave drag caused by hull deformation ( or is that the same thing; I do
> apologise), and the other in a stronger cross-wind is the the angle at
> which the boats run up the course relative to the course itself. A
> left to right cross wind as you know will push the stern to the left.
> Which perhaps requires a new thread; The Position& shape of the fin/
> rudder.

Aha! Which is why we developed the AeRowFin steering foil. Another
story for another time maybe?

[stan...@gmail.com]

Apologies for the threadromancy.

Having moved form the Tideway to Dorset I'm now sculling in Christchurch Harbour where depths vary from 0 to 1.8m and based on outing times are usually about 1m deep in the channel

Would anybody like to ball park what this would to a predicted training split (other than make it a lot slower)

Specifically I'm thinking of me (M lwt sculler in a boat 8m long).

Thank you

stan

[Leo]

Wave resistance depends on the depth-based Froude number,
F = U/sqrt(g*h), where U is boat speed, g = gravitational
acceleration, and h is water depth.

For F < 1 wave resistance increases quite dramatically as
F approaches 1. It is higher than wave resistance in deep
water.
Wave resistance then begins to decrease for F > 1, and it
can be lower than the wave resistance in deep water.

There might also be effects on the boundary layer if the
water is very shallow.

See Figure 32, page 36 of the FIRM manual for how depth
affects an 8+ and some discussion on page 34.
You can also change the water depth for the LM1x example
in FIRM and compare the results to the deep water case.

If you want some mathematics and pretty pictures see:
E.O. Tuck, D.C. Scullen and L. Lazauskas
"SWPE report Part 4: Extension to multihulls and finite depth"
http://www.cyberiad.net/library/pdf/tsl00b.pdf

and

http://www.cyberiad.net/wakefd.htm

[James HS]

Surely though in your move from tideway to bay depth is not the only variable change - apart from the fact that the tideway has a huge variety of depths (due to schoals and other underwater features that become all to evident at low tide) it also has a hugely variable flow rate and even if you reduce that effect with an impeller you have a very turbulent piece of water that you are floating on, and often the very direct effect of wind.

So IMHO there would be so many factors to separate out that 'noise' would probably throw them off.

If you have noticed a change in your splits and are using an impeller with a measurement system that you have calibrated, then it is probably worth notionally cosidering wind, tide - salinity of the water? and the huge effects of rowing without the stream (I always have to do adjustments to technique going from upstream to downstream with a slightly different blade entry technique :)

So I would hate to predict which way it would go as I think they are apples and oranges :)

James

[stan...@gmail.com]

Interestingly from rowing on the Tideway for 8 years I had a pretty good idea of what the numbers should be- which bore little resemblance to actual 2k speed for what was good or bad.
I managed a 1:25 for about 500m a couple of times due to a fast tail wind and ebbing winter flood tide.

Here the splits seem to bear little relationship to the previous ones, I think it's probably a combination of higher salinity, a lot of sediment and water depth, combined with considerably lower flow rates than the tideways tides as they are in general a lot slower.

Thanks to you both though, especially for the maths- which I'm playing with now

s

[James HS]

with an impeller or gps?

I think a tide is going to be giving you 4 metres a second and my current reasonably competitive quad has the fastest speed at 1:38/500 - so either you have been hiding your light under a bushell or have been using gps and are now getting nearer to actual water speed as opposed to the helping hand of mother Thames!

James

[stan...@gmail.com]

I used both as a result of jumping between various different hulls and yes obviously the speeds at the top end were certainly GPS and VERY tide assisted.
conversely into the stream and wind splits >3mins could reflect a really good outing.
I'm finding in Christchurch that all splits are slower regardless of stream/tide direction and whether on GPS or impeller. :-(

[carl]

On 21/05/2015 10:36, James HS wrote:
> I always have to do adjustments to technique going from upstream to downstream with a slightly different blade entry technique

James, that just caught my notice and I wondered what changes you'd need
to make?

While one may perceive oneself to be moving faster with the stream, that
really ain't so. The water moves as a body & you are floating in that
body of water, so all that should matter to you is your speed within
that body. In the first analysis the only way that can be greater
downstream is if there's a following wind or a very marked steam surface
gradient.

Of course, there are other differences which depend on stream depth &
width, due to the water's internal velocity gradients (shear). Thus it
is always harder to stay in the stream when going downstream & harder to
keep out of it going upstream. That's because, if you are slap in the
middle of the fastest flow then the flows on either side of you will be
a little bit slower then that directly under your boat, so if you go
slightly off-centre the blade that moves closer to the mid-stream will
stay immersed a little longer & pull you away from the centre. And when
rowing up-stream near the bank the blade nearer the mid-stream will be
in the faster water & have a shorter stroke duration.

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: tinyurl.com/2tqujf
Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

[James HS]

Carl,

It is a wierd one - but I have often preferred going against the stream because I find it easier to get connected earlier in the catch sequence, whereas with the stream I always felt like I had less connection in the early phases of the catch sequence.

The difference was a bit like the difference I feel between a fixed head erg and a dynamic erg - the dynamic needs a much faster leg drive to get connected and put the pressure on.

When I spin round from an against the tide session to go with the tide there is an an adjustment to make to turn what was a sluggish pattern into a more sprightly pattern.

A technique I have recently been coached to achieve is a fractional' digging of the tip into the water while the blade is still travelling bowwards. While I have been given all sorts of mumbo jumbo as to why this works (from a pseudo physics point of view) what it actually does is ensure that the blade is inserted and ready to load as the last part of the recovery and gives me the same thing to push against as I experience while rowing against the tide.

If I do the technique while I am rowing against the tide I am slightly more punishes for it as I guess the timing of it is more critical.

As most of my racing is with the tide and most of my warm ups are against the tide, I have noticed and learned to counteract the sudden loss of connection by ensuring I am way more 'on' the blade entry.

For the first time I also tried it at Nottingham last weekend on a lake (a rather windy lake) and it rewarded my quad with a win! - though I can't guarantee that was the only contribution, as just holding over white tops was half the game :)

But I notice the difference a lot on the tideway - and the secondary factor is that I am also wanting to go faster with the stream and so perhaps am feeling it even more!

Hope that garbled mess makes sense!


James

[carl]

James - it's fascinating!

I just wonder if, with eyes closed & in still-air (or air-speed = stream
speed), you'd be able to tell the difference? We're so susceptible to
visual inputs that perception of stream movement, together with a
generally-held (but I believe unfounded) view that stream really does
affect stroke action, may mislead you.

Of course, I could be quite wrong & I'd welcome all explanations as to
why that might be so.

What further caught my eye was your comment on fixed vs static ergs.

On a static erg you must load up the stretcher before the catch, just to
absorb the sternwards kinetic energy of your body before you can then
begin the next stroke. So, quite unlike in the boat, your legs are
pre-loaded even before the stroke begins. And that leads to the popular
but probably misguided coaching mantra: "take the catch with the legs!"
Which may explain why so many crews now look as if they are still on
their ergs.

On dynamic ergs there can be no load on the feet before you load the
handle - just as in the boat. So if you're accustomed to static erging
you'll be looking in vain for that firm plant of feet on stretcher with
which to start the next stroke - & it just ain't there. That leads the
static erg user to drive even faster with the legs in attempting to get
that leg loading at the catch. Which takes me back to the last sentence
of the previous paragraph.

Cheers -
Carl

PS As you were a 2012 volunteer, James, have you chanced to read Andrew
Zimbalist's Circus Maximus? I have yet to do so, but this Guardian
review provides a disturbing precis:
http://www.theguardian.com/books/2015/may/22/circus-maximus-andrew-zimbalist-review
C

[John Greenly]

As Carl says- this is fascinating!

I was out rowing this morning on still water with a very gentle, perhaps 3-5 mph breeze. I started out with the wind, and when I turned around against the wind I found as I always do that the catch felt heavier against the wind- more instantly connected, I think would be the way you would say it? I think the reason is simply that the added air resistance means that I slow down more during the recovery when I am going against the wind. If my action at the catch is unchanged then I will naturally feel a heavier and quicker connection when my speed through the water is lower at the catch.

It's always been interesting to me that this is noticeable with even a very light breeze. But remember that if the breeze is, say, 4 mph and I am going 8 mph through the water, then the net headwind speed on my body is 4 mph downwind and 12 mph upwind, and if wind drag force goes very roughly like the square of speed, then there will be 9 times the wind drag force going against even this very light breeze, enough, apparently, to slow me down noticeably during the recovery. The relative importance of wind vs water drag is big for me compared with a heavyweight rower. I am a very lightweight so less water drag, but I'm fairly tall, so probably about the same air drag. I hate headwinds!!

So, the question came to me this morning as I was rowing- is it possible that there is a bit of breeze associated with the tide in your rowing place that could be having this effect? In your case that would mean a breeze blowing in the same direction as, and at least a bit faster than, the tidal flow, so that going against the stream you'd have more headwind than going with the stream.

Your last sentence- if you are indeed going faster through the water, for whatever reason, when you go downstream, then this effect of boat speed at the catch would be happening.

Cheers,
John

[James HS]

Carl,

I take your point about telling the difference - and I think much of the difference in feel is described by John - but rather than wind it is tide - as there must be an appreciable drag difference (with a typical flow rate of 4 knots approx 2 m/sec )

Where I row from (Putney Town) I typically warm up on the outing against the tide and then spin to do the work portion of the outing when I am going to be aiming to get better splits (depending on what I am doing).

If I do not prepare then the 'change' feels really strange - and to prepare what I do is the first 6 inches of the drive - to adjust to the fact that I am not pushing against a "resistance" and adjust my grip.

I had not accounted on the dynamic for the body coming forward which you describe well, but I had always attributed it to not having to counter the inertia of pushing the body mass back, rather than the 14Kg footplate which can just dissapear from under you. (I mainly train on a dynamic) and this is what the turn on the tide suddenly feels like.

Fortunately, on a multilane the experience feels a bit like a mix of the two 'water' conditions, so then we only have the wind to deal with!

John - the wind is definitely there - and also tends to be a westerly (so on my back) but as an example of how much the river bends where I row, the first 2K is north west, then a bend and south west, then a bend and west, and then a long bend to go south! (reverse on the way back) and so the wind is not only changing, bu reacting to the tide to give interesting and variable water conditions.

While I think that wind generally has more effect than tide on this stretch of river they often combine :)

So we have a tide that can be in the order of 2 m/s and wind that is typically at your stated 8mph (3.6 m/s) and then my warm up boatspeed at about 3.2 m/s ..... then I turn around and it feels different - no wonder!


James

[John Greenly]

Ah, this is great! I looked at your stretch of river on Google maps. Beautiful, and very twisty!
Using "street view" I looked at several places, and in a view from near the Kew Rd. bridge there are some single scullers on the water! You are so lucky to have such a wonderful place to row!!

Cheers,
John

It would be fun to have other people post the location of their rowing waters so we could look.

Mine is Cayuga Lake: look up Myers Point, Lansing, NY, that's where I row from. No street view available, but you can look at the satellite view. There's no dock, just wade in on whichever side of the point is in the lee.

VERY different from your beautiful Thames - wide open, almost 40 miles long, and often whitecaps and 2ft seas, but also often enough flat water if you choose your time right.

[stan...@gmail.com]

> Mine is Cayuga Lake: look up Myers Point, Lansing, NY, that's where I row from. No street view available, but you can look at the satellite view. There's no dock, just wade in on whichever side of the point is in the lee.
>
> VERY different from your beautiful Thames - wide open, almost 40 miles long, and often whitecaps and 2ft seas, but also often enough flat water if you choose your time right.

As featured in Assault on Lake Cassitas.
do you ever visit the famous "storm channel"

I used to row where James still does but now row here:
https://www.google.co.uk/maps/@50.7254535,-1.7578777,15z
you can row on the Stour from the A35 Iford Bridge down and out to sea- and the costal boats frequently do. The top 2k is a bit windy though, the section from tuckton bridge to blackberry point is usually ok though- providing there is enough water- there was not enough water to bury a cleaver outside of the channel on Monday morning!!!

[John Greenly]

I have rowed on the Cayuga inlet- AKA storm channel- it's a 20 km round-trip from where I launch on the lake, and you can stop off and get good snacks at the Ithaca Farmer's Market down there by the water. There's a straight section, great for intensive sprint training as Brad Lewis described. Here it is:

https://www.google.com/maps/place/Cayuga+Inlet,+Ithaca,+NY+14850/@42.4595166,-76.5121633,14z/data=!4m2!3m1!1s0x89d083d0e92bd69d:0x6a2d77d2969f40d9

Your Stour looks wonderful! Draining and filling Christchurch Harbour must generate a ferocious current at that narrow entrance when the tide runs! What is the tidal range? Reminds me of a harbor in Maine called Biddeford Pool- a large area goes dry at low tide except for some deeper channels. Tide range there is about 4m and the current generates horrifying standing waves in the entrance channel, impossible to enter or leave except at slack tide for woefully underpowered sailboats like the one I used to cruise in, and you had to be very careful where you anchored a deep keel boat so as not to end up careened on your side in the mud at low tide....

--John

[andymck...@gmail.com]

Goring Gap row on this section of the Thames

https://www.google.co.uk/maps/place/Basildon+Park/@51.488352,-1.0629759,2308m/data=!3m1!1e3!4m2!3m1!1s0x0:0x421370a5d76a535d!6m1!1e1

Generally the most waves we have to complain about are the ripples from a duck splashing down, although in a good winter flood our facilities, such as they are, can be under 60 centimetres or more of water!

Andy

[James HS]

Holy moly - that lake looks amazing!

Street view recently was introduced to the middle of the thames - amazing!

My club is Putney town rowing club (google that) and you can see the huge concrete construction I built to get river access!

Also - bear in mind this is tidal - just about 5 metres of height change twice a day, so you are seeing it at high tide - and when ti is low tide you can almost wade across bits of it with large areas of stony shoreline!

James

[John Greenly]

Yes, the "street view" on the Thames is great fun! 5 m tide range- wow, now I understand your stairs that disappear down into the river. How do you even manage to get into a boat with the water tearing by at 2 m/s?

You can see that I don't have to worry much about careful steering where I row- just have to avoid hitting slow sailboats (and being run down by fast motorboats). I would not be competent to row on the Thames with all the bends, bridges, etc to hit- and that very fast current too- all that would be a whole new world to me! The rivers I have rowed on here, including the Connecticut river in our Putney- Putney, Vermont- are quite placid and easy by comparison, excepting the mighty Hudson around the famous cliffs that the early explorers called the "end of the world", because large sailing ships couldn't make the turns, above West Point- I've been swamped by some interesting, fast and turbulent water there.

--John

[Chris A]

On Wednesday, August 10, 2011 at 1:52:55 AM UTC+1, Carl Douglas wrote:

>
> These effects can become very noticeable, which is why variations in
> depth along & across rowing courses are undesirable.
>
> Cheers -
> Carl

Carl FISA prescribes that where the depth is less than 3.5m it should be uniform. Is that deep enough?

[John Greenly]

Wow, another glorious stretch of river! Zooming in, I can see a double scull followed I think by a catamaran coaching launch, and also numerous motor vessels, some making wakes that look much bigger than duck-splashes! Do boats slow down for rowing shells, or do you have a lot of wakes to deal with?

By the way, I once rowed by mistake right into a flock of a few thousand diving ducks that spend the winter on the lake. The ensuing mass panic and takeoff was quite overwhelming. Sometimes we have had 50,000 snow geese in one giant raft- that would be downright dangerous to row into. Don't underestimate the hazards of ducks!!

Cheers,
John

sorry- I got us way off-topic here. maybe we should move this rowing waters discussion to a new thread?

[carl]

Chris -
While I will defer to Leo on this, I think 3.5m is effectively deep
water for us and that depth effects would start to creep in for rowing
shells when depths are less than 3m.

However, I do wonder at the concern over the influence of variable
depths in what are mostly man-made courses as these effects are likely
to generate less unfairness in racing that the often severe variations
in wind force across 8 lanes.

It seems to me, therefore, that a wider recognition of the fundamental
unfairness of most multilane courses, except in still weather, is being
papered over with money spent on researching & eliminating a rather
second-order source of competitive inequity.

Cheers -
Carl

[James HS]

John -

Speeds are a) way less at the bank and b) those speeds are middle stream and show why steering in river races is so important and c) you point your bows against the stream and then it is fine - though once you have pushed off you are at the mercy of stream and wind while you stow your wellies :)

You would love the tideway - we are actually part of the sea - so the COLREGS apply to this stretch of river, but there are local bylaws that allow rowers to 'row the slacks' - i.e. against the tide on the inside of any bend - but guess what a) it bends all over the place (so you cross the river to go from one inside to another) and the tide changes twice a day, and so, therefore, does the direction of travel in these bends. This leads to there being three lanes - 2 COLREGS ones that all river users should operate, and then a varying rowing lane that changes direction twice a day - plus heaps of bridges that have arches that are navigable at different tide levels and not at others ..... so navigation is both tricky, and in my club subject to theory and practical navigation tests before being granted!

Adds to the fun!

James