Geoff wrote:> Why do rowing skiffs have a long trailing edge at the stern? The> design is usually almost symetrical to the bow in length and very> similar in shape. This sort of design is not seen in many other marine> applications such as yachting (excluding catamarans). So why in rowing> couldn't a wider (providing more stability), shorter, stubbier stern be> used?>
Regards,>
Geoff.>
Geoff,
It seems you understand intuitively why the bow would be knife-shaped. The stern needs to be shaped similarly, because if its profile changes too quickly, (as in the case of a blunt/wide stern) the flow of water over the hull may separate from the surface. Separated, or detached flow is very inefficient. Separated flow allows for eddy currents which cause water to actually flow forward. This all creates an enormous amount of drag. It's much better to maintain an attached flow with a gradually changing hull profile.
Yachts do not use this, I assume, because they NEED to be very wide. Therefore to maintain attached flow, the stern would be REALLY long, and heavy and expensive, and possibly add more to drag (wetted surface, etc) than the blunt shape. But rowing shells can be (and are) very narrow, so they can maintain a good profile at the stern, without becoming TOO long.
Every boat design is an iterative optimization process, where many different factors result in different hull shapes for different applications.
In message <pxrhe.1624$BF5.787@trndny06>, Kieran <kc_news@sonic.net> writes>Geoff wrote:>> Why do rowing skiffs have a long trailing edge at the stern? The>> design is usually almost symetrical to the bow in length and very>> similar in shape. This sort of design is not seen in many other marine>> applications such as yachting (excluding catamarans). So why in rowing>> couldn't a wider (providing more stability), shorter, stubbier stern be>> used?>> Regards,>> Geoff.>>
Geoff,>
It seems you understand intuitively why the bow would be knife-shaped. >The stern needs to be shaped similarly, because if its profile changes >too quickly, (as in the case of a blunt/wide stern) the flow of water >over the hull may separate from the surface. Separated, or detached >flow is very inefficient. Separated flow allows for eddy currents >which cause water to actually flow forward. This all creates an >enormous amount of drag. It's much better to maintain an attached flow >with a gradually changing hull profile.>
Yachts do not use this, I assume, because they NEED to be very wide. >Therefore to maintain attached flow, the stern would be REALLY long, >and heavy and expensive, and possibly add more to drag (wetted surface, >etc) than the blunt shape. But rowing shells can be (and are) very >narrow, so they can maintain a good profile at the stern, without >becoming TOO long.>
Every boat design is an iterative optimization process, where many >different factors result in different hull shapes for different >applications.>
Hope this helps.>
-Kieran
Good explanation from Kieran. Just a few points to add:
You taper the stern of a boat to allow the water to close back together sweetly into the hole the boat made. In this process you get a degree of pressure recovery, the reverse of the pressure loss on the frontal, expanding section of the boat. Imagine pushing a greased wedge between your squeezed fingers - you have to push to force the wedge in & that's the pressure loss at the bows. Then while you keep squeezing suppose you stop pushing - the wedge is squeezed out & the work done in that process is the pressure recovery. You never get complete pressure recovery, but every little helps. And you can afford a longish stern because on such a long boat the local surface friction per unit area near the stern is much lower than it is further towards the bows
That's fine for rowing shells, & keeping the boat long also helps reduce their tendency to pitch - that little bit of variable displacement a long way from the centre of gravity being a very effective stabilising factor
However, water finds it rather hard to follow a rapidly contracting stern taper - such as you get on most boats which are, relatively, short & fat. On fat, rapidly tapering sterns the main flow becomes "detached" (can't follow the shape) & your pressure recovery is less or lost. That makes it worth reshaping the hull to shorten the stern & make it broader, giving you the chopped off transom stern, but you may still taper up from underneath for pressure recovery, instead of from the sides.
As Kieran says, hull design involves many compromises.
HTH Carl -- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK Email: carl@carldouglas.co.uk Tel: +44(0)1784-456344 Fax: -466550 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Richard Gladwell 15 May 2005 08:10:02 [ permanent link ]
Yacht design is generally subject to some sort of rating or class rule.
If there was no length restriction on yachts, then they would probably have tapered sterns like rowing boats.
Yachts also need hull form stabilty to help offset heeling moment from the rig - hence they are wider than they would otherwise be. The cut-off stern design is trade-off amongst several factors, but is as low drag as possible. Further yachts are often planing hulls rather than rowing skiffs which are displacement hulls.
The nearest comparision is with catamarans - which have much more tapered ends, or the International Canoe - which is sailed by one person on the end of a sliding plank/seat - and those have a tapered stern - similar to a rowing skiff. Catamarans rely on beam fro stability. The Int. Canoes only need for hull form stability is to support the crew between tacks and prove a base for support of the sliding plank.
Where is is no need for form stability the hulls will get narrower and narrower (depending on the class rule). On area to watch will be the foiul-bourne Moths (11ft long) which are very narrow and gain stability when they are foild bourne and it is to their advantage to get flying as quickly as possible. So a low drag hull shape is required - which is now evolving as they can now be got onto foils in s loittle as 5-6kts of breeze.
RG
"JD" <johnndavis@yahoo.com> wrote in message news:1116118021.992469.99700@g14g2000cwa.googlegroups.com...> Don't sailing hulls measurements have to follow complex and rigid> displacement, length or other measurement formulas? I understood that> the chopped off sterns were a way of getting around these formula by> the marine architects & designers.>
Racing shells are not subject to these design limitations and, with the> exception of a couple builders, none have transoms.>
"Geoff" <geoff_durham@hotmail.com> wrote in message news:1116115910.528270.242930@f14g2000cwb.googlegroups.com...> Thanks Kieran and Carl,>
I am still having some issues really understanding this however:>
Kieran, can you please explain your comment "the flow of water over the> hull may separate from the surface" for me please.
Let me take a shot at it. There are 2 types flow of a liquid over a hull. Laminar flow and turbulent flow. Laminar flow is characterised by the absence of a boundary layer. The boundary layer in turbulent flow is fluid moving more slowly than the normal flow of the water past the object. Consider the hull a stationary object and the water moving past it. Laminar flow (very low drag) moves along the hull at the free stream velocity. Turbulent flow moves slower than free stream flow. It can be caused by hull form, surface roughness or a number of other factors. Basically the water slows down because it is "sticking" ( a very un-technical term) to the hull and causing drag at the skin of the vessell. There is another high drag junction between the boundary layer and the laminar flow where those two sheets of water interact. A shell uses a very fine entry and stern exit in an attempt to not "trip" a boundary layer event by causing a local surface pressure spike. The stern shape MUST allow for attached flow to exist to keep hull drag to a minimum. Stern shapes like "IOR sugar scoops meet rating rules but pay large penalties as they do not reattach flow as well as other finer hull forms. I hope this helps.
Let me take a shot at it. There are 2 types flow of a liquid over a hull.> Laminar flow and turbulent flow. Laminar flow is characterised by the> absence of a boundary layer.
Imagine the boat standing still, and the water flowing past the boat at the speed of the boat. (The is the perspective the coxwain has while sitting in the boat.) The boundary layer is the region close to the surface where the velocity of the fluid flowing past the surface goes from zero at the surface to the full boat velocity. If the boat is moving, the boundary layer always exists, and is either laminar, turbulent, or in transition from laminar to turbulent.
The boundary layer in turbulent flow is> fluid moving more slowly than the normal flow of the water past the
object.
Laminar flow in the boundary layer also moves more slowly than the normal flow of water past the object.
Consider the hull a stationary object and the water moving past it.
Laminar> flow> (very low drag) moves along the hull at the free stream velocity.
No, see above.
Turbulent flow> moves slower than free stream flow.
Yes, as does the laminar boundary layer flow. However, the turbulent boundary layer has more energy, so it actually exhibits slightly faster, than laminar, flow nearer the surface.
It can be caused by hull form, surface> roughness> or a number of other factors. Basically the water slows down because it
"sticking"> ( a very un-technical term) to the hull and causing drag at the skin of
vessell.> There is another high drag junction between the boundary layer and the> laminar flow> where those two sheets of water interact.
Again, the boundary layer is either laminar or turbulent or in transition, so this phenomenon doesn't exist. No offense intended.
A shell uses a very fine entry> and stern exit> in an attempt to not "trip" a boundary layer event by causing a local> surface pressure spike.
The "fine" stern exit can reduce the level of the pressure recovery, and , as you say, minimize the pressure-rise-induced transition from laminar to turbulent flow. However, this gradual (shallow) stern closure angle also helps reduce the tendency of the flow to stop following the shell surface, called boundary layer separation. This flow separation is caused by the increase in pressure that the boundary layer flow encounters as it flows past the maximum width of the shell (point of minimum pressure) and moves toward the stern.
If the pressure recovery isn't complete, due to the flow no longer following the contour of the shell, there is a net pressure difference between the fore and aft parts of the shell, and this results in what is called pressure drag. So, you want a long, slender shell shape to minimize flow separation, so that you minimize pressure drag.
The stern shape MUST allow for attached flow to exist to keep hull drag to
minimum.> Stern shapes like "IOR sugar scoops meet rating rules but pay large> penalties as they> do not reattach flow as well as other finer hull forms. I hope this
helps.>
Scott.>
I hope no offense is taken in my comments. If you want substantiation, any basic fluid mechanics textbook will further illuminate you on this topic.
"Geoff" <geoff_durham@hotmail.com> wrote in message news:1116138425.465256.46490@o13g2000cwo.googlegroups.com...> Thanks Scott, but if the vessel is travelling at a speed that means> that the water is converging well after the face of the stern (in event> of square cut off on stern) then the turbulance would occur in an area> of water the hull is not longer in contact with.>
Your thoughts??>
Geoff.>
You're right. The wake region aft of the stern has little effect on the boat. The drag due to a cut off stern is due to the pressure imbalance fore and aft. The cut off stern doesn't allow for a gradual pressure recovery, to balance the pressure on the forward part of the boat.
