Max Hull speed and waterline length

silver-fox

silver-fox

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It is generally accepted that a boats maximum hull speed, without planing, is 1.35 x square root of LWL, ( or thereabouts), after which huge amounts of energy is consumed trying to climb an ever bigger bow wave.

If that is the case why doesn't it have the same effect the ducks? I have some near me, that swim past, definitely not planing, but at a speed at best guess between 1-2 knots without much apparent effort and a LWL max 6 inches.

Obviously a subject ripe for humour........ but if there is anyone out there that can explain this to me I would be grateful.
 
This term "hull speed" is seriously misunderstood and often represented as something equivalent to the sound barrier for aircraft. According to Wikipedia:

"From a technical perspective, at hull speed the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed."
 
I think the op is failing to take into account the lift effect of the webbed feet. A duck does not rely on a bow wave to get it up into the air but more on the speed & lift from its feet. You will note that untill the wings can be used there is a point where the duck seems to be running on water
This magical effect was often used in the past to check for witches
If a woman suspected of witchcraft weighed more than a duck the case was proven. This is demonstrated in the film" search for the holy grail." A useful source of information where one will also find proof that the world is banana shaped. This being the basis for some navigation techniques clearly adopted by some forumites when marina manouvering
 
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Duck have hard lives. Apparently, flying is so stressful that it is not unknown for them to die in flight. A women was knocked from her bike by a falling duck while cycling in Richmond Park.
 
hull speed formula related to speed of gravity waves on water. Longer boats generate longer bow/stern waves that travel faster. However Gravity waves only apply to waves longer than about 1ft. Below that you are dominated by Capillary (or hybrid) waves that have a different speed equation. Any other thoughts?
 
Blokatos said:
Slightly off topic but, according to its LWL, the Malango 1045 (http://www.idbmarine.com/en/malango/1045/malango1045.php) should have 7,8 knots speed but as you can see in the link above it is advertised to be almost twice as fast at 14,8.

What am I missing here?
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If you plot drag vs speed you will see that at hull speed the graph changes significantly, so an extra knot takes a whole lot more push. There is another turning point on the graph at planing speed too, this time the drag drops right off then slopes up again.
The boat you link to is obviously a duck with more paddle than is needed for hull speed.
 
Dougal Tolan said:
If you plot drag vs speed you will see that at hull speed the graph changes significantly, so an extra knot takes a whole lot more push. There is another turning point on the graph at planing speed too, this time the drag drops right off then slopes up again.
The boat you link to is obviously a duck with more paddle than is needed for hull speed.
Click to expand...

So is it safe to assume that she is (partially) planning?
 
Blokatos said:
So is it safe to assume that she is (partially) planning?
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Thats a bit open to interpretation of planing.

Whats happening (in duck terms*):
As the duck swims it creates a wave, the bow wave. The water has to get out of the way of the duck, but it washes back. At low paddling rates, this return wave (the bounce back of the neck wave [bow wave on a boat]) slaps back against the sides. As paddling increases the wave meets at the DA and due to the tapered end of the duck adds a bit of push. As the duck paddles even harder and moves faster then it is no longer pushed by this wave, thats hull speed.To go even faster the duck effectively has to paddle up hill. Planing is when the duck has swum to the top of the hill and is in effect riding on top of its own bow wave, so it is no longer in the water, rather on top of it.

So, is the boat partially planing? depends on what you call the bit where a boat is faster than hull speed but not skipping over the water.


* for large ducks >1ft since the waves for small ducks work different a posted by MJWF
 
sailorman said:
Had she been observant, she may have ducked ;)
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Would not have made any difference . A duck falling vertically would not have been avoided by someone bending lower unless the duck was in some sort of swallow dive & coming in from the side.
In which case a real hard impact could have ended up in her swan song. But only if it was two African swallows carrying a cocoa nut between them,, otherwise there would not have been enough weight for the impact.
Now if it had been a cow lobbed over a castle wall that would have been different !!!
 
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