Boat speed

[Mr Cassandra]

In recent posts we have been told of boat speeds up to if I remember 17kts on a hull length of only 41ft .If I am correct if underway and the waterline length is 39ft , this would give a hull speed of aprox 8.5 kts .What I would like to know is how does one get these very high speeds ?I dont think, that even if the crane droped mine, would it attain this speed. cheers bobt


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[Robin]

I'll answer since it was me reporting 16.7kts from a Sun Legende 41!

We think we actually saw 19.5kts but both of us can confirm 16.7 and our log is set to average over 10 secs, initially all eyes were on where we were going rather than the log. The location was just one wave off Poole Bar, wind East F6 gusting F7. We had just a half rolled genoa and no main.

Displacement hull speed is governed by the bow wave wavelength and the formula is 1.34 x squ rt of waterline length (in ft). I.4 and even 1.5 x sq rt is also possible (just) given enough power applied (engine or sail). Any more than this requires the hull to break free from displacement speed and plane - like a powerboat.

Then there is SURFING. This is where yachts can get up and go! If you have watched a surfer riding a wave you will see the initial movement is paddling to get some movement to catch the wave, then bingo off you go (if you are not upside down breathing saltwater trying to dodge a flying board......) Its the same with the yacht, catch the right wave at the right time and YEEEE HAAAAAA. Look at the Volvo boats et al in the Southern Ocean, that's how they get their speeds up to 25kts and more.



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[Evadne]

a) the hull speed is not the maximum speed, even in displacement mode, it's just that the energy required per knot above it increases sharply.
b) in planing mode - well, the skies the limit for a sailing boat.
c) surfing is a non-displacement mode as well, but if it happens to me, I've usually got better things to worry about than checking the log readout!

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[Mr Cassandra]

Hi But are you only traveling at the speed of the wave that the boat is riding on?

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[AlanPound]

... Along it, usually...

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[zephyrsailor]

its not so much the speed of the wave, generally when you exceed max hull speed something like gravity has taken over as you "fall" down the face of the wave.

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[Talbot]

It certainly is for a Cat. This is the point of biggest danger because they can accelerate so much faster than a mono, on a big wave, gravityt takes over and adds to boat speed and under the right conditions you get to catch up with the next wave - then if you are really unlucky, or pushing much too hard - you dig the bow in and pitchpole!!!!

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[Robin]

The log is recording boatspeed through the water, that is all that it can do. The boat has briefly broken out of it's chains (of displacement speed) and been propelled forwards very rapidly, for as long as you stay on the wave.... In our case the ride came to an end when the bow buried in the back of the wave in front, and the previously friendly one doing the pushing jumped into the cockpit and got my nice dry feet very wet! In hindsight if I had steered say 20 degs off straight down wave we might have had an even longer ride - and kept our feet dry!



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[BlueSkyNick]

The theoretical maximum speed of a displacement hull is the square root of (2xLWL), which in Robin's terms is (1.4142 x the sq root of LWL), regardless of how much power is applied. So for a boat of 32' waterline, the maximum is 8 knots.

(I failed a job interview on this point as a 17yr old school leaver seeking a career in marine physics. I remember the correct answer being explained to me quite clearly - and ended up in telecomms instead!)

However, I anticipate somebody will tell me I'm wrong!



<hr width=100% size=1>The problem is that God gave men a brain and a penis but only enough blood to use one at a time.

 

[Mr Cassandra]

SO if a wave is traveling at 10kts and a boat at hull speed of 8.5should the log not read -1.5kts

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[Robin]

I hadn't seen that version of the formula before but the results are much the same.

The mathematical fact is that for monohull yachts, other than a very few specials like the Melges, 707s, J24s etc which can plane like dinghies, if you want to go faster then you have to get a boat with a longer waterline! This is why there has been a design trend with steeper bows and stern scoops that immerse at very low speeds, all in the name of extending the waterline. Classic long overhangs are very pretty but very slow, though again the LWL increases from the static one as the boat heels, and mostly these boats heel more quickly than modern designs with more form stability.


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[AlanPound]

... a wave travelling at 10kts does not mean that water is travelling at 10kts (unless it is breaking - which is when it really gets dangerous)....

... In fact, I seem to recall that, as the wave passes, water on top of the wave is actually going backwards....

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[AlanPound]

"The theoretical maximum speed of a displacement hull is the square root of (2xLWL), which in Robin's terms is (1.4142 x the sq root of LWL), regardless of how much power is applied. "

... weeell, I think it is:-
(some fudge factor) sqrt(LWL in feet)

with the fudge factor generally taken to be some figure around 1.2-1.5 (depending upon who you are trying to impress). In any event, this is really not a hard and fast limit, but merely an indication of what you might roughly expect under some (unstated) conditions. It is a point on a curve beyond which it takes increasingly more power to go faster, and below which the hull is relatively easily driven, without a great deal of science or mathematical support for why the figues are as they are said to be.

You can observe though, the general position of bow wave and stern wave, and why it is increasingly a problem to go any faster.... but its tough to relate that (mathematically) to any particular fudge factor...

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[Robin]

Nope. I think I need lessons in descriptive writing to explain this concept or help from someone else!

Forget the wave, that just provided the means to get the yacht out of its hole in the water and for a short period of time up onto the top of the water, planing like a dinghy or speedboat. The speed on the log IS the speed the yacht is travelling at through the water.




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[AuntyRinum]

"SO if a wave is traveling at 10kts and a boat at hull speed of 8.5should the log not read -1.5kts "

No, the log will read 10kts. The wave is moving but the water is stationary (assuming no tide). Exactly as if you took the edge of a carpet in two hands and flipped it to create a wave. The wave would roll across the carpet but the carpet would remain stationary.



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[Bergman]

Re: Yes but

Surfing is not different to being powered by sails, at least in that no different set of rules apply.

Sailing down a wave is simply being assisted by gravity which adds to the force applied by sails/engine.

The 1.? x sqrt WL only describes a point where the drag starts to increase at a greater rate. Thats why there is a debate about the constant.

Certainly not accurate to 4 decimal places. Suspect memory has thrown up sqrt 2 - which is used lots and lots in telecoms.

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[Mr Cassandra]

OH dont fret! I do understand [I think]Its just that in the Aegean where we don`t have currents to propell us we have to rely on the wind to attain hull speed ,in anthing around 12kts of wind my boat goes at about 6.5kts. with a 140% genoa i carry about 1000sq ft of sail, My yacht weighs in at 10,000kgs.giving me a SA/ dis of about 21. I can carry this sail up to about 22-25kts.Still my boat speed rarly goes over 10kts on the Gps. When being stupid [good fun though] and Running downwind in say 30plus kts I have managed 13.9kts gps but it did feel out of control . Do you think people exaggerate their boat speed?cheers bob t

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[jimi]

Och ...everyone quotes max speed rather than average speed .. and if they're really cheating they quote speed over the ground rather than through the water.

<hr width=100% size=1>O wad some Power the giftie gie us
To see oursels as ithers see us!

 

[Robin]

Sorry but that is incorrect. If the boat is doing 8.5kts, the log will READ 8.5kts. Anything else is irrelevant, this is speed of the boat through the water.

Your analogy using the example of the wave in the carpet is correct, however it has nothing to do with boat speed. The wave acts like a sudden boost to the power propelling the boat and because the boat is upright it can use the power (doesn't just heel as if a huge gust hit) and it takes off down the face of the wave.

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[Sybarite]

" Do you think people exaggerate their boat speed?"

Perish the thought...

John

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