Hull Speed

Achillesheel

Achillesheel

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31 Dec 2004
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Boat; Falmouth, Home: Gloucestershire
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I found the formula for hull speed (1.34x square root of LWL)the other day, and worked out for my boat (Achilles 24, 19'6" LWL) it is 5.92 knots - which would seem about right as we have hit 6knots on the log once or twice but never seem to go faster than this.

But how much of a maximum is this formula?
 
'tis applicable whilst in displacement mode.

If however you manage to get a boost from a passing wave you can surf on the wave - effectively planing and can go much faster but if you drop below the max displacement speed again there you stay until the next time you can get a boost from another wave.
I once stayed surfing on the same wave in the channel for five hours and we covered 57 Nautical miles by GPS in that period (7 miles were due to tide), that was in a Gib'sea 84. It was fab but once we lost the wave we didn't get back to those speeds again.
 
I also seem to recall a Robber surfed on the back of one of the maxis in the channel race and stayed surfing 2 metres from the stern of one of the Yeomans all the way across until the turn for home - at which point he lost the "wave"
 
Who are you calling tubby!!!

Actually, the Achilles is anything but tubby, beam is only 7'1" which is why there is so little room below. As someone on the Achilles website has said, the boat is limited in every aspect apart from sailing ability, but she is great fun to sail.
 
In practice hull speed can be greater than that calulated, it depends upon the hull shape. My boat has a calculated max of around 7 knots but the manufacturer quotes 8.3 under power. This puzzled me, what happens is as the speed increases a stern wave forms which moves backwards extending the water line and supporting the stern. There is a good explanation in Nigel Calder's cruising manual.
 
Also, a boat can exceed hull speed with additional power, and without the assistance of a wave..... hull speed is the maximum theoretical 'displacement' speed, and additional power can move the boat towards 'planing', which increases speed. This does however require significant amounts of extra power..... and is often referred to by dinghy sailors as 'going over the hump'.... the reason keelboats (usually) don't plane is the enormous amounts of power required are beyond the means of ordinary sails or engines.... but it is theoretically possible...
 
My 1975 25ft heavy tub motor-sailer ...... with Perkins 4-107 in ....

s-anne05-07-02.jpg


will do .... and has been observed by others who can confirm ... 9 kts under engine ....

Hull speed .... wadat ?????? /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
This subject re-occurs regularly. The formula is only an approximation which should be roughly accurate for a boat like yours but is not a maximum. It indicates the probable speed at which hull resistance rises to a level which will make it necessary to input a disproportionate amount of extra power to increase speed further. It's not a brick wall you hit. It should be pretty accurate for your boat though in normal sailing conditions although if you had a big spinnaker and going downwind in a F6 you would exceed it (not recommended /forums/images/graemlins/frown.gif)
 
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if you had a big spinnaker and going downwind in a F6 you would exceed it (not recommended /forums/images/graemlins/frown.gif)

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Thast how they win their races isnt it.... /forums/images/graemlins/crazy.gif Hard work for the foredeck crew (thats what they are paid for) and often spectacular to watch! /forums/images/graemlins/ooo.gif /forums/images/graemlins/smile.gif
 
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