Speed of an average sailboat

Discussion in 'Sailboats' started by Hebron Watson, Jul 13, 2007.

  1. Hebron Watson
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    Hebron Watson New Member

    Does anyone know just how fast the average sailboat is( when I say average I mean a monohull cruiser ). I would LOVE to believe that it is about twenty mph.

    If anyone here knows how fast the average thirty footer( my preferential lenght ) might be, I would love to know.

    P.S. Does anyone know how I might speed up such a boat. i have already mulled over the obvious stuff( i.e. bigger sails, smaller, lighter boat.)

    THx!!!!!!!!!!!
     
  2. USCGRET/E8
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    USCGRET/E8 Senior Chief

    Top speed for a regular cruiser is pretty much 7 knots tops. A racing boat can do a lot better depending on several factors. There's no speeding up a displacement hull. You have to get up on top of the water for speed.
     
  3. USCGRET/E8
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    USCGRET/E8 Senior Chief

  4. Crag Cay
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    Crag Cay Senior Member

    Your question mentioned the 'average' sailing boat of about 30 ft LOA.

    Well if you think in terms of distance sailed in 24 hours, it will give you a good idea of average speeds that are sustainable. Any run of less than 100 miles in 24 hours is depressing (less than ave 4.1 knots) but not unusual in light airs. (These are also straight line figures where as 100 miles made good to windward might be considered good going.)

    120 miles a day is good progress. (5 knots average) 150 miles a day (6.25 kts) is dream stuff, and my record in a Contessa 32 is 183 miles noon to noon, with the wife in the trade winds, but we did carry the spinnaker all night which was not our norm. But we were young...

    Breaking 200 miles a day (8.3kts) is only possible in a modern 30ft race yacht being pushed hard. And as for 20 mph, well...........
     
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  5. Hebron Watson
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    Hebron Watson New Member

    Yeah i had that feeling. Thx :)
     
  6. tspeer
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    tspeer Senior Member

    You could average 20 mph if your boat was more than 200 ft long.
     
  7. alan white
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    alan white Senior Member

    The waterline length of an average sailboat determines its maximum speed generally. This is estimated to be about 1.34 x the square root of the waterline length. So if a sailboat is 144 ft long on te waterline, the square root is 12, and the boat will probably not exceed 12 x 1.34, or about 16 knots.
    A 100 footer will do 13.4 kts and a 16 footer will do 5.36. A thirty footer should do 7.34 knots.

    Alan
     
  8. alan white
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    alan white Senior Member

    At a great enough length, however, all bets are off. It's called ultimate displacement speed. The boat drops into a trough created by it's own bow and stern beaches, and ceases to move.
     
  9. Trevlyns
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    Trevlyns Senior Citizen/Member

    Hi Hebron – and welcome to the forum! :p
    I’d just like to add some thoughts on the second part of your question about increasing boat speed. Alan has already pointed out that speed is directly related to length (LWL) so lighter and smaller will not equal faster. Similarly increasing sail area could result in a broach – or even worse, in a gust. Other ratios such as Displacement to Length and Sail Area to Displacement need to be kept within norms too in order to have a well behaved boat.
    Take care!
     
  10. Pierre R
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    Pierre R Senior Member

    Boat speed can be increased by trailering the boat from one location to another.

    It's about the same as when I use to ask my father for a raise when I was young. He would point the staircase and say " Son, there is 13 steps, ought to give you a raise of about 9 feet":)
     
  11. PAR
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    PAR Yacht Designer/Builder

    As has been mentioned, sailboats can be restricted to a speed governed by their LWL and estimated speed/length ratio (S/L). For most modern, displacement sailboats this S/L is accepted as 1.35 and is the qualifier against the LWL's square root.

    Sailboats are not limited to this 1.35 S/L and most modern performance cruising sailboats can move past this generic qualifier, some by a great amount (yep, better then 20 knots).

    On a modern 30' production cruiser, you may have a maximized LWL and possibly as much as 28' in LWL, making a theoretical maximum hull speed of 7.14 knots (about 8 1/4 MPH). In beam or freer winds, you may have the ability to climb over the bow wave and begin to plane, which would permit much high speeds.

    There are also other things that can increase speed, but these are the type of things you can't just buy, at the local West Marine store and bolt onto your yacht, suddenly going 30% faster. Sailboats are all about the big picture, in the balancing act, that is required to motivate her and keep the yacht upright.
     
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  12. BarendGrobler
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    BarendGrobler Junior Member

    All the above responses assume a diplacement hull; why can't one design a planing or semi-planing sailing hull?
    Another question; how come those amaising sailing tri's can reach such high speeds when they to are displacement hulls?
     
  13. Trevlyns
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    Trevlyns Senior Citizen/Member

    Hi Barendt!
    Well this is quite a complex question and would take quite a while to explain fully. But let me have a stab at it! Basically what we are dealing with is wave resistance and something called a Froude Number (try googling it to find out more). This is defined as a dimensionless speed where the velocity in meters per second is divided by the square root of waterline length, times the acceleration of gravity. The Froude number determines how many waves (a form of hull resistance) there are along the hull. If plotted on a graph, the graph would climb in humps and hollows as the Froude number increases. Most ballasted smaller yachts would meet resistance at the first major “hump” and are therefore limited to the basic formula discussed above (1.34 x the square root of the waterline length). However, other craft such as catamarans; tri’s and some dinghies with semi or planning hulls are able to cross that barrier. Hull shape also plays a part.
    The formula discussed above indeed does refer to displacement hulls. But to quote James Wharram, on his cruising catamaran design the Pahi 42, “High speeds of 3 X square root of the waterline can be achieved. But comfortable cruising speeds of 1.5 to 2 X will be the norm”. This type of craft is then able to overcome that first resistance hump and move on to a higher Froude number.
    Those amazing tri’s you mentioned are not ballasted and rely on the leeward ama for “form stability” as opposed to the stability supplied by ballast on a monohulls.
    Hope this goes some way to answering your question.
    Lekker bly! :p
     
  14. BarendGrobler
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    BarendGrobler Junior Member

    Tnx Trev
    So what you are saying is that the cats and tris are in fact not pure displacement hulls?
    I found another explanation on the internet which goes as follows:
    "While the speed of a [non planning] displacement hull is a function of 1.34 x LWL and the hull's of a multihull would be no exception [&#x221ALWLx1.34], except that their extreme narrowness (between 11.1 - 20.1 beam to length ratio), results in very little water to be pushed aside, with such a small bow wave to mount, coupled [the theory goes] by the force in the sails [as the hull reaches the critical hull speed], forcing the bow down and the narrow stern up, leaving little for the water trough to 'grasp' and consequently, a multihull can continue to accelerate.*"
    Does this sound correct? any coments anyone?
    Geniet hom!
     

  15. Trevlyns
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    Trevlyns Senior Citizen/Member

    Hey, that makes sense to me Barend.
    Remember we are dealing with wave resistance. A short fat hull will create more wave (hence, resistance) than a long narrow one. As I mentioned, hull shape does play a part too. The slenderness ratio (LWL divided by waterline beam) is an important factor too – the lower the figure, the fatter the boat. Chris White in his book The Cruising Multihull suggests that a ratio of 12:1 makes for a very efficient hull with very little wave making. James Wharram’s 63 footer has a 17 to 1 ratio and Proas and tri amas can be a whole lot higher still – I know of one at 50:1 – super slippery! :p
    Take care!
     
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