C&C 24 hull speed? - SailNet Community

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  #1  
Old 06-16-2010
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C&C 24 hull speed?

Does anyone happen to know the actual hull speed of a C&C 24? Flew the 155 today for the first time and managed to hit 7 knots. We were at slack tide too so no current effect. Great fun
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Old 06-16-2010
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The theoretical hullspeed of the C&C 24 is 1.34 * square root of lwl. LWL for that boat is 19'5", or 19.42, and sq rt of that is 4.41, so 1.34*4.41= 5.91 knots. Of course, if you were surfing you can greatly exceed that speed.
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Old 06-16-2010
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Yes; except that as you go faster, the boat sinks in and the waterline length gets longer. I don't know what the max waterline length is for the 24; but if you assume 23' you get a max hull speed of 6.4kts.

I'd say you were pushing above theoretical hull speed a bit (.5-1kt); but not planing.
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Old 06-16-2010
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Here's the spec sheet if you want to have all the numbers.

C&C 24 - Used Sailboat Market in Canada

Our resident mathemeticians have given you the 'theoretical' numbers already. Was this a through-the-water knotmeter speed or GPS numbers? In your area there's almost always some tidal influence.
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Old 06-17-2010
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"Hull speed" is always a great bar-talk subject for sailors. I wrote about it with Steve Killing in our book "Yacht Design Explained." I mention this not to slyly sell more copies because it's currently out of print, although we're planning a second edition (some day...).
The standard square-root calc as stated is of course correct. It is a good rule of thumb because it ties speed to the wavemaking ability of the hull. The idea is that the length of one wave (bow to stern) is as much wave as a displacing hull can generate, and the length of a wave is tied to its speed (and that of the hull creating it). As the boat tries to go faster, the stern wave has to move aft into the wake, denying buoyant support to the stern. The stern drops, and in that bow-up stance it takes a lot more power than a sailboat can normally generate to go any faster without surfing or planing.
But it's just a rule of thumb. One of the factors that can increase a hull's theoretical speed is volume in the stern. Boats with wider sections aft have more buoyant volume and can stretch out the wavelength more. Another big one is what the effective waterline length is when a boat heels. Meter class designs have long overhangs for a good reason. When the boat is upright, the measured waterline is short and the rating rule thinks the boat is slow. When the boat heels, a whole lot of unmeasured waterline is put to work in the overhangs. Most every keelboat has "more" waterline when heeled than when vertical.
There are a whole bunch of other factors here as well, including the massive increase in wavemaking drag as a hull approaches hull speed that places increased demands on power to overcome the theoretical limit, and an increase in form drag as the stern drops. But in the end, most every keelboat seems capable of exceeding its theoretical hull speed in the right conditions, for example on a reach.
For what it's worth, I sail a C&C 27 mk 1, which has an LWL of 21 ft and very little effective overhang at the stern. (It's an earlier generation of C&C than the 24. The hulls are not really comparable.) Square root is 4.58, which gives a theoretical speed of 6.13. That's a pretty fair assessment of my redline speed going to windward. But I can routinely hit 6.8 on a white-sail reach.
To keep measured waterline length's influence in perspective, later versions of the 27 had the stern stretched, increasing LWL 8 inches, which would seem to be a lot in a small keelboat. But all 27s race level in class competition, which tells you there's a lot more to performance over a range of conditions than a raw number like LWL.
And yes, it's important to use GPS speed. I also have an impellor speedo, and it routinely disagrees with my GPS by half a knot.
Cheers
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Old 06-17-2010
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Something I meant to add on waterline length is that you must be very careful not to be misled by published LWL, especially as found in original brochures for a design. Companies routinely stretched the actual waterline length of the hull form by including anything that was "wet," by which I mean a small swelling or skeg ahead of or atop the rudder or even the rudder itself. The C&C 27 is a classic case in point. The class website gives the LWL of the later version of the boat as 22 ft 10 inches, but that is achieved by including the swelling of the hull where it fairs into the top of a new rudder design. Inspecting the actual original design drawings shows that the new LWL was actually 21 ft 8 inches. It's the length of the hull form that matters. So in the case of the later 27, this translates into a difference in theoretical hull speed of about 0.16 knots (6.4 vs 6.24). That's significant, especially if you compare it to the theoretical of 6.13 for the original 27 hull. A buyer should not assume he is getting a significantly faster hull with a later 27 than an earlier one, based on the published LWL.
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Old 06-17-2010
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Quote:
Originally Posted by Diva27 View Post
...And yes, it's important to use GPS speed. I also have an impellor speedo, and it routinely disagrees with my GPS by half a knot.
Cheers
Better to properly calibrate the transducer to the GPS at slack tide, or over a measured course. In tidal areas, the GPS/SOG will (almost) always be influenced by tidal current and produce an incorrect speed for the boat's true through the water performance.
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Slack tide, not surfing waves, taken with a GPS. I was pretty please, she's a fast little boat
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