Based on my times in various boats over our Thursday evening time trial in flat to small chop, here is less than 5% between the learner boats (XT, Mentor) and the racing boats.  Over a longer downwind paddle of around an hour and a half, the difference increases to about 10%.  I am not an elite paddler by any means but I am perfectly comfortable in my Icon.  In rough conditions, even an elite paddler can benefit from a more stable boat, note Jasper Mocke's record run in the Mentor.

Also, please lets get the misconception that longer boats are faster, out of the way.  This only applies to displacement vessels whose length to beam ratio is less than 7:1.  Any boat worthy of being called a surfski has a ratio of well over 12:1.

The things that effect the speed of the ski are the underwater shape, the buoyancy distribution, the centre of gravity, the amount of rocker and a host of other details.

As has been said before, it is more important to be stable on your boat than to paddle the flashest one available.  I think it is way easier for a novice to take 10% off your time with some quality training or a course in technique than by shelling out for a new boat.

Rob, do you think that's more a result of less weight/mass or hull efficiency? I'm thinking a good way to test boats is measure HR of the paddlers at given speeds. It would be an excellent way to measure efficiency of the hull. Throw several paddlers in the same boats and see if the trends are the same with them all. My guess it the lighter boats are considerably easier to keep at higher speeds based on paddling can kind of be broken down into a rapid series of short accellerations (followed by boat glide). 

I don't think that what you are saying here is quite true.

The speed of a displacement hull is primarily governed by two things:
#1. Friction resistance (proportional to the wetted surface area)

Reducing friction resistance is very simple because you must simply reduce the surface area of the boat that is underwater.  The optimal shape with respect to surface area is a half sphere.  With regard to a typcial surf ski, you can reduce friction resistance by making the boat shorter and by making the bottom more rounded (optimally, it will have the cross section of a half sphere).

#2. Wave resistance (roughly governed by the length and pointy-ness of the hull

Reducing wave resistance is also very simple.  Make the boat longer and more pointy.
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As you can see, the two goals run somewhat against each other, so boat designers have a challenge of striking the right balance.  If you make the boat too short, it will have a lot of wave resistance.  If you make the boat too long, you will have too much friction resistance.

To make things more complicated, wave resistance increases somewhat exponentially with increasing hull speed, wheras friction resistance increases in a more linear manner.  This means that at different speeds, the ratio between friction resistance and wave resistance changes--so a hull that is optimal for sprinting at over 10 MPH will not be as fast at 7 MPH as a boat designed optimally for 7 MPH.  Also, for the same hull, the ratios will be different depending on the weight of the paddler (plus the weight of the hull)--so a boat designed optimally for a 200 lb paddler (at a certain speed) will not be optimal for a 150 lb paddler.

The 21 ft, 17 inch wide surf skis are generally designed for heavier racers paddling at very high speeds (8+ MPH).  I agree that making these boats longer is likely not going to make them faster (a longer kayak has been made--the X-Par missile--and it is not proven successful).  However, making them shorter will definitely make them slower for their intended occupant.

A lighter paddler would likely benefit from a shorter hull (witness the 18.33 ft Huki S1-A, designed for <150 lb paddler).  A paddler intending to cruise at slower speeds would also likely benefit from a shorter hull (proportionally less wave resistance at lower speeds means that friction resistance is relatively more important).

For a given weight, a given speed, and a given hull shape, there exists an optimal hull length.  For most surfski racers, that optimal length is somewhere near 21 ft--any shorter and the boat will be slower, but any longer and the boat will also be slower. In that sense, the "longer is faster" is a myth..

However, a typical elite surfski racer, the current elite boats are very close to optimal.  In the range of available boats, longer really does equal faster.  The reason why elite racers use 21 ft boats instead of 19 ft boats is because the 21 ft boats are faster--and there aren't many boats out there longer than 21 ft.  In that sense, the "longer is faster" is not a myth.  

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Obviously, you are coming from having the experience of a 17ft K1 being more effecient at 7 MPH compared with a 21ft ski.  However, if you could take your 21 ft ski and shrink the width so that it is as narrow as the K1, I think that the tables would likely be turned (depending on your weight of course)--particularly if you were paddling at 8 MPH rather than 7 MPH.  (I say this realizing that I personally can't hold 8 MPH long.)

Just a thought....what if the paddler were to lose weight? some could lose a lot. Would a heavier boat then go faster? There would be considerable $ savings in buying a lighter boat, and even more savings with reduced beer consumption. Maybe the paddler will become fitter and stronger- and faster. Maybe close and regular attention to technique would bring even more speed gains.

Yes, of course.  However, some would argue that if you had a choice between carying an extra 10 lbs on your boat, or on your body, you would be better off with the weight on your body.  There are two logical reasons for this: One is that invariably, some % of those 10 lbs will be muscle.  The second reason is that the weight distributed near the ends of the boat will increase its rotational inertia (making it less responsive to steering) which causes you to need to use more rudder which causes more drag.  Granted the same weight on your body will also slow the boat's turning ability, but not as much--because all of your weight is distributed very close to the center of the boat.  Furthermore, the increased rotational inertia may also effect responsiveness in riding waves (I don't know about this).

In any case, the MOST important thing is strength to weight ratio, and the best way to do this is to loose your body fat AND get the lighter ski!

Thomas Yonley- you have made a very good point about the age factor. Reaction time to stimulae definitely degrades, but can be offset by regular training. The other thing about good balance is the ability to relax both physically and mentally. A lack of flexability/range of motion in hips, [esp. adductors], and hamstrings can cause a lot of instability. There is nothing like a bit of careful stretching after every workout. Sprint boat stability increases with speed. You have to take control of the thing and paddle it with confidence. Head down, short strokes and a lot of bracing will dump you. I expect the faster surf skis are the same.
You are also quite right about the faster boats- all other things being equal. Still, there is a lot of time to be picked up with attention to good paddle technique and water reading skills.
I too am trying to figure what kind of surfski to buy. The more stable ones make sense- but the long skinny carbon boats sure are beautiful!

Xpat,
10% would be significant in my book on a longer downwind run. I agree with what your saying in your reply about most folks benefitting from a more stable boat. I'm guessing only the very elite paddlers have a real handle on their boats in rougher stuff. Some of us only wish for that kind of boat control.