Re: ski speed

I have paddled a number of different skis (Beginner, Spec, Racing) and I think the real difference comes in the effort you have to put forth to get the boats up to speed. The beginner and spec skis require, in my opinion, quite a bit more effort to keep "up to speed". I agree that racing skis might only be marginally faster (5%) but they are definetly more efficient and require less effort from the paddler. Just like my my K1 requires even less effort (shown via heart rate response) to paddle at 7mph than my Mako 6....

-Rob

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 would have to say hull efficiency. Once a boat is up to speed, the weight should not matter. The lighter boats should only have an advantage upon acceleration and on downwind runs.

-Rob

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.

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.)

I would have to say hull efficiency. Once a boat is up to speed, the weight should not matter. The lighter boats should only have an advantage upon acceleration and on downwind runs.

-Rob

This is absolutely not true. Lighter boats are faster at constant effort, or require less effort at constant speeds. Why do you think that marathon K1s are built even lighter than sprint K1s? When you add more weight, the boat sinks lower into the water. The extra part of the boat that is now underwater is adding additional resistance. The physics is undeniable.

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.

Jim, your absolutely right. You'll see the real speed gains by increasing your fitness, learning good technique and training. There is no substitute for that, IMO. What I'm getting at is basically about boat stability vs boat speed (or potential). In order to extract the potential out of the boat you have to be stable. You're giving up speed if your not thoroughly stable in a boat. I'm sure all of us want the fastest boat that we can paddle. The trick is in defining that "line" between boat speed and stability that helps us choose boats that are best for each of us. That's basically why I'm asking these questions. When I'm ready to upgrade to a new boat from my XT I have a hunch that, although I can paddle a Mako6 or S1-X, I'll be faster overall in one of the more intermediate boats (Evo, V10sport, etc). That would probably take minimal acclimation time to get used to those boats. However, if the elite boats really have alot more potential speed then I'm not against taking the extra time to develop my skills in those boats.

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!