Sandpiper Sailboat

posted November 01, 2007 06:29 PM               

This brings up our previous discussions about moving crew forward. Here is what I have learned. The justification of moving crew forward was based upon getting the stern out of the water to reduce turbulence. Reducing the turbulence behind the boat will reduce the energy loss and increase the speed of the boat. This argument is valid in light wind when conserving energy is important. If the wind is strong enough to reach hull speed, conserving energy is NOT the issue. In fact, it turns out that that big bubble of turbulent water behind the stern increases the effective waterline length of the boat and thereby increases the hull speed. Exactly how it does this remains a bit of a mystery to me, but the turbulent bubble seems reflect the hull wave and increase the effective waterline length. Yacht design books recognize this phenomena.

So, if you are sailing in windy conditions and you are bumping the hull speed of the boat, the best thing that you can do is increase the hull speed! Move to the back of the boat. If the wind velocity drops and your speed drops below hull speed, then energy conservation become important and you should move forward to eliminate turbulence at the stern.

posted November 01, 2007 07:18 PM            

Sapphire's heel may have been a factor. Although, she was sailing on a reach and she was staying relatively flat except when hit by a gust. Your "sub hull speed trim" and "super hull speed trim" explanation sounds logical. I can go with that even if I don't understand the physics behind it.

Ab and I had tested the "ballast forward/ballast aft" theory at the MLTs. When we did not get an increase in speed by moving 1 crew forward, I declared the test to be inconclusive. Maybe what we may have seen was an increase in speed by moving the crew aft. Windy Island was already sailing at hull speed.

posted November 02, 2007 05:34 AM               

I was thinking about this concept, trying to understand it. Here goes. Tell me if I'm on the right track.

As I look at the above shot of WINDY ISLAND, I can't help but observe the sheer of the side of the hull. From this perspective, if I were to imagine the boat heeled, as when sailing at speed, it seems logical that the stern would be "deeper" in the water if the crew's weight were in the stern...effectively lengthening the waterline (and, therefore, increasing the hull speed). With a deeper stern, in effect there would be a more definitive wake. So, this would imply that the wake itself causes nothing, and effects nothing. Rather, it merely represents the consequence of the stern riding deeper in the water, as a function of the crew weight distribution.

By contrast, with the crew forward, the stern would be elevated, reducing wetted surface and reducing hull speed. Assuming this to be true, the resultant wake would be less.

Does any of this make any sense? ...or have I revealed the true level of my stupidity....especially, in the shadow of that Tilly hat, looming so large.

posted November 03, 2007 04:47 PM            

Last weekend while motoring Sapphire to the boat launch to put her on the trailer, I gave the 6Hp Evinrude full throttle. I don't usually open the throttle wide -- 1/2 throttle is usually enough to get to hull speed. I don't know how fast we were going, but, the stern squatted so low that the motor cover was almost touching the water.

So here is the phenomenon. Apply 6Hp at the transom and the boat wants to go bow up. Apply 6Hp to the sail plan at a height of about 14 feet above the water and the boat wants to go bow down.

This may be another reason why extra ballast is required in the cockpit.

posted November 03, 2007 09:25 PM               

There are several ways to look at the affect of waterline length on maximum speed of a displacement hull, but the most visual is to imagine the wave created by the bow. A crest of water is pushed aside by the bow. Behind the crest is a trough. if you look over the side of the boat you'll see the trough appears stationary relative to the boat; that is, the trough might be located below the shrouds at low boat speed. As the boat speeds up, the bow wave gets bigger, the trough gets deeper and moves farther astern. At some higher speed, the trough appears stationary at the stern of the boat. At this speed, the boat is squating in its own wake. The stern is down in a trough and the bow is pointing up. Forward motion requires the boat to climb out of this trough, but that requires a lot of energy. Actually, a displacement hull can never climb out because the boat can never outrun its own wake. If the boat is pushed hard enough, it begins to surf or ride on top of the waves and it is no longer a displacement hull.

Imagine the force required to push a boat graphed against the speed of the boat. At low speed the force is very low. As the speed increases, the force required to push the boat increases. When the trough discussed above reaches the stern of the boat, and the boat sits down into the trough, the force required to push the boat becomes very large. Actually a fourth power function of speed. This effectively prevents any further increase in speed, and is called the hull speed; it depends upon the length of the boat.

Different boats of the same waterline length have the same hull speed. This doesn't mean that they require the same force to move them through the water. It only means that the required force increases dramatically for both boats at the same critical speed, that is, the hull speed.

A light racing boat can reach hull speed in lighter winds than a Sandpiper; however, once the race boat reaches hull speed, it (effectively) can't accelerate unless it begins to plane. The Sandpiper might need stonger wind to reach the same speed.

Sailing heeled, or doing anything that creates turbulence, may increase the force required to push the boat, but it doesn't affect the hull speed at all. All hulls of the same length will experience the same huge increase in resistance to motion at the same speed.

posted November 04, 2007 05:06 AM            

quote:

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Originally posted by elmet3:

. . . In fact, it turns out that that big bubble of turbulent water behind the stern increases the effective waterline length of the boat and thereby increases the hull speed. Exactly how it does this remains a bit of a mystery to me, but the turbulent bubble seems reflect the hull wave and increase the effective waterline length. Yacht design books recognize this phenomena. . .

---

Arthur, do you have a reference for the "bubble of turbulent water"? I searched for it on the web and got nothing. A diagram would be excellent.

Here is a collection of emipirical formulae for estimating boat speed:

http://www.mediafire.com/?ftexatnive4

The calculated speed for the Sandpiper is typically in the range of 5.2 - 5.4 knots. So as our boats exceed this speed, are we getting into semi-displacement or semi-planing mode?

Semi-displacement boats usually have a flat or v-bottom hull with hard chines. There is an interesting discussion on this West-Wight Potter site.

posted November 04, 2007 11:17 AM               

Having said that, I'll pick a sentence out of the middle of the chapter.

Page 68 discusses the separation of flow at the stern. "A thick stern boundary layer (on a bluff afterbody) makes the hull appear longer than it really is, and this effect is even more pronounced if separation occurs. Some designers have therefore produced very bluff sterns with some separation just to decrease the wave resistance."

As I said, this is really taken out of context. The authors are discussing contributions of frictional resistance, wave resistance, and pressure resistance. They say that the hull design must be optimized for the intended conditions and speeds. They state "The higher the speed the more important the wave resistance and the bluffer the stern."

I read this chapter several times with the intent of evaluating the Sandpiper. I finally concluded, in agreement with the authors, that there is no such thing as a "best" hull shape. The "best" shape depends entirely upon the intended use of the boat.

Having reached that conclusion, I was still interested in understanding the behavior of the Sandpiper hull. I didn't make much headway in this respect because it requires calculations based upon data I don't have.

I was able to conclude the obvious; the Sandpiper was not designed for speed. But I also developed a respect for the way it is designed. The Sandpiper is quite sea-friendly, has a gentle motion, is stable, and doesn't pound over waves. These are not accidental attributes; they are the consequence of hull design. Our experiences at Pelee Island and Bruce Peninsula have highlighted these qualities.