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The Chine Effect

Baltic Sails - Progresss through Research
Published by Yannic-Tim Noack in hull mechanics · 1 March 2021
Tags: researchplannedhull_shapechines
This is a question inspired by the especially accentuated chimes on the 49er hull.
How does chine shape influence the switch from displacement to planing?
Content:
  1. What are chines?
  2. Displacement vs planing
  3. What role do chines play in planing?
  4. Designing Chines
  5. Research Question

1. What are chines
Chines describe the "ledge" along the waterline, that progressively weakens towards the aft of the boat:
 
 
 
 

Note that I am not talking about the angle but instead a negative before the angle:
hull shape with negative inset to demonstrate a typical chine
2. Displacement vs planing
Typically, upwind boats with a deep, slim hull move through the water in displacement mode. If you were to follow two water particles split at the bow, you would find that they meet again at the transom of the hull. These boats are fairly slow due to the vast amount of surface area in the water and the water that they have to displace.

Planing hulls on the other hand have a very shallow and flat hull shape. Especially the transom is very flat. In planing modes, the two water particles, split at the bow will not meet up again at the transom but instead will be shot away to either side of the hull and will never meet again.

You can compare displacement to pushing your hand through a water tank slowly and planing to skimming a rock on the surface of a pond. (Skimboarding uses planing only if you have ever seen that)
Half a hull showing a rounded, smooth deep shapeA flat but wide hull with a chine and steep angle
Displacement hull shape vs Planing hull shape
3. What role do chines play in planing?
Surface tension and Chines
Water is not the kind of medium that easily likes to let go of its mates. That is referred to as surface tension. This means that groups of water particles have to be faced with a vast amount of force in order to split and fly away like they would in planing mode. If you have a softly rounded and flat contour along the hull, the particles are not experiencing a lot of force. That is also why hulls are mostly flat and softly rounded. If we did have chines going perpendicularly to the heading of the boat that would slow the boat down dramatically because of the surface tension and turbulences happening below the waterline. That slowing force is what we also call drag.

The chines apply a great force on the water surface being propelled to the side of the hull so that surface tension is broken and the water flow breaks away from the hull. Yes, this explanation is not scientific but I hope it makes sense.

Chines and Drag
Since the chines are not below the waterline (if you didn't forget to plug in the bung, that is) they usually don't introduce drag into the direction of the heading but only towards the sides of the boat and since the boat is symmetric, they pretty much cancel out from side to side. (Unless not sailing level)
Of course, there is some contribution to drag since the hull is not a long straight pipe but also has surfaces that are angled at the water but for now, we are just looking at the sidewards effects of chines.

4. What can be considered in the design of chines?
Chimes exist in all kinds of different shapes and sizes but the general shape is generally somewhere along a sharp edge, curving upward along the waterline of the hull.
The angle at which water starts to spray away from the hull, however, can be altered by changing the shape of the chime or even pitching the boat aftwards.

Different possibilities of chine shapes and sizes
5. Research Question
We want to answer the following questions:
  • How does the angle of the chine, relative to the natural water flow direction at that position, change the min. water speed needed until it splashes away from the hull?
  • Does it matter if the chine edge is radically changing (with a tight bend) or if it has some organic smoothing to it?
  • How does the approach angle of the water to the chine change whether it splashes away or not? (Optimisation of shape, what shape gives the greatest range)
  • What is the drag effect on the water being redirected by the chine?


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