Bottom strakes & water flow

[mbha]

I have noticed that most commercial boats on pictures seems to have their bottom structure (strakes), of some kind pretty straight along the bottom. But the water doesn't move in that direction at all from what I understand. Near the front the water seems to move very much sideways hitting the strakes at a very sharp angle creating a heavy spray. The attached image illustrates what I mean. This just has to cost HP's and furthermore, destroying quite a lift. I cannot find any discussion about this.

Red, darkblue, green & yellow is supposed to be water level at various speeds and loads.

Lightblue, common strake. (is this good?)
Brown, my idea of a better shape ??

[BilltheCat]

I don't have an answer...only a question.

Does the water flow pattern change as you go from displacement to planing speed? And therefore, does your strake design negatively effect performance of the hull during displacement (greater drag - more fuel consumption), or vice versa?

[mbha]

According to my theory, it does! I'm into building a model to study the flow but I have my theories. I believe that the water closest to the hull bottom moves sideways very fast. creating a sort of vortex whereas the underlying water layer stands still. The diameter of this vortex is logically much greater at lower speeds meaning that the sideways movement is slower.

On the image, the area between the darkblue and red is supposed to be the water flow at displacement speed whereas the area between the green and yellow is on plane. The problem as I see it is that there are two vortexex, one on each side that kinds of creates a groove in the middle of the water. This means that the boat is supposed to plane on something that simply isn't there anymore.

According to my belief the OD flat design should be very efficient and the moderate deadrise on the V hulls should be pretty effective too. The question is how to design the bottom to achive a smooth ride on the chops yet providing a linear lift rising depending on the speed. Another issue is how the water flow affects the forces when banking in turns and waves hitting the bow a bit sideways. I guess it wouldn't be too popular if the craft changed direction instantly under those circumstances.

To sum it up. The strakes should either be much bigger then normal, way over an inch in height directing the water flow in the desired direction or smaller followning the water flows natural path or no strakes at all. As I see it. smaller strakes not following the flow only reduces the hulls lift capacity and forces the use of spray rails to stay dry.

Remember, this is not facts. Only my ideas!

[jacquesmm]

We know exactly how the water flows along a planing hull: not parallel to the CL but along a diagonal.
The strakes should not be lined up with the flow. For lift, tracking and spray deflection, they should run parallel to the CL on the planing surface and wherever the designer prefers forward.