Turbulence at the waterline as a fraction of total resistance

[JonathanCole]

Does anyone know what waterline turbulence as a component of the total resistance that a hull encounters? It would be interesting to compare the relative resistance of completely submerged hulls versus a displacement hull of similar WSA and slimness ratio. There is a thread about SWATH started by Yipster, but SWATH is a submerged hull concept for comfort in rough seas at moderate to high speeds. Might there be other advantages to submerged hulls such as fuel efficiency? I know many boat people care less about efficiency than speed, but if you have a budget for power, then efficiency is indistinguishable from speed. With fuel prices on the rise toward infinite, efficiency might keep mere mortals capable of have enough money to fill their hole in the water!

Aloha,

Jonathan

[yipster]

think you mean wave resistance by turbulance at the waterline? when you do a search on hullspeed, wave and frictional resistance on the forums there is planty of info. fiona sinclair's "freds hull resistance calculator" isnt on the net anymore i see, its a need shareware java applet that gives a good indication between hullform and speed versus wave and frictional resistance.

[jehardiman]

First, lets define some terms. Total resistance is made up of skin friction (i.e. tangent to the skin) and residual resistance. Residual resistance is comprised of pressure distribution over the skin (i.e. a force normal to the skin) and air resistance (which again is divided into skin friction and pressure) but we will ignore air resistance for now. One must always remember that the wave/wake train is nothing more than the physical manifestation of the pressure disturbance in the fluid. Even submarines and SWATH demi-hulls make surface waves.

Because the two major components of drag are perpendicular to each other, hull shape is the single largest player between the two. If I had a very thin, smooth, wide plank and towed it to measure it's drag, I would be measuring almost all skin friction and no pressure drag because the pressure drag not only is perpendicular to the direction of travel, it is null because the force on the port side is equal to the force on the starboard side. This is how the constants for skin friction were developed by Froude and others 150 years ago. Now if I had a parachute sea anchor or a thin disk and towed it, I would be measuring almost all pressure drag as the skin friction is now null because it is all offsetting as the flow is radial out from the center.

Of course there are other considerations that vary the relationship between the two. These are some times presented as separate issues but are actually corrections. Such things as surface roughness, kinematic viscosity, water temperature/density, and pressure gradients effect skin friction, and water depth, wave orbitals and fluid motions, hull trim and list, and again water temperature/density for pressure drag. This is why for most drag calculations the assumptions are "isodense, inviscid, and irrotational" for the fluid. The difference in drag between these three assumptions and real world is normally ~5-10% at "normal" speeds but can be as high as 50-200+% at the extremes (slow or fast) depending on what exactly you are trying to measure.