Recent content by tspeer

Sorry to be late to this discussion, but I think the explanation for the starting vortex is simple and the same as for the trailing vortices: conservation of mass. Conservation of momentum says the lift is due to the wing imparting a velocity to the fluid that is at right angles to the free...

Xfoil can be used to model the section characteristics of an articulated rudder that has no slots between the elements. You will still need to calculate the induced drag associated with the planform shape and estimate the wave drag from surface effects. Reducing the rudder area will shift the...

The lower hull speed of the smaller boat is the same thing as the lower speed from operating at the same Froude number. Say you built a quarter scale AC75 - a dinghy sized 19 footer instead of a 75 footer. If the AC75 takes off at 18 kt, the dinghy will take off at 9 kt. Both boats will have...

I misspoke in my description of Froude scaling. What I should have said is with Froude scaling, velocity is scaled with the square root of length. So the hydrodynamic forces that scale as area*velocity^2 now scale as area*sqrt(Length)^2, which is proportional to Length^3.

Talking scaling alone, Froude scaling is appropriate for hydrofoils. If weight scales as length^3, foil area scales as length^2, and you want to maintain the same lift coefficient, then the speed needs to scale as sqrt(length). In other words, the two craft need to operate at the same Froude...

Instead of thinking of aspect ratio, you should instead consider span and area separately. Unfortunately, you are constrained in span by the board trunk, and that rules out the most effective way to reduce the induced drag. So almost anything you do will result in less area than the original...

VLM would be the simplest. A panel code would be the next step up.

The Vortex application is this lifting line spreadsheet. You can read about its derivation here. Although lifting line analysis does assume high aspect ratios, I believe the induced drag results are accurate for the same spanwise downwash and lift distributions. The planform shapes that...

Actually, there is a very good reason for considering aerodynamic lift in the context of yacht performance. Here is what the lift and drag vectors, both aerodynamic and hydrodynamic, look like: If you are concerned with yacht performance, it does not matter what the leeway angle is. What...

I don't believe it would work on a displacement hull. Research with hydrofoils has shown that flow separation is necessary for ventilation, and air that is injected onto a hydrofoil is simply swept away. All of the air injection hulls that I've seen have a step that results in flow separation...

A flexible trailing edge, or flap mounted with a flexure, can act passively to transition smoothly and automatically between high and low speed positions without any control system. There are basically two design points to consider. The first is the high speed shape you want to have. The...

An exponential function has the property that the rate of change is proportional to the value of the function. They arise naturally as the solution to first order differential equations. A good example is population growth, where the increase in the population is proportional to the size of...

Polymer injection into the boundary layer can substantially reduce the skin friction drag. In 2010, BMW Oracle Racing experimented with polymer injection on the BOR90 trimaran. It was found to give the yacht a 3 kt improvement in Vmg upwind and down, in light winds and heavy. However the...

There are fast dynamics and slow dynamics to a boat's motion. Most performance related dynamics are comparatively slow and can be considered to be quasi-steady. One method I've used to model the acceleration of sailboats is to apply a fictitious D'Alembert force in the velocity prediction...

The wake trails at the apparent wind angle. This is due to the wake being convected with the true wind while simultaneously being generated at the boat's velocity. Fast boats are sensitive to small changes in the true wind speed. If the boat is going 3 times wind speed and encounters a 1 kt...