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the physics of blokart^® sailing

recommended skill level for this guide

intermediate

The power of a blokart^®, like any other sailing vessel comes from the way in which the sail catches the wind. A sail is in fact a vertical wing. It operates in the same way as a wing on a plane. A blokart^® uses this wing (the sail), and the wheels (in place of a yachts centre board) to propel it forward.

The power comes not from wind blowing on a sail but pressure difference around the sail (high pressure on the side towards the wind, and low pressure on the side away from the wind). In nature, high pressure moves towards low pressure, as a result, there is a perpendicular force exerted on the sail. This is shown in figure 1. The forces on the wheels which track the blokart^® in a single direction (forward not sideways) much like the centre board on a yacht, act in combination with those on the sail to power the blokart^® forward.

The combination of the wheels and sail do result in a forward motion, but there can be some slipping depending on the surface being sailed upon. The term for this slipping is "drift". On a loose surface, as shown in figure 2, the blokart^® will not travel in the exact direction in which it is pointing. The looser the surface, the more drift the blokart^® will experience. This is an important concept to be aware of, especially if there are hazards nearby.

The last aspect of the physics of sailing that we will discuss is the concept of true and apparent wind. These terms refer to the wind and its changes due to the wind direction and the speed of the blokart^® in relation to its direction. True wind is defined as the direction of the wind to a stationary observer. Induced wind is the wind experienced due to the forward movement of the blokart^®. A good analogy is riding a bike. When riding a bike on a day with no wind, the rider still feels a wind. This wind is induced by riding the bike through the air. A blokart^® experiences wind in the same manner. The combination of these two winds is the actual wind experienced by the blokart^®. This wind is called apparent wind. Figure 3 outlines the three winds and how they relate to one another.

As mentioned earlier, the knowledge of these basic physical principles are not needed to be a proficient sailor. They do, however, help sailors understand how the wind conditions affect a blokart^®.

introduction to steering and adjustment

There are two important parts of the blokart^® that sailors must be aware of before starting to sail. First, the sheet. This is not the sail, but the rope that controls the position of the sail. By pulling in the sheet, the sail moves towards the centre of the blokart^®, and by letting out the sheet the sail moves away from the centre. Figure 4 and figure 5 outline the movements of both the sheet and the sail.

The arrows indicate the movement of the sheet and the resulting movement of the sail.

The second part of the blokart^® to be aware of is the handle bars. This isn’t a trick question… they simply steer the blokart^®.

basic sailing terminology

Unlike the physics of sailing the terminology of sailing is something which blokart^® sailors should be familiar with. Most sailing terminology refers to the position of your blokart^® in relation to the wind. Figure 6 identifies four sailing terms that refer to locations on the blokart^®. To identify the left side of the blokart^® sailors use the term port. Conversely, to refer to the right side of the blokart^® sailors use the term starboard. Figure 6 also indicates the terms leeward and windward. These two terms refer to the sides of the blokart^® in relation to the wind. The side towards the wind is called the windward side, and the side which is away from the wind is called the leeward side.

When sailors use the term tack, they are referring to the side of the blokart^® on which the sail is positioned. Figure 7, shows the two different tacks; port tack and starboard tack.

A blokart^® is on a port tack when the wind flows over the port side of the blokart^® first, and then hits the sail. A blokart^® is on a starboard tack when the opposite occurs.

tip Are you struggling with the Port & Starboard thing? Try to remember this….

“There is a little bit of port left in the bottle”

The terms used to describe a the movement of a blokart^® in relation to the wind are outlined in figure 8. It is a good idea to be familiar with all of these terms, as there is no standard term used.

basic sailing techniques

points of sail

Points of sail is the term used to describe the course of a blokart^® in relation to the wind direction.

First, there is a distinction between the port tack and the starboard tack. If the wind is coming from anywhere on the port side, the blokart^® is on port tack. Likewise if the wind is coming from the starboard side, the blokart^® is on starboard tack. With the exception of Head To Wind, a blokart^® will be on either port or starboard tack while on any point of sail.

The points of sail are as follows:

Head To Wind

At this point of sail the blokart^® is headed directly into the wind. A blokart^® turns through this point of sail as it performs a tack. The blokart^® is on neither port nor starboard tack. Since a blokart^® cannot sail directly into the wind.

§ No Go Zone

The blokart^® is pointed too close to the wind for the sail to generate any power. The sail will be luffing "flapping" in the breeze and making noise, like a flag.

The size of the no-go zone will differ based on the setup of the sail (battern shape/tension etc) and the configuration of mast sections used. For example, a race tuned blokart^® can usually sail much closer to the wind than a standard blokart^®. This is known as "pointing higher." Pointing ability can be very important for racing a blokart^®. Depending on wind direction in relation to the set course, often a real goal in a race is velocity made good (VMG). VMG is the speed at which the blokart^® is approaching the destination (usually a buoy or mark) as opposed to the straight line speed at which the blokart^® is moving along the track (kart speed). These two speeds almost always vary because, during a race, a blokart^® often cannot sail directly to the next mark. VMG may also refer to the upwind vector of kart speed (this is often the VMG expressed on sailing instruments).

§ Close Hauled

A blokart^® is sailing close hauled when its sail is trimmed in tightly and it is sailing as close to the wind that it can without entering the No Go Zone. This point of sail lets the blokart^® travel diagonally upwind. This is a precise point of sail. However, the exact angle relative to the wind direction varies due to setup. A blokart^® is considered to be "pinching" if the sailor tries to sail above an efficient close hauled course and the sail begin to luff slightly.

§ Close Reach

This is any upwind angle between Close Hauled and a Beam Reach.

§ Beam Reach

This is a course steered at right angles to the wind. This is a precise point of sail.

§ Broad Reach

The wind is coming from behind the blokart^® at an angle. This represents a range of wind angles between Beam Reach and Running Downwind. The sails are eased out away from the blokart^® as much as possible.

§ Running Downwind

On this point of sail, the wind is coming from directly behind the blokart^®. This is the only point of sailing that the wind simply pushes the sail along, it is often called the "don't go zone". The high aspect ratio sail used on a blokart^® (unlike the big fat spinnaker that “wet” yachts use downwind) is very inefficient on this point of sail. This combined with the minimal surface friction (compared to “wet” yachts) which enables the blokart^® to generate much faster speeds, basically rule out this point of sail in all but the lightest of breezes.

basic turning

When turning a blokart^®, the direction relative to the wind is as important as the direction overall. Thus all turns can be described by one of the following terms:

Heading up (or luffing up) is turning the blokart^® to sail closer to the direction the wind is coming from. In order to keep the sail correctly trimmed, it must be pulled in towards the centre of the blokart^®. Continuing to head up will bring the front wheel so close to the wind that the sail will no longer fill - this is called "being in irons", or "the no-go zone".

If the turn is continued through the no-go zone and out the other side, the blokart^® is said to have tacked. Thus, a tack is a turn that takes the front wheel of the blokart^® through the eye of the wind.

Bearing away (or falling off) is turning away from the direction the wind is coming from. As with luffing up, the sails must be adjusted during the turn, in this case let out away from the centre of the blokart^®. If the turn is continued, the blokart^® will end up running directly away from the wind, with the sail at around 90º to the blokart^® and acting as a simple inefficient wind-catchers rather than aerofoils.

If the turn is continued such that the back of the blokart^® passes through the wind, a jibe results. Jibing causes the boom to swing from one side to the other, sometimes rapidly, as the wind catches the leech of the mainsail on its new upwind side.

sailing upwind

Using a series of close hauled legs to beat a course upwind

A basic rule of sailing is that it is not possible to sail directly into the wind. Generally speaking, a blokart^® (like all other yachts) can sail up to 45 degrees off the wind. When a blokart^® is sailing this close to the wind, it is close-hauled or beating (beating to weather).

Since a blokart^® cannot sail directly into the wind, but the destination is often upwind, one can only get there by sailing close-hauled with the wind coming from the port side (the boat is on port tack), then tacking (turning the blokart^® through the eye of the wind) and sailing with the wind coming from the starboard side (the blokart^® is on starboard tack). By this method, it is possible to reach that destination directly upwind. How close a blokart^® can sail to the wind depends on the mast configuration, sail trim and the wind speed, since what the blokart^® "sees" is the apparent wind, i.e., the vector sum of the actual wind and the velocity of the blokart^®. As the velocity of the blokart^® increases, the apparent wind direction moves further forward (toward close-hauled) and is what your wind indicator clipped to the front of the mast shows. A good analogy for this increase in apparent wind would be walking through an indoor room and feeling the "wind" on your face.

sail trimming

As noted above, sail trimming is a large subject. Basic control of the mainsail consists of setting the sail so that it is at an optimum angle to the wind, (i.e. no flapping at the front, and tell tales flowing evenly off the rear of the sail).

More detailed aspects include specific control of the sail's shape, e.g.:

altering sail shape to make it flatter in high winds
bending the mast back by increasing main sheet tension when going upwind (to tilt the sail towards the rear, this being more stable)
providing sail twist to cope with gusty conditions