Fast Electric Model Monohulls

By Paul Williams.
Article posted 17th January 2009.

Monohulls

A Monohull, as the name suggests, has a single hull. Monos are broadly divided into stepped monohulls which have one or more transverse steps in the hull to reduce wetted area and hence drag, and conventional monohulls which do not have steps.

A typical stepped monohull.

Wetted areas of conventional (left) and stepped monohulls. A stepped mono has less wetted area, and should - in theory - be faster than a conventional monohull. A stepped monohull can be setup to prop-ride, reducing wetted area to the absolute minimum.

Strictly speaking, a mono with a stepped hull is actually a two-point hydroplane, but this is a distinction that is a bit of a grey area, and somewhat open to one's interpretation of the rules. Under Naviga rules, monos can have steps, but I believe that in the USA this is not necessarily the case.

Adrian Hingston's ToySport Syncron. The Syncron is a rare beast: a stepped monohull that handles well. This boat is setup for 7 minute runtimes for SWAMBC Formula 4.

Deadrise Angle

The angle of the bottom of the hull from the horizontal is known as the "deadrise" angle. The amount of deadrise determines to a large extent how the boat handles rough water. A "rule of thumb" is that the more acute the deadrise angle, the better the boat will be in the rough, and conversely the shallower the deadrise the more the boat will struggle in rough water. The flipside to this is that shallow vee boats, in calmer conditions, are usually faster and because a shallow vee boat won't roll as much when turning, handling problems caused by the rudder are not so pronounced.

Section through a typical deep V monohull.

Spray Rails

A typical monohull design (stepped or conventional) will incorporate devices known as "spray rails" which deflect water downwards and away from the boat, both to reduce wetted area (and thus drag) and to generate lift to make the boat plane and to keep it planing when running at speed. The edge of the boat where the sides meet the bottom is known as the "chine".

Chine and spray rails deflect water downwards and away from the boat, reducing wetted area and generating lift.

Handling Problems

I consider monohulls to be the most difficult of the three hull types to setup. Unlike catamarans and hydroplanes, monohulls lean into a turn like a cyclist leaning into a corner. (At least, most monos do this - I've seen one which actually rolled outwards when turning. Needless to say, this particular design proved a complete disaster as a racing boat!)

The fact that monohulls roll from side to side when turning is a serious problem because this alters the angle at which the rudder is presented to the water line. If the rudder is at an angle to the planing waterline, it will typically lift the back of the boat, causing the back of the boat to wobble and hop around and increasing the tendency of the boat to spin in the turns.

When a Vee hull banks over in a turn, the rudder angle relative to the water changes, causing lift as water flows down the blade. Lifting the transom can cause the boat to spin out.

The effects of rudder angle relative to the planing waterline.

Strut Angle

By altering the strut angle, and hence propeller angle, you can change the way the boat rides. For example, angling the strut upwards pulls the transom down and elevates the bow, reducing wetted area. Angling the strut downwards does the reverse.

Effect of strut angle on hull ride attitude.

Chine Walking

Monohulls can exhibit a behaviour known as chine walking. This is when the boat rolls violently from side to side. Usually the boat will sort itself out without the driver needing to do anything, or the driver may need to close the throttle and allow the boat to settle before accelerating. Chine walking can sometimes cause a mono to flip out.

Monohulls can sometimes "chine walk", or bounce from chine to chine.

Chine walking can be worse on boats which have the weight biased heavily to the left to counter prop walking and propeller torque effects. If you think you have the boat too unbalanced, look at alternative methods of curing propeller effects such as angling the drive or using torque wedges.

Porpoising

Porpoising - a rapid up and down pitching motion.

Porpoising is a rapid up and down pitching motion caused either by rocker, a radiused transom or a CofG (longitudinal balance point) that is too far rearward. Porpoising kills speed, as the propeller is fully immersed as the boat pitches up, which also drives up current draw and places more stress on the speed controller.

Self-righting

If monohulls have one single redeeming feature that absolves them of their sins, it is the ability to be made self-righting. A mono typically has a narrow hull and most European monos come with, or can be fitted with, a flood chamber. The principal of the flood chamber is simple: when the boat flips, the chamber fills with water which sinks one side of the boat, rolling it far enough that the boat rolls upright.

Flood chamber running down the left side of the boat.

Catamarans and hydroplanes are usually impossible to make self-righting. The much wider track of a cat or hydro means they are very stable both upright and when flipped over. Flood chambers work best on monohulls when the semi-scale canopy on the deck is offset to the right and the weight is biased to the left. Self-righting can fail to work if the battery pack moves inside the boat when it flips, or if the boat has partially filled with water.

Monohull Turn Fins

Oval racing monohulls that need to turn at full speed often require a turn fin fitted to the transom to stabilise the back end as the boat rolls into the turn, mainly to stop the boat spinning. Mono turn fins are usually mounted such that fin is upright when the boat is rolled over in a turn:

Monohull turn fin.

A turn fin is a source of drag, and you should use the smallest sized blade that stops the boat spinning. As with hydroplanes, the turn fin should be able to pivot upwards if the boat rides over something.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:06