General
Description
Class
Rules
Contact
The 750mm Radio Controlled Yacht
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General
Description |
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Brief
History
The 750mm model yacht class began in 1986 in Sydney Australia. What
was conceived as a one off race event grew into a monthly race
series.
The first boats had rigs up to 1.2m, keels of around 300mm and bulb
weights of less than 1kg. The more lightly ballasted boats fared well
at this stage as the limiting factor for the ability to carry sail
was nosediving when running downwind. The nosediving problem was
largely overcome in early 1988 with the introduction of the foil
rudder.
From this time on the more heavily ballasted, longer keeled boats
began to prove superior. This led to the current restrictions being
placed on these two variables.
750 designers have experimented with wing masts, multiple rudders,
una rigs and wing top rigs. This experimentation, which is the basis
of the class, continues today.
Currently, the only venue for regular 750 regattas is Queenscliff
Lagoon, Pittwater road, Queescliff, NSW Australia. Racing at this
venue is usually held on the second Sunday of each month. The
organisational style of the 750mm Model Yacht Association is 'very
casual'.
As with all restricted class sailing vessels, the appearance of the
750mm model yacht is governed by the class rules.
The maximum hull length of 750mm produces boats with vertical stems
to gain maximum waterline length (for speed). The maximum
bulb(ballast) weight of 1.5 kg means that in general the beam will be
around 200 to 300mm and the hull depth is about 100 to 150mm.
The sail area is unrestricted. In moderate winds, rigs of up to 1.5m
in height are being used and even taller rigs are used in light
conditions. The height restriction for the rigs is 1.8m. The style of
rigs used varies but all are of the 'balanced' type to reduce loads
on the actuating servos. The use of standard two channel radio
equipment is encouraged. Sometimes up rated servos are used for sail
control, but sail winches have not been shown to be necessary.
The keels are quite long in proportion to the hull. The maximum keel
depth that has been used is 750mm but something around 600mm seems to
work well at present. These small models consistently sail in winds
that scale to gale force for a full size vessel, so maximum stability
is essential.
Probably the most distinctive feature of the 750 is the rudder. Being
quite short for the amount of sail they can carry, 750's suffer from
nosediving. Early attempts to cure this by using fuller bow sections
or moving the weight further aft were largely unsuccessful. The
answer was to attach an inverted foil to the bottom of the rudder
which prevents the stern from lifting. Now, instead of nosediving, it
is possible to drive a 750 completely underwater whilst maintaining
perfectly level trim. A side benefit of the foil is that is possible
to sail upwind at up to 60 degrees of heel with a neutral helm. The
rudder may be fitted on an outrigger as long as the overall length of
the vessel does not exceed 1000mm.
Due to its relatively light weight the 750 can be very difficult to
tack in strong winds.
Typical Dimensions
Sail area: Rig height for moderate wind conditions is 1400 to 1600mm. Main boom length 400 to 500mm. Jib boom length 250 to 350mm. Light weather rigs up to 1800mm high. Storm rigs around 1200mm high.
Hull beam: Wide range of variation, 80 to 300mm
Draft: 400 to 650mm
Total Displacement: 2.0 to 2.5kg. Bulb weights, 1.2 to 1.5kg.
Construction Materials: Balsa wood. Carbon fibre composite.
Foils: The keel is usually a very high aspect ratio fin with ballast attached as a bulb. Chord lengths for the keel range from 80 down to 40mm. A 40mm chord keel will usually have a short canard foil attached to the hull just forward of the leading edge of the keel to prevent stalling at low speeds.
The rudder is usually around 250 to 350mm deep with a chord similar to or greater than the keel.
In moderate conditions and above, an inverted 'T' foil is usually attached to the bottom of the rudder to prevent nose diving downwind, and to improve helm balance upwind. The foil has a span of 250 to 350mm and a chord similar to that of the rudder. It is set at an angle of around 0 to 2 degrees relative to the waterline.
1. Objectives:
(a) To provide rules which allow the maximum freedom for design experimentation consistent with the provision of close racing.
(b) To allow the maximum performance consistent with low cost.
(c) To promote a spirit of friendly rivalry between competitors.
2. Unrestricted Items:
(a) Sail area
(b) Hull beam
(c) Draft
(d) Total Displacement
(e) Construction Materials
3. Restricted Items:
(a) Hull overall length shall not exceed 750mm.
(b) Maximum overall length shall not exceed 1000mm (allowance for overhanging booms, rudders etc).
(c) The rig shall not exceed 1800mm in height, measured from the deck at the mast base.
(d) The weight of material used specifically for ballast shall not exceed 1.5kg.
(e) Radio equipment may have a maximum of two (2) channels.
Note: Definition of restricted items is subject to clause 5.
4. Prohibited Items:
(a) Multihulls
(b) Bowsprits or other projections attached to the hull forward of the bow. A resilient bumper may be added forward of the stem provided that it does not form part of the hull.
5. Disputes:
The legality of any yacht claimed to be a 750mm Class Model Yacht may be challenged, at a race event, by a 750mm Model Yacht Association member or members. In such event the legality of the disputed yacht shall be verified (or not) by a majority ruling of the members present at that race event.
Any yacht or item thus declared legal shall form a precedent for future rulings. However the ruling may be overturned at a future race event, if there are sufficient grounds for doing so, by the same process.
Notes.
1) Prohibited items:
Multihulls are prohibited due to the general belief that multihulls will not provide close racing (objective a) due to capsizing and large speed differences.
Projections forward of the stem are prohibited to minimise collision damage.
2) These rules are subject to revision at any time (see clause 5).
750mm Model Yacht Association
Secretary: Malcolm Smith
mal@cybernautics.com.au