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Impala 28 (Thomas)

1977 — 1984
Impala 28 (Thomas) insignia
David Thomas
Hunter Boats Ltd.
Impala 28 Class Association
Hunter Association (UK)
# Built
Transom hung


Length Overall
27 11 / 8.5 m
Waterline Length
23 1 / 7.1 m
9 3 / 2.8 m
5 6 / 1.7 m
4,300 lb / 1,950 kg
Drawing of Impala 28 (Thomas)
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Rig and Sails

Reported Sail Area
335′² / 31.1 m²
Total Sail Area
335′² / 31.1 m²
Sail Area
191′² / 17.8 m²
33 5 / 10.2 m
11 5 / 3.5 m
Air Draft
Sail Area
143′² / 13.3 m²
31 0 / 9.5 m
9 3 / 2.8 m
Forestay Length
32 4 / 9.9 m

Auxilary Power

Fuel Type
Fuel Capacity


Water Capacity
Holding Tank Capacity


Hull Speed
8.3 kn
Classic: 6.45 kn

Hull Speed

The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.


Classic hull speed formula:

Hull Speed = 1.34 x √LWL

A more accurate formula devised by Dave Gerr in The Propeller Handbook replaces the Speed/Length ratio constant of 1.34 with a calculation based on the Displacement/Length ratio.

Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio.311
Hull Speed = Max Speed/Length ratio x √LWL

8.29 knots
Classic formula: 6.45 knots
Sail Area/Displacement
>20: high performance

Sail Area / Displacement Ratio

A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.


SA/D = SA ÷ (D ÷ 64)2/3

  • SA: Sail area in square feet, derived by adding the mainsail area to 100% of the foretriangle area (the lateral area above the deck between the mast and the forestay).
  • D: Displacement in pounds.
<16: under powered
16-20: good performance
>20: high performance

Ballast / Displacement Ratio

A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.


Ballast / Displacement * 100

<40: less stiff, less powerful
>40: stiffer, more powerful
100-200: light

Displacement / Length Ratio

A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.


D/L = (D ÷ 2240) ÷ (0.01 x LWL)³

  • D: Displacement of the boat in pounds.
  • LWL: Waterline length in feet
<100: ultralight
100-200: light
200-300: moderate
300-400: heavy
>400: very heavy
Comfort Ratio
<20: lightweight racing boat

Comfort Ratio

This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.


Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam1.33)

  • D: Displacement of the boat in pounds
  • LWL: Waterline length in feet
  • LOA: Length overall in feet
  • Beam: Width of boat at the widest point in feet
<20: lightweight racing boat
20-30: coastal cruiser
30-40: moderate bluewater cruising boat
40-50: heavy bluewater boat
>50: extremely heavy bluewater boat
Capsize Screening
>2.0: better suited for coastal cruising

Capsize Screening Formula

This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.


CSV = Beam ÷ ³√(D / 64)

  • Beam: Width of boat at the widest point in feet
  • D: Displacement of the boat in pounds
<2: better suited for ocean passages
>2: better suited for coastal cruising


Partial history from Class website:
The initial design for the IMPALA evolved as a direct result of the formation, in 1977, of the Offshore One Design Council which was set up by a group of leading yachtsmen to encourage One Design racing. There were thirty designs submitted, including the IMPALA, of which three were given the support of the council. The IMPALA remains as the strongest supported class of those craft selected and the only one still in it’’s original form. The class pioneered standard ratings within the I.O.R. which has been copied by other classes. The first National Championshipwas held in Cowes in 1979 and was won by David Thomas, the designer. The event has been held every year since. The class rules have changed surprisingly little since the class was formed. The first main change was brought about by the change in the I.O.R. resulting in a rating increase. David Thomas, designed a lead shoe for the keel and some internal ballast which had the desired effect on the rating. Most other changes were to make the rules more rational without altering the meaning too much. One change to follow progress was the allowance of Mylar No 1 genoas. 1988 saw the introduction of the inboard diesel. This has helped bring the class up to date. The engine adopted was the YANMAR 1GM (9 hp), (see rules). In 1989 the blade no 3was adopted along with some other minor changes. The following year saw a full width main sheet traveller, and 250 lbs of lead added to the outboard to standardize both inboard and outboard C.H.S. ratings. 1993 saw the new mast from Kemps, and for 1994 a new full hoist No 2 is allowed,also the Luff Perpendicular rule has been rewritten.

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