# Gem 550

1976
Designer
Rob Humphreys
Builder
Central Boating Services
Association
Micro Class (IMCCA)
# Built
40
Hull
Monohull
Keel
Swing
Rudder
?
Construction
FG

### Dimensions

Length Overall
18 0 / 5.5 m
Waterline Length
16 8 / 5.1 m
Beam
7 10 / 2.4 m
Draft
0 9 / 0.2 m 3 3 / 1 m
Displacement
1,150 lb / 520 kg
Ballast
400 lb / 180 kg

### Rig and Sails

Type
Sloop
Reported Sail Area
199′² / 18.5 m²
Total Sail Area
?
Sail Area
?
P
?
E
?
Air Draft
?
Sail Area
?
I
?
J
?
Forestay Length
?

Make
?
Model
?
HP
?
Fuel Type
?
Fuel Capacity
?

### Accomodations

Water Capacity
?
Holding Tank Capacity
?
?
Cabins
?

Hull Speed
7.8 kn
Classic: 5.47 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.

Formula

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

7.8 knots
Classic formula: 5.47 knots
Sail Area/Displacement
29.1
>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.

Formula

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.
29.07
<16: under powered
16-20: good performance
>20: high performance
Ballast/Displacement
34.6
<40: less stiff, less powerful

### 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.

Formula

Ballast / Displacement * 100

34.62
<40: less stiff, less powerful
>40: stiffer, more powerful
Displacement/Length
110.6
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.

Formula

D/L = (D ÷ 2240) ÷ (0.01 x LWL)³

• D: Displacement of the boat in pounds.
• LWL: Waterline length in feet
110.55
<100: ultralight
100-200: light
200-300: moderate
300-400: heavy
>400: very heavy
Comfort Ratio
6.6
<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.

Formula

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
6.56
<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
3.0
>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.

Formula

CSV = Beam ÷ ³√(D / 64)

• Beam: Width of boat at the widest point in feet
• D: Displacement of the boat in pounds
3.02
<2: better suited for ocean passages
>2: better suited for coastal cruising

### Notes

Originaly designed as a European IOR Micro Tonner, but has become most popular in Australia where it is was still built, at least, until 2003.

John Stockton
Central Boating Services
10 Jenkins St
(08) 8242 0066

### For Sale

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