1969 — 1977
Designer
Sparkman & Stephens
Builder
Alpa Yachts
Associations
?
# Built
15
Hull
Monohull
Keel
Fin
Rudder
Skeg
Construction
FG

Dimensions

Length Overall
41 6 / 12.7 m
Waterline Length
29 11 / 9.1 m
Beam
11 5 / 3.5 m
Draft
6 9 / 2.1 m
Displacement
18,640 lb / 8,454 kg
Ballast
9,700 lb / 4,400 kg (Lead)
Drawing of Alpa 12.70

Rig and Sails

Type
Sloop
Reported Sail Area
788′² / 73.2 m²
Total Sail Area
?
Mainsail
Sail Area
?
P
?
E
?
Air Draft
?
Foresail
Sail Area
?
I
?
J
?
Forestay Length
?

Auxilary Power

Make
Lombardini
Model
?
HP
45
Fuel Type
Diesel
Fuel Capacity
25 gal / 95 l

Accomodations

Water Capacity
100 gal / 379 l
Holding Tank Capacity
?
Headroom
?
Cabins
?

Calculations

Hull Speed
7.6 kn
Classic: 7.33 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

7.58 knots
Classic formula: 7.33 knots
Sail Area/Displacement
17.9
16-20: good 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.
17.94
<16: under powered
16-20: good performance
>20: high performance
Ballast/Displacement
52.1
>40: stiffer, more 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

52.05
<40: less stiff, less powerful
>40: stiffer, more powerful
Displacement/Length
310.6
275-350: heavy

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
310.61
<100: ultralight
100-200: light
200-300: moderate
300-400: heavy
>400: very heavy
Comfort Ratio
33.4
30-40: moderate bluewater cruising 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
33.42
<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
1.7
<2.0: better suited for ocean passages

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
1.72
<2: better suited for ocean passages
>2: better suited for coastal cruising

Notes

Thanks to ‘rmanganaro’ for providing additional information including the following commentary:

The design was commissioned by 12 individuals who wished to purchase the latest world class racer. (S&S was certainly the ‘cutting edge’ at the time). Alpa, selected as the builder, absorbed some of the cost of the tooling with high hopes of building and selling many more.
The boats are said to be have been built to a very high standard, (with inspections by S&S, Lloyds of London at every stage of construction.) Costs also proved to be higher than expected.
When the first 12 were finally delivered, the International Offshore Rule (IOR) had just been introduced, and these boats, originally designed to the RORC, could no longer be sold as the latest racing design.
Alpa only sold 3 more besides the original 12 and the whole project created a huge financial strain on the company.

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