1974 — 1982
Olle Enderlein
# Built


Length Overall
30 8 / 9.4 m
Waterline Length
24 8 / 7.5 m
9 4 / 2.9 m
4 7 / 1.4 m
9,250 lb / 4,196 kg
4,200 lb / 1,905 kg
Drawing of Monsun 31
  • 1 / 2
  • 2 / 2

Rig and Sails

Reported Sail Area
414′² / 38.5 m²
Total Sail Area
414′² / 38.4 m²
Sail Area
189′² / 17.5 m²
32 9 / 10 m
11 6 / 3.5 m
Air Draft
Sail Area
225′² / 20.9 m²
37 9 / 11.5 m
11 10 / 3.6 m
Forestay Length
39 7 / 12.1 m

Auxilary Power

Volvo Penta
MD 11 C
Fuel Type
Fuel Capacity
32 gal / 121 l


Water Capacity
43 gal / 163 l
Holding Tank Capacity


Hull Speed
7.2 kn
Classic: 6.66 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

7.15 knots
Classic formula: 6.66 knots
Sail Area/Displacement
<16: under powered

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
>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.


Ballast / Displacement * 100

<40: less stiff, less powerful
>40: stiffer, more powerful
200-275: moderate

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
20-30: coastal cruiser

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 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.


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


Launched in 1974 the Hallberg-Rassy 31 Monsun is a no-nonsense offshore cruiser with excellent build quality and sturdy enough for the needs of any long distance sailor. The boat is blessed with good accommodation, a secure feel, and eye-catching lines. These all had a parts to play in her notable success as the all time best seller for the prestigious Hallberg-Rassy brand.

Performance is mediocre due to her relatively heavy displacement, she does not point very high in light winds but this is not the reason sailors buy the Monsun 31. It’s her stability in the water that inspires confidence in offshore conditions – she gives an almost leisurely feel, and it’s this that’s earned her an excellent reputation among sailors.

Overall this 31-footer from the 1970s is sought after and this is reflected in her high price. You’ll find few owners looking to sell their boat.


The Monsun 31 bares the distinction as the first all-new design from the Hallberg-Rassy yard after the merger of the Rassy and Hallberg boatyards in 1972 that formed the company name. The new company came into existence when German expatriate Christophe Rassy bought the competing and well-respected Hallberg yard when it was time for its founder, Harry Hallberg, to retire.

At the time, Rassy was successfully building his Rasmus 35, a sturdy centre-cockpit cruiser, while Hallberg was producing four boats ranging from 24 to 33 feet. Of particular trouble was Hallberg’s Mistress 32, a very racing oriented design that competed successfully in the IOR half ton class. She was in full production and was at boat that was both expensive to build and sold relatively cheaply – this was terrible business and Hallberg-Rassy needed something new.

Rassy approached Olle Enderlein to draw up a boat similar to the Rasmus 35 but smaller and sleeker. The new boat had to have an interior that appealed to couples and for that it was important to have levels of comfort. Enderlein based the layout of the new interior on the Mistress but was much simpler, more streamlined and straightforward to produce and therefore affordable.

Production started in 1974 and a total of 904 boats were built until production ceased in 1982 when the boat was superseded by the Hallberg-Rassy 312 (launched in 1979).


LOA: 30′ 9″
LWL: 24′ 8″
Beam: 9’ 5″
Draft: 4′ 7″
Displacement: 9,250 lbs.
Ballast: 4,200 lbs.
Sail Area: 430 sq. ft.

Water: 43 US Gal.
Fuel: 32 US Gal.
Engine: 23hp Volvo Penta MD11C

Designer: Olle Enderlein
Builder: Hallberg-Rassy
Year Introduced: 1974
Year Ended: 1982
Total Built: 904

Also Known As: Monsun 31

Similar Boats

Hallberg-Rassy 312

Hallberg-Rassy 310

Hallberg-Rassy 31

Moody 31

Hallberg-Rassy 352

Hallberg-Rassy 35 Rasmus

Links, References and Further Reading

» The official Halberg-Rassy website, information on the Hallberg-Rasssy 31 Monsun
» Danish Monsun 31 Owners Club

For Sale

Have a sailboat to sell?
List it for free and it will show up here.

Great choice! Your favorites are temporarily saved for this session. Sign in to save them permanently, access them on any device, and receive relevant alerts.

We will occasionally send you relevant updates. You can opt out or contact us any time.


Get occassional updates about new features or featured sailboats.
You can opt out or contact us any time.

Made with ♥ by the founders of Refit Guide
©2023 Sea Time Tech, LLC

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.