Below decks, the Mark I exudes the traditional craftsmanship of the West Sussex shipyard, characterized by substantial teak joinery, robust laminates, and a sense of cozy security. The interior is organized around a raised deck saloon, which bathes the living space in natural light and offers panoramic views of the anchorage—a massive psychological benefit during cold, rainy stints at anchor. However, the lower profile of the deckhouse on this early model means that navigating the interior passageways requires a bit of agility. The walk-through to the aft cabin features a low-headroom "duck-through" corridor, a design compromise accepted by the builders of the era to keep the exterior lines low and sleek.
Design Brief & Intent
The core mission of the Southerly 135 Mark I was to offer an offshore-capable, luxury blue-water cruiser that could safely sit flat on its bottom. In the early 1980s, the market for large variable-draft monohulls was small, primarily contested by aluminum builders like Ovni in France. Northshore positioned the fiberglass Southerly as a more refined, traditional alternative. Dick Carter approached the design with a focus on stability and volume, utilizing a wide beam of nearly 14 feet and a low-freeboard profile.
The defining engineering feature is the massive cast-iron grounding shoe bolted to the bottom of the hull. This plate serves as the structural foundation of the boat, protecting the hull laminate when dried out on sand, mud, or rock, while housing the pivoting swing keel. Inside, the accommodation plan features a raised C-shaped dinette to port, a linear galley to starboard, a forward V-berth, a midships bunk cabin, and a master cabin aft. The presence of the keel trunk in the center of the saloon is cleverly disguised by the cabinetry, though it does dictate the social flow of the living space 6.
Variations & Configurations
While later versions of the Southerly 135 saw modifications to the superstructure and rudder configurations, the Mark I kept a relatively uniform profile. It was configured primarily as a masthead sloop with a tall, conservative aluminum mast. Draft options were fundamentally binary: with the heavy cast-iron swing keel fully retracted, the boat drew an astonishingly shallow 2 feet 9 inches, whereas lowering the blade to its maximum angle extended the draft to 7 feet 8 inches.
Crucially, the Mark I utilized a single, shallow-draft rudder. To prevent the rudder from drawing more water than the hull when the keel was retracted, the blade was designed with a wide profile. This differs substantially from the twin-rudder configurations designed by Rob Humphreys for later versions of the 135, which were introduced to prevent rudder ventilation under heavy heel and to accommodate taller, more powerful sail plans. The Mark I was also the direct precursor to the Southerly 145, which was essentially a stretched ketch-rigged variant built on the same hull form.
Sailing Performance & Handling
On the water, the Southerly 135 Mark I behaves as a traditional heavy-displacement cruiser. With a displacement of 23,000 pounds and a ballast-to-displacement ratio of 42.48 percent, the boat is remarkably stiff and has a reassuringly gentle motion in a seaway. A capsize screening ratio of 1.96 places it comfortably within the safe limit for ocean voyaging, and its comfort ratio of 28.81 reflects a hull shape that resists jerky, high-acceleration rolling.
The displacement-to-length ratio of 261.24 indicates a moderate-to-heavy displacement profile. Under sail, this means the Mark I is not a light-wind racer; it requires a decent breeze to find its stride. Upwind performance relies heavily on having the swing keel fully lowered. Dick Carter’s early keel design was a low-aspect "wedge of cheese" shape rather than a high-performance foil. While it provides excellent stability, tacking angles are wider than those of contemporary deep-draft fin-keel cruisers. Off the wind, the boat tracks beautifully. In heavy conditions, the single rudder can feel heavy and may lose traction if the boat is allowed to heel excessively, requiring early reefing of the main to maintain helm control.
Market Snapshot & Economics
The Southerly 135 Mark I is a rare find on the brokerage market, as production runs in the mid-1980s were limited before the yard pivoted to subsequent models. Consequently, clean examples command a solid premium among coastal cruisers who value the ability to navigate European canals, explore the Bahamas, or dry out in the tidal harbors of Great Britain and Northern Europe.
Prospective buyers should anticipate that a significant portion of the purchase and refit budget will center on the specialized keel mechanism. While traditional fiberglass hulls of this vintage are relatively simple to survey, a Southerly requires a specialist who understands the mechanical, hydraulic, and structural realities of the massive cast-iron grounding assembly. A neglected keel assembly can turn into a financial black hole, meaning that buyers often trade on the overall mechanical integrity of the lifting system rather than aesthetic upgrades.
Known Issues & Triage
The most critical area of triage on the Mark I is the swing-keel pivot pin and the hydraulic lifting mechanism. Over decades of service, the stainless steel pivot pin can experience crevice corrosion, and the pivot hole within the cast-iron keel can become slightly elongated. This wear manifests as an audible "thumping" sound when the boat rolls at anchor or tacks in a chop. Remedying this requires haul-out facilities where the boat can be chocked high enough to lower the keel completely out of the grounding shoe—a labor-intensive job that often requires custom scaffolding or digging a dedicated pit beneath the vessel.
The hydraulic ram, which is through-bolted to the grounding plate, is prone to seal failures and rod pitting over time. Slow retraction, hydraulic fluid leaks, or a keel that slowly drifts down after being pumped up indicate that the cylinder requires rebuilding or replacement. Furthermore, the massive cast-iron grounding plate itself is subject to severe scaling and rust if its protective epoxy coating is breached. Standard maintenance demands that this plate be sandblasted back to bare metal and recoated with a robust multi-coat epoxy system every decade.
Additionally, the deck saloon windows of this era are large, flat panels of tempered glass bedded directly into GRP frames. These are notorious for developing leaks as the hull flexes over time. Triage of any Mark I should include a thorough inspection of the headliner and the wood cabinetry beneath the forward-facing saloon windows for signs of water ingress.
Modernization & Upgrades
Many veteran owners of the Mark I are modernizing their electrical systems to better support long-term off-grid cruising. Replacing old lead-acid battery banks with high-capacity Lithium Iron Phosphate (LiFePO4) chemistry is highly viable, though it requires upgrading the alternator charging system on the auxiliary engine to prevent overheating. Given the substantial electrical demands of the hydraulic keel pump, a robust 12-volt domestic bank is essential.
The original propulsion—frequently a Buch or early Yanmar diesel—is often approaching the end of its reliable service life. Repowering with a modern 50- to 75-horsepower Yanmar or Volvo Penta diesel is a common upgrade. When doing so, owners often upgrade the hydraulic pump assembly and hoses, as the engine compartment access panels must be removed anyway. The addition of modern solar arrays on a stern arch, wind generators, and low-draw watermakers has transformed many of these vintage boats into highly self-sufficient, long-distance voyagers capable of staying in remote, shallow anchorages for weeks at a time.
The Verdict
The Southerly 135 Mark I is an exceptional, niche cruiser that rewards its owners with unparalleled navigational freedom. While it lacks the upwind agility and modern hull dynamics of its younger Humphreys-designed siblings, its rugged construction, reassuring heavy-weather motion, and light-filled deck saloon make it a premier choice for short-handed couples seeking to explore shallow frontiers without sacrificing blue-water capability.
Pros
- Unmatched draft versatility, allowing access to extremely shallow bays, canals, and drying tidal moorings.
- Stout, heavy-displacement construction with a high ballast ratio for an incredibly sea-kindly and stable ride.
- Raised deck saloon provides excellent natural light and 360-degree visibility while sheltered from the elements.
- Heavy cast-iron grounding plate offers peace of mind and structural hull protection during accidental groundings.
- Safe, secure deck layout with high bulwarks and well-placed grab rails for offshore work.
Cons
- Single shallow-draft rudder is prone to ventilation and loss of control under heavy heel if the boat is over-canvased.
- Low deckhouse profile results in a cramped, "duck-through" passageway to the aft cabin.
- Upwind sailing angles and light-wind performance are sluggish compared to fixed-keel cruisers of the same era.
- Maintenance of the swing keel, pivot pin, and hydraulic lifting ram is complex and highly expensive.
- Salon window seals are a known source of leaks and require meticulous re-bedding over time.
