Sailboat Calculations

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Understanding the Average Motoring Speed of a Classic 70-Ton Sailboat: A Detailed Analysis with Engine Use Insights

When evaluating the performance of a classic 70-ton sailboat, understanding its behavior under motor power is essential for owners, designers, and sailing enthusiasts. While these vessels are built to harness the wind, their auxiliary engines are critical for situations where sailing isn’t feasible—such as calm winds, headwinds, or tight maneuvers in harbors. A key question arises: what is the typical speed distribution when motoring, and how does engine use vary across different types of sailors? In this article, we’ll explore a realistic speed distribution for a 70-ton sailboat equipped with two Cummins QSB6.7 550 hp engines, calculate its average motoring speed, and examine how engine use—averaging 30% across a mix of top-level and day-to-day sailors—shapes its operational profile.

The Role of Engine Power in a Sailboat

A 70-ton sailboat, with a length overall (LOA) of approximately 22 meters (waterline length around 18-20 meters), is a substantial cruising yacht designed for long-distance travel. Its sails are the primary means of propulsion, but the auxiliary engines—totaling 1100 hp (820 kW)—provide essential power when sailing isn’t practical. Operational data for this sailboat indicates that, on average, the engine is used 30% of the time, with the remaining 70% under sail power. This 30% average reflects a mix of sailing behaviors: top-level sailors, who use their engines only 10% of the time, and day-to-day, less competent, or “lazy” sailors, who rely on their engines 50% of the time due to challenges like headwinds or a preference for convenience. Our focus here is on the speed distribution during the 30% of the time the engine is running, examining speeds from 1 to 11 knots to understand the vessel’s motoring behavior.

Engine Use Across Sailor Types

Engine use varies significantly based on sailor skill and intent:

Top-Level Sailors (10% Engine Use): These are experienced, competitive, or professional sailors—think racers or circumnavigators. They prioritize sailing performance and fuel efficiency, using their engines minimally. Skilled at sail trim, tacking, and course planning, they can make progress in light winds (e.g., 5-10 knots, achieving 4-6 knots under sail) or headwinds (tacking efficiently to maintain a velocity made good of 2-3 knots). They might motor only in dead calms (0-3 knot winds), for harbor maneuvers, or in emergencies, totaling just 10% of their time underway. For example, in a 24-hour passage, they’d motor for 2.4 hours—perhaps 1-2 hours for docking and brief periods in no wind.
Day-to-Day Sailors (50% Engine Use): These are recreational or casual sailors—weekend cruisers or less experienced owners. They often motor in headwinds (e.g., 15-20 knots, where sailing upwind yields a slow 2-3 knots VMG, but motoring at 7 knots is faster), light winds (to maintain speed), or for convenience (e.g., avoiding tacking). They might motor out of a marina, into headwinds, and back to port, totaling 50% of their time. In a 10-hour trip, this could mean 5 hours of motoring, especially in coastal cruising where headwinds or schedules dictate engine use.
Average Engine Use (30%): The 30% engine use figure is a weighted average, reflecting a mix of these groups. If we assume an equal split (50% top-level sailors, 50% day-to-day sailors), the average is (10% + 50%) / 2 = 30%. This aligns with the operational data, representing a balanced view of how a 70-ton sailboat is used across a diverse user base.

This 30% engine use means that, over the sailboat’s total operating time, the engine is running for 30% of the time (e.g., 7.2 hours in a 24-hour period), with the remaining 70% under sail power (where fuel consumption is zero). Our speed distribution below applies to the time the engine is on, providing insight into motoring performance.

Speed Distribution Under Engine Power

To calculate the average speed under power, we need a realistic speed distribution—how often the sailboat operates at each speed when motoring. Based on operational data for this 70-ton classic sailboat, the following distribution reflects the percentage of time spent at each speed (from 1 to 11 knots) during engine use:

1 knot: 0.2%
2 knots: 0.3%
3 knots: 0.5%
4 knots: 0.5%
5 knots: 11.6%
6 knots: 20.1%
7 knots: 54.8%
8 knots: 5.0%
9 knots: 5.0%
10 knots: 1.0%
11 knots: 1.0%

This distribution sums to 100% and represents the sailboat’s motoring behavior during the 30% of total operating time when the engine is on. For example, 54.8% of the time the engine is running, the sailboat is at 7 knots, which is 54.8% of that 30%—or 16.44% of the total operating time (0.548 × 0.3).

Why This Distribution Makes Sense

The speed distribution aligns with the practical realities of a 70-ton sailboat under motor power:

7 knots (54.8%): This speed dominates the distribution, and for good reason. A sailboat with a waterline length of 18-20 meters has a theoretical hull speed (the maximum efficient speed for a displacement hull) of approximately 8-9 knots, calculated using the formula: Hull Speed (knots) = 1.34 × √LWL (feet). For an LWL of 65.6 feet (20 meters), this is 1.34 × √65.6 ≈ 10.85 knots. Motoring at 7 knots—about 65-80% of hull speed—is a sweet spot for fuel efficiency and comfort, making it the most common speed for cruising under power. At 7 knots, the sailboat consumes 54.8 L/h of fuel, a reasonable trade-off for steady progress, whether a top-level sailor is motoring briefly in a calm or a day-to-day sailor is avoiding tacking into a headwind.
6 knots (20.1%) and 5 knots (11.6%): These slower speeds account for 31.7% of motoring time. Sailboats often reduce speed to conserve fuel, navigate in moderate conditions, or maintain control in busy waters. At 6 knots, fuel consumption is 40.4 L/h, and at 5 knots, it’s 27.9 L/h—both more economical than higher speeds. A day-to-day sailor might motor at 5-6 knots in light winds (e.g., 5-10 knots, where sailing yields only 2-3 knots), while a top-level sailor might use these speeds briefly during harbor entry.
8-9 knots (5.0% each): These speeds approach the hull speed limit, requiring significantly more power (71.7 L/h at 8 knots, 90.4 L/h at 9 knots). Sailboats rarely motor at these speeds for long, as the exponential increase in drag makes it inefficient. A top-level sailor might avoid these speeds entirely, switching to sail power (where 8-9 knots is achievable in moderate winds), while a day-to-day sailor might motor here briefly to fight a current or headwind.
10-11 knots (1.0% each): These speeds are at or beyond the hull speed, consuming 111.6 L/h at 10 knots and 134.7 L/h at 11 knots. Such high speeds under motor power are rare, reserved for emergencies or specific situations (e.g., avoiding a hazard). Both top-level and day-to-day sailors would typically sail at these speeds, as it’s more efficient and aligns with the vessel’s design.
1-4 knots (0.2% to 0.5% each): Low speeds total just 1.5% of motoring time. These speeds (4.5 L/h at 1-2 knots, 10.1 L/h at 3 knots, 17.8 L/h at 4 knots) are used for short bursts during docking, harbor maneuvering, or slow motoring in confined areas. Top-level sailors might minimize this time by sailing onto a mooring, while day-to-day sailors might motor longer at these speeds for safety.

Calculating the Average Speed Under Power

To find the average speed under power, we calculate the weighted average speed based on the distribution, reflecting the time the engine is running:

1 knot: 1 × 0.002 = 0.002
2 knots: 2 × 0.003 = 0.006
3 knots: 3 × 0.005 = 0.015
4 knots: 4 × 0.005 = 0.02
5 knots: 5 × 0.116 = 0.58
6 knots: 6 × 0.201 = 1.206
7 knots: 7 × 0.548 = 3.836
8 knots: 8 × 0.05 = 0.4
9 knots: 9 × 0.05 = 0.45
10 knots: 10 × 0.01 = 0.1
11 knots: 11 × 0.01 = 0.11

Total Weighted Speed: 0.002 + 0.006 + 0.015 + 0.02 + 0.58 + 1.206 + 3.836 + 0.4 + 0.45 + 0.1 + 0.11 = 6.725 knots.

Average Speed Under Power: 6.725 knots, which we can round to 6.7 knots.

This 6.7-knot average applies to the time the engine is on—whether that’s 10% of the time for a top-level sailor (2.4 hours in a 24-hour period) or 50% for a day-to-day sailor (12 hours in 24 hours). The 30% average engine use means that, across all sailors, the engine runs for 7.2 hours in a 24-hour period, and during that time, the sailboat averages 6.7 knots.

Impact of Engine Use on Overall Operation

The 30% average engine use provides context for the sailboat’s overall operation:

Top-Level Sailors (10%): They motor for 2.4 hours in a 24-hour period, averaging 6.7 knots while motoring. The remaining 21.6 hours are under sail power, where they might average 5-8 knots in typical conditions (10-20 knot winds), or 8-11 knots in stronger winds.
Day-to-Day Sailors (50%): They motor for 12 hours in 24 hours, also averaging 6.7 knots while motoring. Their 12 hours under sail might yield lower speeds (e.g., 3-5 knots) due to less efficient sailing techniques, especially in headwinds or light winds.
Average (30%): Across all sailors, the engine runs for 7.2 hours in 24 hours, averaging 6.7 knots. The remaining 16.8 hours under sail power contribute to the overall speed profile, which depends on wind conditions and sailor skill.

The speed distribution (e.g., 54.8% at 7 knots) remains the same regardless of engine use percentage, as it applies only to the time the engine is running. However, the overall fuel consumption scales with engine use: a top-level sailor uses less fuel (10% of the time at 58.29 L/h weighted average), while a day-to-day sailor uses more (50% of the time).

Why 6.7 Knots is a Realistic Average

The 6.7-knot average speed under power aligns with the operational realities of a 70-ton sailboat:

Hull Speed Constraint: At 10.85 knots, the hull speed sets a practical limit. Motoring at 7 knots (just below this limit) for 54.8% of the time keeps the average close to 7 knots, but the 31.7% of time at 5-6 knots pulls the average down slightly.
Fuel Efficiency: Higher speeds (8-11 knots) are fuel-intensive (71.7-134.7 L/h), so the sailboat spends only 12% of motoring time here, minimizing their impact on the average.
Practical Use: Sailboats often motor at 5-7 knots for efficiency and control, especially in calm seas or when wind conditions aren’t favorable for sailing. The 6.7-knot average reflects this balance, whether a top-level sailor is motoring briefly in a calm or a day-to-day sailor is motoring into a headwind.

Implications for Owners and Designers

For owners of a classic 70-ton sailboat, this speed distribution and 6.7-knot average provide a clear picture of motoring performance. It highlights the vessel’s efficiency at 5-7 knots, where it spends 86.5% of its motoring time, consuming 27.9-54.8 L/h. The 30% average engine use underscores the balance between sailing and motoring: top-level sailors (10% engine use) maximize sail power, while day-to-day sailors (50% engine use) rely more on the engine for convenience, especially in headwinds or light winds. For designers, this data emphasizes the importance of optimizing hull design for 5-7 knots, as these speeds dominate real-world motoring use. The minimal time at 10-11 knots (2%) suggests that pushing beyond hull speed under power isn’t practical—sail power is better suited for higher speeds.

Conclusion

The average speed of a classic 70-ton sailboat under motor power, based on a realistic speed distribution, is 6.7 knots. This figure is derived from a detailed operational profile: 54.8% of motoring time at 7 knots, 20.1% at 6 knots, 11.6% at 5 knots, and smaller percentages at other speeds from 1 to 11 knots. This distribution applies to the 30% of total operating time when the engine is running—an average that balances top-level sailors (10% engine use) and day-to-day sailors (50% engine use). Whether you’re a skilled sailor motoring minimally or a casual cruiser relying on the engine half the time, the 6.7-knot average provides a solid benchmark for understanding motoring performance in real-world conditions. For planning voyages, comparing efficiency, or designing new vessels, this data offers a clear and practical foundation.