45 Catamaran Sailboat

Introduction:

The appeal of the open sea, combined with the sustainable promise of renewable energy, makes the prospect of solar-powered electric boats an exciting idea. This paper explores the feasibility and considerations of re-powering a 45-foot catamaran sailboat into an entirely solar-powered electric vessel.

This project involves a relatively comprehensive transformation of a typical mid-sized catamaran, such as a Leppard or Lagoon. The sails and rigging are completely removed, and in its place, we will install a prefabricated and robust tube frame, made of stainless steel. The frame serves as the base of a mounting structure to accommodate a maximally sized array of solar panels. The solar array is setup to be a static (non-movable) array that will harness the sunlight and transform it into the primary energy source for the vessel.

The catamaran has a length of 45 feet and a beam of 24 feet. This area provides ample space for installing the solar array. The solar panels we select are highly efficient, with each capable of producing approximately 490 watts. The solar array will not only power the propulsion system but also meet the energy requirements of all other onboard utilities.

The following sections of this paper delve into the specifics of the propulsion system, including energy calculations, and considerations related to the transition from conventional propulsion to a solar electric system. The aim is to provide an overview of the design considerations, challenges, and benefits of this green marine venture.

IMPORTANT NOTE:

While this paper focuses on the energy calculations and propulsion considerations for transitioning to a solar electric system, the successful implementation of this project requires a holistic approach that considers all elements of the system and vessel. This paper does not cover in enough depth, many areas involving the electrical and mechanical modifications required for the described conversion.

Power Analysis of a 45-Foot Catamaran Propelled by Two 25 kW Electric Motors 

Assumptions and Inputs:

1. The boat motors for 10 hours per day, consuming a total of 9.4 kW while running.
2. The solar array produces peak power (20.5 kW) for 5 hours per day, considering the “peak sun hours” concept.
3. There is a ‘house load’ of 7 kWh per day.
4. We allow for one day of only 10% sunlight, before it returns to 100% sunlight the next day.
5. The battery bank is 48V DC LFP and will not be discharged more than 90%.
6. The battery bank is comprised of 5.1 kWh 48V DC LFP modules.

Part I: Solar Power Input Calculations

The solar array, when operating at peak power, generates 20.5 kW for 5 hours per day, leading to:

     20.5 kW * 5 hours = 102.5 kWh/day

On a day of reduced sunlight (10%), the solar array produces:

     10% * 102.5 kWh = 10.25 kWh

Part II: Motor Power Output Calculations

The motors consume 8 kW over a 12-hour period of operation, which is:

     8 kW * 10 hours = 80 kWh/day

Part III: House Load Power Calculations

     The house load uses 7 kWh each day.

Part IV: Recommended Battery Capacity

The total energy required daily, considering both the motors and house load, is:

     80 kWh/day (Motors) + 7 kWh/day (House Load) = 87 kWh/day

To maintain operation with reduced sunlight for one day, the battery bank should store the energy deficit:

     87 kWh - 10.25 kWh = 76.75 kWh

To ensure we never discharge the LFP battery more than 90%, we need to adjust the size of the battery bank. Dividing the energy deficit by 90% gives:

     76.75 kWh / 0.9 = 84.04 kWh

Therefore, we recommend a battery capacity of 102.0 kWh. This ensures that the motors and house load can be powered for at least a day with reduced sunlight without discharging the battery bank more than 90%.

Part V: Required Battery Modules

Given that each module is 5.1 kWh, the number of modules needed to reach our recommended capacity is:

     102.0 kWh / 5.1 kWh/module ≈ 20 modules

So, we’ll need approximately 20 modules of the 5.1 kWh 48V DC batteries to ensure you can power your boat for a day of reduced sunlight while maintaining the discharge limit.

NOTE: This analysis assumes average conditions for solar panel energy generation and battery storage. Actual performance will vary due to factors such as panel orientation, temperature effects, and battery efficiency.

Part VI: Charging Depleted Batteries:

Let’s now calculate how long it will take to charge the batteries back to 100%, given the assumed consumption rates? in other words, how many days of sunlight will we need before we can have another day of 10% solar production, without reducing the motors power output or over discharging the batteries.

First, we need to understand how much surplus energy is generated each day and how long it will take to replenish the energy used during the day of reduced sunlight.

Daily Surplus Energy Calculation

On a typical day with full sunlight, the solar panels produce:

     20.5 kW * 5 hours = 102.0 kWh/day

The daily energy consumption by the motors and house load is:

     9.4 kW * 10 hours (Motors) + 7 kW (House Load) = 101 kWh/day

Therefore, the surplus energy generated each day is:

     102.0 kWh/day - 101 kWh/day = 1.0 kWh/day

Battery Recharge Time Calculation

From the previous calculations, we know that on a day of 10% sunlight, the energy deficit is 102.5 kWh. This is the amount of energy that needs to be replenished from the surplus generated on full sunlight days.

So, the time required to recharge the battery back to 100% is:

     102.0 kWh / 1.0 kWh/day ≈ 102 days

Clearly, that isn’t a practical recharge time.  Therefore, when the solar array isn’t producing at full capacity, we don’t want to be powering the motors at their regular cursing speed for 10 hours per day.

In this situation, if we cut the motoring time in half, or run them at half power, we get a surplus and recharge time as follows:

     102.0 kWh / day – 50.5 kWh/day = 51.5 kWh

So, the time required to recharge the battery bank back to 100% then becomes:

     102.0 kWh / 51.5 kWh/day ≈ 1.98 days

From this, we see it would take approximately 2 days of full sunlight to completely recharge the batteries after a single day of 10% sunlight, given our assumptions for ½ motor power consumption rate and still allowing for full house loads each day.

Part VII: EV verses ICE, in terms of Torque, Power, and RPM Differences:

Electric motors and internal combustion engines (ICE) have significantly different power and torque characteristics, which often necessitates propeller changes when migrating from an ICE to an electric propulsion system. Here are some general trends and considerations:

Torque Curve:

Electric motors can deliver their maximum torque from zero RPM, unlike ICEs that need to reach a certain RPM to deliver peak torque. This means electric motors can efficiently turn larger propellers from a standing start, which may not be the case for ICEs.

RPM Range:

Electric motors often operate at lower RPMs than ICEs. This means that a propeller designed for an ICE, which typically reaches its peak efficiency at high RPMs, might not be the best choice for an electric motor. A propeller with a larger diameter and lower pitch might be more suitable for an electric motor, as this type of propeller can provide the same thrust at lower RPMs.

Noise and Vibration:

Electric motors are much quieter and produce less vibration than ICEs. A propeller designed for an ICE might be optimized to minimize noise and vibration at the expense of efficiency. With an electric motor, you could choose a more efficient propeller design without worrying as much about noise and vibration.

Continuous vs. Peak Power:

Electric motors often have a lower continuous power rating compared to their peak power rating, while ICEs are typically rated by their continuous power. This means that a propeller designed for the continuous power of an ICE might overload an electric motor once the electric motor drops down from its peak power to its continuous power.

When transitioning from an ICE to an electric motor, it’s advisable to reassess your propeller. While there are many factors to consider, having a larger diameter and a higher pitch propeller tends to be more suitable for an electric motor. Detailed propeller selection will be covered in another article separately, and of course a naval architect, marine engineer, or other propeller expert should be able to provide specific advice based on the exact parameters based on a given boats propulsion system.

Important Safety Note about the LFP Modules:

The installation and management of the recommended 20 LFP battery modules require specific attention and care. The positioning, securing, and connection of the modules can significantly impact the safety, performance, and longevity of the batteries. Moreover, ensuring adequate ventilation, temperature regulation, and safe access for maintenance are essential considerations during the installation process.

Furthermore, the integration of these battery modules into the vessel’s electrical system is a complex task. It requires a comprehensive understanding of the system’s requirements and the proper configuration of charging controllers, inverters, and other system components.

The detailed aspects of battery module installation, as well as the broader electrical system design and installation, are beyond the scope of this paper. Specialized guidance should be sought from professionals experienced in marine-grade electrical systems and the specific characteristics of LFP battery technology.

Considerations for Purchasing Hurricane Salvaged Catamarans for Conversion

Purchasing hurricane salvaged catamarans can be a cost-effective way to source boats for conversion into battery electric vehicles (BEVs) powered by solar energy. However, it’s important to carefully consider the following points:

Condition Assessment: The extent of the damage caused by a hurricane can vary greatly from one vessel to another. It’s crucial to conduct a thorough inspection of the structural integrity of the hull, deck, and other key components. This can help to avoid hidden repair costs that could make the project more expensive than initially planned.

Rebuild Costs: Although the initial purchase price may be low, the cost of rebuilding and retrofitting a salvaged boat can be significant. It’s important to estimate these costs upfront and factor them into the overall project budget.

Insurance and Legal Factors: In some regions, it can be challenging to ensure a boat that has been salvaged and rebuilt. It’s recommended to check with insurance providers before purchasing a salvaged boat. Additionally, there may be legal requirements or restrictions associated with salvaged boats that need to be considered.

Expertise Required: Rebuilding a salvaged boat and converting it to a BEV requires a wide range of skills, including boat repair, electrical engineering, and possibly naval architecture. Having these skills in-house or finding reliable contractors is crucial to the success of the project.

Time Commitment: Rebuilding a salvaged boat is a significant undertaking that can require a substantial amount of time, especially if significant repair work is needed. This should be factored into the project timeline.

Despite these challenges, the potential benefits of this approach are substantial. Not only can this be a cost-effective way to source boats for conversion, but it also aligns with the sustainability ethos of solar-powered electric boats by giving new life to vessels that might otherwise be discarded. As with any complex project, careful planning, budgeting, and risk assessment are key to success.

About the Author:

Walt White is the founder of Newport Electric Boats and systems engineer, who provides design and consulting services for electric boats, electric charging systems, and other engineering analysis including software and systems engineering. Walt can be reached at walt@newportelectricboats.com