AC System Builder Walkthrough: Sizing a 42ft Grand Banks Trawler with Steve Harmon

Marine AC systems aren't one-size-fits-all. A 42-foot trawler with four zones, a pilothouse, and a galley that doubles as a command center has completely different cooling requirements than a 30-foot express cruiser with one cabin. That's why we built the AC System Builder — a free tool on our site that walks you through your boat's layout and tells you exactly what system type, compressor, circulation pump, and air handlers you need. No guesswork, no overselling, no undersizing.

To show how it works, we're going to walk through a real scenario: Steve Harmon's 42-foot Grand Banks trawler, docked in Fort Myers. Steve called us because his 20-year-old Marine Air system was losing the battle against a Southwest Florida summer. Instead of quoting blind, we ran his boat through the AC System Builder first — and what came out shaped the entire project.

Why a System Builder Matters

Most marine AC quotes start with a phone call: "I've got a 42-footer, what's it gonna cost?" The problem is that boat length alone tells you almost nothing about cooling load. Two 42-foot boats can have wildly different BTU requirements depending on hull type, glass area, number of zones, insulation quality, and how the interior is laid out.

The AC System Builder eliminates that ambiguity. It uses three inputs to generate a complete recommendation:

1. Boat type — trawler, sailboat, express cruiser, sportfish, catamaran, or mega yacht. Each has a heat-load multiplier based on typical glass area and insulation.
2. Boat length — used to auto-suggest zones based on what's typical for that size vessel.
3. Zone configuration — you add each cabin, salon, pilothouse, flybridge, or galley individually, with adjustable BTU values per zone.

The output includes system type (self-contained, split, or chilled water), compressor model, primary and alternate circulation pump recommendations, number of air handlers and thermostats, installed price range, and estimated timeline. Everything a boat owner or yard manager needs to make an informed decision before anyone picks up a wrench.

The Walkthrough: Steve Harmon's 42ft Grand Banks Trawler

Step 1 — Select Boat Type

Steve's Grand Banks is a classic trawler — moderate glass, standard hull construction, decent insulation for a boat of that era. In the AC System Builder, we select "Trawler / Cruiser" which applies a 1.0x heat-load multiplier. That means no adjustment up or down — the BTU values we assign to each zone are taken at face value. A sportfish or express cruiser with more glass would bump that to 1.1x; a well-insulated sailboat would drop it to 0.85x.

Step 2 — Enter Boat Length

We enter 42 feet. The builder immediately suggests a starting zone layout based on what's typical for a 42-footer: V-berth (6,000 BTU), salon (16,000 BTU), master stateroom (12,000 BTU), and aft cabin (9,000 BTU). That's a solid starting point, but Steve's boat has a pilothouse — so we'll customize.

Step 3 — Configure Zones

Here's where it gets specific. Steve's Grand Banks has five distinct areas that need cooling:

We bumped the salon from the default 16,000 to 18,000 BTU because Steve's Grand Banks has a particularly open salon layout with large side windows. In Southwest Florida, that glass is a heat magnet from March through October. Better to size slightly over than to have a system that can't keep up on a 95°F afternoon at the dock.

Step 4 — Build the Recommendation

Hit the "Build My System" button and the tool generates the full recommendation. Here's what came back for Steve's boat:

At 57,000 BTU across 5 zones, the system falls squarely into split system territory. Self-contained units top out around 24,000 BTU, and chilled water doesn't make sense until you're north of 96,000 BTU. The P120 compressor has headroom up to 72K BTU, which means Steve has room to add a zone later (like a flybridge unit) without replacing the compressor.

Step 5 — Pump Recommendation

The builder also recommends a raw water circulation pump based on the total BTU load. At 57,000 BTU, Steve needs at least 855 GPH of raw water flow. The tool recommended:

Steve's pump was installed below the waterline, so self-priming wasn't required. The March LC-5C-MD is a sealed magnetic drive pump — no shaft seal to leak, quieter than open-drive alternatives, and rated for continuous duty. The Scot 65/66 is the OEM pump that came with his original Marine Air system, so it's a direct bolt-in replacement if he prefers to stay with what he knows.

Installed Price Range

The builder generated the following estimate for Steve's 5-zone split system:

That's a wide range because it depends on the condition of existing wiring, plumbing, and through-hulls. Steve's boat needed new raw water plumbing (the old hoses were original and hardened), which pushed him toward the middle of the range. A boat with newer infrastructure would land closer to the low end. The point is: the builder gives you a realistic window, not a lowball number that doubles once the work starts.

What Happened Next

After running the builder, we scheduled an onboard visit to Steve's Grand Banks at Legacy Harbour Marina in Fort Myers. The diagnostic confirmed what the builder predicted — his existing system was a single-compressor Marine Air unit that had been undersized from the factory and was now 20 years into its service life. The compressor was drawing high amps, the raw water pump was cavitating, and two of the three air handlers had corroded evaporator coils.

We installed a new Dometic P120 split system with five individual air handlers, a March LC-5C-MD circulation pump, new raw water plumbing from the through-hull to the compressor, and five digital thermostats for independent zone control. The job took five days. Steve now has 57,000 BTU of cooling capacity with room to grow, and every zone is independently controllable.

His words after the first weekend aboard: "I've owned this boat for twelve years and it's never been this comfortable. The pilothouse used to be an oven by 10 AM — now it's 72 degrees while I'm running the ICW."

Try It Yourself

The AC System Builder is free and takes about two minutes. You don't need to create an account or provide any contact information. Enter your boat type, length, and zones — and you'll get a complete system recommendation with equipment specs, pump sizing, and installed price ranges.

If you want to move forward, hit the "Request a Quote" button on the results page and we'll follow up with a site visit to confirm the configuration and provide a firm quote. That's the diagnostic-first approach — we verify everything on the boat before committing to a number.