Raymarine ST900 Speed/Temperature Low Profile Thru-Hull Transducer, For Airmar P120/B120, 0.3-45 Knots, Analog Output

Overview

The Raymarine ST900 is a low profile thru-hull speed and temperature transducer for boats that need responsive analog speed data with water temperature from a retractable sensor body. It records speed and temperature, works through a 51 mm (2 in) hull opening, and reads speed from 0.3 to 45 knots, with a water temperature range of -10°C to 40°C.

The ST900 uses Raymarine's Gen2 paddlewheel for better startup response and smooth readings across the speed range. The housing sits nearly flush with the hull, extending only 5 mm (3/16 in) outside the surface, so it keeps drag low on sailing yachts and powerboats.

A retractable insert makes paddlewheel cleaning and inspection more practical while the boat is in the water. The valve assembly helps limit water entering the bilge when the insert is removed, and the transducer core can retrofit into existing Airmar P120 and B120 housings.

Key features

• Speed and fast-response water temperature sensing from one low profile thru-hull transducer.
• Gen2 paddlewheel starts reporting from 0.3 knots and maintains speed output through 45 knots.
• Analog output for Raymarine instruments including i40, i50, iTC-5 and legacy systems.
• Retractable insert gives access to the paddlewheel for cleaning and routine service.
• Valve assembly helps reduce water ingress into the bilge when the sensor insert is removed.
• Nearly flush housing extends 5 mm (3/16 in) outside the hull to help reduce drag.
• Retrofits into existing Airmar P120 and B120 housings for many upgrade or replacement installations.

Variants

This ST900 range is offered in three configurations, with the main differences being housing material and cable length.

The plastic versions are listed for fiberglass or metal hulls, while the bronze version is listed for fiberglass or wood hulls. All three versions share the same ST900 speed and temperature function, low profile thru-hull mounting style, analog output, 51 mm (2 in) hole size, and 0.3 to 45 knot speed range.

• E70673: Plastic housing with 13.7 m (45 ft) cable. This version suits installations where a plastic thru-hull is appropriate and the instrument run does not require the longer 20 m cable.
• E70674: Plastic housing with 20 m (65 ft) cable. This version gives extra cable length for longer routing paths from the thru-hull location to the instrument system.
• E70686: Bronze housing with 13.7 m (45 ft) cable. This version uses a bronze thru-hull body and is listed for fiberglass or wood hull applications.

Installation and fit

This is a thru-hull transducer, so the hull material, existing housing, cable route, and service access around the mounting location should be checked before ordering. The 51 mm (2 in) hole size and retrofit compatibility with Airmar P120 and B120 housings make it a practical replacement path on boats already using those housings.

The low profile body works well where drag matters, such as sailboats, performance cruising boats, and powerboats where clean water flow across the paddlewheel helps maintain consistent speed readings. It should be installed where the hull shape gives the sensor a steady flow of water and where the insert can be reached for cleaning.

System integration

The ST900 outputs analog speed and temperature data for compatible Raymarine instrument systems. It is a good match for boats using Raymarine i40, i50, iTC-5 or supported legacy instrument setups rather than a networked smart transducer connection.

The cable length should be matched to the route from the thru-hull position to the instrument interface, allowing for service loops and safe routing away from sharp edges, high heat, and moving equipment.

Maintenance and care

Paddlewheel speed sensors need occasional cleaning, especially in saltwater, warm water, or areas with fouling growth. The retractable insert helps make that service easier, and the valve assembly helps reduce the amount of water that enters the bilge during removal.

For dependable readings, keep the paddlewheel free to spin and inspect the housing during regular haul-out or service intervals. This unit is well suited to owners replacing an older analog speed sensor or upgrading an existing P120 or B120 installation while keeping the same general thru-hull format.

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