Marine transducers are critical components of sonar systems, enabling boaters to navigate safely, locate fish, and map underwater environments. When upgrading a boat’s sonar system, such as adding a Garmin GSD20 sounder to a Garmin 3006C, choosing the right transducer is paramount. Two primary options dominate the market: thru-hull and in-hull transducers. Each has distinct advantages, limitations, and installation requirements, making the decision complex yet crucial for optimizing performance. This guide explores the differences between thru-hull and in-hull transducers, their pros and cons, and key considerations for selecting the best option for your vessel, with a focus on solid fiberglass hulls like that of a 1984 Trojan International.
Understanding Marine Transducers
A transducer converts electrical energy into sound waves and interprets the returning echoes to provide data on water depth, fish locations, and bottom structures. This technology underpins fishfinders, depth sounders, and chartplotters, making it indispensable for safe navigation and successful fishing.
Transducers vary by mounting style, frequency, and sonar capabilities (e.g., traditional, CHIRP, ClearVü, SideVü). The two most common mounting styles are thru-hull and in-hull, each suited to different hull types, boating habits, and performance expectations.
Types of Transducers
- Thru-Hull Transducers: Mounted through a hole in the hull, with the sensing element directly in the water.
- In-Hull Transducers: Installed inside the hull, transmitting and receiving sound waves through the hull material.
- Transom-Mount Transducers: Attached to the transom, typically for smaller boats.
- Tilted Element Transducers: A variation of thru-hull, designed to compensate for hull deadrise angles without a fairing block.
This article focuses on thru-hull and in-hull transducers, as they are the most relevant for a solid fiberglass hull and the Garmin GSD20 sounder.
In-Hull Transducers: Functionality and Features
In-hull transducers, also called shoot-through or glued-in transducers, are mounted inside the hull, typically bonded with epoxy or placed in a liquid-filled well. They transmit sound waves through the hull into the water, receiving echoes to generate sonar data.
How In-Hull Transducers Work
The transducer sends high-frequency sound waves (e.g., 50 kHz or 200 kHz) through the fiberglass hull. These waves reflect off underwater objects and return through the hull to the transducer, which converts them into electrical signals for the sounder to interpret. The hull acts as a medium, but its thickness and composition affect signal efficiency.
Pros of In-Hull Transducers
- No Hull Penetration: Installation requires no drilling, preserving hull integrity and eliminating leak risks.
- Ease of Installation: Mounting is straightforward, often completed without hauling the boat.
- Versatility for Hull Designs: Suitable for stepped hulls or complex designs where thru-hull installation is challenging.
- Low Maintenance: Once installed, in-hull transducers require minimal upkeep, as they are protected inside the hull.
Cons of In-Hull Transducers
- Signal Loss: Sound waves lose energy passing through the hull, reducing depth range and sensitivity compared to thru-hull models.
- Hull Material Limitations: Only effective on solid fiberglass hulls; not compatible with cored hulls, wood, metal, or foam-sandwiched designs.
- Performance at High Speeds: Readings may degrade at speeds above 30 knots due to hull-induced signal interference.
- Installation Precision: Air pockets, voids, or improper bonding can significantly impair performance.
Installation Tips for In-Hull Transducers
- Location: Place in the aft third of the hull, near the centerline, in a solid fiberglass section free of voids or core materials.
- Testing: Use a water-filled zip-lock bag or K-Y Jelly to test signal transmission before permanent mounting.
- Mounting: Bond with slow-set epoxy or use a liquid-filled well (e.g., non-toxic antifreeze or mineral oil) to ensure a bubble-free interface.
- Hull Thickness: Verify thickness (typically 0.25–1 inch for 200 kHz) to ensure compatibility with the transducer’s frequency.
Thru-Hull Transducers: Functionality and Features
Thru-hull transducers are installed through a hole drilled in the hull, with the sensing element directly exposed to the water. They often require a fairing block to align the beam vertically and reduce drag.
How Thru-Hull Transducers Work
The transducer’s acoustic window is in direct contact with the water, allowing sound waves to travel unimpeded. This direct exposure minimizes signal loss, providing superior depth penetration and target resolution. Frequencies range from 50 kHz (deep water) to 200 kHz (shallow water), with some models supporting CHIRP for enhanced detail.
Pros of Thru-Hull Transducers
- Superior Performance: Direct water contact ensures minimal signal loss, offering better depth range and fish detection.
- High-Speed Reliability: Maintains accurate readings at speeds up to 50 knots, ideal for fast boats.
- Rough Water Capability: Performs well in choppy conditions, as the transducer is unaffected by hull interference.
- Broad Compatibility: Suitable for various hull materials (fiberglass, wood, metal) with appropriate models.
Cons of Thru-Hull Transducers
- Complex Installation: Requires drilling a hole, precise sealing, and often a fairing block, increasing time and cost.
- Leak Risk: Improper sealing can lead to water ingress, damaging the boat.
- Maintenance Needs: External exposure requires periodic cleaning to remove marine growth.
- Hull Design Constraints: Not ideal for stepped hulls or boats with limited underwater clearance.
Installation Tips for Thru-Hull Transducers
- Location: Mount forward of the propeller, rudder, and keel to avoid turbulence, ensuring the transducer remains submerged.
- Fairing Block: Use to align the beam vertically, especially on hulls with significant deadrise angles.
- Sealing: Apply marine-grade sealant to prevent leaks, and check regularly for wear.
- Professional Help: Consider hiring a marine technician for precise installation, especially on larger vessels.
Key Differences Between In-Hull and Thru-Hull Transducers
To aid decision-making, the following table summarizes the key differences:
| Feature | In-Hull Transducer | Thru-Hull Transducer |
|---|---|---|
| Installation | No drilling, bonded inside hull | Requires drilling, external mounting |
| Performance | Reduced depth range, less effective at speed | Superior depth and high-speed performance |
| Hull Compatibility | Solid fiberglass only | Fiberglass, wood, metal (model-dependent) |
| Maintenance | Minimal, protected inside hull | Requires cleaning, sealing checks |
| Cost | Generally lower | Higher due to installation and hardware |
| Risk | No leak risk | Potential leaks if poorly installed |
Performance Comparison
Thru-hull transducers outperform in-hull models in most scenarios due to direct water contact. Airmar, a leading transducer manufacturer, notes that in-hull models compensate for signal loss with oversized ceramic elements and deadrise correction, but they still fall short of thru-hull performance in deep water or high-speed conditions. For a 1984 Trojan International with a thick fiberglass hull, an in-hull transducer may suffice for shallow-water fishing (up to 200 feet), but a thru-hull is preferable for deeper waters or faster cruising.
Installation and Maintenance
In-hull transducers are simpler to install, requiring only surface preparation and epoxy bonding. Thru-hull installations demand precision, including hull drilling, fairing block alignment, and robust sealing. Maintenance for in-hull models is negligible, while thru-hull transducers need regular inspection for fouling and seal integrity.
Choosing the Right Transducer for Your Boat
Selecting between in-hull and thru-hull transducers depends on several factors tailored to your boat and boating habits.
Factors to Consider
- Hull Material and Thickness:
- Solid Fiberglass: Both in-hull and thru-hull are viable. For a thick hull (e.g., 1 inch), verify the transducer’s frequency compatibility (200 kHz is typically effective).
- Cored or Metal Hulls: Thru-hull only, as in-hull transducers require solid fiberglass.
- Boating Habits:
- Speed: If cruising above 30 knots, a thru-hull transducer ensures reliable readings.
- Water Conditions: Rough waters favor thru-hull models for consistent performance.
- Depth Requirements: Deep-water fishing (beyond 200 feet) benefits from thru-hull’s superior range.
- Budget:
- In-hull transducers are more affordable, with models like the Airmar P79 starting at $199.99.
- Thru-hull models, such as the Garmin GT15M-THF, start at $399.99, with installation costs adding $200–$500.
- Installation Constraints:
- If avoiding hull drilling is a priority (e.g., due to existing plugged holes or aesthetic concerns), an in-hull transducer is ideal.
- If a plugged thru-hull hole exists, as with the 1984 Trojan, reusing it for a thru-hull transducer minimizes additional work.
- Sonar Needs:
- For basic depth and fishfinding, an in-hull transducer like the Airmar P79 paired with the Garmin GSD20 is sufficient.
- For advanced features (e.g., CHIRP, ClearVü), a thru-hull model like the Garmin GT17M-THF offers better resolution.
Expert Recommendations
Marine electronics professionals often recommend starting with an in-hull transducer for solid fiberglass hulls if drilling is a concern, as it’s less invasive and reversible. Airmar’s FAQ supports this, stating that their in-hull models perform comparably to thru-hull transducers when properly installed. However, for maximum performance, especially on larger boats or in demanding conditions, thru-hull transducers are preferred. For the Garmin 3006C and GSD20, the Airmar P79 (in-hull, $199.99) or Garmin GT15M-THF (thru-hull, $399.99) are compatible choices, with the latter offering better deep-water performance.
Case Study: 1984 Trojan International
The 1984 Trojan International, with its thick solid fiberglass hull, presents a unique scenario. The owner’s reluctance to drill a new hole aligns with the appeal of an in-hull transducer. However, the availability of a plugged thru-hull hole offers a compromise, allowing a thru-hull installation without additional drilling.
- In-Hull Option: The Airmar P79, epoxied near the centerline, is a cost-effective choice. Users report reliable performance up to 200 feet and 40 mph, suitable for coastal fishing. Installation involves testing with a water-filled bag to ensure no voids, followed by epoxy bonding.
- Thru-Hull Option: Reusing the plugged hole for a Garmin GT15M-THF minimizes invasiveness. This model supports CHIRP sonar, enhancing fish arch clarity and bottom detail, ideal for deeper waters or faster speeds.
Given the hull’s thickness, a 200 kHz frequency is recommended for both options to optimize signal transmission.
Pricing and Specifications
Below is a comparison of popular transducers compatible with the Garmin GSD20, focusing on in-hull and thru-hull models:
| Model | Type | Frequency | Power | Max Depth | Price (USD) |
|---|---|---|---|---|---|
| Airmar P79 | In-Hull | 50/200 kHz | 600 W | 800 ft | 199.99 |
| Airmar B619 | Thru-Hull | 50/200 kHz | 600 W | 1,000 ft | 249.99 |
| Garmin GT15M-THF | Thru-Hull | 85–165 kHz | 600 W | 1,900 ft | 399.99 |
| Garmin GT17M-THF | Thru-Hull | 80–160 kHz | 1,000 W | 2,500 ft | 499.99 |
Note: Prices are approximate and exclude installation costs. Thru-hull installations may require additional fairing blocks ($50–$100) and professional labor.
Chart: Decision Flow for Transducer Selection
This flowchart guides boaters through the decision process, prioritizing hull material, drilling preferences, speed, and depth requirements.
Real-World Insights from Boaters
Boaters’ experiences highlight practical considerations:
- In-Hull Success: Users like “djy” and “FASTFJR” report excellent results with the Airmar P79 on solid fiberglass hulls, maintaining bottom lock at 40 mph. “Ray Zor” successfully epoxied a transom-mount transducer as an in-hull, demonstrating flexibility.
- Thru-Hull Preference: “Guard Bum” initially leaned toward a thru-hull due to local experts’ advice but reconsidered an in-hull after weighing Airmar’s claims. The plugged hole ultimately made thru-hull feasible.
- Installation Tips: “Reefmagnet” and “CarinaPDX” emphasize testing with water bags or K-Y Jelly to avoid voids, while “FPNC” recommends polyester resin for large transducers.
These insights underscore the importance of hull-specific testing and proper installation for both transducer types.
Conclusion
Choosing between in-hull and thru-hull transducers for a 1984 Trojan International with a Garmin 3006C and GSD20 sounder involves balancing performance, installation ease, and hull integrity. In-hull transducers, like the Airmar P79, offer a low-cost, non-invasive solution suitable for shallow waters and moderate speeds. Thru-hull transducers, such as the Garmin GT15M-THF, provide superior performance for deeper waters and high-speed cruising but require more complex installation. The availability of a plugged thru-hull hole tips the scale toward a thru-hull model for optimal performance without additional drilling.
By considering hull material, boating habits, and budget, and leveraging expert advice and real-world experiences, boaters can select a transducer that enhances their sonar system’s effectiveness. Whether prioritizing simplicity or performance, a high-quality transducer from reputable manufacturers like Airmar or Garmin ensures reliable navigation and fishfinding for an enjoyable boating experience.
Happy Boating!
Share Difference Between In-Hull and Thru-Hull Transducer with your friends and leave a comment below with your thoughts.
Read Garmin vs Humminbird vs Lowrance until we meet in the next article.