Quick Answer
A transom transducer bolts to the back of the boat, suits any hull material and works best below about 34 mph. A thru-hull transducer goes through a hole in the hull and gives the best signal at speed and depth, but needs a professional install. An in-hull transducer sits inside a solid fibreglass hull with no hole. A trolling motor transducer fits on or inside the motor and shows what is under the bow. Choose by hull material, top speed and where on the boat you fish.
Who This Guide Is For
This guide helps you if you are choosing a transducer for a new fish finder, replacing a damaged one, or adding a second transducer at the bow.
It will not replace your transducer's installation instructions. Always follow the manufacturer's manual, and use a qualified installer for any job that involves drilling through the hull.
The Main Mount Types
Humminbird's model codes show how common each mount is: TH for thru-hull, XNT and XM for transom, XTM for trolling motor, XP for in-hull, XPT for portable and XI for ice transducers.
| Mount | Where it goes | Hull materials | Speed | Install |
|---|---|---|---|---|
| Transom | Outside the transom at the stern, just below the water surface | Any, according to Garmin | Best results below 34 mph and away from turbulence | Usually the simplest, with no hole through the hull |
| Thru-hull | Through a hole drilled in the hull, in the smoothest water near the keel | Check the transducer's manual | Excellent signal even at higher speeds and greater depths | Garmin recommends a professional installer |
| In-hull | Bonded to the inside of the hull, with no hole | Solid fibreglass only, up to 16 mm (5/8 in.) thick. Not for metal, wood or cored hulls | Performs well at speed if away from turbulence, but signal strength and depth are reduced | No hole, but bubbles in the adhesive spoil the signal |
| Trolling motor | On the motor shaft or built into the lower unit | Not affected by hull material | Used at trolling speeds | Clamp or bracket, or built in at the factory |
| Portable and ice | Suction, clamp or float mounts that move between boats or holes | Not affected by hull material | Low speed or stationary | No permanent fitting |
How to Choose Your Mount
- Check your hull material. Aluminium, wood and cored hulls rule out in-hull transducers.
- Note your top cruising speed. If you regularly run above about 34 mph, a transom mount may not give its best results, and a thru-hull may suit you better.
- Decide where you fish from. If you fish from the bow on the trolling motor, a transom transducer shows the water behind you, not in front.
- Check your hull shape. Garmin does not recommend transom mounts for sailboats or stepped hulls.
- Look at your stern. A transom transducer should sit at least 38 cm (15 in.) from the propeller, on the side where the propeller blades move downwards, and nearer the centreline for higher speeds.
- Think about damage risk. Garmin notes transom mounts are exposed to damage from trailers and underwater obstructions.
- Decide who will install it. Drilling through the hull for a thru-hull is a job for a professional.
- Check the transducer works with your fish finder in our compatibility checker before you buy.
Worked example
Choosing transducers for an aluminium jon boat
You have a 16 ft aluminium jon boat with an outboard that tops out at about 28 mph. You fish both drifting from the stern and from a bow-mount trolling motor.
- Hull material. Aluminium is metal, so Garmin's guidance rules out an in-hull transducer.
- Speed. At about 28 mph you are below the 34 mph point where Garmin says transom mounts give their best results.
- Install. A transom mount needs no hole through the hull, so you can fit it yourself following the manual.
- Placement. Mount it at least 38 cm (15 in.) from the propeller, on the side where the blades move downwards.
- Bow coverage. Add a trolling motor transducer, or use the motor's built-in sonar if it has one, so you can see under the bow.
- Result. A transom transducer at the stern plus a trolling motor transducer at the bow covers both ways you fish, with no drilling through the hull.
When the Usual Advice Does Not Apply
- Very fast boats. Garmin says transom mounts give their best results below 34 mph, so faster boats may get a weaker picture at speed even with careful mounting.
- Cored or thick fibreglass hulls. In-hull transducers are only recommended for solid fibreglass up to 16 mm (5/8 in.). Cored hulls need a different mount.
- Two transducers close together. Humminbird notes that a trolling motor's built-in transducer can interfere with an external transducer mounted near it, and two transducers that close may interfere with each other.
- Trolling motor electrical noise. Garmin warns that trolling motors are a source of electromagnetic interference that can affect any transducer, so route transducer cables away from motor power cables.
- Imaging transducers. Side imaging transducers need a clear view to both sides, so check the manufacturer's placement guidance for your exact model.
Action Checklist
This page contains no affiliate links. It was researched from the manufacturer documents listed below, and the products were not tested by us. We will recheck it by May 2027, or sooner if Garmin or Humminbird update their placement guidance.
Common Questions
Which transducer mount is easiest to install?
A transom mount is usually the simplest because it bolts to the outside of the transom and needs no hole through the hull. Garmin says it works with any hull material.
Can I use an in-hull transducer on an aluminium boat?
No. Garmin recommends in-hull transducers for solid fibreglass hulls only, up to 16 mm (5/8 in.) thick, and does not recommend them for metal, wood or cored hulls.
Which mount works best at high speed?
Garmin says thru-hull transducers give excellent signal quality even at higher speeds and greater depths. Transom mounts give their best results below 34 mph.
Can a trolling motor transducer replace a transom transducer?
It covers a different area. A trolling motor transducer shows what is under the bow, while a transom transducer shows what is under the stern. Many anglers run both.
Does a thru-hull transducer need a professional installer?
Garmin says thru-hull transducers require a professional installer for best results, because the job involves drilling through the hull.
Sources
- Garmin Transducer Placement Guide (PDF)
- Humminbird Help Center, How to Identify Your Transducer
- Humminbird Help Center, Mounting a Transducer to a Trolling Motor
- Minn Kota Help Center, Identifying the Cables Coming from Your Motor
Manufacturers change part numbers and firmware support. Confirm with the manufacturer before you buy.