Andrew Pace
Installing a transducer on a fiberglass boat can be a challenging task, especially if you want to avoid drilling holes in your boat. While some transducers can be installed without drilling, such as with a mounting plate or by gluing it on the inside using epoxy, most installations require drilling pilot holes and sealing them with marine-grade sealant. The positioning of the transducer is crucial, as it needs to be level with the waterline and not interfere with the cooling water flow to an outboard engine. The type of screws and sealant used are also important considerations to ensure a secure and watertight installation.
How to Install a Transducer on a Fiberglass Boat
| Characteristics | Values |
|---|---|
| Mounting method | Transom-mount, trolling motor mount, or thru-hull mount |
| Mounting plate | Minimises holes in the transom and allows for future installations without new holes |
| Positioning | On the transom, ensuring contact with the water without interfering with the cooling water flow to an outboard engine |
| Distance from the waterline | Bottom of the transducer should sit about one-sixteenth of an inch below the extended waterline of the hull |
| Hole drilling | Pilot holes should be drilled and filled with marine sealant; holes should be big enough to allow screws to thread without excessive force |
| Screws | Use top-grade stainless steel screws |
| Cable routing | Drill another hole above the normal waterline to route the cable to the display; seal and cap the hole |
| Gelcoat cracking | Prevent by running the drill in reverse until it reaches the fiberglass, then use a countersink to chamfer the edges |
| Sealing | Use a quality marine sealant approved for below waterline use, such as 3M 52000, Sikaflex 295 UV, 3M 4200, or Sudbury Elastomeric Marine Sealant |
| Transducer bracket | Bed the entire bracket in sealant to prevent crevice corrosion |
| Transducer alignment | Ensure the bottom of the transducer is parallel to the bottom of the hull |
| Transducer performance | Lowering the "tail" end of the transducer a few degrees can improve performance at speed |
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What You'll Learn
- Drilling holes: use a drill to make pilot holes, fill with marine sealant, then use stainless steel screws
- Positioning: place the transducer on the transom, ensuring it's level with the waterline and doesn't interfere with the engine
- Sealing: use a quality marine sealant to seal the installation, preventing water penetration
- Cable routing: route the cable to the display by drilling another hole above the waterline, securing it to the transom
- Transducer mounting: use a mounting plate or aluminium mount to minimise holes in the transom and allow for adjustments
Drilling holes: use a drill to make pilot holes, fill with marine sealant, then use stainless steel screws
Drilling holes in your boat can be a daunting task, but it is often necessary when installing a transducer. Here is a step-by-step guide to help you through the process:
Drill Pilot Holes:
Start by marking the spots where you need to drill. Carefully drill pilot holes at these marked locations. These holes should be just large enough for the screws you will be using. It is important to drill above the waterline and to avoid any internal wiring. If you are drilling into fiberglass, be cautious of any foam or wood that may be behind the drilling area.
Fill with Marine Sealant:
Once you have drilled the pilot holes, fill them with a quality marine sealant. This step is crucial to prevent water penetration and ensure a watertight seal. Choose a sealant that is approved for below-waterline use, such as lower-strength polyurethanes like Sikaflex 295 UV and 3M 4200 or synthetic rubber formulations like Sudbury Elastomeric Marine Sealant.
Use Stainless Steel Screws:
After the sealant has cured, you can fasten the transducer mount to the boat using stainless steel screws. Remember to use top-grade stainless steel screws specifically designed for marine use, as regular steel screws may be weaker. Do not overtighten the screws to avoid snapping them.
Additional Tips:
If you are hesitant to drill holes in your boat, consider using a transducer mounting plate or an aluminium mount that attaches to the top of the transom. These options can reduce the number of holes needed and make it easier to adjust the placement of your transducer. Additionally, always refer to the installation manual for specific instructions and recommendations.
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Positioning: place the transducer on the transom, ensuring it's level with the waterline and doesn't interfere with the engine
When positioning the transducer on the transom, it is important to ensure that it is level with the waterline and does not interfere with the engine. The transducer should be placed in a spot that will have constant contact with the water but not disrupt the cooling water flow to an outboard engine. This is especially important if your boat has twin engines, as you will need to mount the transducer near the hull centerline.
To achieve the correct positioning, you can use a horizontal guideline on the transom, such as the bottom edge of a jackplate, along with a framing square to ensure the transducer is level side-to-side. You may need to rotate it slightly clockwise. It is also crucial to get the bottom of the transducer parallel to the bottom of the hull. Most installation manuals recommend that the transducer be parallel to the hull bottom and either flush with it or 1/8" below.
If your transom has a straight, horizontal line, such as a bottom paint line or an aluminium transom cap, you can use that as a reference to mark a vertical line and then align your transducer bracket accordingly. If there is no obvious horizontal mark, you can use a string to create a line across the transom from one chine to the other.
Keep in mind that you want to avoid drilling holes in your boat unless necessary. Consider using a transducer mounting plate, which minimises holes in the transom and allows for future installations without creating new holes. Additionally, make sure to use high-quality stainless steel screws that are suitable for marine use.
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Sealing: use a quality marine sealant to seal the installation, preventing water penetration
Sealing is an important step in installing a transducer on a fibreglass boat, as it prevents water penetration and potential damage. To start the sealing process, you will need to drill pilot holes. It is recommended to use a drill bit that is slightly smaller than the screws you will be using. Once the holes are drilled, coat them with a quality marine sealant. This will protect the holes from water ingress and corrosion.
It is important to choose a sealant that is suitable for below-waterline use and is compatible with fibreglass. Some recommended sealants include lower-strength polyurethanes like Sikaflex 295 UV and 3M 4200, and synthetic rubber formulations like Sudbury Elastomeric Marine Sealant. These sealants provide a strong and flexible waterproof seal, preventing water from entering the boat through the screw holes.
After applying the sealant to the pilot holes, mount the transducer bracket and screws. Ensure that the screws are coated with sealant as well. This will create a watertight seal around the screws and prevent them from corroding. Once the transducer is mounted, you can apply additional sealant around the bracket to ensure a complete seal.
In addition to sealing the screws and bracket, it is also important to consider sealing the cable entry point. If you are routing the transducer cable through a hole in the transom, be sure to seal this hole as well. This will prevent water from entering the boat through the cable entry point. A marine-grade sealant or epoxy can be used for this purpose.
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Cable routing: route the cable to the display by drilling another hole above the waterline, securing it to the transom
When it comes to routing the cable to the display, you will need to drill another hole in the boat, but this time it should be above the waterline. This is an important step to ensure that the hole is above the normal waterline to avoid any issues with water penetration. The hole should be large enough to pass the transducer cable terminal end through the transom. You may need to cut the transducer cable to make the routing process easier, but be sure to check with the manufacturer before cutting, as some cables cannot be cut.
After drilling the hole, pass the cable through it and secure it to the transom. You can use the hardware provided with the transducer to secure the cable and seal and cap the hole. If you're routing the cable forward to a centre console, you can usually pass it through the rigging tube that carries engine controls and wiring.
It's worth noting that some people prefer to avoid drilling holes in their boats. In that case, you can use a transducer mounting plate, which minimises the number of holes in the transom and allows for future installations without creating new holes. However, keep in mind that you may still need to drill holes to attach the mounting plate to the boat.
Additionally, it's important to use high-quality marine sealant to fill the holes and prevent gelcoat cracking. This will help maintain the integrity of your boat and ensure that water doesn't penetrate between the gel and the fibreglass.
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Transducer mounting: use a mounting plate or aluminium mount to minimise holes in the transom and allow for adjustments
When installing a transducer on a fibreglass boat, using a mounting plate or aluminium mount is a good idea. This method minimises the number of holes that need to be drilled into the transom and allows for adjustments to the transducer's placement without drilling additional holes.
Commercially available options include screw-on and stick-on plates, or you can make your own out of starboard or expanded PVC. A transducer mounting plate is typically made from polyethylene plastic or a similar durable, synthetic material. It is fastened to the hull with two small screws and a little marine silicone adhesive to seal the holes and keep the plate in place. The transducer is then mounted onto the plate, with the added benefit of being able to adjust or move it without drilling more holes in the hull.
If you don't want to use a mounting plate, an alternative method is to glue a King Starboard (poly) block, such as a Stern Saver, to the hull with a special adhesive. The hardware is then screwed into the block.
When drilling holes in the transom, it is important to measure the length of the screws minus the thickness of the mounting bracket and mark the necessary depth on the drill bit. This will ensure that you don't drill too deeply. It is also recommended to tighten the screws by hand to avoid stripping.
To seal the installation, coat the screws and fill the pilot holes with a quality marine sealant approved for below-waterline use, such as lower-strength polyurethanes like Sikaflex 295 UV and 3M 4200, or synthetic rubber formulations like Sudbury Elastomeric Marine Sealant.
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Frequently asked questions
The first step is to position the transducer on the transom in a spot that will have contact with the water at all times but not interfere with the cooling water flow to an outboard engine. Then, you can either drill pilot holes and fill them with marine sealant or use a transducer mounting plate to minimise the number of holes. Finally, mount the transducer and apply more sealant.
To avoid gelcoat cracking, try running your drill in reverse until the bit reaches the fiberglass. Then, use a countersink to carefully chamfer the edges around the hole. Seal the installation by coating screws and filling pilot holes with a quality marine sealant approved for below-waterline use, such as 3M 52000 or Sudbury Elastomeric Marine Sealant.
Use a horizontal guideline on your transom, such as the bottom edge of a jackplate and a framing square, to ensure your side-imaging transducer is level side-to-side. You can also use a string to mark a line across the transom from one chine to the other if there is no obvious horizontal mark.
Two common problems are properly locating the transducer so it reads well at speed and routing the cable to the sonar display unit. To avoid these issues, make sure the transducer is positioned correctly and that the cable is routed through a hole above the normal waterline.
