Transducers
Thru hull vs in hull transducer: what does each one give up?
Quick answer
A thru-hull transducer sits in a hole cut below the waterline and reads through open water, while an in-hull transducer glues inside the hull with no hole at all. An in-hull mount only works on solid fiberglass with no core and no air gap, and it cannot read bottom water temperature, which a thru-hull always can.
Garmin Airmar P19 plastic thru-hull transducer
$161.59A hole below the waterline, reads any appropriate hull material and true water temperature.
Check before buying: A thru-hull means a hole below the waterline. Plastic housings go in fiberglass hulls only, never aluminium.
Best for: Fiberglass hulls that run fast enough to aerate a transom transducer
Check price on Amazon
Lowrance shoot-thru-hull CHIRP transducer
$96.53Glues inside solid fiberglass with no hole, but no core, no air gap, and no bottom temperature reading.
Check before buying: It only works on solid fiberglass with no core and no air gap, and it cannot read water temperature at the bottom.
Check price on AmazonA thru-hull transducer sits in an actual hole cut below the waterline, in open water, and an in-hull transducer skips the hole entirely by gluing inside the boat and shooting its signal straight through solid fiberglass. The in-hull option sounds like the easy win: no hole, no haul-out, no risk of a leak. The tradeoff is that it only works on the right hull, and it gives up two things a thru-hull always has: a true bottom water temperature reading and, generally, a stronger signal.
Both are legitimate choices, and choosing between them comes down to hull construction first and installation tolerance second.
What a thru-hull transducer requires
A plastic thru-hull like the Garmin Airmar P19 needs a hole cut through the hull below the waterline, bedded and sealed properly, which is a genuine job with a genuine consequence for getting it wrong. In exchange it reads through open water on any hull material appropriate to its housing, in any conditions, with no signal loss to hull thickness or lamination, and it carries a working temperature sensor reading actual water in contact with the housing.
What an in-hull transducer requires
Lowrance's shoot-thru-hull CHIRP transducer glues to the inside of the hull with an adhesive puck and shoots its signal through the fiberglass itself, with no hole cut anywhere. It only works on solid fiberglass hulls with no core and no air gap between layers, since any foam core, balsa core or air pocket blocks or scatters the signal before it ever leaves the boat. It also cannot read water temperature at the bottom, because the sensor never actually touches the water, it is reading through a wall of fiberglass instead.
Thru-hull vs in-hull fit
A thru-hull fits any appropriate hull material through an actual hole, while an in-hull only fits solid fiberglass with no core and no air gap, and gives up a true water temperature reading.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Mount | Hull requirement | Water temperature |
|---|---|---|
| Thru-hull | A hole cut below the waterline, on the hull material the housing is rated for | Reads actual bottom water temperature |
| In-hull (shoot-thru) | Solid fiberglass only, no core, no air gap | Cannot read bottom water temperature |
Why anyone chooses in-hull at all
No hole below the waterline means no haul-out, no bedding compound, and nothing to leak, ever, which matters on a boat the owner is not willing to have hauled or is not confident cutting into. It is also fully reversible: the puck glues in and can be removed without leaving a hole to patch, which a thru-hull installation cannot offer. On the right solid fiberglass hull, that tradeoff, giving up bottom temperature for zero hull penetration, is a genuinely reasonable one.
Frequently asked questions
How do I know if my hull is solid fiberglass or cored?
The builder's specification sheet or a call to the manufacturer is the reliable answer, since cored construction varies by hull section and by model year even within the same boatbuilder. A rough field check is tapping the hull and listening for a dull, deadened sound where a core is likely present versus a sharper sound over solid laminate, but that is not a substitute for confirming with the builder before gluing in a transducer that will not work through a core.
Can an in-hull transducer read side imaging?
Generally no. Shoot-thru-hull transducers are built for straight down CHIRP sonar, not for side imaging or down imaging, which need their transducer element in direct contact with open water at a specific angle to build a picture. A boat that wants side imaging on a solid fiberglass hull without a hole is choosing between a limited feature set from an in-hull unit or accepting a transom mount or true thru-hull instead.
Does an in-hull transducer lose signal strength compared to a thru-hull?
Yes, generally. Shooting a signal through solid fiberglass costs some strength compared to a thru-hull sitting directly in open water, though on a hull thin enough and solid enough, the loss is often not noticeable at recreational depths. In very deep water or on a thick hull layup, that loss becomes the limiting factor, which is one more reason to check the hull thickness against the transducer's own specification before installing.
Is a thru-hull transducer harder to install than a transom mount?
Yes, by a wide margin. A transom mount typically bolts on in an afternoon with the boat still in the water. A thru-hull needs a haul-out, a hole cut to the correct size, proper bedding compound, and enough care that many owners choose to have it done by a qualified installer rather than doing it themselves, given the real consequence of a poorly sealed hole below the waterline.
Why can't an in-hull transducer read bottom water temperature?
Because its sensor never touches the water. An in-hull transducer glues to the inside of the fiberglass and reads its sonar signal through the hull material itself, and a temperature sensor built into that housing is reading the fiberglass and the bilge air around it rather than the lake or the sea outside. Any transducer style that sits in actual contact with the water, like a transom mount or a genuine thru-hull, reads true water temperature instead.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.