Where an underwater boat light mounts decides how hard it is to install, how big a hole goes in your hull, and how much water it can light. There are two ways to do it: a surface mount on the outside of the transom or hull, or a through-hull light set into a hole cut through the hull. Black Oak Fathom lights are surface mount. This guide shows how each one sits against the waterline, how many lights your boat needs and how to pick a color, so you can decide before you buy.
- Surface mount: the practical default for most boats. One small cable hole and three screws on the outside of the transom or hull.
- Through-hull: a flush look, but a large cutout below the waterline and a much harder install, usually a yard or rigger job.
- How many: 2 lights for an 8 to 11 foot boat, up to 4 or 5 for 16 feet and up, mounted 6 to 12 inches below the waterline.
- Color: blue travels farthest through water, white shows true color on bait and structure.
Every light in this guide is in our underwater boat lights collection, and the sections below link straight to the right model and kit for each boat length.
What are the two ways to mount an underwater boat light?
The two styles differ in where the light sits and what you have to cut. The side view below shows each one against the waterline.
Surface mount and through-hull lights compared by what they need from your hull.
Surface mount. The light bolts to the outside of the transom or hull. The Black Oak Fathom series installs through a single cable entry hole and three stainless screws, with no large cutout and no fairing block, on any flat surface above or below the waterline. You seal one small hole instead of cutting a big one.
Through-hull. A through-hull light is set into a hole cut through the hull below the waterline and held by a sealed fitting. It sits flush, which is the cleanest look, but it is the biggest hole you can put in a boat. The cutout has to be exact, the fitting has to be bedded and sealed from inside, and on many boats the work means hauling out, so most owners hand it to a yard or rigger. Black Oak does not sell through-hull lights.
A pair of deep blue surface mount lights on the transom, lighting the water behind the boat at the dock.
Surface mount vs through-hull: which fits your boat?
| Mount | Install effort | New hole in the hull | Good fit |
|---|---|---|---|
| Surface mount (transom or hull) | Moderate: mark, drill one cable hole, set three screws | One small cable hole | Trailer boats, center consoles, pontoons, larger vessels |
| Through-hull | Highest: large cutout and sealed fitting | One large hole below the waterline | Refits and new builds with a rigger |
Surface mount specs are from the Black Oak Fathom product pages. The through-hull row describes the mount style in general.
Pick your boat to see the setup that usually makes sense.
Small hulls have little room and a small electrical budget. A pair of Fathom FL6 lights draws 3.5 amps each at 12 volts. If your jon boat is aluminum, ask your installer how to isolate the bronze fixture from the hull to prevent galvanic corrosion.
Surface mount a pair on the transom, then add lights along the hull sides using the counts in the next section. The kits below bundle lights for each boat length.
Fathom lights mount on any flat surface, which suits the flat faces on a pontoon hull. Browse pontoon boat lights for the full range, and use the placement counts below for the number of lights.
The Fathom FL24 is rated for vessels up to 150 feet and draws 10 amps at 12 volts, so it needs its own fused circuit. Kits are sized for the larger hulls too.
How many underwater lights does your boat need?
Black Oak's placement guide counts lights by boat length and drive type, with every light mounted 6 to 12 inches below the waterline.
| Boat length | Outboard | Inboard |
|---|---|---|
| 8 to 11 feet | 2 lights | 2 lights |
| 12 to 15 feet | 3 lights | 4 lights |
| 16 feet and up | 4 lights | 5 lights |
Source: Black Oak LED underwater light placement quick guide, shown below.
The placement guide: lights spaced evenly along the hull, 6 to 12 inches below the waterline. Tap the image to enlarge.
A pair on the transom is the most common setup, and larger boats and dedicated fishing rigs usually run three to four. Space the lights evenly so the glow overlaps and you do not get dark gaps. The FL6 and FL12 are rated for vessels up to 65 feet and the FL24 for vessels up to 150 feet.
Plan the circuit before you buy
Total current is the number of lights times the draw per light. At 12 volts:
| Light | Draw per light | Pair | Four lights |
|---|---|---|---|
| Fathom FL6 | 3.5 A | 7.0 A | 14.0 A |
| Fathom FL12 | 4.6 A | 9.2 A | 18.4 A |
| Fathom FL24 | 10 A | 20 A | 40 A |
Draw per light from the Fathom product pages. On a 24 volt system the draw per light is 1.8 A, 2.35 A and 5 A. Size wire and fusing for the total load.
Which underwater light color should you choose?
Water filters light by color, so the color you pick changes how far the glow reaches. NOAA Ocean Exploration reports that blue light penetrates best, then green, yellow, orange and red.
Order of travel through seawater, from NOAA Ocean Exploration.
Real photos of Black Oak underwater lights on a transom at night.
- Deep blue: the color that travels farthest in clear water. In one Japanese coastal field study that tested red, green, cyan and blue LEDs, blue attracted the most fish and red the fewest. Species and water clarity change the result, so treat it as a starting point.
- White: shows true color on bait and structure. Coastal water often looks greenish because algae and silt can absorb blue, so in murky water white is a sound choice for working light.
- Blue/White: both arrays behind one lens on the FL12 and FL24. Flick the switch off and on to step between them.
- RGB: cycles through the full spectrum from an ordinary switch with no controller, on the FL6 and FL24. Green is available this way.
- Red: absorbed first in water and attracted the fewest fish in that study, so Fathom lights do not come in a dedicated red.
Sources: NOAA Ocean Exploration, Ocean Facts on light and color; Sakamoto et al., Fish attracting effects of LED light of different colours (2017); Sierra College, Introduction to Oceanography, Light. Color options per model are from the Fathom product pages.
Fishing is the most common reason for choosing color. Our night fishing lights guide covers placement above and below the water, and the fishing light collection has the above-water lights.
How do you install and wire underwater boat lights?
- Choose the spots. Mark each light 6 to 12 inches below the waterline and space them evenly.
- Check behind the hull. Look inside for stringers, wiring and tanks before you drill. Drilling is one cable entry hole per surface mount light.
- Seal and mount. Use marine sealant on the hole and under the fixture, then fasten it with the three stainless screws that come in the box. Stainless hardware also avoids metal-on-metal corrosion.
- Add the harness and switch. Each light ships with a spliced pigtail and needs both. The underwater light wiring kit and a waterproof rocker switch cover them.
- Size the circuit. Use the table above, then fuse and wire for the total load.
- Test. The lights are rated for use out of the water, so a quick bench test on deck is fine.
Fathom lights are milled from AB2 bronze, the alloy used for propellers and thru-hull fittings, and they are IP69K rated and military grade. If you want to know what IP69K means, our IP rating guide explains it. Keep the lens clean with a soft cloth to prevent algae and barnacle buildup, and check the fasteners for corrosion each season.
Which Black Oak underwater light fits your boat?
All three Fathom lights share a low profile and a surface mount. Choose by boat size and by how much water you want to light. Browse the full underwater LED lights collection for every color.
| Model | Raw lumens | LEDs | Power | Size | Colors | Vessel up to |
|---|---|---|---|---|---|---|
| FL6 | 4,100 | 6 | 40 W | 4.3 in | White, Deep Blue, RGB | 65 ft |
| FL12 | 8,000 | 12 | 55 W | 4.3 in | White, Deep Blue, Blue/White | 65 ft |
| FL24 | 12,000 | 24 | 120 W | 5.12 in | White, Deep Blue, Blue/White, RGB | 150 ft |
All three have a 65 degree beam, a 0.7 inch profile and 12 or 24 volt operation. Specs from the Fathom product pages.
Fathom FL12: six optics, two LEDs in each, in a milled bronze body.
Shop the lights, wiring and kits:
Fathom FL64,100 raw lumens, 6 LEDs, boats to 65 ft
Fathom FL128,000 raw lumens, 12 LEDs, boats to 65 ft
Fathom FL2412,000 raw lumens, 24 LEDs, boats to 150 ft
Underwater Light Wiring KitHarness for your Fathom lights
Waterproof Rocker SwitchAlso steps Blue/White and RGB colors
16-20 Foot Boat Lighting KitCenter console kit
21-26 Foot Boat Lighting KitCenter console kit
27-32 Foot Boat Lighting KitCenter console kit
33-36 Foot Boat Lighting KitCenter console kit
37 Foot and Larger KitCenter console and sportfish kit
Underwater boat light FAQ
Do underwater boat lights need a big hole in the hull?
Not with a surface mount light. Each Black Oak Fathom light needs one small cable entry hole and three stainless screws, with no large cutout. A through-hull light needs a large cutout below the waterline and a sealed fitting.
How deep should underwater boat lights be mounted?
Black Oak's placement guide puts each light 6 to 12 inches below the waterline, spaced evenly so the glow overlaps.
How many underwater lights does my boat need?
Black Oak's placement guide calls for 2 lights on boats 8 to 11 feet, 3 (outboard) or 4 (inboard) on boats 12 to 15 feet, and 4 (outboard) or 5 (inboard) on boats 16 feet and up. A pair on the transom is the most common starting point, and larger boats and dedicated fishing rigs usually run three to four.
What color underwater light attracts the most fish?
In one Japanese coastal field study that tested red, green, cyan and blue LEDs, blue attracted the most fish and red the fewest. Species and water clarity change the result, so treat blue as a starting point, not a promise.
Does an underwater light come with a harness and switch?
No. Each Fathom light ships with a spliced pigtail and needs a harness and a switch to operate. The underwater light wiring kit and a waterproof rocker switch cover both.
How do I change colors on a Blue/White or RGB light?
Use the switch that powers it. Flick it off and straight back on and the light steps to the next color, then keeps cycling. No controller or extra wiring is needed.
Designed and engineered in Sarasota, Florida.
