Prismatic Kelp Window

The Signet Marine Prismatic Kelp Window allows you to easily

see if kelp or line has fouled your keel, rudder or prop.

Kelp Window Field of View

A prism works by refracting light, which slows when it travels through denser materials.

Light travels faster through the air than it does through the acrylic of the Kelp Window. When it exits the oblique angle of the viewing port, the light coming from the short end of the prism reaches your eyes sooner than the light coming from the tall end.

This differential slowing of the light along the gradient of the viewing port causes the light waves to bend, increasing the field of view.

Dimensions and Installation

For keel viewing, the Kelp Window is installed forward of the keel with the viewing port facing forward. Choose a location that will be easy to access from inside the boat.

It is not uncommon for a boat to have several Kelp Windows installed for viewing both sides of the keel and the rudder.

The Prismatic Kelp Window is a machined acrylic thru-hull viewing port that is bonded through the hull and must be installed when the boat is hauled out of the water. Once installed and bonded, it can not be rotated.


Prismatic Kelp Window: (PKW-1)  $139


Vertical fin keel designs, such as T and L keels found on racing sailboats have an annoying tendency to snag and hold kelp. For the racer, that can mean the end of the race before passing the starting line. On smaller boats, even one kelp leaf can noticeably slow the boat.


Why a prism?

The optical characteristics of the Prismatic Kelp Window allow you to see underwater at an angle, not just straight down, using a flat cut surface that is flush with the hull. The 3D animation at the top of the page simulates how the Kelp Window sees beyond its immediate boundaries. The angle of view in which you can see through the Kelp Window is approximately 50 to 75 degrees beyond it's vertical axis.