Recreation
Fat Sac
A soft, fillable PVC ballast bag placed in a boat's lockers or floor to add water weight for wakesurfing, named after FatSac, the original and best-known ballast-bag manufacturer.
Fat Sac is a term from limnology — the scientific study of inland waters. The detailed explanation below covers the definition, why the concept matters to lake water-quality interpretation, and how it intersects with the LakeGrade rubric. Reading Fat Sac alongside the LakeGrade rubric helps make the rubric's structure intuitive — each grading parameter maps directly onto an established limnological concept.
The per-lake pages on LakeQuality always show the specific Fat Sac value for that lake, so you can move from the general concept to the specific lake without leaving the site.
What It Means for Your Lake
A fat sac is a flexible ballast bag filled with lake water to add mass to a wake boat. FatSac is the original and most well-known ballast-bag manufacturer in the world and is still made in the USA, and its name has become a generic term for soft ballast. Bags are placed in rear lockers, the bow, or under seats and are filled and drained via plumbed pumps. Unlike hard tanks, soft bags can be added, removed, or resized to tune weight, but they take up storage and seating space and can develop mildew. Every bag requires a vent or overflow connection; without one the bag will trap air and not fill fully, or it will overfill and grow larger than its compartment, risking damage.
Frequently Asked Questions
What is fat sac?
A soft, fillable PVC ballast bag placed in a boat's lockers or floor to add water weight for wakesurfing, named after FatSac, the original and best-known ballast-bag manufacturer.
Why does fat sac matter for lake health?
A fat sac is a flexible ballast bag filled with lake water to add mass to a wake boat. FatSac is the original and most well-known ballast-bag manufacturer in the world and is still made in the USA, and its name has become a generic term for soft ballast. Bags are placed in rear lockers, the bow, or ...
Related Terms
Added weight (water-filled bags or hard weights) placed in the boat to sink the hull deeper and build a bigger, more defined wake.
A rigid, molded ballast tank built below the floor of a wake boat that holds water weight without occupying storage or seating space and avoids the mildew issues of soft bags.
A permanently installed ballast system with fixed plumbing—pumps, hoses, intakes, vents, and drains—that fills and empties hard tanks or sacs automatically without manual hoses.
The overflow tube on a ballast bag that lets trapped air escape so the bag fills fully and prevents it from overfilling beyond its compartment.
The electric pump that moves water into and out of a boat's ballast tanks or sacs, available as reversible flexible-impeller pumps or one-directional aerator pumps.
Source: EPA National Aquatic Resource Surveys, 2026.