FrostList

When to Winterize Your Boat in San Antonio, TX

ON TRACK157 days until first 28°F freezeDec 15

In San Antonio, target December 8 to drain, antifreeze, and lay up the boat, about a week before the median first 28°F freeze of December 15; a surprise cold snap is exactly how freeze claims happen. Plan for the early end: the one-in-ten date lands about 26 days before the median.

OUTLOOK

Typical first first 28°F freeze near Dec 15; local deadline about Dec 8. The live 10-day outlook loads here.

Local freeze dates for San Antonio

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Nov 7Nov 28Dec 21
28°F (hard freeze)Nov 19Dec 15Jan 20
24°F (severe)Dec 3Jan 1Feb 4

NOAA station: San Antonio Incarnate Word · 3.4 mi away · 700 ft elevation.

San Antonio draws its numbers from San Antonio Incarnate Word, 700 feet up and 3.4 miles away. Its median first-freeze dates are 32°F by Nov 28, 28°F by Dec 15, 24°F by Jan 1. That hard freeze has landed anywhere from Nov 19 to Jan 20, a swing of roughly 62 days. Spring's last 32°F freeze clears around Feb 25. Snowfall averages 0 inches a year.

Expect the first frost near Nov 28 in San Antonio and the first hard freeze by about Dec 15. That first freezing night has ranged from Nov 7 to Dec 21, roughly a 44-day spread. On the spring side, the last 32°F freeze clears around Feb 25 and as late as Mar 19 — the green light for reopening water and de-winterizing. With almost no snow in a normal year, cold — not plowing — sets the calendar, and it centers on Feb 25.

Your boat checklist

  1. Change the engine oil and filter while the motor is warm so contaminants leave with the old oil.
  2. Run the engine on antifreeze until pink fluid reaches the outlet, protecting the block and manifolds.
    Helpful gear: Marine antifreeze — Recommended pick
  3. Fog the cylinders with fogging oil as the engine runs down to coat internal parts against corrosion.
    Helpful gear: Fogging oil — Recommended pick
  4. Top off the fuel tank and add stabilizer to limit condensation and protect ethanol-blend fuel.
    Helpful gear: Marine fuel stabilizer — Recommended pick
  5. Drain the raw-water system, the livewell, and any washdown lines that can trap water.
  6. Remove the battery, charge it fully, and keep it on a maintainer over the layup.
    Helpful gear: Battery maintainer — Recommended pick
  7. Support the cover so snow and rain shed off, and leave vents so the interior can breathe.
  8. For an inboard or sterndrive, follow the manual closely — those blocks are the ones freeze claims hit hardest.

What to have on hand

Marine antifreeze
Propylene-glycol fluid rated for engine blocks and systems.
Recommended pick
Marine fuel stabilizer
Additive that protects ethanol-blend fuel over the layup.
Recommended pick
Fogging oil
Spray that coats cylinders against corrosion in storage.
Recommended pick
Battery maintainer
Keeps the cranking battery topped off on the trailer.
Recommended pick

What this means locally

San Antonio freezes later than New Braunfels (Dec 2) and close to San Marcos (Dec 15) — a reminder that even nearby towns differ by days. Statewide, Texas prep dates run Jan 1 through Dec 29, which is why San Antonio gets its own number rather than a Texas-wide average. The same freeze also decides when to winterize an RV and store a motorcycle.

Other winter jobs in San Antonio

Every task below is dated to San Antonio's own freeze and snow normals.

Sprinklers → Dec 5Frozen pipes → Nov 7Car battery → Dec 15Sump pump → Feb 25RV → Dec 5Motorcycle → Nov 28

See the full San Antonio winter checklist, in order →

Frequently asked questions

At what temperature does a boat engine block crack?
Trapped water in an engine block or manifold can freeze and crack the casting once temperatures fall below 32°F and stay there long enough. Inboard and sterndrive blocks hold the most water and are the most vulnerable. Draining fully and adding antifreeze before the first hard freeze near Dec 15 is the protection.
Do outboards need winterizing?
Yes. An outboard should be drained so no water sits in the powerhead or lower unit, and the fuel system stabilized. Outboards self-drain better than inboards when tilted down, but fogging the cylinders and changing the gear oil are still part of a proper layup.
Fogging oil: is it necessary?
Fogging coats the cylinder walls and internal parts with a film that resists corrosion during months of storage. On engines that sit all winter it is cheap insurance against rust, especially in humid or coastal storage. Follow your engine maker's guidance on how to apply it.
How much does boat winterization cost?
A professional winterization varies with engine type and boat size, and inboards cost more than a simple outboard. Doing it yourself costs mainly antifreeze, oil, and fogging oil. Either way, the bill is far smaller than repairing a cracked block, which is the failure winterizing prevents.
Do boats in TX really need winterizing?
If temperatures reach freezing where the boat is stored, yes. Freeze claims are common in milder states precisely because owners assume they are safe and skip the step, then a rare cold snap cracks a block. Check the local freeze dates above and protect the engine before the first hard freeze.
When should I de-winterize my boat?
De-winterize in spring once the risk of a hard freeze has passed for your area. Reconnect and charge the battery, replace drain plugs, check hoses and clamps, and inspect the fuel system before the first launch. Reopening too early leaves the block exposed to a late cold snap.

Data: NOAA 1991–2020 normals via San Antonio Incarnate Word, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.