I would have to guess + 5 minutes unless you site a shark in which case maybe -15 minutes

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I'll be what I am
A solitary man

Wetsuit makes a huge difference. Whether or not you do flip turns makes a big difference. How good are your turns (flip or otherwise)? Are you good at drafting? Are you good at sighting? Are you good in rough water? Do you swim reasonably straight? What is the current like? (The ocean has STRONG currents parrallel to the shore. Many people think only of the tide...)

Plus, triathlon courses are NEVER marked correctly for distance.

So the answer is, you really have no idea! Honestly. It's not worth worrying about. Focus on how you feel at your pool pace, and try and duplicate that feeling. Then try to swim straight and draft as much as possible. Whatever the time is, it is.

I would say you would be at 20-30 minutes is the range you can expect for most courses.

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"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp

I grew up swimming across my lake when I was a kid and hadn't really swam laps in a pool until my 20s. Therefore I swim much faster in open water (wetsuit or not) because I can't turn for crap and am more used to long straight sets of 1500+ rather than 200's for example.

For me is a probably -5 on a 1500 in open water vs pool.

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Don't hold back

Interesting to crunch the numbers:

I would say it will NOT take you a lot longer.

Remember a genuine mile in a 25-yard pool is just about 1800 yards, not 1600 yards as many people think. Olympic triathlons are supposed to start with a 1500 meter swim, which is close to 1600 yards or about 9/10 of a mile.

My best time for 1000 yards in a pool is about 13 minutes and a few seconds; That's like 1:20 pace ... My best time in an olympic triathlon is like a 21:30, which is also about 1:20 pace ... I did one-mile open water swim race under 24, which is a bit slower ... I have pretty decent turns but I also believe wetsuits make lake swimming easier and faster ... so for me its a wash and pool times give me a decent indication of how I swim in lakes.

I find I swim about 5 or 6% slower in open water than in a 25-yard pool. I also find that a wetsuit more than offsets this difference and allows me to swim about 7% faster than without.