Seneca sailor here - since age 7. I know Seneca like the back of my hand. Been there my whole life - (well not yet
There are many launch sites but not all of them sailboat friendly. Some launches are in harbors. The best option in that case is find a place on the beach where you can pull up the boat. Leave your mast and beer on the beach. Go launch the boat and paddle it over to the beach where you can rig it and launch and land all day. You have to be careful in some areas because there are boulders. However, there are also plenty of shale and sand beaches too. If I have an idea of your itinerary I can be of better help (east side/west side/north end/south end, camping etc) - or if you want an itinerary feel free to email me.
Briefly- if you're camping - The east side of the lake has more sailboat friendly launches. About 14 miles up is a a quaint little place called Smith Park near Peach Orchard Point. There's great places to hang out and eat like Big Johnson's and the StoneCat cafe' at the top of the hill. The launch at Smith is sailboat friendly. We used to have our Seneca Sailing Association races there every Sunday. Much further up the lake on the east side is Sampson State Park. It's a really nice spot and there is a sandy launch site - the only problem is that you have to temporarily dis-obey a "no vehicles" sign and drive down a walkway -road to launch. The Ranger doesn't really seem to mind however. The west side has almost nothing that works well for a beach and launch site. Check my post elsewhere on the forum on the Places to Sail discussion. A search for posts by author widerisbetter should do the trick.
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