I'm learning how to spray boat bottoms but I get over lapping vertical
stripes. Does anyone have any tips to avoid this?

Cover 1/2 of your previous pass and start and end with a wide mask, so your coverage on the ends is even. Resist the temptation to lay a thick layer - Multiple, thin layers are vastly better because they won't run, puddle, etc. and are just more even. Also, the amount of thinner is a lot more in a sprayer than with a brush or roller - up to 70%, depending on a large number of factors. Read instructions on paint manufacturers' websites - Their tutorials are terrific! Best wishes!
- Jerr

very well said