You need one
All sailboats need a long shaft outboard. The shaft length is as important as the horsepower. IMHO, a long shaft is not just a nice feature, it is an absolute necessity. A 20" shaft is the standard long shaft length (15" is the standard length shaft). However, at least one manufacturer also offers a 25" extra-long shaft. Bottom line: buy the longest shaft engine offered. You will understand the importance the first time you cross a powerboat wake or are operating in any kind of swell.