It depends...
Your decision will be affected by how you use the boat (cruising or racing), how much motoring you have to do and what your local wind, wave and current conditions are like. Of course, price is also a big factor.If you don't have to deal with adverse weather conditions, tides or currents and you don't motor very far, then a smaller motor (5hp)is OK.However, if you even think that you may be in a situation where you have to rely more on your motor, then I suggest something with more "grunt", and possibly an alternator.In my case, I deal with a wide variety of sailing conditions, from flat calm to 25 kts. Going out, I often have to deal simultaneaously with a headwind, waves and current. The trip to sailable water is almost two miles, and there are three bridges (one of which is a drawbridge) along the way for good measure. My kids sail with me and I often sail at night. My motor is important to me.Four months after I bought my 88 H23, I replaced the original Tohatsu 5hp with a Nissan 8hp, and I must say that I am very pleased. The price was right (under $1200), it is light and powerful (it will move the boat at hull speed below half throttle in a light chop). The color also matched the trim on the boat.One huge advantage is the external fuel tank. At 3 gals. it gives me plenty of motoring range and if I have to, I can easily refill it in the comfort of the cockpit. The built-in tank on the 5hp would only gave me about an hour of motoring, and refilling it while hanging over the transom in waves was a pain in the @$$.I hope this helps you make your decision.PeterS/V Raven