Sales brochure says the gear ratio is 2:1 and available props for that engine are 8-19P. You are in a 17' boat, telling us you don't have a hole shot, then telling us that it hasn't been calm enough for you to go out which speaks volumes about a heavy load on the engine. You said you have a 13P prop. or average pitch for those available/recommended from Mercury. We don't know how well your engine is running otherwise but some data would really help. A gut feel would be that you need to get closer to 8P but with no data, who knows how close.
If you could give us some numbers on speed and rpms. Other things is how is the engine trimmed? How does the horizontal plate just above the prop line up with the bottom of the boat if you take a yard stick and run it out from flat with the bottom of the boat and across the plate? Is the plate parallel to the yardstick? Is it above or below? If so, how far?
What kind of load do you have in your boat. What's at the transom in the way of you (tiller steering engine???) how much do you weigh, gas- how much and where, live well- how big and where, batteries- how many, how big? What's up in the bow?
