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Re: How Fast Should My Boat Go Under Diesel
Hull speed is the theoretical top-end speed on flat, still water. You may or may not be able to achieve hull speed, depending on your engine's horsepower, prop size/pitch, gross weight of the boat and its contents, and amount of growth on the bottom. Without experimenting with your boat, nobody will be able to tell you how fast you should go.
I think the question you should be interested in is: How hard should I run my engine? Consult the engine owner's manual for that. If you bought the boat used and don't have a user's manual, a good rule of thumb is to cruise at 80% of your RPMs at maximum throttle. Bring the engine up to full throttle, check the RPMs and speed through the water, then throttle back to 80% of your max RPMs and re-check your speed through the water (rather than your GPS speed over ground because that will be affected by currents). If you reach hull speed before hitting full throttle, then there isn't any reason to run the engine all-out. And as your speed increases toward hull speed, the amount of additional power needed to make the boat go faster increases. To put this in financial terms, it might cost you $3/hr to cruise at 5 knots and $5 or $6 an hour to go 6 knots (i.e., a 100% increase in fuel costs for only a 20% increase in speed). These numbers are just for illustrative purposes; your actual numbers will be different but will likely follow a similar pattern.
Pay attention to the relationship between RPMs and speed. If you notice an unexpected drop in speed, ask yourself if you are bucking a current, have added weight to the boat, or if it is time to clean the bottom. (BTW, the slightest amount of growth on your prop will ruin its efficiency).