Hi all
What rpm do you typically maintain at cruise.
depends. there is hull speed cruise. i think thats 9.7mph for our 39'9" hull? please check my math.
then there are faster cruises. highly depend on conditions, wind, currents, propeller pitch/tuning, load (fuel, water, gear, persons), etc.
typically on longer trips we hover around 3000 rpm and if conditions and load arent bad, maybe push to 3250 if we are trying to make a deadline and aren't heavily loaded. some times we start at 3000 rpm until we are down to half fuel and then push a little harder. at 3000 rpm we see between 17-22mph, average 20mph, depending on the variables. at 3250 we see 20-25mph, average 23mph, again depending on the variables. i was on a river with strong current, 3250 rpm, barely making 16mph, river had a current of 7+mph against me.
i think volvo says technically you can do 1 hour WOT for every 8 hours. we only do WOT to exercise, up to 5 minutes at a time, and to check WOT RPM. higher rpm/load on an engine usually equates to lower engine life but lower rpm, especially with a turbo, also isn't good for it, so best to find a happy medium. they also recommend a cruising speed 10% below the maximum rpm at WOT (which again can vary depending on conditions, load, etc).
We usually see what WOT RPM we can get when starting each trip. Some times its 3650, some times 3590. Tides, wind, fuel level, changing during a trip can also cause it to change during a trip. some times one engine will vary from the other too, usually depending on which direction the wind, current is coming from, and some times where people/gear is on the boat. we will take the lower RPM WOT and consider that the RPM WOT even if the other engine is higher RPM WOT. they match pretty identically in neutral conditions and moderate load.
generally with turbos you want to exercise them and have some turbo psi to dry out condensation and at the same time you want to allow them to spool down and cool down after a longer trip.
you'll also see your fuel burn vary wildly depending on conditions, load, etc. some times we see 3250 rpm giving us 23mph and 23gph, so 1mpg, other times that could be 25-26+ gph at same rpm. we aim for 1mpg or better but some times conditions and time lines just dont allow it.
