This is from the perspective of a 1972 43' MY with 310HP rated Johnson and Towers 6-71N's. They are rated at 2,500 rpm max, but the most I have ever seen is 2,450 rpm. I use the rule of thumb that max cruise is max RPM at WOT, minus 200 rpm. However, I have found that for some reason she is "happiest" at 2,150 rpm, which is where I cruise.
Sidebar is that reduced rpm approach includes some oil consumption issues, plus that the right engines runs a little warmer, only 2-3 degrees, but runs just below indicated 180, at 2250rpm. However, I have addressed some of those issue sources. E.g, oil leaks at old hoses and seals, since replaced and oil consumption is quite a bit lower as of late. I had always heard that 1-1.5 qts /10 hours run time was acceptable, while some owners say never used "any". Lately, I seem to be using much less than before, almost none really, so I am curious if that is for real.
Factors that affect my over the ground speed include:
> depth of water...a pushback bow wave effect kicks in about 25 foot depth water, knocking off about 1/2-3/4 mph.
> cleanliness of hull...she is slower for a few miles, after she has been sitting for a couple weeks, as some of accumulations run off. We have a little slime, plus collections of water weeds, as I am moored in a relatively fast moving river, whish brings weeds to us.
> load in the vessel: She feels like she is flying, if I am low on fuel, the water tank is low, as well as the holding tanks. Those tanks when full add several thousand pounds of weight: 400 gals, plus 120 gals, plus 2@135 gals or so: approx 800 gals at 8#'s=6,400 #'s. Also, passengers add weight, so it all adds up.
> uphill or downhill, i.e. into a sea or running with the sea. A 1-3' sea can reduce or increase speed by 1-1.5 knots, if running into the sea or running with it.
At any rate, when running at 2,150 rpm, ground speed can vary from 14.4 to 16.0 knots, usually closer to 14.7 as a rule, but as high at 15.3 knots. These are GPS observations. I haven't made the knot to mph conversion here.
Last year when running into 4-5 foot slightly quartering seas, I did check down to 2,000 rpm for a better ride at about 12.5-13 knots. We did not pound, but did throw our share of spray and some green water. Hull speed of around 9.0 knots is at about 1,400rpm.
Weekend was fabuous in Northern Michigan!
