Does anyone out there know a good way of checking a tachometer, or better yet, determining the correct propeller size? A little over a year ago we purchased a 39 ft. CHB. On the bridge the previous owner had noted the cruising tach setting, along with the corresponding tach reading, at the lower helm. This was 1850 and 1725 RPM, respectively. After running for an hour we opened the throttle for a minute and the bridge tach showed 2200 RPM. The surveyor said that the bridge tach was the correct one. Further reading stated that the 120 Lehman had a top end of 2500 RPM, and that a throttle setting of 70 - 80% was a good cruise speed. We knew that the propeller was a little oversize, but "within limits." Last month we installed a new inverter system. This included a new 100 amp alternator. That change made it necessary for the tachs to be readjusted at the boatyard. When bringing the trawler home I ran it up to 1700 RPM which sounded like it was too much. That thought was confirmed when the heat gauge reading began to creep up. When almost home, with the engine well warmed, I opened her up and could only get 1950 RPM. I called the boatyard and discussed the settings and results with them. They said that I was overpropped. They also said that a good (theirs) tachometer calibrating instrument costs about $600. If they are correct, we are definitely overpropped. The CHB is 39 ft. (actually 39.5 ft.) and has a 14 ft. beam. I don't know the weight. We are running a single 26x20 prop (26" diameter). Any ideas at all would be of help. Sandy and Dawna Floe M/V Sea Eagle Puget Sound

         http://www.miwheel.com/InboardSizing.htm


                                              Jim Wujciak
                                              MT 36' Sedan
                                              Hard to Get

You also need the the transmission reduction to determine the proper sized propeller for your boat. Michigan Wheel Corporation has a free sizing service. I used them for my 'new' prop and they sized it just fine. They have a forms based page at: http://www.miwheel.com/InboardSizing.htm Jim Wujciak MT 36' Sedan Hard to Get ============================================================= On Mon, 27 Apr 1998, Sandy wrote: > Does anyone out there know a good way of checking a tachometer, or > better yet, determining the correct propeller size? > > The CHB is 39 ft. (actually 39.5 ft.) and has a 14 ft. beam. I don't > know the weight. We are running a single 26x20 prop (26" diameter). > > Any ideas at all would be of help. > > Sandy and Dawna Floe > M/V Sea Eagle > Puget Sound >