Consensus on cruise RPM?

Rick · Post by **Rick** » Thu Jul 07, 2005 2:28 am

I have to guess because I'm not sure, but I think my tranny's are 1.5:1. Will check the next time I'm on board. Although I haven't done it in a while, the engines are spec'd to run at WOT of 4400 - 4800 and I top out at about 4500. So I think that says that I'm okay but if something else can save me some gas then I'd definitely look at it. But props aren't exactly cheap either. Frank, I'm one of those 1 3/8" guys. The 21 X 19's were on the boat before I re-powered, I'm surprised nobody else has them.

photo finish · Post by **photo finish** » Thu Jul 07, 2005 6:05 am

Rick, my guess is that your original props were 21x20 which is common, and that someone had cut them down to get more rpms as the engines got hours on them Tim Stamm Photo Finish 1981 33' Bertram FBC

Rick · Post by **Rick** » Thu Jul 07, 2005 7:23 am

Thanks Tim.....that's probably explains it. So here's a question for ya....how does one calculate what you need for a prop ? I found quite a few websites that sell calculating software, but do we really need it ? Rick 85 SF Falmouth, MA

photo finish · Post by **photo finish** » Thu Jul 07, 2005 9:38 am

go to michigan wheel's web site i think they have a form you can fill out and they email the info back to you....or easier i called them and they were very helpful with all my questions. however in a nut shell my findings are that as long as you are in your rpm window you are fine ...now that is if you are at 4500 fully loaded. from all my research i found that your target should be at 4600 fully loaded if 4600 is your middle range.so if you are not loaded then you might consider cutting them down to 21x18...you might lose a bit of speed maybe 1knot but you should gain a little efficiency. Tim Stamm Photo Finish 1981 33' Bertram FBC

lobsta1 · Post by **lobsta1** » Thu Jul 07, 2005 12:47 pm

Rick, My 78 came from the factory with 1.91 gears, 1 3/8ths shafts & 21x20 3 blade props. At that time Merc recommended 3800 rpm as WOT. Bert tested the boat as set up to 4000 rpm fully loaded according to the chart in my owners manual. Prior owner had engines & trannys rebuilt & a hardtop & full enslosure put on. At same he put on 21x19 3 blade props. He THOUGHT he was turning 4600rpm WOT. But, he had put in Mallory dist. & did not realize that the wiring harness was modified by Bert to work with the old Thunderbolt I ignition & the Detmar tachs, Actually he was only turning 3650 WOT. Since I've cleaned up the bottom a lot & had the props scanned & tuned to 21x18. After my launch last week, with full load & enclosure up, & two people on board, we were turning 3650 rpm at WOT. That puts us in the ballpark for my engines. I see you have MPI engines. Don't know what year your boat is. IF it is another oldie like mine, you may not be actually getting 4500 rpm. Have you checked with a digital tach connected at the engines? That was how I verified that my old Detmars were off. I put in new Teleflex on the bridge with a new wire run directly to the coil. Al 1978 33 FBC NITES OFF

Rick · Post by **Rick** » Thu Jul 07, 2005 2:49 pm

Al, I re-powered in 2000, rpm's were verified with external guages. 4500 is fully loaded, but I'll be confirming the next time that I take her out ! Thanks, Rick Rick Ticket 85 SF Falmouth, MA