I have a "84" commander 410. What rpm should I be able to achieve if I'm propped properly and If I fall short am I under or over propped? Also how much would problems with the condition of the props affect this rpm?
Thanks
RPM and prop pitch ( gear ratio, weight, hull type, and other good stuff )
November 26 2008, 2:06 PM
Hi Rick,
Just saw your note and your other notes, which I'll repond to in a moment. The general rule is maximum rated engine rpm should be achieved at wide open throttle. This means, that when you factor in your engine type (I think you have the 427, which is rated at 4000, and the GM 454 may be 4200 better verify) you will find it is rated at a particular max rpm. With the gear reduction (I believe you have 2.5:1) and prop size, weight of the boat, hull type, etc., you should be able to reach that max rpm at wide open throttle in realtime operation on the water.
If you are only able to reach 3700 rpm as I am now with my overpropped 4-blade props, then you are clearly running too much pitch. If you are able to run a motor at higher speeds than the rated rpm, then you are underpropped. Chris Craft did exhaustive testing on the water with each hull, changing out props to achieve what they thought was the best top end and best efficiency. It is very difficult to beat them at their own game with second guessing what prop is best for a given boat.
The rule of thumb I'm (painfully) experiencing now, is the fact that my 23x24 4-blade props should actually be 23x23, because the specified prop for a 38 is the 23x25. That is why I cautioned about the need to compensate between the pitch of a 3 blade or 4 blade.
To restate this for you again to be clear: a 23x25 three blade will limit the rpm of a 38' Commander Express to real close to the 4000 rpm max allowable by the engine manufacturer; while a 23x24 four blade will cause the rpm to drop around 3700, and therefore should have been a 23x23.
You always give up a little top end with a 4-blade prop, but you (supposedly) get better mid range cruising and less vibration. I didn't really notice all that much difference in vibration, but the boat sure gets onto a plane nicely at lower rpm as you might expect. Under some engine speeds I'm causing the motors to work a little harder than they should, and I suspect fuel economy is suffering a bit. In the springtime I'm going to swap back to 3-blade, and have the 4-blades repitched to 23x23.
His is 427 powered, but the props are most likely going to be the same between the 427 and 454 as torque figures are very similar. I'll drop him a note to see what props he's using on his 41, so you can use it as a reference.
427 and 454 motors are going to be pretty much the same performance and fuel consumption
November 26 2008, 2:56 PM
Power, torque, compression rations, cubic inches are essentially the same, especially when looking at the torque transmitted to the prop. The 427 actually produces more power than 300 just as it sits, but the 300 is the rating at the 4000 limit they put on it for public release in a boat. The 300 number is simply the rated output at 4000. If you look at the torque and power curve you'll see what I'm talking about, the power curve is a pretty straight 45-degree angle with little indication of peaking out abruptly. They could have just as easily called this a 310 or 320 hp motor if they adjusted the rpm limits accordingly, as I believe it would easily produce more if it were put on a dyno and run at a true wide open throttle. Naturally, a tweak on the cam would have helped too.
I sent Bill a note, hope he is in during the holiday and able to respond. It will give you an excellent starting point to see what props he is using with his 427 powered 41, and then we can always find someone with an identical boat to yours for the final confirmation of true on-the-water performance numbers.
Rick,
I have the factory sea trial notes from Chris Smith on this model with various engines from that era. If you want them I can mail to you.Please e-mail off list for info.Prop tuning and engine state of tune are very important.
Tim
Now I remember THAT is the sea trial you handed out to everyone at Huron! Rick will be VERY interested in having that in his file. Looking at the data now it appears that 22x22 4 blade are THE right prop for the boat after all. For some reason I took Rick's earlier postings to suggest the 22x22 4-blade was the wrong prop, not sure why I assumed that, but your post clarified this nicely.
Looking at the test data, interesting to see one test (at least) was done with 427 power running at 4200 rpm with 22x22 4-blade props, and with what appears to be the 454 at 330 hp (28.5 mph) and 350 hp (30.8 mph) looks like they got a 41' hull to step out very smartly. I honestly didn't recall the 41 was able to achieve that speed, and I'm impressed! As they say, "Chris Craft didn't build a slow boat".
Good stuff, should allready be posted in the data base, I'll try to scan it next week. I think you may have just saved Rick a bunch of his hard earned cash, and some heart-ache too.