1993 chris craft 380 continental performance question

Hello,

I recently purchased a '93 ChrisCraft 380 Continental. The boat had 454 gas engines which were in poor condition so I changed them with some Yanmar 6 lps. I am trying to get the proper propellers but am not sure what speed this boat cruises at. I see a published speed of 15 knots but I see people say they cruise closer to the 25 knot range (or higher). By chance, do any of you have a similair Chris Craft and if so what kind of performance do you get? I currently have 3-blade 20X18 props which give me the correct WOT rpm of 3900 but at 3400 rpm she seems slow at aroudn 14 knots (fully loaded) and also seems she is not able to trim out properly.

Thanks

Thanks for the reply and of course you rasie valid points. I do not have access to fuel flow. I tried a larger set of props (I think they were 20 X 20?) that were 4 blade and while I got a better speed - 20 knts at 3400 rpm, I could not get to the 3900 rpm at WOT and that is certianly the "magic number".

The Yanmars are 300 hp and they replaced 330 hp gas engines so while I lost 10% HP I almost doubled torque. I am in contact with several people such as Yanmar and a local prop shop. In both cases I need to give them an idea of how fast the boat should go and that is where I am hitting a wall. I never knew what the boat with the gas engines ran at and I am seeing posted values of 15 knts to 30 knts cruising speeds. My thought is that at 14 to 15 the boat is not ableto trim properly and this is reducing my rpms.If I can increase the speed to around 25 knts I would be "surfing" the wave instead of trying to go up it and this could mean that I can get my 3900 rpm with a higher pitched prop.

I was told by the Yanmar person that I need to have a range between 3800 and 4000 rpm. I figure that 3900 is the ideal target. I would be happy though with good performance and 3800 since I tend to cruise at 3400-3500 and never go to 3600.

I have the same gear as on the gas boat. Also, the boat is a semi-pocket drive. With the currentprops (20x18) i have only about an inch or so clearance. One thing I notice with the current props is that the boat jumps up to the 14-15 knots fast. Based on this, I hope that the next round of consulation results in a recomendation to decrease diameter and increase pitch.

Still, I would like to know what speed the boat should really cruise at.