RPM'S vs Speed

[capthorace]

1995 Volvo's with jack shaft.on 28 CC tower and full curtains

Cruise RPM's = ? Speed = ?

Wide open RPM'S = ? Speed= ?

Not sure I am getting full rpm's. What say the others?

[backman]

What Volvo's?

If they are KAMD 42's they must turn 3900 for full RPM.

[capthorace]

Engines are 7.4 gas Volvo's on a jack shaft. Sorry I omited this.

[hammerhead]

I have a 1997 28 with same setup, I cruise at 3200 rpm and will go about 23to 25 knots, seems to be my sweet spot and not burn too much fuel, after 3200 seems fule burn goes way up. Sometimes I do go to 3400 when its a nice flat day. Also depends on how many people and fuel on board but I would say 24 knots at 3200 Harry

[ckennedy]

I have a 1997 28 with same setup, I cruise at 3200 rpm and will go about 23to 25 knots, seems to be my sweet spot and not burn too much fuel, after 3200 seems fule burn goes way up. Sometimes I do go to 3400 when its a nice flat day. Also depends on how many people and fuel on board but I would say 24 knots at 3200 Harry

What is WOT for you in RPM? What props do you have?

[Bruce Clarkson]

Engines are 7.4 gas Volvo's on a jack shaft. Sorry I omited this.

The 7.4's were spec'd for 4200-4600 for both the Volvo and Merc versions unless you got the high HP version.

Bruce Clarkson

[Mackey]

Also need to figure in your gear ratio and we are assuming the 7.4 are carb'ed. As Bruce noted, you should be turning up no less than 4200rpms on a season'ed bottom & well loaded. If you are not and the engines are running fine, then you are over prop'ed.

I have a 1995 CC28 that was orginially a gaser (454) but the previous ower repowered with Yanmar 4LHA (200hp) and 290 DP-E drives. I have SS C4 props and I believe the gear ratio is 1.95:1
Depending on the load and condition of the bottom, I can cruise at 23kts @ 2950rpm, my fast cruise is 25kts @ 3100rpm and WOT 29.5kts @ 3400rpms.
I generally find myself running at 2950rpm since I am only burning 15gph total and 23kts is fast enough for me.

[Bluefin 32]

You might want to get a bit scientific about this to see whether you really have a problem. On a calm day take two-way (to neutralize effect of wind and current) speed and rpm readings at several rpm points between minimum planing speed and WOT (let's say 2500, 2700,3000, etc). Then take the average of the two speeds at each rpm level and plug it into a slip calculator with your gear ratio. Based on the slip reading, you can gauge whether you have an issue with your props, excessive bottom drag and so on.http://www.rbbi.com/folders/prop/propcalc.htm is a simple on line prop slip calculator you can use.

Ideally you would see less than 15% slip at cruise and 10-12% at WOT.