engine upgrade prop help pls.
 

hi

i have a 34foot non stepped sterndrive sport boat, italian make with 7.4L 310hp bravo1 engines, i have 19p mirage props on and i get a max speed of 49mph@4400 rpm.
now i am upgrading my engines to 8.1L 420hp volvo penta's and i need to know which props and pitch i need taking into consideration that the 8.1L max rpm is 5000 compared to the 7.4L's 4400rpm

can anyone help me or have experience in such a situation advise on which props to buy. i was thinking of the bravo1 props or revolution4 or just stick to mirage plus higher pitch

my local dealer does not have any props for trials so i need to pick the right safe bet ones

anyone pls.

Re: engine upgrade prop help pls.
 

The best way to get started would be to use the props you have now, if possible. What is the part number on the side of the props. It may be possible to use those props with your new motor and drive with the proper hub it. Some of the older Mirage props use a square rubber hub. Those props can use the new flo torque 2 hubs after the rubber hub is pressed out.

Since your new motors have a rev limiter of 5150 you can try the props you have now and see what happens. After that you can get an idea of what prop may work best for you.

Matt

Re: engine upgrade prop help pls.
 

thanks for the quick help, i bought my props in 1995, and i have a sort of a heavy boat, generally would'nt the bravo 1's be a better choice for such a boat? usually how close to the rev limiter should my max rpm be?

Re: engine upgrade prop help pls.
 

The new motors will be very happy at 5000 rpm.

As far as the type of prop it depends on what type of boater you are and under what types of water conditions you boat in.

What drives are going on the new motors? Unless I'm mistaken only the Duo Prop is available with the 8.1 motor. If thats the case you will be limited to Volvo Duo Props as your only option.

Matt

Re: engine upgrade prop help pls.
 

i'm using my old bravo 1 drives, and water here is always choppy with small waves, and i like to get the maximum of my cruising speed but keep the engines comfortable, about 80% to 85% of rpm for no more than 2hrs at a time. so cruising speed is more important to me than flat out. by the way can you tell me for how long at a time can i run flat out on the 8.1L 420HP'S without damaging the engines?

Re: engine upgrade prop help pls.
 

I'd say if the current props are running 4400 rpm with 310 hp they should be pretty close to 5000 rpm with 420 hp.

I would do as Julie suggests and try those and see where you're at. I'd bet you'll be between 19 and 21 pitch.