Prop upgrade for Horse Power upgrade

[drwillis]

I have a 24ft Cobalt 2004 with Volvo Penta Duo Prop's, size F6. The boat had a 350 cubic inch 270 hp (blew a piston). I am putting in a 450 HP 454. I know that I will be under prop'ed.

I'd like to know if someone could recommend (best guess) at what to try first. F7 or F8?

Trying to save some time and money in getting the prop right. If someone knows of any charts that explain these props, that would be great. I could guess with a Merc, but these Volvo Penta's are a different story.

Thanks for your help.

 

[Maclin]

Re: Prop upgrade for Horse Power upgrade

Regarding the pitch steps in between the Volvo numbered system, you can figure 1.5 to 2 inch in between the numbered steps, closer to 2 inches.

You will still need to estimate what the extra Torque/HP will allow you to do using the performance numbers from the 5.7 as baseline. What RPM could the 5.7 attain at WOT with the F6's? If it was at the top of the RPM range then I would try F8's. Otherwise if the 5.7 was at the low end of the top RPM range then I would only go one step up to F7. That is the logic I would have to apply in a case like this.

I would also put out there that the larger cubed engine, had it been a VP option, may have come with a different ratio drive. The DP drives came in 2.3 (V6), 1.98 (5.7) and 1.78 (Diesels, and maybe Big Blocks as well). After experimenting if you find yourself at F9's and still can over-rev then a ratio change to get back into the F6/7 range may be your next step after that.

Regarding the pitch step in inches numbers, those numbers are from personal experience, and also a website from Solas that can make those props now, and they put a pitch# in their description. The numbers I got from my personal experiences more or less align with the Solas pitch#'s. They do not offer F series, but the numbers between F and B and C align regarding the pitch steps. Like I said earlier, figure not quite 2" increments between the VP series numbers.

 

[Maclin]

Re: Prop upgrade for Horse Power upgrade

I have attached a prop guide from VolvoPenta, it has a bunch of graph type representations of the DuoProp RPM ranges, but no real explanation.

 

[hwsiii]

Re: Prop upgrade for Horse Power upgrade

Willis, in order for anyone to help you we need a lot more information,you don't mention any speeds at what RPMs or even what exact outdrive you are using as well as the gear ratio of the outdrive. If you will give us some real numbers there is a good chance we can help you. You are making a major change with the HP, a 67% increase in fact, and that will require a lot of information to try and find the right prop set for you.

After you install the new motor if you can give us some speeds with RPMs we can help.

H