Should we switch from F6 to F7? Volvo DP-SM DuoProp

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Well the long and the short of it is, that we turned our 350 into a 383 stroker and now it is putting down a lot better power. It will accelerate to top speed faster than before. We have the F6 on there now and that produces 50mph (GPS) top end (before and after). And, it will hold top speed easier than before also.

There have been a couple F7s on craigslist and ebay going for half price. With the new added low end torque, I don't think I need to be geared so high, and won't notice a huge loss down low from changing out to the F7s. I'm looking to pick up 3-4 mph. Will this prop do that?

What are your suggestions? Should I swap them? I'm new to the propping area so any comments and suggestions are welcomed! Thanks in advance. Cheers - Adam
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Forgot the essential information....sorry! 5,000 max RPM, 1.95 gear. 2003 Larson Escape 234 23' deckboat. Online says dry weight is only 4,000lbs, but it pulls a lot heavier than that. When we got it I took it to the scale and weighed the boat and the trailer at 6,700lbs with no fuel... can't imagine the trailer weigh 2700#. :eek:

Before doing the stroker the boat would top out at 49.9mph, 3 people, perfect water, and running on fumes.

As I've been tuneing the fuel for this stroker, i've noticed the most RPM via the ECU at about 4850 and GPS produces 50.5 mph with 3 people and 3/4 tank of fuel.

I've read that the F6 is 25" and the F7 is 27".

I've also read that by making a 2" change up in prop you lose 300-400 rpms so my RPM would drop to 4450 and based on a calculator @ 4450 with 27" I'd be at 51mph.

Is this correct? Can any of you provide some help please. Do it don't? Don't Do it? And any other suggestions or things I need to consider are welcomed. I'll provide any other info you need.Thanks again.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Rangoneer, I know what I am going to say is not going to make you happy. But from reading your posts it appears that you only gained about 1 MPH carrying an extra weight of 250 pounds in gas. I found the base weight to be closer to 4,500 pounds base weight not including fresh water or fuel.

I believe you possibly have a cam that is not performing to its full potential or you have the wrong cam installed or a myriad of other potential problems when trying to increase HP and torque for a marine engine. But quite frankly I am not qualified in that field.
What I do see is NO real increase in performance for the money spent, you might check with people in the engine forum for help in that area.

But if you change to the F7 all you will be doing is lowering your RPM and lugging the motor, in my opinion. Right now you are running the motor at the top of it's potential RPM range and that is where you ought to run it. If you can get the RPM up over about 5,300 or more it will be the right thing to change props.

H
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Prop and RPM is going to limit top speed.

I would argue that the setup is very strong. It accelerates to top speed in a fraction of the time it did before.

In this situation, the increased power will not change my top speed. Gearing will. On a boat, props are a component of our gearing. The questions is, how much will I get or lose by re-propping?

Look at it this way. When going down the road in your car at say 65mph your tach is around 2,000RPM. Just because you add a supercharger for another 150 horse doesn't mean that at 2,000RPM you are going any faster. If we swapped out the 3.73 gears for say 3.55s then at 65 your tach would be around 1,800.

Peak TQ now, on the conservative side is 475tq/304hp @ 3400.
Peak HP now, conservative, is 418tq/355hp @ 4200.
Peak attainable RPM is 4850 = 380tq/350hp

If my understanding of propping is correct, changing from the 25" F6 to the 27" F7 will reduce my RPM to 4450 (425tq/350hp). Yet at 4450 with the 27" prop the prop calculator says I'll be at 51.5 mph instead of 50.5 at 4850rpm. Is that correct?

At this point, I need someone to say, yes doing it makes sense or no, don't do it. As well as correcting me on any of my thought-process. I have access to a clean F7 set for about $450. Come on prop pros...help a guy out ;)
 

Dave1251

Petty Officer 2nd Class
Joined
May 18, 2009
Messages
151
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Assumming you are hitting rev limit at 5,000 rpm, than the F7s will help your top speed. If you are not hitting rev limit and the F6 are limiting your engine RPM, than the F7s will not help you that much.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Rango, the problem I see you have is that you are saying that you increased your low end torque substantially, but how does that help you when you didn't affect your high end RPM. Theoretically yes, you could change your gear ratio to take advantage of your new low end torque with a higher pitch prop, BUT you won't be able to turn it at high RPM so you won't be able to change your gear ratio. And that is the difference in boat motors and car motors, boat motors do use close to maximum RPM on a regular basis.
And since you can't change your gear ratio you won't be able to take advantage of your new found low end torque. You didn't help your top end speed at all.


H
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Our goal was to increase low end grunt without falling off at the top. And that is what we got. The top end power now allows it to hold that 4850 RPM and speed completely where as before it would simply touch 4800. We knew that more power will not increase top speed, understanding that until max RPMs are increased, top speed will remain the same. With the cheap F7s available I posted here to check other opinions to see if my thoughts were correct and if I was missing something.

I thank you guys for the comments, and feel free to continue, I appreciate them very much.

Additionally; if we switched to a F7 our max RPM respectivley will change to 4450. At that RPM I am currently at 45mph but will change to 51 with the F7. I am a little rich on fuel at 4450, and am burning 18.5 Gallons/hour. The question is, would the F7 increase the manifold pressure too much and in fact 'load' or 'lug' the motor and make the GPH significantly worse or could it remain near the same? Thanks again. :)
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Rango, I now believe I understand what you were trying to accomplish, I just don't understand spending that much money for so narrow an increase.
The only way to know if you got what you intended is to take the boat out and run it in two opposite directions and write down the RPM and speed from 2,500 RPM to WOT in increments of 250 RPM on both sets of props, and then we can do an analysis to compare them.
And I guarantee you if I spent that kind of money I would definitely buy the other props and run the analysis.


H
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

I know we were successful building a motor that is far far better than stock through the entire range, Without high compression, thru hull exhaust, or burning significantly more fuel; so I don't call that 'narrow' by any means.

I agree completely with the proceedure on working with both props. I enjoy this kind of thing. I'll look into those again and get back with some results.

Thanks. Adam
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Adam, I am not trying to be derogatory about your rebuild, but looking at what I wrote with complete neutrality it sure looks like I am. Sorry.


H
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

H, no need for apology. It did seem negative, but a man has a right to an opinion, and I was defensive; though I'm not always correct. ;)

I'm still battling whether to get the other prop or not. If I can adjust my throttle stop for another 400 rpm then I'll probably do it, but if not, then less likely. I'd like to see what it will do for fuel consumption. I'll let you guys know. Cheers! - Adam
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Adam, I always say " I NEVER make a mistake, UNLESS I am breathing." :eek:


H
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

:D Agreed!

I checked out the throttle stop and it is non adjustable. Porting the throttle body will only go so far also. So, now it is a question of should I try it to test fuel consumption and maybe help a few folks out with the idea...or just be happy with how well it runs. Hmmm...more pondering ahead. :)
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Adam, I believe this will help you get your fuel flow information cheaper than anyone else sells these, it is less than $ 120 and that is way below any other ones on the market. Navman use to make these but Northstar bought the rights somehow.

http://www.boatersland.com/f210.html

Northstar F210

NorthstarFuelSensorf210.gif



H
 

rangoneer

Cadet
Joined
Jun 5, 2009
Messages
20
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

That looks like something I should get for the Subaru! I'll keep that in mind. Thanks!

For the boat. I use the data from the ECU through a program called ScannerPro. The program has dozens of parameters that it monitors. I take that information and use that in conjunction with a dynojet wideband A/F datalogger and then make changes to the fuel/spark mapping with a program called TunerPro...then load the new tune to the boat with MefiBurn. Works super slick!
 

Attachments

  • Screen data.jpg
    Screen data.jpg
    75.8 KB · Views: 0

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Should we switch from F6 to F7? Volvo DP-SM DuoProp

Adam, I have never seen anyone setup a motor like you have in your boat. The more I think about it the more interest I have in it, because it is not close to the NORM (LOL) that I am use to working with.
But after giving it due consideration I can see some advantages to your rebuild. The NORMAL rules don't apply I believe, and that may mean that just because you can't reach maximum WOT RPM with a set of props that does not necessarily mean that you are lugging your motor as would be the case normally. Theoretically with a lot of increased torque in the lower to upper mid RPM ranges the only time you might be lugging the motor is when you are at WOT and only then and not when you are at cruising speeds or lower RPM.
Do you have a dyno test of the motor, as I would like to run some numbers and make a few charts to see what is actually happening at different RPM comparing the prop power requirements versus the motor power output. But in order to do that I need some torque and HP numbers at approximately 250 RPM increments throughout the whole RPM range.


H
 
Top