Volvo duo prop on Eliminator boat

[otowniron]

I have an 2002 Eliminator skier 215 or sometimes called Eagle 215, It is 21.5' long. Powered by a Volvo 7.4GI. Rated at 310 HP It has a Volvo duo Prop outdrive with a 1.95 gear ratio. It has polished stainless steel "F8" propellers on it. Boat runs great. Has only 160 hours on it. The problem is it only revs up to 3900 rpm at WOT. Top speed after trimming out and a good 60 seconds of build up is 50 mph. The motor is rated at 4600 WOT. Pulls the same top speed and hole shot ( which will pull a skier up no problem) whether there is 2 people in the boat or 5 people.
I am trying to maximize performance by getting closer to that WOT rating of 4600 rpm. I think I need a smaller prop. But they are very expensive so I need to get some sort of recommendation. Does anyone out there have a boat of similar size and power train. If so , What size props are you running. I was thinking of going with "F6". That is 2 sizes smaller.

Thanks for any input in advance

 

[Fun Times]

Re: Volvo duo prop on Eliminator boat

Hi there, In the link below there is an attached thumbnail of an informative Volvo propeller guide to help you through the process with making an informed decision of what propeller size to choose from courtesy of iboats member Captain Maclin.

http://forums.iboats.com/prop-questions-topics/what-size-duoprop-526434.html#post3571805

Also, you will want to ensure that both your tachometer gauge and speedometer are accurate.

Hope this helps, Good luck.

 

[otowniron]

Re: Volvo duo prop on Eliminator boat

Thanks for the input and answering my first thread. I have got a lot of info through this forum by reading other threads. I have seen that volvo chart you attached. When I look at that chart it basically says you can put any size prop (f4-f9) on any engine and the bigger the prop the faster you go. It seems to me that everyone would just want the biggest prop to go the fastest. The chart says you can put an F9 on a 4.7 L. I have a 7.4L and am having trouble spinning an F8. So am I missing something?
I checked my Tach on dash with an external tach. At idle it seems to match. Cant check the speedo until I put it in the water again. Let me also add to this that I have a brand new fuel pump. I just bought the boat 6 months ago from a guy who had it stored for 3 years and could not get it running correctly. After searching this thread I discovered the black paint problem. Took apart the fuel cell and discovered the paint blockage. Boat fired right up and ran like a champ after that was replaced.
Have ran the boat at 3000 feet elevation and at 6500 feet elevation with very little difference in performance.

My next question to you would be. If i put the muffs on it in my driveway, take off the props, That engine should rev up to close to the max rpm of 4600 rpms and the limiter should kick in. Being under no load from the props this motor should hit that rpm limit. Correct? If it cannot do that the prop size may not be the problem.

Is this safe to do? I am not worried about bodily injury safe. I dont want to damage my drive train.

Let me know your thoughts

Thanks Again

 

[Wind dog]

Re: Volvo duo prop on Eliminator boat

Others will chime in but it's my understanding is never off low idle while on muffs. There is a chance the moror will runnaway with no load on it.

 

[Maclin]

Re: Volvo duo prop on Eliminator boat

My first thoughts are that the engine is not at full potential. F7 is a typical duoprop set for that setup with 280-300 HP 5.7l in a boat that size. With 310HP on tap and the bigger torque-ier engine an F8 set should be close to ideal. The fact that it hits the same top RPM with light load and heavier loads indicates to me that RPM is being limited either electronically (rev limiter out of tolerance) or artifically (tuning components failing). I do not know what diagnostics are available with any onboard computer with your setup, but it may be worth finding out and having any fault codes extracted.

Does it come out of the hole like a tiger? If so, what RPM does it start to flatten out at? What do the spark plugs look like after a high speed run?

That Volvo prop guide is more of a technical simulation what-if of RPM vs. Speed IF the engine can spin the propset to the top rpm. Notice the size/weight of the boat is not in the graphing parameters.

Regarding blipping the throttle, I would not do it on muffs as you could outpace the water flow to the impeller. If you feel brave you might try it in the water in neutral, bring it to 2000 rpm steady for a second or 2, then blip to see if the tach goes past 3900. Still that has some inherent risk.

 

[Maclin]

Re: Volvo duo prop on Eliminator boat

Where are my manners, Welcome Aboard!

 

[Fun Times]

Re: Volvo duo prop on Eliminator boat

I just bought the boat 6 months ago from a guy who had it stored for 3 years.

The problem is it only revs up to 3900 rpm at WOT. Top speed after trimming out and a good 60 seconds of build up is 50 mph. The motor is rated at 4600 WOT. Pulls the same top speed and hole shot ( which will pull a skier up no problem) whether there is 2 people in the boat or 5 people.

The fact that it hits the same top RPM with light load and heavier loads indicates to me that RPM is being limited either electronically (rev limiter out of tolerance) or artifically (tuning components failing).

Thank you Maclin for helping out, much appreciated.

Quick thought here, the way the description sounds of how the boat is performing brings up the question of if the boat / stern drive is being trimmed up properly once on plane?

Otowniron, since the boat is new too you, and we're not aware of your past boating experience, are you aware that once on plane, you should be raising the stern drive up a little bit to gain better overall performance? which will increase both your speed and RPM.

 

[otowniron]

Re: Volvo duo prop on Eliminator boat

Thanks for the input. I am aware of the trim adjustment. This is the first sterndrive I have owned. All previous were jet drives so I know how important trim is. I was Just simplifying things in the first thread. Boat hits plane very fast maybe 3 seconds. Slowly creeps from 3500 rpms at about 45 mph to 3900 rpms at 50 mph. This is over a period of 15 to 30 seconds. Then I trim the drive up sliglty bow comes up and I can get maybe 4000rpms at 53 mph. Remember this is on the dash speedo not gps. But thats all she has.
My local volvo dealer doesn't even have the ECM reader. So I need to find another dealer close by.

 

[otowniron]

Re: Volvo duo prop on Eliminator boat

And once again Thanks!!! you guys are very helpful. I will keep you posted.

 

[Fun Times]

Re: Volvo duo prop on Eliminator boat

My local volvo dealer doesn't even have the ECM reader. So I need to find another dealer close by.

Maybe you could look into buying a scanner yourself someday?

Look up Rinda technologies techmate scan tools.

Rinda Technologies Marine Diagnostic Tools

Rinda Technologies - MerCruiser Scan Tool Spec..

Have you hooked up an fuel pressure gauge to see what both your low and high fuel PSI is at especially while under load/at speed?

 

[otowniron]

Re: Volvo duo prop on Eliminator boat

I have checked fuel pressure on the muffs up to about 2000 rpms. But not on the water at full throttle. I get 10 psi on the low and 40 psi on the high. I see comments in the forum that people are getting 55-60 on the high. The local volvo mechanic said 40 psi was in spec. Although I would love to see something in print on that subject. I assume the pressure regulator should keep it at a constant of whatever the spec is. I think the people with higher readings might have some black paint clogged in the regulator. I read a thread of a guy that was getting 95psi on the high due to paint in the regulator screen.
I am going back in the water next week and I will test fuel psi at WOT. If anybody has a printed spec for the high side psi chime in!
Thanks otowniron

 

[Fun Times]

Re: Volvo duo prop on Eliminator boat

Yes you are correct, a restriction around the fuel pressure regulator usually causes higher fuel pressure readings.

Yes you're system should be 39psi +/- 2psi awansed by another marine professional that looked it up since the Volvo service manual to look up the 7.4gi EFI fuel pressure specifications appears to be harder to find online now.
http://www.justanswer.com/boat/58wa2-2000-cobalt-7-4-efi-volvo-penta-last-season.html

While doing some research, I ran across some gear ratio information from the late Don S saying that typically the 7.4gi engines usually run a 1.78:1 gear ratio. Also if you re-lookup the attached thumbnail from Maclin, it also mentioned the 7.4gi using 1.78:1 gear ratio. While the 7.4's may also use the 1.95:1 gear ratio, it seems less uncommon.

So ultimately you may be trying to overcome a less uncommon gear ratio to propeller size combination which wouldn't be unusual in an eliminator running a Volvo.

http://forums.iboats.com/volvo-penta-i-o-inboard-gas-engines-outdrives/2000-volvo-7-4-gi-453455.html

Boatinfo - Volvo Penta SX and DP-S SternDrive Service Manual