WOT 8.1 Gi

[Driller]

I have an 8.1 Gi in my 2011 242.

Yesterday WOT was around 4250 rpm. This seems low to me, all the reading I have done specs between 4500-5000 rpm.

Do any of you have more detailed information and what do I need to check to verify my engine is functioning properly?

[NautiGirl]

Original Props or at least the same spec's as the originals? Also, the tach could very well be off a few hundred RPM's. I think ours is listed around +/- 250

[AsLan7]

Yes-- engine RPM's primarily depend on propeller pitch. I would start there before suspecting an engine problem. However Volvo does not normally list pitch -- only part numbers.

Are you running the manufacturer's props that came with the boat ?

[cmattj]

I recently was discussing my WOT and at 4500 rpm it seams the Out-drive creates a cavatation wall.. Pulls hard right to 67mph then it then stops..
My other boats three blades are always faster than 4 blades and 4 where faster when I tried 5 blades in full throttle runs.
I did notice I would loose take off bite and my cruise speed suffered with less blades..

So in a sense maybe with the Duo prop and the fact it essentially is a 6 Blade with counter spinning blade maybe its just inefficient at higher speeds unless you have serious power to push past it?

[AsLan7]

You are correct Matt.

The more blades, the more "bite" you can get from the water to propel you. This is especially advantageous for hole shot as the additional props "grip" that water to propel you forward. However those additional blades begin to work against you as speed is increased.

For top end speed, the limiting factor is the drag produced by the additional props as more and more water "runs" into the blades. This is one of the main factors ( other than engine horsepower; hull tyoe; water conditions, etc. ) which limit top speed.

The manufacturer's try to find a balance between hole shot and top end speed.

In the aviation world, constant speed propellers use hydraulic fluid to keep the blades flatter during takeoff for best "hole shot" then increase the pitch of the prop to increase maximum speed.

Recreational boating has tried this concept using props that have spring tension but with limited success.

[Big Block Power]

What pitch driller is on it now? Also what gear ratio do you have? I saw a chart I found awhile ago what came factory.I can find it again. I have a good set of hill marine that may work for you? I'm hoping you don't have a fuel pump issue starting to cause it also.

[Big Block Power]

I have an 8.1 Gi in my 2011 242.

Yesterday WOT was around 4250 rpm. This seems low to me, all the reading I have done specs between 4500-5000 rpm.

Do any of you have more detailed information and what do I need to check to verify my engine is functioning properly?

Driller here it's your wot spec. You really should be pushing the 4600 rpm range.