Removed intake grate... WOW!!!

[cfnnslsq]

My 06 C180 has the inlet clearance (weedless) type of intake grate and it has always bothered me that nothing seemed to "lock" it in place while in its normal "off" position. For example I am able to pull it down without much effort from under the boat. It has a wing toward the rear of the grate and I got to thinking that when at speed this wing would probably generate enough force to cause the grate to open up and hang down causing alot of drag. This weekend I decided to test my theory and remove the grate just to see what difference it made as it is only held on by three bolts.
I gained 3 MPH!!! I am also all over the limiter both outa the hole and at top speed.
It was also quite windy this weekend and I noticed that it really helped keep the boat from "porpoising" over the two foot average waves. The speed gains BTW were realized on a part of the lake that is well sheltered from the wind not in the 2 foot chop.

Soooo...
The speed part I can understand but I have no explanation as to why it caused me to hit the limiter with WOT starts. It will cavitate and bang the limiter about 3 or 4 times and then take off like crazy. If I feather on the throttle I can avoid this and still get better holeshot than before with the grate.
Why would removing the grate cause me to cavitate? I would have expected it would have just allowed more water to be delivered to the impeller and thus reduce cavitation if anything....
At this point I plan on adding 1 or perhaps 2mm to the impeller and probably have the venturi nozzle machined and opened up 1 or 2mm as well. I realize this will probably result in lost top speed but I am more interested in holeshot anyhow. Could the teeth on the pump support be causing this? They are all in perfect shape, but....
Does anyone with a better understanding of hydrodynamics than me have any other suggestions?

[eoc1212]

very interesting... I also hate that grate down there and the whole ICS weed clearing system all together. I am interested to hear from anyone else who has revomed it too!

[cyoungesq]

Been running without weed grates for 2+ seasons with no problems whatsoever.

[holdmeback]

but wouldnt it be easier to suck up bigger sticks and rocks? the lake i ride on is sometimes very trashy.

[cfnnslsq]

Yeah I thought about that, but the lake I frequent never has any sticks so no problem there. I think most floating sticks would also be pushed to the side by the hull before they reached the intake area.
As for rocks it is not going to matter if there is a grate or not.
Ask me how I know....

[Jeepster]

That's funny because i tried removing the grate on my C180 (2005) and actually there wasn't any speed improvement doing so (nada...zero more)....in fact it was slightly cavitating a bit more, but no gains there for me...?!?!

But my shark fins inside my pump shoe are all removed, and grinded flush maybe that's the difference...???

[cfnnslsq]

Well... I definitely have more cavitation but the top speed gains are quite real. Again, I think its because the weed grate is getting pulled down at speed. Mine really doesn't take much effort to pull down when I am under the boat. I may try putting a small pair of vice grips on the VTS mechanism so I can be sure it wont move and try the grate again.
I still dont understand why it would cavitate more without the grate though. The only theory I can come up with is that it is processing more water and can't get it out of the venturi fast enough so the backup is causing cavitation. Sounds reasonable in my head anyhow.....
Once the long weekend is over I will be taking the venturi in to a machine shop have a couple of millimeters machined out. I was also planning on getting rid of the ridges in the steering nozzle and did attempt to do so using a cylinder hone but it did not seem to be working out so I hope the machine shop can do that as well.
What was the pitch on your prop at the time?

[gerapau]

I am not sure that the weed grate could get pulled down given the very high flow of water through it and into the pump. Where it is I just can't see it moving once you start the engine and get a flow of water through it. I guess I could be wrong but I don't think that the flow of water around it could overcome the much greater flow of water through it.

[cfnnslsq]

If you look at the wing on the grate you will see what I mean. It would have an increadible amount of down force put on it by catching and directing water up. Of course I am only guessing as to what is going on because I can't very well stick my head down there at 50+ mph. Would be nice to be able to mount some kind of camera down there but that is a little to James Bondish for me.

[cfnnslsq]

Just dawned on me that I can actually see whats going on at speed by looking in through the starboard hatch in the rear. From here I can put a line on the exposed portion of the grates cable with a sharpie and monitor for movement while the wife drives.