v4 x-flow cav plate(s)?

[pcrussell50]

My 135hp v4 crossflow has what I'm pretty sure is the anti-cav plate as the first plate above the prop. It contains that little curved "fin" for countering torque reaction, I think. Next about two inches above that is another "plate". Don't have any earthly idea what that's for, but I think my engine runs too low in the water. While under way up and planing at around 40mph gps speed, if I go aft and look, the upper plate is just skimming the white water. The actual cav plate is completely buried, with no trace visible. I wonder if that is too low. It's in the deepest mounting hole right now. Maybe I should raise it up one?

The boat _should_ be much faster. It spins a 24" pitch omc raker right up to the 6k rpm redline doing only 40 mph? Something is wrong.

-peter

 

[Chris1956]

Re: v4 x-flow cav plate(s)?

The top plate is likely a spray deflector. When running at normal trim and load, the antiventilation plate (lower plate) should be just on top of the water. If it is under, you might raise the motor a hole or two to try to achieve this.

 

[emdsapmgr]

Re: v4 x-flow cav plate(s)?

If you mount the engine according to Chris's directions, you should significantly reduce your lower unit drag, probably reducing water spray at the same time-a good idea. Any time you do this, you will pick up speed. Since you are running a high rake prop, you should be able to pick up slightly more speed if you mount the engine one bolt hole higher than just above the anti-ventilation plate. (less lower unit in the water=more speed.) High rake props like the Raker maintain their bite, even if part of one of the blades is out of the water. When you start raising the engine to this point-or higher, it is a good idea to add a water pressure gauge so that you know the engine is getting adequate water pressure at all throttle settings.

 

[tashasdaddy]

Re: v4 x-flow cav plate(s)?

totally agree. rent a cherry picker to lift the motor, it is too heavy to do manually.

 

[kenmyfam]

Re: v4 x-flow cav plate(s)?

I have to agree with the above. Set up is a huge part of performance (and it costs nothing or next to nothing to try it)

 

[pcrussell50]

Re: v4 x-flow cav plate(s)?

The insights you guys bring is solid gold.

Td, I was able to raise it by tilting the CMC setback/trim plate all the way up to nearly 90deg. Then it was a simple matter of sliding the motor up to the hole I wanted.

I decided to raise it by two holes from the very lowest/deepest. That puts the anti-cav plate about a half inch above the lowest part of the transom. Guessing this may be ok, since the motor is set back 6" on a CMC setback/tilt plate.

Heading out for a test now. Be back in a couple hours with some results.

Oh yeah, there indeed was a lot/too much forward spray before I raised it. We'll se how that changes too.

-peter

 

[tashasdaddy]

Re: v4 x-flow cav plate(s)?

you may have to go a hole higher due to the set back.

 

[pcrussell50]

Re: v4 x-flow cav plate(s)?

ok, back from testing. anti-cav plate is now visible, though still throws a wake off it's bottom surface. I'd love to install a water pressure gauge. how big a deal is it? do I need to tap a hole into a water jacket on a head?

the spray situation is MUCH better now, so that's a BIG win.

but damn! only picked up a couple of mph on the handheld hiker's garmin. sooo, 6000 rpm with a 24" pitch to get 42mph?

?????

-Peter

 

[bktheking]

Re: v4 x-flow cav plate(s)?

What year is the motor, does it have a tell tale, if so you could put in a different fitting with 2 outlets instead of 1, 1 for the gauge and 1 for the telltale.

 

[ezeke]

Re: v4 x-flow cav plate(s)?

The water pressure gauge uses a dry line tee'd into the tell tale line. If you don't have the tell-tale (overboard indicator) on your 1975 135HP motor, follow this link: http://forums.iboats.com/showthread.php?t=222677

 

[pcrussell50]

Re: v4 x-flow cav plate(s)?

Why yes, it is a '75 135! Is it in my signature file? If so, it's not showing on this device's browser. Or do you remember a previous post of mine?

Anyhoo, I'm completely stumped as to how you can spin a 24" wheel at 6k engine rpm and only go 43mph. At a 10% slippage rate that's still 64mph.

Is it possible to be having 60% slippage?

Sheesh.

-peter

 

[kenmyfam]

Re: v4 x-flow cav plate(s)?

What boat are we on here ?? What speed are you getting at 5,500 rpm ??

 

[ezeke]

Re: v4 x-flow cav plate(s)?

"....Anyhoo, I'm completely stumped as to how you can spin a 24" wheel at 6k engine rpm and only go 43mph. At a 10% slippage rate that's still 64mph.

Is it possible to be having 60% slippage?.........."



Propeller gear ratio is 13:25 or .52.

It is certainly possible to have extraordinary slippage if you are drastically overpropped.,

 

[Chris1956]

Re: v4 x-flow cav plate(s)?

I would question the Tach, in this case. A 26" pitch Prop is very tall for a normal boat. The propeller calculator shows 46% slip, which is hard to believe.

 

[pcrussell50]

Re: v4 x-flow cav plate(s)?

I was wondering when someone was going to broach the subject of what kind of what kind of boat it is. it's a 16' sport/performance hull. kinda wide and and flat v-hull like a bass boat. and it's almost gutted. power tilt and tach are the only systems. I think it's pretty light because it's hand laid, not chopper gunned. I just ASSumed that if I gave the prop pitch and the rpm, the fact that the motor could swing the prop all the way to redline was sufficient.

fwiw, the prop is a 24" pitch, not a 26" pitch, but you bring up something I hadn't considered when you say too much prop can cause a lot of slip. I am missing the intuition to see how this is possible unless the motor is so powerful it just blows the prop right out, (don't know the techniccal term). on the other hand, I CAN see how too much prop would bog down the
motor and cause dangerous lugging. with this 24" prop and three souls on board I can open it up to full throttle and achieve 6100rpm. I tried a 22" prop from the same manufacturer, (omc raker) and I could not maintain full throttle without going a good deal past 6k rpms. that's when I went to the 24" pitch and I COULD use full throttle without going too far past 6000rpm.

can you elaborate more as to how you can have large slippage with too much prop and still not have the prop blown out? or do you think my little old '75 crank-rated 135hp could blow out a 24" prop with the reputation for "holding" the raker has?

 

[Dhadley]

Re: v4 x-flow cav plate(s)?

What is the tach set on? Assuming it's the correct tach it should be set on 6. If it's set on something else like 4 - or not the right style tach - then you'll get some wierd readings.

 

[pcrussell50]

Re: v4 x-flow cav plate(s)?

What is the tach set on? Assuming it's the correct tach it should be set on 6. If it's set on something else like 4 - or not the right style tach - then you'll get some wierd readings.

lol, if that's true, then it's a hell of a trap for a noob like me... because my motor is a v4, not a v6. just want to be sure before I go mess with something. again. my motor is a v4. are you sure it should be set on "6"?

-Peter

 

[Dhadley]

Re: v4 x-flow cav plate(s)?

Assuming you have the correct tach, it reads off of the stator, not the number of cylinders. You have a 6 pulse stator, the tach is looking for pulses from the stator. It's not counting spark plugs.

 

[pcrussell50]

Re: v4 x-flow cav plate(s)?

Assuming you have the correct tach, it reads off of the stator, not the number of cylinders. You have a 6 pulse stator, the tach is looking for pulses from the stator. It's not counting spark plugs.

well I'll be... so if I have the tach set on 4, it's expecting 4 pulses/revolution. so when it sees 6 pulses it will think there has been 1.5 revs? or if the 6-pulse stator sees two revs, (12 pulses), the tach will see 12 pulses and think there has been 12/4, or simply 3 revolutions, when in fact there have been 2 revolutions? is it as arithmetically simple as that? or is the reality that the readings will be "all over the map, screwy"?

-Peter

 

[Dhadley]

Re: v4 x-flow cav plate(s)?

Again, assuming you have the right type tach - and you haven't told us what you have - it needs to be set on 6 for your motor. There's all kinds of ways to (over)-think how it works. You have a 12 pole, 6 pulse stator. The tach reads pulses per revolution. It really doesn't matter how many cylinders the motor has.