cavitating?

[beason]

On our last outing we were headed back up river to the ramp, and i wanted to see what my "new" boat would do.

It is a sea ray 180 with the merc 3.0l 4 cyl. it has a 21 pitch prop. I had 2 adults and 2 13 year old girls onboard.

I ran up to about 33mph with the trim down, then started trimming it out. i could feel the boat lift, and the speed increase. I could also hear the rpm increase. i got up to 40mph (on the speedo) and about 4200rpm, then it felt like the prop was free spinning. RPM went up, and speed went down. i trimmed it back down just a bump of the switch and it caught again and we ran at that speed for a few minutes.

My old boat (170 sea ray with the same engine) had a 17p prop on it. It took off a lot harder, got on plane faster, but didnt have near this top end.

I assume this was cavitation from to aggressive trim, but i have a couple of questions.

Does that speed/rpm sound right?

I think i want to step down to a 19p prop for more pull when pulling the tube, how much mph would i loose?

my prop is a little rough around the edges, (no missing chunks). would this cause the cavitation?

would an anti cavitation plate help get me any more speed out of it, or am i pretty much tapped out?

Im not terribly worried about the top speed, and i know this is no speed boat. I just want it running optimal.

I have heard good and bad about the "whale tails", but arent anti cavitation plates different?

Thanks in advance.

 

[Maclin]

Re: cavitating?

You already have an anti-cavitation plate on the outdrive, that is what the wide plate above the prop on the outdrive is.

The MPH sounds high for a 21p at that RPM, but if the MPH reading is from the speedo then actual may be closer to 36-37-38 which is where a prop calculator (see link below) puts it.

Regarding the prop cavitation or blowout, you probably found the trim limit for your boat.

I think you are over-propped some, meaning you should have a lower pitch prop on there. The engine should be able to get to 4600rpm I think. With a 19p the speed would be about 36mph (actual) at that RPM which is probably close to what you got to with the 21 at 4200rpm. My gut feeling is that with a 19p you will get good low end take off and still be able to hit close to that same MPH you saw in your test.

Sometimes tabs like Smart Tabs can keep the stern up and give the outdrive a better trim angle to work from and you can get the same or better speed with no blowout, but that is a trial and error type of tuning. They help the boat to plane at lower speeds, so even if you would not get any MPH increase they may allow you to keep the 21p and still do watersports without a prop change.

But in any event the engine, if in good condition and state of tune, should be able to get to a higher RPM than you are seeing.

I personally recommend against the whale tails or fins.

Here is a link to a prop calculator, try it out, you can see what a 19 vs. 21 gets you in theory....:

http://www.marksmarineinc.com/prop_calc.aspx


To "run" your numbers I used a 2.0 ratio drive and .12 or 12% slip.

 

[H20Rat]

Re: cavitating?

just as a FYI, there is a huge difference between ventilation and cavitation. 99% of the time when someone says cavitation, they mean ventilation. (including the anti-VENTILATION plate, it isn't an anti-cav plate)

Cavitation is the result of a low pressure area formed by the power of the prop. Either too much power, or scratches/knicks/etc causing a vacuum to form via the pressure drop, water will actually boil away. The vacuum eventually collapses out, and takes a microscopic chunk of metal with it. Repeat it enough times and bad things happen. Jet impellers are often subject to cavitation, and I've had numerous impeller hubs welded up because of cavitation damage.

Ventilation is air that reaches the prop. It isn't nearly as destructive as cavitation to metal parts, as the vacuum and collapse isn't nearly as extreme.

 

[QC]

Re: cavitating?

x2 ^^^^^^

 

[Texasmark]

Re: cavitating?

You were ventilating for the reason you mentioned. That is where you want to run....trimmed out just before it blows out...best speed, fun and mph.

Mark

 

[beason]

Re: cavitating?

well, i guess i am learning something on here!

From my limited research the 21p is at the top end of the props for this engine, and a 17p is the bottom. The 19p is the happy medium.

The prop i have is more bent then i realized, as i was just out there staring at the boat for no good reason. I think my plan will be to get a 19p and keep the 21p as a backup spare. i want to stick with aluminum, because the lake i frequent is a shallow lake and i dont want to tear anything up. After IKE, there is no telling what is under there.

I did notice that this boat takes longer to get on plane and the bow rise is significantly more then the old boat.

I tossed around the idea of smart tabs, as i dont think my boat is large enough to need power tabs, but that will be in the future.

Thanks for the info, i will start looking for props, and check out the tabs.

Any suggestions for best bang for the buck props? stick with 3 blade? 4? rubber hub? I am a prop buying virgin.

Thanks!

This one looks promising. It comes with the washers, hub, nut, and wrench, and is only $9 more then the cheepo.

http://boatpropellers.iboats.com/Me..._992004_Propeller/437/1130/?*******=626107417

comments?

 

[QC]

Re: cavitating?

test

 

[Maclin]

Re: cavitating?

Blowing out and cavitation definitely not the same, good explanation my smokingcrater, and you hit the ventilation (blowout) point. And in my opinion you are definitely thinking the right way about staying with aluminum in your lake situation.

 

[jimdogg1269]

Re: cavitating?

i have a bayliner 18ft with the 3.0 merc. 21p prop and i run 47mph on gps at 4900 rpm

 

[NYBo]

Re: cavitating?

i have a bayliner 18ft with the 3.0 merc. 21p prop and i run 47mph on gps at 4900 rpm

Bayliners are lighter than Sea Rays of the same length.

 

[FastFission]

Re: cavitating?

I'm glad I saw your post. I was out with my family this weekend trying out our new (to us) boat. Bayliner 1952, Merc 3.0 and a 19p prop. I had an almost identical experience, down to the same RPM and MPH. This is my first motorboat (other than small outboards). I was running about 4200 and 40 indicated when we crossed a wake and the bottom dropped out. Initially, I couldn't get the boat back over about 15 MPH, but once I thought to trim back down, everything went back to normal. I've been trying to figure out what I did since...

It didn't help that we were kind of heavy aft, so initially I thought we might have been taking on water. Nothing like learning a new boat to get your adrenaline going.

 

[John3]

Re: cavitating?

The prop i have is more bent then i realized

Think you should get that prop repaired & keep it as a spare.
You might want to check out Piranha Props. They have a unique Prop where you can try out a Prop of the pitch you think might work, change blades (Pitch) until you get it right. I just bought one, hope it works as advertised. DiscountBoatPropellers.com has them.

 

[Texasmark]

Re: cavitating?

I'm glad I saw your post. I was out with my family this weekend trying out our new (to us) boat. Bayliner 1952, Merc 3.0 and a 19p prop. I had an almost identical experience, down to the same RPM and MPH. This is my first motorboat (other than small outboards). I was running about 4200 and 40 indicated when we crossed a wake and the bottom dropped out. Initially, I couldn't get the boat back over about 15 MPH, but once I thought to trim back down, everything went back to normal. I've been trying to figure out what I did since...

It didn't help that we were kind of heavy aft, so initially I thought we might have been taking on water. Nothing like learning a new boat to get your adrenaline going.

Quickest recovery is to cut the throttle to N. Immediately put it back in gear and firewall it. You will regain thrust immediately.

Mark

 

[zagger]

Re: cavitating?

i have a bayliner 18ft with the 3.0 merc. 21p prop and i run 47mph on gps at 4900 rpm

At 4900 you are operating over your rpm limit (3.0 = 4600/4800). The best I got out of a 3.0 was 43 on gps running Laser II, 4600rpm, 3 blade SS 21P on my Maxum 1700SR. The dry weight was 1800lbs, 1/2 tank and 300lbs passenger/gear. If you are getting 40 with an aluminum prop (best I ever got was 36 with stock al.) I would not change anything. To get the best prop recommendation check your speed with a GPS (iPhone ap is great) and post your drive ratio. Depending on which way your prop is bent you might be pitched at more than 21" which would actually help your top end but severely degrade hole shot. Best thing would be to get it fixed and re check your numbers.

 

[FastFission]

Re: cavitating?

Thanks, I'll have to try that next time out (probably next spring at this point, since we're coming up on cold weather season here). I think just trimming down for a couple seconds would have recovered it too.
Having never really driven a planing hull, it was surprising how quickly these things slow down when you drop off plane.

Quickest recovery is to cut the throttle to N. Immediately put it back in gear and firewall it. You will regain thrust immediately.

Mark

 

[chriscraft254]

Re: cavitating?

Get the smart tabs, you won't regret it ever!