Chaparral prop cavitation

[Richard Petersen]

Before I spend a bundle on a Mercruiser HI FIVE prop as recommended by them. Pass some judgement. 2002 Chap 186 SSI 5.0L 2 bbl carb 220 hp, me alone in it. Trim must be full under, tucked in, to slowly power up on plane. Prop blows out instantly if I goose it to 3000 quickly. Start triming out at plane of 25 mph ok. Slam the gas - blow out. accelerate ok to 40 or 43 mph now tach reads 4800 rpm and is being "goverend". Trimout more will go quickly to 48 mph still at 4800 rpm. 2 light trim taps out for more speed causes cavitation to slowly creep in and a loss of speed. Quick turn of the wheel at almost any speed causes a prop blow out. It is a great boat with 4 or more adults on board, does 44 then. Prop is a original 14 1/2" X 21" X 3 blade Alum. I can't even make a guess on this one. HI FIVE is to be a 13 1/4" X 21" X 5 blade they guarentee no blow outs.

 

[MGallik]

Re: Chaparral prop cavitation

Sounds like a spun hub, bent blade, or the blades have chewed up leading edges. You stated everything is OK with weight in the boat, Hmmmm. If you've added a sensor for a fish finder, etc, within 12 inches of the props edge, especially on the starboard side, it could cause trouble. It sounds like a disruption to the flow of water to and through the prop. A new or loose zinc on the front of the drive, weed bed growing on the hull, damage to the hull or a repair job thats not flat and smooth. It also sounds like too many ventilation holes uncovered on a Rapture. <br /><br />Your boat, engine, prop combo should not blow out, cavitate, or ventilate. What, if anything changed before this problem first arose?

 

[Richard Petersen]

Re: Chaparral prop cavitation

Always did it. I bought the boat for my wife to sit in the bow and read a book. 200' away from the dock and back. I started running it alone and found it does not like me.oooooooooooooooooo I called Chaparral, not much help. This boat normally has a V6 as std. I ordered the 5.0. No chance I got a 4bbl or a 350 by mistake to cause such easy blows. Prop looks new and even edged. 90 hrs. As I remember the boat says it has a planeing pad at back of V. At 48 mph boat gets LOOSE. Lean a few degrees left, lean a few degrees right, bow lift a little, comes down again. Each change is about 2 to 3 seconds long.

 

[MGallik]

Re: Chaparral prop cavitation

So the prop blows out as soon as you hit the throttle even with the drive fully down?

 

[Richard Petersen]

Re: Chaparral prop cavitation

Yes.Down-level-anywhere

 

[Richard Petersen]

Re: Chaparral prop cavitation

Mercruiser never wrong labeled the impressions of Pitch and dia. on the wheel.

 

[Richard Petersen]

Re: Chaparral prop cavitation

Yes.Down-level-anywhere

 

[phaeton]

Re: Chaparral prop cavitation

Firstly you seem to be experiencing ventilation rather than cavitation<br />second - just try the different prop; you should get a number of advantages from SS rather than ally on a rig with that sort of power to weight ratio.<br />Look forward to hearing the results.....

 

[Richard Petersen]

Re: Chaparral prop cavitation

Is it possible the blade area is too small for the engine size? What the difference between cavation and ventilation? How does each normally occur? Can one cause the other. Thanks.

 

[tommays]

Re: Chaparral prop cavitation

richard<br /><br />this will tell you more than you could think about props from invention to all current types<br /><br />if your prop was to small the motor would excede the recomened WOT of about 4800 rpm <br /><br /> http://www.mercurymarine.com/everything_you_need_to_know_about_propellers5 <br /><br /><br />tommays

 

[gspig]

Re: Chaparral prop cavitation

Not an expert, but had the same thing happen to me. I bought a prop for my boat, 13 dia x 21 pitch. If I brought the throttle above 3000 rpm, the prop would loose its bite. Bought another prop, I think this one is closer to 15 dia. Boat runs perfectly. Later found out the first prop, even though it fit the mercruiser shaft, was for a 50hp mercury outboard.

 

[Doug Durako]

Re: Chaparral prop cavitation

If you go with 5 blades, you would want to increase the pitch by at least 2 inches. More blades, more air (turbulance) circulating around the prop. If it was me, I would borrow a stainless steel 3-blade, 23 pitch Merc Laser 2 and try that first.<br /><br />What does Chaparral say the top speed of this boat should be? With your power and the right prop, you should be running in the upper 50s at wide open throttle.

 

[bigbrownbuku]

Re: Chaparral prop cavitation

could be a spun hub, happens even to new props.<br />could also be under pitched i would try a 23 stainless to try and get those revs down. boats i rig of similar size with the mpi v6 run 21 stainless from the factory. try a stainless prop of the same pitch. better bite, less flex, lower rpm. if you still hit the rpm limiter move to a 23.

 

[Richard Petersen]

Re: Chaparral prop cavitation

I just realized something from TOMMAYS. My engine is way under propped. If my engine had no governer or rev limeter which is active at 42 mph, I would start to over rev from there up. DDBOATER says it should do in the 50's. My storage place checked the prop for me. Said it was a 14 1/2 X 21 X 3blade. Also said there was noMerc part # stamped into the hub. Said it was not a Merc. Sound right about the part #?

 

[Northern Eclipse]

Re: Chaparral prop cavitation

Richard, <br /> Check with your local prop shop or marinia and see if you could try a couple of props such as a Quiksiver Mirage 13 3/4 X 21p, a Mirage 14X19 and in the aluminum 4 blade try a 18p and a 20p, in 3 blade try a 19p a 21p and 23p, I know this seems like I'm all over the map with pitches but within the these 3 styles of props, there are many different varibles other than pitch and diameter, such as the shape and length of the blade and cupping just to name a few, so if you can try props has I'v stated above I'm sure you'll discover that for sure one of them is going to work and you might even find two. Just a reminder that if you are able to test an assortment of props be sure that the correct back spacer is being used for each one of them and that none of them will strike the zinc trim tab. Selecting the correct prop is mostly a science of trial and error, I'v been lucky enough to have a cousin that owns a prop shop, so I'v tried many props on my boat some good some not so good, but you won't really know until you try.

 

[Richard Petersen]

Re: Chaparral prop cavitation

Thanks Northern Eclipse. I have a great prop shop near by.

 

[phaeton]

Re: Chaparral prop cavitation

sorry was short of time last time - <br />OK you sound underpropped (very) and resolving that should sort out all the issues you are having (except the skittish'ness when trimmed excesively at high speed which will get worse but simply means trim it in a bit!<br />re ventilation/cavitation - the former is a result of air being sucked into the prop area (by design or because of a problem with hull design, drive mounting or use). The design element is where the prop utilises the through hub exhaust via some hub slots to deliberately mix in some gas to the water at slow prop revs generating slip - engine spins faster, generates torque and off you go and the exhaust gases stop mixing in the aea of the blades........<br />cavitation is caused as a result of pressure changes either side of the propellor blade causing the acceleration of gas bubbles to the extent that they can actually destroy the blade - normally triggered by changing angle of attack in cornering manoevres I believe.<br />Probably should have cut and pasted a link to a professional article on the subject but I am sure that will follow with the corrections!<br />Best of Luck with the new prop and enjoy the new speed!

 

[Doug Durako]

Re: Chaparral prop cavitation

I agree with Northern, except if you have a good prop shop, have them measure the exact pitch of your current blade. From the blowout you describe, my guess is your current prop is in the area of 17-19 pitch.<br /><br />Then, move up two inches in pitch and start your tests from there. When you find what you want, get a good stainless prop (or two) and tweak 'em.<br /><br />I did not get the best results from my Merc stainless 23-pitch, so I had them beat it back to a 22-pitch.<br /><br />A man can never have too many props.

 

[Richard Petersen]

Re: Chaparral prop cavitation

See if I am getting the hang of this. Me alone in the boat and a 1/4 tank of gas. Find a prop that runs at 4,700 rpm ( puts me 100 rpm under REV LIMITER ) and max speed. A little more out trim on my trim gauge should cause only a slight loss of speed but no cavitation. To take 5 others out, it should be doing no less than 4,400 rpm at any speed . Will a HI FIVE 5 blade pull this off, or are 2 props more likely? Thanks again. Rich

 

[Richard Petersen]

Re: Chaparral prop cavitation

If I had my choice I would rather be able to cut the wheel hard at any speed, have the boat lean at maximum speed to a 45 degree angle and pin me to the seat for the full turn. I passengered in a 2000 Smoker Craft that could do it perfectly. It was set up for water skying. Does that require a different type of prop ? Rich