new prop is cavitating bad

[amosh]

hi, I have a 91 Malibu flightcraft 21 ft with a 200hp 2 stroke merc on it. 19p stainless prop is what came on it, and it does well, but I want a little better hole shot. I bought an 18p 4blade stainless and put it on. took it out for the first time last night, and it ran like crap. the boat was porpising like ive never seen and at about 3000 rpm it would be in full cavitation mode, not even barely pushing water. did I go the wrong way with this prop? should I have gone with a 20p? or did I just go too far dropping an inch and adding a blade. any help would be great, thanks

 

[steelespike]

What is the diameter of reach prop?

 

[Chris1956]

Gee, is it trimmed all the way down? A different prop shouldn't cause porpoising, but one that is overtrimmed could.

 

[jimmbo]

What exactly, make and model, was the old prop? What is the set up of the engine, is it all the way down on the transom, or is mounted higher? Also what exactly is the new prop?

 

[amosh]

it was definitely trimmed all the way down, the original prop was a lazer 2. the writing I saw on it says quicksilver 48 16544/a40 19p and the new 4 blade that is cavitating is a solas 1553-141-18. the engine is mounted as high as it can go, which is where it was when I bought the boat. the 19 does fine and runs out at almost 50mph, but like I said I'm just hoping to get a little better hole shot for water skiing. thanks

 

[Bondo]

Ayuh,.... Ya didn't answer SS's question,....

What is the Diameter of the props,..??

 

[SkiDad]

Why not just run a 17p laser for skiing? Maybe eBay has one. Or have the 19p turned down to 18. It's probably the cupping that's keeping the laser alive at the engine height.

My boat is setup for skiing and the engine is deeper in the water as speed is not the main concern.

 

[amosh]

they are both a 14" diameter. skidad, good thought, but I would like to know for sure that a 17p laser would work correctly before i go buy another prop. if a 18p 4blade cavitates so bad that my boat wont even plane out, wouldn't going to a 17 be an even bigger jump and make it worse? the cups of the blade look to be very close to the same to the untrained eye (mine). thanks

 

[ondarvr]

Your prop is not cavitating, it's ventilating, they are two very different things/terms that are frequently used incorrectly. While what you're experiencing is more extreme than in most situations, depending the exact prop design you can get this type of a result, it's why many shops will let you exchange props until you find the right one for you needs. Exact motor high can be very dependent on the prop, not all props work well when the motor has been raised up higher.

 

[SkiDad]

can you try lowering the engine ?

 

[Sea Rider]

Try lowering the engine one hola at a time while testing each one till ventilation/aeration stops and prop grips right on water.

Happy Boating

 

[Chris1956]

Well, it sounds like the Laser is better at "gripping" the water than the Solas. So either get another Laser, or lower the motor until the Solas "bites and holds" the water better. The Rake of the prop will influence how well the prop grips the water. You might compare the two for Rake.

I have a high performance ski boat. I found that raising the motor on the transom detracts from the ability to trim the motor and lift the bow. There was a "sweet spot" where the motor was raised less than the max on the transom, but the trim allowed very good speed. In addition, with the motor high, I had ventilation problems when trying to tow a skier.

 

[amosh]

Ok sounds like a lower pitched laser is the way to go, as I don't really want to fight lowering the motor. What is the difference between cavitating and ventilating? And does anyone want to buy a Silas 4 blade?

 

[SkiDad]

ventilation is when you suck air into the prop path. Cavitation is when you boil the water around blade edges. You have ventilation it seems.