Cavatation

[paulrfrancisco]

I have a 30' Cruisers with twin 7.4L/ bravo I drives... and with the drives buried and the trim tabs all the way down, I have a problem when coming out of the hole... especially when trying to go realativly conservative.. The drives lose bite and cavatate making the engines difficult to keep synchronized... the props are stailness and in fairly decent shape (very minor knick or 2 only) One drive will cavitate and cause the engine to increase rpm's - so I back off on that one, then the other one does the same... back and forth it goes until I get on plane.. On the other hand, when I get on it hard out of the hole, it performs much better, but then it sucks the gas big time... Any thoughts?

 

[rodbolt]

Re: Cavatation

hello<br /> I would suspect they are ventilating rather than cavitating. the tabs maybe creating to much turbulence near the props. as far as the fuel consumption you have 7.4's. they drink fuel at their rated horsepower. look for any transducers or other intakes thatmay be causing turbulene. you may have some cup added to the props to help thm bite better. and has it always done this or is this a new problem?<br /> good luck and keep posting

 

[paulrfrancisco]

Re: Cavatation

Mmmm ... Well the boat was taken out last week.. so to verify this I cannot do till next season.<br /><br />I really cannot answer the question of 'if it has always done this'... because I just bought the boat in August and when I got it I was like WHOOOO HOOOO!!! 454's POWER!!!!.. and I drove it hard and used the 4 barrels alot on take off, then the wallet was hit hard and I learned to be far more conserative.. thats when I started to notice it.. LOL..<br /><br />I had thought that the drive was too buried, and tried to trim them up just a tad, but that didn't help.. and I also tinkered with the trim tab positions a little bit.. but that also did not help... it almost seem the drives might be set a little to high out of the water...but probalbly not likely.. <br /><br />the transducers are through hull type, and the raw water pickup is also through hull and set about 5' forward of the transom. Could bottom growth and or a flaky bottom paint job cause this condition?

 

[paulrfrancisco]

Re: Cavatation

 

[tommays]

Re: Cavatation

the bottom condtion will have a big inpact on overall performance and it will use much more fuel if a lot of slime is allowed to build up<br /><br />if the bottom was really bad it could easely make it much harder to get the boat on plane its like driveing a car with a flat tire your trying to push 12000 pounds of boat over the bottom finish<br /><br />on my J24 sailboat one week of mid summer slime will take over 10% of the top speed and even more on comeing up to speed <br /><br />tommays

 

[Bt Doctur]

Re: Cavatation

Was anything installed within the arc of the propeller.Anthing in the path to the prop can disrupt water flow including barnacles.You may find it better to keep the tabs/planes in the fully up position when getting up on plane and then use them to level out the boat.

 

[LubeDude]

Re: Cavatation

OK, Ill chime in but its not anything you want to hear.<br /><br />This is a heavy boat, and it has two heavy engines in the back to go along with it. You will have to power full throttle to get a good hole shot, babying it is causing the problem IMHO. Trim tabs fully deployed, both engines full throttle, then back off when it breaks over and is on plane. Pedal to the metal, or is that glass.

 

[Don S]

Re: Cavatation

If you are trying to "Baby it" out of the hole, you are taking longer and using more fuel than if you just gave it full throttle and backed off after you got on step. Less time, less fuel.

 

[waterone1@aol.com]

Re: Cavatation

Paul, 30 ft and larger cabin cruisers are what I am used to running. I have never,well almost never treated my cruisers like a 20 ft ski or fishing boat (throwing the throttles all the way down out of the hole). It should not be behaving as you describe. The only time I have experienced what you have described is after I have hit a submerged object (we're on the ohio river...land of logs) and the drives have been pushed up. They will do exactly what you are describing, untill you cut power, raise both drives all the way up, then lower back down again. Last year I had a 320 regal continue to do that, we pulled her out of the water and found a chunk of log wedged into the gimbal ring preventing the drive from going all the way down, even though the gauges showed both drives were down. Another thought was a few years ago we brought a Carver 350 up here from Florida, the whole time we were in the Gulf, she behaved as you described. Once we got her into the Mississippi, she behaved properly. The moral of that story was that she just was not set up to deal very well with 3-5 foot waves, the drives were just not far enough below surface.

 

[palermo1]

Re: Cavatation

Paul,<br /><br /> You are expereincing ventilation. I hear about this quite often with twin I/Os of about this size boat. My Rinker 280 does it sometimes but only in heavy following seas. The design of your boat and the placement of the outdrives are causing the problem. The only way I know that anyone was able to solve the problem was to experiment with diferent props. One case I know of was the person went from three blade to four blade props and the problem disapeared.

 

[paulrfrancisco]

Re: Cavatation

Thanks everyone for your input.. I have a few things to check.. first, the bottom paint was in not good shape.. I pressure washed it this weekend and blew a ton of it off the bottom... it was very flaky... in the spring, I will sand and paint it properly.. next, there is is very small knick/bend in the starboard propeller... I need to get this fixed, infact I'll have the other one dressed up as well while I'm at it.. If next spring this is not happening, I'll leave well enough alone, however if it changes by the end of the next season, it is likely slime/bottom related... if not, then it might be props, and I'll experiment with the 4 bladed props... thanks for your advise...

 

[kennyv]

Re: Cavatation

cavitation solution:<br /><br />I had same problems with my twin 280 outdrives, on my 27 Cabin cruiser.<br /><br />after 2 years of changing and checking everything, we resolved the problem by changing the angle between the outdrives. They should have a "toe in" angle; The distance between the drives at the front (between transom centers) needs to be about an inch larger than the distance measured at the props. While doing this measurement, you need to pull the outdrives apart by hand. <br />This is the only thing that needs to be done. Your boat will plane much easier, and the cavitation will be history.<br /><br />Kenny

 

[mercrewser]

Re: Cavatation

You might want to see if you have vented propellers. They have holes in the hubs to allow them to ventilate a little, kinda like riding the clutch. If the holes are to big, they will ventilate to much. Look for a hole near each blade. There are plugs you can put in which will save you the purchase if a new set of wheels.