Prop Help?????

[Mr. Carp]

I am doing some research for a friend of mine. He has a 24' Motion Marine Sled with two 200hp Mercury Optimax offshore XXL motors on it. He's having problems cavitating all the time. The motors need to be trimmed down all the time or it'll cavitate. If you hammer on it to get out of the hole, they will both cacvitate instantly. This shouldn't do that. We have checked the depths of the motors and they are in the water the correct amount. He is running 21 pitch props. It is bad in rough water because you can't bring the nose up at all. I was wondering if any of you have had any experience in these types of problems or if you have any insight on who we should contact to see how to correct this problem. Any insight would be greatly appreciated!

[Albacore Tuna Captain]

I may be able to help a bit but recommend he contact the manufacture of the engines to get the best solution. Cavitation can be caused my a number of reasons but I suspect that if the cavitation occures immediately when hammered, the pitch is incorrect. A 21 pitch means the prop is suppose to move foward 21 inches per revolution. That is a lot of pitch! What may be happening is that the prop is trying to grab too much water and when it can't it will cavitate. Try reducing the pitch and see what happens. You may just want to contact a propeller repair shop and get their imput. They have a lot of experience on this matter.

Captain Mike

[Pilar]

Mr. Carp, I've had this happen when turning too sharply with too small a pitch on the prop.

Call Ampro in Clackamas, those guys know props.

[Mr. Carp]

Thanks for the info guys. Albacore Tuna Captain, I thought the same thing, but the boat does have 400 hp. That doesn't mean a lot, but it can play a role in the size I'd think. I mean the thing does 60mph and that is not on one of those hokie speedo's. We thought about the same thing, but with dual motors, we are looking at almost a grand so we thought we'd ask first. Thanks!

[ 04-21-2003, 11:33 AM: Message edited by: Mr. Carp ]

[Miss B Haven]

You may just need to move some weight around in the boat. If the boat runs great trimmed down on smooth water but cavitates when you try to trim it up I'm not sure I'd mess with anything on the motor (height, pitch or diameter). :smile:
Are the trim tabs clear up when you try this? (Sounds kinda obvious - but had to ask). [img]graemlins/stupid.gif[/img]

Put all your cannon balls and weights in containers and move them to the transom and see what happens. Or take some of us old overweight dogs fishing and you can use us for balast.

[Pilar]

Mr. Carp .. another thought. You could go to Cascade Marine at 149th and Stark in Portland. They have a whole rack of 'Loaner' props, painted blue, including those 5 blade stillettos and various stainless props.

The idea is borrow a prop and try it to see if you get the right wide open RPM out of the motor. That is the procedure I use to size prop pitch. The right prop should limit wide open RPM to just below or at redline for the motor. Props that do not have enough pitch will jump the boat out of the water on a start but will cavitate at near full throttle and on turns.

[Mr. Carp]

yeah that is kinda how we were going to do it as well. At top speed, the boat is right under redline like it is supposed to be. The problem is that is if you don't have it completely trimmed down at any speed it will cavitate if you try to accelerate.