prop/cavitation problem

[fowl intent]

I recently purchased a used 25 foot center console with bracketted 225 Mercury EFI Saltwater series, with a Michigan Wheel "Rampage" prop. The first few times I took the boat out I noticed a number of times that the prop seemed to be cavitating, especially in a following sea going close to cruising speed. The rpms would jump, with no power to the prop. If I cut the throttle, I was able to start back up and get to speed with no problem. Two weeks ago I spun the hub and had to be towed in. I have had a new hub pressed in, and the first time I took the boat out I experienced the same "cavitation" problems. Is this a sign that the hub is about to "spin" again, or is this something that is to be expected. Is there a possibility that the prop/hub assembly is defective? Any help would be appreciated.

 

[ob]

Re: prop/cavitation problem

Are you just experiencing this situation in a following sea scenario or does it break loose in calm waters.If you prevail offshore frequently,given your boat size ,it may be beneficial to look in to a four blade prop,which will keep its bite a bit better running shallower to surface.Also ,check the motors transom placement and insure it isn't mounted too high for your application.

 

[Dhadley]

Re: prop/cavitation problem

The Rampage isnt really known for having a lot of bite. I would, as suggested, look into a 4 blade prop of higher technology. If you are going to use that prop you may indeed have to, as suggested, lower the motor. Then your top rpms may drop so be sure to check.<br /><br />Did the hub burn out or simply spin?<br /><br />Good luck!

 

[fowl intent]

Re: prop/cavitation problem

Any recommendations as to prop manufacturers?

 

[Dhadley]

Re: prop/cavitation problem

I would look at the Turbo props. We need more info about the hull and set up. Did it burn out the hub?<br /><br />Good luck!

 

[Reel Appeal]

Re: prop/cavitation problem

Fowl Intent,<br /> Michigan Wheel is an excellent prop manufacturer.I have questions on the Rampage prop though.What size prop are you using?A boat in that size range with 225 hp should be around 15x17 or 14.5x19 max.pitch.If pitch is greater than 19,the prop is probably "breaking lose" or losing it's bite in the water(over-pitch).A cupped prop should also help.Before doing any prop swaps make sure the cavitation plate on your motor is even with the boat bottom.It can be slightly higher or lower,maybe 1/2" either way.

 

[fowl intent]

Re: prop/cavitation problem

We may be on to something here. The boat is a 26 foot SeaSport, 3400# deep v, with a factory mounted Armstrong bracket. The engine is mounted to the bracket with only an additional 3/4" or so for downward adjustment. As it is presently mounted the cavitation plate is a full 6" above the lowest point of the v hull. Is the very lowest point of the hull the reference point? If so, then it would seem that this is a design flaw, unless the bracket was designed for a long shaft (if there is such a thing in a 225 hp). The only way I see to lower the motor more than 3/4" is to have someone cut the top 2-3" of the Armstrong bracket, but that still wouldnt get the cavitation plate flush with the bottom of the hull. As for the prop, it is a 14 1/4" x 19 Stainless Steel Rampage. As I mentioned, I had noticed the cavitation problems during the first several outings, until the hub spun while running at cruising speed. I was in the waterway, and don't believe I hit anything. Does this info change any opinions, or offer any explanations?<br />Thanks to all for your help and advice.

 

[fowl intent]

Re: prop/cavitation problem

I meant to ask, and forgot, but does the fact that the motor is mounted on a bracket, some 2+ feet from the transom affect the desired motor placement as far as the cavitation plate being flush with the bottom of the hull????