Will I Experience Cavitation?

[trentmh]

My 1991 Cobra outdrive is non-standard. My boat has a 3.0L. The issue I'm having at the moment is that the lower portion of my outdrive has been replaced with a V6 version of the Cobra outdrive and it has a 15 spline shaft. My boat came equipped with a 13 1/4" x 17 prop but it is almost impossible to find this prop that will fit a 15 spline shaft. I spoke today with a gentleman who works for a prop manufacturer and he was telling me that I could use a hub kit designed for a V6, but use a 4 cylinder prop. However, he theorized that since the 4 cylinder prop would be smaller than the V6 housing of the Cobra drive that I would experience excess exhaust blow-by and therefore have cavitation. You think this manufacturer rep may be on to something? BTW, I have a 15x17 prop on my boat now, but I'm at least 400RPM's below the recommended range at WOT. Several had said that by reducing it down to 13 1/4, I would probably gain about 400RPM's.

 

[cobra 3.0]

Re: Will I Experience Cavitation?

You should gain 200 rpm with every inch of shorter pitch. So a 15 pitch should be fine.<br /><br /><br />Look here for your prop:<br /><br /> http://www.boat-props.com/propellers/ <br /><br />I think these should fit:<br /><br /> http://www.boat-props.com/propeller... Outboard Marine/rotation-Standard/index.html

 

[trentmh]

Re: Will I Experience Cavitation?

I wish I could order something that would fit correctly from there, but it seems that I cannot.<br />I need a 13 1/4" prop, but that is made to fit a 15 spline shaft. And, if I did find that prop, would I experience cavitation since the prop housing would be smaller in diameter than my Cobra 800 outdrive?

 

[cobra 3.0]

Re: Will I Experience Cavitation?

Why do you feel 13 1/4 is the necessary diameter? How much space do you have? I run a 14 x 17 and a 13 1/4 x 19 off my 1986 Cobra.

 

[trentmh]

Re: Will I Experience Cavitation?

I was told by my boat manufacturer that a 13 1/4 x 17 is what the highly recommended for my boat and they had done extensive testing with that prop. I'm not sure what you mean by how much space. The housing is from a V6 so if a 4 cylinder prop were installed it would fit inside the housing and although I have not done this yet, I would guess there would be about a finger width or less between the outer edge of the prop housing and the housing of the outdrive.

 

[Bondo]

Re: Will I Experience Cavitation?

Well,.......<br />You Can't use a prop for a Small Dia. gearcase on a Large Dia. gearcase.....<br />You're Absolutely Right........ It'll cavitate like Crazy......<br /><br />So,..... Just Forget About the Diameter(of the blades)......It's a # that You have No Control over.... It's determined by the engineer who designed the prop.....<br />Just buy the Right Pitch........<br />The Diameter will work itself out.......<br />

I was told by my boat manufacturer that a 13 1/4 x 17 is what the highly recommended for my boat and they had done extensive testing with that prop.

You've got a self described Mongrel........<br />All that time,+ money spent by the builder is Not Applicable to Your boat....<br /><br />I don't do OMCs,... So I've never seen the difference between the 4cyl. model,+ the 6cyl. model.......<br />On a Mercruiser, The props from the Large gearcase Outboards, Mod.1s, Alpha 1s, Alpha 1, GenIIs,+ the Bravo 1s All use the Same Props..........<br />And,...... JonnyRude props in the 15 spline, 4 1/2" gearcase size will Also fit a Merc......<br /><br />I Guess this is 1 More Reason I Don't Like OMCs............