Different Props?

J

JAYROCK

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Sep 18, 2007
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I was hanging out at my local boat shop when the mech and I noticed that the Bravo 3 he was servicing had 2 different pitch props on it. It was late 90s 25ft Searay with a 7.4L and the Bravo 3 drive. The front prop was a 26P anf the rear was a 24P. We stood scratching our heads for a while.....Bringing up the point I had read in Boating mag about Trips vs Quad outboards, running different pitch on inside and outside motors. Neither one of us had ever seen or heard of a Bravo with different pitches. The owner said he was aware or the difference and didn't notice any ill performance issues. He said the boat would run low to mid 50s! Has anybody seen or tried this?

I am VERY aware that the Bravo 3 is a 2 prop drive and the concept of 2 different pitch props makes no sense......OR DOES IT?
 
I'd imagine if the rear prop on a B3 was the same pitch it would add 0 thrust as the water is already moving at that speed due to the fwd prop. The only way to add thrust is to push the water with the aft prop faster than it arrives, thus it stands to reason it would require higher pitch.

I hadn't heard of different pitch on inside and outside motors but presumably it factors in water speed at each location as well as increased density at lower levels, and perhaps control factors as well.
 
The only way to add thrust is to push the water with the aft prop faster than it arrives, thus it stands to reason it would require higher pitch.

The aft prop is a lower pitch.
 
Nice try , Sandy .:D

Hmmm..... Harlan, this seems to indicate we are both in error:
http://www.mercurymarine.com/media/mercury/documents/Bravo-Three.pdf

Aft prop smaller diameter, but same pitch?? That would presumably give the thrust of a larger diameter prop of that pitch(less slip than either of the BIII props individual, with added benefit of neutralizing steer torque.

The aft prop has final effect on the much of the thrust water flow ( and torque ) so it's probably smaller to keep combined prop steer torque closest to neutral.

Just trying to dig my way out of the previous hole & into this nice big new one...
 
From the Merc document, the props need to be of same size & pitch. All I was saying in my previous post was, the aft prop was of smaller pitch. I hae Bravo III's & they are same dia. and pitch.
I have been told on some new Merc o/d's they put a 4 blade on front & a 3 on aft. Just what I have been told. Dunno.
 
Can't speak to Merc, but I would assume they're similar to my volvo DP's - both forward and after props have same MODEL number, cuz the're a matched set (the inner prop is opposite thrust, w/ a different hub, but same prop designation) - but they appear to be different pitch. In my case, both props are F4's. I guess you could put a F3 forward prop w/ a F4 after prop, but not sure why you would. I would think the props are designed to work in tandum without adding undue stresses to the geartrain . .....
 
quote:
Originally posted by Harlan

From the Merc document, the props need to be of same size & pitch. ...........
Click to expand...


There's no telling what some fool owner or mechanic might have done to the boat, but the manufacturer knows best so look up what's supposed to be on it and buy and install what the manufacturer calls for.
 
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