2 questions regarding prop mods on a 28' tunnel with 300XS's


1) I see the flare or bell end of a Bravo prop cut off. what is the purpose and the result in boat action.

2) Also, I see Bravo style thru hub props with an extended hub, no flare or bell. what is the purpose and result.

Thanks guy's & gals. ;-)

From my testing and experiences, cutting of the diffuser usually will help the stern settle. The longer diffusers are to help ad lift to the entire boat.

Im sure the prop pros will give better explanations

I'm thinkin you're hittin on all 8, I wanted to hear somebody say what I was thinking. (I can't try them till santa's gone)

Thanx

I do have one prop that contradicts this though. A stock Maximus carries my boat nicely, so I bought a same pitch Maximus with just 5/8” of the diffuser cut off, nothing else done to it. Well the one with the 5/8” removed lifts the back of the boat at any thing above 70 and shoves it around, not a pleasant ride at all. But it’s probably worth getting the prop checked out to see if there’s something not right with it.

I assume you're running a v-bottom, sounds like the long hub might work for you. (lift the whole boat)
On the Skater I may be able to use the extra (full length lift) I have tried the short hub 5 blade cleavers and it does like them. I was just wondering what the long hubs might do for me.
As you suggested the missing 5/8 should have dropped the rear. NP :-)

IGetWet is accurate as far as the physics go.
However, boats can react differently than planned as he shared.

I could write a short story on the history of long and short diffusers as well as the divergent or flared(bell) end. I will spare you the long story but will tell you the diffusers were almost always flared and long enough to go past the blade tips for almost always. This was because outboards came along before stern drives. Exhaust running out of the propeller could be drawn back around to the high pressure face of the blades and cause blowout or cavitation. The flared end changed the high speed exhaust from low pressure to high pressure which the faces of the blades dislike. Flairs also create more drag and more stern lift on prop surfacing set ups.
We have been modifying diff. tubes for decades to create more lift or less drag.

These and other reasons are why we elected to go to a medium non-flared exhaust tube on our new 4-SPEED propeller line. The length aids in planning quickly while the non-flair decreases drag. Of course the tube can be trimmed to any length we determine to be the best for you.
The long tube props you see from Mercury are nothing new. They are the original cast diff tube the Bravo 1 and Maximus have always had. The short tub versions are just the original casting with the flair removed. People may think it is new because they are now getting used to the "short" tubes. Just marketing not new prop castings.

If you search back a year or 10, you will find volumes I have written about the Maximus long tube......I mean the original prop. Oh yeah, they're the same thing.

Brett

Thank-you Brett quick and easy ... 28 Skater, 300XS's, on the transom, 5 blade, thru hub, cleavers. Would you recommend long tube or short.

Thanks again for your valued opinion, Steve

Nice Pair,

Normally shorter diff rings or exhaust tubes. What cleavers are you running?
Where do you trim the engines at speed? A lot of trim, too long of tube. Full negative trim, potentially too little of tube. However, there can be other issues affecting this as well.

Brett

Brett the boat has run its best with -0- trim sometimes a little negative. I ran a set of borrowed 5 blade cleavers (pointy tip) motors are 1.5 above bottom directly in front of lowers.

Winner-winner-chicken-dinner!
Long hub cleaver on a boat that is close will stand it on its nose hard enough to scrub off a bunch of speed.
Thanx everybody ... back to the drawing board.