CLE Gearcase Hydrodynamics

[Rootsy]

so i've looked at a few Alpha SS gearcases and the bullet on the gearcase is smooth from nosecone to propshaft.

The SS i purchased from GeneD is a bit different. this gearcase is from a MOD VP racing outboard. It has the serial # stamped right into it.

this gearcase differs from the SS gearcase in a few ways, first it is a bit less "beefy" than the true alpha SS. Secondly it has side water pickups instead of low water pickups. This feature i actually love! cause i can use good ole rabbit ears and a hose :) .. thirdly and the topic mainly of this discussion, is the square lip at the end of the bullet on the gearcase before the propshaft. Check out the photos below and you will see it.

The Actual SS gearcase does not have this feature.

In educated opinion would it be beneficial to remove this step? and fair in a straight line across the gearcase into the prop hub? what is it actually there for?

I am thinking that this is possibly one of the reasons why i experienced such poor low and mid range and even high rpm performance with the small exhaust hub Hydromotive quad IV O/T... the flow was diverted out away from the hub to the outer blades resulting in HUGE slip #'s.. we are talking 33% at 3500 rpm... 17% on the top... pure fluid dynamics and i am looking for reenforcement before i break out the DA :rolleyes:

the full sized quad IV on the other hand is 8 - 9% across the FULL rpm spectrum.

the quad IV O/T on a TRUE SS gearcase does not experience this performance... at the same X dim...

so what do ya think?

http://www.donzi.net/photos/jaroot_250.jpg

http://www.donzi.net/photos/jaroot_251.jpg

[ToonaFish]

I think you got a funny looking pinata there.

In my educated opinion.

Bunches,

Celene "oh, and you need a graph..."

[Digger]

I think you should put some margaritas in a blender, drink four of them, and do the naked outdrive dance. then it will come to you.

[MOP]

Me thinks you could Dremmel it off by spring! :D Wonder why its there, to me it has to cause turbulence I have seen a drain screw with an extra washer cause burn marks on props. I think you are right, the trailing edge should be square and sharp so water breaks clean.

[ToonaFish]

Digger:
...do the naked outdrive dance. I'm certain that a few of our more recent board members may be unfamiliar with this dance. Would you be so kind as to demonstrate it for us? :D

Bunches,

Celene 'Off to see the leaves change. Not sure into what.'

[Rootsy]

well gee wiz... i ask a serious question and all toona wants is to see is all the guys naked running around beating on things with big sticks :p

so where is our extra talented prospective naval engineer who i KNOW remembers waaaaay more about good ole fluid dynamics than me! i think i slept through most of that course... :rolleyes:

my trigger finger is itchy... diegrinder and a sanding disk could do some serious material removal before the snow flies wink

[terry]

Jamie, like I told you in the chat room last night. You dont see one on the Imco lower do you?

I know they did extensive hydro-dynamic testing in the development on their Extreme lower.

[Rootsy]

and ted speaketh... and there was a great thunder and a massive red and white flash upon the waters...

Root headth the gosphel of ted... and the ring was no more...

well eventually :)

PS you are welding the damn thing back up and shaping it if it doesn't work wink

i honestly believe that if the shortie and the SM's and the alphas and the bravos and the trs' do not have them... then neither do i need one... as long as she falls in line with the hub :)

damn gotta visit the paint store again :rolleyes:

[terry]

Uh Oh! Boy did I step in it now....sure hope I'm not wrong eek!

[ToonaFish]

I'll have it noted that I said nothing about big sticks. Or small sticks. Or beating. Or knobs, for that matter. Or even Ted holding a torch for Jamie. But now that you mention it...

And I know better than to believe everything the Donzi guys tell me. I mean, really, if a 16'7" boat is only called a "sixteen", I'm sure you guys are most likely underestimating the size of other things.

Just trying to keep you on top JARoot.

Bunches,

Celene "now, that didn't sound right either..."

[BigGrizzly]

The bass boat guys use a special tapered ring to keep the space from the unit to the prop to a minimum. However like ted said says about imco they spend a tremendous time in testing and Hydrodynamic flow. With my engine combo on a 24 foot Python the difference between the bravo and and a Imco is over 12 miles an hour. Anyway get rid of the ring, it is there to induce venelation comming off a conner in outboard racing.

[Tomahawk]

The ring on the trailing edge of that particular gearcase was put there for a reason. It is commonly refered to as a "blowout ring" and serves a similar function as the flair at the trailing edge of the propeller hub on most props. In this case though, we're talking about gearcase blowout which can cause a sudden and sometimes violent turn to the left (assuming a right hand turning prop). The issue for some gearcases is cavitation (low pressure) bubbles just from the design of the gearcase or, as Phil points out, from little imperfections like bumps in the casting or paint or the oil drain hole or water intakes. The higher the speed, the more bubbles and the more likely gearcase blowout will occur. High engine height on outboards, or high X dimension on I/O, and over trimming will hasten the onset of blowout. The violence happens when the cavitation bubbles from the gearcase are able to link up with the exhaust gases and draw the exhaust forward into the propeller blades and screws up everything the prop is doing like, lift the bow and steering torque to the right. That little blowout ring is there to keep the exhaust from from linking up with the cavitation bubbles from the gearcase and moving the onset of blowout to a higher speed.
That all being said, with later gearcase designs, blowout shouldn't be an issue until speed approach something like 80 mph. That's why you don't see the ring on Bravo, Imco or most anything else these days. The cases are a little longer (different aspect ratio) and some, like the Bravo, have the drain plug in the bearing carrier rather than on the nose or side of the gearcase. The CLE (cresent leading edge) gearcase also helps with all this by increasing the length and adding rudder area. Your gearcase may have been updated by the addition of the nose cone and CLE. If that be the case, as suggested in earlier posts, the blowout ring would seem to be no longer needed. If you do grind it off, just be sure not to roll the trailing edge over at all. That would just encourage blowout at a lower speed.

[Rootsy]

thanks for the input guys... this is a TRUE CLE gearcase.. straight from mercury... the side water pickups are unique to this gearcase ONLY which forced the MOD VP racing class of that time (1985 vintage) to keep the drives down in the water. the plug for the oil is where the low water pickups are located on the new sportmasters.. on the bottom side of teh bullet.

i run straight thru-hull and nothing through prop and no diffuser rings on my props. it's important to maintain a straight transition into the prop hub as tom said. plus a good sharp edge makes for better flow break off of the gearcase, as in sharp chines and transom joints..

good winter project maybe :)

now how bout some theory behind skeg shaping? this thing has at one time been played with a bit and who knows if it is right or wrong...
:rolleyes:

[MOP]

Rootsy awhile back I found an article on re-shaping the skeg into a foil which helps to offset the prop torque. Could not find the original but here are couple that have interesting points for you to ponder.

http://www.hydrostream.com/wwwboard/messages/1744.html

http://www.screamandfly.com/screamandflymagazine/evaluations%20sportmaster%20mod%201.html

http://www.bassboatcentral.com/nosecones.htm

Phil

[AMAROK]

Hi Rootsy,

I'm new to the boards, but hopefully I can help out a bit. What you are showing is called the "blowout ring." It was designed to reduce exhaust from getting sucked to the propeller blades at very high speeds causing cavitation. High speeds are like over 80mph. It is a pretty complicated issue, but it is well described in the Mercury book "everything you wanted to know about propellers." I scanned in the pages, but have not been able to post the jpeg. I'll try and email it to you.

Cheers,

[AMAROK]

Let's see if this works....


http://www.donzi.net/photos/ppranzo01.jpg http://www.donzi.net/photos/ppranzo02.jpg

[AMAROK]

OK, I think I got it this time.....

http://www.donzi.net/photos/ppranzo03.jpg
http://www.donzi.net/photos/ppranzo04.jpg

[Rootsy]

Thanks for all of the input. really, thank you.

the decision has been made... the SS is losing the blowout ring, it really is NOT needed in this application. it is going to be faired into the bullet and cut to a sharp corner as it should be :)

now, who wants to talk about the bottom of the 16 wink

[MOP]

Hmmm! Maybe I can find that E-Mail with Lennys pad dimensions! :D

[Rootsy]

THE BLOWOUT RING IS HISTORY!!!!!!!!!!!

the gearcase is bare aluminum and not the smoothest thing in the world since someone previously got a bit carried away with some form of powered paint removal tool :rolleyes:

i swept the skeg at the torque tab and sharpened the leading edge of the skeg, Crescent and top of the nosecone.

i removed the blowout ring and faired it in straight with the bullet

i ran the beast tonight...

it handles better, and is more stable and easy to control but i still need to apply right rudder to keep her pointed straight... hydraulic steering is in the skunkworks right now...

i generally cruise at 3500 - 3600 rpm which used to yield 48 mph or 9% slip... 49.8 mph gps now @ 3550 rpm... which equates to 6% slip... holy cow... that quad IV really grabs...

top end i lost a few rpm (5400 - 5500 vs 5600) but speed remained the same with the full blown windmill.. 75.5 mph before i ran outta lake.

i'll be testing a 25 O/T hopefully before naptime... when swapping the Quad IV and the O/T on an 18 / SS equipped hull the slip was the same for both props but the O/T was 4 mph faster... wink

so hopefully by spring we have a prop ready to be finally dialed... have some extra strake length and we'll be steering with oil :D

Root - i have my fingers crossed...