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W. Tripp

Gearcases

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If you are trying to get the most from your Mercury Powered STV, knowing about the different Mercury gearcases, and modifications to them, will help with better driving, acceleration, and top speed. We are concerned with fast lake use, not racing here.

CLE - The first High Performance gearcase released from Mercury was the Crescent Leading Edge (CLE) unit. This can be a very fast and good driving case on an STV. The best ones came with 2 low water pick-up holes. These are hard to find these days, and many have a lot of time on the skeg. Since a high speed STV will "crab", or run best with the engine cocked at a slight angle, the skeg is a very important issue. I am not going to get into details on this case as few good ones are around, but if you have one, keep it and watch the skeg for signs of cracking.

200 - There have been some performance cases that came on the 200hp engines, but they cannot be purchased new anymore. Most had small upper driveshaft bearings, and a 2 piece propshaft. The most noticeable part on this case is the thin, swept back skeg (like an F14 compared to a Cesna). A nose cone on this case can be very fast, but the skeg will not take a lot of abuse. Most 200 cases have a side oil fill hole - for true performance, this needs to be filled. These also have gearsets that are NON-ratcheting. In otherwords, the prop does not continue to spin when you let out of the throttle, and can be a bit startling if you chop the throttle.

Torquemaster - The TM case is the latest regular production case. Internally, the parts will swap between the Tm and Sportmaster (SM) case. They both have a bigger upper driveshaft bearing and a one piece driveshaft. The skeg is more upright and thicker than the 200 cases. With a nosecone, this case can be very fast and reliable.

Sportmaster - The SM case is the latest High Performance gearcase to come from Mercury. It has a longer nose and low water pick-up holes only (no side holes). The skeg is thick, with a tapering torque tab. This case is know for the transom lift it provides, this is not needed or wanted in a fast STV. This can be a very good case with a few modifications.

XR - This case came on the XR 150hp engines. it has a smaller diameter than the other V6 gearcases, and is desireable for racing with some work - lets leave it there.

Nosecones - As far as I am concerned, there are only 2 nosecones worth working with - Bob's and JC's/Tooter's. The issue here is who is doing the work, and how much detail they are putting into it. The amount of angle that exist between the tip of the nosecone and the bottom of the gearcase effects the lift from the gearcase. The curve between the tip and the water intake ports is crucial to how much drag the gearcase has, and how well it drives.

Most nosecones are not put on perfectly in line with the propshaft. Sometimes this is intentional, many times it is not. Offsetting the cone to the side can help on a fast V-Bottom - not an STV. However, moving the cone up, or down relative to the propshaft will effect the amount of tail lift the gearcase provides. My opinion is that the STV likes the nosecone centered 1/16" above the propshaft, and lined up directly from side to side.

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Modifications -

Lets say you already have a good gearcase, either a nosecone was put on using the guidelines above or you have a good Sportmaster. What can you do to make it drive better, and go faster? In most cases, A LOT. If you have an engine that is hitting its rev limiter, you will not gain more speed, but it will handle and accelerate better. If you do not have a limiter, you can gain a few MPH by trying a few things, and working out the details. Don't sweat the details, right? - WRONG! With a little sweat on the details, you can be much happier with your gearcase. The amount of water pressure running along the gearcase is amazing, details here all add up.

1.) Since our boats "crab" quite a bit, the nosecone should not be pointed. take some time to work the tip to a gentle radius with sandpaper. Do this to the 4 inches above the tip as well.

2.) Work the area near the front of the water intakes so that it forms a very gentle curve all the way up to the cone tip. The area just in front of the water intakes usually needs the most work. DO NOT flatten this area!

3.) Fill in the right side water intakes. Clean this area well to get a good bond and use Devcon, JB Weld, or Marine-Tex, etc. Straighten the water intake openings so that the ribs are sharp and flat sided leading into the water holes. Lay the trailing edge of the water intake holes back with a 1/8" radius in a tear drop shape. This will reduce drag and cavitation bubbles along the case.

4.) Any depressions along the case must be filled in to smooth out the water flow. The bottom of the case on either side of the skeg must be worked to be as flat as possible. This is where the gearcase will ride on the water.

5.) On the right hand side of the case, fill in the area where the skeg joins the case so that there is a 1/2" radius, to prevent cavitation in this area. On the left side, make a 1/4" radius. MAKE SURE the entire length of this area is evenly worked.

6a.) I like to cut 1/2" from the bottom of the skeg - making only as long at the props I use. The bottom 3/4 of the skeg should be laid back or "swept" like the 200 or XR cases. The bottom of the skeg needs to be FLAT and parallel to the bottom of the gearcase.  

6b.)The left side of the skeg should be worked in an "aero" or "foil" shape, like a wing. The left side needs to be as flat as possible.

7.) I like to put a small notch in the top of the skeg, above the torque tab, to prevent stress cracks from forming. This is a half moon shaped semi-circle that is radiused on all edges. If a larger torque tab is added, the top of the tab should be rounded off to reduce loading for high speed runs. Without the "rounding", the load can eventually lead to cracks where the tab was welded in place (It actually drives better without the rounded off skeg).

8.) All trailing edges need to be flat and sharp. This includes the skeg/torque tab and the back of the case - in front of the prop. The "Blowout ring" only needs to be about .030" high, and angled correctly.

9.) I like to coat the gearcase with laquer primer, and lightly sand it in the direction of travel. This way any cavitation areas or skeg flex will be noticeble by the paint flaking.

These mods will add up to make a great driving gearcase, and give you more efficiency with less drag.

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COOL...

now we just need some pics :)

to go along with these great articles

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Here are a few photos from my modified Sportmaster. This case has been on TWO ODBA World Championship Modified Production winning boats. (Walter Wolf and Trent Blocker)

post-2-1079460093.jpg

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Ask...

and Receive...

THANKS WAYNE

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Great 2 articles!

BUT you better show a decent picture and an explaination of how adding a stress riser prevents stress cracks????

Number 7.

Adding a notch or sharp edge is adding a stress riser in about anything.

I assume this is a rounding of an existing notch?

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Techno - I NEVER said I added a stress riser - however, I was unclear on the subject. The notch mentioned must be rounded. The photos show an exagerated rounding where any stress is applied - to prevent crack forming. The Sportmaster pictured has a torque tab added, and does not need the "notch", but does need to be rounded at the top of the tab to reduce load - a hard learned lesson. ( I have edited the post for more clarity - thanks Techno)

As soon as I can shoot some photos of my XR cases, I will post them here. I am also prepping a 200 gearcase to test against my modified Sportmaster and XR cases on an STV.

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I have noticed on my Euro with the Sportmaster that about 105mph the hull feels very light in the rear. Is this the lift from the Sportmaster... and if so what should I do? Presently I'm more than happy with only hitting 110 and have no plans seaking higher speeds.

Later

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The transom lift in the Sportmaster case can be reduced by reducing the curve on the bottom front of the gearcase. Look at the photos and notice how this case has been worked in the area from the water intake ports to the tip of the bullet.

If you would like, I can make a sheet metal template of this area, and send it to you. However, I have loaned this gearcase out (again) to Russ Bentley to try on his STV. He does not want to give it back until AFTER the first ODBA race in Meridian, MS. He said that it drives much better than his stock Sportmaster. He has also picked up several MPH in the 1/4 mile while using it in his testing. It will be early May before I can make a template.

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Wayne,

Would love to see pics of your XR6 case, or talk to you about them.

Currently I am having handling problems with the one I have.

Thanks,

Dennis

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Cool info!!!

Randy Pierson was right he told me that when he owned my  Euro the CLE was the best gearcase for it. I run a stock 200 case with a Hydromotive nose cone. It was a "freebee" seems to work good for me!!!

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Great stuff Wayne,

In pic #2 you shot the blowout ring. You stated .30 and angled the right way. It seems as though I can only see the ring at the bottom, but it is flush with the sides. Is this intentional and could you explain just a little more ??

I assume pluggin the starboard pickup as long as it doesn't affect pressure or temp.

Greatly appreciated

Rick

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Rick - Yes, the blowout ring has been removed from the sides intentionally. It gradually is tapered in thickness from the 4 o'clock position to the 6 o'clock position, then tapers back out at the 8 o'clock position.  At the speeds where blowout will occur, only the lower areas of the case are riding on the water, and only the areas shown need the water disturbed by the blowout ring. Any other areas will simply add to the drag of the case. In fact, the case shown has much more blowout ring than any of my other cases, which I regularly take to much higher speeds. My XR racing cases have MUCH less.

A blowout ring acts like a vortex generator or "gurney", and causes the water to continue its flow to the prop blades, rather than rolling the flow into the low pressure area at the back of the gearcase.

Plugging the water intakes as pictured has not shown to reduce water flow to the pump. In fact, in some cases, it has helped to increase the flow. If the boat crabs excessively, the side pick-up holes are in a lower pressure area than the opposite side. Since all of the holes are connected to the same cavity in the nose of the cone, filling these holes can prevent water from flowing back into this lower pressure area. At speed, the water pump is doing very little to assist the flow of water into the engine.

A highly modified engine makes a lot more heat than a stock or mildly modified engine. During break in, my Pro Gas/Pro Fuel engines will run for 2 hours under 2000 rpm. Then run for 2 more between 3000 and 5000 rpm. Using these mods to my gearcases, I have NEVER had cooling problems at low or midrange speeds.  At higher speeds, water flow takes care of itself.

On another issue, all of these mods are what I have found to drive better, accelerate quicker, and allow a higher top speed for me. This does not make them the end all and be all of gearcase work. I am constantly finding small details that add to this. I have simply shown many things that I feel a performance boater would want to know. If you are going to run over 100 mph, then the gearcase needs to be addressed. I hope this helps those in the quest for a better handling boat, or those trying to get a few more mph. The mods shown to the Sportmaster are night and day better for an STV running over 100 mph.

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Something mentioned before. By plugging those 2 holes it also acts a bit like a wedge. The 2 open ones are drag on one side of the case and the filled ones are not so helps a tad.

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Mr. Tripp I was wondering your opinion on a CLE 2 hole with an "Allison" cut skeg running on a PCS. I have a Brand new sporty ordered and would like some advice on modifications when it arrives, like the sheetmetal templates idea....but was considering keeping the CLE as a backup. Can the Ally cut skeg be filled/repaired and worth of such.

my email is gottahaveanstv@hotmail.com

Thanks in advance.

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