IMCO Lower Decisions
 

Good morning. Back to drive decisions. Looking for advice on what lower to choose. The boat is a 2004 American Offshore 2600 NSX. The power is an Eddie Young 850SC. 547ci Merlin Block, 871 Roots, B&M intercooler, Brodix Aluminum 345cc heads, running 6lbs. of boost with 2 750cfm carbs. This engine makes every bit of that power and is turn key so downtime due to engine problems is minimal unless something breaks (which is usually drive related). We put approximately 50 hours of run time on the boat per year. We accumulated almost two seasons out of an IMCO SCX upper/rebuilt SC lower. The SC lower developed a stress fracture in the lower unit this month and is leaking gear lube. We purchased the boat 4 years ago with an IMCO raised 3" box, Bravo upper and SC lower. The boat has been dialed in with the IMCO 3" raised box, SCX upper/SC Std. length lower, and a 1.5" spacer. The drive has a 1.5 gear ratio and we're spinning a non-labbed 34 pitch B1 prop. The boat with this setup runs low 100's and pulls skiers etc. meaning this prop stays on the boat during all activities which is nice. Now's the time to think about going to the SCX lower and potentially a cleaver style prop on a number 6 prop shaft. Again the boat is used for generally boating, water sports etc and we'd like to keep the 100mph performance. Should we gamble on another SC lower for performance and chance reliability or gamble on the SCX lower for reliability and chance performance?

Drive hasn't treated you that badly. Doesn't sound terminal. I assume you have looked into replacing the lower case and repairing the internals? Repairing the case would probably be a bad idea as at least one dimension has changed when it cracked.

Your gonna loose speed with the SCX Lower , Why not a new SC Lower and atleast 4 more years?

How deep is your drive?
How far below the hull is your prop shaft?
Is it possible that your prop is too deep and your slip numbers are too low.
This is causing to much stress on the drive.
A little bit of slip is good for the drive.
If you go to an SCX you will want to raise the prop shaft height and run a larger diameter prop.

Yes would definitely replace the case if we go back to SC lower.

I agree doesn't sound like a terrible idea. IMCO recommended tearing down and inspecting the SC lower after every season with this setup. That doesn't sound great.

The prop shaft is even with the bottom of the center pod. The center pod on this hull is slightly deeper than the sponsons at the transom (tunnel hull).

Slip is in the double digits. We tried 3 different spacers and multiple props to dial in the X-dimension to where it currently sits with SC lower.

If we go to SCX we would remove the 1.5" spacer for starters (I think) and run a larger diameter prop like you suggest. The problem is the expense of the cleaver style props and availability. I'm a boater on a budget and don't want to try and reinvent the wheel here with this hull design. The hull was setup as a Bravo style boat from the builder. I'm not sure how a cleaver style prop will effect handling and performance.

Another idea is going back to a new SC lower and installing a Drive Guardian. Maybe that's the ultimate compromise.

With how well your setup performs currently, triple digits plus being able to pull tubes and skiers, inspecting the lower after every season sounds like a fair trade off to me.

If you do decide to go with one of the scx lowers, most likely the 1400, I'd get it up as high as possible, which would be 3.5" above the bottom of the center pod if I read your first post correctly? I haven't read of many or any scx lowers retaining the same speeds of an sc lower if they aren't raised above the bottom of the hull. Imco also makes a #6 propshaft adapter that could be used for testing #6 props as well if you couldn't get a bravo style prop to work out.

I'm going to have to try an sc lower on my outlaw next season because 800hp for lower 80s with the 1300scx lower just isn't cutting it if you ask me. My lower is 2.25" below which I believe leaves too much of the massive lower unit to drag.

Not sure how much a drive guardian will help in this situation as this is not a boat that sees air time. Where are your torque spikes going to come from that would cause the guardian to slip?

Not sure if this post was for me or not. I was told the DriveGuardian will help with torque spikes from planing to prop re-entry and everything in between. The prop shaft is approximately even with the center pod behind a 12 inch box when the drive is set for neutral on the trailer. When traveling at speeds over 80mph the boat likes positive trim. The prop comes out of the water more than expected when managing boat wakes, etc. The worst is trying to keep on plane with a full load of passengers at LOTO or Lake Michigan in rough water. The boat comes out the water so much in waves over 2 feet I won't even try and get on plane in those conditions anymore. Live and learn!

I would be willing to bet the SC lower will put your boat in the low 90s.

Guess I didn't realize how much your prop comes out of the water, Rob. If you are always experiencing the prop breaking loose then by all means a drive guardian should help.

This is the type of water that kills the drive. We were taking a customer and his wife to the City Winery on Chicago's Riverwalk in late July. I couldn't keep the prop in the water to save my life so I finally just took it off plane and idled to the lock. Super fun trip though. We parked behind a yacht with someone playing the violin off the stern of the boat. We pulled up looking like the Beverly Hillbillies, good times!

I forgot all about your love of Chicago boating! And obviously LOTO during the SOTPR would be hard on the drive.

I'd stay with the SC lower for now. If you decide to go SCX lower you can get adapters made to where you can run Bravo style props instead of cleavers. Not sure how that would work out given the shape of the SCX bullet.

Even this afternoon got a little dicey at LOTO with very few other boats on the water (July 5th). I guess we were one of the few who decided to go to Backwater Jacks with a storm moving in. Fun ride home to say the least.

One thing you may want to consider if you stay with a SC is REM and Cryo of the drive components. A lot of info on the web about it. Did them to my SC lowers 5 years ago. Been trouble free behind 800's. Relatively inexpensive to do and added reliability and performance.

Secondwind, were you running the 1-7/16" shaft? I've heard better results from that lower.

We were running 1-1/4” propshaft before. I’m not sure if a 1-1/4” or 1-7/16” propshaft is going back on. Performance Boat Center is installing the DriveGuardian and new lower. I called IMCO about 1-7/16” propshaft over the Summer and they mentioned customers order them if they’re worried about throwing prop blades. The larger diameter propshaft is less likely to bend if you throw a blade. I was under the impression that was the only advantage. I didn’t even ask PBC what SC lower they ordered. I’m trusting the Coil brother’s at PBC to order whatever they think I need.

I’ll look into that. Who did you use to strengthen the gears?

George at AADS. He rebuilt my drive in June. Mercruiser OMC Cobra Volvo Penta rebuild and repairs

Good to know, thanks.

Drive Guardian installed, new SC lower on and engine is back in! Things are looking good.

Well that was quick. Can you put 100hrs on it quick and let us know how the lower is holding up? Great, thanks.

If the lower does begin to fail again is there any worry about large metal pieces getting into the upper? Did you have the gears of the upper investigated for damage? That'd be one of my biggest worries if I were to go to an SC lower, failure in the lower causing damage or complete failure to the SCX upper.

Looking good Roberto!

Ok so now that he is all good and it's cold as hell here in ohio I have a question. I went from standard bravo 1 lower to standard imco sc lowers sense the drives where off and one thing lead to another. Didn' want to change the hight of the drives because finding out thro other people trying it before me and saying my boat didn't like it. So I think it was set up pretty good form the factory. Is the sc lowers way more efficient then the bravo and are they faster in most cases? Or did I just spend 5k to look cool. Lol

Mach1 I purchased this boat with the SC lower already installed. I read on this site a few times that if your boat is traveling at speeds of 75mph and above the SC lowers will be faster (around 5%). So if your boat use to do 80MPH now it should do 84mph. Sounds like money well spent to me. Let us know how she does.

Why change your current setup? Sounds to me that you have a 100 MPH performance boat that is already more versatile than most. Want more low cost versatility?....buy some fishing gear.
Ben

Ben,
We stayed with the SC lower and installed a DriveGuardian. I couldn’t be more pleased with the performance of this boat just seems to beat up the SC lower. We’ll see if the DriveGuardian helps. I was responding to Mach 1 installing SCs’ on his Donzi. Fishing isn’t fast enough for me, a lot of waiting around.
Rob

I will go ahead and call BS on this, I would argue they are the same if anything. In fact in the basement 80 range, I bet a merc lower would be faster.

I don’t know, that’s what I’ve read on this website. Appreciatiate you calling me out though. I’m not bull**** I promise you that.

Not trying to bust your chops, Ive heard the same type of comment but the speed additive was when you got above the 90 mph range. I still don't buy it, I've kinda tried all that stuff by default. Probably not apples to apples, but in my boat for instance (28 Nordic) it takes about 17HP per mph. So, an imco lower would be almost like adding 70HP in that case.

Now, a blueprinted lower? Theres some data to suggest a couple MPH with those.

I'm not sure how much speed is increased with a SC lower, it's all hear say. I will say that prop shaft height has made a huge difference with our boat. Taking a 1/2 inch out of the drive increased our top speed 7mph, not's not bull****. If someone would post this info without my testing I would call bull****.