Stern Jack on a 16 w/Alpha

[woobs]

On another thread discussing the ride difference between the classics (specifically the 16 vs, the 18), it was mentioned...

"Well technically it's not 2 feet it is 16 1/2 inches longer, all added fwd of midship. and the 16 18 and 22 are all 24X7 boats so they are all 7 feet wide but yes without the lounge seat the cockpit area has more width on the 18. with the changes in running surface the 18 rides bigger than a 16 and so on as the 22 rides bigger than an 18".

So, I got to wondering if a "Stern Jack" as manufactured by Land & Sea for the Alpha might help a 16 act like a bigger boat? These are basically a 12" bolt on extension to the drive. Claims are for speed increases of 3 - 10 mph with improved handling. Benefits being greater with smaller hulls vs. larger ones.

For context: On my little 16 custom mod project (which will not be a daily driver), I'm considering/planning all manners of modifications including a pad, longer inner strakes, a 3" shortened drive, alloy heads (any/all weight saving measures) as well as moving the Cg rearwards with a reconfigure to a benchseat. It will be a 4.3 V6 and I'm aiming for 300Hp.

My question would be: What experience do you have with Stern Jacks (or extension boxes) and do you think that it could help a 16 act more like a grown up 18? Comments encouraged.

[BUIZILLA]

get ahold of Lenny, he put one on the 18LE I sold him

pretty impressive results if used with an Alpha SS in front of it

[Lenny]

get ahold of Lenny, he put one on the 18LE I sold him

pretty impressive results if used with an Alpha SS in front of it

I believe I got 10 gps MPH out of it. You lose some bow lift ability obviously. Oh, and,... it is a 7 3/4" entension, not 12". ( I am talking about the SternJack :lookaroun:

It is still on the boat, with the SS.

Woobs, on your coffee break today, here is some 6 year old " reading" to keep you busy.

http://www.donzi.net/forums/showthread.php?54178-Best-prop-for-an-APLHA-SS-SternJack-install/page2&highlight=stern+jack

Bottom line, an IMCO 3" shorty on a Stellings or Mayfair or whomevers' 7" stand off box would be killer on an 18...

[woobs]

Thanks for the read Lenny. That's some intersting stuff. I suppose it's ALL really been discussed before.

In theory, the stern Jack looks good and by testimony it still looks good speedwise. However, there is not much there on the behaviour of the boat as far as handling goes.

I imagine the Stern Jack set up is just like figuring out props...you just have to try it.

After further thought, I may not be able to use a Stern Jack as my upper case is shortened 3" ...this means I'd have to shorten the stern Jack the same amount (or have a new shorter one made). That means this may be an "either-or" situation. So, which is a better concept...raised X or greater setback ?

Any Imco product is budget prohibitive to this project... That's why I'm looking into every area of Alpha modification... gotta work smarter not harder (or more expensive). Maybe I shoud start a thread.... "1000 things you can do to modify an ALPHA" - Go! :nilly:

I suppose the whole 16 Ski Sporter project is about doing more with less. The little 16, the lightweight 4.3 and an Alpha (as a light sterndrive) is the baseline. Less motor-Less weight-less money....No idea is silly.:bonk:

[Morgan's Cloud]

OK , call me dumb , compared to some things I've been called it's almost a compliment , but I've never understood where the speed comes from with the Stern Jack .

How does pushing the drive further astern into the deeper water as it rises off of the keel of the bottom result in spectacular speed increases ?

Even if you got more +trim leverage the (standard , not shortie) outdrive is in deeper water now probably rising up and over the anti ventilation plate .

[Ghost]

Just speculating: does it allow raising the X (reducing drag)? Of course, if so, that's transom work. Great on a blank slate, PITA on an existing cutout.

Also, does it add torque to lift the nose, on boats where that's a challenge? Dropping the wetted surface to raise WOT speed.

Again, just speculating on what it seems like it might do. Don't know, zip for experience.

[Carl C]

It raises the drive higher in the water as you trim up kind of like a jack plate. That's where a lot of the speed comes from. Reduced drag.

[Morgan's Cloud]

It raises the drive higher in the water as you trim up kind of like a jack plate. That's where a lot of the speed comes from. Reduced drag.

How can it raise the drive when the X is still the same and the outdrive is further from the transom where the water coming off of the keel has had time to rise to a higher level ?
To me it's not like a jack plate on an outboard at all .
(like I said , call me dumb)

[Carl C]

Because the drive now protrudes further out and the sternjack is attached to the upper drive so now when you trim up it brings the drive higher along with more trim. This guy explains it better:

" How It Works
So what exactly is a Stem Jack? As the name (sort of) implies, it is an add-on unit for MerCruiser Alpha and Bravo stern drives that enables the drive to tuck under more for better holeshot, and trim out farther for more top end. It is not a "jack plate," or up/down adjustable plate. Rather, as the photos depict, it is merely a setback unit that fits between the drive and gimbal housing (instead of between the gimbal and the transom, like current extender boxes do). This enables geometry to work its "magic" on the drive's (and therefore the propeller's) placement in the water. Quite simply, the Stern Jack's extension (eight inches of setback) puts the drive deeper into the water when trimmed in, enabling better prop bite and reducing planing time. At high speed, when trimmed out, the increased setback gives two benefits. First, the additional setback reduces the amount of trim needed to carry the bow-this increases propeller efficiency. With less prop thrust needed to lift the bow, more is available to drive the boat forward. Second, the setback after the gimbal housing increases prop height relative to the water surface, thereby reducing drag. Both result in increased top speed. Since the difference in prop height is not as dramatic as that attainable with an outboard and jack plate, the resulting speed gains are a bit smaller. However, they are definitely significant, especially when the price and simplicity of the unit are concerned. Finally, one of the greatest benefits to this piece is that it requires no engine modifications and therefore won't shorten service life, increase stress on critical components, or cause running-quality headaches like many engine hop-up packages do."

[woobs]

How can it raise the drive when the X is still the same and the outdrive is further from the transom where the water coming off of the keel has had time to rise to a higher level ?
To me it's not like a jack plate on an outboard at all .
(like I said , call me dumb)

Here's a picture to save a thousand words and create a hundred questions... :) lol.78663

[Lenny]

Woobs, the SS is 2 3/4" shorter than the stock ALPHA so you with a 3" sectioned upper would be in the ball park. Perfect actually.

You're all missing the point. A stock, 1987 350 carb motor, no work, 400 hours pushing an 18 to 68.4 with 2/3rds fuel and beer and a 50 pound toolbox. Get rid of 20 gallons, toss the tool box, drink the beer and this is a 70-71 mph stock 270 hp18 foot boat...

Too bad it is a static display in my shop :yes:

[Morgan's Cloud]

hmmm ? I still think that they're oversimplifying the diagrams.
You know , the old ' it looks good on paper' thing . Indeed it may work in real life but is it the same as a jack plate ? No , the manufacturer states that .
I think that in the diagram they're omitting that water rises quickly after it leaves the keel and the outdrive is actually now 'deeper' .

I guess the simplest apples to apples would be a photo of both units running at the same speed on the same boat with neutral or a minimal amount of + trim.

[Carl C]

I think it also gives you the benefit of a -2" shorty but still being able to run a top end 3 blade prop. I'd like to try one on my boat.

[woobs]

You're all missing the point.

I don't think I'm missing the point... I'm trying to find one now! :wink:

[Lenny]

When we're all done trying to figure out why it " works" it is interesting to note that almost ALL race boats have someones version of a standoff box out back and a shorter drive. Nowadays they incorporate steering and such but they still hang their drives an extra foot or so back.

???

[Morgan's Cloud]

When we're all done trying to figure out why it " works" it is interesting to note that almost ALL race boats have someones version of a standoff box out back and a shorter drive. Nowadays they incorporate steering and such but they still hang their drives an extra foot or so back.

???

Absolutely true . But I can understand a shortie with regular installation or on the extension working better . But not the standard i/o leg on the extension .

It's also nice to see your input again Lenny . Hope everything is well out there !