2" shorty drives??
 

Could somebody explain what is the advantage of installing a 2" shorty on a Bravo x ??? I dont get the concept other then the mph increase.
Thanx for any input...
mike

2" shorty
 

the closer to the top of the water with out the blades comming above the water line the less volume of water the blades have to turn , more leverge for the boat ,and more speed , less work on the motors. :ernaehrung004:

Raising drive height lessens leverage to the carry the bow. they don't work on all boats. The reasons to them are for the speed increase and in some cases handling when you add a lot of HP.

So, if the X-dems are high enough to where the prop is piercing the surface on a Bravo One drive; then as it was mentioned there is less stress on the engines and drives?...correct??? So, if that is true, I would think (for example) going from a 30" 5-bladed prop to a 32" (or any larger prop within reason) should not be too much more stressful (or torque loading) on a drive vs a drive/prop that was NOT piercing the surface. Am I right or wrong?

What do you guys think? Hopefully, my questions would benefit all on this topic and in this thread. thanks

What are the typical gains in speed from this change? What all needs to be changed??

Thanx,
mike

If you lift the drive (put on a shorter lower unit) so that the propeller runs in the surface, the engine will not have as stable rpms. This stresses the outdrive and engine more and ca often give more vibration. But if you run very fast, the drag itself of the gearcase will slow the boat a lot and prohibt the boat from loosening from the water. Jumping wakes in 100 mph will tear the drive almost off the boat. Then simplicity and little into the water helps for reliability and speed.

Also where to put the thrust also affects the boat's attitude to the water a lot. When you put on a shorter lower unit, you lift that "thrust stream" up. This gives less bow-lift. On many boats, this will slow the boat down. especially under 70 mph where the wind is not "hard" enough to lift the hull up itself, I don't think you will see any increase in speed. And cruise speed will be not as good as the slip increases due to the fact that not much of the area on the propblades are gripping the water.

(This is for a Vee-bottom)

Please correct me if I have not got this right. This is my personal understanding though.

See which boats that has shorties as standard, often boats that do not need bow lift.

Huge gains with my application. I was buried with a very conservative x-dim.

Speed gains can vary from a loss to maybe 5 mph. It really depends on the boat and how deep the prop is in the water.

My 28 Pantera was set with a high X dim and didn't carry the bow well. I lowered the propshaft with a 1" bigger spacer. The boat carried the bow better and picked up about 2 mph at WOT and 3-4mph at cruise speeds.

We first tested our 30' straight Vee with -2" IMCO lower with a 2" spacer to get a base line We picked up 4 MPH at the same X with the same Mirage props. (speed 93 to 97 MPH). Put on B1's at this X and the speed dropped to 95 MPH.
Started raising the X in 1" increments with B1's and ended up at +3" up from the original X at 103+ MPH with no other changes. Each +1" in X added approx. 2 MPH. (prop shafts about 1.5" below the bottom)
You will likely get the most benefit from a IMCO case if you are starting with a 90+MPH boat or an "Old School" set up with the drives buried.
If you have a boat that runs 90+MPH, your X is likely close to optimum. In this case, you could benefit from the hydrodynamic advantage of the IMCO lower vs. the stock Bravo lower, but may you need to couple the IMCO case with a 1-2" spacer to maintain the best performing X.
Test, test, test....

With my -2" IMCO shorties, the propshaft centerlines are 1.5-2inches below the bottom of the boat. Once I added the shorties I lost about 6-7mph in my midrange cruise speed (stock Merc lowers w/32" Bravo One 4-bladed props). However, I moved that same 6-7mph up to my boat's top end speed. I would like to try a 32" P5X vs the 30" P5X's I now have and see what happens. Keep in mind my drives sit on Stellings extension boxes.

Ben, what kind of power and RPM's are you turning and what make of boat?