Inspector

iTrader: (1)

Assuming all are standard length lowers, what is the advantage of an IMCO or Sportmaster over the standard Bravo 1? If hydrodynamics is the only benefit, is installing a nosecone on the Bravo 1 equivalent?


Thanks.

articfriends

iTrader: (1)

I bought a -2 imco lower,boat gained absolutely NOTHING over my well blended hydromotive nose coned drive except it would un-expectedly blow out at 90-95 mph and cavitate on take off,Smitty

Mr Gadgets

Inspector

We have found the Hydromotive nose cone works well for the Checks. I have not personally tried an apples to apples test. I had a 251 Convincer and now a 283 with stand off box. Both lowers were standard Merc, side water pickup and Hydromotive. The 251 went 90mph and the 283 has gone 87.7mph.
What speeds are you seeing and is there any issues?
Hope that helps.
Dick

BenPerfected

I would say it this way. Below 90+/- MPH the -2 lowers are the low cost way to raise the X. With the right props, the higher X = + MPH. At speeds somewhere in the 90 MPH range, at the same X, the efficiency of the IMCO and Merc Sportmaster lower add + MPH vs. the Bravo lower.
In our twin engine Scarab , the boat boat jumped from about 90 MPH with Bravo lowers & Mirage props to 96 MPH with the IMCO lowers w/2" spacers and the same props. After 4 days of testing, we ended up with no spacers and Bravo 1 props at 103 MPH.
At the next test secessions, we raised the extension boxes an additional 1" and ran 105 MPH w/Bravo 1's and then 107 MPH with Hering 5-blades. Turning the props inward was worth 2 MPH in all cases. Not counting parts, this is about $10,000 worth of info probably more

Griff

If you're not running over about 85mph, nose cones are not needed and most likely will not help anything.

Inspector

iTrader: (1)

That is amazing...
A 17 MPH increase in speed with Drive Height and Propeller tuning. What type of horsepower were you working with? Did you calculate prop slip for each condition (those would be interesting numbers to see for each run)? Are you in the buisness of dialing in boats or was this just a home project?

Thanks for the input.

BenPerfected

About 730 HP at the first testing, then about 750 on the last. On the last test, the first + 2 MPH was due to the new HP. The Herings were worth +2 MPH but they made the boat too squirrely because of the increased stern lift. We didn't really gain any/much MPH with the last + 1" in the X. The testing was done in about 2003 at Lake X. The spacers, props, coaching, etc were all supplied by Mercury Racing. Lake X was one unbelievable test facility...RIP. We were active in APBA racing at the time. We did calculate slip with each prop tested and I still have that data. This tested was being done just prior to Merc was coming out with their the new Bravo 5-blade prop...never did get the opportunity to test with it as they only had a few prop sizes at Lake X at the time. The extension boxes allowed is to get on plane with the prop shaft CC about 1" below the bottom. Even so the Bravo 1 labbed 28's don't hook up until about 3500 RPM. You spin them like the old Kiekhaefer 3-blades in the 80's.

BenPerfected

Inspector,
What you do learn over time is that if you are looking for more speed, it is generally available in places not in the bilge. This would make a good thread...."What have you done to increase your boats performance that is not related to more HP". As you noted, we got +15 MPH with no increase in HP. When we were racing, we were about maxed out in HP due to the class rules. It would have taken an additional 300 HP to get the same result and then the Bravo drives wouldn't have lived with the new HP.
In vee hulls, Reggie Fountain was/is the master at getting top performance with set up and bottom work.
Even Mercury Racing didn't learn much of this until the late 90's. The biggest change was the introduction of the Bravo 1 prop that allowed all the production boat builders to raise the X and get "free" speed with just a prop change....and many thought it was mostly due to the of ventilated bottoms! The "new bottoms" were an excellent marketing campaign that mostly added small speed gains but large safety issues. Our straight vee hull efficiency went from about 220 in 1987 to 252 today. Today, the most efficient vees might be in the 275 range.
Remember the attitude in the early and mid 90's? 4-blades were good in the mid range but slower than 3-blades on the top end? This is only true at a fixed low X. We are now +3" in X from where we could barely get on plane with 3-blade props.
It took 15 yrs to figure this out and then 2 years later the APBA production classes died