Cutting Bravo Lower (it's done!!!)
10-02-2002, 10:38 PM
#21
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Maxx
Great work! It's not easy, I know. Research and development is 1% inspiration and 99% perspiration. Let us know the propshaft location below the hull. If it's deep you're probably using a 3 blade prop and raising 2" will not cause blowout. The before/after comparison is really only valid if you can keep the same prop and spin it faster. If you think you might actually increase speed enough to move up in pitch, test the higher pitch prop before switching the drives.
Good luck!! Watching your progress helps keep me moving on the intercooler thing.
Tom
Great work! It's not easy, I know. Research and development is 1% inspiration and 99% perspiration. Let us know the propshaft location below the hull. If it's deep you're probably using a 3 blade prop and raising 2" will not cause blowout. The before/after comparison is really only valid if you can keep the same prop and spin it faster. If you think you might actually increase speed enough to move up in pitch, test the higher pitch prop before switching the drives.
Good luck!! Watching your progress helps keep me moving on the intercooler thing.
Tom
10-02-2002, 10:49 PM
#22
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Thanks Tom, appreciate the comments. I think helping each other and keeping costs down is what this board is all about. We have seen some great posts lately. Alfster showed us his steering system that was truly a fantastic project (we'll be working together on that to get it in to production) Just keep plugging away and don't give up. 
B.R.S. give me a call.
B.R.S. give me a call.
Last edited by Maxx; 10-02-2002 at 11:42 PM.
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10-03-2002, 10:46 PM
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I'll keep you posted, Steve. I am working on the stainless cylinders now. Pretty much have everthing together I'm just waiting for the machine shop to free up a cnc lathe to produce the glands. Give me a call or stop by when you get back down to Norfolk. 
RIGHT ON JOHN!
10-04-2002, 01:09 PM
#26
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THE BOAT IS A 27 FOOT IMP WICH ORIGINALLY WAS A ALPHA SSDRIVE TO ABRAVO KNOW I AM 4''TO DEEP HEAR IS A PIC 454 30 OVER 10/1 PISTONS HYDRALIC ROLLER CAM DART PRO 1 HEADS BRODIX INTAKE AND HOLLEY 850 CARB DYNOED AT 530 HP WITH THE CHANGES BOAT SPEED STAYED THE SAME 54MPH TOP SPEED NOT GOOD
10-08-2002, 10:18 AM
#29
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Maxx
I think you need to find a different boat with less variables for back to back testing. You have a great idea and I would hate to see it ignored because of one inconclusive test. What does 4" inches too deep mean? Where is the propshaft before and after? How heavy is this boat? What is the torque curve of the modified engine, etc.
I'm sure you could find a volunteer on the board with a more "typical" boat, a vee hull with stock power, well known characteristics and best performance with a Mirage three blade. If the boat just starts to blow out on turns then the X dimension is about right. At this X dimension a Bravo four blade will be slower on top end although usually better at everything else.
Form this starting point, when you raise the drive 2", the boat will be faster, but the three blade will probably blow out too much in turns and will be hard to get on plane, especially with any load. Now you put on the Bravo four blade and get back the planing and turning ability while keeping most of the top end gains. This is exactly how the manufacturers have used the four blades and raised X dimensions.
The test report should give the position of the propshaft before and after, GPS speed, verified RPM etc. Any changes in trimming ability with the raised drive should also be noted. You could try Hydromotive props; I found they were a bit faster but did not carry the bow as much and as a result were a little scary handling in some conditions.
I suspect the gains on older, heavier boats will be small. Since you only have one drive, the test boat is obviously single engine. It may be that you have to be going faster to begin with. At slow speeds, hull drag is the biggest factor. At higher speeds, as the hull rises out of the water, drive drag becomes a bigger percentage of overall drag, so improvements here may be more noticeable. I suggest stock HP500 power in a smaller boat. You would probably be going 70+ already.
You may have already come to these conclusions yourself. In any case, good luck!
Tom
I think you need to find a different boat with less variables for back to back testing. You have a great idea and I would hate to see it ignored because of one inconclusive test. What does 4" inches too deep mean? Where is the propshaft before and after? How heavy is this boat? What is the torque curve of the modified engine, etc.
I'm sure you could find a volunteer on the board with a more "typical" boat, a vee hull with stock power, well known characteristics and best performance with a Mirage three blade. If the boat just starts to blow out on turns then the X dimension is about right. At this X dimension a Bravo four blade will be slower on top end although usually better at everything else.
Form this starting point, when you raise the drive 2", the boat will be faster, but the three blade will probably blow out too much in turns and will be hard to get on plane, especially with any load. Now you put on the Bravo four blade and get back the planing and turning ability while keeping most of the top end gains. This is exactly how the manufacturers have used the four blades and raised X dimensions.
The test report should give the position of the propshaft before and after, GPS speed, verified RPM etc. Any changes in trimming ability with the raised drive should also be noted. You could try Hydromotive props; I found they were a bit faster but did not carry the bow as much and as a result were a little scary handling in some conditions.
I suspect the gains on older, heavier boats will be small. Since you only have one drive, the test boat is obviously single engine. It may be that you have to be going faster to begin with. At slow speeds, hull drag is the biggest factor. At higher speeds, as the hull rises out of the water, drive drag becomes a bigger percentage of overall drag, so improvements here may be more noticeable. I suggest stock HP500 power in a smaller boat. You would probably be going 70+ already.
You may have already come to these conclusions yourself. In any case, good luck!
Tom
