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drive spacers/cleaner water/slip

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Old 06-20-2002, 09:07 AM
  #21
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Tread-

I hear you about the additional drag with the standard xr case.

Jassman,

I hear where everyone is coming from. From the information that I have gathered it should be real simple, if we add the spacers and our slip goes down, our speed goes up, then we are going in the correct direction.

Conversely, if we add the spacers and our slip increases then we are going the wrong direction by lowering the drives and they need to come up.

So this afternoon we should know the answer to part of the question...
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Old 06-20-2002, 11:07 AM
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thunderdan

Man, you've got some high-powered advisors helping you out here. And they all agree! Each one has raised a valid, interesting point that pieces the puzzle together. That's what I love about this board! I too am trying to understand how all these pieces fit together. This is what I think I understand.

To go a certain speed (100 mph, why not) you must overcome hull drag. Hull drag is made up of aerodynamic drag, water drag on the hull itself and the water drag on the drive. Each of these drags can be broken down into different types of drag but let's not. I'm going to say that you can't do much about aerodynamic drag, you trim the drive to reduce wetted surface (drag on the hull) and you raise the drive to reduce drag on the drive. We all know that. Whatever this ideally low hull drag is, we now need an equal amount of thrust to overcome it and achieve the speed.

Now it gets interesting. If you ask a naval architect what the most important factor is in choosing a prop, he will say "blade area". That's where the thrust comes from. Blade area in the water. We manipulate the amount of blade area by changing diameter, the number of blades and by raising the drive and prop. Too much blade area and slip % is low, but you are slow because you are using to much of your power turning the blade. Too little blade area and slip % is high and you know the rest.

In the old days, the only way we would trim the drive to get the bow up, and even though we were giving up some of our forward thrust, the net result was more speed because we reduced wetted surface drag and maybe drive drag. We all knew how to find this sweet spot. This was with three blade props because with the drives deep they gave us all the blade area we needed.

The high drives on todays vee hulls reduce drive drag, but they also reduce leverage for trimming the bow up. Trimming up too much just causes more slip and/or blowout. So steps are used to reduce hull drag. The only drawback of the high drives is the props are now too high, so we have go to four blade props to get the required blade area in the water.

If raising the drives is good, why not raise them some more? We are doing this, and the results are good, but once again we have raised the props, so five blades are needed to get the required blade area in the water for the thrust we need.

The fastests boats use aerodynamic lift to reduce wetted surface drag, and surface drives to reduce drive drag. They are using huge diameter four and five blade props to get enough blade area.

Each of these different hull types has a different range of ideal slip %. The old vee hull with submerged three blademight be 15%, the step vee with raised four blade might be 10% and the racing cat with surface drive might be 5%. (Just guessing)

So, to summarize, your boat shouldn't need much drive trim and you don't want to give up the forward thrust anyway. Let the hull design take care of hull drag. When these guys are telling you to try four blades, or get Hering to rework the props, or maybe even raise the drives more for the five blades, they are saying get the blade area right for whatever drive height you are running.

It has been said on this board before, I don't remember who, that you start out with too much blade area, cut until the speed stops increasing, then buy a new set of props and don't cut them quite as much! Not something I'd want to do to a set of Herings.

Anyway, thanks for sharing your dial-in experience with us, and good luck.

Tom

Hey Ted, how's it going?
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Old 06-20-2002, 11:59 AM
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thunderdan,

are you turning your props in or out.

also, no matter what you do don't do the spacers. your just gonna go backwards. putting the prop further in the water will slow you dowm.

now you don't live to far from me. if you want i could help you dial it in if you want. i have many tricks in the garage.

Treadwell
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Old 06-20-2002, 12:12 PM
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Treadwell, on your 37 OL, what motors, drives, gear ratio, and props are you spinning. Thanks Jeff Jassby Share with me your speeds at rpms if you could also, Thanks again
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Old 06-20-2002, 12:18 PM
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Dan If it helps any based on the above formula's my 32 slips 11% with merc labbed 15 1/4 x 30 straight from mercury. I have no clue on the x-dim and again I am not stepped but just something to chew on or maybe reference.

BTW they called me yesterday when they fired the motors. Its getting close
 
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Old 06-20-2002, 12:24 PM
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This is interesting. I am fallowing this because my old hydrostream I always thought had a high X dim. I could never get teh bow to really float after I had the new motor built and the floors redone. The old floors were rotted and heavy. I always thought that when I put the new floor and core into the boat I saved so much weight that now my X was to high. ANYONE??????
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Old 06-20-2002, 12:36 PM
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Good input Tom. I was wondering when you were going to chip into this discussion.

When can we expect TOMCAT to be floating instead of collecting leaves in the driveway?
 
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Old 06-20-2002, 05:04 PM
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sam,

The props are 32p hering 5 blades on xr drives without nose cones. We were dancing at 96 mph at the boxes with 1:5 gears. That is all of the testing we have done so far with the 1:5 gears.

We tried several pitch 4 blades but all with 1:36 gears. We spun lab 30 at 92 mph but hitting the rev limiters. We then went to non labed 30's and ran 92 mph at 5250 rpms. We also tried labbed 32 and ran almost 95 at 5000 rpms, we then tried non labbed 32 and could spin them 4900 at 89 mph.

Tomcat,

Thanks for the input.

Treadwell,

Right now we are spining the props out. I understand your thoughts on adding the spacers are going to slow us down. I appreciate your offer of helping me to dial it in, and I may take you up on your offer once I get it her to Jersey. Thanks.

I will keep you all posted and Thank you all for your valuable input.
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Old 06-20-2002, 06:19 PM
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ALL THESE EQUATIONS AND THEORYS ARE WONDERFUL FOOD FOR THOUGHT...ON PAPER WE CAN MAKE THAT THING BALLISTIC--IN THEORY --CMON PEOPLE LETS PUT ALL THE DAYDREAMING ASIDE AND GET OUT THERE AND TEST !!!!GET SOME REAL ACTUAL FACTS FOR YOUR PARTICULAR BOAT-CAUSE WE KNOW WHAT WORKS FOR ONE DOESNT WORK FOR ALL...GOOD LUCK POPPING THAT 100MPH MARK IM SURE YOU WILL..
 
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Old 06-20-2002, 08:05 PM
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turning them in will certainaly help. and then have herring dial in the props. and you should break 100 as is. then go to the new merc gear cases and you will get better control and mabye 1 more mph. your setup withthe right things should do 100 to 101.

when you get it to jersey let me know and i will bring my bag of tricks.

Treadwell

as to the outer limits we are running the new merc drives. 1:50 gear ratio. when i have it dialed in i will post speeds. hate to say before it's done.
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