drive spacers/cleaner water/slip
 

As you all already know my set up is a 37 AT, with KE 675 motors.

This is where we are, we are dialing it in right now and getting ready to add spacers in the drives to try and get them a bit deeper in the water, into cleaner water to reduce the slip on the props, and ultimately increase the speed. My best so far was about 97 mph, at 5450 rpms but 14 percent slip with hering 5 blades. Hering told me the average slip factor for his props is somewhere around 9 or 10.

So far we have had high slip numbers, around 14 percent. Now this is the formula as I understand it.

RPMs divided by gear ratio X prop size divided by 1056= 100% efficiency.

Figure the slip from there. High slip, high drives. Low slip, low drives.

So we have some high slip and going to sink the drives a bit with spacers. There is the update so what do you all think?

Did the spacers help, or not, and are you still on the 1:35 or 1:50 with the herrings.

We are still with the 1:5 gears and the herings. We were slightly delayed due to the difficulty procuring the drive spacers.

Tomorrow we will continue our testing Jassman..

Dan, help me out! So, theoretically if you get the slip down to 9 to 10% and calculate everything out where do you think you will end up speed wise??

roughstuff,

Not real sure as to where the speeds will be but theoretically if I lower the slip, then my speed should increase unless I am dragging too much. So from what I understand, if I lower the drives and my slip goes down then I am going in the right direction, if i lower the drives and my slip goes up then I need to raise my x dimesion....I am pretty sure that is correct...

Testing is the only answer.....luckily for me Pat is spending the time to dial it in for me.

Thunderdon Have hering rework props to lower slip .

If you lower drives they will slow you down.

Revers props that may help.

RPM xPITCh DIVID GEAR RATIO = 100%

figre the slip from there.



sam:cool:

:eek: :eek:

rpm X pitch DIVID by GEAR RATIO DIVID 1056 =100%

FIGURE the slip from there .

sam,

but what if i add the spacers-lowering my drives -that could possibly lower my slip, increasing my speed, no?

In the reverse if I lower the drives and my slip increases than I need to raise my drives, No?

Remember the 92-95mph "wall" I mentioned in the previous
thread ? You've found it. How many pins of trim can you run
before it starts to blow out when your running with the highest
X ?

JD

"Not real sure as to where the speeds will be but theoretically if I lower the slip, then my speed should increase unless I am dragging too much."

I'm sure you are doing fine, but this statement gives me the impresion there may be some confusion. If you are using the same prop, if you lower the slip, by definition you are going faster. No "unless." The problem with "reduced" formulae is it is easy to forget the factors.

Engine RPM / drive gear ratio = prop RPM

Theoretical speed = prop RPM x (pitch/12) x 60 / 5280

pitch in inches, 60 minutes per hour, 5280 feet per mile

so ts = 102.27 if you use a 30" pitch prop.

Prop eff = (102.27 - GPS speed) / 102.27 (times 100 if you like)

Asyour speed (GPS speed) is one of the factors, you can't lower slip without going faster.

Make changes that give you less slip...it will not be limited to just up - down.

Ted

I ewas afraid that I may be hitting that wall, the only confusing thing to me is the slip being over 14 percent with hering 5 blade props. I am fairly certian that we can run it up to about 6 on the indicator, but I will verify that with Pat.

hey, Ted, good formula. One I prefer, but GPS is SPEED OVER THE GROUND!!! Put it this way...I showed an increase of 10 mph on my Apache running with a 25 knot wind at my back and five knots of current going with me. Everyone on the boat went WHEW!!! Thats GPS for you. Turn the boat around in the same conditions and its all the other way. I'd prefer he used a pressure speedo. As a pilot, I file my IFR airspeed with ATC as indicated airspeed. I could return from Detroit in my plane showing 265 knots across the ground GPS but only 165 on the airspeed indicator for that altitude. The book says 175, so that what I file. All air traffic is travelling in the same 80-90 knot tail wind, so its relative. Good numbers TDan. Jerseyboygoesfast.

Right! Lower the slip, increase the speed...

... but that's only part of the equation. With reduced slip the motors will be working harder to reach the same RPM.

My guess is as you reduce slip you will also reduce RPM's slightly. The fastest speed will be found at the best comination of low slip and high RPM. Also remember the difference that conditions can make, temp, humidity, surface of the water, weight, fuel load, etc.

My .02.

Good Luck with the testing!

Dan for you the wall is simply not there by way of, Thunderstorms DDC Bottom Also there is a fairly large horizontal area down at the keel (as we know nothing supports a load like horizontal areas =really neat things like Lift strips a Pad bottom and your water skis) your only limited by simply the amount of force applied by the Engines and the manner of the accompanying setup:)

Best Regards

Thanks for all the clarification, I am starting to understand all this. I am pretty confident that we can reduce the slip, thus increasing speed. And i know we were right at the rev limiters, even bumpingthem, so if we reduce the slip, reduce the rpms and increase the speed I will be one happy mo fo....

I will keep you all posted, and thanks

Your drives are ALREADY too deep, need to get them up a little so you can start spinning the right prop...

Thunderdan, let's start from the beginning.
What pitch props are you running? What is the setup on the props? Right on right or left on left? Are you running nosecones on the drives, what type of drives are you using?
If trimming drives too high, rpm will increase and slip will increase. Perhaps props are a tad too big. Depending on pitch, I have experienced high pitch props will want to paddlewheel in the water. Maybe if you step down a inch or two in pitch and not trim drives out your slip will be lower, mph will be higher.
Have you tried some Bravo I 4-blades?
Over trimming will raise bow and cause props to slip more.
I have experienced similar problems that you are having and achieved my goal of 103 mph 25' Checkmate.
I hope this helps. Get back and let us know how it's going.
SAM

Hi all,

Usually you dial-in in an area with flat water. If there is a current use 2 way averages. GPS is the only way to go when a change gives you a half a mile an hour etc.

Ted

herring props like to run on the surface not below the water. ssince your is about to crack 100 you might try the new merc lower unit with nose cone built in.. it has no wtaer pickup on it thus creating less disturbance to the prop IE you will go faster and reduce drag. probabley almost same cost as buying drive spacers.

Treadwell

Dan, I agree with Treadwellmotorsports, I think the nose cone will help, also check out what this Blue Moon guy says, he also has a valid point, call me on my cell when you get a chance:D

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...

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?

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

Treadwell, on your 37 OL, what motors, drives, gear ratio, and props are you spinning. Thanks Jeff Jassby:D Share with me your speeds at rpms if you could also, Thanks again:D

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

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??????

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?:D :D :D

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.

BENCH RACING
 

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..:D

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.

alomost forgot. i know for a fact if you change your lower gear cases to high water pickups at the minnimal a 2 mph gain. if you go to a xz drive with high water pickup your at 3.8 mph gain. and if you do a new merc gear case with nose cone and no water pickup with xz gears your at 4.5. just remember each boat acts diffrent so you could get more or less. i have done this on several boats and they were all within 3/10 of a mph witch is not bad.

Treadwell

Thanks Treadwell, how is the new merc drive, the one with the nose cone in comparision to the Imco Extreme SC drives. Do you have any idea if that drive that is going on my Thunder going to help or be a power robber also. I know it is stronger, but thought the bmax was over kill, so I elected the Imco, but have not purchased yet. I too am having the same motors as Thunder Dans, just with possibly different drives.

Thanks Treadwell, I will let you know when I get it her in Jersey. :D

Dan call me later on, JJ:D

Merc drive gains??
 

Treadwell- sounds like you've seen small but measurable gains with the merc sportsman case. If I read correctly and there is no water pickup in these drives- wouldn't you risk losing all the gains once you install transom mt water pickups?

How would you compare that to the Imco lower which has a pickup and is said to be a more flow/speed friendly design than merc's LWP?

Great thread- I'm sure many OSO members will benefit.:D

Treadwell?
 

Merc sportys with no pickup. How does the drive stay cool?

Mine (merc sportmaster)have 4-strake intakes on each drive.

Cattitude - Have you seen an imco after it hits the saltwater. "Filler" spots all over it.


Maximus

fyi
 

Bravo One XR Sport Master drives. Modeled after the race-proven Dry-Sump Six drive gearcase and designed for boats capable of reaching speeds in excess of 90 mph, the Bravo One XR Sport Master features a Precision CNC machined, satin-finished aluminum housing. The new gearcase, available in standard and short shaft models, features a parabolic-shaped torpedo, which reduces drag and increases blowout. The four-slot, low-water pickup design reduces the ingestion of weeds and other debris. Standard and short shaft Bravo One XR Sport Master drives enhance overall boat performance in select applications.

MAXIMUS
 

I believe the filler spots you are referring to are typical of any casting with internal cavities etc. Saw them all the time when I repaired engine gearboxes for commercial aircraft. I would not consider that a "flaw" of any type.

I suppose you would want painted lowers for salt water apps? I guess the Merc sporty is milled from a single billet or is it milled from a casting only? If it's cast it likely has the same plug welds in different areas. If it is a billet unit it sure would eat up profit margins in a hurry.:eek:

well guy's

actually you would put the water pickup in front of the steps so you get clean water. no you will not lose speed from it. my opinion is the merc is better than imco because you get a larger fin witch gives you beter control with such high x dimensions. less chance of spinning out or rolling the boat. the only draw back of the merc is they are a little more pricey then imco but somtimes you have to go the extra buck. if you don't need the shortie drive then the cheap way would be to just buy the cases and swap gears. no more expensive then a set of spacers or a set of good props.

remember these are my opinions and the numbers i shoot at you guy's are from my testing. i can't gurentee it will work on all boats but you don't know till you try.

Treadwell

you can get the gear case with no water pickups on it like the speed six. this is the best case because there is no disturbance to the prop. as far as staying cool don't use a drive shower they rob speed and don't always work. run a line from your motors to spill over top of drive. thats all you really need. i race for an hour straight and never use drive showers. they were created because years ago the drives were failing from heat related problems. today you have stronger gears and better synthetic oils to keep drive cool.

Treadwell
P.S.
if you have an older boat still use drive showers