More speed by dropping pitch?
 

I have a 2000 Carrera 257 cat with a naturally aspirated 632 Merlin that puts out 901hp @6600. Currently I turn a 30 bravo 6100 @100 mph through 1.50 gears. Power @6100 is 860hp. I'm thinking about going to a "lab" 30 to pick up some r's. What do you think about going to a 28 bravo to buzz the motor harder and pick up the extra 40hp? Acceleration is outstanding with the 30, I would think the 28 would be even better. I have a very high "X" and so far have not found anything other than a bravo that can get me on plane. INPUTS/suggestions???

Hi,

The only way to know for certain is try one. Before you do figure out your current theoretical top end VS actual and thereby determine slip. Then do the same thing for theoretical top end with a 28 and apply the same slip factor. You might find that by dropping 2 inches of forward travel (minus your slip factor) per prop RPM, you won't pick enough RPM's to make up the difference.

You will gain your HP (possibly) but not any speed. You'd think HP would increase your speed, but ultimately it's prop speed, moreso than HP that determines your top speed.

If you need any help with these calculations, feel free to call.
You've sure got an awesome boat!

Steve

jdnca1, You should be much faster with that motor. Your current setup - theo. speed is 115 @6100 so your slip is 13.5% which is high for a cat. Interesting is that at 6100rpm your tq is 740 and at 6600rpm your tq is 716. There is much debate about hp and tq but if you prop for peak tq I believe you will go faster. What was your peak tq and at what rpm? Spinning your bravo drive over 5500 rpm is a great way to destroy it. I would go up 2-4" in pitch or go to a 1.36 drive. Tell us your dyno numbers. My 30' single eng cat does 100mph with a stock 502 efi -supercharged with 725HP and 800TQ at 5100.

cory your buddys bravo drive is ready call me so you can pick it
up.
:p

jdnca1, in my opinion, peak horsepower gives you your top speed. HP is power, or energy/time being output from the engine. Power is also Force x velocity. Now the Force to move a boat is a function of velocity squared .... so the Power to move a boat a given velocity is therefore a function of velocity cubed. this is physics. the equilibrium is the peak power output of the engine offsetting the power requirement for your boat at a given velocity. if you could find the right prop/gear to get to your 6600 rpm, you should theoretically see a speed of 101.5 (4.6 % power increase = 1.5% speed increase (cube root of 1.046=1.015)

Like Cobra Marty said ther is much debate

now for some usefull answers to your question.
Talk to John at York Propellor. he lab finishes props very reasonably and has all kinds of insite into what you should do with your props. He will ask for your peak power at what rpm, boat characteristics (weight, current top speed, current prop, etc)and then suggest what to do with the prop. He typically says you gain about 100-200 (maybe 300 in your power range) from lab finishing. do you want to spin that engine to 6600 for any length of time?. It seems there is a reason that all factory warranteed engines cam for about 5200 rpm tops.

keep us posted.

DYNO Numbers;
Merlin 632; 10.5:1; Full CNC port and polished dart 360; dart tunnel ram; 2X775 Race Demons; Comp solid roller 280/288 @.050 .785in/.750ex 115 centerline +2 Advance.

Torque Power
4900 805 758
5000 803 764
5100 800 777
5200 795 788
5300 792 799
5400 791 813
5500 783 820
5600 783 835
5700 780 842
5800 771 848
5900 765 852
6000 753 860
6100 743 862
6200 730 868
6300 727 875
6400 718 886
6500 711 893
6600 706 897
6700 704 901
6800 694 900
6900 685 900
7000 668 888

Interesting dyno curve. What and where was your peak TQ? Who built your motor? ?Not a boat guy?

Marty;

Peak TQ was 805 @4900. My Engine was built @Hawkins speed shop in Richmond, IN. They build mostly "Big Blocks" for Drag racing, Alcohol and gasoline Blown and unblown. I almost went with a 540 blower motor but prefer naturally aspirated power..its fun to burn down single engine blower power being natural. I also thought the bravo would handle the "lower" torque of a natural motor better. This motor was actually the first "LOW compression" (if you call 10.5:1 low) 632 built by this guy..all of his were 14+:1 motors. I really wanted to stay with 93 Octane. I did a lot of homework to get the right heads, cam, induction, and carb combination to keep drivability reasonable. For example the builder was thinking about "big chiefs", cam duration in the upper 290's, 1050 dominators etc. I felt it was important to keep my velocities higher for idle, hence the 2X775 race demons. The reason I went with Tunnel Ram was to fill up my "blower" hatch. Otherwise I would have run a dart single plane with 1250 Dominator. Long story made short is drivability is EXCELLENT. Idles at 950, 650 in gear, never bogs loads up etc and is very snappy through out the band. I had one problem with the motor and it was kind of a fluke...my oil pickup fell off, the tack weld broke...spun 3 center mains and 2 rod bearings, ...very frustrating. The motor started life @10:1 witha 272/278 @.050 cam on a 114 centerline, similar lift. On the dyno the motor was simply running out of "cam" @ 6000, so I took the opportunity to correct that at the rebuild.

I just feel this thing might come alive @6500-6700, plus I have Merlin 540 on retainer if I scatter a drive. Actually I don't plan for sustained runs at those r's, 45-60 seconds tops.

OK, You need to try a 26", 28", 32" and 34" prop and let us know. Brench racing is one thing but testing is the proof. Let us all know.

jdncal1, i’m in agreement with cobra marty and stevedavid, i believe you need to increase prop pitch. the reason is, i think rambunctious left out a crucial portion of the equation. that is the speed that the prop will need to spin. an increase in prop rpm will mean an exponential increase in required power. the resistance of an object moving thru water is increased by the square of the % of increase in velocity that the object moves thru the water. since the velocity is a squared function it will be greater than the equivalent of a gearing change you achieve by dropping pitch, (a liner function). so all other factors remaining the same, going from a 30” to 28” you would need to spin the prop 7.143% faster just for the boat to maintain the same speed (100mph), since each revolution of the prop would travel 2” less. the power required to achieve this would be something close to (30/28) = (1.07143% squared) = (1.148% power increase minus the equivalent gearing advantage of the 28”pitch which is 7.143%) = (1.07143% total power increase x 860hp) would be 921hp. required to spin the 28” prop @ 4357rpm as opposed to spinning the 30” prop @ 4067rpm. to maintain the same speed. the speed for each boat using each prop is going to be different because it’s the ratio between prop slip and prop velocity that will allow the best results, in my opinion. testing, testing is the only real way to be sure. any way i agree with cobra marty in the fact that with the kind of power you have, you should be going close to 115mph, things aren’t adding up, maybe your tach. is off, or speedo, i doubt it but maybe the timing has slipped a little, something isn’t right, also verify that the throttle plates are opening all the way.

Not to bore jdnca1 with our technical combats but.... i think to some extend we are in "violent agreement". Iagree with Steve david completely that ....

"...You might find that by dropping 2 inches of forward travel (minus your slip factor) per prop RPM, you won't pick enough RPM's to make up the difference. ...."

Dean, you are right that the FORCE to move a prop through the water is a function of velocity squared. Therefor by physics, the POWER required is force x velocity or now a function of velocity cubed. i feel The increase you talk about is the additional increase to "Free spin?" the props blade projected frontal area through the water. the force to move the boat at a specific speed stays the same.

What the power is to "free spin? " a prop in moving water would be very valuable information for the optimization we are trying to find. because like you said Dean is true. it will require more power to spin a prop faster. if this is more that the change in power output of the motor due to new operating rpm, then you didn't gain anything.

If you lab finish, you are reducing the power required to "free spin ?" a prop through the water. this frees up that power to move the boat faster. by moving the boat faster to a new terminal velocity, it moves the engine rpm to a new power point, thus allowing additional speed till a new equilibrium.

for $150 each. york will lab finish and polish props. that still remains my suggestion.

If you have a lot of slip. maybe a larger prop diameter is needed. Here you balance the parasitic power requirement to swing a larger prop with the efficiency gain you will see .

my more than asked for $0.02

thanks for listening

this technical combating is fun right :) what else to do when the snow is still melting.....

Rob
Rambunctions
B.S Physics
M.S Mechanical Engineering

Guys;
I appreciate all the insight, especially the scientific stuff. I'll just have to do some testing this spring with 28's - 32's on radar to see what happens. My guages are all Gaffrig and I have tested my tach with the MSD ignition tester..its dead on. My speeds have all been verified on radar (my Gaffrig reads 2 mph optimistic) Timing is through MSD Crank trigger system, (Verified @32 degrees) Butterflies open all the way. I've actually seen 109 with a 175hp shot of NOS, taking total HP close to 1100. Turned my 30 (not labbed) bravo 6700. So in that respect I think 115 natural is very optimistic to say the least. Maybe a 32 will be the ticket, I'll let you all know. thanks again for all the insight.

how fast we will see wont we??????:p

Anyone ever see an actual propeller efficiency curve for the props we all play with, Mirage, Bravo, Hydromotive? It might help us when we are trying to optimize a setup.

Prop efficiency = Thrust HP/propshaft HP

Mercury says that 80% is about the best you can get and that value varies according to slip(angle of attack). If slip is too low or too high, then the amount of propshaft HP that actually gets converted into forward thrust is reduced. From the one efficiency curve I have seen efficiency dropped to 65% in a fairly narrow range of slip(angle of attack). This would make a huge difference in speed since it is ultimately prop thrust that overcomes the drag of the hull and drive.