hp vs mph
 

is there any rough way to calculate the amount of hp it takes to get 10mph more ? does this change when speeds will be over 70mph ? i know there are different variables that come into play . am thinking of pro charging my boat with 502 mpi,(415hp) am running 70mph gps right now and hope to see 80 mph to 80+ mph ,is it possible?

SuperCharge
 

The only way to get to those numbers are either build your motor more H/P or Super Charge. You will need at least 600 or better horse. To get that hull over the 80 mph mark,

physics says that power is a function of Velocity cubed (i.e. to the third power) for viscous fluids (air and water)

so the theoretical answer to your question is.

if 415 hp to go 70mph, then to get 80 (14.3% more speed)
will require (1.143 cubed) or 1.49 x power, or 619.38 HP.

This assumes same prop slip/efficiency. so being realistic, you will probably need a little more that the 619. but 619 is the minimum requirement.

you can use this function in reverse too. if you build an engine up from 415 to 500Hp for example you should expect a speed of approximately ((500/415) cube root) x 70 =

74.48mph

Where does torque fit into this equation? Supercharging has dramatic effects on boats because they add a lot of torque, which is needed to push through the water.

You will definately gain more than 4.5 mph!

Where's Tomcat? he's the man to ask.

Screw the math. Old rule of thumb, not considering hull variables, is 15 ponies per mph gained....Anyone ever hear anything different?.....:confused:

Rick

Here are two formulas that are suprisingly accurate.

New Speed = Old Speed X [the squareroot of (New Horsepower / Old Horsepower) ]

or

New Horsepower = Old Horsepower X [ (New Speed / Old Speed) squared]

It takes alot more horsepower for each mph.

Note - These formulas are simular to the ones that Rambunctious stated. However, he uses cubes and I use squares.

Good Luck!

Clay,
I'll have to agree with your method above. I've also used that formula before and like you said is surprisingly accurate. I think the hyperbolic(square) function is more accurate:

Horsepower is what we're all accustomed to using as a determinant in our speeds, however, in essence what you're really interested in is how many pounds of thrust you can translate that horsepower into.

Maximum speed will be achieved at the precise point when the thrust produced by your engine equals drag(hydrodynamic=hull, outdrive & aerodynamic=hull).

And drag(hydro, aero) is a function of velocity squared.
:)

There you go. That's the first explanation I've heard that sort-of takes into account hull variations...I'm jottin' that down dude...;)

Rick

Just stick a blower on it and see what happens. Remember you will need a bigger prop too. Go to 650 HP so you get over your 80 MPH. mark.

Put a Batman jet on the back.:D

Clay, I think We're in vilolent agreement :D

thrust ( or force) IS a function of velocity squared. Power, by definition IS force x Velocity. therefore HP (power) is a function of velocity cubed.

1 horse can lift 550 lbs at 1 ft/sec. that same horse can pull 1100 lbs at .5 ft/sec = both are one horsepower. the force is doubled in one case, while it's velocity is halved.

Torque x rpm is the equivelent to force x velocity. 600 ft-lbs of torque at 3000 rpm is 1/2 the power as the same 600 ft-lbs turning at 6000 rpm.

through our boating experience we have come up with "emperical calculations " that approximate these physical equations and in many cases are more accurate because they take into acount efficiencies and such that we choose to neglect in the physics calcs. things like prop effeciency chances, trim angle, aero lift in cats, etc.

ie 15 hp per mph. for an example. but this estimate applies only around say the 400--500 hp range, or 60 mph range. At 70-80mph it is a difference story because the drag is not linear.

(61mph/60mph) cubed = 1.05
1.05x 300 = 315 !!! there's the 15hp needed.

(81mph/80mph) cubed = 1.037
1.037 x 600 = 622.
22 hp more from 600 to get 1 mph


any of the calc mentioned in this string will give you an estimate to stat with. I'm an engineer so I like to try to take the physics approach to understanding this. hopefully saving my money by reducing or eliminating trial and error. One thing that is evident here . It takes a lot of HP to gain a little speed.


Where's Tomcat. He always has insight

Yes Rambunctious. We are in close agreement. However, my calculator does not have a cubic root, only a square root. So, I use squares! :D

Your math is correct. But, boats never understood math! ;)

It is all a close guess. As you say, a starting point.

There is also another formula: It take $1000 for each 1 mph on the water. How fat is your wallet? :D

20 degress last night in Dallas, TX. Will winter ever be over? :mad:

Calculator? Who needs a calculator?....:rolleyes:

thanks to sharky
 

http://www.boatramp.com/prop_applet/...erAppletG.html

hey clay, you can always consult the base 10 log tables for roots!!!
:p

Hey 26Sonic - Asking the speed question; and everybody's answers are right in one way or the other. There are so many variables that empirical equations based on many, many tests by Mercury and others are the easiest way to go. If you want a true calculation based on physics, you better save up and buy Hydrocomp software for thousands of dollars. No offense Rambunctious; I have to figure it out by first principles too, or I don't really understand what's going on.

As many of you know I wrote a program based on empirical equations, that not only predicts speed, but optimizes the prop choice to go with that speed and the actual dyno curve for the engine. When you calibrate with your own test data, the program will create the hull coefficent and slip for your current hull/drive/prop and makes pretty good predictions. The only thing I don't like is that the empirical equations are based on tests where only estimates of prop HP were known, not the actual thrust of the prop/drag of the hull. That means the program does not incorporate prop efficiency curves or drive heights and becomes less accurate when you deliberately put the wrong pitch into the program. I'm working on it. Check out my propeller efficiency thread and the Bravo vs. Hydromotive thread for more info on that.

I'm also working on getting this program into a form that can run on a website. It looks like a java calculator with interactive graph is the best way to go, just costs more money than I was planning to spend on a software "toy".

Anyway, down to business. Based on 26Sonic's test data and a few assumptions, I ran his boat with a 502 MPI and then supercharged it. The results are shown in the attached graph. With a true 700 HP kit, he will reach about 87 mph and spin a 30" pitch. I don't know what prop you are using now, but if it's a three blade and blows out a bit now, you will need four blades to handle the power. If you can give me data on weight, prop, drive ratio, max RPM and expected HP, I can finetune this number a bit.

I have a nice, little program that will give you an idea of the prop pitch that you will need for any given set of data...

WinProp

You can either run it in place, or save it to your hard drive.

The biggest "unknown" is prop slip. However, if you plug in your current numbers, you can calculate your current prop slip. Now that you know your approximate prop slip, you can determine the proper prop pitch (and gear ratio) for many rpm and speed combinations.

Of course, this all assumes that you have the horsepower to turn the prop at the rpms that you specify. For that, please refer to my previous post! :D

Oh, and one more thing...

26Sonic, Go with a Whipple Supercharger

I have one, and I LOVE it! :D :D :D

Keep it up guys and you will allmost be ready for my ON-BOARD-DYNO with real time HP and Tq readings while you drive. 0-1000 guages are being made now.

Wow Marty! That sounds awesome!
So, how's it gonna work...Will it be computational hp or measured. I guess you only really need torque readings to derive hp. Is this going to be a strain guage set-up on the shafts to measure torsional displacement? :cool:

Exactally, it will measure shaft Tq with a strain guage and convert it to HP and then output it in a 0-5volt signal to a guage, computer or data logger. I am having Norgskod guages made now. Right now it is for a Stellings bravo extension box. There is no room in the stock mercruiser/bravo one setup.
I am one you thinks and agrees with TQ being the critical factor. HP is only a calculation of TQ. TQ is what is measured and what matters.

Thanks Marty! Your project has me intrigued. Where are you pullin' the readings from? Are you using old-fashioned piezoelectric strain guages or some kind of strobe set-up.

I remember from working with turbines some setups we would use photodisplacement cells to measure torque on high-speed totating shafts. Another method would use a scribe line paralell with the shaft and we would use a diode(kind of like a timing strobe) to measure when the line would pass by the receptor. Naturally, it the shaft was torqued one end of the line would pass by sooned than the other side. By measuring the time difference and knowing the rpms you could determine torsional displacement.

This all sounds really Cool! Good Luck!:cool:

Ya gotta Whipple it! Get the torque pick up all the way down the curve (I believe). One of you math guys ought to figure out the payback time considering the gas saved at cruise - considering most of our time is spent at cruise any ways. Dustin, have you done any calculations on this? This will be especially important for me as my cat needs about 75mph to really get up on air and that speed is pushing the rpms pretty high pre blowers.

p.s. Cool thread, thanks for the info guys.

CM? did you consider a trust load type sensor? are you trying to measure drivetrain power output net from the outdrive? or power thrust being delivered to thrust the boat. One would be output of powertrain. the other the net power after all inefficiencies.................... I guess both are useful.

Do you pick up the rpm from the motor to calculate the power?

sounds cool !!

Wow,
you guys are WAAAY to smart for a dumb designer like myself !!:D

Clay ,
I tried both of your formulas and they were VERY close.
My real new speed was 80.1 and the formula said 81
My estimated HP was 675 the formula said 650.

I should have paid more attention in math class !!

tom cat speed vs hp info
 

tom cat here is the info you requested to fine tune my hp to speed . boat weight 5500 with full tank of fuel, 23p mirage plus 3 blade lab finished, 1:50 bravo 1 drive , max rpm 5000 wot, expected hp 650 at crank with procharger @5psi.

26Sonic ,
With that much HP and that gear ratio you are going to be WAAAAAAY higher than 5000rpm at WOT with a 23 mirage.
You will probably need a 25 Mirage plus or a 26 Bravo 1, even with the B1 you will probably be around 5400-5500.
I'm making between 650-670hp and with a labbed B1 I'm spinning 5800rpms.

Hey cobra marty - Good for you on the dyno, good luck with it! But once you get it working and know your torque at a given RPM we are going to take a look a some prop efficiency curves (power out/power in) and pick/lab a prop to deliver more thrust at the same RPM and same torque/power. It's thrust that matters, we just have no way of measuring it directly.

Hey Rambunctious - Propeller efficiency is the last frontier! There are research labs in the world that have propeller test tanks instrumented for both propshaft torque and propeller thrust. With these two measurements you can calculate power into the prop and thrust horsepower out of the prop, therefore propeller efficiency curves. If cobra marty does add a thrust sensor/washer he will be able be able to do the same thing.

Some of you guys are too smart out there. I am using a piezoelectric strain guage and some fancy electronics with wireless connection and conversion to 0-5volt output. This will measure the torque or twist of a shaft. The housing will pickup the rpm which is crank rpm. By knowing tq and rpm the electronics will calculate hp. Current applications in top fuel measures up to 10,000 foot-lbs of TQ!! In a car application I have to pick up rpm from a different source.

26 scarab
 

26 scarab the 23p mirage plus labbed is for my current set up non pro charged . i plan to have to
use a bigger prop , not sure what size yet. the rev limit will be kept the same 5150 rpms after installing pro charger .sorry i did'nt give enough info.

26 Scarab, Very close will be +4" from your current prop which at the same rpm will be about 12mph. Might need +6" if youre at the rev limiter now. When going from 3 blade to a 4 blade you need to -1" pitch. So for you 23" + 4" -1"= 26" bravo prop and ?maybe a 28" for more speed but less mid range.
Remember one prop will never do it all. I carry 2 different pitch props and own 3 different pitches. Depending on the load, fuel, water conditions, temp etc I pick a prop.

26Sonic - I used your actual test data, and a 650 HP engine that I built by adding a centrifugal blower to a 502 MPI on Desktop Dyno. Because of the small cam in the MPI engine, I had to use 8 psi, not 5 psi boost. More cam and a bit of head work would help this engine a lot.

Your speed would be about 85 mph @ 5000 RPM with 28" pitch. Going down to a 27 " pitch would still be 85 mph at about 5200 RPM. You would likely be happier with the bottom end with the centrifugal blower by using less pitch. This is assuming a 3 blade prop doesn't blow out. If it does, then a Bravo 28 might be your best bet.

Marty,
I think you meant to reply to 26Sonic, I'm all set with props.
I also suggested those props it will fall somewhere in between a 26 28 B1.

Here's a question though I'm spinning a labbed 26 B1 5800 , would it be better using a 28 B1 and spin it around 5500 rpm ?

26scarab

what does your hp curve look like. if you are still making more power past 5500 on to 5800, then that will probably be the fastest the way you have it propped. if the power curve is flat or falling off between 5500 and 5800, then prop for 5500, reducing prop parasitic losses by slowing a prop down and adding pitch, also it seems there's a reason why the Merc factory racing power (500hp's and such) only cam for about 5200 for reliabilty.

two bottom lines.

1) pitch for your peak power point
2) trial and error to optimize

Tomcat

just tell me your curve for hull power required vs speed fits through the one data point and is a function of velocity
[B]cubed[B], not squared.:)

also, can you plot the power curve for a 330 carb

I am getting my props labbed by john at York and want to specify the work. I am at 4600 right now at 64.9999 with 25's. when he labs them I want to know if I should raise the pitch slightly to taked advantage of the "reduced drag" or leave the claimed "200 rpm gain" because the motor makes more power at 4800. supposedly the 330 are pretty flat between 4200-4600.

Must...........have............data ............:)

Sorry Rambunctious, the equation is:

Hull HP requ'd = weight x (Speed/Constant)Squared

If this was a displacement hull being pushed through a fluid, I believe the cubed rule would apply, but this is a planing hull operating at the interface of two fluids, one of which is 600x denser than the other, so friction at the wetted surface takes over. I believe the drive dragging through the water operates on the cubed rule, that's why raising the drive a little bit can have a big effect. The boat moving through the air is also operating on the cubed rule, but that component must not be anywhere near as important as the friction at the wetted surface.

I know that this equation holds up to 200 mph, because entering the data for a 36 Skater running at 110, 150 and 190, with three different engine packages, produces three curves that overlap almost perfectly. It would be interesting to know at what speed aerodynamic drag becomes important. I know in cars aerodynamic drag begins to exceed rolling friction at 55-60 mph. Whatever its effect it seems to be included adequately in the empirical equation up to 200 mph.

I doubt if you will have any more power at 4800 RPM, but the lab prop will be faster because it will be more efficient. With thinner, identical, vibration free blades a higher percentage of propshaft power in will be converted to thrust HP out. More speed means more RPM. I had a 7.4L - 310 HP, which is a very similar engine, and it went faster spinning a 23 @ 4600 than a 21 @ 5000, so asking them to increase pitch to maintain 4600 might work, I don't know.

The real answer is to port the heads, bump up the cam, change intake and open up the exhaust. See below for more data...

thanks for the reply tomcat.

as long as you are saying that the drag (force) due to hull/water friction is a linear function of speed, then I am in total agreement with you on the sumation of forces. but (i thought) the friction drag is also a function of velocity squared and therfore the power (force x velocity) is a function of velocity cubed.

lets agree, (either way, I reserve the right to get smarter) so I'm not tootin my horn and it's full of poop. i want to understand the details. This is our (my and your) only discrepancy. upon resolution, we will be in "violent agreement":D

can you resend the graph of my 330's to fill the whole screen like a previous post of yours, I can't quite see the numbers.

As far as engine mods, my buddy waterfoul has bought his boat and engines three times over now. Keeps breakin stuff. I like stock for now. ( i am faster than him. he hates that)
Heck, I'm still savin pennies to lab the props! Next, drill out the silent thunder some, maybe new intakes, and upgraded ignition. after that. sell the formula for a newer one with 502's or 500hp's. that's the 5 year plan.

thanks

Rambunctious - I tried to post your graph the same large size, but the board would't accept it. Funny thing is both the small and large size show as less than 640 x 640 pixels under properties. What gives?

640X480 Max. :)

OK, got it figured out. For some reason if I downsize to be less than 640 x 480, then convert to JPEG, the file properties remain at the larger size. Convert to JPEG first then downsize works.

Rambunctious - I'm assuming drive ratio of 1.5 and weight of 6500 lbs. Here's your new graph. How about a pair of HP500s?