hoozeyurdaddy

Back to the original post, any exaust change is going to be a increase, (power to weight ratio and exaust flow) just get rid of the cast stuff, other than that, make sure your props are running properly for you, a little adjusting on them can help alot.

stevesxm

the boat came with gills already. please note that the weight is only one small component of the total drag... thats why that formula is condensed to that point. the real formula has a great many more variables but since we are considering an " apples to apples, those variables fall out ( so to speak). each boat has its own drag profile on that day and moment in time. my boat , for instance will be much slower w/ full tanks then w/ out... why ? because it will sit lower in the water and have much more parasitic drag. thats why just comparing lengths and and weights is not a valid comparison. a bad 24 ' lightweight hull will have a higher drag profile than a well designed slippery heavier 30' foot hull. thats why everybodys gains will be dependent on their boat specifically. but again... having said all that, once you start going really fast... over 60 mph ... the drag numbers get really big really quick. and thats where a lot more power is required... WHICH is not available ( in my opinion) from bolt on bells and whistles. the exhaust is the single real exception to that rule as you point out. they are heavy and they are horribly designed. putting ANYTHING else on actually FIXES a problem and is worth doing. beyond that tho i think its all just throwing money away as the dollars spent per performance gained ( again in my opinion ) doesn't justify it. the single biggest improvement i got on my boat was by increasing the airflow thru the engine compartment by an order of magnitude. that reduced the inlet temps hugely, the inlet manifold temps hugely and stabilized the oil temps precisely.

that bought me substantial performance.... NOT because i was making MORE than the 425 that the motors started out with but because they wern't breathing 150 deg contaminated air anymore and only making 395 or so as a result. all i did was make them work properly.

i just ran your numbers backwards... at 445 the numbers suggest for that drag profile as derived from your base line , a predicted top speed of 76 mph give or take... less than a 2 % error.... thats all you can ask from these sort of rough numbers.... thats 135 hp to go 13 mph faster... or more than a 43 % increase in power required for about a 20 % increase in top speed....

if we assume those sort of numbers are representitive you are talking about 830 x .43 or 356 MORE hp nec to go 13 mph... in my application, or 178 a side.... and if you plug THOSE numbers in and work them backwards into my original estimate of 125/side to do 10 mph youll find them almost spot on....

so... you see ? your observed data supports the math completely.

articfriends

iTrader: (1)

Steve,I have talked to smartys dad on the phone and the difference between his 34 ft boat and most others is it runs around 90 mph with stock 502's because of the light lay-up and race hull design so my point without getting into co-effiecient of drag or anything else is that it is a extremely fast boat to begin with that responds very well to small changes unlike most other boats so anything is possible,Smitty

stevesxm

of course. absolutely correct. but the drag profile for THAT boat will remain consistant for that boat. you could find their drag number using their 90 mph numbers and 502 power as it sits and very accurately predict what their boat would do with whatever power increase you gave it. thats all im saying.

not trying to sell anybody anything. just keep the mythology apart from the science.

Pat McPherson

Hey guys,
We can go around and around in circles. I think in the end we agree to some point. Whether we’re talking theory or practical experience, adding a little bit of power is going to push a boat faster to some degree.
Obviously making a small upgrade like better flowing exhaust (20-30HP) is going to help more on my little 24' Super vs. a 38' Cig. In this case, a 32' Fountain, probably somewhere in the middle.
The man asked what to do to gain power while leaving his engine basically stock. My answer is still the same; change the exhaust. Now if the 2-3mph increase is worth 2K per engine, that's up to him to determine.
Happy Boating.

onesickpantera

iTrader: (1)

Actually you MULTIPLY your new projected hp by your drag figure since you are backing it out.

I was going to stay out of this one since it was already way off-topic now but I couldn't resist trying the formula.

My boat runs a consistant 70mph with a stock 502 mag. I used 445hp since this is suppose to be the crankshaft hp #. This gave me a drag factor of 2398.792. With 500hp my boat would have a projected top speed of 72.76. If I use pshp I get a projected speed of 72.82. I would hope I would gain more that 2.6-2.9 mph! I have used the speed calculator before and have found it incredibly accurate IF you use accurate data to get your constant. Speed calc says 74mph using crank numbers and 75 using prop hp numbers.

Regardless this debate got started saying a boat needs 18% increase in power to pick up 3mph. This is simply not true "across the board". Some boats it is, but others it is not. And more importantly not true for the Fountain in question. Which is why we have these formulas and calculators.

I don't know what his fountain weighs or what the current speeds are. But I would guess that 60-80hp should get him 3mph. However, will exhaust give you the hp needed? IMO no but as Pat stated what else is there if he doesn't want to get into the engine?

I'm out!

stevesxm

no. hp = drag * v cubed.

if you know the drag and you know the v cubed, your NEW v cubed will be the NEW hp DIVIDED by the same drag number.

and what the number show absolutely is that getting the drag down in whatever way you can is a lot more effective at buying you speed then just adding more power. and a lot cheaper generally. for instance... i just stopped being stupid when i gassed up. i started by always filling... duhhhh boat slowed right down. i started doing 1/2 fills.... boat came right out of the water and drag went down and everything was much better. simple and cost me nothing.

none of these calculations are aimed at having a 0 % margin of error. they are aimed at the decision making process.

this is what i have

this is how fast i go

this is how fast i WANT to go......

is it cost effective to make the power to do it ?

if the numbers say you need 50 hp... then it doesn't matter whether its really 55 or 45.... all it means is that it is an easily achievable number. if the number ( as was my case ) was 125 hp per motor or more, then that told me everything i needed to know about the cost effectiveness of that decision.

onesickpantera

iTrader: (1)

You originally had to divide velocity cubed by your horsepower. So, you have to MULTIPLY to work it backwards.

70mph x 1.46 = 102.20(Velocity)
Now cube that number = 1067462.648
Now divide that by current hp, 1067462.648/445=2398.792

To work it backwards using the drag number

MULTIPLY, NOT DIVIDE your projected hp by the drag number, 500x2398.792 = 1199396.233
Cube root of 1199396.233 = 106.24
Now DIVIDE Velocity by 1.46 = 72.76

It's a decent formula but I'll stick with the speed calculator. More accurate and MUCH easier! LOL

stevesxm

your algebra needs work.

if you divide by hp you leave nothing on that side of the equation and end up having to move the drag number to that side as an inverse function.

hp ( 455) = d * 70 cubed ( don't even bother going to fps...)

455 = d * 343,000

455 / 343,000 = d = .00132

so working backwards from KNOWN d and hp

500 = .00132 * v cubed

DIVIDE 500 by .00132 to get V cubed = 378,887

cube root 378,887 = 72.04 mph

onesickpantera

iTrader: (1)

Ahhh, my bad I read this incorrectly:

I did the opposite and divided the hp into the total number(343,000/445=). And FYI since we are using the same numers you get the same answer no matter which way you divide them.

Like I said, I'll stick with the calc since it's faster and more accurate! But that is a nice formula and it was nice to strain my brain!