Regarding bravo drive losses, I've dyno'd many engines on the engine stand, put them in the boat and redyno'd at the prop. In every instance there has been at least 10% power loss. As you can see in my article the 496 HO made 387 hp at the prop. Pulled it out of the boat and put it on the engine stand and made 431. I've dyno'd 525s at around 510 at the prop. I've also dyno'd several Raylar 496's(with headers) and they all made right around 490 hp at the prop, 540 at the flywheel. These runs are all using the standard correction factor as opposed to Merc using the SAE correction factor which is more conservative.

Hope this helps

Bob Lloyd
Full Throttle Marine

After reading what Ray posted, I would say the published info is wrong

Bob, thank you for the post and information.

Thank you for looking up my information!

61 was around 4900-5000 with the stock 496HO, no E-box or shorty.

With the Mirage Plus 23P and the new set up 560HP (@ the crank), E-box, and shorty I was getting 5400RPM and 66MPH. Same set-up with the labbed B1 26P I was getting 4900RPM and 66MPH.

I saw an article recently regarding driveline losses. Mercury Marine said the I/Os lose about 13%, and the V-drives about 8%.

Michael

There is no horsepower per mph. It is completely non-linear. On a planing V-hull, horsepower goes up by the cube of the speed.

Let's take the above figures.

425 * .87 (13% loss) = 370 hp at prop

New engine

560 * .87 (13 loss)= 487 hp at prop

(487/370)^.333 x 61 mph = 67 mph

You are only down a mph. Of course, you could do some tuning to get it up higher.

Michael

The energy (hp) required to move anything is proportional to velocity squared, not cubed. E=1/2 mv^2 from my physics book. 70 would probably be top speed assuming the dyno numbers are correct which may be a huge assumption.

See bams website: http://www.go-fast.com/boat_speed_predictions.htm

The slip before and after the mods is 15.5%. That looks like a place to after some efficiency.

The way I look at it is this IMO.

You have a given boat weight, beam, hull design, HP/Torque. This particular boat has a given MPH (GPS/Radar). As an example, the boat does 61mph with 400hp and you proceed to add 80 HP and now your gps reads 65 mph in identical water and air conditions. 80/4 = 20 which tells me that this PARTICULAR boat requires approx 20 HP to go 1 mph faster.

On a related math topic.

I have to say, I find it funny that math formulas are always accepted for so many aspects of a boats performance build such as degreeing a cam, out drive gear ratios, comp ratios, ignition advance and percent of HP loss due to outdrives.

However, when it comes to Carb Cfm formulas so many look at them as fuzzy math? Even Holley has a carb selection utility which asks for Cubic inches and maximum rpm to formulate carb recommendations.

Sorry for getting off topic.

Been following and trying to read this post and threads carefully but I have to say something here. At Raylar we get 560HP and about 600lb/ft. of torque at 4100 rpms out of a perfectly stock 496 with our 103 kit its small camshaft and a set of good headers so I think you are suffering from two things on the engine.

1. That stock intake will not make enough power to do what you want, the low torque number shows that. You're down about 50 lb/ft. of torque from where you probably should be and believe me when I say torque loss in a boat means speed loss!
Lots of 496 owners want to know why they need and why Raylar needed a better than stock GM intake when going for bigger power, this is exactly why! Engines are air pumps, if you can't get it in your not gonna get it out with good power!

2.The air fuel ratios should be to fat at 52psi fuel pressure especially with that stock intake that can't flow the air your head, camshaft, header combo needs and wants.

Kieth has chimed in here and his Raylar HO600's like many other Raylars make about 640-650HP on a reworked 496 block and its been done many times over. You are missing some badly deserved power here ( You paid good money for it!).

As for the hull and props, I leave that assistance to others here who are infinitly more qualified to opine than I !

Best Regards,
Ray @ Raylar

QUOTE: texasrpm:On a related math topic.

I have to say, I find it funny that math formulas are always accepted for so many aspects of a boats performance build such as degreeing a cam, out drive gear ratios, comp ratios, ignition advance and percent of HP loss due to outdrives.

However, when it comes to Carb Cfm formulas so many look at them as fuzzy math? Even Holley has a carb selection utility which asks for Cubic inches and maximum rpm to formulate carb recommendations.

Sorry for getting off topic.[/QUOTE]

Not to beat a dead horse but s.b also known as Scott /CFM has pretty much explained this to you on the other thread in fairly good detail. You might not Know this but he IS a expert at airflow and owns a company that does cnc development work on flow technology/manifold work, carb spacers etc and he is in the know. If you listened to 1/2 of what he has to offer you might be enlightened, Smitty

Drag forces are typically a function of speed squared, making power indeed a cubic function, planing boats often do a little less than cubes due to the reduction of wetted surface area at higher speed, and reduction of induced drag coefficient at lower trim angle as speed increases. These effects don't get it down to squared though.

A Kenetic energy equation has nothing to do with the power required to overcome drag. Except for applying Bernoulli's theorem which exchanges potential and kinetic energy of a fluid for pressure, which equates back to lift and drag forces.