Boy, is this ironic! I just finished on the dyno today with a 383 Chevrolet small block combo. Engine has 10.2:1 compression, hyd. roller camshaft, 1X4 750 Holley carb, Brodix Mild ported heads. Single plane intake. - 485 ft.lbs of torque and 508 HP. Marine application. Engine was dyno'd on 91 octane unleaded.

What was your cam specs? Which Brodix heads?? Mine is about the same, except flat tappet cam, Dart heads, and dual plane. Same compression here.

Maybe there is hope for my motor!

ok..this I doent get I have 376 hp (188x2) my boat is supposed to weigh 5000..
so we have 376/5000= 0.0752 square root of 0.0752 =0.2742 (rounded ) 0.2742*250=68.55 my boats best is 47.8 gps light 1' chop 1/4 tank 2 people thats by 21..... what am I doung wrong??

Don't feel bad, I'm not getting 77.8 out of my 3400 pound, 330 horse Donzi.

Cause there are WAY too many factors beyond the scope of the ideal model this equation represents. What happens on paper and what happens in the real world are typically far apart when it comes to unknown outside forces acting on the object.

lots of times a single has more top speed than a low power twin.......eg a 29 Fountain with 470 hp HP500 will run 75-76 mph but the same boat with twin 350s making 640hp total will only run 72.......thats right out of the Fountain book, although the twin will out accelerate the single by a signifigant margin......Im pretty sure it has to do with not enough Hp to overcome the increased drag of the second engine/drive

Could someone please provide the derivation of this formula? I'm not getting the relationship here.

Michael

Mid 70's at that weight is going to take a big block. A max effort SBC could get close but it will probably end up costing you more than a BBC.

mrhorsepower1, Specs on that 383? Cam? Rpms? Idle?

http://www.go-fast.com/boat_speed_predictions.htm

This the easy calculator and it says I require 500hp at prop which means I'll need 550 at crank to be sure..SO BBC it is.

http://www.rbbi.com/folders/prop/propcalc.htm

Here's another