Honestly, unless you REALLY like the look, need the hatch clearance, and/or dont care about lost space on your sun pad, there are probably better ways to get more cold air in there.
Check in some aftermarket boating catalogs. I've seen the triangular shaped vents that could be easily added to the side of the hull above the rub rail.

Well how about this for a reason......
A 502 Cu inch motor, turning at its 5400 rpm WOT sucks in 22590 cu inches of air every second. Thats a cube of about 28" x 28" x 28"!
A four stroke motor running at 5400rpm fills all 8 cylinders with every second rotation of the crank. So 2700 times per minute is 45 times per second. 45 x 502 cu ins is 22590 cubic inches. Then add some air going through the motor compartment to transport the heat from the block and the headers, and you can start to see why cracking open the hatch at speed (apart from being dangerous) can add a mile to your top end on a warm day. A hood scoop could bring the same improvement......

welllllllllllllllllllllllllllllllll..... yes and no. certainly your arithmatic is correct... but the motor is doing all its breathing thru the two annular openings in the throttlebody ( on your 502) so, in theory, all you would need is the same annular openings to atmosphere anywhere in the engine comp for them to breath... so as long as thats the case, you aren't going to pull a negative pressure in the engine compartment... but thats not what we are talking about... your idea of cracking the hatch is exactly correct... i did that on my gun in the very beginning... dropped it on a couple of pieces of 2x4 as a test and everything got so much better so fast it was scary... and thats when i started working out how to do it for real.

my belief is that is is stricly a fresh air and air temp issue... get lots of air in and get lots of air OUT. keep the intake air and engine compartment as cool as possible and you are way way ahead of the game.

That's the great thing about the panel. You can't tell it's even there. It is part if the upolstered pad. You can walk, sit or lay on it without even knowing it's there.


Darrell.

Let's say I crack the hatch (mine splits down the middle) for a full throttle run. Will the hatch stay in place? I figure that if I notice any gains during a back to back runs, that the motors need additional air?

You are quite right about the fresh cool air issue, and also correct with the observation about how small the intake opening on the motor is, whether its a carb or fuel injection. However, now work out at what velocity the air enters the motor. It is way way faster than the boat speed, for example. The air going in through the air intakes has to speed up ALOT.
This acceleration causes heat and uses energy.
Then there are the aerodynamic qualities of the boat itself, that come into play.........

again... yes and no.... that acceleration you note happens on the other side of the venturi and is a naturally occuring phenomena. and all the other issues you note are correct and a direct result. which is why a scoop is generally preferable to no scoop in those applications where you can get away with it for whatever practical reasons. a positive pressure on the intake is always better in some degree then anything else for all those reasons.

my position would be this. in a race application or a boat where performance is everything and practicality mean nothing then a hatch mounted scoop will have to be the correct solution.

for the rest , then getting LITTERALLY as much fresh air moving in and out of the engine compartment by conventional means i.e. much larger side mounted ducting similar to stock but scaled up as much as the boat allows along with sufficient fan driven exits will bring big gains in every sense. that hinged panel deal is a great idea.

on another note, i figure a lot of the blower motor issues that show up here are related to this issue and i don't think i would ever do a blower motor without some direct outside scoop or air intake.... but thats a whole other story and not an " apples to apples " deal.

For example if on August 29 at 4 pm on a lake at 915 ft elevation the air is 90*F the dew point is 68*F and the altimeter setting is 30.1"Hg the relative hp will be 95.4%. If the air in your engine compartment is 120*F then the relative hp will be 92.5% or 2.9% difference. If the engine compartement air temperature is 165*F the relative hp will be 88.6% or 6.8% difference. If your engine hp rating is a 525hp then at 95.4% relative hp at the 90*F that would be 500.8 hp or at 120*F 485.6 hp or at 165*F 465.1 hp. The 120*F is a stabilized real temperature for my engine compartment with hatch closed and 90*F ambient. The 165*F is a real engine compartment temperature for twin 496HO engine installation fit in there with a shoe horn.

There are two choices. Try to fully ventilate the engine compartment so that the engine sucks ambient temperature air from there or direct feed the ambient air to the engine intake. My opinion is that the latter will be more successful assuming CFM air flow capability is equivalent to the engine's draw.

Ok, just for the fun of it, lets work it out.

The total surface area of the throttle body opening on my 525 is about 8 square inches (2" x 4").
The 22590 cu inches of air that it sucks in, will have to be doing 2824 inches per second, or 169425 inches per minute, or 10165500 inches per hour.

Thats 847125 feet per hour.......which works out to about 160 mph........double the speed that my boat is running at. However, as the air intakes on my 25 Outlaw are at least double the size of the throttle opening, the amount of air that the motor needs will only have to enter the engine compartment - at boat speed.

So, you are quite right! The scoop isn´t neccessary! Twins and blower motors might need much more air, but it still doesn´t appear to be the major factor.

I´m fitting a hood scoop to my Outlaw, but partly because in my opinion, it suits the Factory 1 race look of my boat.........
and just try lying ACROSS the sunpad of an offshore boat anyway....you just roll straight off:D :D :D :D :D

close but no cigar. your math is based on the assumption that the area of the scoop only double the area of the throttle opening. in fact the scoop area is routinely 4 to 6 times the throttle opening. that is a linear offset ( as you note) so the speed necessary to equivilancy would only be 1/4 to 1/6 that of the nominal air inlet velocity to begin to generate a positive pressure.

i did a lot of testing with airboxes sealed to the intake and pressure sensors and the results were always very interesting and the degree to which scoop placement was important was just amazing. but equally so is the effect of the inlet tract temp... before the extra venting i believe i could litterally have fried eggs on my intake manifold when i opened the hatch... post venting i can lay hands on it comfortably after wot for 15 mins.... and the improvements in all respects were dramatic.