f242

Hi!

I need to discuss the dimensioning of the sea strainer (really the whole cooling system)...

Some prerequisites:
My setup is a (not yet) built BBC, 598cu ~850hp
Drive leg water pickup outlet = 1-1/4"
Thermostat input = 1-1/4"
Seawater pump i/o = 1-1/4"

I'm concluding that a 1-1/4" system should be adequate...
As a rule of thumb the cooling should be 1/10 of engine hp in gph, i.e. 800hp equates to 80gph. The source of this statement I think I read in this forum. I also combined/"verified" this fact along with the fact that most mercs are dimensioned @ 40gph and most engines are about 3-400hp.

Checking data on a 1-1/4" connection strainer tells me it will flow 37,6gph... (The 1-1/2" flows ~50gph). Well known brand and not a nische product.

The (jabsco)pump is rated 20gph @ 1500rpm. Probably too simplistic but this should mean that it flows 40@3k, 60@4,5k and 80gph @ 6000 rpm..
In other words- this should be sufficient... IF my logic about linear flow vs rpm holds true... (Pressure increase may of course affect this but I chose to pull that out of the equation for simplicity)

Flowing media is subceptible to "resistance" much like current in a wire which leads to "pressure drops"(compare voltage drop).. i.e. the pressure rises as you try to push more volume through. I've kind of not taken this into account at all and just chose to blindly trust the factory dimensioning that a 1-1/4" tube is sufficient.

I can't get my head around the low number on the strainer and it is hard to find any data on other strainers for comparison...

For example Hardin SS strainers is connected with 1" connectors and is sold as "performance"... How much do they flow? What happens pressure-wise when pushing through a 1"?

Anyone with data on strainers? Hands on experience rigging high performance engines is also helpful since it will tell what works and not, theory put aside...

To sum it up in one sentence; what strainer should I chose?

Maybe I should build a larger tubing system due to pressure/flow issues in between the endpoints?

Griff

I'm pretty sure your flow numbers and math are way off or you are not taking water pressure into account.
https://www.slideshare.net/raju175/w...low-pipe-sizes

f242

As I wrote, I did not take pressure into consideration... That's probably why I can't seem to conclude on how to proceed..

Good chart though. It seems like 1.25" hose/pipe can take about 50gph without pressure going through the roof...(and a suction pump renders useless).

I'm open for any suggestions and corrections to get to the final result; a well dimensioned and thus working system.

Crude Intentions

Any decent strainer will work. Don’t overthink that part.

underpsi68

I'm not an expert on this or anything for that matter. Your flow rate is definitely off on flow of a stock merc sea water pump. I believe the pump flows 50-60gpm- yes per minute, not hour. My Hardin sea strainer has no problems keeping up with demand.

f242

Oops... I meant gpm on all numbers, a bad typo/brain fart...

I'm not trying to overthink really, just want to back what I install with some scientifical facts. It's easy to for example chose a larger diameter of piping, if that is what the numbers tell...

It feels kind of stupid to put a "factory" cooling solution on a high end engine... It becomes a risk by just guessing.

An engine puts out a lot of heat... A 800hp engine doubles the dissipated energy from a 400hp. All merc original systems are designed to cope with up to around 430hp.

How does the cooling system look on a merc racing 700SCI or 860? That would be of interest as someone has done the math behind that solution... Anyone with insight in such installations?

1MOSES1

Do you have a bravo setup and pulling water thru drive? Or low water pick up?

If it’s bravo setup, have you seen the the water fitting on the transom assembly. The neck of it is less than 1”. It’s probably closer to 0.75”. Kills me why boats have 1.25” lines when the pinch point is the transom fitting.

f242

I've noticed that on my bravo also... . This time it will be replaced by a big fat NXT drive. As a side note I can't find the water intake on the drive. 😬. It has a hose fitting on the inside of the transom so it will most likely draw water from somewhere but there are no holes at all on the lower...

I will install a thru hull pickup in tandem. Better safe than sorry as on a previous boat I had troubles with bravoitis. If anything obstructs the intake, overheat is just seconds away(mabe a minute)...