Merc uses straight walled hose from the transom to sea pump, and from the sea pump to the coolers. Why do they use flared hoses at the thermostat housing to exhaust manifolds?

Here's a pic if you're not familiar with what I'm asking about. https://www.marineengine.com/parts/m...mostat-housing

I'm going to be doing some custom water hose routing to get around my clearance issues with the alternator and CMI header cooling. The thermostat barbed fitting is 1" OD and the hose reduces down to 3/4" ID for the remainder of the length, except when it attaches to the exhaust manifold fitting which is 1" OD. It's clear to me that they are maintaining a 3/4" ID opening the whole way from thermostat to ex manifold.

The question is WHY?

Am I going to be introducing some type of drop in water pressure by installing straight wall 1" ID hose from thermostat to header cooling bar?

Or should I be using 3/4" hose and heating the ends up to force them over the 1" OD barbs?

Looking to order the parts i need, just don't know which way to go?

Thanks for any input you guys might have.

I've always wondered that too, I simply use 3/4" hose, your assumption is the same as mine: pressure regulation

Air flowing from a small to a larger tibe will drop pressure a bit and slow down the flow.

Water will just slow down as it enters a larger tube, but not enough to really notice...as it has velocity, and in this case the increased diameter is not large enough to affect it.

What's even more interesting to me is the feed hose is 3/4" ID and the 4 fittings feeding each cyl on the headers is 3/4" also. How any water makes it to the back cyl is beyond me?

because if they didn't they would not be able to sell them for 4 times the price of a regular hose bent in that shape. not really sure that is the reason but Mercury likes doing things to make you think you have to buy just from them at inflated prices.

Well, it did work on me. I have been holding onto a back up for every hose that came on my set up.