Time for a heater...

I can report I'll loose heat if idle speed in gear is around 650, around 750 the heat returns, and plenty of it.
When I place the core at feet level, no loss of heat occurs at low idle

I imagine with a t return to raw inlet, the still- warm return water is diluted with the main flow, and most these btus dumped out the exhaust, very little return heat makes it back into the engine replacing the exiting water, like having a stat bypass putting cold water into the engine continuously
The orifice could possibly inhibit higher flow the core would see under nonidle conditions.

Well the way I see it is....

TRBenj wrote: Having no real background with thermodynamics, my back of the napkin logic would have thought that moving hot water through the core more rapidly (before it has a chance to cool) would allow the most heat transfer. I would have guessed the reduction at the return would slow the flow though the core down and make it less effective at pulling heat out, but apparently that logic is flawed.

I like your explanation Gary

When I bought my boat my heater feed was connected to a T at the intake like Tom's. The return went into the pipe plug drain of the U. No heat at idle. Put the Y in before the raw water pump. Hugh difference- originally was going to install their optional 12v pump Quinner told me to get the Y. The restriction consists of a 1/8 or 3/16 hole drilled into the 1" then a 5/8 braised over that hole on to the 1". I think it is the suction of the raw water pump that pulls the hot water through the system. That restriction might serve 2 purposes,it stops the hot water from being completly pulled out of the block and slows the flow down so the heat transfers to the heater core?

TRBenj wrote: FYI the key component of the heatercraft Y is the restriction on the return... which is how it retains good heat at idle. It’s fairly easy to make your own tee with a restriction.

Probably not as cheap as Tom’s but I’ve found these to be fairly reasonably priced and effective. Just add a heater tube and one flange for the drivers feet (plus the engine side stuff).

https://www.summitracing.com/parts/sum-991102-1

I’ve done the same as Tom with opposite sex quick connects on the engine supply/return. Mcmaster sells nice brass ones.

Certainly Mark,

If you don't see pictures tonight, I'll post them tomorrow, may be easier than from the phone.

I am still using the original blower since making my first heater in '87.
I think they still make the same brand, tmc! Just found it...

https://arnolds-boat-shop.shoplightspeed.com/tmc-bilge-blower-flange-mount-momentary-12v-90cfm.html

I am still using a core from an 80s fox mustang no air, because that's what I had available (used) at the time.
The first wrecking yard brass core lasted almost 25 years . The latest aluminum replacement I don't think will last very long, but they are inexpensive af.
It suits a two outlet system, one can search a larger core from something popular, newer. Best is short, uncomplicated outlets. if you desire three outlets go bigger. Most anything works!

In the last three years, I've added heater craft pullout tubes, they are reduced now, grab a couple!

https://www.bakesonline.com/heater-craft-h509b-1st-generation-pull-out-hot-tube.html?gclid=Cj0KCQiA0rSABhDlARIsAJtjfCd-t9WFrt_qFXn7fYLneZNKifDG_aFTxvVz84XpDEgEmkz8ZkdwHeoaAvM8EALw_wcB

The box is compact as possible, no more than 12x9x3 outside dimensions, but size the inside to your core of choice. A little foam and weatherstripping keeps all the air going through the core

You will see a couple pvc flanges for the 3" outlets I found at HD, with a portion cut off so both outlets can compactly fit side by side on the face of the box. Three outlets would be similar.

Engine side, I'll add a picture of each gazinta and gazouta

Winterizing, I shopvac extract water
from a hose at the engine then push some af forward. alternatively I've also connected the core with reversed garden hose connections, you can both bypass the core if it fails and keep going, and also bring the whole unit inside for the winter.

Good to learn Tim!
My exposure to the tee must have been flawed or something else amiss

I'll try to take a picture of mine, and also the ground-down street elbow on the circ pump, so one can route the water more like it is on an auto, and get excellent heat at idle by not continuously loose high proportion of heated block water out the exhaust water and cooler water replacing it, as happens with the y