Seafoam, Stabil, Techron and products like it are good at cleaning the internal passages of carbs and bowl areas which can get clogged with varnish or deposits over time but it's not a total cure...some mixture needle passages may still have blockages or crud in them which require further disassembly and cleaning...or are partially blocked by sediments or hardened deposits which Seafoam, etc.. does not dissolve.

I'd suggest using a double or triple dose of product per gallon...usual mix is 1 ounce per gallon but see label instructions,,, and run the engine for a while at medium speed to clean out passages, Use some spray carb cleaner and clean out the throttle plate and bore area of the carb and linkage areas. Replace the fuel filter is there is one and drain/clean the fuel tank of any sediments....drain, remove and clean the carb bowl as well if possible...some bowls have a drain plug you can remove to drain out any sediments in the bottom which can get sucked up into the carb fuel intake and cause rough idle or poor performance.

If all those measures don't help, you may need to have the carb rebuilt or replaced.....or possibly have the idle mixture screw and low speed/high speed adjustments checked.

I always recommend using a funnel with a built in fine mesh strainer or a separate strainer to strain out any dirt or sediments when refilling the gas tank which can later get brought into the carb and clog passages. Products like Seafoam and Stabil can help prevent fuel from getting stale and varnishy if left sitting for a few months but a better option is to drain the fuel system/carb or run the engine until it quits and store with no fuel in the system to prevent buildup of varnish in the carb....fill with fresh fuel mix every season.

In most marine heat exchangers round tubes are used as they are more readily available. They also allow the greater surface area for heat transfer. The external shel is also usually round as this allows for the least internal wasted space. If the vessel is pressurised then a round unit allows for a greater pressure to be utilised. This is best seen within marine boilers especially in the super heated steam section.

I have a 150 evenrude 1989 model and it runs great on top end but it shudders and almost shuts down before finally taking off.?

Heat exchanger, why cylinder and not square form?

Why are marine heat exchanger build in a cylinder diagram rather in a square (box) form? Please support answer with facts if possible.
Thank you in advance