Calaloo -
Proper Heat transfer depends on several things:
1. mass flow of water in the Hx --- the rate of cooling water
2. The temp of the inlet cooling water and the fouling characteristics of the Hx
This year again on the Ches. barnacles have become a problem in most areas, sometimes affixing and growing on and IN the inlet through-hulls especially (despite the use of appropriate bottom paint IN the entrance of the through-hulls); thus restricting flow
The FIRST assay of the problem with Hx is always to check the water flow. Go to your engine shop manual (or contact Bayshore Marine in Annap.) and look for the "specification water flow vs. 'target rpm'". It will be somewhere near 3.0 gallon per minute at 2400 rpm but verify to make sure for your particular engine model. Take a bucket and stopwatch and run the engine at the spec. rpm and measure/catch the flow as its coming out the exhaust through-hull - do this 3 times, and average the results.
1. If the output flow is less than spec. (±2%), then check for water flow resistance/partial blockage, etc. Start with the inlet through-hull and assay each and every component (in sequence) through to the injection elbow. You can 'momentarily' disconnect each component in the series to assay with the bucket/stopwatch.
2. If the output flow is within spec. (±2%) then consider that fouling is a probable cause - and then consider to 'de-scale' the engine and exhaust manifold, etc. with an appropriate boiler descaling compound ... (Rydlyme, etc.).
FWIW - Im located on the upper-Ches. and have had barnacles periodically blocking (inside) my raw water inlet since July. Micron CSC, etc. is no longer adequately protecting hulls, etc. on the upper Ches. due to the heavy 'slime' deposits. Most there are changing to Micron-extra. These are 'barnacles' and not 'zebra muscles' (yet, although the are moving down the Susquehanna towards the Ches.). Up here, the barnacles are getting worse and worse every year ... probably because the 'slime' deposits are getting worse and worse due to 'stagnation' of the water and extreme high turbidity - the Ches. is not 'turning over' due to the prevailing westerly winds instead of prevailing southerlies and SWers ---- hence SLIME then barnacles. Ive had barnacles growing many 'feet' up & well into the inlet cooling lines of my engine, refrig., AC ... and Im having to periodically use Rydlyme in most of the rubber cooling inlet lines to keep everything 'clear' - the damn barnacles are 'way' INSIDE the inlet cooling lines and 'beyond' the stainers.