I cannot find a reference to an IH D-90. The IH parts web site shows the D engines started with D-178 or maybe 155 if you go back far enough.
LandRover, in the late 1980s, made a D-90 (Defender Series) that used a 2.5 liter IH diesel, which seems to have come in an NA and Turbo version. Is that possibly the engine you are asking about?
However, in the 1950s Lathrop built a D-90 diesel that was rated at 90 HP at 900 RPM. It was 6-cylinders and 998 cubic inches with a weight of 4,100 pounds.
That does not sound like a sailboat engine.
Do you have any more particulars?
I have a computer program that does a very good job of predicting fuel consumption for a diesel engine. It needs
BSFC for the particular engine (I can estimate this pretty closely knowing the age / build date of the engine)
Propellor curve / HP delivered to the water
(I use a cubic curve for default and it works well)
Max engine speed
Engine power curve (this is only a fine detail for a better approximation)