[katman2860]

A 1998 Seamaster 21 WA with a 2000 Yamaha HPDI 150 cruising at 30mph burns 7-8gph almost 4 miles per gallon depending on the sea conditions and it is fairly lightwieght to tow

 

[katman2860]

The carbed 200 Johnson is not nearly as economical as a Yamaha hpdi probably like burning 16-17 gals per hour at the same speed and burn about 30% more oil.
The other problem with a carbed engine is ethonal fuels tend make the needle and seats stick together if gas gets left in it without running it for 30-45 days.The Yamaha hpdi is a high pressure direct injection engine which is why it is more economical to run.

 

[katman2860]

Svence The Johnson 150 and 175 up to 2001 are the same block,the 200,225 and 250 up to 2001 are the same block.
Most of the v-20's I have worked on came in at about 3600lbs loaded,not sure about the 4200lbs.
I do know your boat with a Yamaha F150 4-stroke will get a little over 4mpg cruising at 30-32mph