The Dolphins are a highly modified version of a German diesel electric submarine. Israel owns part of the design rights to the Dolphins because they are built to Israeli standards its not an off the shelf version which everybody else buys and the best bit is Germany asks no questions and places very few conditions of sale. American weapons come with all kinds of strings attached. There are already 5 Dolphins and the navy wants a fleet of 6 subs you don't buy 6 of one type and 1 of another. Also the US only builds nuclear submarines these days. so there isn't even an alternative because a nuclear sub other than being contreversial because Israel hasn't signed the NPT and it would be much more expensive than the Dolphins and the US subs are designed for deep water operations the Dolphins are designed mainly for use in littoral waters which is what Israel needs. Also Israel was planning to buy the US littoral combat ship as its next generation ship but the costs were to high ( around $500 million each ) even with military aid that kind of money is to much for Israel. Germany makes some good boats and much more affordable than the littoral combat ship. Israels navy has been virtually all either domestic or German for decades.