How can I make part-time work a success?

Question

In all likelyhood, starting next year I will work at reduced hours (at 60%) for some time as a form of parental leave. This is nothing I have to negotiate, the law here being what it is. I did roughly the same thing when my first kid was smaller, and while my boss was generally supportive he's no big fan of his people working less than full-time. I want to solve several problems:

1. The perception that I'm far less useful than when I'm fulltime.

2. The fact that I'm less useful than when working fulltime:
   The projects we work on are usually big chunks, a week worth of fulltime work or more. So me being in the office less does not only mean that I do less work, but that one project will take longer to complete. We also often need to work with people outside the house, when someone has a question for me and I'm not around for a day or two this is an additional block.
   Also, the individual projects we work on are highly dissimilar, so it's hard for a coworker to take over and cover for me. Also, we have to work on stuff as it comes up with about a month or two of rough planning ahead.
   At another place I worked, part time was no problem as an individual task were smaller.

I want to go to my boss and say: "Hey boss, I know some of the work can be difficult with only part of the time. I have a few suggestions to ease the situation a bit, maybe we can talk about it soon so we still have time to make adjustments." But I don't have real ideas right now, where do I find any?

Edit to add
Looking at the answers so far, I want to stress that I don't have to negotiate or even ask for part time. This is parental leave, the law in germany is that they have to give me the time (and that I'm protected from beeing fired from 7weeks before the parental leave starts until it's end).
I also want to clarify that my parttime period will probably be half a year, my position is not so that a new hire could start right away and they would have to hire the replacement as a temp or move him/her to another department so I don't think they'll do that.

Answer 1

How can I make part-time work a success?

Part time work can be successful if both you and the company are flexible.

Job Sharing - have you ever explored the possibility of hiring another part-timer to cover the hours you are giving up? Sometimes two part-timers can equal one full-timer.

Changing Roles - sometimes your current role demands full-time coverage. But often there are other roles within the company (some at lesser pay rates) that don't.

Changing Hours - sometimes you can cover hours that are in need of coverage by working a different schedule. Occasionally covering that need can make you valuable enough to offset the loss in total hours per week.

Taking Longer - if you cut your hours, sometimes projects just take longer to complete. Sometimes that's acceptable.

Only for a While - Sometimes a combination of any/all of the above is acceptable for a short period of time.

Answer 2

In my experience, bosses tend to be more comfortable with full-time, in the office work. The standard 40 hour work week has a long and extremely hard fought history behind it. However, you have the law on you side (tips imaginary cowboy hat) so: How can you make everyone feel better about the situation.

Now comes the big question: Has you boss (or co-workers) expressed negativity towards your situation or are you expecting them to?

If you have been the target of negativity, figure out the source and politely address it. Otherwise, be generally professional about the change and address negativity as it comes. Negativity often comes simply because we expect it to. Being positive sometimes solves these kinds of problems before they happen. Help people to feel good about the positive instead of trying to make them feel better about the negative. If your work environment is healthy, people should feel good about paternity leave and support you. If there is negativity, this flavor of positivity will bring it out of the shadows. Then it can be faced with with the support of other positive, like minded people.

Answer 3

I think all you can do is ask for a trial period to show you can get things done. Often managers are reluctant to make these types of changes if they feel like it will be difficult to change them back. Now if you get too far into the project only to find out this is not working, he may have to replace you, but at least you can get him to try.

We don't know your boss or they relationship and history you two share, so it's difficult to say how to approach him.