Kyushu ferry - reservations needed for car?

I am wondering if we need to reserve a spot on the ferry (for our car) or if we can just show up a bit early and be reasonably assured that we and our car will get on the ferry with no reservation.

We plan to take the Kyushu Ferry from Kumamoto Port to Shimbara Port in mid-May (this will be on a Saturday, almost 2 weeks after Golden Week). We will have a rental car. I expect we will get there late morning or mid-day. I see on the ferry website that you can make a reservation, but since we do not have very detailed information about our car, we do not know the car "length" which is required for the ferry reservation (we have paid a deposit for the car, but car rental companies do not specify the exact car model you will get - I just know it will be a "compact" car).

The ferry runs often (about every 90 minutes). Since I don't know the time we will be there and don't know the length of the car, I would prefer to NOT make a reservation, and just show up to get in line for the next ferry. On a Saturday in mid- to late-May, is it reasonable to assume we won't have to wait for hours in a long line to get on this ferry?

I know some ferry systems do have long waits if you have no reservation, others always have space if you just show up. Any experience with this line (Kyushu Ferry Co., Ltd) so we can plan? Thanks.

You need 890 yen per person travel by 60 minutes ferry ride,2750 yen for car transport

Thank you - yes, I know you have to pay to ride a ferry. Costs and other details are provided on their website, which I have seen.

What I'm asking is if I really need to reserve a spot on the ferry for our car, or if we can just show up at the ferry without a reservation and be reasonably sure we can get on the next ferry (or if they are so busy we will be stuck waiting for hours without a reservation).