Unity3d no-turning RACETS

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Racet : MonoBehaviour
{
    float countshoot2;
    Rigidbody racet;
    // Start is called before the first frame update
    void Start()
    {
       racet = GetComponent<Rigidbody>();
       //  GetComponent<Rigidbody>().velocity = new Vector3(0, 0, 20);
    }

    void Update()
    {
        racet = GetComponent<Rigidbody>();
        GetComponent<Rigidbody>().velocity = new Vector3(0, 0, -30);
    }
    void OnTriggerEnter(Collider other)
    {
        countshoot2++;
        print(countshoot2);
        if (countshoot2 > 3)

        {
            countshoot2 = 0;
            if (other.tag == "shiptrig")
            Destroy(other.gameObject);
            Destroy(gameObject);
        }

    }


}