http://www.bukkit.fr/index.php/topic/13128-probl%C3%A8me-velocity/

Arrows.java

package fr.neatmonster.arrows;

import org.bukkit.block.Block;
import org.bukkit.block.BlockFace;
import org.bukkit.entity.Arrow;
import org.bukkit.entity.EntityType;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.ProjectileHitEvent;
import org.bukkit.plugin.java.JavaPlugin;
import org.bukkit.util.BlockIterator;
import org.bukkit.util.Vector;

public class Arrows extends JavaPlugin implements Listener {

    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void onProjectileHit(final ProjectileHitEvent event) {
        if (event.getEntityType() == EntityType.ARROW) {
            final Arrow arrow = (Arrow) event.getEntity();
            if (arrow.getVelocity().length() > 0.5D) {
                final BlockIterator path = new BlockIterator(arrow.getWorld(), arrow.getLocation().toVector(), arrow.getVelocity(), 0, 3);
                Block next = path.next();
                Block previous = arrow.getLocation().getBlock();
                while (path.hasNext() && !next.getType().isSolid()) {
                    previous = next;
                    next = path.next();
                }
                final BlockFace face = next.getFace(previous);
                final Vector velocity = arrow.getVelocity().clone();
                if (Math.abs(face.getModX()) > 0D)
                    velocity.setX(-1D * velocity.getX());
                if (Math.abs(face.getModY()) > 0D)
                    velocity.setY(-1D * velocity.getY());
                if (Math.abs(face.getModZ()) > 0D)
                    velocity.setZ(-1D * velocity.getZ());
                arrow.getWorld().spawnArrow(arrow.getLocation(), velocity, (float) arrow.getVelocity().length() * 0.6f, 0f);
                arrow.remove();
            }
        }
    }
}

plugin.yml

name: Arrows
version: 1.0
author: NeatMonster
main: fr.neatmonster.arrows.Arrows