“(4.) The vessel a she is constructed, and all her engines, boilers, and machinery, and all materials from time to time intended for her or them, whether in the building yard, workshop, river, or elsewhere, shall immediately as the same proceeds become the property of the purchasers, and shall not be within the ownership, control, or disposition of the builders; but the builders shall at all times have a lien thereon for their unpaid purchase money.