OPINION OF THE COURT
WEIS, Circuit Judge.
Damage to cargo transported by water has been a subject of litigation since the dawn of nautical history, yet the legal problems engendered by this activity continue to perplex the courts. In this case, the carrier contends that once it established lack of responsibility for damage done to a barge’s hull as it struck a submerged object, the burden of proving negligence causing cargo damage shifted to the plaintiff shipper. The trial judge, however, charged the jury that the burden remained with the carrier to show due diligence in caring for the cargo after the incident. The district court also held that a clause in the towing contract requiring the shipper to secure insurance on the cargo with a waiver of subrogation against the carrier was exculpatory and unenforceable. And finally, since double-skinned barges were not in general use on the rivers, the trial judge ruled that a single-skinned barge used for carrying a water soluble cargo was not unseaworthy per se and withdrew that issue from the jury. Finding no reversible error in the district court’s rulings, we affirm.
On March 2, 1971, Ashland agreed to transport a cargo of antifreeze owned by PPG Industries, Inc. from Beaumont, Texas to St. Paul, Minnesota. Canal Barge Company later entered into a separate towing contract with Ashland, agreeing to tow the barges containing antifreeze from Baton Rouge, Louisiana to St. Paul. Upon delivery in St. Paul, the antifreeze in one of Ashland’s barges was found to be contaminated by river water.
PPG brought suit against both Ashland and Canal. In early stages of the litigation, it was determined that because of the incorporation of the Carriage of Goods by Sea [140]*140Act (COGSA), 46 U.S.C. §§ 1300-1315, in the PPG — Ashland contract, the limitation period had run before suit was commenced. Judgment was accordingly entered in Ash-land’s favor against PPG. The claim for cargo loss asserted against Canal, however, continued to be viable as were the cross-claims between Ashland and Canal. PPG Industries, Inc. v. Ashland Oil Co.—Thomas Petroleum Transit Division, 527 F.2d 502, 505-07 (3d Cir. 1975).
On remand, the case proceeded to trial in the district court1 before a jury which found a general verdict in favor of Canal, but by special interrogatories determined that Canal was negligent and that the cargo loss was $110,000. Ashland’s claim for damages to its barge was denied. The district court entered judgment n.o.v. in favor of PPG for the cargo loss and added interest.
The evidence established that about four days before arrival at St. Paul an accident occurred near Lock 14 on the Mississippi River and the Ashland barge was damaged. While maneuvering before entering the lock, the barges and towboat unexpectedly turned around in the river, a movement called “topping.” As they did so, at least one barge scraped the river bottom or hit some submerged object. Inspection of the Ashland barge hull after it reached St. Paul and had been taken to dry dock revealed three holes more than one inch wide and a number of cracks varying in size from a fracture line to one-quarter inch in width. None of this damage could be seen while the barge was in the water.
Members of the crew testified that after the topping incident and during the remaining days of the voyage routine external checks did not disclose any signs that the Ashland barge was taking on water. No steps were taken, however, to examine the cargo or have the hull thoroughly inspected. At St. Paul, it was discovered that water had entered some of the barge compartments and contaminated the antifreeze, a highly soluble substance.
The trial judge removed from the jury’s consideration Canal’s contention that the barge was unseaworthy because a double-skinned vessel should have been used to carry cargo of this nature. The district court also decided that a provision in the PPG contract with Ashland requiring that the cargo owner purchase insurance for its own account with waiver of subrogation to the carrier did not relieve Canal of liability for the loss.
The case was submitted to the jury on instructions that upon proof the cargo was in good condition at the voyage’s beginning, and was delivered at its end in damaged condition, a prima facie case for PPG was established. The jury was further charged that the burden shifted to Canal to establish due diligence and care for the cargo, but if it were shown that the loss was caused by a peril of the sea, then PPG would not be entitled to recover. The jury was told to submit a general verdict.
During their deliberations, the jurors sent a note to the trial judge reading:
“If in our deliberations we consider that the damage to the barge took place during the topping at Lock 14 and that no negligence existed on the part of the captain and pilot of the Caroline [Canal tug], must we at that point stop any further deliberation, or can we consider what is believed to be negligence on the part of the parties involved following the topping?”
After consultation with counsel, the trial judge instructed the jurors in substance that if they found Canal was not at fault for the topping, they should stop at that point. However, in addition, the judge asked that certain interrogatories be answered. In general they asked, assuming Canal had shown due diligence up to and [141]*141including the topping incident, what part of the cargo damage took place as a result of the topping; what portion occurred from the failure of due care thereafter; which parties failed to exercise due care; and what was the amount of damage.2
The jury returned a general verdict in favor of Canal, but in answer to the interrogatories, found that Canal failed to exercise due care after the topping. According to the jury’s responses, none of the cargo damage resulted from the topping, but rather flowed from failure to exercise due care thereafter. After consideration of post-trial motions, the district court entered judgment n.o.v. in the amount of $110,000 against Canal.
I.
BURDEN OF PROOF
Canal contends that in submitting the interrogatories to the jury, the trial judge erred in allocating the burden of proof. Canal’s position is that if it met the burden of proving a peril of the sea, then PPG was required to prove that negligence of the carrier caused the cargo damage. The judge did not accept this argument. In connection with the interrogatories, he delivered these instructions:
“So what I am asking you to do is that, if you are satisfied that Canal met its burden by a fair preponderance of the evidence that it exercised due care and diligence up to the time of the topping, and if you are satisfied that Canal has met its burden of showing by that standard that the topping itself was not due to the lack of due care or diligence, but to the peril of the sea, if you will, or whatever, then you decide that Canal didn’t establish to your satisfaction that it exercised due care and diligence during the rest of the trip, — I think there were four days beyond the topping incident, and if you decide they didn’t establish that to your satisfaction, then I want to know what percentage of the damage to — the contamination to the cargo occurred as a result of the topping itself and what percentage occurred as a result of the failure to exercise diligence and due care after the topping.”
The towing contract required Canal to exercise due diligence in the handling, care and delivery of the cargo but reserved for the carrier the limitation of liability granted under § 3 of the Harter Act, 46 U.S.C. § 192.3 Section 3 would relieve Canal from [142]*142liability if the loss was caused by a peril of the sea.4
The jury verdict established that the damage to the barge was caused by the topping incident. Cargo loss, however, was another matter. The jury apparently believed that the damage to the hull was not so extensive as to be the sole and proximate cause of the contamination and that the subsequent neglect of the carrier was the immediate cause. In granting judgment n.o.v. to the cargo owner, the trial judg;e necessarily implied that there was adequate evidence to support the jury’s conclusion.
Where damage to the cargo is not discovered until after it has been delivered and no substantial evidence of the cause remains, allocation of the burden of proof and attendant risk of nonpersuasion in proving that the loss arose from a cause excepted under law or by the parties is often crucial in determining liability. Our task here is to determine whether the judge’s charge properly directed the jury on this issue. Since the exculpatory provisions of the Harter Act were incorporated into the towing contract by reference, the decisional law regarding burden of proof under that Act and COGSA,5 is pertinent to our review.
A seminal case in the United States, a pre-Harter Act decision,6 is Clark v. Barnwell, 53 U.S. 272, 13 L.Ed. 985 (1851), in which there was a claim for damaged goods shipped under a bill of lading that excepted liability for “all, and every the dangers and accidents of the seas and navigation . . . .” Id. at 280. The Court explained that upon a showing that there had been delivery of goods in damaged condition, the burden was on the carrier to show that the loss was caused by an excepted peril. Once that was established, the shipper was required to prove that the damage could have been avoided by the exercise of reasonable skill and attention by the carrier. “But in this stage and posture of the case, the burden is upon the plaintiff to establish the negligence as the affirmative lies upon him.” Id. The Court then quoted from the charge of Lord Chief Justice Denman in Muddle v. Stride, 9 Car. & Payne 380:
“[I]f on the whole, it be left in doubt what the cause of injury was, or, if it may as well be attributable to ‘perils of the sea’ as to negligence, the plaintiff cannot recover.”
This latter reference must be considered to have been modified by The Folmina, 212 U.S. 354, 29 S.Ct. 363, 53 L.Ed. 546 (1909). In that case cargo was damaged by sea water, and the Court said, “while there was a certainty from the proof of a damage by sea water, there was a failure of the proof to determine whether the presence of the sea water in the ship was occasioned by an accident of the sea, by negligence, or by any other cause.” Id. at 363, 29 S.Ct. at 365. The Court held that the carrier had failed to meet its burden and accordingly, was liable. See also Commercial Corp. v. New York Tank Barge Corp., 314 U.S. 104, 108-09, 62 S.Ct. 156, 86 L.Ed. 89 (1941). Moreover, as the court explained in Lekas & Drivas, Inc. v. Goulandris, 306 F.2d 426, 431 (2d Cir. 1962): “The respondents in Clark v. [143]*143Barnwell had not only established a peril of the sea as a cause but had negated all others.”
Since the carrier, and not the shipper, usually has the means of knowing most about the events of the voyage, it is sound to place on the ship owner both the burden of showing the inclusion of loss within an exception and proving the exercise of due diligence to prevent harm to the cargo. Application of the general rule at times appears inconsistent because of the varying factual circumstances leading to cargo loss and the diverse exemptions sought to be invoked. Two patterns discernible from case law and pertinent to our analysis here surface.
The first is where the cargo is damaged by forces completely unrelated to its inherent nature. For example, a cargo of steel is lost because the ship sinks. In this situation, inquiry focuses upon the cause of injury to the vessel, not the cargo. The damage claim, therefore, centers on cause and to avoid responsibility the carrier must prove that the damage to the cargo stemmed from an exempted occurrence, such as a danger of the sea. The burden rests upon the carrier to show that the cargo damage resulted from the excepted peril, and the exercise of due diligence to avoid the damage caused by the described hazard. E.g., States Marine Corp. v. Producers Cooperative Packing Co., 310 F.2d 206 (9th Cir. 1962); Tri-Valley Packing Ass’n v. States Marine Corp., 310 F.2d 891 (9th Cir. 1962); Schroeder Bros. v. The Saturnia, 226 F.2d 147 (2d Cir. 1955).
Second is that where the nature of the injury to the cargo indicates in and of itself inclusion within a specified exemption from liability provided by the Act or bill of lading, such as decay or rust. For example, if steel cargo is delivered in rusty condition and the bill of lading exempts the carrier from liability for rust, the burden is on the cargo owner to establish negligence as a cause once the carrier shows that the nature of the damage brings it within the exception. In order to recover, the cargo owner must carry the burden of proving the carrier’s negligence brought about the damage. If the cause of injury is left in doubt, the carrier stands excused since by virtue of the exemption the shipper accepted the risk of loss of that kind. There is nothing additional the carrier need prove to place the loss within the clause which exempted liability — the nature of the injury speaks for itself. See, e.g., The Monte Iciar, 167 F.2d 334 (3d Cir. 1948). As the Supreme Court said in The Folmina, supra at 362, 29 S.Ct. at 365:
“Of course, where goods are delivered in a damaged condition, plainly caused by breakage, rust, or decay, their condition brings them within an exception exempting from that character of loss, as the very fact of the nature of the injury shows the damage to be prima facie within the exception, and hence the burden is upon the shipper to establish that the goods are removed from its operation because of the negligence of the carrier.”
See generally The Patria, 132 F. 971 (2d Cir. 1904); A. Knauth, The American Law of Ocean Bills of Lading 193-96 (4th ed. 1953); Comment, Cargo Damage at Sea: The Ship’s Liability, 27 Tex. L. Rev. 525 (1947).
The case sub judice falls into the first category. Canal’s defense is that the damage was caused by a defect in navigation or peril of the sea exception under the Harter Act. We look therefore to cause, not result, and the carrier must meet its burden of persuasion that the cause of the damage fell within the exception.
The way in which the issue is posed often determines the outcome of the case. Here, the issue could be framed as whether the failure to mitigate damages to the cargo was a separate act of negligence which the plaintiff was required to prove; or, alternatively, whether the carrier fully discharged its obligation to meet the plaintiff’s prima facie case by showing that a peril of the sea caused some hull damage during the voyage.
Case law provides no reliable chart. In G. Gilmore & C. Black, The Law of Admiralty 170 (2d ed. 1975), the authors characterize the failure to use due care in prevent[144]*144ing the spread of mischief as negligence. Moreover, they reject the proposition that the mere happening of damage through an excepted peril releases the carrier from responsibility to take due measures for proper cargo care thereafter.
The jury’s findings established that the carrier met the plaintiff’s prima facie case up to and including the topping incident, and the defendant would have been exonerated if the matter had ended at that point. It did not, however, since the jury found that thereafter the carrier failed to exercise due diligence and cargo damage resulted. This is a close case, but in our view the trial judge did not err in imposing the burden on the carrier to demonstrate its exercise of due care in caring for the cargo to the end of the voyage. Immunity conferred by the Harter Act must be strictly construed. Gilmore & Black, supra at 156 n.50. The prima facie case of negligence which the cargo owner made out by showing damage to the cargo at destination was not completely answered by the defendant’s evidence of the topping incident and its conduct for the remainder of the voyage.
Moreover, the basic policy underlying the shifting of burden to the carrier to explain damage, see Schnell v. The Vallescura, 293 U.S. 296, 307, 55 S.Ct. 194, 79 L.Ed. 373 (1934), supports imposition of the burden on Canal. The carrier knew what happened after the topping incident to the end of the voyage and was in a far better position than the shipper to present evidence of due care.
This case differs from those in which the burden shifts back to the shipper to prove that antecedent or concurrent negligence on the part of the carrier contributed in bringing about the loss. In those instances, after the carrier produces evidence of the excepted cause which brought about the damage, the shipper may go forward and show that other factors for which the ship was responsible played a part. But when the carrier’s evidence does not show that the excepted cause produced all of the damage, as here, the shipper is entitled to a complete explanation. In Armco International Corp. v. Rederi A/B Disa, 151 F.2d 5, 8 (2d Cir. 1945), the court, concerned with the problem of determining whether damage occurred aboard ship or ashore, said:
“So far as there is any uncertainty as to the extent of the damage which happened while the [cargo was] on board, the ship has the burden of proof. She did not, and indeed could not, prove what part was done aboard and what ashore.”
Similarly, here the burden was on the carrier to prove the damage caused by the excepted hazard. “It is sufficient, if the carrier fails to show that the damage is from an excepted cause, to cast on him the further burden of showing that the damage is not due to failure properly to stow or care for the cargo during the voyage.” Schnell v. The Vallescura, supra at 305, 55 S.Ct. at 196. In Schnell, because the claim was for an excepted result, the shipper initially did produce evidence of negligence in order to sustain its burden. The court there held that the carrier was liable for all the damage in view of its inability to apportion the loss between an exception and its negligence.7
In the case at bar, having made out a prima facie case of negligence, the plaintiff stood in as good a position as the shipper in Schnell. The burden shifted to the defendant to prove whether all or an ascertainable part of the damage was the result of an excepted cause. This allocation of respective burdens is supported by sound public policy and is consistent with the general rule permitting a cargo owner to use a prima facie case to show lack of care in delivery of the goods. We find no error in the court’s charge in this respect.
[145]*145II.
THE INSURANCE CLAUSE
In the transportation agreement between PPG and Ashland, the parties agreed that
“Hull and P & I insurance will be carried with a first class underwriter by Carrier [Ashland] for Carrier’s account. Cargo insurance will be assumed or carried by Shipper [PPG] for his own account, with waiver of subrogation to the Carrier.”
In the contract between Ashland and Canal, there was no mention of cargo insurance but only a proviso that Ashland was to carry Hull and P & I insurance. PPG did not take out cargo insurance and Canal contends that it is therefore relieved of liability. The district court ruled that even assuming arguendo that the plaintiff was contractually bound to maintain coverage, that provision was unenforceable as a matter of public policy, citing Bisso v. Inland Waterways Corp., 349 U.S. 85, 75 S.Ct. 629, 99 L.Ed. 911 (1955), and Dixilyn Drilling Corp. v. Crescent Towing & Salvage Co., 372 U.S. 697, 83 S.Ct. 967, 10 L.Ed.2d 78 (1963).
Requiring the cargo owner to insure the goods and providing there shall be no subrogation is an effective way of insulating the carrier from liability regardless of culpability. The cargo owner can be expected to make a claim against his insurance company- in anticipation of reasonably prompt payment rather than proceeding to contest the matter with the carrier. If the insurance company pays the claim, it cannot sue the carrier and, thus, the matter is at an end. As the district court acknowledged, however, such an arrangement runs afoul of Bisso and Dixilyn Drilling which bar contract provisions that shift responsibility for tower’s negligence. See also Boston Metals Co. v. The Winding Gulf, 349 U.S. 122, 75 S.Ct. 649, 99 L.Ed. 933 (1955).
We are unpersuaded by cases tending to place a different focus on this issue. Although the Court of Appeals for the Fifth Circuit has treated the problem as essentially one of bargaining over which party shall pay the insurance premium, see Twenty Grand Offshore, Inc. v. West India Carriers, Inc., 492 F.2d 679 (5th Cir.), cert. denied, 419 U.S. 836, 95 S.Ct. 63, 42 L.Ed.2d 63 (1974), and Fluor Western Inc. v. G & H Offshore Towing Co., 447 F.2d 35 (5th Cir. 1971), cert. denied, 405 U.S. 922, 92 S.Ct. 959, 30 L.Ed.2d 793 (1972); cf. Dow Chemical Co. v. Ashland Oil, Inc., 579 F.2d 902 (5th Cir. 1978), we are inclined to view Dixilyn Drilling, supra, as proscribing indirect as well as direct exculpation of negligence by carriers. See Dow Chemical Co. v. M/V Charles F. Detmar, Jr., 545 F.2d 1091 (7th Cir. 1976). Moreover, even if the Fifth Circuit approach were used here, it would not help Canal because it never bargained with PPG about cargo insurance, and therefore, cannot show reduction of its charges on that basis. Compare Twenty Grand Offshore Inc. v. West India Carriers, Inc., supra at 681-83.
We are also not convinced that the insurance clause in the transportation agreement extended to anyone other than Ashland. In the first appeal in this case, we found that COGSA provisions were not intended to apply to Canal and that contract references to “persons employed by carrier” extended only to individual employees of Ashland, not Canal. 527 F.2d at 506. The rationale of that opinion requires adoption of the same approach in construing the insurance clause of the transportation contract. Accordingly, we conclude that Canal is not entitled to judgment because of PPG’s failure to secure cargo insurance.
III.
SEAWORTHINESS
Canal produced an expert who opined that the antifreeze should have been carried in a double-skinned barge and that for this cargo the single-skinned vessel used was unsuitable, and consequently, unseaworthy. The expert said that to his knowledge double-skinned barges had first been built about six years before the topping incident and were used to transport lubrication oil.
[146]*146The marine superintendent for Canal testified that although it had some double-skinned barges in 1971, most of its equipment was of the single-skinned variety. He said that the Coast Guard had not issued any pertinent regulations, and at the time of the incident, custom was beginning to change in favor of using double-skinned vessels. When double-skinned barges were not available, Canal would use singleskin vessels to carry soluble cargo. The witness also stated that Canal would never furnish an unseaworthy vessel.
During the trial, the district judge ruled “there is nothing unseaworthy about a single skin barge so long as the single skin holds and in view of the testimony that at the time this was going on, most barges were single skinned.
Then there was another question of whether it could be a single skin on the bottom and double skin on the side. I’m not prepared to rule that a single skin barge was ipso facto unseaworthy, and I have so ruled.”
Although the jury was directed to consider unseaworthiness in' other aspects, the issue of whether a single-skinned barge was seaworthy was not submitted to it.
Seaworthiness is a relative term which looks to such matters as the type of vessel, character of the voyage, reasonably expectable weather, and navigational conditions. May v. Hamburg-Amerikanische Packetfahrt Aktiengesellschaft, 290 U.S. 333, 346-49, 54 S.Ct. 162, 78 L.Ed. 348 (1933) (The Isis); J. Gerber & Co. v. S.S. Sabine Howaldt, 437 F.2d 580, 596 (2d Cir. 1971); Horn v. Cia de Navegacion Fruco, S.A., 404 F.2d 422, 431 (5th Cir. 1968), cert. denied, 394 U.S. 943, 89 S.Ct. 1272, 22 L.Ed.2d 477 (1969). The vessel must be reasonably fit to carry the cargo she has undertaken to transport. The Silvia, 171 U.S. 462, 19 S.Ct. 7, 43 L.Ed. 241 (1898).
Generally, the issue of unseaworthiness is one of fact and is submitted to the jury. Luckenbach v. McCahan Sugar Co., 248 U.S. 139, 145, 39 S.Ct. 53, 63 L.Ed. 170 (1942); The Wildcroft, 201 U.S. 378, 387, 26 S.Ct. 467, 50 L.Ed. 794 (1906); The Carib Prince, 170 U.S. 655, 658, 18 S.Ct. 753, 42 L.Ed. 1181 (1898). Here, however, there was no dispute about the facts. Essentially, what the defendant was asserting was a new development in construction which had not come into common use in the industry. It was not such an innovation it had to be adopted by all shippers immediately, making the older barges obsolete for transporting this particular cargo. Compare The T. J. Hooper, 60 F.2d 737, 740 (2d Cir.), cert. denied sub nom. Eastern Transportation Co. v. Northern Barge Corp., 287 U.S. 662, 53 S.Ct. 220, 77 L.Ed. 571 (1932), with Marshall v. Ove Skou Rederi A/S, 378 F.2d 193, 201 (5th Cir.), cert. denied, 389 U.S. 828, 88 S.Ct. 86, 19 L.Ed.2d 84 (1967). Nor was it shown that scraping the barge and holding the bottom was reasonably to be expected in a voyage on the Mississippi. Under the circumstances of this case, the trial judge did not err in withdrawing the design issue from the jury.
The defendant also contends that it was error to award interest to the plaintiff. This was a matter within the discretion of the trial court and we find no abuse in its exercise.
Having carefully reviewed the case, we conclude that the district court did not commit reversible error, and accordingly, the judgment will be affirmed.