(Last Update 17 Oct 01)
“For military purposes a boat designed on this [hydrofoil] principle is therefore unsuitable. It even appears questionable whether the pursuit of the principle after the war has any point at all... The problem will be running against the swell, and it will never be overcome.”
Among my papers is a report on the trials conducted by the German Navy on various hydrofoil boats during World War II. This report is an English translation summary of the German Navy's conclusions from trials conducted during 1940 to 1944 on various designs. The report was written by operational officers of the German E-Boats who studied the development and participated in the trials of this new type of high speed motorboat.
I am not completely certain just who gave me this report, but I believe it came out of my 1953 visit to Sachsenberg and von Schertel as discussed in the Summer 1996 IHS Newsletter. I recall their discussions at that time regarding these trials and the attitude of the German Navy toward the use of hydrofoils. The E-Boat commanders loved their high speed craft and could not visualize these beautiful planing boats being replaced by hydrofoils. The hydrofoil concept was primarily accepted by the naval engineers and the designers who supported the continued development of the hydrofoil. Similar attitudes were evident in the German Navy as late as the 1970s.
The report does reveal the following-sea problem of the early von Schertel hydrofoil designs. This was solved much later when Rodriquez installed flaps on surface-piercing foils controlled by an automatic system. Also, this problem was one of the reasons that the U.S. Navy adopted fully-submerged foils on all of their hydrofoil designs.
Anyway, what follows is the English translation (not made by me, and in fact I don't know who made the translation) slightly edited for clarity:
The following information was extracted from various papers dated 1940 to 1944 which covered development and trials of this new type of high speed motorboat. To record as much as possible of the German experience, the available information has been translated and included in this report.
Papers dated June 1940 contain lists of the following experimental E-boats fitted with hydrofoils:
Further records, dating up to November 1944, show the following types:
After the trials of VS 8 in August and the VS 6 in October 1944 it was decided to suspend work on VS 6, 7, 10, and 14 immediately, and that “every drop of fuel expended on further trials would be regarded as wasted.” If continued, it would have required 1-1/2 to 2 years to prepare as a combat weapon. It was, however, generally agreed that trials should resume after the war.
“After putting out, the boat was brought onto its hydrofoils between the beach-heads, i.e. in calmer water, and then proceeded out into the bay of Danzig, into the open sea north-east of Hela. With the wind NE, strength 5-6, swell 4, sometimes more, the boat proceeded on hydrofoils against the sea and was stable, and there was no spray, whereas an ordinary E-boat would have been buffeted about and shipped water. That created a very good impression and is a great recommendation for the sea-going characteristics of the boat. When the boat was turned slowly in a slight sea, 10-12 degrees abaft the beam, it dropped from hydrofoils. The attempt to bring it onto hydrofoils again, when heading the boat in all directions, failed in a calm sea as well as with a swell. The boat, therefore, only rose on to its hydrofoils once during the whole trial. The reason was said to be the insufficient performance of the engines. The trials therefore were brought to a close, and the results yielded nil.
Wind Force 5-6, Sea Force 3-4. When running against the sea or during sharp turns, the VS 8 always dropped down from its foils. However, it was proved that a hydrofoil boat of this size could maintain a 35 knot speed in sea force 5, and could probably increase this performance to 40 knots under similar conditions if proper engines and propellers were provided. The craft was remarkably stable even in sea force 3 from ahead. However, even with two 2500 hp. engines it was considered doubtful that the craft would stay on hydrofoils with the rudder turned hard in either direction; a five-degree turn seems to be the maximum rudder for staying on hydrofoils.
To test the speed when hydrofoils were damaged, it was found that 20 to 21 knots was absolute maximum on both motors because of the braking action of the submerged foils. Technically, this is not due to drag but mainly to the lack of a variable-pitch propeller suitable for running both on and off hydrofoils.
At the end of trials, it was decided to abandon VS 8 in favor of VS 6, which had an improved hydrofoil. The craft itself was subsequently stranded at Stolpmunde after engine failure. It was planned to cut off the hydrofoils and lay up the hull at Maureb, Swinemunde.
VS 6 was tested in Bay of Travemunde on 24 and 25 October 1944. “Wind 1 - 2, swell 1!” The boat rose on to its hydrofoils, in all directions, in 6 -10 seconds calculated from a medium speed. That means that the hydrofoil's principle with this [gentle] swell is solved, and that the engine installations and the performance attained from them, was not only sufficient but very high (2 engines totaling 1300 hp!)."
A second trial showed that the boat became foilborne in 14 seconds from a start with engine cut and helm turned 10 degrees. When the helm was turned hard over, and remained thus, the boat rose onto foils with difficulty.
On the 25th October, the following results were recorded with wind 2 - 3 from the NE and swell 2: “On courses against the sea, and with the sea running abeam, the boat rose on hydrofoils and was buffeted badly by the heavier seas. The performance of the engines was completely sufficient, and the boat rose onto foils in approximately 15 seconds, calculated from a low speed.
“When course was set with the sea, in not one instance was it possible to bring the boat back onto the hydrofoils. As soon as the forward part of the boat came just in front of the crest of a wave, and the stern in the trough, the boat rose for a brief period on to its hydrofoils, and when the stern was lifted by the next wave it dropped completely from the hydrofoils again, and even caused the boat to pitch down to a certain extent. In these cases the revolutions had to be decreased immediately to prevent the bow from submerging completely because of the braking effect of the hydrofoils.”
“It is established that the boat, which has a displacement of approximately 13 to 17 tons maximum with 2 engines totaling 1300 hp, sets out the most favorable conditions for the proof of the hydrofoil principle. According to a report of the Representative of the Sachsenberg Yards, all the latest developments of the hydrofoil principle are included in this boat. The trials ought, therefore, to be pursued fully.”
“It has now been established that the hydrofoil E-boat can only be used in favorable weather (swell 1). For military purposes a boat designed on this principle is therefore unsuitable. It even appears questionable whether the pursuit of the principle after the war has any point at all.
“The Navy Department and Ships Construction Commission, from which Departments a representative was sent to take part in these trials, do not concur.
“The problem will be running against the swell, and it will never be overcome. A seaplane, for instance, cannot land or take off against the swell. The incline of a hydrofoil boat to the sea depends on the wave crests and troughs. As soon as the stern is lifted by the wave the boat always drops from its foils and threatens to dip. This failing will never be offset by increasing engine power. The hydrofoils will always tend to go into a dip no matter how many alterations aligning the foils are tried to control the movement of the forward part of the ship, when the stern is lifted by the swell.”
[Contrast these mid-1940 statements on military unsuitability of hydrofoils with achievements of the US Navy's developmental hydrofoils (HIGH POINT, PLAINVIEW, FLAGSTAFF, TUCUMCARI) and later its PHM Squadron. Despite their technical success, the PHMs were a political failure... always struggling for existence and ultimately felled in their prime by the budgeteer's ax. -Ed.]
The main role finally proposed for the hydrofoil E-boat was that of a fast offensive mine-layer invulnerable to all German types of sea mines. The main concern was with the DM (pressure box) mine, but it was found in trials that the pressure effect of the hydrofoils was insufficient to make the DM react, even if adjusted to maximum pressure sensitivity. At high speeds the mine is unaffected because of the short time factor (note - the normal E-boat will detonate D-mines at speeds under 10 knots with a draft of less than 2.0 meters).