Lurrson effect rudders for Bismarck?

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Billy
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Lurrson effect rudders for Bismarck?

Post by Billy » Thu Sep 12, 2019 11:16 pm

So, come across a rather interesting tidbit of information. German S boats or E boats used a very interesting arrangement to help reduce wave making resistance at speed. Basically a system whereby two outermost rudders were canted out slightly to create a cavitation bubble. The wake of the ships props would fall into the hole thus created and disturb the wave making profile. The effect started at from if I remember correctly about 25 knots and got more effective with increasing speed, could be carried down to at least 20 knots as the threshold to induce the effect was greater than keeping it going. Overall the system was very effective. Increasing top speed of the boats from 37-38 kn to 43-44 kn and helped reduce stern squat at speed and especially stern wake/wave profile. Besides the two rudders there was a third central one to maintain control without upsetting the effect. So the question arises. If the effect was known, why was there no carryover to larger ships? Was this an oversight or lack of transparency between competing sections (which is very believable given the lack of cooperation and info sharing between sections of that time), or just overlooked as a fast boat curiosity, or was it down to plain old snobbery, as in what could the lowly powerboat section teach us. Or perhaps it was considered and excluded although the effect does not seem scale specific so I don't buy that argument. So the question being is what sort of effect like that would have on Bismarcks top speed? And could Bismarcks rudders be moved independently to induce the effect separately (Yes, I know Bismarck only had 2 rudders and didnt have a third to keep the effect going in a turn. This is just to explore the potential of what was possible). So if a person uses the same percentages of the E boats speed (which is very much a stretch given the broad disimilarities between the two, not least of which both size and top speed), we are left with the rather startling assumption of a potential top speed of 33.94 kn. Although definitely not a given the effect is at least illustrative and suggestive of the potential to be had given the potential adoption of such an effect. It would very interesting to see if anyone could predict the behavior given a simulation program. Or if anyone out there has knowledge or documents thereof, would be very interesting to hear from you.

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marcelo_malara
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Re: Lurrson effect rudders for Bismarck?

Post by marcelo_malara » Sat Sep 14, 2019 5:08 pm

Hi, it seems an effect applicable to small vessels, the stern is lifted a little and this reduces resistance. Nowadays we have trim tabs for the same. Don´t think that a ship of the Bismarck size would benefit from it.

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Byron Angel
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Re: Lurrson effect rudders for Bismarck?

Post by Byron Angel » Fri Sep 20, 2019 11:08 pm

Hi Billy,
Interesting! I was familiar with the canted rudder arrangement in use with German S-boats, but never contemplated whether this method would prove practicable with capital ships. My gut instinct suggests probably not - there after all being HUGE difference between an S-boat and a Bismarck, in terms of both tonnage and hull shape. But one never really knows.

B

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