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Subject: Oahu Blockship AH Proposal
robdab    8/4/2008 6:22:00 AM
Gents, I know that this topic was roughly handled a few months ago in the "Rising Sun Over Hawaii ..." thread here but I'd like to re-visit it if anyone else is interested ? --------------------------------------------------------------------- I'd like your comments/suggestions for a Japanese blockship design suitable for an alternative history Pearl Harbor attempt on Dec.7'41. Historically the Japanese received pre-war (but post-embargo) US permission to send 4 liners across the Pacific as repatriation vessels to return American and Japanese citizens to their home countries during that time of crisis. The 4th and last Japanese vessel scheduled to make that Yokohama-Honolulu-San Francisco crossing was the 17,000 ton 21 knot NYK cargo-liner Tatuta (also spelled Tatsuta) Maru. Which can be seen at Historically the Japanese wished her scheduled to arrive outside Honolulu at 0700 on Dec.9'41 as a way to keep the Americans there relaxed. Who would expect an attack on Oahu just before a big civilian liner was scheduled to arrive ? In fact she did depart Yokohama on schedule, with paying US passengers, but turned around in mid-Pacific at midnight on Dec.6-7'41 so as to avoid the Kido Butai's Dec.7th air attack on Pearl Harbor. Gordon Prange's book, "At Dawn We Slept" presents these details. I would like to explore, here, the "what if" impact of the Japanese instead requesting (and being granted) US permission for an 0700 Dec.7'41 arrival outside of Honolulu for a (disguised) 585' long Tatuta Maru (my blockship). This being my point of departure from the origianal timeline. The Pearl Harbor entrance channel and it's surrounding restricted sea zone had been historically guarded 24/7 since 1939 by a peacetime gateguard destroyer. That morning the USS Ward followed a 15 knot patrol box circuit box of some 2 miles by 2 miles thru which travelled a fairly steady stream of smaller USN vessels including transports, garbage scows and minesweepers. So many and so regularly in fact that the twin anti-submarine nets strung across that entrance were usually left open between 0500 and 1900, the daylight hours, every peacetime day. The channel dimensions at the nets were 700' wide by 72' deep which quickly shallowed to a constant dredged depth of 40' thru the entire channel leading into Pearl Harbor itself. Only one US Coastal Artillery battery, consisting of one 60" searchlight on a 50' tower and 4x155mm GPF guns stood on Sand Island, at Honolulu Harbor, watching over the entrances to both harbors, simultaneously. Although the PH entrance was some 6 miles away. This sandbag protected "Examination Battery" was also guarded against air attack by an emplaced (but not usually manned) US battery of 4x3" AA guns, also located on the Sand Island Military Reserve. I'd like your suggestions on how the Tatuta Maru could have been fitted out by the Japanese to become Adm. Kimmel's worst nightmare ? And any operational details that you'd care to suggest as well. Please remember that drydock time in Japan at this time was at a premium as war approached. Also that the overflight of Tatuta Maru by USN PBY long ranged air recon. aircraft, as she approached Oahu, was nearly guaranteed. As had been the case for the three previous historical repatriation trans-Pacific cargo-liner trips. Just to make the scenario more plausible I would also propose the alteration of the historical orders given to the 5 Japanese minisubs present that morning. Rather than attempting to sneak into Pearl Harbor itself, they would instead be ordered to torpedo the patrolling gateguard US destroyer, USS Ward, by 0730 ambush. Twice, to insure that her torpedoes could not offend the now charging Tatuta Maru. The Japanese consular spy, Yoshikawa, could have observed the gateguard's regular patrol box and reported it's habits to Tokyo. With the distraction of having to stop to place a Honolulu Harbor pilot and a US Marine inspection team aboard her via the captain's gig, I'd doubt that the Ward would be watching for nor be able to evade the Japanese secret minisubs that were completely unknown to the US at the time. Any/all minisubs still having torpedoes after Ward was hit twice, would follow my Tatuta Maru into the PH entrance channel and takeup ambush positions around her, on the bottom there. Any US warship attempting to slowly squeeze past that anchored and scuttled blockship might thus join her in further jamming that channel. How would you (and indeed could you) prevent my corking up of Pearl Harbor with the Tatuta Maru on Dec.7'41 ? And could I do so without giving away the remainder of the historical Japanese 0755 surprise air attack ?
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robdab       8/6/2008 4:02:11 PM
Hmmm ? I have 126 views but nobody has a comment to post ?
Ok, I'll try tossing out a little bit more bait ... some maps.
The first scaled map from shows a 1941 surface view and lists the location of Sand Island as well as most of the other major US military bases on Oahu at that time. The only significant one missing is Fort Barrette which was located in the "V" shaped highway notch shown just to the NE of Camp Molekoli on this map.

The USS Ward's historical 2 mile x 2 mile patrol box can be imagined just below the "LA" and just to the right of the "BAY" lettering shown to mark Mamala Bay on that first map. Thus she could keep a close watch over any vessel approaching the entrance to either harbor.

It must be noted that the US Coast Guard did keep three cutters based inside of Honolulu Harbor (one of which, the Taney, had the size and firepower of a destroyer escort) to aid in the policing of civilian traffic in and out of that busy port thus leaving the USN's "gateguard" destroyer mostly free to watch the military harbor entrance and it's approaches.

At night however it was a differing story as none of the three USCG vessels (two were just 25'-35' launches fitted with only a single .30 cal mg and fit only for chasing fishing sampans & smugglers) had any depth charges, sonar or radar equipment and were thus mostly blind after sunset. The USN helped out during the hours of darkness by watching both approaches then (and also during stormy weather) and by transfering pilots for either harbor when requested to do so by the USCG.

The second map that I suggest from is a modern day NOAA navigation chart for Mamala Bay. The Oahu shoreline has changed considerably since WW2 but the navigation channels dredged thru the coral reefs so long ago still show clearly and deliniate the path that my proposed Tatuta Maru mission would have had to follow. This modern chart shows the channel dredged to 50' instead of 40' depth and some 1050' wide vs the 700' width on Dec.7'41. Right down to the peacetime channel buoys on both the Pearl Harbor and Honolulu Harbor (the one at the southern tip of Sand Island is the WW2 shipping entrance) approaches.

I hope that these two maps will clarify some of the scenario details that I have already suggested.
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JFKY       8/6/2008 4:26:24 PM
I'd say you've done all that needs doing for alternative history......Fill it's compartments with concrete to make it harder to break apart or to raise.  It's a blockship, an inert lump.  What "special characteristics" does it need, but mass and open sea cocks?  It blocks the channel.
I think you don't get a lot of takers on input because the story is a little improbable.  1) The repatriation ship isn't going to Pearl.  If it tries to go to Pearl alarms are raised.  Just like driving into Area 51 or onto a missile launch reservation.  You can be in a UPS truck all you want, but the security forces are STILL going to stop you. 2) I wouldn't bet that the midget subs are going to hit the USS Ward.  If you miss, you alert the Americans...
For me, the blockship is just a little "too cute."  Sounds good, BUT...what if the Kido Butai is delayed?  It's under strict radio silence...the blockship doesn't sail with the do you coordinate the strike? And again, if the blockship is heading for Pearl somebody's going to notice and begin an intercept.
If you're writing a novel, go ahead...if you're seriously entertaining this as a "what if" realize the reason no one tried it is that it would be darn near IMPOSSIBLE to pull off, simultaneously with the air attack, but the only way it works is if it does happen simultaneously with the air attack.  Strike on Dec. 6 and the US is warned that Japan is up to no good....and possibly the attack fails, so all you do is alert the Pacific Fleet who aren't there on 7 Dec. to be bombed...strike on Dec. 8 and you just get sunk...and in fact, you have to get there on Dec. 7th in a fairly strict time frame, do sinking at 05.00 not so good, sinking at 10.30 not possible.
Bottom-line: neat idea killed by facts...if you're writing write away, just know folks like me are going to be pulling their hair out, ESPECIALLY if this is some great central plot point.  And I think most readers of the discussions think, "Didn't we talk about that last month?"
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robdab       8/6/2008 7:29:48 PM
JFKY, thanks for your reply. I was beginning to feel very lonely.
What "special characteristics" does it need, but mass and open sea cocks?  It blocks the channel. - I was thinking a radio jammer might prove useful in choking off any calls for help by Ward. A couple of deck guns hidden as per the axis merchant raiders might give her some way of suppressing the fire of the lone US examination battery that was manned on that peacetime Sunday morning. Some Japanese style floating "smoke candles" could be useful in screening her from that fire as she moves away from Honolulu and towards the Pearl Harbor entrance channel shown on the chart that I referenced.
Explosive scuttling charges (with double wiring) to blow her bottoms out and so sink her much faster than the houirs it would take a few open seacocks to do the job. An extra stern anchor or two to hold her firmly in a channel blocking position as she sinks. An alternate or at least armored steering position since such will draw American coastal artilley fire in an effort to knock her off her course to a good blocking position.
A couple of fast motor boats to allow her scuttling crew some small forlorne hope of escaping back out to sea, alive. ?
I'm sure that there must be more items that would be useful.
 The repatriation ship isn't going to Pearl.  If it tries to go to Pearl alarms are raised.- If you look at the maps that I presented you will see that approaching Honolulu Harbor also means approaching the entrance to Pearl Harbor too. It's just geography. And my research shows what Yoshikawa could have easily observed on that same ground some 66 years ago. There were few alarms inplace to be raised and historically the Americans there were very tardy in doing so.
I wouldn't bet that the midget subs are going to hit the USS Ward. - Why not ? There were 5 of them carrying 2 torpedoes each. They were a secret weapon that the US knew nothing about so Ward wouldn't have been even watching for them.. Yoshikawa could have given Tokyo the exact details of Ward's patrol box at any time. Ward would have had to have stopped in order to launch a small pilot boat to carry a Honolulu Harbor pilot over to the Tatuta Maru. And last but not least, the 5 full sized Japanese "mothersubs" were still lurking in Mamala Bay and could have aided in sinking the USS Ward.
... the blockship doesn't sail with the fleet... - At 21 knots there is no reason at all that she could not have led the Kido Butai across the North Pacific to Hawaii. Historically Nagumo had three I-boats assigned for just that task but they couldn't keep up their surface speed thru the heavy weather that the Japanese chose to travel thru (and hide in). Since she was authorized by the US to be traveling to Hawaii there would have been nothing at all unusual about radio messages originating from an ocean liner in mid-Pacific. Nagumo would then have had a usable radio link to Tokyo !!
if the blockship is heading for Pearl somebody's going to notice and begin an intercept. - Not at all. Look at the maps. Except for the last 15 minutes at 21 knots approaching Pearl Harbor from the southwest is exactly the same as approaching Honolulu Harbor. Once she left the Kido Butai behind some 400 miles north of Oahu, any US PBY overflying her would have seen just what they expected to see ... a Japanese cargo-liner on her authorized way to Honolulu.
...if you're seriously entertaining this as a "what if" realize the reason no one tried it is that it would be darn near IMPOSSIBLE to pull off, simultaneously with the air attack, but the only way it works is if it does happen simultaneously with the air attack. - I would like to point out that the Tatuta Maru was an trans-Pacific cargo- liner that had been running the Yokohama - Honolulu - San Fransicso route once a month for NYK Lines of Japan since she was built in 1929. Nearly 12 years worth of back and forth. Her most recent visit to Honolulu had been in late October of 1941. She was recorded as arriving at the pilot launch there a mere three minutes ahead of her scheduled arrival time. Just three minutes over a 3,400+ nmile route. A "good enough" dress rehersal, I'd say.
And I only ask the Japanese to take the same risk that they did historically by dispatching the 5 minisubs to sneak into Pearl Harbor well ahead of the historical Japanese air raid. Historically Ward DID find and sink one of them but some 85 minutes later the USN was still caught by surprise inside of Pearl Harbor. Why do you
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