Civil War history has a bad habit of comparing apples and oranges when strength categories are in contention. Invariably the "effectives" of the Confederacy is compared with the "present for duty" strength of the Federals. This distorts matters. Here we will adopt the approach of accepting the Confederate measurement and moving the Federal measurement to conform. We will also analyse the arrival times of various units to determine strength ratios.
2a. Confederate Effective Strength and Arrival Times
To determine Confederate strength the partial return Johnston provided ca. 30th April in the OR (ref) is used. To this some cavalry is added from the OR (ref) and Livermore (ref). Steven Newton estimated that between 10 and 20% of the army went sick in the trenches, and thus a correction factor of the lower bound (10%) is used in the sure knowledge this is not overcorrecting.
Arrival dates were ascertained by a general survey of biographies, regimental histories etc., and the last unit to arrive (Pettigrew's brigade) essentially walked the whole way, whilst most units got a train and then a steamer to King's Mill Wharf in Williamsburg. Results are in table 1. The entry "present before campaign" indicates the unit or formation had joined by 5th April, in some cases barely (such as Colston's brigade from Norfolk, and Pendleton's artillery).
Table 1: Effective Strength of Johnston's Army at Yorktown with Arrival Dates
2b. Strength at Various Dates
Accepting Newton's lower bound of 10% and correcting for a constant decay down to the 30th April estimate by Johnston we get the following strength at Yorktown (inc. Gloucester Point, Williamsburg etc.) for various dates:
Table 2: Estimated Effective Strength of the Yorktown Defences at Various Dates
3. Federal Effective Strength
One problem facing us estimating Federal effective strength is what did Johnston mean be "effective strength"? He appears (from previous returns) to be estimating his officers and men that were combat effectives, so that's the measure I will use.
Secondly there is the issue of what constitutes arrival? Do we count Casey, whose division consists of newly raised troops who haven't even been issued tents and so are suffering in camp near Fort Monroe whilst the QMG in Washington sorts his act out and don't move forward to a mile behind the main army on the 11th? When did Hooker, Richardson and Franklin actually arrive?
In his diary entry for 11th April la Comte de Paris states Hooker and Richardson have just arrived at Ship Point and have been held there for the moment. My notes indicate the two divisions completed disembarkment 10th-11th April and were held at Ship Point until 16th, when McClellan decided to adopt "regular approaches". Kearny's brigade of Franklin boards ship on the 17th, with the rest boarding on the 24th as a turning force.
Thus I will count Casey, Hooker and Richardson as arriving on the 11th, Kearny's brigade on the 17th (when they board ship) and the rest of Franklin on the 24th (ditto).
The third problem is the lack of divisional or brigade returns for the period. Hence I will estimate divisions at that fraction of their corps. I will take the corps strength, deduct an estimate for cavalry (800 men per regiment) and artillery (20 men per gun) and assume the infantry effectives are 75% of the residue.
Table 3: Calculation of Infantry Effectives in Each Federal Corps
We shall now calculate effectives by date.
McClellan intended to have a much larger body of cavalry, forming a brigade in each Corps taken to the Peninsula, and a reserve of 2 brigades (ref). He ended up with about 6.5 regiments. This of course had a significant effect. Here it's just left at 6,000 for all cavalry in the absence of better data.
The artillery is rated by number of guns. The strength of the artillery reserve (100 guns) is estimated with the rest. However, one would observe without gaining or losing guns or batteries its strength (PFD) has gone from 2,731 on 31st March to 1,888 on 20th May, or 69% of starting strength. This may be a good indicator of wastage. However, a simple analysis shows essentially no Federal wastage during the Yorktown operations, whilst the later Chickahominy operations show heavy losses.
Table 4: Federal Effectives by Date.
4. Force Ratios and Conclusions
Hopefully this is informative. It should be observed that McClellan's infantry force ratios start out in Grant territory of ca. 2:1, but within a few days is closer to 3:2 and by the late siege (with Franklin) are nearer 4:3. The Confederates were able to reinforce quicker than the Federals and by the time McClellan committed to regular approaches the infantry ratio was 4:3.
The question must remain was an immediate attack possible? It might have been, and McClellan actually ordered it. When Keyes reported Smith had met resistance at Lee's Mills his order to the courier bringing the news was “Ride back as fast as you can, and tell General Keyes to attack with all his force if only with the bayonet.”; he had just ordered Porter (leading the other column) “Attack with all your forces as soon as you arrive.”
However, the Confederates laid down such a wall of artillery fire that they smothered the advancing Federals, who settled down to counterbattery, and of course lost because the Confederates had works. The infantry found the approaches barricaded and covered by heavy fire.
Anyone criticising McClellan must provide a viable means of assaulting Yorktown. Personally I believe that after the failure of the navy McClellan hit upon the only viable plan - construct our own works and being up big guns to blast them out and get my infantry across the killing area. The assault was set for the 5th May, with six divisions assaulting the Yorktown fortifications above the Warwick river, but the rebels skedaddled even as the Federal assault forces were moving into their assembly areas awaiting the crossing of the LD. However I'm open to better options....