Back (American Ordnance)

American Hand Grenades of W.W.I
Germany was well prepared for trench warfare in 1914, having produced quantities of many types of grenades as early as 1913. The Allies had failed to foresee the extended use of hand grenades and were forced to hastily construct improvised types. During the next three years hand grenade design progressed from crude bombs to refined weapons.

With the entry of the United States into the First World War it also found itself without many of the "modern" devices necessary on the battlefields of Europe. With time short, the U.S. Army was supplied by the French and British with many weapons missing from its inventory, including the hand grenade.
America, evaluating those weapons, developed versions better suited to the desires of the War Department.
Four were produced in time to see combat use.
American WWI Grenades

Left to Right:
Mk.II Defensive Fragmentation (Mk.II Fuze/Mk.I Body)
Mk.III Offensive Blast
Mk.II Gas
Mk.II Phosphorus

Defensive fragmentation grenades were the most common type in use by all nations, typically made of heavy cast iron with external and/or internal fragmentation grooves. Because of its extended lethal range they had to be used from protective cover.
Offensive types used a non fragmenting body, in the case of the American Mk.III this was made of cardboard. It caused casualties by blast effect, effective only within a limited radius, so could be used in the open.
Gas grenades were for clearing tight twisting confines of the trenches where an explosive grenade was ineffective.
Phosphorous grenades were used in offensive barrages to create walls of thick white smoke to screen advancing infantry.

French F1 - US Mk.I
French F1 ("Fusing One") and the U.S. Mk.I

Mk.I HE Offensive Fragmentation Grenade

Of particular historical interest is the Mk.I fragmentation grenade, which was inspired by the French F1.
However, American designers felt that there were several shortcomings with foreign fragmentation grenades in general:
o Cast iron bodies which did not fragment well.
o Safety levers difficult to grasp firmly, tending to slip prematurely.
o Insufficient safety features

" The full-hand grip, prescribed for foreign grenades which have very strong levers, like the F1 and Mills grenade, is a very awkward and unsatisfactory grip for Americans, and is taken from necessity rather than choice."
Grenade Training Manual - Army War College, Jan 1918
With little time, designers set to work and by August 1917 were ready with the Mk.I hand grenade. Production was started immediately and at an accelerated rate. Grenades were rushed into the hands of troops training for deployment to France, without any significant field evaluation. There was an extremely high degree of confidence that this concept made all existing grenade designs inferior, as indicated by this glowing excerpt from an Army training manual of the time:

"When thrown in the prescribed manner... it is armed automatically as the grenade leaves the hand.... without any conscious effort on the part of the thrower."
"This is the simplest grenade that has ever been invented and may be called a fool-proof grenade."
Grenade Training Manual - Army War College, Jan 1918

Ironically it was not long before the shortcomings of the Mk.I became all too apparent. Rather than the anticipated "automatic" and "fool-proof" function it proved just the opposite.
A field report from the AEF, in May 1918, condemned the grenade as overly complicated and a failure. Evidently many times the grenade was thrown before being completely armed, only to be returned by the enemy in a properly functioning state.  
Grenade production came to a grinding halt in July 1918.
More Details.... Mk.I Cut-Away & Detailed Comparison to the Mk.II

Mk.I Mk.II Fuzes
Mk.I Twist-Lever Fuze and the Mk.II "Cut-Back" fuze.
Mk.II Fuze
With the failure of the Mk.I to perform adequately an immediate redesign effort commenced. The fuze was modified by eliminating the twist lever and its supporting arm and creating the now familiar one piece fly-off lever. There were also some minor grenade body changes. The new grenade and fuze were designated the Mk.II.
The Mk.II fuze became the standard for other U.S. grenades.

At this time, in inventory, there were over 4.5 million Mk.I fuzes already assembled as well as 15 million rough Mk.I body castings.
The fuzes were reworked into a Mk.II configuration by removing the pivot arm from the body and replacing the lever. Existing frag bodies were finished and used as is. New components were produced and introduced as existing inventories were depleted.

US Mk.II & Mk.V Gas Grenades
Mk.II Gas, w/ Transit plug and Mk.V Gas
Mk.II KJ Gas and Mk.V CN Gas Grenades

Here is the Mk.II KJ gas grenade, shown next to its post war successor the Mk.V CN gas.
The KJ gas type was filled with a pressurized stannic chloride filler at 50 p.s.i.. It was used to clear bunkers and make dugouts uninhabitable.

Production difficulties lead to the redesigned non-pressurized Mk.V grenade using a solid CN filler. This was a combustible mix which burned and produced gas that vented out holes drilled around the top of the body (covered with tape). This concept continues to be used today.

These are shown together as Mk.II bodies were reworked into the Mk.V configuration with a modified fuze well. Note the Mk.V with the later fuze style.

More about the Mk.II Gas More about the Mk.II Gas Grenade

Production Figures
U.S. made WWI grenades are a desirable addition to any collection. They were made in far fewer quantities than many European types. The Mk.I in particular is of interest because of its notoriety and short production life.

Statistics from: America's Munitions 1917-1918,  U.S. War Office 1919
Nov 1918
Feb 1919
Shipped Overseas
Mk.I & II Defensive
Mk.III Offensive
Mk.II Gas
Mk.II Phosphorus