The African Queen

Samuel Sayer (Robert Morley) and his sister Rose (Katharine Hepburn) are British Methodist missionaries in the village of Kungdu in German East Africa at the beginning of World War I in August/September 1914. Their mail and supplies are delivered by a small tramp steamer named the African Queen, helmed by the rough-and-ready Canadian boat captain Charlie Allnut (Humphrey Bogart), whose coarse behavior they tolerate in a rather stiff manner.
When Charlie warns them that World War I has broken out between Germany and Britain, the Sayers choose to stay on, only to witness the Germans burn down the mission village and herd the villagers away. When Samuel protests, he is beaten by a German soldier. After the Germans leave, Samuel becomes delirious with fever and soon dies. Charlie returns shortly afterward. He helps Rose bury her brother, and they set off in the African Queen.
Charlie mentions to Rose that the Germans have a gunboat, the KÃ¶nigin Luise (German for "Queen Louisa"), which patrols a large lake downriver, effectively blocking any British attacks. Rose comes up with a plan to convert the African Queen into a torpedo boat and sink the KÃ¶nigin Luise. Charlie points out that navigating the Ulanga River to get to the lake would be suicidal: to reach the lake they would have to pass a German fort and negotiate several dangerous rapids. But Rose is insistent and eventually persuades him to go along with the plan.
During their journey down the river, Charlie, Rose and the African Queen encounter many obstacles, including the German fort and three sets of rapids. The first set of rapids is rather easy; they get through with minimal flooding in the boat. But when they pass the fortress and the soldiers begin shooting at them, the bullets pierce the top of the boiler and cause one of the steam pressure hoses to disconnect from the boiler. This causes the boat's engine to stop. Luckily, Charlie manages to reattach the hose to the boiler just as they are about to enter the second set of rapids. The boat rolls and pitches crazily as it goes down the rapids, leading to more severe flooding in the boat. However, they make it through.
While celebrating their success, the two find themselves in an embrace. Embarrassed, they break off, but eventually succumb and strike up a relationship. The third set of rapids comes up. This time, there is a loud metallic clattering noise as the boat goes over the falls. They dock on the river bank to check for damage. When Charlie dives under the boat, he finds the propeller shaft bent sideways and a blade missing from the propeller. Luckily, with some expert skills and using suggestions from Rose, Charlie manages to straighten the shaft and weld a new blade on to the propeller, and they are off again.
All appears lost when Charlie and Rose "lose the channel" and the boat becomes mired in the mud amid dense reeds near the mouth of the river. First, they try to tow the boat through the muck, only to have Charlie come out of the water covered with leeches. All their efforts to free the African Queen fail. With no supplies left and short of potable water, Rose and a feverish Charlie turn in, convinced they have no hope of survival. Before going to sleep Rose prays that she and Charlie be admitted into Heaven. As they sleep, exhausted and beaten, heavy rains raise the river's level and float the African Queen off of the mud and into the lake which, it turns out, is just a short distance from their location. Once on the lake, they narrowly avoid being spotted by the KÃ¶nigin Luise. The KÃ¶nigin Luise departs, but Charlie believes that she will return as she makes her normal rounds of the lake.
Over the next two days, Charlie and Rose convert some oxygen cylinders into torpedoes using gelatin explosives and improvised detonators that use nails as the firing pins for rifle cartridges. They then push the torpedoes through holes cut in the bow of the African Queen as improvised spar torpedoes. The KÃ¶nigin Luise returns as Charlie predicted, and Charlie and Rose steam the African Queen out onto the lake in darkness, intending to set her on a collision course with the KÃ¶nigin Luise before diving overboard and making their way to safety to the Belgian Congo on the west side of the lake. A strong storm strikes as they head toward the KÃ¶nigin Luise. Unfortunately, the torpedo holes they cut are not sealed, which allows water to pour into the African Queen, causing her to sink lower and lower. Eventually the African Queen capsizes, throwing them both into the water. Charlie loses sight of Rose in the storm.
Charlie is captured and taken aboard the KÃ¶nigin Luise, where he is questioned by the captain. Believing Rose to have drowned, he makes no attempt to defend himself against accusations of spying, and the German captain sentences him to death by hanging. However, Rose is captured and brought to the KÃ¶nigin Luise just after Charlie's sentence is handed down. Charlie hollers her name, then tries to protect her by pretending not to know her. The captain questions her, and Rose confesses the whole plot proudly, deciding they have nothing to lose anyway. The captain sentences her to be executed as a spy, too. Charlie asks the German captain to marry them before executing them. After a brief marriage ceremony, the Germans prepare to hang them, but there is a sudden explosion and the KÃ¶nigin Luise quickly capsizes and sinks. The KÃ¶nigin Luise has struck the overturned hull of the African Queen and detonated the torpedoes. Rose's plan has worked after all, if a little belatedly, and the newly married couple happily swims to safety on the west shore of the lake.

Question: What do the soldier's bullets hit?
Answer: a boiler