In a model of a ship, the mast is 9 cm high, while the mast of the actual ship is 15m high.

Let $x \mathrm{~m}$ be the length of the model of the ship. Then, we have:

$1 \mathrm{~m}=100 \mathrm{~cm}$

Therefore, $15 \mathrm{~m}=1500 \mathrm{~cm}$

$35 \mathrm{~m}=3500 \mathrm{~cm}$

Clearly, if the length of the actual ship is more, then the length of the model ship will also be more.

So, this is a case of direct proportion.

Now, $\frac{1500}{9}=\frac{3500}{x}$

$\Rightarrow x=\frac{3500 \times 9}{1500}$

$\Rightarrow x=21 \mathrm{~cm}$

Therefore, the length of the model of the ship is 21 cm.