Visuddhimagga VI-86

Yasmā pana dasavidhepi etasmiṃ asubhe seyyathāpi nāma aparisaṇṭhitajalāya sīghasotāya nadiyā arittabaleneva nāvā tiṭṭhati, vinā arittena na sakkā ṭhapetuṃ, evameva dubbalattā ārammaṇassa vitakkabaleneva cittaṃ ekaggaṃ hutvā tiṭṭhati, vinā vitakkena na sakkā ṭhapetuṃ, tasmā paṭhamajjhānamevettha hoti, na dutiyādīni.

次于十种不净之中，譬如在水不静止而急流的河中，由于舵的力量可以停止船只，若无有舵想止住它是不可能的；如是因所缘的力量弱，由于寻的力量，止住于心而成专一，若无有寻想止住他是不可能的；所以在十不净中只能获初禅，不能得第二禅等（第二禅等无寻故）。

Ñ(VI,86): But as regards the tenfold foulness, just as it is only by virtue of its rudder that a boat keeps steady in a river with turbulent [(aparisaṇṭhita)] waters and a rapid current, and it cannot be steadied without a rudder, so too [here], owing to the weak hold on the object, consciousness when unified only keeps steady by virtue of applied thought, and it cannot be steadied without applied thought, which is why there is only the first jhāna here, not the second and the rest.

Sayādaw U Sīlānanda: [rudder => pole].