Passengers would not accept longer ferry sailings between the mainland and the Western Isles to cut CO2 emissions, according to the islands' council.
The Scottish government has suggested that slowing down the boats could help reduce the vessels' carbon footprints.
But Comhairle nan Eilean Siar said mainland links to the isles were long enough already.
Lengthening journey times further could cause problems for commercial drivers, the authority said.
The comments are contained in draft responses to questions in the Scottish government's ferries review.
Councillors on the comhairle's transport committee have been asked to consider the answers to a series of questions which form part of the review.
Officers drafted the responses following public meetings on Barra and Benbecula and in Tarbert and Stornoway.
On slowing ferries to reduce carbon emissions, officers have urged caution.
In the draft response, they said the mainland links were already the longest in ferry operator Caledonian MacBrayne's network.
Long distance lorry drivers and other commercial drivers could also have problems meeting limits on their hours of work if trips were extended, officers said.
They added that consideration should be given instead to speed reductions on short routes such as journeys taking less than an hour.
Transport committee chairman Donald Manford told BBC Radio Scotland that it was important to tackle emissions, but this had to be balanced with the need for quick and dependable services.
He said: "Sea transport are our highways and the over-riding importance is to get efficiently, safely, quickly and dependably to where you want to go.
"What has been seen in the past few years is what a huge boon from the lowering of ferry fares have been to the Western Isles."