LOUISVILLE — The Coast Guard has received responses from only 10 houseboat manufacturers out of 85 that were asked for ways to fix a problem with carbon-monoxide buildup near generator exhausts.

        Carbon monoxide was blamed for the deaths of seven people who drowned while swimming near houseboats on Lake Powell in Arizona. The study at Lake Powell, and a follow-up study at Kentucky's Lake Cumberland, found dangerously high concentrations of the gas around the swim platforms of boats with rear exhaust systems.

        Randolph J. Doubt, a Coast Guard engineer, said Tuesday he was disappointed with the low response rate but added that more responses could still be in the mail.

        The Coast Guard's deadline to respond was Monday. About a dozen companies that were sent letters by the Coast Guard are no longer in business.

        Manufacturers who fail to respond could be fined $1,000, Mr. Doubt said.

        As houseboats have acquired more amenities demanded by buyers, such as refrigerators and other appliances, the generators that run the devices have grown more powerful — and their exhausts more dangerous.

        One of the largest houseboat makers, Sumerset Custom Houseboats in Somerset, Ky., is experimenting with a system that vents fumes from the top of the boat.

        Two weeks ago, the company offered to convert its older-model boats with rear exhausts to side-vent systems free of charge. Sumerset also offered to make the same modification at cost on competitors' boats.

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