1985 61 Hatteras MY with De-Tuned 12-71 TIs 2000 Hours

[happyours]

I am planning to look at a 1985 Hatteras MY which appears to be in excellent condition. The listing asserts that the engines are 12-71-TIs 650 HP De-Tuned to Commercial Work Boat Standards (New Rated HP is 450 HP). Engine have approximately 2,000 hours.

I have been asking but have so far not found anyone who recognizes this designation and/or able to say if this is a net positive on this boat. Since we would use the boat primarily in trawler mode it seems like a positive but it would be nice to confirm that. If we decide to proceed we will certainly have a proper engine survey and hopefully that will clear up the subject. However it would be nice to have the best understanding possible before proceeding with an offer and survey.

Any suggestions or comments would be appreciated.

[oscarvan]

Never heard the term, but I don't know everything. If I was a betting man I'd say some of the old Hatts are gonna come along and raise the BS flag. All I can think of is smaller injectors.

[happyours]

Never heard the term, but I don't know everything. If I was a betting man I'd say some of the old Hatts are gonna come along and raise the BS flag. All I can think of is smaller injectors.

Thanks

[Cricket]

Pretty common to "detune" engines, especially 2 stroke DD"s which were probably "tuned" too high in the first place for some of the applications. Think it's a good thing for what you're trying to do. Guess my one question is would it be better/helpful to have a higher gear ratio in this case?

[happyours]

Pretty common to "detune" engines, especially 2 stroke DD"s which were probably "tuned" too high in the first place for some of the applications. Think it's a good thing for what you're trying to do. Guess my one question is would it be better/helpful to have a higher gear ratio in this case?

Thanks for the insight. Good to hear from someone who is familiar with de-tuning. Most folks react with a quizzical stare when I bring it up. Might take the survey to know abut the gear ratio. Brokers don't seem to have a wealth of specific information like that immediately available. Someone did bring up the subject of the props needing to be tweaked.

[jim rosenthal]

Some marine engines were offered in different power ratings, along with instructions regarding how hard and long you could run them. I think a C or D rating means commercial or workboat use, where they could run at WOT all day, all the time. Pleasure craft use restricted you to something like "no more than 1/2 WOT out of five or six hours" or something like that. I'm sure when they built the engines to a lesser rating, fitting smaller injectors was part of the plan.

450hp is a rather modest rating for 12-71s, but as long as they'll turn up to rated RPMs and they pass an engine survey by a guy who knows DDs of that vintage, I would not be worried about them.

Actually, 12-71TIs at 650 hp are not at the highest power rating, either. Seems like they were offered in higher numbers than that, but not certain.

You might be able to find more information on boatdiesel.com, too.

[happyours]

Some marine engines were offered in different power ratings, along with instructions regarding how hard and long you could run them. I think a C or D rating means commercial or workboat use, where they could run at WOT all day, all the time. Pleasure craft use restricted you to something like "no more than 1/2 WOT out of five or six hours" or something like that. I'm sure when they built the engines to a lesser rating, fitting smaller injectors was part of the plan.

450hp is a rather modest rating for 12-71s, but as long as they'll turn up to rated RPMs and they pass an engine survey by a guy who knows DDs of that vintage, I would not be worried about them.

Actually, 12-71TIs at 650 hp are not at the highest power rating, either. Seems like they were offered in higher numbers than that, but not certain.

You might be able to find more information on boatdiesel.com, too.

Thanks for your help.
Visited the boat today and everything looks pretty nice. In talking with the broker (who apparently also captains for the owner) he indicated that the engines only turn up 1500. I have also heard that de-tuning might also involve adjusting the governor but obviously that leads to more questions.

[scottinsydney]

My previous boat had 8v92Ns, but was derated from 388 to 356 hp using a different injectors. This is the only way I know of to reduce the hp. This also reduced WOT from 2300 to 2100RPM.

Some form of throttle stop preventing the motors going past 1500 might have been installed. In theory they should go past 2100 or so.

You could try spinning them up in idle to see if they go past 2100RPM.

Or the boat has very odd propellers installed. Or the present owner just does not like them going past 1500RPM.

A reduction of 600 RPM or so from WOT is possibly more than what smaller injectors could do and I doubt any real fuel savings.

[rtrafford]

I don't see any real fuel savings, as horsepower is needed to push the boat at a given speed, and horsepower is achieved by fuel consumption. Detuning, you're limiting the potential stress on the engine, allowing it to live a perhaps longer life (usually), and lessening the attachments necessary for maintenance and repairs. 12v71TA can be rated at 750 HP, but peel away the turbos and after coolers etc, swap the injectors, and reset the fuel pump and you'll only get the 450HP.

However you'll run her at a higher RPM to achieve the HP needed for the wheels to spin and speed to be achieved. If you compare the horsepower curves o the configuration, you'll likely find that your 14-1500 RPM max displacement cruise at 750 HP is now requiring you to push her at 2000 RPM to hold the same speed.

But, if you then back down to a slower displacement, say 9 knots instead of 11, you're saving fuel and long term maintenance. In theory.

[happyours]

My previous boat had 8v92Ns, but was derated from 388 to 356 hp using a different injectors. This is the only way I know of to reduce the hp. This also reduced WOT from 2300 to 2100RPM.

Some form of throttle stop preventing the motors going past 1500 might have been installed. In theory they should go past 2100 or so.

You could try spinning them up in idle to see if they go past 2100RPM.

Or the boat has very odd propellers installed. Or the present owner just does not like them going past 1500RPM.

A reduction of 600 RPM or so from WOT is possibly more than what smaller injectors could do and I doubt any real fuel savings.

Thank you for the insights. If we decide to proceed we will have a good engine survey and expect to get a complete understanding.