48' Yachtfish

Does anyone have any experience with replacing
the DD 6-92 TA's with smaller engines, say 140 HP? i.e. converting to a LRC?
I am OK with 8-9 Kts. cruise speed, looking for reduced fuel flow.
Thoughts and idea's appreciated.

Duke, Why re-invent the wheel. Your boat as is a fine LRC if you choose to run it that way. Actually at an 8k cruise you will burn a little less than 6 gph. The exact amount can probably be verified by Bobk who uses his '81 48 MY as an LRC and runs back and forth from Baltimore to the Fla keys every year at 8k. Unless your engines are shot and need to be rebuilt/replaced, I'd leave it alone and enjoy it he way it is.

Walt
1982 Series 1 48
New Jersey

Just thinking of what to do when these engines wear out and diesel prices are at $5.00 per Gal.

If you run them at 1500 RPM's they will never wear out!

You will likely never get your money out of a repower in fuel savings. Keep what you have and run at hull speed.

I did a little research on changing out my 43 to 110hp J.D.'s the short of it is it's just not doable. Between the gears and the engines, wheels and install just to much money to put in a boat that for all intents and purpose (at this time) is worth very little compared to the repower. It can only be even slightly pratical if you totally lose an engine. The savings in fuel are almost non exsistent maybe a half a mile or so to a gallon. The most practicle thing is face hatteras every morning give the 6-71 prayer and run slow. Bill

I'll say the 6-92 prayer and just keep going at 1500 RPM until they blow up.

Ahh, Duke, I see you have a 48CMY, not a 48YF. I wondered why you had 92's in an early 1970's boat. At first I thought you must have a repower, but now that I looked at your profile, I understand.

The later model 48 MY is a good boat. I like the layout. Just run the the little 692s slow and you're good to go.
More LRC envy.
Skooch
Worton Creek MD

Sorry about the YF misspeak. I'll keep running the 6-92's @1500 until they die.

Duke,

Actually 1500 rpm's is probably very inefficient. Either go slower around 1200 r's or bite the bullet and get up on plane.

If you check out the brochures section on our host's web site (Sam's), check out the Series 1 MY, which is basically the same hull and machinery. There is an article which reports an early test when these boats first his the scene as well as a full brochure. The article has a pretty accurate chart on fuel consumption/performance.

Walt

Good article, 1250 rpm it is.
Still wondering what engines to re-power with if my 6-92's blow up?

Duke...you don't say how many annual hours you run and that is an important factor. If you run like most at under a hundred hours you would not likely recover new engines costs in a lifetime. But resale would possibly be better. At 2,000 annual run hours it could make a lot of sense in fuel savings.

You also don't mention the condition of your 92's. Are they turbos? If so 1250 RPM might be a bit slow...check your turbo exhaust temps at that RPM with an infra red thermometer to see if you are at Mike P's recommended 500 degrees or above...I doubt it...I have to run my older 8V71TI's at around 1500 to keep that temperature....naturals do better at lower RPM's as the injectors are smaller....
Unless either engine has a serious problem repairing one would likely be far less costly than replacing both. Something to consider: put in hotter thermostats: Get advice from others here. 71 series TI's apparently have 160,170 or 180 degree thermostats...according to my looooong time DD mechanic. I use 170's because I run at modest RPM and I like the temperatures where I operate. If I was running fast in tropical waters, I'd definitely change them to 160's..

I would NOT hold my breath waiting for well maintained 2 cycle detroits to wear out at 1500 RPM or so. If you WANT to kill them off just run them at WOT whenever you go out. Otherwise, run your engines frequently, keep them filled with fresh heavy duty DD approved antifreeze, heavy duty DD approved lube oil and monitor operating temperatures when underway and you'll be able to post your question again in twenty years!!!!

Duke, here is my experience and thoughts with my 1981 48MY. She had 26X22four blade props when I bought it (Hatteras manual called for 26X23 but most all of these seemed to have 26X22). Not sure they were tuned right as I had some vibration at 2000 rpm during the survey but was assured it was in 'normal range'. I made two round trips to Florida from Baltimore at 1000-1100 RPM. Just barely 8 knots. Fuel burn averaged 1.2-1.3 mpg. Lots of generator time included in that, maybe three hours a day. I had some transom soot and got some smoke on the first trip each time I cranked it up to 2000 rpm to clear the carbon.

On the second trip I began adding Howes MPK, and soot was really reduced as was the smoke. For the third trip, I had my spare set of props tuned to 26X23.5 and printed. You may have seen my post. Vibration gone. Speed came up a bit. Fuel burn seems about the same. No more transom soot, virtually no smoke when running them up and I'm getting proper WOT rpm and 15 kts at 2000. 18.5 at 2300. I now run them up once every other cruising day and just for a few minutes unless I'm trying to make a bridge.

If you look at the fuel burn curves, you will get very happy at 1000 rpm. Incrementally you pay dearly to push it to nine knots from 8.2-8.3.

I have almost 1000 hours on her at these speeds in the last couple of years. By the way, my 6V92's are rated at 425 hp and had 200-300 or so hours SMOH when I bought her.

I agree with Walt, you do not want to run 1500 rpm, and while 1250 will get you to nine knots, why bother. It's quieter and more relaxing at 1000 rpm.

Bob