48' Yachtfish

[Walter P]

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

[Duke54]

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

[REBrueckner]

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!!!!

[bobk]

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