Fuel for thought - putting thirsty DDs on the wagon

[richardoren]

Hi Guys,

Well, after hand scraping the hull and watertite/repainting the old gal finally took to sea. The 8V53s run strong and she seems as seaworthy as they get.

Although she's just a baby Hatt she has a noble name, the teak ladder steps are a custom carved gift from Paul Starzynski (Starman) who refused payment - how's that for Hatteras community commitment!





The trip across the Med from Saint Tropez to Corsica took just over 6 hours, and I was glad that the fuel tanks had enough capacity to make it. However, I was a little stunned by the consumption for a 38 footer, some $2000 of diesel fuel for a short jaunt. Granted diesel costs close to $10/gallon this side of the pond.

As I didn't want to go alone, I hired a skipper who insisted at running WOT all the way, that probably didn't help. However, I noted that in the former owner's log book that his gallons per hour were indeed lower at slower rpm but his knots per gallon were not. This corresponds to 200 gallons for 100 nautical miles at between 15.5 and 16.5 knots.

BTW this is not a fuel cost lament thread, just a quick check with other forum members here as to whether this is normal consumption for the smallest normally aspirated DD naturals in terms of cylinder/injector size, even if they are V-8s.

Other posts in this forum have shown much heavier Hatts with 8-V71Ti engines getting roughly the same gallons per nautical mile numbers. It makes me wonder if my assumption that a smaller Hatt would be cheaper to run fuel-wise was incorrect?

If it is merely a matter of adjusting the injectors or what not, then it pays for me to fly to Fort Lauderdale and take the Customer Mechanics class at DD. In Corsica it is going to be darn hard to find a proper DD mechanic anyways so I'm resigned to doing the grunt work myself.

There is no way I would consider repowering. Aside from the unrecoverable costs involved, part of why I fell for this boat was the rumble of its DDs. But i do wonder if there'd be a way of retaining its exhilarating planing speed while saving somewhat going slower. I fear the wise will reply that going 5 knots should reduce the fuel flow significantly, and running only one engine at a snail's pace may become my only option due to double digit gallon prices... until I yearn for THE RUSH of those thirsty screamers!

[Boatsb]

I believe the 16 Kts at 22-25 GPH makes sense. My 41 gets about that with the 8V53's.

As for the captain running on the pins. Not good. First the engines were not made to run that hard all the time. They should be run a few hundred RPM's lower. Also the Captain on my vessel will never be overriding me on my vessel unless there was a safety concern. Do not let a hired captain dictate improper handling of your vessel to make his life easier. I know a delivery captain here that does the same nd he never gets a second call from 90 % of his customers. I wonder why.

[jim rosenthal]

Richard, you should feel proud, the boat looks great. I like the passarelle, very Mediterranean. Got any more photos?

[SKYCHENEY]

Our old DD mechanic that worked for us for 30 years always used to say to push those girls to the floor. A DD likes to run at high rpm. Those, of course, were naturals with low HP ratings. I would not push a turboed marine version to WOT for any length of time. But, his naturals are probably just fine at that pace.

Hull speed will be the best efficiency. Try running at about 8kts.

[Brian Degulis]

Most of our boats even those with naturals are set up at " Maximum Recreational Boat Rating". With that rating they should not be run on the pins more than one hour out of every 12 of operation. Only the 1800 RPM "continous commercial rating" can safely be run on the pins all the time.

Brian

[frank]

does that come out to 33.3 gph?

[spartonboat1]

These are observations of a 1972 43'MY w/6-71N's (naturals, non turbo). These are J&T (Johnson & Tower) engines rated at 310hp at 2,500rpm. I usually can only get about 2,450 digital indicated at WOT and other 43's say that also true for them.

Although I have often run at 2,150rpm, due to a slight overheat in the stbd unit, I now run at 2,180 or 90, now that the heat exchanger is cleaned and the temp is stable. So that is the 200 rpm under max rpm at WOT I often see as the best cruise rpm.

I see 12.7-12.9 knots at that rpm, althought the other day, with about half load of fuel and full holding tanks I was surprised to see 13.5kts, in to a 1 ft head sea in deep water (15-25 foot of depth will scrub off about 0.5 to 1.0 knot). I did convert over some of the hull to VC-17 bottom paint, so maybe that helped as well.

However, this year I have been running at hull speed (8.9kts) gps observed more often, with runs up to full cruise for the last 4-5 miles of the run. Even over the life of the boat, I expect 1mile/statute mile. My two tanks are about 185 gals (370 total) and measure 18 inches high, i.e. 10 gals/inch. So in 100 miles of running, I will expect to use 100 gals or 50 gals/side, i.e. 5 inches each. That usually holds true, plus I expect to use about 20% from the port tank, since that is the feed to the genset.

Usually these are rated at 0.7 mpg, at full cruise. So the mix of cruise and idle up rivers and going into ports increases the overall to 1.0 mpg.

My 11 years of use remarks.

[Will]

I believe that HP ratings are usually down-graded on commercial engines because most of the commercial guys run them on the pins full time. Most 71 series engines I've seen were governed to 1800 RPM - turbo'd or not. Depending on injectors, that's about 185 HP and anything less than 10,000 hrs was considered a premature failure. Remember however, commercial boats are run regularly so the engines are not as prone to corrosion as on boats only run occasionally. Although my 6-71s are rated at 485 hp at 2500 RPM, the power curves show them to be around 185hp at 1800. Regardless, being a bit on the frugal side, I usually run at 1200, 1600 if in a hurry but never more than 1800. The engines have about 2,000 hrs and show no signs of age or fatigue.

As for a Captain running them on the pins, if I tell him not to and he refuses, he is relived.

Will

[Capt K]

She looks great, man! I had a 41' TC with 6 53's before my current 50'MY. That 41' was probably the best sea boat I ever had. She had no bridge so the CG was low and she ran great in all seas!

Best of luck (not that you'll need luck) and have loads of fun!!

Capt K

[Pascal]

the captain had no business "insisting" to run at WOT, unless there is a safety issues (weather), he has to listen to you the owner, or else. Cruising speed, route, etc... needs to be discussed before the trip/hiring.

fuel burn is all about speed... at WOT any engine will burn a ridiculous amount of fuel, what an idiot!

the worst speed to run at is from hull speed (8.5 or so on your boat) up to when the boat is on plane, probably 15kts. you're just pushing a big wake and burning a ton of fuel.

for max range and economy, slow it down to hull speed or just under... that boat shoudl get well over 1 NMPG. with the 53 and 8V71s we get just under 1nmpg at 9kts (hull speed is 9.3)

beautiful boat!!! in Corsica... great cruising grounds!