8V71ti's fuel consumption

[jcrlaw]

I take delivery of my new (to me) 58' YF with 8v71 ti's on Sunday. Does anyone have any fuel consumption information or know a site which might provide me with some general information?

 

[SKYCHENEY]

should be about 10gph at 9 knots or 35gph at 16knots.

 

[Dustoff44]

Real basic numbers .75 x horsepower = burn at max cruise
.25 x horsepower for displacement speed
Works for most boats. Double the speed triple the burn.

 

[Nonchalant1]

For planning, I think it's best to think of it in gal per mile. The hull speed of your boat is about 9.8 knots. That's the most efficient speed for your boat. You'll get a little better than 1 gal per nautical mile at hull speed. But that's really slow. If your hull, engines, props, etc. are in good shape, your boat will plane by taking it up to 2300 RPM until it planes and then backing down to 2000 or 2050 (but watch the engine temps to stay below 190 degrees). Planing, you can probably do 15 knots while burning 2 gal per NM. So, 50% faster is twice the fuel per nautical mile.

Doug Shuman
1978 53MY (8V71TI's)

 

[jcrlaw]

Thank you all very much.

 

[first230sl]

My experience is bit different. I have a '71 58 YF with the 8V71TIs.

To be honest, I am not sure I actually ever get the boat fully on plane. At my usual cruise of 1500 RPM, I get between 10.5 and 11 knots out of her. I burn about 15 gallons per hour at that speed (both engines combined).

If I increase the engine speed, the boat speed increases linearly up to a maximum of about 15 knots (at most) with the engines between 2150 and 2200. My engines never get up to 2300 - which almost certainly means I am over propped. Of course, while the engine speed and boat speed change in a linerar relationship, the fuel consumption, alas, does not

I can send you the engine speed <-> boat speed numbers if you like.

I will be extremely interested in the experience you have with speed and fuel consumption to compare against mine. I posted a similar request to yours in this list many months back and, while I got some excellent responses, no one with a 58 YF was able to tell me their actual experience.

So do let us (or me, if no one else is interested) know.

Take care - Murray (doing some consulting in Barcelona - but looking forward to two weeks on the boat when I return).

 

[Cobra29]

Does anyone have the same kind of estimates for a 43C with 6V92ti's.

Thanks,
Gene

 

[Cobra29]

Does anyone have the same kind of estimates for a 43C with 6V92ti's. 19 knots @ 1950 turns

Thanks,
Gene

 

[Genesis]

45C with 6V92TA 500s turns 18 kts @ 1900 RPM burning about 36gph (combined)

Estimated off long runs, no flowscans.

On the pins I can do 22/23 with my usual load, but I never run over 2000 (which is in the 19ish range) Anything more than that requires 30+kts of wind at my back

She saw 25kts once - at sea trial, with about a gallon of fuel, no water, no waste, and three guys on board.....

 

[Nonchalant1]

Murray,

Either over-propped or fouled bottom, or wrong injectors, or, or..........

Your engine ID plates will show the max RPMs for which your specific engines are rated. You should be able to get there at no load and close to it with a good load. Karl (Genesis) has opined numerous times in previous threads about overloading engines, high exhaust temps, etc.

You'll know when she actually gets up on plane because the bow comes way down to level attitude again. At all speeds before that, the more power you apply, the higher the bow goes and you're pushing a mountain of water in front of you and digging a bigger hole behind you.

Doug Shuman

 

[Genesis]

I can hold my 45C on plane at about 13kts. Another way to know if you have a clean view of the stern is that the wake will separate from the transom cleanly when you are out of the hole.

 

[Traveler 45C]

What do you mean by, “the wake will separate from the stern cleanly”? I can’t picture that in my head.

 

[Genesis]

If you look at the transom while running, when the boat is on plane you can see all the way down to the bottom of it - the water comes off the stern of the boat cleanly.

When you're not all the way out of the hole, there will be a big roiling boil back there.

 

[Traveler 45C]

Ah, ok, I’ve seen both. Thanks.

My last boat had a swim platform so I never really saw what the transom looked like on plane. Traveler does not have the swim platform attached yet. I first time I saw the trim tabs on the surface I freaked. I didn’t expect that. I was completely amazed that I could see all the way down to the bottom of the transom.

 

[first230sl]

I'll take a look down the transom, but I believe I never get onto plane. The bow continues to rise as I apply more power and never comes back down - even at full throttle. I wonder if I can coax it onto plane by putting the trim tabs all the way down at maximum throttle? I'll try that.

I know people with 53s get them on plane no problem. But can anyone tell me for sure that the 58 YF actually does plane with the 8V71TIs? If so - what RPM/Speed does it take to get it up there for you, and how much can you slow down afterward and still remain on plane? Can you do so despite the setting of your trim tabs?

I'd really *love* to hear from a 58 owner who can give me some of this info. It would be tremendously useful.

I am in Barcelona now, but get home tomorrow and immediately go away on the boat for two weeks (hurray!). Hope I have some network connectivity while on the boat. Not sure if I will.

Take care - Murray

Murray,

Either over-propped or fouled bottom, or wrong injectors, or, or..........

Your engine ID plates will show the max RPMs for which your specific engines are rated. You should be able to get there at no load and close to it with a good load. Karl (Genesis) has opined numerous times in previous threads about overloading engines, high exhaust temps, etc.

You'll know when she actually gets up on plane because the bow comes way down to level attitude again. At all speeds before that, the more power you apply, the higher the bow goes and you're pushing a mountain of water in front of you and digging a bigger hole behind you.

Doug Shuman

 

[MikeP]

Planing and fuel burn...

Our 53 does not plane very easily. In fact, with full fuel and water, it's a real struggle to plane at all. When nearly empty, it will hold 18 knots planing but last week it struggled to get to 15 and varied between 14.5-15. that was at WOT, 2350RPM. If I bring the rpm down much at all, she falls off plane. So, for all practical purposes, she won't plane under "normal" load.

The bottom could be fouled although there is no growth on the side where the anti-fouling is located. I haven't checked the bottom since we brought the boat lto NY last November.

Fuel burn is not impressive compared to what other folks have reported - at 1500 RPM she does about .6 Nautical Miles per gallon. That's based on the amount of fuel actually used - (dip stick measurement when full, dipstick measurement after trip, tank then filled to exactly the previous full dipstick measurement) for the nautical miles traveled.

The boat was de-propped by the previous owner which reduces the load on the engines but also reduces the boat speed at any given RPM compared to oem. Since that is the case, fuel burn for any speed is higher than with OEM props since DD fuel burn is based on RPM and doesn't change appreciably with loading.

I've been thinking about putting OEM props back on - there are two brand new oem spares on the boat that have been there since it was new. They are still wrapped in plastic. After I complete my SCUBA certification I might do some prop switching!

 

[Genesis]

Two things...

Oh yes fuel burn does vary with load - and not all that much with RPM. You put the props on, you'll see the fuel burn go up!

If you're running in the transition zone (where there's a huge roiling boil at the transom, rather than clean water or a nice quiet slipping through) you're running in the WORST possible place for fuel economy, other than at screaming-WOT. You are literally trying to lift the boat over the water!

If your boat is propped such that you can't get out of the hole cleanly, then run at displacement speeds or slightly above. You'll get far better fuel economy and save a lot of wear on the engines.

As for prop swapping underwater, I wouldn't. There is a significant risk of ending up with a prop riding on the key, while will lead to shaft failure. Further, props are HEAVY and while they can be changed underwater, this is not a job for the new diver.

I am a pretty-experienced diver, but I don't change wheels in the water except in an emergency. I pull the boat for that. Yes, it costs money.

How much does a broken shaft and lost wheel cost?

 

[MikeP]

Sounds like good advice re prop switching! I'll leave it alone in the water.

I was under the impression that Diesel fuel burn, unlike gas engines, was essentially RPM based, not load based. That is, if the engine is turning 2000RPM the fuel burn does not vary all that much regardless of load. I admit I find this counter-intuitive. But the fact that there are RPM/Fuel burn rates published for diesels where no such thing exists for a gas engine seems to support that.

But I have to admit, it makes sense to me that it would be load-related...

 

[Geoff58yf]

Murry,
Our Kemosabe is a '78 58yf with 871/ti's. I agree with you that getting my burdened beast up on plane is wishful thinking. Reminds me of the old Star Trek where Scotty is shouting "ay captain, im givin ya all shes got, but she wont take it much longer..."
We keep the boat fully loaded with all long distance cruising spares -including things like a starter, alternator, turbo,water pump etc etc-, 3 staterooms full of clothes,Rib with a 40hp Merc on the FB, and every other storage place jammed. We also have a teak cockpit and aft deck and (im not sure if standard or not) two water tanks, one aft of the rudder gear and the other midships.
All that said, we cruise at approx 1650/1700 rpm and burn, combined about 20/22 gals per hr (from floscans) at
10-11knots. Weve been depropped a 'little' for long range cruising and reduced load but the thought of cruising on plane, regardless of fuel consumption (where here in N.Calif i just paid $2.60per gal !!) remains a dream..
On the 7th of Aug we have a 6hr run to make and Ill keep accurate notes on speed/rpm/burn and will report back.

Have a great trip
Geoff
kemosabe 78 58YF

 

[Pascal]

Mike

I have naturals but i don't think it make a big difference in fuel flow. if it take X hp to push a 53 at X kts, the amount of fuel = energy needed shoudlbe pretty much identical..

what speed do you get at 1500 rpm?

I get just under 10kts at 1500rpm (9.7/9.8kts) in deeper water (a least 12 to 15'). since i usually cruise on biscayne bay where the depths rarely exceed 10 to 11', I run at about 1450rpm and get 8.8 to 9.0 kts. that's usually with full water, 1/2 to 3/4 fuel...

at that speed, I average about 0.85 NMPG, if the water was deeper, I'd be over 0.9nmpg.

estimated consumption is measured from gallons needed to top the stbd tank used only for the stbd engine.

with the naturals, I never run faster than that, there is just no point. WOT gets me about 14kts, just doesn't make any sense...

Geoff

you should try runing a little slower... 20/22 gpg at 10/11kts stinks