Are marine engines getting more efficient

[andrewa]

I have posted about repowering my boat and have been thankfull for all the advise I have received as is probably Grumpy1 as he ventures out soon. While reading mags and looking on the web especially at the American sites over these long winter months I have noticed quoted consumption markably lower than I would expect. For instance a Wellcraft 26ft cruiser weighing in at over 2.6 tons with a 5.7 MPI Volvo Duo quoted at 2.5 US MPG at 26 knots. I thought surely this can't be right but Sea Ray equivalent with the 5.0l MPI merc Bravo III about the same. Yet a Diesel merc as you see below is slower and burns not much less. Only at full tilt is there a big difference but also a big speed difference. The Sea Ray quotes 16 US gallons at 30 Knots and 19.5 at 35 knots. Cars have been getting more efficient for some years but I didn't think boats had changed much in the last 10 years or so. I have put an example below. Only really posted incase of interest to people and to see if boaters have seen a difference in the real world. Maybe this is why US boaters don't really have much interest in diesel power when the added cost of engines are put into the equation.

Performance*
350 Mag MPI BII 4.2 L D BII

Cruise mph (Fuel)
26.5 mph (14.5 gph) 21.2 mph (11.3 gph)

WOT mph (Fuel)
39.6 mph (25.8 gph) 27.8 mph (13.8 gph)

Andy

 

[DepSol]

Yes and they are also using lower compression and lower injector pressures.

Dom

2003 is going to be a good year for me

 

[spannerman]

When you consider it boat petrol engines aren't really stressed, take a typical 5.0 l V8 which cranks out around 230 hp, yet some of the modern cars are pulling 180hp out of a 2.0 l block. Obviously the engine is more efficient and this is done by running at higher temps and pressures. Whereas a boat engine is under full load all the time, and runs very rich mixtures compared to car engines, some of the fuel being used as a means of cooling, hence the low mpg. But the figures you saw about Wellcraft are about right, I have a Bayliner 26 ft, with a reworked motor, I ported the heads and put in higher compression pistons and a different camshaft, and converted it to freshwater cooling and it now runs better than before, at idle you can't feel or hear its running, and it is much smoother through out the whole range, and best of all at 28 knots it now does 3 mpg, 20% up from before, I have a fuel computer which measures to a thousandth of a gallon, call me paranoid but I can see immediately if its off tune.
Some of the new diesels are very efficient now, giving goog mpg and power output, I just pulled two Volvo 41's out of a Nimbus 36 Coupe and put in two Mercruiser D 300's with B3's it weighs 9 tons and goes like the wind! I clocked it at 42 knots (48mph) on the GPS and it uses 9 gph at 36 knots (41mph) best cruising speed, and 11 gph at full throttle. (per engine) Thats what I call real performance and economy. The new KAD 300 is quite efficient too, but then you are pulling 275hp out of only 3.6 litres.

 

[BrendanS]

Most people aren't aware of the most efficient cruising speeds of their boats. Mine is about 30 mph ( that's my speedo, it's US, and isn't in knots). This has gained me a reputation for speed with my club, but the boat just isn't comfortable cruising at 20 knts.

If you look at www.boattest.com, they show charts for a wide variety of boats and engines. The peak efficiency is nearly always a peak, you have to be in a very small slot of speed, or you will be burning far more fuel than you need to, and it's usually far higher in planing boats than people believe.

I can do the trip from Lymington to Weymouth and only use 60 litres of petrol, and that includes a few belts at far higher speed. It's when I slow down that fuel consumption increases, unless the conditions are really bad, when 4 knots is nearly as fuel efficient.

 

[andrewa]

So are the MPI's more efficient with what they are given fuel wise, i.e 5.0L Volvo block now kicks out 270 pshp but doesn't seem to burn much more fuel than the carb model at 220 pshp in fact less at higher rpms i.e below 4500. Interesting because a big thing was made about optimax outboards but i/o units are rarely mentioned unless they are Diesel. I have been looking at the same site as Brendan and didn't realise that the optimum speeds for most planning boats was quite so high. Isn't the Internet great.

 

[Guest]

Is there anything that you can attach to your engine to measure fuel consumption e.t.c. like the compters in cars, whether on a temp basis or permanent. It would be good to know the most economical speed.

 

[andrewa]

Have a look on www.boattest.com and try and find a boat near to your model or the same and look under the test results tab. Its free to get a login. By the way the joke was funny.

Andy

 

[DepSol]

I have a Navman GPS and fuel flow meter, I will be soon upgrading to a chartplotter and will be selling this unit, I had the fuel flow meter with the unit but it doesnt work with diesell so never bothered with it.

I understand that they work well and the gps isnt that bad altho i prefer garmin units.

Dom

2003 is going to be a good year for me

 

[BrendanS]

You don't need anything to measure fuel consumption, you can 'feel' optimum cruising speed quite rapidly once you get used to your boat, and you can pay for quite a lot of fuel by not spending money on gizmos that are relatively worthless.

There is stufff you can put on the engine to measure fuel flow, the boattest.com website gives details, but I cannot imagine this being of any benefit to most boaters in terms of overall cost to boating. Get a feel for where your boat feels most happy. Mine is when it's flying over waves, not stuffing into them.


Had a race with an Extreme in fairly rough conditions a couple of years back...they said that I looked more like a plane than a boat. They had a far bigger engine, and a deep V hull that was cutting through the waves. I've got a deep V front, and almost flat at back with a couple of interesting features on the way, that help at high speed. I suspect my trip was far rougher than theirs, but I was the first boat to keep up with them, and they throttled back before me, because I was actually using less fuel, and they were gobbling it up. (OK, I'll be fair, if I'd had any passengers, I'd have slowed down, but it was wonderful to take them on).

My fuel consumption for the trip was slightly more but theirs was enormous.

Try out boattest.com, play around with different boats of similar size with different engines. You'll soon see where most boats are efficient, and see how narrow the speed efficient range is, and then feel how your boat reacts to different speeds. The test results only confirmed my gut feel

 

[Guest]

Thanks to you all, I will report back on my boattest results. I guess mine will be similar to yours Brendan, I know you are about 6" bigger than me, and that your 4.3ltr is an EFI, mine is the 190hp volvo. I would be interested if you could PM me with a brief idea of your fuel consumption. Perhaps some others would be willing to name their engines & sizes and average fuel consumptions, just for interest.

 

[Guest]

Cannot find one like mine, can anyone help, I have pasted the boats info below.

Engine: 4.3 GL Volvo Penta, Prop: 14 1/2 x 19 RH
RPM MPH GPH RANGE*
2000 15 2.8 217
3000 25 3.8 266
4000 35 6.9 205
WOT 43-45 - -
0-20 MPH Acceleration: 3.6 sec.

SPECIFICATIONS

Length: 20’ 1-3/4” (6.13 m.)

Beam: 94-1/4” (2.39 m.)

Depth: 50” (1.27 m.)

Approx. Weight: 2852 lbs. (1283 kg.) w/5.7 GS Volvo Penta

Cockpit Depth: 35-1/4” (.90 m.)

Transom Width: 90” (2.29 m.)

Transom Deadrise: 19 degrees

Draft (drive down): 33” (.84 m.)

Fuel Capacity: 45 gal. (170.3 L.)

Maximum Persons: 8 or 1150 lbs. (518 kg.)


Standard Features - GS 209
SSV hull with BiTex construction foam-filled fiberglass stringer system combination bucket/sleeper seating with aft jump seats automotive-style contoured dash tilt steering wheel AM/FM/cassette stereo upholstered sleeping berth cabin lounge seat cabin light screened deck hatch lockable cabin doors bow access step anchor locker ski tow ring integrated swim platform automatic bilge pump Sunbrella soft top and boot Canvas package camper canvas package mooring cover cockpit cover bow pad Clarion stereo CD radio w/2 speakers porta-pottie galley package including sink/stove/insulated coolbox telescopic stern light stainless steel telescopic 3-step boarding ladder convenience package.

Thanks,

 

[Col]

Ah , so that's how the hull got broken /forums/images/icons/laugh.gif

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[Col]

Grumpy, you will find on a boat like yours, the trim will make a big difference to the fuel consumption (and speed)
To find the optimum trim, drive the boat at say, 3250rpm and note the speed. You may have to go for a couple of miles to let it settle.
Then, trim the drive out a little, again allow time for speed to settle. You should notice the speed has increased, without opening the throttle.
Repeat this trimming until speed starts to drop off, you have now gone past optimum trim, bring the drive back in slightly, and experiment at around that angle of trim, till you find the best angle. This will give you maximum speed/ efficiency.

You will need to alter this angle slightly for differing sea conditions, but you will soon get a *feel* for it.

The mechanics of it are-

As you trim out, this lifts the bow, reducing the length of wetted hull (hull in the water) reducing hull friction. You can of course over do this, and the prop will ventilate.
You will have to "trim in" as sea conditions get rougher, this will also make the ride more comfortable, albeit wetter for you.
The bows will punch through the waves more, so the boat will not take off so much.
With the drive trimmed out still, you would be taking off like a moto cross bike on every wave, then belly flopping into wave troughs.
Hope this helps, and all the best for launch day.

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[Guest]

Blimey Col, you should be an instructor /forums/images/icons/laugh.gif that is great info, I have printed it off and will have it for the MAIDEN voyage, if I get out the lock that is /forums/images/icons/wink.gif

Cheers,

 

[spannerman]

Yes BrendanS, I agree with your observations about boats cruising speeds. Its all to do with wetted area, seeing as water is 17 times as dense as air it makes sense to get as much of the hull out of the water as poss. And you are right about fuel consumption at lower speeds, lots of boaters don't realise that a planining hull has an optimum running angle and if you go too slowly you begin to fall off the plane so the boat slows down even more, they then give it more throttle but its just hanging there trying to climb back over its bow wave and not quite making it, so you are driving 'uphill' burning loads of gas and running the engine and transmission hard. If they just accelerated up to max let the boat get going and then throttled back to usually around a 1,000 rpm below max with a petrol motor. Then we come to trimming the boat, again a sadly mistunderstood art, that some never bother to learn. You often see boats trimmed too high and they porpoise along with the bow nodding up and down and the engine wailing up and down as the load changes. Its great when you get your boat trimmed out to the sweet spot and it sits there as though its on rails. One guy I know bought a used Nimbus and drove it back from Oslo to Stavanger about 350 miles with the drives trimmed full down all the way and wondered why the steering was so sensitive! Another I know has a Sealine Statesman and when I was calibrating his autopilot I noticed he left the drives trimmed out to 4 degrees positive the whole time and wondered why it took so long to plane! I am not sure who is to blame here, is it the seller for not demonstrating the controls, or is it the owner who hasn't taken the trouble to find out? No doubt this will raise a whole new discussion!

 

[BrendanS]

I've tried measuring fuel consumption several times by the old tried and trusted method of filling the tank, going on a trip, then filling up again. Lymington to Weymouth (about 60 miles?) I use 60 to 70 litres in flat conditions at optimum speed. So about 1 litre a mile. Most boats in the same length/engine power will be somewhat higher, I've got an unusually efficient hull.

Out with the club, they tend to cruise slower due to the variety of boats, so I burn a lot more fuel, 20knts is too slow for my optimum.

If the weather is rough, and I slow down, I can multiply this level of consumption several times over.

 

[BrendanS]

Agree trimming is an art that few seem to realise even exists.

Mine is very sensitive, and and just a little too much causes bad ventilation, but now I just adjust when needed without even thinking about it - apart from the couple of times I came to a halt, and forgot to trim in. Trying to get a boat on the plane with a trimmed out leg I have discovered makes lots of noise and white froth, but isn't an efficient mode

 

[BrendanS]

Nah! that was a couple of years ago.

Doing the same thing repeatedly, in ever rougher water, is what probably did it in /forums/images/icons/wink.gif

 

[Guest]

Wow, I didn't realise that the weather could have such an effect, I wonder if I need to fit bigger fuel tanks, or get reserve tanks built in? I only have a 45 gallon tank, I don't want to run out. I was in fact thinking of this, but I was torn between SS and plastic tanks. I must check out the weight as another 45 gallon back up tank could cause problems. I was concerned about fuel contamination and thought it would be better to be able to switch between tanks in an emergency.

 

[BrendanS]

It's not the weather per se, the weather just means you have to slow down.

The slowing down is what causes the problem, if as Spannerman says, you have more hull in the water, you need more energy to push your boat along.

There was a thread a while back where I was given advice on putting a fitting on to allow me to use outboard portable tanks to extend range. I wouldn't even think about putting another large tank on as a permanent installation - in a boat that size, where on earth would you put it?