saving fuel by going slow?

[duetto]

hi all,

we travel about 4500 sm each year. the recent rise in diesel cost will have a large impact on us. we have kept detailed fuel and mileage statistics since we bought the boat 3 years ago but we have never cruised any distance at less than 11 mph. our boat has 75hp engines and we usually travel with a pretty full load since we live on the boat from oct-may. underway we usually cruise at 12.5-13 knots and at that speed loaded up we consistently get 3 sm/gal.

i'd be interested in hearing about mileage stats from others especially those who have run at lower (i.e. displacement ) speed. i know that rhumbline cruises on one engine.

any help will be appreciated.

[ThomKat]

Hi John & Diane,

Sorry we missed you on your run north this year!

We generally run ThomKat at 1800 - 1900 rpm except for sprints across larger bodies of water (i.e., Gulf of Mexico, large bays, etc) with short weather windows. With 1459.7 engine hours and 212 genset hours, our fuel usage is has been 2861.3 gals or 1 .71 gals per engine hour. FWIW, I use 0.5 gph for genset consumption for record keeping purposes, but suspect it is actually a bit less.

Hope this is useful.

Anice & Tom
ThomKat MV3475

[Mishigas]

Hi John & Diane, This year I have decided to "slow" down a bit myself from time to time. I also have the 75 Yanmars and I normally run a pretty loaded boat as well on the Chesapeake. My normal cruise speed / RPM's are 13-14 knots at 31-3200 RPM's. This season I have been running at 10 knots at around 28 - 2900 RPM's. Due to the fact that we use our Generator often and transfer fuel from my forward tank aft often to keep it fresh, I don't have any accurate fuel burns lately. With all this said, I am very thankful each time we use our PDQ due to it's efficiency....at any speed. It's truly an economical boat especially in these times with fuel on the rise. My dock mates are crying with their high fuel burns and I have noticed many are just staying tied to the docks as we cast off. Sure it costs more this year to run our boats but it's all relevant when looking at other powerboaters. We sure get asked alot abouts Catamarans now more than ever. It's hard though not to grin when your fellow boated tells you he burns 30+ gph and he asks you what does your PDQ burn?......
Regards, Sandy Kramer
MV MISHIGAS Hull #68 PC34

[Ross Bowling]

Vita Bella came up the Pacific Coast from San Diego to San Francisco Bay over the last 2 weeks. While on the way up, the boat was running between 15-18 knots, we needed to take advantage of a weather window, and the fuel burn rate was a little below 4.75 gallons per hour. The boat used 38.9 gallons between Monterey and my slip on San Francisco Bay, and I am sure that the tanks were not quite full in Monterey. We are running the 110 TD's, and she loves to cruise between 15 & 17 knots.

Last Saturday I helped a friend with a Carver take his boat to the fuel dock, his bill was $1575.00, and he told me that he burns 30 gal/ hr. When I told him that we burned 39 from Monterey on up, he damn near pushed me into the harbor.

Gosh I love this yachting stuff!

[Leadfree]

Hi guys. We too, slowed down coming up the ICW this spring. In the past, if we traveled at 13 MPH, we would get 2.2 miles/gal. Now that we have slowed down to 8-9 MPH, we get 4.0 miles/gal. Obviously, it works if the purpose is to save fuel.
Sharon & Larry of Lead Free Too

[Sno' Dog]

Fuel economy at speed was a major factor in our choice of a powercat - and the PDQ 34 in particular. Now that we've had Sno' Dog for a couple of years, we've collected a bunch of our own data. (Much of this can be found on our website on the "Performance" page: http://www.snodoglog.com/Performance.html )

The bottom line, as expected, is that fuel milage does decrease somewhat with higher speed, but not nearly as much as you might expect, and certainly MUCH LESS than it does with a monohull. At 1500 RPM, just under 7 kts, we burn about 2 gallons/hour and get well over 3 nautical miles to the gallon. But at 3100 RPM, Sno' Dog cruises at 15 knots, burns about 6 gallons and hour and still manages to get an amazing 2.5 n miles per gallon.

The curve is by no means linear, however. Slowing down from 3000 RPM to 2500 RPM actually results in decreased mileage. I'm quite sure that speeds between 8.5 kts and 11.5 kts (2200 to 2600 RPM) are less efficient than higher or lower speeds. So, slowing down from 3000 RPM to 2500 RPM will NOT save you money. But slowing down from 3400 RPM to 3000 will save a bit, and slowing down from 2000 to 1600 will help - and slowing all the way down to 1200 RPM really helps!

I wish someone would outfit a PDQ with fuel sensors and collect some good data. Powerboat Reports did publish some data (there's a link to their data on our website) but it skips from 2000 to 3000 RPM; I suspect there's actually a bit of an "S" curve between these points rather than a straight line.

Finally, I occasionally see discussion of saving fuel by running on a single engine. I wish someone would document this. As mentioned in this Forum, Rhumbline did this on their trip to the Western Caribbean, but I gather they have no data to support any savings, they just believed it would save fuel. Personally, I suspect there's little-to-no savings to be had here - unless the boat were fitted with feather-able props. I believe the best way to maximize efficiency and range in the PDQ is to run (both engines) at about 1200 RPM. Cruising speed will be 5.5 knots (in good weather), you'll burn 1.3 gal/hr and get well over 4 nautical mi/gal. This would allow a (theoretical) cruising range of 625 nautical miles - assuming 150 gallons usable.

One final comment... On our way north this spring, I overheard a sport-fisherman running alongside us telling his friend he'd just slowed down a bit to save fuel. He said he'd been showing a fuel burn of 124 gal/hr - now he was down to 96! I'm just glad I don't own a 45-ft sportfish!

Henry
http://www.snodoglog.com

[duetto]

hi henry,

we've noticed the same thing, namely, that you want to avoid the rpms/speeds between where the significant stern wave appears and you get on plane. also my data would also suggest that mileage curve tends to hover between 2.75 -3.25 mpg.

but we still have very little hard data on 6.5-7 knot cruise.

[Mishigas]

Interesting discussion. I agree that we need to come up enough to reduce the stern wave that follows us when we run at certain slow speeds. However I would think that this discusion will differ with our different engine sizes somewhat. Sternwave aside, I would think the higher horsepower engines would be achieving a higher speed at a lower RPM. I have 75's on Mishigas and I think Henry has 100's. Not sure about the others but the HP of these engines will vary the RPM / SPEED for this discusion. I agree with all of our engines that a reduction in RPM will net a lower fuel burn once out of the hole.
Smooth Seas...Sandy Kramer MV MISHIGAS Hull 68

[Sno' Dog]

OK guys, here's the scoop...
Being an engineer and all, I decided to gather up all my info and combine it with data from Yanmar and Powerboat Reports to create a table of speed vs mileage for our Sno' Dog. The complete table can now be seen at: http://www.snodoglog.com/Performance.html. Even more interesting (to me) is the graphical representation which I will attempt to paste in below:

So it does appear that the answer is YES, you do save fuel by slowing down. Although, as I expected, there's not much saving to be had by slowing down from 12 knots to 8 knots - but the curve does not invert as I though it might, it merely flattens. Although the numbers will vary somewhat for the 75-HP boats, I suspect the curves will have the same general shape. There's more discussion on our website - which I updated again today.

Finally, I also used the data to consider the the question of saving fuel by running on a single engine (which was discussed earlier under a separate topic). My conclusion (also posted on the website) is that it may save a bit of fuel at speeds below 6.5 knots. Above this speed, it appears that the drag of the "dead" propeller, combined with the yaw of the hulls, offsets any potential savings.
Happy - and economical - cruising to all,
Henry
http://www.snodoglog.com

[Mishigas]

Henry, thanks for that excellent chart. It's nice to have an engineer on board to illustrate what's going on. I have the 75 horse engines so I don't quite get the turn of speed at those RPM's but I think we can all pretty much agree that we can tell where the slower "sweet spot" is on each of our vessels. Thanks again...Sandy Kramer MISHIGAS MV34 HULL 68

[Marleetet]

At the 1st PDQ U- the Yanmar rep stressed that you should not run the turbo'd engines at low rpm's for too long; due to turbos inherent need to be run a lot. More than a few people were pretty upset at that idea: either because they wanted to save gas or had to go slow due to speed limits in there area (Rideau and trent-severn canals). Yanmar was pretty insistent that you should run on one engine at higher power to go slow. They had no answer for saving gas.

Neither of those were a concern for me, so I didn't pay real close attention. So you may want to find out from your mechanic or Yanmar if you are shortening the live of your turbos.

[ThomKat]

Despite the fact that when on transits, we generally drive ThomKat slowly, we will ALWAYS run at 3400 rpm or more for a short period of time (15 - 30 min) at the end of the day to ensure that the turbos are brought up to temperature and the crud is burned off. The hazard with running at low speed is a build up of residue in the turbos because the temps are too low to burn them. I have discussed this issue with Yanmar mechanics on several occasions and they all agree that this is an acceptable alternative to running "hot" (i.e., burning lots of fuel) all the time.

Tom

[Moonstruck]

Is It posssible that you are doing damage to (burning) valves when running on one engine with a high throttle seeting and rpm in the 25-2700 range because of high exhaust temp? As I recall Yanmar requires that props be pitched to let engines run at the 3800 rpm at full throttle to reduce the possibility of burned valves. Isn't running one engine near full throttle at 2700 rpm causing an overload condition? After all if your boat had just a single screw wouldn't your prop be adjusted to allow 3800 rpm to forestall valve damage?

[duetto]

hi,

unfortunately we don't know how loaded the single engine is at 7 knots and 2600 rpm. i do know that rhumbline has run in this configuration for quite some time. it would be good to know if rhumbline has suffered any adverse effects from it.

[Moonstruck]

I think that probably the only way to check ii is an use exhaust temperature probe whict boat yards use whe re-powering to certify that the engine is installed per Yanmar specs. They usually run for a specified time at full throttle and compare this to factory limits.