Idle Speed Cruising?

Strange question of the day:

I like going fast on the water and I spent much of my working life doing just that as a yacht captain and running crew boats in Louisiana, but someone else was always paying for the dead dinosaurs to power the boats.

I also don't mind going slow, and it has some advantages over going fast, especially if you're running in places like the ICW or rivers. You get to see things the go-fast boats miss. I once ran an old classic yacht that wouldn't go faster than 10 mph (not knots, mph)if it went over Niagara Falls and I did three trips from Cape Cod to Ft. Lauderdale in her. I also owned a 26-foot sailboat and if I could make good 5 to 6 knots I was feeling that I was making good time and now, since I'm retired I don't have to be anyplace at any given time..

I need to get back on the water again but with COPD and three stents in my coronary arteries I don't think I can physically handle a sailboat again. But in looking around for boats recently I've seen some real good deals on small powerboats with single Mercruisers and one with a big outboard.

Now I know all the bad mouthing that's done about trying to run a planing hull at displacement speeds, and that there would be no savings on fuel doing so, but here's my question:

Since I'm going to be running almost exclusively on the ICW and rivers like the St. John's in Florida and the Tenn Tombigbee I really don't need to be a speed demon. If a small powerboat like a Wellcraft or a Tiara, for example, could make five knots at idle speed wouldn't there be an appreciable savings in fuel costs? I mean when you run the ICW there are probably a few hundred miles where you're running at idle speed because of wake restrictions, anyway, so why not do it as a way of life? And you'd STILL have power to git up and go if you needed to to get into shelter from an approaching storm.

Any thoughts, folks? Is it practical?

 

My feeling is that the efficiency penalty for a planing hull at displacement speeds may be a lot percentage wise, but be pretty minimal in terms of fuel burned. The real downside is the boat will handle like crap. Wandering all over the place and requires a lot of steering input to keep strait.

If you go this route, take a look at a power cat. At least they tend to track pretty strait even at idle speeds.

 

You won't make five knots at idle speed, you can just about cut that in half. And slow speed wander does bedevil some planing hulls, but particularly anything with minimal chine immersion aft. Of course adverse winds can cut your forward speed at idle to almost zero.

 

Stumble is right. My 18 foot Sea Ray with a 165 Mercruiser wanders like crazy at low speeds. I spend most of my time going less than 10 mph because my wife doesn't like fast boats. But occasionally I actually get it up to plane. As far as fuel goes I doubt seriously that I am saving any fuel by going slow because it simple isn't designed for displacement speeds. At idle it goes about 3-4 mph, but feels most comfortable at about 10 but throws one hell of a wake, which is what eats up the fuel. At that speed it's fighting it's own wavemaking. Actually it probably gets it's best fuel economy at about 20 which is on plane but only about 2500 rpm.

 

Hi,

You define semi displacement hull. My suggestion would be lobster boat.

Regards

 

The best speed for fuel economy would be "dead slow", but it does not make sense to run a planing hull at crawl speed, all or most of the time, especially as by the time you arrive at the destination, it will be time to return home !

 

Outboards and shaft drives ard usually fine at low speed. Sterndrives are frustrating even duoprops.

 

You could fit an aux outboard for slow running. Something like a yamaha 9.9 4 stroke .then you would sip fuel at disp speeds and have the bonus of a backup engine. Rig up a tiebar or changeover tap so it can steered from the helm.

 

oldsalt1942,

I sent you an email.

 

Old salt,I've been doing this for years in the interest of range.

Where I am is very fast (6 to 18 knots) tidal currents,ocean currents,and storms can blow up quickly so I can speed up and get to a hidey hole if need be. Trawlers either have to wait hours for the tide to change,or get up at 4am or some rotten hour (on holidays...really?) to avoid them. I merely ease on the throttle for a few minutes (cost of $20?) and go though.

Of course there are many varying vessels and some are pigs while others aren't.

Points:
-When they're bigger and heavier they don't wander about.
-my past boat,a 48' (capable of 33 knots) would get 3.5 mpg at 7 knots.
-my current 68' (capable of 34 knots) still gets over 2 mpg at 8 knots and 7 knots about 2.4.
-don't know behaviour of smaller boats,but both of them once they pop up on plane the MPG varies little all the way up to 28 or so. The 68 pops up on plane at 15+ and it's at .6 mpg, and at 27 knots it's at .5 mpg,but at 34 less than .3. Needless to say I never run it there.

Looking at trawlers in the 40'-45 range-much smaller with vastly less interior space and not the luxury-usually get about 2 to 2.5 mpg or a bit more IIRC at 8 knots...

Edit:

I did a quick look and am shocked/disappointed at the mpg on a 35 Nordhavn coastal boat...half the length of the 68 and worse mpg much of the time
https://books.google.ca/books?id=9Z...KcQ6AEIKDAB#v=onepage&q=nordhavn mpg&f=false

 

For most folks the AICW speed will be the same as with a sail boat.

The bridge tenders only open for a gaggle , so any faster boat soon learns he must get well in front , out of sight , of the gaggle from the last bridge.

As a 25 ft LWL is not uncommon 5 or 6 mph is the usual cruise .

A gas engine does well at low power outputs , as do most lawn implement sourced diesels.

A 40 ft displacement hull will be close to 36LWL and do just fine at 6 K or 7Mph.

Enjoy your retirement!!

 

Fred,

I know you have made extensive trips on the Atlantic ICW. Since I live on the ICW, I have a different view of common speeds that are seen here. You are certainly correct about the speeds of sailboats which do not vary a lot even for the larger ones. 6 kts+or- seems to be the common speed. For powerboats which is what the OP is interested in, the scene is quite different with speeds of 6 to 10 kts common for trawlers and up to 25kts and more for the boats capable of that.

The price of fuel is a great determinate of speed also. It does seem that the price becomes less important with time though as people get used to it. Still the cost of running boats of various size and design varies far more than a automotive vehicle, even including motorhomes. I routinely see fuel use rates from less than 1 gal/hr to 200 gal/hr or more.

An electric sailboat conversion was at the town dock yesterday and it's speed will be well below the slowest of IC powered boats.
http://towndock.net/img/21092.jpg

 

Our latest boat is a great AICW cruiser.

It is a Uniflite US Navy Utility 50 ft , with a 6-71 for power.

The air draft is about 10 ft so although we put along at 1300 RPM we do 7statute with under 3gph fuel burn.

The usual swift trawler has to get most of the bridges to open , so although they are happy to burn 25GPH for 20K , we get about the same distance as they do many days.

We easily travel from CT to about Port Orange FL with out a need to pay for a slip.

South of there its a bit tricky to find anchorages .

As a live aboard for almost 23 years we really prefer to anchor out although not paying for 50ft x $1.00 to $3.00 per ft per night pays for lots of diesel.

The sport fish folks usually never have a good fast trip as the plodders scream as they kick up huge wakes while passing.

Since 1968 I have seen only 2 or 3 Sports that knew the technique to pass a slow boat , with no complaint.

We just get RFS , ready for sea , and let them blast by any way they desire.

Its ALL great fun!!

 

Yes Fred,

Getting out of their way has proven the most practical response. There are many sportfishermen here in NC and delivery captains tend not to watch their wake as well as some others. My worse encounter was with a fast trawler running right on his hump with tremendous wake. I sat behind for a while but wanted to pass and finally gave up waiting for him to help. Insufficient room to take the wake as a sharp angle and he did not move out of the center. I made it past but the high wake and angle gave me a fit in the narrow cut just north of Beaufort.

My boat which is tiny compared to yours, can run economically at any speed but I like 12mph in narrow areas and 15 to 17 in open water. Averages about 7mpg for all cruises. The number of homes, docks, moored boats and no wake signs on the ICW has multiplied several times over the last 40 years. Still enjoy the journey though. We think the run from Charleston to Savannah is one of the favorites. Plenty of other great cruising areas though.

 

I wonder what size boat the OP is thinking about. An Ablin 27 or Cape Dory 28 might be the right boat for him. Diesel power with fairly decent fuel economy.