Cruising in a power boat

[sailcrazy]

We have been sailors for 20+ years, and are thinking of converting to a power boat in the 45'-55' LOA range. What are the goods and bads of using just one of the two engines when under way? Fuel consumption would be less, and the engine in use would be loaded higher (a good thing, at least according to Yanmar) to reduce carbon buildup in the exhaust elbow, etc, and to reduce glazing in the cylinders, but is it a common practice to just use one engine? If not, why not? Also, where can a sailor go to learn about power boats...is there a forum or?? folks use? And what are the advantages/disadvantages of a trawler over a twin screw?

[Steadman Uhlich]

Originally Posted by sailcrazy

We have been sailors for 20+ years, and are thinking of converting to a power boat in the 45'-55' LOA range. What are the goods and bads of using just one of the two engines when under way? Fuel consumption would be less, and the engine in use would be loaded higher (a good thing, at least according to Yanmar) to reduce carbon buildup in the exhaust elbow, etc, and to reduce glazing in the cylinders, but is it a common practice to just use one engine? If not, why not? Also, where can a sailor go to learn about power boats...is there a forum or?? folks use? And what are the advantages/disadvantages of a trawler over a twin screw?

This forum site, CF, has a related or sister forum site called "Trawler Forum" where you will find answers to all of your questions. You will find a link here.

http://www.trawlerforum.com/forums/

I have enjoyed reading that forum too, and found the members there friendly and helpful. The site software is the same, so easy to navigate.

It is a smaller forum membership, but there are several members from CF (which also has a power oriented forum here too) who are members there too.

While the focus is on "Trawler" styled yachts, the members there have lots of experience on power yachts and so good advice on many topics. Several of the members moved from sailing to power.

I am not a trawler owner. But I have a strong interest in them. Here are a few quick answers:

The merits of twin versus single engines is debatable. You will find that topic has many answers.

The use of a single engine on a twin powered boat is thoroughly discussed for reasons of engine wear and fuel efficiency.

Cruising issues and destinations are discussed throughly too.

Boat brands and construction quality and recommended boats are discussed at length.

Boat design types are discussed, along with the cost differences of running large gas versus Diesel engines and related boats.

I think searching this CF site "Power" forum and also joining and searching (use Google Custom Search to do forum searches for better search results) the Trawler Forum too, will yield you many good answers

Good luck!

[FSMike]

We, like many others, use only one engine when motoring for a distance on our catamaran sailboat. Cuts the fuel bill and the wear and tear in half.

[neptunesjester]

As a fellow powerboater, I can answer some of your questions:

1) There is no real fuel savings running on one engine. Between the parasitic drag of the non-running prop, the need to 'crab' for the offset thrust and loading the running engine harder to maintain steerage and speed (hull speed), you lose most if not all fuel savings benefit to running on one motor.

If you're willing to take the hit to your SOG and dont push the running engine harder then you can realize fuel savings along with less vibration/noise.

I have done both single and twin long runs and much prefer to run on both engines. The overall performance of the boat is optimal with both running

2) Trawlers come in both twin screw and single screw configurations. I prefer the twin screw for manuevering in tight quarters and the built in redundancy factor, but there are many single screw trawlers out there and it is a matter of preference.

The redundancy factor has come in handing on more than a few occassions where one engine developed an issue (started to over heat, lost a belt, etc.) and I was able to continue to a safe anchorage or harbor on one engine and wait for the engine room to cool down before starting repairs. I was not left adrift wait for a tow or in a hot engine room attempting quick repairs while being tossed around on unprotected waters.

Most sailboats have similar redundancy between the sails and motor. Having two means of propulsion goes a long way towards turning mechanical issues from an emergency to an inconvenience.


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[darylat8750]

Try the trawler forum.

I have a 42 ft. trawler with twin 120hp Lehman's. I have put over 1800 hours on the engines. I've done a couple of 400nm crossings and have tried several different configurations and speeds. I have a friend with a similar boat with the same engines and he has crossed between Ft. Meyers FL and Isla Mujeres several more times than I have. With fixed pitch props we have found no noticeable difference in fuel burn per mile between these configurations at our maximum speed before mile per gallon start dropping off rather dramatically. On one engine the fuel burn increases even more dramatically above this speed. (For my boat that is 7.2 knots, for his approximately the same.)

Both engines running

One engine running one engine shut down with the prop secured so it can't windmill.

One engine running one engine shut down with the prop allowed to windmill.
In this configuration you must be absolutely sure that your transmission will not be
damaged with the output shaft turning without the input shaft turning. Most
transmissions have the oil pump on the input shaft. My manual says that free wheeling
at low speeds will not hurt it. I didn't have the courage to try it like that but my
friend did and on a 400 nm leg had essentially the same burn that he had with
one prop locked off and on that leg under the same conditions (almost smooth with
mostly light quartering tailwind) my boat with both engines running used the same amount of fuel.
In shorter experiments the drag from the locked prop seems to increase very steeply
with small increases in speed.

So my conclusion (based on an admittedly small sample) is that with fixed pitch props at low speeds it is basically a wash. There may be some advantage in drive train wear to operating on a single engine with one prop locked. In my case I figure my engines will die of old age rather than wear out so I don't see that as significant. Less oil used do to oil changes I don't see as enough of a saving to concern me. I do think that controllable pitch props that could be feathered would be a whole different ball game.

[ranger58sb]

Originally Posted by neptunesjester

1) There is no real fuel savings running on one engine. Between the parasitic drag of the non-running prop, the need to 'crab' for the offset thrust and loading the running engine harder to maintain steerage and speed (hull speed), you lose most if not all fuel savings benefit to running on one motor.

Yep, yep, and yep.

Running on twins are "trawler speed" -- not the theoretical maximum hull speed, but rather closer to 1.0*SQRT(LWL) -- can be relatively economical, without too much regard to the hull form. Some hulls are more comfortable at slow speeds in sloppy seas, though.

Agree, ref Trawler Forum, too. When you get there, first do a search on the bazillion or so threads about twins vs. single. There's some religion involved in the opinions in those threads, but lots of factoids as well.... so you can at least pick your religion with eyes wide open.

"Trawler" -- in the recreational boat market -- is more of a marketing term than a boat description. Lots of leeway in the what is/what isn't department.

And.... most of us will say fuel will likely be the least expense.

-Chris

[reed1v]

Up here in Maine a single prop works best so you do not get tangled in lobster buoys which are really unavoidable since many of the buoys are underwater at high tides. Also the single prop skeg protects against the inevitable tree(deadheads) floating just below the surface. Lots of things to bump into.

Add a bow thruster and you got as much maneuverability as a twin prop setup.

[vlathom]

We made the sail to power transition a couple of years ago. It's not a slam dunk. Handling our trawler is very different from handling our sail boat. Maneuverability is much more compromised by the windage. Get a trawler instructor to go out with you before you start using the boat.

Here's an article I wrote about the transition.

Vicki Lathom

A Journey from Sailboat to Trawler - ALL AT SEA

[Scaramanga F25]

I would consider a catamaran power or sail before a rolly trawler.

[reed1v]

Originally Posted by vlathom

We made the sail to power transition a couple of years ago. It's not a slam dunk. Handling our trawler is very different from handling our sail boat. Maneuverability is much more compromised by the windage. Get a trawler instructor to go out with you before you start using the boat.

Here's an article I wrote about the transition.

Vicki Lathom

A Journey from Sailboat to Trawler - ALL AT SEA

We tried converting. Bought a nice Grand Banks. Could not get use to the constant engine noise, smell, nor the rollypolly ride. Also it is a vastly different experience driving across the ocean than sailing across her. We sold the Grand Banks after two years. Back to sailing.

[jmschmidt]

I have to disagree with Neptunesjester in a couple of areas. Robertson & Caine deliver all their power cats from South Africa to Europe and the Caribbean on their own bottoms. These operations are done single-engine, some with temporary rudder extensions. There is a Y-Tube video on such a delivery clearly showing only one throttle in the mid-position and the other fully aft. There are two large plastic fuel barrels at the bridge deck after rail.
Some years ago a friend of mine delivered two power boats from Honolulu to Seattle using this technique. These were trawlers (Grand Banks and a Choy Lee) so asymmetric thrust and rudder offset were not as big an issue as with a cat. In both cases he removed one propeller and at the halfway point he went over the side with scuba gear and swapped the prop to the other side thereby reducing drag and saving fuel. He noted that the rudder offset was negligible, costing only one-quarter to one-half knot in speed at the power setting he used. He did a significant about of sea trials in the calm and current-less waters of Kaneohe with experts from the University of Hawaii to prove out his theory.
He made both trips on the internal fuel capacity of the boats without resorting to the use of his emergency fuel. He also discovered the added safety of this configuration when he arrived in the North Pacific Gyor which is full of floating junk. He quickly realized that if contact with floating debris took out his operating engine it most likely would not damage the unused one thereby allowing him to continue to his destination or return to Hawaii.



Also a wind-milling prop, regardless of what some on this forum think, will create drag equal to a solid disk the diameter of that prop and may cause transmission problems. That's probably why some have reported no fuel savings during single-engine operations. If you don't want to actually remove one prop as my friend did you could use folding or feathering props for long trips or at least lock the shaft or transmission to prevent water-milling.

[neptunesjester]

Interesting. My post came from my own experience with my fuel burn calculations with a couple thousand miles under the keel using a combination of single and twin engine runs over the years mostly due to mechanical issues forcing a single engine distance run. The dynamics must be different for a powercat vs a mono semi-displacement hull.

I agree about the drag and some power boating magazine did testing a few years back and concluded that a locked prop provided less drag than a rotating prop.

How do they switch the prop from one side to the other? Shouldnt one be right hand rotation snd the other left hand rotation? Do powercats use the same rotation on both engines? Or do they just run one engine in reverse?

Having swapped props in the water myself, I commend them on doing it mid ocean. Between the cotter pin, castle nut, and shaft key, I'd be too nervous about dropping an important piece into the deep blue never to be seen again.


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[reed1v]

Practical Sailor did research on the free wheeling vs fixed position prop years ago. Fixed was better. Counterintuitive.

[Lepke]

I sailed in my youth. Since then ship, tugs and larger boats most of my life. Docking, running, anchoring is easier with almost any power boat. You can usually see better and have a decent anchor winch.
Twin screw gives you backup. I came back on one engine twice. Once in this boat. I have an 83' 80 ton boat. With twins I don't need a bow thruster. I've been in some very tight spots. Single screw can make docking and undocking harder. There are many good books on the differences. Find an experienced local and get some lessons on the boat you buy.
On the running on a single engine... I had the opportunity to do that for several hundred miles in the North Pacific during gale and small craft warnings. I normally run 1800 on both for 10 knots using less than 8.5 gallons/hr. On one engine at 1800 I got 4.5 knots freewheeling one prop. Locked I made 4 knots. 1/2 the fuel. I have old style Twin Disk transmissions. Gears and bearings sit in oil. 33x30 4 blade props.

[marlin driver]

Originally Posted by Lepke

I
I normally run 1800 on both for 10 knots using less than 8.5 gallons/hr. On one engine at 1800 I got 4.5 knots freewheeling one prop. Locked I made 4 knots. 1/2 the fuel.

Slow down to 4 knots (or as slow as you can) with both engines running and your fuel burn will probably be half also. Run at the same speed both ways and see if you save anything on one engine.