Twin vs single

Soo-Valley said:

Willy, can you explain why post 69 is FD. I read the brochure on it and it says FD, but any description of FD says rounded bottom.
Looking to learn something.

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By rounded bottom you mean soft chines?
The only roundness the leads to FD is the shape of the stern on the bottom of the boat. From 1/3 to 1/2 way fwd from the transom. As in this rowboat example. See that the bottom “rounds up” to the transom and the transom is usually out of the water even when loaded. It’s all about the bottom aft of amidships. It can be a flat bottom, V bottom or a rounded soft chine bottom.

Most associate a soft chine with a slow boat and transfer slow and round to FD. It’s a bit of a trap. Also FD boats can have any number of chines. Some stitch and glue constructed boat have 3-4 chines. Usually a substitute for a round chine/bottom.

Soo-Valley said:

Willy, can you explain why post 69 is FD. I read the brochure on it and it says FD, but any description of FD says rounded bottom.
Looking to learn something.

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Yes,
The water line rises in the stern to the end of the boat (transom). Go to the thread FD/Sd and see the pic of the rowboat w a FD stern. The rise in the stern is obvious and even profound.
This FD/SD chat should be on the new thread specifically for that to quit the off topic content.

There's always the third solution, for those with the necessary funds. Check this one out with a 70hp Daewoo auxiliary engine with its own dedicated propeller shaft:

https://www.passagemaker.com/cruiser-reviews/pisces-one-mans-long-range-working-class-hero

https://images.atlanticyachtandship...53/1189034/jenkins_marine_ltd-pisces-full.jpg

I like the twins for redundancy and manouverability. Some transmissions allow freewheeling which means you can run on one engine with the other turned off. I do this often when running at displacement speed on my Mainship 43 with ZF transmissions. The fuel burn is about the same because of the added drag of the freewheeling prop but the maintenance costs are halved. Best of both worlds with twins.
Be sure to check with the tx manufacturer before trying.

It is a simple matter of physics and hull form. Most so called trawlers you see on here are Simi-displacement and not a true round bottom full displacement hulls.

It takes X amount of HP to drive a hull to displacement speeds plus running gear drag. If the rudders are designed with very low drag and the shafts have low drag. Then if it requires for instance 65HP to drive the hull at FDS then you need 65HP on one engine or 1/2 65HP on twins plus the additional RG drag. If the diesels a properly matched to the application and running @ 80% load ( which is considered correct for most manufactures ) then to BTUs of HP required would be the same with singles or twins. Fuel burn equals BTUs energy which equals HP which equals thrust.

In most applications these formulas will hold true every time.

I've always preferred the smaller (135 hp) twin naturally aspirated bullet proof diesel engines and that's what we got in our DeFever 49 CPMY. We took three years to cruise the great loop and only lost an engine twice. Once, oops, duh!, and the other was someone else's bad idea for a hose fix that was hidden until it wasn't. Both times it took less than 20 minutes to get the engine back online.

Where are you going to go cruising?

If you're an ICW traveler, with runs to the Bahamas, now and then, a single is fine. Get gold level towboat insurance. If you're crossing the ocean and it's not a Nordhaven you've got, then go for easy to access and maintain twins. Or, a newer but not new boat with known engine reliability.

I think differently today. Now, it's how easy is it to get around the engines to maintain them. The more difficult, the less maintenance will be done. The 44 and 49 DeFevers have walk-in, stoop over engine rooms (5'8") with easy access to everything. Since we had 1100 gallon fuel capacity and burned relatively nothing, we added a Gulf Coast Fuel Polishing system.

Maintained diesels are very reliable. Our current 28 foot boat is a single spit shined continuously inspected and maintained Yanmar 315 diesel with bow thruster and our next boat will be a single with bow and stern thrusters. Most likely with a Cummins 370 or 380.

With a proper engine room design you can still have a twin engine configuration with ample room around the engines. The problem is that the people who design the engine room and the ones that have to perform maintenance are not the same and have very different opinions of how much space is needed.
Attached picture is the engine room on my trawler, designed by a marine engineer and not a naval architect ��.
Twin Vetus (marinized Hyundai) 65 hp each which give the ship (40’ / 15 tons) a speed of 11 knots.
Going from a single engine to a twin engine ship, I absolutely enjoy the better maneuverability, especially when you’re entering a small marina, which we happen to have a lot of in The Netherlands.
Also, being a marine engineer myself, I like the redundancy it provides. During my 40 years at sea I have seen all sorts of engines spontaneously fail, from 35 hp to 16,000 hp, most of them were being professionally maintained.

IMHO, Get a single, bow and stern thrusters and for the ultimate, an articulated rudder. Would never get twins.

That is one nice engine room. Plenty of space to move around. Congratulations.

No doubt twins are nice in tight spaces. The people who bought our twin screw DeFever added both a bow and stern thruster. And, I have to admit, that made maneuvering seem like you had a joy stick.

What is typically talked about is one or two engines of the same relative size (double the cost, etc.). This is not true if you are willing/accustomed to travel at trawler speeds.

I went from a single 400hp boat to one with two 54hp engines. Instead of 8+ gallons of oil in the single engine, I now use 1.25 gallons --- times two. Went from $95 impellers to $20 impellers --- times two. Costs for ALL components and most services are much less, even with two engines.

Certainly affords plenty of engine room space with the small engines (attached pic).

Speaking of maneuvering, having the two engines roughly 13ft apart has HUGE advantages. Here is a video of us leaving a dock - moving the boat completely sideways using only the engines (rudder right, starboard engine fwd, port engine reverse). Bow thruster was only used at the very end of the video to avoid a fishing boat very close by.

The boat I bought had twins, I added a bow thruster after 5 years. I need all the help I can get getting in and out of slips, or doing locks with wind and current.
If had a single and bow and stern thrusters it would be fine.

Cost and redundancy of twins are factors but insignificant.

I have limped to marinas on one motor more than once for fuel and mechanical issues. I Have unlimited towing and use that too.

Any boat you buy will have an operational learning curve no matter how it is configured. Single or twin your boat challenge will depend on wind, current and what you do with it, your skill set, the boats size, draft, maneuverability.

Buy the boat that checks your boxes and learn to love it.

kind of ironic that the big benefit of twin screw maneuverability as touted here,also have bow and sometimes stern thrusters.

TwoToTango said:

Ok I know this is an age old debate- Twin engines vs Single. . I know most of the stated advantages and disadvantages like more maneuverability and back up power for twins. I get that . But given two identical boats w a full displacement hull., one with a single Lehman 120 and the other with twin 120s , how much extra fuel do the twins use at say cruising speed of around 6 nts? Twice? My gues is not that much more but does anyone have some real experience to answer that for me ? Also just how often really end up using the twin in a back up situation ? I’ve been told if one goes down the other is likely because it’s usually fuel issues that cause a diesel to stop . Thanks for your help . Just trying to educate myself. I like the idea of a single primarily because it’s easy on fuel. Done really have desire to go fast.

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Since I now have an Endeavour 44 Cat Trawler and previously had a Leopard 38 cat sailing vessel, there was no choice- twin engines. But the major advantage of twins in any boat is maneuverability and control. In a cat you can turn in your own length, easily back into the slip, dock, etc. my good friend has a Beneteau 44 Swift Trawler and the twins on her make her infinitely easier to handle. While the fuel consumption might be slightly higher, you have more power and more efficiency, plus if one engine fails you have a backup

No brainer for me
Jack

Sailor of Fortune said:

kind of ironic that the big benefit of twin screw maneuverability as touted here,also have bow and sometimes stern thrusters.

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In the 60's/70's all I ever piloted were single screw. Back then "thrusters" weren't discussed.

Since then, I've had boats with twins for quite some years. Enjoy the heck out of twin screw. Still, never had nor used a bow or stern thruster. Don't plan to ever do so.

I can handle a boat with a single. It's super easy with twins!

When I did my re-power I considered the option of going to a single. As you can see in the pic, the keel was designed and built with a single in mind. But I'm not aware of any Mk1's that have a single.

I went with twins because the yard's quotes for the twins was cheaper, largely due to the higher cost of the larger HP single engine. Afterwards the broker I bought from said that it was a good thing I installed twins - in his words, had I installed a single I would never have been able to sell the boat.

I would have been comfortable with either route, and expected maintenance and fuel costs to be more or less the same, but market sentiment or bias definitely favours twins in the >45' size range.

TwoToTango said:

Ok I know this is an age old debate- Twin engines vs Single. . I know most of the stated advantages and disadvantages like more maneuverability and back up power for twins. I get that . But given two identical boats w a full displacement hull., one with a single Lehman 120 and the other with twin 120s , how much extra fuel do the twins use at say cruising speed of around 6 nts? Twice? My gues is not that much more but does anyone have some real experience to answer that for me ? Also just how often really end up using the twin in a back up situation ? I’ve been told if one goes down the other is likely because it’s usually fuel issues that cause a diesel to stop . Thanks for your help . Just trying to educate myself. I like the idea of a single primarily because it’s easy on fuel. Done really have desire to go fast.

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Rule Number 1 - Always have an alternative means of propulsion
Rule Number 2 - Remember Rule Number 1


That said I've come over to what most sailors would call the dark side - Grand Banks 36 - twin Lehman 120's. My previous boat was a Westsail 42 with a nice little Perkins when things got ugly or the wind died.



But Rule Number 1 and Rule Number 2 are still appropriate.


Fuel Economy - A good trawler with the right hull form will make hull speed pretty easily with either single or twin engines. Mine is a Grand Banks 36 with twin Lehman 120's. Fuel consumption is 2.8 GPH at 8 kts. And that's measured. Sorry to everyone out there - I'm an engineer.



So - this is my when the smelly sticky stuff hits the rotating device which it always does out on the water answer to 1 versus 2 engines. Fuel economy is transparent when the **** hits the fan.


Just my $0.02 for whatever it's worth.

PopeyeGB36 said:

Rule Number 1 - Always have an alternative means of propulsion
Rule Number 2 - Remember Rule Number 1


That said I've come over to what most sailors would call the dark side - Grand Banks 36 - twin Lehman 120's. My previous boat was a Westsail 42 with a nice little Perkins when things got ugly or the wind died.



But Rule Number 1 and Rule Number 2 are still appropriate.


Fuel Economy - A good trawler with the right hull form will make hull speed pretty easily with either single or twin engines. Mine is a Grand Banks 36 with twin Lehman 120's. Fuel consumption is 2.8 GPH at 8 kts. And that's measured. Sorry to everyone out there - I'm an engineer.



So - this is my when the smelly sticky stuff hits the rotating device which it always does out on the water answer to 1 versus 2 engines. Fuel economy is transparent when the **** hits the fan.


Just my $0.02 for whatever it's worth.

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Your calc = 2.86 nmpg.

As comparison...

Our 34' Tollycraft, flying bridge - tri cabin, with twin 350 cid Mercruisers gets:

1. Running one engine, at 4.5 to 5 knots... just shy of 3 nmpg.

2. Running twins, at 6.5 to 7 knots... just over 2 nmpg... [calced hull speed is 7.58 knots].

3. On full plane, doing 16 to 17 knots... she gets 1 nmpg.

We seldom run at #1 speed. Often run at #2. And, love to run at #3 when we want to save time getting some place.

Having 200 gallon tankage [considering 20% safety hedge; i.e. 40 gallons remaining fuel]:

#1 = 480 mile range
#2 = 320 miles
#3 = 160 miles

Another item I love about our planing hull Tolly is its 23 nmph at WOT... when required. That speed = OMG nmpg!!! Never gone that fast long enough to calc nmpg. As s guess, I'd have to take a stab at 0.5 nmpg??? That would only = 80 mile range. I'll never push her engines that hard [5400 +/- rpm] for that long.

Few times I've hit WOT were for few minutes each. Things like equipment testing purposes and a couple times we simply needed to alter our position due to other boats' incorrect actions.

Single vs twin.

TwoToTango said:

Are you saying there are other advantages to a single? Maintenance costs?

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I have a very competent mech perform the annual svc on my single 120, transmission and generator for several reasons. As an aside, while I could do it, he probably saved me $30,000 this year by finding hidden problems I would never have discovered doing the std service myself. I digress. Annual svc for 1 engine, xmission and gen is $1,000. You can almost double that for 2 (almost since there would still only be 1 gen). Every so often there’s the matter of stuffing box and cutlass bearing. Double that for two engines.

Additionally, I can literally walk (crawl) all the way around my engine and everything on it is very accessible. That’s a huge benefit. And yes, generally speaking, Diesel engine failures are usually fuel related, which means that second engine isn’t going to help you. This isn’t always the case but it usually is. I don’t know what handling like on 1 engine when the other fails on a displacement hull but I imagine it’s pretty bad.

I have 1 engine, no thrusters. Backing is always an adventure and here, two engines would be nice, but not enough to justify the downside of having 2.

When I was in the market for my boat 1 engine was a requirement.

Having said all that, in my opinion fuel cost should not even be on the radar screen when deciding between 1 or 2. Listen to the 2 engine guys and their reasoning and experience and then decide which philosophy fits your needs.

Finally, there is a saying in aviation. “That second engine gets you to the accident scene faster”.

PopeyeGB36 said:

Rule Number 1 - Always have an alternative means of propulsion
Rule Number 2 - Remember Rule Number 1


That said I've come over to what most sailors would call the dark side - Grand Banks 36 - twin Lehman 120's. My previous boat was a Westsail 42 with a nice little Perkins when things got ugly or the wind died.



But Rule Number 1 and Rule Number 2 are still appropriate.


Fuel Economy - A good trawler with the right hull form will make hull speed pretty easily with either single or twin engines. Mine is a Grand Banks 36 with twin Lehman 120's. Fuel consumption is 2.8 GPH at 8 kts. And that's measured. Sorry to everyone out there - I'm an engineer.



So - this is my when the smelly sticky stuff hits the rotating device which it always does out on the water answer to 1 versus 2 engines. Fuel economy is transparent when the **** hits the fan.


Just my $0.02 for whatever it's worth.

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Hi. I have a GB 36 Classic with 1 120. Hull number 712. What is yours?

IMHO - forget gas. 30 years made me a diesel believer (after 20 years of gas).
Get a T dock and maneuverability becomes moot for most of your experiences.
Put a little extra into maintenance (still not equal to twins) and you should be fine.
A "get home" engine or genset attachment would be nice. Get experienced recommendations for onboard spares and keep a lot (cheap insurance). Definitely join Boat US insurance for underway, anchoring failures FOR SURE
Good Luck - with your forethought you'll have lots of fun (and some terror as well)

Oh Yeah ! Reduce your "fuel expense" concern, fuel cost will become a small percentage of your boat ownership. Personal experience taught me that the more you can competently do yourself is your BEST cost reducer.

dhays said:

2. While I have never used a twin engine as back up for an engine failure, you might also ask how many of those of us with singles have had an engine failure that left us stranded. In 30 years of operating single engine diesel boats, I have had that happen once.

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For newer boats with electronic engines this becomes more problematic. These engines are software controlled and have tons of sensors. They are designed to shut down (or in some cases de-rate) rather than hurt themselves.

So for example my engines have a seawater outlet pressure sensor. Why, I cannot fathom. Stupid. Anyway they are failing all over the country. When it fails, the ECU in all it's wisdom shuts down the engine.

Was quite glad I have two engines when it happened to me!

Personal Experience....
IF I didn't have spares aboard, or good drill/easy out capability, patience to deal with multiple engine flameouts a day due to fuel system air ...in busy waterways, etc....


Issues I have had in 18,000 miles and 3,600 eng hrs and last 8 years of cruising.



1. Fuel pipe pin hole on 1000 hr pipes. Massive atomized spray...could have been a fire to those less experienced.
2. Fuel pipe crack on 2000 hr pipes. Could have been a manufacturing defect or the need to bend it a bit more than designed.
3. Alternator bracket bolt sheer.... required through drilling with cobalt drills and replacement bolt.
4. Bubbles in coolant suggesting head gasket replacement.
5. Dampner plate failure stopping engine cold in a bad seaway.

6. Multiple engine shutdowns due to elusive air in fuel system.
7. Alternator failure/engine lug to alternator adjuster arm failure. Difficult to find/replace lug.


Some of these could have been avoided with better engine history from the PO. Maybe. All can happen even with experienced, liveabord, active cruiser eyes watching things.


Do I hate single engine? Naw...the gasser on my towboat was actually more reliable.


So much for Lehmans being "bullet proof"...sure maybe the block is but you are at the mercy of marinizing manufacturer after market parts sometimes.


If Sea Tow or BoatUS wasn't available in the areas I cruise...my experience would have me leaning twin....not for any other reason though.

JustBob said:

So for example my engines have a seawater outlet pressure sensor. Why, I cannot fathom. Stupid. Anyway they are failing all over the country. When it fails, the ECU in all it's wisdom shuts down the engine.

Was quite glad I have two engines when it happened to me!

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I'm not familiar with that particular sensor...what was the fix, (other than replacing it.)

Guess I just love old-school simplicity!

In all the power vehicles I list below... the only one that can be an expensive PITA is the last one - because - although it's a comfortable classy rig... it's all computer interfaced and chips/features go south way too often [currently in shop for new chips and new backup camera = $1,200, and, something I could not have sussed out and cannot accomplish]. The rest of the rigs I maintain well... they breakdown very seldom. Mostly I can repair breakdowns and if expert assistance becomes necessary it usually does not cost an arm and a leg.

Vehicles we have:

1 - 1967 Buick Wildcat, AT with nearly all options inc tilt wheel, electric seat/windows and AC w/ 430 cid, 400 hp engine [5 years ago, at 125K miles I had her rebuilt by an expert - jacked-up her power range from factory original 360 HP to around 400 HP] - 4 bbl Rochester with enlarged internal fuel ports. Increased air intake, Twin 2" straight exhaust pipes with V-crossover pipe and low restriction mufflers - A good sounding, real screamer - See photo.
1 - 1977 Tollycraft tri cabin w/ twin Mercruiser 350 cid, 255 HP engines, single 7.5 kW Kohler gen set... all 3 carbed
1 - 1975 Crestliner fast runabout 15' 4-seater, canvas top, full windshield; a cool tow behind w/ 50 HP Johnson - carbed
1 - 1985 Chevy Cheyenne 1 ton, 4WD, 4 spd, geared transfer box, 4" lift, reg cab, power everything, AC'd, LB pickup w/ 350 cid 325 hp GM engine - Carbed
1 - 1989 19' Bluewater ski boat, cuddy cabin w/ Mercruiser and I/O drive - TBI fueled
1 - 1996 25'6" Tiffin Allegro motorhome w/454 Chevy engine - TBI fueled
1 - 1998 Ford Explorer SUV 4WD w/ 6 cyl - EFI fueled
1 - 2014 Lincoln MKT SUV, 4WD and nearly every option w/ 375 hp - EFI - A screamer!

This post will never end.....

Sailor of Fortune said:

kind of ironic that the big benefit of twin screw maneuverability as touted here,also have bow and sometimes stern thrusters.

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Never had a bow or stern thruster. I have had singles and twins. And, I admit, it's a lot funner parking a twin!

Mooring a twin I do while napping.... a single takes a bit of seamanship in some cases.

psneeld said:

Mooring a twin I do while napping.... a single takes a bit of seamanship in some cases.

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Our last single, a 42 foot sailboat, would only back 20 to 40 degrees to port, so when it was time to back into a slip, you had to have it positioned where those numbers put you into the slip (or wherever you wanted to back up to).

twin vs single

I owned a GB 36 with a single engine, no BT for 6 years and have owned a MS 34 trawler with twins for 3 yr. Although this may never happen, when faced with a choice between twins or single with a BT I would go with the boat in the best condition with price always a consideration. The reason I went with the twins was the boats availability at the time and its condition. Never consider a single lacking a thruster!! I know the old salts of the world will see this as a reduced hormone level on my part but try a 30mph cross wind with a margin of error of less than 2 ft some time with the single no BT!