Single vs Twin: It's Baaaaack!

The one engine- two shafts/props makes absolutely no sense to me. In terms of engine redundancy you have none so no advantage there.

And in terms of maneuverability, you don't have one of the most effective maneuvering tools of a twin which is not only to be able to use differential thrust but to use differential power, too. I am very often adding power to one engine and not the other to speed up a maneuver, counter a wind or current, or improve the accuracy of the maneuver.

While I guess you could split the thrust with a pair of transmissions driven by a single engine, you could not add power to one side while remaining at idle on the other.

So other than having redundancy in the shafts and props I see no advantage to it whatsoever.

A lot of good single-engine boat handlers know how to take advantage of the propwalk from their one propeller. Rather than bemoan it they have learned to take advantage of it and are able to do things that single-engine drivers who don't understand how to make propwalk work for you have no idea how to do. If the two props in the single engine-dual prop setup are counter-rotating, the setup even takes away a good maneuvering tool from the experienced single engine driver.

The cons greatly outweigh the pros in this case, so far as I can see.

Greetings,
How about two engines/motors ONE variable pitch prop just to complicate matters further? I looked into x Swedish rescue boats about 8-10 years ago and a LOT of them had this set up. I suspect for towing or light ice breaking.

Marin said:

Eric--- With regards to your above post, it would be interesting to know how the drag of a twin engine boat like our GB36 compares to the drag of the single engine version of the same boat.

The reason I wonder about this is that while our twin engine boat has a pair of shafts, struts, and spade rudders, the deep keel of our boat is cut short and reverse-curves up to blend into the bottom of the boat. As opposed to the deep keel of the single-engine boat which is carried a fair amount farther back to culminate in a vertical trailing ege and a shoe supporting a big rudder.

Don't know how it works with hydrodynamics but the single engine GB36 has, I think, more wetted surface than the twin. Would this result in more drag?

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Re the drag of GB36 twin.
There's a lot more wave making resistance from the 3000# (approx) of the additional weight required from the extra engine, sea cock, strainer, control cables or hoses, larger tanks and more fuel, hoses, shaft, prop rudder ect ect ect. Perhaps 4000# would be closer.
There is the drag of the propeller shafts and struts.
Decreased wetted surface of the keel.
Engine inefficiency reduced due to engine underloading (25 to 35hp per engine)
Twice as much surface area for rudder drag.
Rudder drag to compensate for prop walk eliminated.
Less weight = lower in the water and more wetted surface.


The wetted surface of the keel and extra rudder could possibly eliminate each other but the extra rudder is in the propwash so probably more drag.

One could find out how much the extra disp from the weight of the extra propulsion system by computing said weight and putting it aboard a single and testing it.


Re the drag of the GB 36 single.
Much reduced drag due to much less wave making resistance due to less weight.
With less weight boat rides higher and has less wetted surface.
half as much rudder drag.
More drag from larger keel.
Engine not over loaded so thermally considerably more efficient.
Increased rudder drag compensating for prop walk.


Marin I almost told you to find an apples to apples pair of boats to compare and that your question would only shed light on the efficiency differences between GB boats. No mater what the above shows it will NOT tell much of anything about wether or not a single engined boat is more efficient than a twin. Twin engined boats in general may be more efficient or singles may be but comparing boats with twice as much and half as much total power is rather pointless. Compare a single w a 120hp engine w a same boat w two 60hp engines and you'd be close to a meaningful comparison.

Anyway I humored you and gave you what you asked ... I think.

Marin re the geared up one engine 2 screws ... ya basically need to read the article. Trust me Marin the maneuvering capabilities of the geared up system is almost as good as the regular twin. And they concluded it is more efficient that either the single or traditional twin. It's worth reading for those that are interested. It's in a PMM.

Chuck Gould,
I'm trying to keep this about twins v/s singles not the efficiency of one engine over another. I did indeed include in my post/statement that the thermo loss of the twin would be far greater ... in the GB example. But if your'e going to compare the drive system efficiency of singles and twins I would think one would want engines w the same disp per hp, weight per hp, power per pound so only the difference in efficiency of the drive system emerges. But first of all I think one needs to compare boats w the same total power. And most all the trawlers on this forum have twice as much hp in twins and half as much hp in singles. So even a good comparison won't be applicable to the boats on this forum.
But re the above basically most all the boats considered here have singles being more efficient that twins, mostly due the engine loading and the drag from increased weight that causes greater wave making losses.

Eric--- While I don't dispute your drag comments there are a couple of points to consider. The spade rudders on the twin GB are each considerably smaller than the one rudder on the single.

The hp of each of our FL120s at our cruise rpm of 1650 with the props we have is about 60 hp per engine. So I'm not sure where you got that 25-35 hp figure unless you are saying that the loading of EACH engine in the twin is 25-35 hp LESS than the loading of the ONE FL120 at the same cruise speed in the single-engine version of the boat.

I don't have PMM magazines to reference, but the only question I have with regards to the single engine-dual shaft/prop setup is: can one of the props be spun up to a high rpm while the other prop remains at a low rpm regardless of which direction the props are turning or if they are opposing each other with one generating forward thrust while the other is generating rearward thrust?

Most everything said here I have heard in other single vs. twin arguments, but Chuck Gould offers something new in that twins are more likely to sink and qualifies it by saying that he has seen a few sent to the bottom. Should't that end this debate? Since twins are likely to sink, what use will having a redundant engine be?

So to those still shopping for a boat, do not buy one with twins unless you are a strong swimmer.

Well, now wait a minute, Mahal. I am aware of some single-engine boats that had their hulls punched open by running into deadheads and reefs and stuff and they sank, too.

So I'm thinking that this is a FAR bigger issue than any of us first thought. Perhaps what we are discovering here is that BOTH single AND twin engine boats can be sunk by slamming into something. In which case the only sure-fire way to avoid this potential catastrophe is to STAY THE HELL AWAY FROM BOATS.

What do you think? Has that finally put the single vs twin issue to rest?

C. Marin is absolutely correct that single engine boats can also be holed by a deadhead and sink.

The critical difference isn't a case where some deadhead comes rocketing vertically up from the depths and makes an unwelcome intrusion into the bilge. There would be no clear advantage, single or twin, in such situation.

Consider the far more common encounter with a deadhead. There's a submerged log, floating more or less horizontally, just below the surface. Strike that with a single, and it will make a lot of noise as it thumps its way along the keel and pops up (giving you the equivalent of a deadhead finger) immediately astern. No holes in boat, nothing damaged, usually, except your pride. (Don't ask me how I know).

Strike the same deadhead with a twin screw trawler. It thumps its way along the keel, but can't pass harmlessly astern because it runs into the strut and the exposed shaft. Hey, maybe 19 out of 20 times even *this* isn't going to tear out a strut. Then comes the guy unlucky enough to statistically be #20. One 12 x 18" hole in the bottom of your boat can wreck your entire day.

Marin said:

The one engine- two shafts/props makes absolutely no sense to me. In terms of engine redundancy you have none so no advantage there.

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Agree, and two engines and one well-protected shaft/propeller/rudder makes more sense even at the expense of paranoic redundancy.

Chuck Gould said:

...., maybe 19 out of 20 times even *this* isn't going to tear out a strut. Then comes the guy unlucky enough to statistically be #20. One 12 x 18" hole in the bottom of your boat can wreck your entire day.

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Every time you pass an oncoming car on a two lane road there is a chance that the driver will be drunk or strung out or falling asleep and he/she will swerve over and slam into you head on. Given the number of times this actually happens on just one stretch of I-5 here, I think it is actually a much greater statistical risk than hitting a deadhead with your boat and having it take out the running gear in such a way as to sink it. So are you going to stop driving your car?

This "it could sink your boat" reasoning for preferring a single to a twin is just dumb as far as I'm concerned. If it was really a credible threat, you would think this would have long been figured out by everyone including the boat manufacturers, yet Grand Banks, Fleming, and just about every boat manufacturer on the planet keep on turning out twin-engine death traps and people keep buying them up and--- gasp-- NOT SINKING.

For another thing, deadheads are called that because they float vertically, not horizontally. That's why they're so hard to see--- just the top end of it is visible lifting in and out of the water. Hence the "head" in the name. And sometimes they float vertically just below the surface of the water.

As such, they can and have done considerable damage to the props and rudders of single engine boats, too. We know one couple who used to be on our dock who had the prop and rudder ripped off their sailboat, of all things, by a deadhead they failed to see. If there's anything out there with a protected prop and rudder it's a sailboat but....... you just never know.

Deadheads almost invariably are hemlock logs. So if you really want to protect your boat from deadheads, the best thing is to figure out a bug or a poison that will kill off the hemlock population of the PNW.

So while I certainly agree that twin drivelines can be more vulnerable to some kinds of waterborne debris, this notion that they are putting every twin engine operator in dire peril is silly and is certainly no reason to pick a single over a twin. There are plenty of valid and sensible reasons for making the decision one way or the other, but i sure don't see this as being one of them.

Tad is right BUT Tugs are not cruising boats.
Our tug fleet consists of Z Tech tugs and Rotor Tugs.
The Zedies have 2 main engines with azmithuing props ( ie 360 deg drive)
Highly manoueverable and with 60 Tonne bollard pull draught of approx 4.5 mts
Rotor tugs have 3 main engines even more highly manoueverable and with a bollard pull in excess of 80 tonnes.
BUT none of you would like to go to sea in one. When we have to deliver one of these harbour tugs down the coast it is difficult to get even seasoned seamen to man them.
Look up Ztechtug.com
Cheers
Benn

Sorry to assualt your sensibilities with my "dumb" remark that a boat that has two sets of exposed running gear could be more vulnerable to damage when striking a deadhead or other debris than a boat that has a single set of running gear that is well protected.

Our Puget Sound deadheads eventually rot enough at one end to reach a vertical position, but unless you want to quibble that a partially rotted and submerged log is not a "deadhead" until it's floating fully perpendicular to the surface or want to challenge my example of striking almost *anything* submerged, having it find its way down the keel to do more or less damage based on the architecture of the boat, my point is valid.

If it makes you feel more at ease, simply imagine the "head" of the submerged log bumping along your keel to meet its destiny back at the strut.

And, furthermore, I never said that exposed shafts, props, and struts were putting every twin engine operator in "dire peril". If you insist on calling my argument "dumb", please restrict your rebuttal to points actually made. I think it's fair to say I estimated the increased chances of a catastrophic encounter at about 5%, (1 out of 20). Don't know about you guys in the north sound, but down this way we don't consider a 5% risk to be "dire peril".

I respect your opinion, even if I disagree. Would you consider returning the courtesy?

I prefer a single engine vessel because I have seen the results of struts torn out of twin engine hulls. I can't imagine why that opinion is so hard for you to tolerate.

Chuck Gould said:

I respect your opinion, even if I disagree. Would you consider returning the courtesy?

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That is great advice Chuck. "Civil discourse" should be Google searched by some.

This is almost as important an argument as "Which came first the chicken or the egg!!" It doesn't really matter, we have chickens and we have eggs, single and twin engine boats. If you want to pursue this, then the question should be, which type of vessel is at greater risk for sinking, all risk, during normal operation?

Through poor maintenance, lack of good seamanship skills, mechanical failure or just plane bad luck any boat can be lost regardless of how many engines or props it has.

Having owned both single and twin engine boats and nearly 50 years of boating experience, I've never sunk a boat. I have struck many floating deadheads, sticks, boards, etc. and never had a significant deadhead pass under the hull and strike the struts or prop(s). With our slower speeds and proper handling skills this just doesn't happen very often. High speed boats, yeah they can take a log under the boat. I've seen a number of boats twin and single run aground due to navigational errors and single engine boats drift ashore because of mechanical failure and suffer significant damage.

There may be some cruising exceptions, such as running in shallow waters in deltas or rivers where you may be at greater risk of hitting the bottom or sunken debris. In which case, you may need a certain type of boat or take extra precautions for safe cruising.

If you think this discussion is worth an answer, then I'll bet you some insurance company can give you that answer.

"Civil discourse" should be Google searched by some.

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I did. And what I learned is that civil discourse is really, really boring.

Of course the running gear of a twin is more exposed to damage. I have no argument with your [Chuck's] point in that regard.

But your experience seeing a couple of boats sink because they got running gear ripped off does not an epidemic make.

If you want to live in terror of the evil deadheads of Puget Sound, that's fine with me. I don't see it as a practical or realistic reason to make a decision on how many engines one's boat should have. But everyone has their own fears and their own priorities and so should absolutely do what works best for them.

But unless one can actually prove that twin engine boats are suffering mightily from running gear getting ripped off by deadheads and sinking, it's nothing more than an individual paranoia.

Sure it could happen. Sure it has happened. Boats have blown up because they have gas engines. Boats have blown up because they have propane on board. Does that mean that people shouldn't buy boats with gas engines or propane systems regardless of all the other reasons that make gas engines and propane desirable or not?

Some people are terrified of gas engines and propane. For them, I guess they shouldn't buy one. I could draw the conclusion that you are terrified of hitting a deadhead. So for you, absolutely, stay well clear of twin engine boats.

But one person's paranoia over gas engines or propane or deadheads is not a reason that everyone else should feel the same way.

I have no quarrel with the notion of bringing the point up. Someone contemplating the purchase of a cruiser should be made aware of the pros and cons of everything, from the number of engines to the fuel for the galley. But at the same time these things are brought up I think they should be kept in perspective.

We are very much aware of the risk having a couple of shafts, props, rudders, and struts out in the slipstream. So perhaps we keep a closer watch on the water ahead of us than the single-engine boater who perhaps blissfully cruises along believing his keel and rudder shoe will protect him. I have no idea if that last bit's true or not. The single engine boater we cruise with frequently is every bit as vigilant as we are but perhaps he's the exception, I don't know.

But while we are well aware of the problems running into a pot line or catching a branch the wrong way can cause, the very real and every-day advantages of having two engines far, far outweigh the distant glimmer of a tiny risk that we might catch a deadhead the wrong way and sink the boat.

Elevating the tiny risk of hitting a deadhead and sinking to the same status of reasons like increased service and maintenance costs, higher fuel costs, less room in the engine room, higher boat purchase costs, and the other actually real reasons for selecting a single over a twin is, in my view anyway, making a massive mountain out of a minuscule molehill.

Manyboats-A single vs a twin in the same hull will not have more wetted suface because of the additional weight of the second engine and running gear. The DWL (design waterline) of the hull is the same irrespective of the power setup-a single will simply have additional ballast if needed to bring the hull to its DWL.

Here's a case of twin engine design where a rock strike merely broke the strut, rather than ripping it out of the hull. Unfortunately, the unprotected mini-rudder was then clobbered by the same rock. It looks like at "almost" tore out.

Maybe striking a molehill can be as damaging as striking a mountain.

Is it unrealistic to place a desire to avoid this sort of (somewhat avoidable) damage up at the same exalted level as "more space in the engine room"? If you think so, then we disagree.

Hmm, looks like my photo didn't go through..

Chuck Gould said:

Maybe striking a molehill can be as damaging as striking a mountain.

Is it unrealistic to place a desire to avoid this sort of (somewhat avoidable) damage up at the same exalted level as "more space in the engine room"? If you think so, then we disagree.

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How'd the boat in your example come home? On the other engine?'

I think it is totally unrealistic to elevate the incredibly remote possibility of a deadhead strike sinking a boat to the same level of importance of "more space in the engine room" which is an everyday benefit to servicing the engine that every single engine owner will be thankful he has.

Look, you're clearly scared to death of hitting something and sinking, and that's fine. I assume you selected the boat you have based largely on allaying that fear. Good reason. For you. Not necessarily for anyone else although there are a hell of a lot of boaters on the planet so I'm sure there are a fair number who share your same fear.

But I think it's misleading to cherry pick a vulnerability of a twin engine boat that scares the hell out of you but that, statistically, is an incredibly remote possibility and hold it up as a primary reason why people should never select a twin engine boat.

Looks like my role as the newbie site lubber precludes me from getting my hole-in-the-bottom-where-the-strut-tore-off photo posted. Here's a link to yet another photo and different source with a series of damage photos taken from a twin engine boat that went aground on some rocks. Notice the final photo, "strut torn off through the hull". The good news, if any, that once aground it is harder to sink. This is a smaller boat, not a trawler but the design principle is essentially the same.

go deep damage « middlebaysailing

C. Marin;

If you tear the strut out of the hull, you're not coming home on the other engine.

Among the many reasons I prefer a single screw is the reduced risk of damage to the hull or the running gear. If everything else were a wash, that alone would be enough for me, based on my personal preferences, to choose a single over a twin.

PS, C. Marin

Once again you assume too much. Please confine your rebuttal to what is said, not what you imagine has been said or wish was said. Nothing I posted indicates that the prospect of hitting something with a boat "scares me to death". It would be as ridiculous for me to assert that you bought a twin engine boat because you were "scared to death" you couldn't learn to handle a single. Sounds pretty absurd, doesn't it?

Have a nice evening.

Chuck Gould said:

C. Marin;

If you tear the strut out of the hull, you're not coming home on the other engine.

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You need to tell that to the owner of the GB66 that tore an entire stabilizer out of his hull on a rock in SE Alaska a few years ago. Big ass hole, stopped the water ingress, and came back 600 plus miles on the other engine. Actually he came back on both engines since no running gear was damaged.

But maybe that was an illusion in his mind and in reality his boat is on the bottom up there? I'm voting for the owner's sanity, however, as I saw his boat being repaired in the yard after he got back.

These blanket "this will happen," "you'll never" statements are absolute crap as far as I'm concerned. For every example Chuck or anyone else can stick up of a twin in trouble there are examples of equally disastrous things happening to single engine boats. In fact I know someone who DID have a strut ripped out of his hull by a deadhead and who DID bring the boat home on the other engine.

So get a sense of reality and stop all this Chicken Little silliness about things that are so remote in the overall scheme of things as to be not a valid factor in making a realistic decision about what kind of boat to buy.

If you are personally scared sh*tless of stuff in the water, which I can only assume some of you are, fine. Do whatever you have to do to make yourself feel better. Buy a sailboat without an auxiliary engine if that lets you sleep better at night.

Again, I ask you if this hitting stuff and sinking is such a common occurrence and major risk that some of you seem convinced it is, why are the boat manufacturers still making and selling twins, probably more these days that singles? Why haven't the insurance companies shut them down? Why hasn't the USCG banned twins?

The only sensible post that's been put up here lately is Eidelweiss'. Go back and read it for a shot of reality.

What a particular boater 'worries' about may be related to what they already have.

If you have propane for cooking and it has been accident free for many years, you may have a lower 'worry level' that someone who has diesel cooking and considers propane to be very hazardous and would never have it on their boat.

I have a single engine with the prop shaft and rudder protected by the keel. I would 'worry' about having twins with exposed shafts, props and rudders, however Marin is not 'worried' about having twins. Twin engine owners 'worry' about having a single engine fail and be adrift.

The thing is to decide what you are comfortable with and then do what you can to reduce the dangers of what you have. If you decide to have propane, put in an ABYC rated system, and exercise vigilance. If you have a single engine, take good care of the fuel and cooling systems, and the belts and hoses. If you have twins, be careful to not run over logs!

You pays your money and takes your chances!

Tad Roberts said:

Draft......Overall draft (with equal power and thrust) will be less with twin drives than with a single prop. This adds versatility to the job envelope......

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I would think that, everything else being equal, you;d weigh more with twins even if you went down in size (twin 250's weigh more than a single 500) with all the accessories attached. With the same hull design, you've just moved the props to the side. The keel isn't going to be higher (less deep?) if the only difference is twins -v- single. And once you've started tinkering with the boat design then there are going to be other variables to get a true reading on how twins fare better than a single.

Marin said:

I don't have PMM magazines to reference, but the only question I have with regards to the single engine-dual shaft/prop setup is: can one of the props be spun up to a high rpm while the other prop remains at a low rpm regardless of which direction the props are turning or if they are opposing each other with one generating forward thrust while the other is generating rearward thrust?

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You still have the shift capability of the original transmissions. So one transmission could be left in neutral. Would take a slight alteration in technique, but the overall maneuvering capability is very nearly the same. Interesting concept which could probably be done for less than the cost of a second engine. The Ramsey chain setup in most appealing in terms of simplicity and very likely cost. I spoke with the engineer who sized the chains on the twin 40ish trawler that I mentioned earlier. A 4 inch wide chain would easily carry the full output of a single 400 HP engine in the event of one drive chain failing. For our boat, the packaging was best if the engine was backwards in the hull....and a V8 like the 4.5 diesel that GM has sitting on the shelf would have been the perfect choice.

Based on this newly offered theory in this age old twin vs. single argument that twins are mole likely to sink due to exposed running gear, What does Chuck Gould think about serious single engined passagemakers with fin type stabilizers and, hold your breath, a get-home engine with an exposed shaft and prop like Nordhavns? Sinking candidates?

By the way, the fear of an engine failure and not having a back-up is much more realistic than the fear of sinking because of having a twin screw. How many here with twins has sunk because of hitting debris?

skidgear said:

A 4 inch wide chain would easily carry the full output of a single 400 HP engine in the event of one drive chain failing.

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But then you would need a propeller capable of absorbing the full power and that prop would be way too lightly loaded in normal operation to produce much thrust.

Such a setup would require a pair of CPP wheels to be of much use and that setup would negate any cost savings plus add another layer of complexity and maintenance demands.

What's the point?

There is a good reason that the opposite of that scheme is the only method with a history of successful marine application, that is two engines (or more) driving a single shaft.

Dear Prudence:

Although struts can be bent by hitting something, avoiding that "sinking" feeling from a 3' hole seems prudent. By having a well made vessel and using care it seems pretty remote. I'd like to know the details of a deadhead ripping out a strut on a well found vessel if you'd care to share them. I have seen it happen following a grounding on dry rotted wood vessel on the Mississippi.

Of 3 major MV groundings in teh PNW I am familiar with in the past two years - they were single engine high end boats striking rocks - careless in all cases. PM me if you care to and I'll share the gory details. Yes, I know of a few bent struts during that same period, but no serious damage except to $$ and pride

Also, civil discourse when tossing up a very big gauntlet should not be expected in work, play or on the internet.

Lots of good points/posts but I'm still thinking a proper twin engined boat could be as efficient (or more) than single. But w a double hp boat .... no way. I saw a 36 GB on Yacht World w her 2 120 FLs replaces w 2 Yanmar 55hp NA engines. That boat is probably more efficient than a single. But still there's the shafts dragging through the water and the Yanmars are probably more efficient than the FLs. The difference, whatever it is and to which way it goes couldn't be important enough to wallow on but the conversation is and has been rather interesting.

Rambler,
500hp engines are a bit out of relevance here. Aren't 50 to 150hp engines comparable in this respect? Close to spot on I'm sure compared to the double hp boats that are so common here. And w the double hp boats the keel will be lower. With an extra 3000# why wouldn't it be?

Chuck Gould,
Your posts are really good and I sincerely hope you stick around. And Marin frequently dosn't confine his rebuttle to what was said nor do many others.

And Marin don't go around telling really good posters they are "scared to death". Guys don't like that me included. But you make a good point that deadheads don't frequently sink boats. There's a big Sea Ray type in our yard with incredible damage done to it's struts and shafts but the mounting pads for the struts look undamaged. Those struts were REALLY ATTACHED but the average boat may not be so. Oh I see you said Gould's posting was "Chicken Little silliness". Wer'e not all as brave as you .. haha.
Can you access a PMM article on the internet?

Seems to me the geared up twin shaft single engine setup was mostly aimed at repowers. It was to be cost effective in a certain way.

Greetings,
#360........