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Old 15-03-2016, 07:47   #31
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Re: Different Size Engines

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Originally Posted by Dockhead View Post
Weight is the main saving -- crucial in an unballasted boat. At least half a ton.

Secondly better loading of the engines, which results not only in better fuel efficiency, but in better service.
1/2 a ton? Where are you saving that much?

Shutting down one engine in calm to moderate conditions, already dials you in pretty well for appropriate loading of the engine and keeping it near peak efficiency with matching HP conventional drives.

You've already ruled out the only option that would result in significant weight savings (2 conventional propulsion and 1 generator to 2 slightly larger conventional and a hybrid that doubles as a generator)
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Old 15-03-2016, 07:58   #32
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Re: Different Size Engines

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Originally Posted by valhalla360 View Post
1/2 a ton? Where are you saving that much?

Shutting down one engine in calm to moderate conditions, already dials you in pretty well for appropriate loading of the engine and keeping it near peak efficiency with matching HP conventional drives.

You've already ruled out the only option that would result in significant weight savings (2 conventional propulsion and 1 generator to 2 slightly larger conventional and a hybrid that doubles as a generator)
Well, in theory diesel-electric sounds cool -- what I wrote was that I don't know whether it is developed enough to consider using, and also that I don't understand the efficiency issues.

But direct drive using two unequal main engines would save exactly the same amount of weight, or probably even more -- the same diesel engines but no electric motors. A 6 or 7 kW heavy duty genset weighs 350kg or so -- if you save 150kg on downsizing one main engine, then you're at half a ton of weight saving.


I agree with you about loading on normal cats -- that's one of many significant benefits of dual propulsion. I like it so much that I might consider trying to have dual propulsion on a monohull, if my next boat is a mono.

Interestingly, Moody's "gentleman's motor sailers" of the '50's and '60's, gorgeous things design by Laurent Giles, often had twin engines.

To avoid the extra drag and, especially, the gear protruding without a keel in front of it, I would use a retractable drive like on Open 60's for the smaller secondary drive.

I don't know whether the cost of that retractable drive would be worth it or not, but it would give all these benefits. Making the second diesel engine a second propulsion engine with big alternators, instead of a genset like I have now. We had a thread about it.

Obviously on a cat this is easy peasy.
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Old 15-03-2016, 08:20   #33
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Re: Different Size Engines

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Well, in theory diesel-electric sounds cool -- what I wrote was that I don't know whether it is developed enough to consider using, and also that I don't understand the efficiency issues.

But direct drive using two unequal main engines would save exactly the same amount of weight, or probably even more -- the same diesel engines but no electric motors. A 6 or 7 kW heavy duty genset weighs 350kg or so -- if you save 150kg on downsizing one main engine, then you're at half a ton of weight saving.

.
Agreed, a marine hybrid is going to be a custom job. In principal is should work well but with any custom work, unexpected issues could arise. At least in theory, the hybrid option allows you to eliminate an engine while maintaining the full capability.

Unequal conventional engines doesn't really save you anything as the savings from the smaller engine will have to be balanced against a larger engine on the other side. There might be a little savings but I wouldn't count on much, certainly not a 1/2 ton.

It's only if you can eliminate the genset that you gain weight savings but if you don't need a genset, it doesn't matter if you go with equal or unequal drive engines.
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Old 15-03-2016, 08:34   #34
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Re: Different Size Engines

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Originally Posted by valhalla360 View Post
. . .
Unequal conventional engines doesn't really save you anything as the savings from the smaller engine will have to be balanced against a larger engine on the other side. There might be a little savings but I wouldn't count on much, certainly not a 1/2 ton.

It's only if you can eliminate the genset that you gain weight savings but if you don't need a genset, it doesn't matter if you go with equal or unequal drive engines.
There are two reasons for downsizing one of the mains:

1. To size one of the engines so that it can be used for long periods of time for generating power. I wouldn't want to use a 70 horsepower engine for hours at a time generating 4kW or 5kW -- it's not enough load. But a 30hp or maybe even 40hp would be ok for this. This works well with LiFePo batteries which can accept large currents and can be charged in short, efficient generator runs.

2. To have one engine sized well for very light power requirements -- motorsailing for example, or very flat water.



If you can achieve 1, then you can do away with the generator (350kg right there). If you have two 70 hp engines, then you kind of need the generator if you are a significant electrical power user (and any 60 foot cat is going to be I think). This gives you the double weight savings -- eliminate generator, downsize one engine.

Achieving 2 just makes motoring with light power requirements more pleasant and efficient. You'll probably save some fuel, and you'll definitely load the engine better, and it will be happier and longer-lived.


Hybrid would have the advantage -- a significant one I think -- of being able to drive both screws with either or both diesel engine.

Conventional shaft drive would have the advantage -- probably the overwhelming advantage, for me -- of simplicity, economy, well-proven technology, no science experiment or beta test. At the expensive of driving only one screw per engine at a time.
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Old 15-03-2016, 08:48   #35
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pirate Re: Different Size Engines

What I fail to understand is the reason for say a Lagoon420 to have 2 x 75hp engines..
Surely 2 x 45's would be more than capable of the job..
My boat, 40ft LOA and weighs 27000lbs, has 6.5ft draft yet her 47hp John Deere will bang her along at 6-7kts at 1900revs..
Seems over the top to me considering all things..
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Old 15-03-2016, 09:53   #36
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Re: Different Size Engines

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What I fail to understand is the reason for say a Lagoon420 to have 2 x 75hp engines..
Surely 2 x 45's would be more than capable of the job..
My boat, 40ft LOA and weighs 27000lbs, has 6.5ft draft yet her 47hp John Deere will bang her along at 6-7kts at 1900revs..
Seems over the top to me considering all things..
Good thing Lagoon agrees since their website calls for a pair of 40hp engines as standard.

The 62'er has a pair of 110hp engines but I'm betting the guys buying a 62' boat new, don't mind burning a few extra gallons to add a extra knot to the cruise speed. If it was a priority, I'm sure you could talk them into fitting something in the 60-80hp range.
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Old 19-03-2016, 04:18   #37
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Re: Different Size Engines

Your thought process is off in my mind. When deciding what overall power to put on a catamaran say a 45 ft. Calls for 80 up. You put in 2 40 hp. Not one 60 and one 20 hp.
Lagoons need more power per weight due to the amount of windage. I would not like to try a lagoon 42 with 40's motoring into 25 knts, and 5' seas.

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Old 19-03-2016, 05:38   #38
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pirate Re: Different Size Engines

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Originally Posted by valhalla360 View Post
Good thing Lagoon agrees since their website calls for a pair of 40hp engines as standard.

The 62'er has a pair of 110hp engines but I'm betting the guys buying a 62' boat new, don't mind burning a few extra gallons to add a extra knot to the cruise speed. If it was a priority, I'm sure you could talk them into fitting something in the 60-80hp range.
Just to check my facts I searched 420's for sale and the one I opened had 2 x 75's so that's what I posted..
No.. I'm not going to find and post the link to prove it..
Believe me.. or don't..
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Old 19-03-2016, 07:56   #39
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Re: Different Size Engines

fwiw,

I spent the better part of 24 hours motoring into Gulf of Mexico 30kt and 6-8' sloppy GOM short-period waves...

...with my two-bladed-propped 29hp Volvos.


I'm sure the mileage may vary, and there are lots of factors, but one of the nicest things about a catamaran is that you don't need large engines.

Having two different-sized engines makes absolutely no sense.


btw: it isn't the smartest thing to be travelling northwest in the GOM in February
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Old 19-03-2016, 08:18   #40
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Re: Different Size Engines

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Just to check my facts I searched 420's for sale and the one I opened had 2 x 75's so that's what I posted..
No.. I'm not going to find and post the link to prove it..
Believe me.. or don't..
Sure you didn't pull up a trawler version? Those usually upsize the engines.

No need for proof. I'm sure for the right dollar amount, you can get a pair of 75's and maybe someone retrofit them but the manufacturers website calls for 40's.
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Old 19-03-2016, 08:40   #41
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Re: Different Size Engines

I think that what gets lost in these conversations is what are you designing the engines to do. Is the intent to simply help out on a light air day, or to allow you to bash into 40kn breeze and big seas for days at a time. Much like electric power conversions, there is a use profile that may justify a mismatched set of engines, but they won't work very well for other situations.

In my head all engine conditions come down to motoring into a 70kn wind while trying to reset a dragging anchor prior to a hurricane rolling thru. I have been in this situation, and it scarred the crap out of me, so if we are talking about a cruising boat, this is what I need an engine to do. If whatever proposed system can't meet these requirements then I am not interested.

On the other hand for a race boat that just needs to get in and out of the harbor, then the use profile changes dramatically, and some combination of electric and smaller engines may be ideal.
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Old 19-03-2016, 09:03   #42
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Re: Different Size Engines

I wouldn't do the different sized engines, I'd do two of the same, with the idea of a Life-PO bank that can accept massive amps during charging, don't forget to add house loads in with the charging load, you may be able to load a 40 hp motor more than you think, I believe your issue will be how much can your alternators generate, what is their duty cycle at max output, not engine, you may have to go with "better" than those school bus alternators, but ones that can do it are I'm sure are available. I'd also have both engines set up as generators too as I like redundancy.
Plus as has been bought up, with the size of a Solar farm you can put on a Cat, it may be that you don't need that generator as often as you may think. I think Cats change the equation, different animal, gotta think differently. I know one goal is to be a pure electric boat, no propane and that of course means lots of electricity available.

Key here I think is a massive battery bank, and inverter or inverters, as much Solar as is possible, then the engines as generators


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Old 21-03-2016, 04:58   #43
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Re: Different Size Engines

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Originally Posted by Sailingcouple13 View Post
Your thought process is off in my mind. When deciding what overall power to put on a catamaran say a 45 ft. Calls for 80 up. You put in 2 40 hp. Not one 60 and one 20 hp.
Lagoons need more power per weight due to the amount of windage. I would not like to try a lagoon 42 with 40's motoring into 25 knts, and 5' seas.

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One thing does not follow the other in this argument, in my opinion. We were not talking about total horsepower. We were talking about the possible benefits of distributing total horsepower non-symmetrically.

We're also not talking about Lagoons at all, but large performance cats like the Atlantic 57.
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Old 21-03-2016, 05:19   #44
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Re: Different Size Engines

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Originally Posted by Stumble View Post
I think that what gets lost in these conversations is what are you designing the engines to do. Is the intent to simply help out on a light air day, or to allow you to bash into 40kn breeze and big seas for days at a time. Much like electric power conversions, there is a use profile that may justify a mismatched set of engines, but they won't work very well for other situations.

In my head all engine conditions come down to motoring into a 70kn wind while trying to reset a dragging anchor prior to a hurricane rolling thru. I have been in this situation, and it scarred the crap out of me, so if we are talking about a cruising boat, this is what I need an engine to do. If whatever proposed system can't meet these requirements then I am not interested.

On the other hand for a race boat that just needs to get in and out of the harbor, then the use profile changes dramatically, and some combination of electric and smaller engines may be ideal.
Well, I don't know any boat that can motor against a 70 knot wind, not to mention the sea state that would produce. I think that's an impossible task. You would need 1000 horsepower and a wave-piercing hull. Better a submarine.

Two boats with the same total horsepower will have roughly the same ability to get upwind against a storm, whether or not the engines are the same size, or not. The asymmetrical thrust will somewhat reduce effectiveness at full power, but not by much unless the difference is very large. An interesting question is whether this becomes a bigger problem at very low speeds with less rudder authority -- I reckon it would.

I motored around on a catamaran with only one engine for a week, after one prop fell off, which is how I know that the symmetry of the thrust is not all that important.

So again -- how much total power is a separate and unrelated conversation.

For the record -- I do believe in having a large reserve of power. I have 100 horsepower in a 20 ton boat (24 tons or so loaded), and have found it to be not quite enough in a couple of situations such as trying to bash through Borkum Riff against a F8 with wind against tide and horrendous steep seas.

It is possible that I would have been able to use this horsepower better with a different prop, but I hated the feeling that I could have been driven onto shoals if things had been only slightly worse.

So if my next boat is a mono (not yet definitely decided), I am planning that it would be a bit longer but about the same displacement, and 150 horsepower instead of 100, and a Hundested variable pitch prop.



So I definitely believe in having significant power reserves. And having different size engine on a cat would be one way to do that, without chronically underloading an engine.


Concerning "what are the engines designed to do" -- on a boat for me, that would be:

1. Provide the ability to motor efficiently for long distances in normal conditions.

2. Provide the ability to motorsail efficiently in light conditions.

3. Provide the ability to get upwind and make headway against wind and sea in at least F8 conditions.

4. Generate electrical power efficiently and without damaging the engine due to underloading it.


Using two engine, not three, while optimized for weight.

Commonality of spares is not nearly as important as the other considerations here.
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