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Old 14-03-2016, 06:02   #1
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Different Size Engines

Thinking idly about catamaran design --

Why do all cats have the same size engine in both hulls?

Long distance motoring, you guys usually shut down one engine anyway, don't you?

A smaller engine in one of the hulls would make a better generator, with jumbo alternators on it, and would be good for economical cruising in calm water.

It seems to me that there would be a lot of advantages to having a choice between two different sized engines.

I guess you would have asymmetrical thrust when using full power, but is this a big problem? I have asymmetrical thrust at full power on my mono because of the offset shaft, and it doesn't bother me much.

You would save a considerable amount of weight, but you'd get trim problems. I guess the trim problems could be solved by distributing tanks and equipment disproportionately into the hull with the smaller engine.

A cat set up that way, with very large alternators on both engines, and LiFePo batteries, would have a very, very elegant plant, with no separate generator required, fully redundant propulsion and generating ability, and diesel engines used at near optimum efficiency, all the time.
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Old 14-03-2016, 06:52   #2
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Re: Different Size Engines

I had considered the same designing my propulsion.

A very good idea and definitely a way to reduce weight.

The spot it gets a little tricky is when maneuvering dockside in "tank mode" using each engine in forward and reverse.

To spin in place, you'd need to use differing throttle settings on each engine, which would also have different prop sizes.

Probably could get used to it, but I didn't want to experiment with that.

Second, if the big engine went out in an adverse situation, the little one might not be adequate...unless the little one is the size recommend by the designer, rather than undersized. (with the bigger engine oversized)

Trim wouldn't be an issue, as you mentioned, easily solved by distribution of items in the hulls.
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Old 14-03-2016, 07:08   #3
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Sorry... makes no sense to me whatsoever..
When motoring cats I run engines alternately to keep engine hours balanced as much as possible.. the extra/double fuel consumption is not worth the knot or so gained..
The only time I will use both is when the situation requires it in port or heavy seas where the rudders keep losing their grip and I need to keep bows to sea's.
Why not just fit the bigger alternators on the existing engines and save all the hassle of rebalancing the boat.. dis-separate thrust loads on the beams that are not calculated into the design spec.. not to mention the cost..
Mind.. the above comments come from an ignoramus on things technical...
Be gentle with me..
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Old 14-03-2016, 07:08   #4
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Re: Different Size Engines

Quote:
Originally Posted by cruisersfarm View Post
I had considered the same designing my propulsion.

A very good idea and definitely a way to reduce weight.

The spot it gets a little tricky is when maneuvering dockside in "tank mode" using each engine in forward and reverse.

To spin in place, you'd need to use differing throttle settings on each engine, which would also have different prop sizes.

Probably could get used to it, but I didn't want to experiment with that.

Second, if the big engine went out in an adverse situation, the little one might not be adequate...unless the little one is the size recommend by the designer, rather than undersized. (with the bigger engine oversized)

Trim wouldn't be an issue, as you mentioned, easily solved by distribution of items in the hulls.
I wouldn't think maneuvering would be a problem.

Just like a single engine airplane which turns faster in one direction, than the other. Or a mono with prop walk. No big deal at all, I suspect.

Concerning the adequacy of the smaller engine -- I wouldn't think this would really be a problem, either. Losing either propulsion engine should be a very rare event. If you do, I don't think it's unacceptable that the "wing" engine is able to provide absolutely full function -- e.g. motoring upwind. It should just be capable of getting you home. You also have sails, of course, so "getting you home" just means motoring in a dead calm, and maneuvering in harbor, which should be possible on considerably reduced power, from the design power.

I would think that the smaller engine would be used several times more than the big one. It would be used basically at all times when you need any kind of diesel power -- whether generating, or motoring in calm weather, or motor sailing. The big engine would be used, together with the small one, when you need a lot of power, which should be relatively rare, and for harbor maneuvers.

So I think the small one should probably be sized so that 80% power will give you hull speed minus two knots in calm weather, which is going to be maybe 20% or 30% of the total recommended power. Unless that would make a too great difference in size between them.
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Old 14-03-2016, 07:23   #5
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Re: Different Size Engines

Quote:
Originally Posted by boatman61 View Post
Sorry... makes no sense to me whatsoever..
When motoring cats I run engines alternately to keep engine hours balanced as much as possible.. the extra/double fuel consumption is not worth the knot or so gained..
The only time I will use both is when the situation requires it in port or heavy seas where the rudders keep losing their grip and I need to keep bows to sea's.
Why not just fit the bigger alternators on the existing engines and save all the hassle of rebalancing the boat.. dis-separate thrust loads on the beams that are not calculated into the design spec.. not to mention the cost..
Mind.. the above comments come from an ignoramus on things technical...
Be gentle with me..
I was talking about designing a new boat from scratch.

Naturally you wouldn't bother on an existing one.

Concerning keeping engine hours balanced -- I don't see any reason in the world why the hours should be balanced. Naturally you don't want one engine to be disused, but I see no problem making one engine, the smaller one, the predominantly used one.

Cheers.
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Old 14-03-2016, 07:36   #6
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Re: Different Size Engines

Having two engines the same allows you to minimize spares.


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Old 14-03-2016, 07:46   #7
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Re: Different Size Engines

Quote:
Originally Posted by Dockhead View Post
I wouldn't think maneuvering would be a problem.

Just like a single engine airplane which turns faster in one direction, than the other. Or a mono with prop walk. No big deal at all, I suspect.

Concerning the adequacy of the smaller engine -- I wouldn't think this would really be a problem, either. Losing either propulsion engine should be a very rare event. If you do, I don't think it's unacceptable that the "wing" engine is able to provide absolutely full function -- e.g. motoring upwind. It should just be capable of getting you home. You also have sails, of course, so "getting you home" just means motoring in a dead calm, and maneuvering in harbor, which should be possible on considerably reduced power, from the design power.

I would think that the smaller engine would be used several times more than the big one. It would be used basically at all times when you need any kind of diesel power -- whether generating, or motoring in calm weather, or motor sailing. The big engine would be used, together with the small one, when you need a lot of power, which should be relatively rare, and for harbor maneuvers.

So I think the small one should probably be sized so that 80% power will give you hull speed minus two knots in calm weather, which is going to be maybe 20% or 30% of the total recommended power. Unless that would make a too great difference in size between them.
I suppose my thoughts had turned to the following situation:

An enormous current, a bridge and the main engine goes out. Death for any monohull as well, but I've become used to the added safefy factor of twin engines over time.

Certainly no worse than any single engine boat and quite a bit better actually, as there is a backup method of propulsion.

The idea certainly is attractive. I had come close to doing it myself.
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Old 14-03-2016, 07:54   #8
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Re: Different Size Engines

Dockhead, I see three reasons for following Boatman's suggestion and going with two large diesels equipped with high-output alternators:

1. Spare part redundancy.
2. Cats can be very difficult to dock in a crosswind due to the increased windage and reduced underwater resistance to side-slip in comparison to a monohull. In these conditions, twin diesels of equal power are a godsend - I typically dock with the helm amidships using only the twin screws for steering. I suspect that the lack of balance in thrust would make that challenging with dissimilar engines.
3. With adequate solar panels (and cats are better than monos for placement of these), your need for a generator should be limited for times when you want to run air conditioning. If that is something you do not expect to do on a regular basis, then surely it is better to have redundant engines for motoring so as to keep the hours down on both diesels and to ensure that you have enough power to motor in boisterous conditions, should one of the diesels (the larger one in your scenario) fail i.e., true redundancy.

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Old 14-03-2016, 09:23   #9
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Re: Different Size Engines

If the goal is to have two engines of unequal size I would frankly rather go with a diesel electric with two generators. One sized for house loads the second sized for propulsion loads. So say a 10kw (house) and a 40kw (propulsion). If you lost eithe of the generators you would still have a reasonable amount of power remaining. 10kw won't get you home fast, but it will get you home.
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Old 14-03-2016, 11:44   #10
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Re: Different Size Engines

I've seen a cat with a single diesel and an outboard mounted on the opposite side for maneuverability. I've also seen a cat with a diesel in one hull and a generator in the other hull hooked up to a hydraulic system that ran a prop in that hull, also used primarily for maneuverability.


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Old 14-03-2016, 13:50   #11
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Re: Different Size Engines

I suspect the issue is there isn't enough to gain.


Let's say the current standard for a design is a pair of 25hp engines. Are you going to save much switching to a 20 and a 30hp engine? Even if you took it to a 15 & 35 hp, I don't think there is much savings available if you want the same performance. If take the smaller engine much lower, it will have trouble giving you much more than 3-4 kts on the small engine (depends on boat size of course).


Without getting into massively oversized engines, you get very close to the same efficiency out of 25hp engine putting out 15hp as you would with a 20hp engine putting out 15hp. Now if you are talking about a 500hp engine putting out 15hp, mechanical losses from turning over the big engine could make for substantial savings but if you have a 500hp engine, you are likely not a displacement vessel.


If you plan to have a separate generator regardless, what could work is conventional diesel in one hull and a battery/electric in the other. In the situation described above, you could hook up a 40hp diesel in one hull and maybe a 10hp electric in the other with a battery bank good for maybe 3-5miles.
- During maneuvering, you would run both and the generator wouldn't need to kick on.
- Long distance you could pick electric (say motorsailing where you just need a little boost) or diesel depending on the desired speed with battery power available on demand if an engine fails.
- If you just want to go out for a day sail, you could turn the diesel off as soon as you are clear of the dock and run on battery till you get to open water and raise the sails.
- 10hp generator should be able to provide for just about any house loads.
- Probably not much more expensive than a pair of 25hp diesels (assuming you will have the generator regardless)
- Doesn't require an exotic or huge battery bank.
- Assuming you put the generator on the electric motor side, it should balance the weights out pretty well.
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Old 14-03-2016, 14:59   #12
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Re: Different Size Engines

Quote:
Originally Posted by valhalla360 View Post
I suspect the issue is there isn't enough to gain.


Let's say the current standard for a design is a pair of 25hp engines. Are you going to save much switching to a 20 and a 30hp engine? Even if you took it to a 15 & 35 hp, I don't think there is much savings available if you want the same performance. If take the smaller engine much lower, it will have trouble giving you much more than 3-4 kts on the small engine (depends on boat size of course).


Without getting into massively oversized engines, you get very close to the same efficiency out of 25hp engine putting out 15hp as you would with a 20hp engine putting out 15hp. Now if you are talking about a 500hp engine putting out 15hp, mechanical losses from turning over the big engine could make for substantial savings but if you have a 500hp engine, you are likely not a displacement vessel.


If you plan to have a separate generator regardless, what could work is conventional diesel in one hull and a battery/electric in the other. In the situation described above, you could hook up a 40hp diesel in one hull and maybe a 10hp electric in the other with a battery bank good for maybe 3-5miles.
- During maneuvering, you would run both and the generator wouldn't need to kick on.
- Long distance you could pick electric (say motorsailing where you just need a little boost) or diesel depending on the desired speed with battery power available on demand if an engine fails.
- If you just want to go out for a day sail, you could turn the diesel off as soon as you are clear of the dock and run on battery till you get to open water and raise the sails.
- 10hp generator should be able to provide for just about any house loads.
- Probably not much more expensive than a pair of 25hp diesels (assuming you will have the generator regardless)
- Doesn't require an exotic or huge battery bank.
- Assuming you put the generator on the electric motor side, it should balance the weights out pretty well.
I agree that in this size range, it wouldn't make much sense. Those size engines are loaded up enough with say 5kW of alternators, and so you just put bit alternators on both normal mains.

I was thinking of larger boats -- like the Chris White Atlantic 57.

But concerning "you will have the generator regardless" -- no, the whole idea is to get rid of the generator. Two diesel engines is more than enough already on an unballasted, weight sensitive vessel.

Someone above suggested diesel-electric -- I have never spent much time on this, since I'm very conservative about propulsion systems and wouldn't want to be a beta (or alpha) tester.

I don't know what the efficiency implications of diesel electric are. You introduce losses into the system, but you gain the ability to run the propeller and the engine at their most efficient speeds independently. How much you gain from that I don't know.

Certainly diesel-electric would solve this problem -- one big generator and one small one driving generators driving electric motors and Bob's your uncle.
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Old 14-03-2016, 15:06   #13
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Re: Different Size Engines

I pretty much always motorsailed my cat, therefore just used the lee side engine, which points the boat 10-15 degrees higher sailing to windward.
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Old 14-03-2016, 15:18   #14
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Re: Different Size Engines

IMO it's a viable option. A home boat builder was pricing a pair of Yamaha 25 HT outboards. I suggested a 60 HT and a 9.9 HT.


If I ever need to re-power, I'll look seriously at it myself.


The overall weight would be less, single engine cruising speed would be higher, with the possibility of very high sprint speeds if needed.


For motorsailing, the 9.9 would work well and economically.


To me, the thrust difference in manoeuvring wouldn't be an issue. I really don't look at where the throttle levers are anyway. I tend to be looking at what the boat's doing instead.


I guess the biggest difficulty would be in selling the boat later. Too many people would just write it off as being "too weird" without giving it a try.


The guy fitted 25's.
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Old 14-03-2016, 15:21   #15
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Re: Different Size Engines

Way too much work/thought for way too little gain.
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