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Old 14-03-2016, 15:33   #16
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Re: Different Size Engines

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Originally Posted by tomfl View Post
Way too much work/thought for way too little gain.
On a boat the size of a Seawind, most likely without any need for much electrical power generation, that would definitely be true.


But for a 55 - 60' foot cat which would otherwise have twin 60 or 70 hp diesels, and which would have a third diesel engine on board to power a generator, the gain could be huge in terms of weight and efficiency.

More fuel efficiency also means less fuel carried, which adds even more benefit.
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Old 14-03-2016, 15:42   #17
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Re: Different Size Engines

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Originally Posted by Dockhead View Post
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.
The "have a generator anyway" takes you from 3 diesels down to 2 with the smaller one able to power house loads or the electric motor as needed and a small generator could be fit in the same location with the electric motor on most designs.

Even on a larger cat, I don't think there is much to gain. Say it's a pair of 50hp engines typically. The smaller one is probably going to have to be upwards of 25-30hp to maintain a 6-7kt cruise in calm conditions. You are still looking at a 30hp putting out 25hp is not going to be significantly more efficient than a 50hp putting out 25hp.

I see this setup all the time...on fishing boats. They have a big 200-350hp engine to get the boat up on plane and a small 5-10hp outboard for slow speed trolling. Of course, the fishermen do it more for the fine speed control that a small engine provides as the big engine wants to idle at upwards of 5kts. The little outboard can often be dialed in to 1kt or less.

A cat with a pair of diesels already provides the ability to dial power into what you need with better efficiency.
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Old 14-03-2016, 15:48   #18
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Re: Different Size Engines

Here's a hp/loa/speed data point to further the discussion:

My cat: 50ft, 30hp per side, 7 knots cruising on one engine. These are outboards in specially designed wells.

I could wring out more speed because I run out of RPMs well before I run out of ability to push the hull. I'm a little under propped.
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Old 14-03-2016, 15:57   #19
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Re: Different Size Engines

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Originally Posted by cruisersfarm View Post
Here's a hp/loa/speed data point to further the discussion:

My cat: 50ft, 30hp per side, 7 knots cruising on one engine. These are outboards in specially designed wells.

I could wring out more speed because I run out of RPMs well before I run out of ability to push the hull. I'm a little under propped.

What kind of cat do you own ? Would love to see some pics of your outboard well setup.


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Old 14-03-2016, 16:44   #20
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Re: Different Size Engines

on a large cat a better bet would be to mount an engine amidships in addition to the 2 engines in the hulls with a sillette catamaran drive or similar and have the 110/240v genset driven off the front end of the engine and cat drive off the back end,extra alternators could be added,and if the engine was the same as the 2 in the hulls then you would have great interchangeability of spares,a coarse pitched prop would allow motor sailing and geneset use at the same time in calmer conditions.
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Old 14-03-2016, 17:46   #21
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Re: Different Size Engines

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Originally Posted by valhalla360 View Post
The "have a generator anyway" takes you from 3 diesels down to 2 with the smaller one able to power house loads or the electric motor as needed and a small generator could be fit in the same location with the electric motor on most designs.

Even on a larger cat, I don't think there is much to gain. . . ..
????

These two paragraphs seem to be a total contradiction.

Two large diesels plus generator, total three

compared to

One large diesel one small diesel, total two


Is a pretty huge difference in weight on board, and on efficient loading of the machinery.
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Old 14-03-2016, 17:47   #22
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Re: Different Size Engines

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Originally Posted by valhalla360 View Post
The "have a generator anyway" takes you from 3 diesels down to 2 with the smaller one able to power house loads or the electric motor as needed and a small generator could be fit in the same location with the electric motor on most designs.

Even on a larger cat, I don't think there is much to gain. Say it's a pair of 50hp engines typically. The smaller one is probably going to have to be upwards of 25-30hp to maintain a 6-7kt cruise in calm conditions. .

I guess the point is to use the larger engine for purely motoring, the smaller one for motorsailing or manoeuvring.
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Old 14-03-2016, 17:52   #23
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Re: Different Size Engines

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Originally Posted by 44'cruisingcat View Post
I guess the point is to use the larger engine for purely motoring, the smaller one for motorsailing or manoeuvring.
Indeed. And add generating power to motorsailing and maneuvering.
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Old 14-03-2016, 18:13   #24
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Re: Different Size Engines

I am wondering how much fuel a person would actually save in a year by doing this?
What is the cost/benefit?
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Old 14-03-2016, 18:14   #25
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Re: Different Size Engines

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Originally Posted by Dockhead View Post
????

These two paragraphs seem to be a total contradiction.

Two large diesels plus generator, total three

compared to

One large diesel one small diesel, total two


Is a pretty huge difference in weight on board, and on efficient loading of the machinery.
Just to be clear, I doubt the weight savings would be all that much. Let's assume DC generators since they can easily be combined where AC gens can't. So you go from three engines (50, 50, 12) to two (80, 12). Plus the weight of the two electric engines. My guess is that the larger generator plus the electrics pretty closely matches the weight of the two diesel propulsion motors.

I also doubt you would get much efficiency gain overall. Transmission-diesel is just to efficient to expect to gain much out of the system.

all in all I just don't see it as a terribly beneficial design, but it would at least be a justification.
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Old 14-03-2016, 18:24   #26
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Re: Different Size Engines

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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..
This! A perfectly elegant analysis...your not going soft on us are you?
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Old 14-03-2016, 18:52   #27
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pirate Re: Different Size Engines

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Originally Posted by IdoraKeeper View Post
This! A perfectly elegant analysis...your not going soft on us are you?
What..?? The technical ignoramus bit..

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Old 14-03-2016, 18:58   #28
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Re: Different Size Engines

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Originally Posted by Stumble View Post
Just to be clear, I doubt the weight savings would be all that much. Let's assume DC generators since they can easily be combined where AC gens can't. So you go from three engines (50, 50, 12) to two (80, 12). Plus the weight of the two electric engines. My guess is that the larger generator plus the electrics pretty closely matches the weight of the two diesel propulsion motors.

I also doubt you would get much efficiency gain overall. Transmission-diesel is just to efficient to expect to gain much out of the system.

all in all I just don't see it as a terribly beneficial design, but it would at least be a justification.
You obviously did not read through the thread and are commenting on a configuration which was proposed and rejected.
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Old 14-03-2016, 19:00   #29
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Re: Different Size Engines

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Originally Posted by Liam Wald View Post
I am wondering how much fuel a person would actually save in a year by doing this?
What is the cost/benefit?
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.
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Old 15-03-2016, 07:37   #30
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Re: Different Size Engines

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Originally Posted by Dockhead View Post
????

These two paragraphs seem to be a total contradiction.

Two large diesels plus generator, total three

compared to

One large diesel one small diesel, total two


Is a pretty huge difference in weight on board, and on efficient loading of the machinery.
Two different lines of thought.
- Conventional drive in one hull and hybrid drive able to handle house loads in the other hull.
- Two conventional engines of different HP (maybe/maybe not separate generator)
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