1 or 2 engines coastal waters?

dragoon said:
Increase the complexity, and increase the points of failure.
Click to expand...

I guess I'm not rationalizing complexity in the same way, they are two independent engines and each one is no more complex than the one in the single engine boat and therefore no more or less likely to fail. Yes there are two engines so two "failure points" but you have redundancy and therefore as you said overall reliability would be better.

As per Hugin's post I have been considering this in the circumstance of being at sea when a failure occurs not whether I would leave port in a boat with a failed engine, which I don't really see anyone doing out of choice. I considered the reliability as more of an at sea situation in which I would prefer redundancy (either twin engines or a "get you home solution")

I agree you obviously have 2 engines so of course you could have 2 failures that could mean the boat is out of action longer, assuming they don't fail at the same time. We can probably all agree in general diesel engines are very reliable and the weakness is likely to be fuel related issue or poor maintenance which is more under the owners control anyway.
 
I would guess that if fishing boats have a problem it is most likely fuel related, or caused by fouling of their propellor or rudder. Two engines woudn't help them much in either situation.
 
alancollins said:
I would guess that if fishing boats have a problem it is most likely fuel related, or caused by fouling of their propellor or rudder. Two engines woudn't help them much in either situation.
Click to expand...

With separate and independent fuel tanks it might, if the problem is fuel related. I don't think fishing boats are exempt from other types of mechanical failure either.
 
Richard.C said:
I guess I'm not rationalizing complexity in the same way, they are two independent engines and each one is no more complex than the one in the single engine boat and therefore no more or less likely to fail. Yes there are two engines so two "failure points" but you have redundancy and therefore as you said overall reliability would be better.

As per Hugin's post I have been considering this in the circumstance of being at sea when a failure occurs not whether I would leave port in a boat with a failed engine, which I don't really see anyone doing out of choice. I considered the reliability as more of an at sea situation in which I would prefer redundancy (either twin engines or a "get you home solution")

I agree you obviously have 2 engines so of course you could have 2 failures that could mean the boat is out of action longer, assuming they don't fail at the same time. We can probably all agree in general diesel engines are very reliable and the weakness is likely to be fuel related issue or poor maintenance which is more under the owners control anyway.
Click to expand...


I don't think we're saying anything very different. Here's some maths though (which I think is correct, but it's been a long while since I looked at this topic);

If the reliability of each engine is 90% during a given year, then the chances of failure are (1 - 0.9) = 10% for each engine. Then;

- the chance that at least 1 is working at any time is (0.9 + 0.9) - (0.9 * 0.9) = 0.99 (99%) ( note the reliability of the boat, is now greater than the reliability of a single engine)
- the chance that 1 fails during this time is (1-0.9) + (1-0.9) - (1-0.9) x (1-0.9) = 0.19 (19%) (see how the chance of failure of 1 of the 2 engines, is much greater than the chance of a single engine alone)

So increasing engines, increases reliability of the overall boat (assuming an engine fails independently of the other engine - e.g. 1 isn't required to work the steering, or they don't both drive the same prop).
Increasing engines also increases the likelyhood of a partial failure (single engine failing in the dual engine case).

If we looked at extreme numbers, we'd find some interesting stuff - for example, with the same reliability levels, if our boat had 1,000 engines, then at any one time 10 of them would statistically be out of order! I wouldn't be surprised if I looked across 500 boats (each with 2 engines), that there are more than 10 boats out of action !

Cheers
 
dragoon said:
I don't think we're saying anything very different. Here's some maths though (which I think is correct, but it's been a long while since I looked at this topic);

If the reliability of each engine is 90% during a given year, then the chances of failure are (1 - 0.9) = 10% for each engine. Then;

- the chance that at least 1 is working at any time is (0.9 + 0.9) - (0.9 * 0.9) = 0.99 (99%) ( note the reliability of the boat, is now greater than the reliability of a single engine)
- the chance that 1 fails during this time is (1-0.9) + (1-0.9) - (1-0.9) x (1-0.9) = 0.19 (19%) (see how the chance of failure of 1 of the 2 engines, is much greater than the chance of a single engine alone)

So increasing engines, increases reliability of the overall boat (assuming an engine fails independently of the other engine - e.g. 1 isn't required to work the steering, or they don't both drive the same prop).
Increasing engines also increases the likelyhood of a partial failure (single engine failing in the dual engine case).

If we looked at extreme numbers, we'd find some interesting stuff - for example, with the same reliability levels, if our boat had 1,000 engines, then at any one time 10 of them would statistically be out of order! I wouldn't be surprised if I looked across 500 boats (each with 2 engines), that there are more than 10 boats out of action !

Cheers
Click to expand...

Oh no, you had to bring math into it!!!!!

Like the look of your avatar by the way, can't quite tell but is that a Storebro?
 
Richard.C said:
Oh no, you had to bring math into it!!!!!

Like the look of your avatar by the way, can't quite tell but is that a Storebro?
Click to expand...


Indeed, sir, and thank you. A 1980 34' Baltic - the picture is from 2002 I think - the first season after we bought her.

Cheers.
 
So getting back to the thread a 30ft boat with a single kad 44 edc compressor is strong enough to go into coastal waters.

jon
 
Thank you BruceK - I am calculating the boat is 4 ton and a kad44 engine is 260hp at the crank and 248hp at the propeller shaft.
so without getting too technical it should plane.

thanks again
jon
 
Right - I have found some information which may be of interest for other forum users.
The boat I am looking at is a single diesel as I explained earlier but on the CE plate that is on the boat which states how many persons and how many persons with luggage above this is the word category which a letter after it.
In my case the boat is a category B which is offshore which can handle up to wind force 8 and significant wave heights of 4mtrs.
Here is the definitions which may help others.

Definitions of Boat Design Categories.

‘A’ OCEAN: Designed for extended voyages where conditions may exceed wind force 8 (Beaufort scale) and significant wave heights of 4 m and above but excluding abnormal conditions, and vessels largely self-sufficient.

‘B’ OFFSHORE: Designed for offshore voyages where conditions up to, and including, wind force 8 and significant wave heights up to, and including, 4 m may be experienced.

‘C’ INSHORE: Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where conditions up to, and including, wind force 6 and significant wave heights up to, and including, 2 m may be experienced.

‘D’ SHELTERED WATERS: Designed for voyages on sheltered coastal waters, small bays, small lakes, rivers and canals when conditions up to, and including, wind force 4 and significant wave heights up to, and including, 0.3 m may be experienced, with occasional waves of 0.5 m maximum height, for example from passing vessels.

Jon
 
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