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Old 09 August 2008, 13:36   #21
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I had discounted twins for all the normal reasons.
Interesting, I bought mine for all the normal reasons. Have I been had?
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Old 09 August 2008, 18:03   #22
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Interesting, I bought mine for all the normal reasons. Have I been had?
The only advantage of having two engines is having a backup engine, say you have twin 90's on one boat, and a single 180 on another identical hull, the latter will have better top end performance due to less hydrodynamic drag induced by the skeg, and as a result better fuel economy.

You really need to weigh up which suits you best - if you're operating a commercial dive boat which is used daily and quite far off shore, I think the cost/benefit of having that backup over the performance and economy would sway me towards having twins. You could always lift one of the engines when it isn't needed after all.
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Old 09 August 2008, 18:08   #23
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If the startup budget was any way flexible, a bigger lump could be run at much lower revs for the same speeds and keep the fuel bill down on an ongoing basis.
Not necessarily, the last thousand revs or so (where the gas really gets guzzled) only adds a couple of knots onto the speed performance so whilst you might be a bit inconvenienced through trimming and giving the boat some time to get it planing, not to mention shifting the weight, I think you'd have roughly the same, or better emissions and fuel economy with a smaller engine. You'd have a slower boat, mind.
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Old 10 August 2008, 05:59   #24
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I'm with you on most of that Hangman, but my previous experience on dive RIBS suggests that there are three critical engine power requirements:

1: The ability to get the hull quickly on the plane (with a huge pile of assorted dive gear and exhausted divers behind you in the stern well, those last trying to have a smoke in the sheltered space beside the petrol tank)
2: To get the boat (as quickly as possible) to just below the speed where divers and kit begin to fall out)
3: To approach the landfall at a speed high enough to convince other Diver Cox'n s that the driver of this RIB is indeed, Special.

Granted, the limiting factors here are generally environmental, rather than engine HP, but you get my drift....

Only kind of joking.
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Old 11 August 2008, 08:43   #25
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I'm with you on most of that Hangman, but my previous experience on dive RIBS suggests that there are three critical engine power requirements:

1: The ability to get the hull quickly on the plane (with a huge pile of assorted dive gear and exhausted divers behind you in the stern well, those last trying to have a smoke in the sheltered space beside the petrol tank)
2: To get the boat (as quickly as possible) to just below the speed where divers and kit begin to fall out)
3: To approach the landfall at a speed high enough to convince other Diver Cox'n s that the driver of this RIB is indeed, Special.

Granted, the limiting factors here are generally environmental, rather than engine HP, but you get my drift....

Only kind of joking.
"Kind of" is close enough, this is clearly pointing keen observational skills or a closer than stated knowledge of the subject, also need to add;

4: If it's not 3 figures it's not a proper engine.
5: To arrive first, with a boat of puking and battered divers is far, far more admirable than the embarassment of poling up 5 minutes later.
6: Anyone who complains about the ride clearly has no idea of how difficult it is to maintain almost full throttle in borderline conditions; force 3, sunshine, sea state slight - It's hell out there - dont you know!
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Old 11 August 2008, 08:49   #26
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Interesting, I bought mine for all the normal reasons. Have I been had?
No!

I discounted the idea for the following;
1. Weight (2x74hp is heavier than 1x150hp)
2. Service cost of consumables is double.
3. A dive rib does not need the built in redundancy that a lifeboat has due to it's operating conditions.
4. Power to weight and effective horsepower due to apendage drag are all better with a bigger single.

If we can afford it an auxilliary would be good.
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Old 11 August 2008, 15:55   #27
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or a closer than stated knowledge of the subject,
OK OK, I've been a Diver since 1990! There, I admit it. But I'm on the way to making a full recovery, one day at a time...
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Old 11 August 2008, 18:16   #28
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No!

I discounted the idea for the following;
1. Weight (2x74hp is heavier than 1x150hp)
2. Service cost of consumables is double.
3. A dive rib does not need the built in redundancy that a lifeboat has due to it's operating conditions.
4. Power to weight and effective horsepower due to apendage drag are all better with a bigger single.

If we can afford it an auxilliary would be good.
Very good points with 1 & 2, I hadn't thought about those, but my question regarding redundancy remains valid in my opinion. Do you operate in waters which are quiet, or shallow enough to anchor if your engine decides to die or a throttle cable comes off in the engine casing (or the battery has had enough and stops working, we've had that one before!). Should you anchor, do you have another boat with a crew which can be mustered for towing you back in?

What size auxiliary are you considering? If you have strong currents as we do, a small one with a full boat will be as effective as, excuse the saying, p*ss*ng on a housefire.

It depends on how often this boat is being used, and the circumstances it's being used in. How far out do you usually go? Are you commercial or a club? What's the worst weather conditions you'll usually go out in (not forgetting coming back in )

I think those questions are the ones which should determine your engine choice, as two engines is better than one is better in some circumstances, although I'll agree one engine is far more practical.
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Old 12 August 2008, 10:46   #29
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OK OK, I've been a Diver since 1990! There, I admit it. But I'm on the way to making a full recovery, one day at a time...
It's ok, you are among friends - just take it one day at a time!

Hangman,

Twins would be my choice, however to truly make a redundant system duplication would need take the form of 2 totally independant (but still connectable system) i.e. 2 batteries twin tanks etc etc. And counter rotating lower units

However, our budget doesnt stretch that far and a big main plus aux run off an aux tank is a better compromise. In terms of conditions divers tend to dive in good conditions in the event of engine failure we have 2 anchors if required at present. If it's not a lee shore and the tide is taking us toward a rescuing facility I might not choose to anchor up and wait. In terms of power an aux is only ever going to be used in displacement mode, covering of distance being second to survivability/endurance. Suffice to say we do not have an aux at present! It's an aspirational goal!
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