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Old 15 March 2003, 20:59   #11
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Twin's props

Er, yes and no.
If the engines were under propped at the outset then this could be the case. But, if correctly propped there will be a problem using just one engine.
Laymans' type of explanation to follow: Imagine you have one 50hp engine driving a boat. If you add another 50hp engine the load on the prop of the first engine will be less because the second engine is helping to push the boat along. If the load on the prop is less, then the engine will find it easier to turn and the engine revs will rise. It would over rev at full throttle. This is, of course, the situation with the second engine too. The solution is to use props of a greater pitch and this also allows a higher top speed to be attained at full throttle.
If this was not the case and, say, the single engine reached max rpm with a 15" prop. The second engine would be the same. How would the boat go faster with twins? Both engines would be turning at the same speed as the single installation and both props have the same pitch. ( Accepting there would be a slight gain because there would be less prop slip.)

OK?


If you had no problems balancing the engines, well and good. As someone who has run twins and tripples for a while, there is a freedom and simplicity in driving a big single. If you get out into waves which are more than a bit bumpy, you'll be working the throttles all the time. You may then change your mind.
JW.
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Old 16 March 2003, 02:22   #12
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Re: Twin's props

Quote:
Originally posted by jwalker If you had no problems balancing the engines, well and good. As someone who has run twins and tripples for a while, there is a freedom and simplicity in driving a big single. If you get out into waves which are more than a bit bumpy, you'll be working the throttles all the time. You may then change your mind.
JW.
I can see that in a heavy sea there'd be more work to keeping the throttles balanced, but ease of so doing must be partly down to the set-up and fine adjustment of the twin engine installation. Whilst I can accept that no two engines will be alike, it can't be beyond the wit of man to get the throttle cables etc. adjusted such that the engines are reasonably balanced with the levers aligned at say 60% -70% throttle opening.
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Old 16 March 2003, 03:59   #13
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Paul you are asuming that the throttles stay set once your on the move. Out on the water this just doesn't happen. To drive a rib at any speed you need to read the waves and constantly adjust the throttles, its fine with one, but two is a real pain espcially if they are stiff.

The debate over twins v singles could run on for ever, but for my money it has to be a single if only on the finanical aspects.

If you need to carry six divers with kit then you will need 6m plus to do it in comfort.

Pete
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Old 16 March 2003, 04:17   #14
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Ere, JW. It doesn't say on your profile what you do for a living! if that's coz your currently unemployed can I suggest you go into teaching!, that description of single/twin stuff was well cool, very articulate, you have a gift. even manos & CH might have understood that!
P.S. If I'm sounding sycophantic tell me to shut up
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Old 16 March 2003, 17:49   #15
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Jon - Ta, nice of you to say so.

Crazyhorse maybe, Manos...Mmmm


Have a look at the joke thread. Yeh, really.
JW.
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Old 16 March 2003, 17:55   #16
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Paul, as Pete7 said, it's more complicated than that. You'll learn.
JW.

Oh yeh, I meant to mention this before but I had brain fade. You'll get that too if you are out at sea for a while. Then twin throttles will become even more difficult.
If, out on the water, one engine fails, what are you going to do with it? If you leave it down it'll cause almighty drag, If you lift it up you need to steer a motor which will fall from side to side and it will be very heavy. That is, if your steering arrangement will allow you still to steer with one engine lifted. Often it doesn't.
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Old 17 March 2003, 04:54   #17
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I suppose if one lump fails whilst you're out at sea, you may be none too bothered about the drag from it - but more concerned about still having some power to get ashore.

Controlling double throttles in arduous conditions will, I admit, be more challenging than a single. That's all part of the fun isn't it?

Seriously thinking of a new or ex-demo boat now - been to see a few this weekend - some sellers views of condition appear to be at odds with mine - mind you my rose tinted specs are away at the opticians.

So will probably spec out with a Honda 130 or Suzuki 140 four stroke. Honda is a bit of a heavyweight though - nearly 40 kilos heavier than the Suzuki!
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