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Old 13 August 2009, 15:23   #11
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Malthouse.

You never know what crops up when you start talking. I have got into other businesses through even less comments than this exchange.
In reality, besides enjoying myself enormously (as I love talking about boats), I am learning from people's advice. And many of these valuable comments made on these forums by knowledgeable people kind enough to share their experiences are important in helping me to choose.
For example, I can now categorically say that am looking for a
7.5 or 8.0 or 8+ rib, one engine ( and auxiliary), cabin or canopied.
I am still negotiating with people re. used and new boats and am still waiting for info from some.
The things above, however, are pretty well fixed.
What is not fixed yet is the final decisions as to the make of the rib and a couple of other technical aspects which I am still exploring and thinking about, albeit things are now pretty clear in my mind.

I should like to get a boat for next week, but I have seen that people take time to send info, etc., and this has to be digested and thought about. Who knows, however, if the right boat crops up, I'll go for it at once.
And that, Malthouse, is the state of play at the moment. I am, as they say, working on it.

Thanks for replying to my thread.
Rupert.
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Old 14 August 2009, 08:55   #12
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The "more drag" argument is an interesting one. As you get to a bigger boat (& therefore more horsepower) the speed will generally increase, as will the size of the leg of your engine.

Drag through water at the sort of speeds most ribs will plane at goes up roughly as the cube of your speed. So, if you had, say a comparison of twin 25s vs a 50 on the back of an SR5.4 you'd see the square root of b*gger all in difference, as you aren't going to be travelling much above 21 knots so the drag is low. 2 metres and 200Hp more, you'll be at 40 odd knots & the drag numbers start to get significant.

You also need to look at the engines - e.g my Merc 60 - Early clamshell has a lower leg on a size par with my old Suz DT25. Yam 50s share their legs with most sizes up to 90, so it's a bit fatter, therefore more drag.....

Also the "it's heavier" argument is usually a 50/50. As can be seen from lots of posts here, as a general rule most O/B makers will use one lump for two or three different Horsepowers, derating it for the smaller engines. If you compare, say twin Mercs with Twin Hondas, you'll probably find that for a given overall HP one manufacturer will end up with lighter single, the other lighter twins, Especially when you take the Aux into account as well. (and it will change for a given overall HP as well.


Like Pol, I'm not saying one or other is best, just some more food for thought.....
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Old 14 August 2009, 09:21   #13
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There was an interesting test in a boat mag the other day - a Grandezza 28WA against a Nord Star 28 - very similar to the Botnia Targas.

The Grandezza had twin D3 160hp and the Nord Star has a single D6 330hp. They said the single engined boat was far better.

Top speed was 36kts v 31kts - fuel consumption was 34lph v 50lp at 26kts.

Not exactly like for like but the single engine boats always come out tops.

And remember - the World's biggest ships only have 1 engine!!!
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Old 14 August 2009, 09:37   #14
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it's generally cheaper and faster having a single engine but having two gives you so much more security at sea. As previous posts have mentioned this only works if all the systems ie fuel tanks, batteries are separated. Having two small engines instead of one large will probably make your servicing and part sourcing easier as more are sold and the engineers have more experience. Our eight metre in Dubai was so much better with twin 75hp 2 stroke Yamaha. When we switched over to a Honda 130hp four stroke was boring to drive. It was much quieter but I preferred it before.
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Old 14 August 2009, 11:02   #15
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This thread

is getting to be very interesting and informative.
Thanks folks.
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Old 14 August 2009, 11:14   #16
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is getting to be very interesting and informative.
Thanks folks.
It is. If you find a nice tidy solution for mounting an auxie on a single diesel outdrive rig - please share!
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Old 14 August 2009, 11:20   #17
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Thanks Willk,

Over to you, Codprawn
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Old 14 August 2009, 11:53   #18
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There was an interesting test ..... Not exactly like for like
And that is my point. Hull shapes? Props? Trim tabs? Who was driving? How much fuel were each carrying? What else wasn't quite the same?

Bottom line is you pays your money, you takes your choice. If you can get trial runs beforehand, that's a huge advantage. Take the chance, then decide.

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And remember - the World's biggest ships only have 1 engine!!!
Errr the QE2 had 9 oil burners driving twin screws.......
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Old 14 August 2009, 12:28   #19
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And that is my point. Hull shapes? Props? Trim tabs? Who was driving? How much fuel were each carrying? What else wasn't quite the same?

Bottom line is you pays your money, you takes your choice. If you can get trial runs beforehand, that's a huge advantage. Take the chance, then decide.



Errr the QE2 had 9 oil burners driving twin screws.......
Yes but the new super tankers and container ships have a single screw!!!

Some of the older massive ships had multiple screws though.

Here is the Emma Mærsk's stern end............
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Old 14 August 2009, 13:37   #20
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Depends what you are wanting to do with the actual boat, if its pleasure one engine, work, saftey or anything else I would have to say two!
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