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Old 26 October 2013, 14:04   #1
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Country: UK - England
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Twin or single?

Hello

Noob here looking for help please. I did a Powerboat 2 course recently on a 21' ex lifeboat rib, with twin 70hp 4 strokes. Awesome boat, great fun.

My question is: do I want twins or a bigger single engine? I like the idea of twins (like my cruiser) in case one conks out, but what about power? The ex-lifeboat did 30 knots and felt really safe. She dug in on hard turns at speed - I was gobsmacked. I have a limited budget, so am I better going for an old Humber with smaller twins, or a bigger single?

Many thanks
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Old 26 October 2013, 14:09   #2
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Here is an email that was sent to me, I'm sure some of the more experienced will throw a few other suggestions in.

Today's question. Should you have twin engines on your RIB?
Well, the answer is not as straightforward as

one engine = bad
two engines = good
It depends... on where you are going to use your RIB and what you are
going to use the RIB for.

Twin Engine Advantages...

Peace of mind that you can easily get home without outside assistance
if one engine fails. Plus increased manoeuvrability.

And the disadvantages...

More expensive, increased maintenance costs, twice the drag in the water
and increased fuel consumption (typically a twin engine setup will use approx
1.5 times more fuel than a single installation of the same Horse Power).

Is A Single Engine Plus Auxiliary The Answer?

If you are not going too far offshore on a regular basis, a good compromise
would be an auxiliary engine (a small 'get you home engine') that will
push the boat along at a few knots in an emergency.
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Old 26 October 2013, 14:18   #3
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Thank you Wibs

Good info. 1.5 times more fuel would make a difference. I would only be going a few miles offshore, so perhaps a 'get you home' auxiliary. I have a 15hp 4 stroke on the back of my tender (3.1 Zodiac rib), which would do a a spare quite nicely I reckon.

Mainly family use (kids 18 & 16) - picnics, fishing, exploring the East coast on days out. We would use our cruiser for longer voyages.

Any thoughts on the Humber 6m(ish) cheapies?
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Old 26 October 2013, 14:38   #4
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Nobby, firstly, Welcome to ribnet!

Before this degenerates into it's usual slagging match of guestimations, do a search for "twins" and filter for posts by me. (can throw "cube" intro the text search box as well if it helps)

I can't be bothered typing it all again, but suffice to say a lot of the numbers bandied about are for BIG (multiple hundred HP) gee gee territory. The"general rules" are a whole different kettle of fish at the "small" end of the scale....

If you are thinking in the 100-ish or below mark, then the speed is lower & the science works in the favour of twins by the square or so of the speed, and the smaller gearboxes win as drag is approx. proportional to the cube of the frontal area. (or put another way, twin 40s vs 80 are a whole different kettle of fish to twin 150s vs 300 )


Plenty more to read here, so do a search, but keep your mind open!

What approx size boat are you looking at?
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Old 26 October 2013, 14:49   #5
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Thanks 9D280.

I will have a look for your posts.

I really want something which will do 30 knots or so. My little jetrib (now sold) was mentally fast, but too small to be able to use the power in a lumpy sea. It was great fun in an estuary, but not in a swell. I think somewhere about 5/6/7 metres would be fun.

Would I get 30 knots out of a 5 or 6m rib with twin 50's or a single 125 for example?
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Old 26 October 2013, 18:04   #6
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My old Ribtec 535 does 30 knots max with (even older) twin 40's
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Old 26 October 2013, 19:06   #7
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Quote:
Originally Posted by Swain Marine View Post
My old Ribtec 535 does 30 knots max with (even older) twin 40's
Ah ideal comparison, my 535 with a carbed 90 2 stroke would just hit 40 knots.
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Old 27 October 2013, 00:51   #8
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Quote:
Originally Posted by Wibs View Post
Twin Engine Advantages...

Peace of mind that you can easily get home without outside assistance
if one engine fails. Plus increased manoeuvrability.

And the disadvantages...

More expensive, increased maintenance costs, twice the drag in the water
and increased fuel consumption (typically a twin engine setup will use approx
1.5 times more fuel than a single installation of the same Horse Power).
I used to run 2 identical 7m RIBs, 1 with twin Honda 90s and the other with a single Honda 225hp, so feel like I know the advantages and disadvantages pretty well.

The single had far better acceleration and about 10 knots extra at the top end, although this probably had more to do with the extra 45hp than having less drag.

In terms of fuel, there was very little in it. We ran both from Plymouth to Salcombe and back on a few occasions and they would always use about the same. The twins were actually more economical if just pottering around, as used to only run one at a time.

Initial purchase and maintenance costs were both substantially higher on the twin.

The twin was far more manoeuvrable, especially in a cross wind/tide.

We never had a mechanical failure due to a fault on the actual engines. The only failures we had were due to fuel contamination and electrical issues linked to the battery/isolators. Twins wouldn't help with the fuel contamination and having twin batteries/isolators would make a single almost as good as a twin on the electrics.

I always preferred driving the twin, but economics unfortunately meant that when the time came to change them we went for 2 single engined RIBs.
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Old 27 October 2013, 05:35   #9
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I think that the twin or single question all comes down to your cruising area.
If you are going on longer trips away from land then twins is the answer. But for say Solent cruising with lots of other boaters and good help to hand such as Seastart, RNLI and other boaters then the single engine is the answer.
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Old 28 October 2013, 15:59   #10
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Brilliant, thank you for your replies. This is exactly the info I was looking for.

Much respect, thank you
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