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Old 30 June 2003, 15:46   #1
Country: UK - Scotland
Join Date: Jun 2003
Posts: 6
Atlantic 21 Meets Single 2 Stroke Engine ??? Advice Please!!

Hi we are in the process of buying an Atlantic 21 from a reliable source and are happy with the rib itself. However we have hit a
stumbling block and require some advice and comments on the engine.

We have a 4 stroke Yamaha on our own smaller rib (its been great) but on taking further advice have been seriously tempted with a single two stroke engine with a longer shaft for the new 21. Does anyone know of or experienced a similar marriage. We spoke to Redbay boats who were very helpful and couldnt have given better feedback on 2 stroke over 4 stroke but they are running with 226HP (Twins) we are looking at a single and we know the boat was designed for twin engines as opposed to one.
Continuing on the theme of twins versus singles we had thought about a 135 or a 150, but spoke to Hullamatic the designers of the boat who reminded us the boat was built for twin engines and the 135 would be scarey to drive given it was designed for 120 tops (twins).

we are now back to square one - do we go two stroke / 4 stroke. If we go 2 stroke and it's looking more favourable do we go single 115, 135 or 150 or do we go twin. Any advice, feedback or personal experience would be very helpful to us. Thanks very much Jackie

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Old 30 June 2003, 16:13   #2
Country: UK - England
Town: West Wickham
Boat name: Aries IV
Make: Scorpion
Length: 8m +
Engine: Etec 250
MMSI: 235036477
Join Date: Dec 2002
Posts: 273
Hello Jackie,

You might want to have a look at a thread which I started recently - "One lump or Two?" - about 3 pages back on this forum.

I was considering a smaller boat at the time but I have since realised that I couldn't turn it into my drive, so I am now looking at 7.5mtr - 7.8mtr boats to keep in dry storage on the coast.

The fuel economy and performance for twins does seem to be significantly worse than for a single, based on other forum users' comments, and on some statistics that I found on the Yamaha site, so I have decided to opt for one for my new boat - although it will be a long time coming.

Good luck with your new boat!



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Old 30 June 2003, 16:21   #3
Richard B's Avatar
Country: UK - England
Town: Berkshire and Devon
Make: Lodestar
Length: under 3m
Engine: Mercury 5hp
Join Date: Jul 2002
Posts: 5,015
Hi Jackie, is the 21 rigged for twins or a single - ie, any steering gear, fuel feed(s), throttle cable(s), electrics, or are you starting with a "blank canvas"?
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Old 30 June 2003, 16:45   #4
Country: UK - Scotland
Join Date: Jun 2003
Posts: 6
Thanks Chris for that have just read the info you pointed us in the direction of.
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Old 30 June 2003, 16:47   #5
Country: UK - Scotland
Join Date: Jun 2003
Posts: 6
Hi Richard, basically we are starting with a blank canvas and will adapt the boat to the requirements for the engine we will choose.
Any info graetly appreciated. Thanks jackie
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Old 30 June 2003, 16:53   #6
John Kennett's Avatar
Country: UK - England
Town: Brighton
Length: 3m +
Join Date: May 2000
Posts: 6,678

I'm surprised that Halmatic reckon that an Atlantic 21 would be scary with a 135hp outboard. There are plenty of RIBs out there around the 6.5m mark with anything up to 150hp.

Unless you have overwhelming reasons to want a twin installation I would go for a single engine (cheaper, more efficient, less to maintain, less to go wrong).

As for 2-stroke vs 4-stroke there doesn't seem to be a definitive answer, but the new generation of 2-strokes do seem to be the most popular option.

There has been quite a lot of discussion about engines oer the last few months. Do a forum search on 2-stroke or HDPI for example to see what other people have written.

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Old 30 June 2003, 17:05   #7
Country: UK - Scotland
Town: Edinburgh
Boat name: D Hollins
Make: pacific 38
Length: 10m +
Engine: Twin 212 hp Diesel
Join Date: Dec 2001
Posts: 176
Jackie, One thing to watch is that if the boat was built for twins as it was, the transom height may be too high in the middle for a single engine. In which case you would need to cut the transom down a bit.

I found this problem the hard way when I bought a large commercial Avon which was designed for twin 90's.

Another call to Halmatic will soon tell you, or you could measure the boat's transom in the middle and compare it to your engine.

I think that 135 hp sounds quite tame for a Atlantic, but if the makers have told you..... Good luck.
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Old 01 July 2003, 06:33   #8
Country: UK - Channel Islands
Town: Jersey
Boat name: t/t
Make: Honda
Length: under 3m
Join Date: Oct 2002
Posts: 253
cutting transom

You will really have to go some to cut the transom down on an atlantic.

They only have about 3 inches of transom above the deck. The transom is open at each end for drainage aswell.

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