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Old 29 June 2007, 10:41   #1
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Re-engining an Atlantic 21?

We are considering re engining an Atlantic 21 and would welcome any advice form someone who has researched or done this before.

The big question is will we get away with a pair of twin (new) 4 stroke Mariner 60s?

Now obviously the RNLI used to run with twin evinrude 70s but have since moved to bigger engines. It seams they used to get 3 hrs at flat out (29 knots) on both tanks (65l each). However they are looking at a slightly different enviroinment from us.

We would be using the boat more for slow speed stuff (with some planning) but always within the Solent and are not therfore worried about the offshore capabilities of the boat, however dont want to spend 8K and find the boat suits our purposes but then find when we come to sell at some point in the future that no none wants it as they think the engines are to small.

I belive the boat weighs about 1 tonne without the donkey on the back. I believe the Mariners will do about 13-15l/hr each at full chat. They weigh 112kg each. I would be happy with the 29knots the RNLI got but not much less as a top speed.

If anyone can confirm the above figures that would be useful, also anyone with any experince of these boats with smaller engines on, your thoughts would be appreciated.
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Old 29 June 2007, 16:49   #2
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The RNLI's Atlantic 21s are still running with 70HP Evinrudes. The slightly larger Atlantic 75s I believe started off with Evinrude 70s as well, but are gradually switching to Yamaha 2 stroke 75s. It is only the bigger Atlantic 85 that runs 115HP Yamaha 4 strokes.

None of which answers your question . Personally, if I had the money an Atlantic 21 with twin 60s would suit me nicely (more so than with a single big outboard), but other people's preferences may well differ.

I'll keep an eye out for it if you decide to sell in the future and I win the lottery in the meantime

Cheers

Chris
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Old 29 June 2007, 17:42   #3
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Would that come up on the plane properly with twin 4stroke 60s? Were the last 70s the RNLI used on them 4 or 2stroke? I can only remember them (way back in the mists of time) with 2 stroke 70's which are considerably lighter.

Personally I wouldn't buy one with twin 60 4/s unless I was going to replace the engines.
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Old 29 June 2007, 18:01   #4
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All the RNLI's RIBs and SIBs have two stroke outboards, the only exception is the Atlantic 85. There are not many of these in service yet - about half a dozen or so.

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Old 29 June 2007, 18:14   #5
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In my mind that'd answer the question then Chris.
Loads more weight and less power from 4strokes on a boat that's a bit underpowered with twin 70 2/s would make for a difficult boat to sell unless a yacht club wanted it for a safety boat.
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Old 29 June 2007, 18:28   #6
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Doug, you may just get away with it but thats about it, 2x 70 2 strokes would put out around 105-110bhp where as 2x 60 4 strokes would put out around 90 bhp. Your not going to get your 29 knots or even near, my 18 ft rib has twin 60 mariner 2 strokes and i get around 35 knots on wot, the Atlantic 21 is much bigger and considerably heavier and its 4 strokes which are heavier too.
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Old 29 June 2007, 19:01   #7
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way back in the late 80's early 90's I had the pleasure of driving many Atlantic 21's in the Standby industry... they were fitted out as the RNLI boats, 2 X Evinrude 70's. This configuration was for reliability as both engines ran from separate fuel tanks & you can manouvere a twin screwed boat with no steering if needed.
If you're using just in the Solent & don't require fast cruising speeds a similar system should surfice.... However in my experience I would look at engine's weight etc & maybe go for 2 X 90 opti's.
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Old 03 July 2007, 05:16   #8
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Thank you all for your thoughts.

Oogs
2 x opti is not really an option as theigh weigh 170 kg each

Tim
Any guess at top speed

Nos
the twin 70s that would have originally been fitted are not avaliable any more so options are 4/s or modern opti/direct injection. Weight difference is also not that great. The same engine in 2/s would have wieghed 100kg vs 112kg.

Is 25kg on the back of a 1 tonne boat going to really stop it getting on the plane? Not trying to argue, this is the sort of discussion/opinions I want to hear.

Dont however agree with your club safetyboat idea, it is a wholly inappropriate boat to put in the hands of a volunteer safetyboat driver at a club.

Chris
If we re engine her will be planning to keep for three yrs
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Old 03 July 2007, 06:24   #9
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Doug, i wouldn't like to say really? it may be close to 20 knots, there must be a formula somewhere though that can be used to estimate the top speed of a craft with power and weight of engines + weight and type of hull= ??? if you know what i am saying? (start a new thread on this topic and i bet somebody would have a solution) Why don't you put a nice 130 on the back, and have a 15 hp aux, that would give you 30+ knots and there are plenty of nice ones for sale at reasonable prices, such as:

http://www.boatsandoutboards.co.uk/view/F149033/



Or 2 of these if you can get them would be perfect http://www.boatsandoutboards.co.uk/view/PPM031/
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Old 03 July 2007, 09:44   #10
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Quote:
Originally Posted by Doug Stormforce View Post
...
We would be using the boat more for slow speed stuff (with some planning) but always within the Solent and are not therfore worried about the offshore capabilities of the boat, however dont want to spend 8K and find the boat suits our purposes but then find when we come to sell at some point in the future that no none wants it as they think the engines are to small.

...
If you aren't using 'offshore' capabilities, why go for twin engines? As someone posted earlier, go for a bigger single - more power for the same weight, only one lot of servicing fees etc etc, and an auxilliary.

If you are putting new engines on, I'm not sure what the new emission rules say for 2 stroke engines - but I think all the new RNLI boats (Atlantic 21 and the IB1) are now coming out with 4 stroke engines on them.

If you are doing a lot of slow 'pootling' around, 4 strokes are much better than 2 strokes anyway - as the 2 strokes seems to oil up very quickly at tickover/low revs.

D...
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