4 strokes on Atlantic 21

[Orkneydive]

New to the forum, currently have a Avon searider 5.4 with yam 90hp.

Looking at a Atlantic 21, it has twin 80hp 4 stroke yams on it.

Been told they will be too heavy if I have divers and kit, better with two strokes.

Any info appreciated on this

Cheers

[beamishken]

In what way are they saying too heavy? The rnli now run 4 strokes & they seem to manage okay & they tend to be fairly weighed down with kit. Granted the 4 strokes are heavier by about 120kg for the pair so I guess the boats carrying capacity is reduced by 120kg ish but the carrying capacity of those boats will be pretty high so unless you have lots of divers aboard I doubt you'll be near the max capacity. You'll also need far less fuel for each trip so that should compensate a bit. The 75/90 yam two strokes are one of the best engines ever built imho but I doubt the difference in weight would significantly harm your use of the boat. Redistributing kit would probably easily compensate for the extra weight of the engines in my opinion

[Orkneydive]

Hi, thanks for that, yeah makes sense indeed.

[Searider]

Originally Posted by beamishken

In what way are they saying too heavy? The rnli now run 4 strokes & they seem to manage okay & they tend to be fairly weighed down with kit. Granted the 4 strokes are heavier by about 120kg for the pair so I guess the boats carrying capacity is reduced by 120kg ish but the carrying capacity of those boats will be pretty high so unless you have lots of divers aboard I doubt you'll be near the max capacity. You'll also need far less fuel for each trip so that should compensate a bit. The 75/90 yam two strokes are one of the best engines ever built imho but I doubt the difference in weight would significantly harm your use of the boat. Redistributing kit would probably easily compensate for the extra weight of the engines in my opinion

But the RNLI don’t use Atlantic 21s any more. They are now called Atlantic 75s and 85s. I don’t know what the difference is but an ex RNLI Atlantic 21 I played with 25 years ago had ver6 little freeboard aft with twin 55hp Evinrude 2 strokes.

[Orkneydive]

Hi

Yes this is my concern is the combined weight of 4 strokes, then add 3 or 4 divers with kit.

My searider actually copes pretty well with three divers, but just no deck space at all

[beamishken]

Originally Posted by Orkneydive

Hi

Yes this is my concern is the combined weight of 4 strokes, then add 3 or 4 divers with kit.

My searider actually copes pretty well with three divers, but just no deck space at all

The larger boat will sink less with your crews weight than the sr so might not be as large an issue as you expect. Most larger ribs tend to struggle with freeboard with 20" transoms & many now have 25" transoms to compensate. When we dive the boat always ends up swiming with water off out kit anyway so a bilge pump is essential.

As long as the powerheads aren't sat too deep at rest it should be fine, you might get water slopping in if its a bit lumpy when stationary especially if your all in the stern getting dressed in but your going in the water anyway so getting your feet wet isnt realy a problem & once your over the side the weight is gone.
Check how the boat sits at rest with no load, if its okay I doubt you'll have a problem, lack of freeboard on a dive rib doesnt bother me but obviously swamped engines if they sit too low will be an issue

[Orkneydive]

Originally Posted by beamishken

The larger boat will sink less with your crews weight than the sr so might not be as large an issue as you expect. Most larger ribs tend to struggle with freeboard with 20" transoms & many now have 25" transoms to compensate. When we dive the boat always ends up swiming with water off out kit anyway so a bilge pump is essential.

As long as the powerheads aren't sat too deep at rest it should be fine, you might get water slopping in if its a bit lumpy when stationary especially if your all in the stern getting dressed in but your going in the water anyway so getting your feet wet isnt realy a problem & once your over the side the weight is gone.
Check how the boat sits at rest with no load, if its okay I doubt you'll have a problem, lack of freeboard on a dive rib doesnt bother me but obviously swamped engines if they sit too low will be an issue

Yeah that's a good point, my searider let's mumps of water in over transom but we spread out kit between bow and stern,