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Old 27 September 2015, 17:30   #1
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Advice on 5.5m RIB

My club has recently purchased a 5.5m Humber assault rib, unfortunately they chose to put a Suzuki 50hp on the back. I just want to clarify that a rib like this shouldn't be topping out at 6,500 rpm at full speed with good trim. Personally I believe the way to fix this is with more power, what about you guys?
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Old 27 September 2015, 17:35   #2
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You need to add at least 2" to the prop pitch for starters. What's its use? When you say "club" are we talking sailing "club" safety boat or diving "club"? If it's a safety boat on inland waters, it'll be fine, if it's for diving it's a lemon.
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Old 27 September 2015, 17:51   #3
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You need to add at least 2" to the prop pitch for starters. What's its use? When you say "club" are we talking sailing "club" safety boat or diving "club"? If it's a safety boat on inland waters, it'll be fine, if it's for diving it's a lemon.

It's a club safety boat but it's not inland waters, you can get 2-3m swells on a bad day
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Old 27 September 2015, 18:12   #4
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I'd say that 50HP on a rib expected to react quickly in 2-3m swells - so I'd guess coastal requirement - is underpowered. I'd want 90-100HP on the back,but it will depend on actual use /expectation.
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Old 27 September 2015, 18:19   #5
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Ollie,

As PD says you shouldn't be doing that many revs so probably want to up the prop pitch.

Why do you think the engine isn't big enough? There is a common mistake at sailing clubs of putting big engines on ribs that spend most of their life at tick over (burning fuel) and are often driven by relatively inexperienced crew (where too many horses is potentially bad). You need to keep in mind the job it is asked to do. Most club ribs would be fine if they could do 25knots lightly loaded. Clubs with big skiffs, fast cats or foiling moths might have different needs. If its your only (or biggest) boat then towing a dozen becalmed oppies back in against a strong tide might be asking a bit much for it too.

2m swell and 2m waves are very different problems. The assault hull doesn't win many fans on here, but its actually designed for the sort of job you are asking it to do (i) used in conditions where sailing dinghies is possible/enjoyable (ii) not jumping waves all day long (iii) using a small engine / fuel efficient design to keep the rest of the cost down.

If its predominantly a coaching, mark laying, type boat I'd say its fine for the job in most environments with most mainstream dinghies. If its primary purpose is a rescue boat then I think the question depends on a whole load of local circumstances (the tides/currents you deal with, the hazards you face, shipping you need to keep the fleet away from, how far from the launch point you sail, the number of rescue (and or coaching) boats in the fleet, the size of your sailing area, etc.) However it wouldn't be unusual to see that sort of arrangement at a council run training centre - which is a good indicator of its suitability rather than being someone's "grant funded toy".
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Old 27 September 2015, 18:21   #6
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I'd say that 50HP on a rib expected to react quickly in 2-3m swells - so I'd guess coastal requirement - is underpowered. I'd want 90-100HP on the back,but it will depend on actual use /expectation.
An assault is only rated for 90HP max. Whilst everyone on RIBnet "specs" always go for max HP its not necessarily the best choice for Sailing Clubs.
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