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Old 08 August 2010, 11:50   #1
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speed

HI guys ,I have a 4.7 metre seapro inflatable with a 30hp four stroke as I dont own a GPS yet I was wondering what sort of speed I should be getting,say with two adults on a calm sea.ThankyouMark Mckee
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Old 09 August 2010, 14:49   #2
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Originally Posted by mark mckee View Post
HI guys ,I have a 4.7 metre seapro inflatable with a 30hp four stroke as I dont own a GPS yet I was wondering what sort of speed I should be getting,say with two adults on a calm sea.ThankyouMark Mckee
At least 60 Km/Hr
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Old 09 August 2010, 15:21   #3
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At least 60 Km/Hr
that sounds a bit ambitious to me.
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Old 09 August 2010, 15:26   #4
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that sounds a bit ambitious to me.
Me too, I'm not a sib expert but would've thought 25 knots at the very most?
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Old 10 August 2010, 09:36   #5
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I would think 25 knots would be reasonable. I used to get 19 off an 18 hp Evinrude fitted to a 4.5m Avon Sportboat.
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Old 10 August 2010, 15:11   #6
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Some numbers

My 380 sib inflated with gauge to 3.5 PSI, keel included with 120 KG (sib & Tohatsu 18 HP, 2 strokes weight), with another 130 KG between 2 mates, 3 gallons of pure unleaded no alcohol 95 octane gasoline giving 250 Kg in total weight, with perfectly balanced sib/mates & engine with proper transom height and well trimmed on a 1 Km full throttle run with no wind and plain sea conditions marked 39.2 Km/Hr on the GPS. My max rpm at wot were 5500, 300 off from max which is 5800, so if going for a 1 pitch less definitely will pass 40 Km/Hr

A Tohatsu 30 HP, 2 strokes on a 370 Rib (150Kg) with same mates and gas, giving a total weight of 280 Kg tested same way, was 62.0 Km/Hr and the engine used had only 5 hours on it while undergoing break in period.

Both engines were factory re tuned as timing advance and butterfly max opening at wot were slightly off from ideal.

A 470 sib with a 4 T / 30 HP engine that weights no more than 150 Kg, (90 + 60) add 2 normal mates and that configuration well inflated and trimmed should give at least 55 Km/Hr, if not, engine is not fine tuned, horrible water conditions or maybe using crappy BP fuels.

1 Knot = 1.7 Km so on both tests:

380 (18Hp) 39,2 Km/Hr or 23.05 Knots
370 (40Hp) 62.0 Km/Hr or 36.47 Knots
Check pic, next time outting with a Tootsie 30 Hp and a 370 rib will keep data for those non believers.

In excelent conditions, these speeds are possible, a guy is doing 102 Km on a 4.00 Mt Cat Hull sib with a 2T/50 Totatsu

Happy Boating
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Old 10 August 2010, 15:37   #7
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With minimal load, trimmed out properly, maybe 45-50 km/h
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Old 10 August 2010, 16:07   #8
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with another 130 KG between 2 mates, 3 gallons of pure unleaded
your friends are small. There are people on here who weight that on their own!

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A 470 sib with a 4 T / 30 HP engine that weights no more than 150 Kg, (90 + 60)
firstly I think you are being optimistic with the weight a SeaPro 470 apparently weighs 125 Kg, and depending on the model a 30HP 4str could be nearer 80 kg. Combined with a couple of well fed northern irish lads you might have underestimated the weight of the boat by 100kg or more.

Quote:
that configuration well inflated and trimmed should give at least 55 Km/Hr
I think if you put 30 HP (with no extra weight) on your 3.8m boat you might get there, but the boat is nearly double the weight and a metre longer.
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Old 10 August 2010, 17:06   #9
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I think if you put 30 HP (with no extra weight) on your 3.8m boat you might get there, but the boat is nearly double the weight and a metre longer.
Indeed ... and even 40 Kmh on a SIB in any case, is a boat I'm not sure I'd want to be on if my engine snagged a pot rope, or hit an unexpected wash
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Old 10 August 2010, 17:27   #10
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I think there is a some difference between SIBs and RIBs regarding speed. This especially when loaded. SIBs have less efficient hull shape, as it flex much more. My Aerotec goes 26 knots when single +normal fuel and safetygear(optimal conditions). With 2 persons onbord, otherwise same, aprox 22 knots, a dramatic drop in speed. Last weekend did a 60 nm trip with 3 persons( but light ones) 50 l fuel + camping gears and food but still managed close to 20 knots. If that boat would have a hard hull, sure it would goes faster than as a SIB, despite increased weight.

Sibs tends to go rather easily 20-22 knots, but faster than that the drag increases. So my slightly pessimistic guess is 22-23 knots for the seapro inflatable with a 30hp+2 crew.
Good luck with sea trials.

One NM/Knot is 1.8520 KM, at least that was the figures I used in school ?
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