speed

[mark mckee]

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

[Locozodiac]

Originally Posted by mark mckee

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

[Polwart]

Originally Posted by Locozodiac

At least 60 Km/Hr

that sounds a bit ambitious to me.

[martini]

Originally Posted by Polwart

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?

[Merrie Thames]

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.

[Locozodiac]

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

[prairie tuber]

With minimal load, trimmed out properly, maybe 45-50 km/h

[Polwart]

Originally Posted by Locozodiac

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!

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.

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.

[Bigmuz7]

Originally Posted by Polwart

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

[C-NUMB]

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 ?