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

At least 60 Km/Hr

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?

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.

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

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

your friends are small. There are people on here who weight that on their own!

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.

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 :eek:

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 ?

My 4.7 Zodiac with 30Hp 2St gives 50km/hr odd with 5 gallons of fuel, 170kg odd for us two, and a load of fishing gear, heavy console, battery, seating etc. etc.

I've often wondered what kind of speed it would do without all the loading, but it can't be far away from 60km/hr.

speed
 

Thanks a lot for the feedback.I have borrowed a handheld GPS so hopefully will do a few trials this weekend.Thanks Again

mark

Some mis concept issues:

My 380 sib with 18 tootsie only weights 120 KG, tested on extreme nice water conditions, 2 mates weighted 120 Kg (60 each) 10 kilos for 3 gal gasoline making 250 Kg total The test was made in a rowing field 1.5 Km long at sea, the drivers were jockey sitting inside middle deck, seems speed is increased a bit over sitting on top tubes and also betters lower center of gravity. Sibbing alone at wot with same conditions get close to 40 Km/Hr and I'm not a feather champ.

Ribs are much faster than sibs, other issue when speeding in a sib or rib you will have at least 1/3 sib out of water, much less drag to move. Running at 30 or 40 Km/Hr over things might get you into trouble, so sib safe. Usually running at 3/4 throttle, that is 30 Km/Hr as don't work for a oil Company.

Best final speed is directly related to water conditions, wind, currents, sib weight, balance and proper engine tuneup & mount. My final bet would be around 55 Km/Hr in best opt conditions.

Happy Boating

Mark, test your configuration at wot, 470 tubes & keel must be well inflated and sib correctly balanced, test with you alone and with 1 more mate of normal weight and let us know you final speed obtained in these 2 conditions were as there is a speed bet on you.

Happy Sibbing