380 inflatable speed - is 20mph OK?

[ak-71]

Is that roughly right?
380 (12.5ft) and 20 HP 4-stroke getting 20mph.

 

[seasdes]

Re: 380 inflatable speed - is 20mph OK?

I have a JP Marine Patrol 3.8 with a 18 hp Tohatsu. The other day I was travelling 30 km/h (gps measurement) in the ocean (calm day) with plenty of throttle left.

 

[ronaldj]

Re: 380 inflatable speed - is 20mph OK?

Is that roughly right?
380 (12.5ft) and 20 HP 4-stroke getting 20mph.

No it is not
It depends on the weight of the boat
The load of the boat
The size engine the boat is rated for (having an engine that is too large causes more problems than an engine that is not large enough
The type of water

 

[ak-71]

Re: 380 inflatable speed - is 20mph OK?

No it is not
It depends on the weight of the boat
The load of the boat
The size engine the boat is rated for (having an engine that is too large causes more problems than an engine that is not large enough
The type of water

12.5 ft, SIB, rated to 25 HP, load - me (~100kg), ~ 80 kg of gaz and sand bags in the bow. Water never got really calm, but got calm enough to go full throttle, don't know if it's a big variable for SIBs.
I just want to know if it looks reasonable, or should I expect much more? 12.5 ft "heavy" SIB, 20HP 4-stroke => 20 MPH (32 kph)?

 

[Sea Rider]

Re: 380 inflatable speed - is 20mph OK?

Plainly possible, my ex 380 sib, well inflated, ballanced & trimmed with a 2 strokes Tohatsu 18 was doing 21 MPH with me alone on flat calm waters. Now it's doing 22 MPH on a 430 rib on same water conditions. GPS measured during a 1 mile spin at a flat rowing water course circuit.

Happy Boating

 

[ak-71]

Re: 380 inflatable speed - is 20mph OK?

Thanks, looks like mine is about right

 

[saabsaviorlee]

Re: 380 inflatable speed - is 20mph OK?

I had a myriad of issues with slow running and not getting on plane. Hull speed a blazing 6.2 mph haha. With my 12.5' sib, 1973 Merc 20hp, 2 men (500lbs) cooler, troller and battery, I'll get just under 20 mph. Still haven't made a solo run but I'd assume solo I could reach mid 20's. Also altitude and carbs has an effect. I lose 2-3 mph at 2k feet vs 1k. Get a tiny tach and make sure you're not missing rpms. With me and my buddy's hefty butts we have to rock a lower pitch prop to stay at max rpm band when fully loaded with fishing and camping gear.

 

[Sea Rider]

Re: 380 inflatable speed - is 20mph OK?

Get a tiny tach and make sure you're not missing rpms. With me and my buddy's hefty butts we have to rock a lower pitch prop to stay at max rpm band when fully loaded with fishing and camping gear.

That's the way to go when standard delivered prop is not doing it's homework and need to select a different prop pitch when boating fully loaded to have engine reving happily inside it's top wot rpm.

Happy Boating

 

[Marqui]

Re: 380 inflatable speed - is 20mph OK?

I have a Seamax 380 heavy duty (extra weight) and a 77 evinrude 25 hp and it tops out at 34 kmh.(21 mph) me alone in the boat. So I would say you're pretty much in the ball park. Prop is 9X9.

Marqui.

 

[ronaldj]

Re: 380 inflatable speed - is 20mph OK?

Another thing to consider is the difference in weight between a 2 stroke engine and a 4 stroke engine. The difference is huge. That will also effect performance and handling

 

[fbpooler]

Re: 380 inflatable speed - is 20mph OK?

I would bet that you are doing well at 20 MPH. Using a tach, you may find a prop which will allow a pitch change which could give you another MPH or so, but probably not much more.

I am one of those nuts who change prop pitch to find the ideal. In doing so, I find that much of the common perception of a few HP difference making a lot of difference in speed is wrong. Playing with a 14 ft fiberglass, I found 23 MPH with 15 HP and 26 MPH with 25 HP. Ten HP difference gave higher gas consumption for 3 MPG. It did however allow a heavier boat load to obtain the relative same top end speed. Sold the 25 HP and kept the 15.

Generally speaking, the more rigid fiberglass hull enables more speed than an inflatable running full tilt.

 

[ak-71]

Re: 380 inflatable speed - is 20mph OK?

Just an update - had calm water yesterday - it goes 23 MPH, so I guess I'm good. Thank you for the replies.
fbpooler , I went with 20 HP vs 15HP because in 4 stroke most have the same weight, I will have 2 adults and 2 kids in the boat at times and it really doesn't take much gas anyway