300HP really ?

tom52

tom52

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I will admit my thoughts have been prompted by the Padstow tragedy but I do not wish to relate this post to that specific event.

I am agog at the power available as an outboard on a rib. Why are these engines so big ? Does the hull shape even when on the plane not impose some max speed irrespective of the engine power ?

What kind of speeds can be achieved by 300HP on a rib ?

Finally although not generally in favour of regulation and licenses for boating can it really be ok to let someone drive something as powerful as 300hp unlicensed or regulated in any way
 
tom52 said:
I will admit my thoughts have been prompted by the Padstow tragedy but I do not wish to relate this post to that specific event.

I am agog at the power available as an outboard on a rib. Why are these engines so big ? Does the hull shape even when on the plane not impose some max speed irrespective of the engine power ?

What kind of speeds can be achieved by 300HP on a rib ?

Finally although not generally in favour of regulation and licenses for boating can it really be ok to let someone drive something as powerful as 300hp unlicensed or regulated in any way
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300 is conservative for a rhib.
 
If you are using the rib for water sports then the power helps you get multiple skiers or bug toys up quickly.

We have a 6.5m rib with 225hp and its great fun, only tops out out at a shade over 40knts though.
 
I agree 300 seems excessive and I can't help thinking a pose, but what about the people in racing mobo's with far more than 300hp on tap ?

I don't think they are subject to any particularly stringent rules, I once had a racing mobo - the catamaran hull, 2 crew in tandem with crash helmets type - come and idle alongside as we were motoring off Ryde in a calm, after a bit of hearing trouble with engine noise and helmet they removed both, and the ' navigator ' in the back seat yelled " Excuse me, where are we, and where's East Lepe Buoy ? "

I was a bit taken aback and pointed out Ryde spire, the forts etc and the direction to E Lepe, at which they started up again and set off at Warp 9...
 
tom52 said:
I will admit my thoughts have been prompted by the Padstow tragedy but I do not wish to relate this post to that specific event.

I am agog at the power available as an outboard on a rib. Why are these engines so big ? Does the hull shape even when on the plane not impose some max speed irrespective of the engine power ?

What kind of speeds can be achieved by 300HP on a rib ?
Click to expand...

Why are these engines so big ? A. For power re water sports skiing etc) and B. Speed, the sheer thrill of it.

Does the hull shape even when on the plane not impose some max speed irrespective of the engine power ? Not really, because the boat hull is planing [explained here http://en.wikipedia.org/wiki/Planing_(boat) ] not displacement. Ultimately there is a top out speed at WOT relating to engine size.

What kind of speeds can be achieved by 300HP on a rib ? I did read a report of 70 knots, but I think that is exaggerated for that RIB (a Cobra 8 metre) in that configuration.
It depends on the hull shape, the weight (gross including fuel, gear and passengers aboard), propellor diameter and pitch and sea conditions. So flat calm, minimal fuel, two persons aboard, it could easily top 50mph.

That could also be 'tweeked' to quite a bit higher by adding trim tabs, type of propellor etc.


BTW more engines do not necessarily mean more speed but :


 
" Why are these engines so big ? A. For power re water sports skiing etc) and B. Speed, the sheer thrill of it ".

Nothing to do with performance, simply a case of ' mine's bigger than yours ' - a graphic on the engine cowling saying 1,000 would be the ideal I suppose. Sad.

When it gets to the several hundred horsepower level, surely an inboard engine with ' jet ' intake & thrust is better for safety & trim ?
 
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tom52 said:
I will admit my thoughts have been prompted by the Padstow tragedy but I do not wish to relate this post to that specific event.

I am agog at the power available as an outboard on a rib. Why are these engines so big ? Does the hull shape even when on the plane not impose some max speed irrespective of the engine power ?

What kind of speeds can be achieved by 300HP on a rib ?

Finally although not generally in favour of regulation and licenses for boating can it really be ok to let someone drive something as powerful as 300hp unlicensed or regulated in any way
Click to expand...

Don't know if you know, but the RYA scheme for motor is split "powerboat" and "motor cruising". The line is a little blurred but a powerboat is usually under 10m and single engine, and for teaching usually a RIB.

I teach motor cruising but not powerboating, so I was surprised to be contacted for a delivery and a week's instruction from a RIB owner.

Turns out it had two, inboard diesel 400hp engines ie 800hp.

Needless to say it was jolly quick. Topped out at about 65, but opportunities to do that speed are very limited.

What it did well was extended passages at c25-40 knots, eating miles with zero wake. It had radar and autopilot so was well equipped for passage making.

Ok I'm not that experienced in RiBs and I've only raced saily boats not power boats, even so I am an experienced motor boater. But this RIB demanded constant respect because of its power. Thankfully its new owner was bright enough to realise he needed to learn.

You do need to remember that a big RIB is to your tender what a family cruiser is to a laser. So in summary yes 300hp is a lot, but it isn't unprecedented or wrong in any way.
 
Seajet said:
" Why are these engines so big ? A. For power re water sports skiing etc) and B. Speed, the sheer thrill of it ".

Nothing to do with performance, simply a case of ' mine's bigger than yours ' - a graphic on the engine cowling saying 1,000 would be the ideal I suppose. Sad.

When it gets to the several hundred horsepower level, surely an inboard engine with ' jet ' intake & thrust is better for safety & trim ?
Click to expand...

If you want 'a mines bigger than yours' you get one of these-

 
tom52 said:
I will admit my thoughts have been prompted by the Padstow tragedy but I do not wish to relate this post to that specific event.

I am agog at the power available as an outboard on a rib. Why are these engines so big ? Does the hull shape even when on the plane not impose some max speed irrespective of the engine power ?

What kind of speeds can be achieved by 300HP on a rib ?

Finally although not generally in favour of regulation and licenses for boating can it really be ok to let someone drive something as powerful as 300hp unlicensed or regulated in any way
Click to expand...

The Dive RIB I've driven on occasion is 8m with 250HP - on flat water it tops out at about 44 knots.
 
Once at the fuelling pontoon I was talking to a RIBist while we were waiting for the pump attendant to come back from taking a whizz. It was a fairly megaRIB.

"What'll she do?" I asked.

"About 70 knots," he replied, "but only for 20 minutes, then she runs out of fuel."
 
st599 said:
The Dive RIB I've driven on occasion is 8m with 250HP - on flat water it tops out at about 44 knots.
Click to expand...

The safety boat I specced for our sailing club has a 50hp and I was able to run it to 35Kts - a brand new engine & prop probably helped ;)
 
In La Linea I met a couple cruising in a modified mobo racing hull. I had twin berths and a galley and toilet, all a bit low profile and sleek. They had two 300hp outboards on the back. They cruise at 40 knots. He told me the hull would take two 500 hp and do well over 50 knots, but, who needs it he said. Trips I was taking a day to cover, he was doing in an hour. Fuel was the problem. No, he was not filling in Gib. - too expensive for petrol. A quick flick across to Cueta and back, cheaper petrol there. Last time I saw them was in Ayamonte at the other end of the season.
 
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