Best small rib for fifty knots plus

bosun · 25 July 2012, 17:17

Hi All
I currently have a humber 5m assault with a 90 yam on it,
I realise the shortcomings of this boat as it is fairly over powered and not really being used as intended.
In the corner of my garage is an identicle 90 yam two stroke.
So I need a secondhand rib around 5 to 6 metres in length with a nice deep v ,high mounted tubes and plenty of chines/spray rails on the bottom that I can bolt the yams to.
I also have a pair of manual 4 inch jack/set back plates if needed.

A possibility is to flog the yams and buy a merc XR2 or similar, but I like the idea of twin engines for 'get you home' ability.

Your thoughts please Gentlemen.......

And any tips on transom strengthening also welcome.

whisper · 25 July 2012, 17:42

I have a 4.7 Searider hull console and trailer for sale although it's only rated for a 70........

chewy · 25 July 2012, 20:33

I bet the SR4.7 wound't do 50kts with a 90hp on....

spartacus · 25 July 2012, 20:42

A Searider 6.0m will probably take a pair of 2-stroke 90s. Thing with twin set ups is more weight (twin batteries), seperate fuel supplies, more maintenance, etc. Here's one with twin 60s for reference. Avon Sea Rider 6.0m Boats for sale UK, Avon Used boat sales, Avon Inflatable Boats For Sale 6M commercial Avon searider - Apollo Duck

50 knots... no idea?

willk · 25 July 2012, 20:55

Interesting one. If you're prepared to throw caution to the wind, I think that you may get quite close to your target.

You need a hull and transom capable of dealing with the weight. The hull will need to be "fast" and lightly loaded. You'll have to balance the load/fuel out carefully. The jack plates aren't the maddest thing ever suggested.

Obviously a large single will be quicker. You are presumably aware that a near 50kt 6m RIB will only be "usable" on the flattest of water and will be as scary as fkk?

bosun · 26 July 2012, 15:07

Yes I know it'll only go that fast in flat water,I had a concorde race hull a few years ago that would touch fifty before trying to chine walk into the next dimension. It was best with a bittle of a ripple on the water to keep it unstuck.
However that could have been sorted if it had been set up professionally.
The 90 yams seem to have the best power to weight ratio around and I was thinking that the jack plates would make setting up easier.
Has anybody ever cut and shut a phantom,ring or similar powerboat into a rib ?
Is it do-able ??
I know a guy who is a hot shot laminator.

Pikey Dave · 26 July 2012, 17:47

Quote:

Originally Posted by spartacus

A Searider 6.0m will probably take a pair of 2-stroke 90s. Thing with twin set ups is more weight (twin batteries), seperate fuel supplies, more maintenance, etc. Here's one with twin 60s for reference. Avon Sea Rider 6.0m Boats for sale UK, Avon Used boat sales, Avon Inflatable Boats For Sale 6M commercial Avon searider - Apollo Duck

50 knots... no idea?

6.4m with twin 115etecs WOT 5750rpm 40kn if I'm lucky with the wind behind me going downhill. If I stripped out all the toys, took the "A" frame down & ran low fuel I might get another 5kn, but where's the fun in that

If I went up a pitch in prop & kept the weight down I might touch 50 in perfect conditions. It's always going to be a compromise between speed & allround balanced, well behaved boat, unless you just want the out & out speed.

geir · 26 July 2012, 18:05

This guy claims he can do 48 knots+ in a Hydrosport 636 with one Suzuki 140, you loose some power in a twin mount, so it might be comparable to twin 90? This is also the old hull design, the new might be faster? The designer is on the forum sometimes...

Roughneck Hydrosport 636 - YouTube

spartacus · 26 July 2012, 18:10

Quote:

Originally Posted by Pikey Dave

6.4m with twin 115etecs WOT 5750rpm 40kn if I'm lucky with the wind behind me going downhill. If I stripped out all the toys, took the "A" frame down & ran low fuel I might get another 5kn, but where's the fun in that

If I went up a pitch in prop & kept the weight down I might touch 50 in perfect conditions. It's always going to be a compromise between speed & allround balanced, well behaved boat, unless you just want the out & out speed.

Fine for the Med, but sea conditions constantly change so the practicalities of 50 knots are restricted to a few flat calm days. Give my a rough weather boat any day.

Pikey Dave · 26 July 2012, 18:30

Quote:

Originally Posted by spartacus

Fine for the Med, but sea conditions constantly change so the practicalities of 50 knots are restricted to a few flat calm days. Give my a rough weather boat any day.

Absolutely

I'd much rather feel comfortable at 20kn in the lumps than cacking my pants at 50 on a flat calm sea (let's face it how many times has that happened this year

) However the OP wanted a small 50kn RIB, I doubt that such a beast exists outside a specialist racing setup. A "bog standard" production RIB would have to be overpowered to get to that kind of speed & would probably chine walk like buggery, clean undies anyone

My last RIB was a RC585 with a 175 FICHT on the back. That topped out at 47kn and was "interesting"

mick · 26 July 2012, 19:37

Why the need for speed I rather be safe and comfortable

kerny · 26 July 2012, 19:39

Quote:

Originally Posted by mick

Why the need for speed I rather be safe and comfortable

chewy · 26 July 2012, 20:38

Stick the other 90 on the back of the Assault, that should have the desired affect....

spartacus · 26 July 2012, 20:56

Quote:

Originally Posted by chewy

Stick the other 90 on the back of the Assault, that should have the desired affect....

Cheers chewy... I've just sprayed my tea up the wall!

chewy · 26 July 2012, 21:01

Quote:

Originally Posted by spartacus

Cheers chewy... I've just sprayed my tea up the wall!

Its ok, I aim to please

9D280 · 27 July 2012, 10:14

Quote:

Originally Posted by chewy

Stick the other 90 on the back of the Assault, that should have the desired affect....

I thought it, you said it.....

Quote:

Originally Posted by spartacus

Thing with twin set ups is more weight (twin batteries), seperate fuel supplies, more maintenance, etc.

It's a 2- stroke 90. Self Sustaining. One battery will start them both. I bet with no batteries he could take the lids off & pull start them both too...... Remember it's speed he's after, not total failsafe.

Quote:

Originally Posted by geir

you loose some power in a twin mount,

Search for my previous notes on these matters. I'm fed up typing the same thing.....
From memory 90 is about the top end of that generic gearbox size, so I guess if you replace with a 140+ the drag argument will likely reduce due to the much bigger 'box on the bigger engine, although at 50 knots the drag will be significant. Maths needs to be done!

Downhilldai · 27 July 2012, 11:10

90 Yam has a 4.25" gearcase. V6 motors have a 4.75" gearcase, so the extra drag of 2 Yam 90s compared with a single V6 200hp motor will be significant.

In terms of power:weight, a single 200 will outperform twin 90s.