Repowering 4.8m Ribcraft

[physdemon]

Hi,

Just wondering if anyone has any strong thoughts on what engine would suit my 4.8m ribcraft. Currently powered by a 75hp 1999 Mercury 2 st, but seems a little heavy on fuel compared to my old boat--5.8m tornado, powered by 115hp 2 st. Two obvious choices are the etec's and TLDI's. I use the boat mainly for diving with one coax and two divers plus kit. The 75hp pushes the boat along well, but seems to gulp the fuel off no matter how much I open the throttle--currently using 20-25mph as the baseline for cuising speed. Top speed is around 35mph. Over the last season I have been considering whether now is the time to repower, realising the best price for the carburettor engine, and opting for a fuel injection model, and thereby hopefully realising better fuel economy. Given my intended use, size of the boat, and current experiences of older v newer type engines, I'm looking for some advice on engine size and type from those more in the know. All advice/comments welcome from you seasonned ribbers.

[HUMBER P4VWL]

Your limit may be 75, but what's the weight limit?

Check, then check this!!! The opti will use just over half the fuel than a carbed 2s. We moved from 75 carb to 90 opti, worlds apart.

This below is a bargain.

http://www.rib.net/forum/f8/whats-it...lly-47288.html

[physdemon]

Hi,

thanks for the reply. Just had a quick look at Mercury's page re: the optimax 75: can't quite understand why the entire range has a weight of 170kg; seems a heavy engine to me. looked on line and the existing engine is weighing in around 138kg; so looking to repower with something around this weight or lighter. the newer 75's all look a shade above this tldi and etec type. Might then need to start looking for 60hp mariner 4 st or 60hp etec; the 50hp tldi might then be a bit light on cc's considering. Any thoughts from anyone, again, welcome.

[TonyC]

afaik, the RC 4.8 is rated to 60hp

[horacebat]

I've got a 60hp 2st Mariner on mine, but according to the owners manual that they kindly sent through earlier in the year the 4.8 maximum is 56kw / 75hp.

[HUMBER P4VWL]

The 75,90,115 and 125 hp are all the same block. A 1.5L 3cyl. Hence other engines that cover the 40-70 range are generally a smaller block, hence lighter. But that engine could be a steal.

[Searider]

My only experience of a 4.8 had a 50hp 4 stroke on it. Not enough power.

[Robbie Diesel]

Yamaha's new F70, only 117 kgs

[TonyC]

Originally Posted by Searider

My only experience of a 4.8 had a 50hp 4 stroke on it. Not enough power.

Your kidding right? The Club has 4 x 4.8's, all with Suzuki 50's on them and they are like shit off a shovel.

[longjohn]

You can buy a lot of fuel for the price of a new motor - so the payback is going to be a long time or maybe never!

[physdemon]

hi,

thanks for all the replies. Appreciate the point about cost of fuel versus initial outlay. however, I think the time has come to start looking in earnst for a suitable modern replacement--wanting to be kind to the sea/environment and all that. Soumds like I'm looking at either a 60 or 70hp 2/4st. I hadn't really considered YAM as a friend had mentioned all the parts were very pricey. that said, I'll do some digging on the internet and see which specification/model ticks all the boxes, if there is such a thing!

[Pikey Dave]

A 75 Etec will sit nicely on there. Sips fuel &oil, light, powerful & 3 year/300hr servicing, works out at less than £100/year.

[physdemon]

Any advantages of the 75 over the 60, apart from the obvious of a little more power, but that is somewhat set off against the just from 138 to 113kg??

[kerny]

I have a 60 Etec on my 5.4 Air-Craft and it's great on fuel and I average 31 knots, top speed 34 knots the difference between the two is the 75 is three cylinders as opposed to two and only £500 difference. Your call

[HUMBER P4VWL]

What's the weight difference captain?

[kerny]

Originally Posted by HUMBER P4VWL

What's the weight difference captain?

Etec 60 109kg Etec 75 145kg

[IndyR1]

Originally Posted by physdemon

Any advantages of the 75 over the 60, apart from the obvious of a little more power, but that is somewhat set off against the just from 138 to 113kg??

I have the Yamaha 60 2st on my 5m Bombard and it goes really well but only weighs about 106Kg 2003 model and 3Cyl. The few long runs I've done show it to be probably more economical than I expected at approx 0.6Lt at 25knots. I doubt you would realy notice any difference if you got a 70 as these add nearly 40kg of weight for the extra little power although the new Yamaha 70 is light too as already mentioned.

[9D280]

Couple of Jura fueled thoughts- if your boat is full of dive gear, is that weight difference going to be proportionally significantly different? If it is, Clamshells only weigh 85kg.....

[Downhilldai]

Originally Posted by IndyR1

I have the Yamaha 60 2st on my 5m Bombard and it goes really well but only weighs about 106Kg 2003 model and 3Cyl. The few long runs I've done show it to be probably more economical than I expected at approx 0.6Lt at 25knots. I doubt you would realy notice any difference if you got a 70 as these add nearly 40kg of weight for the extra little power although the new Yamaha 70 is light too as already mentioned.

Yam 60 & 70 2-strokes are the same motor. The 75 is bigger (& heavier), same as the 90.

[Pikey Dave]

Originally Posted by physdemon

Any advantages of the 75 over the 60, apart from the obvious of a little more power, but that is somewhat set off against the just from 138 to 113kg??

As already said, the 75 is a 3 cyl against the 2 of the 60. The 75 is well regarded & considered a "strong" engine pushing out well over its 75hp rating. The extra cylinder gives more torque than the modest 15hp increase suggests. AND it's a 2 stroke