Powering a 5.8m RIB.

[Teaspoon]

Hi all.

I am soon to be purchasing a new 5.8m RIB to replace my 4.2m toy.
I will use this boat mainly for spearfishing but also want to be able tow wake boarders.

My last 2 engines have been 2 strokes (Mercury 30 and a Yamaha 40). I have had no problems with either of them but the noise and the taste of burnt oil in the water is less than desirable. Fuel consumption has not been a huge issue for me as the majority of my diving is done less than 20km from the boat ramp. I do get out between once a week and ond once a fortnight on average.

I am not sure what engine to look at that will tick the boxes.

- Mid price range.
- Enough torque to pull a fat man out of the water on a wakeboard.
- Would like a quiet motor that is clean burning.
- Reliability is a must. I am not interested in cheap motors and would prefer to stick with the big 4.

The boat is rated 90-150. I am thinking a 90 2 stroke or a 115 4 stroke but am unsure. Any advice will be greatly appreciated.

Kind regards

Darren.

PS, here is my old boat:

[Char]

Get a four stroke .....the criteria your wanting .....a four stroke is the ticket

[whisper]

If you are going to do your own servicing check out the Mercury Mariner warranties

[Mollers]

Originally Posted by Char

.. four stroke is the ticket

Like a parking 'ticket', frustrating followed by annoying.

[Lee argyle]

Originally Posted by Mollers

Like a parking 'ticket', frustrating followed by annoying.

Same can be said about two strokes - I've been on 3 boats over the years where the large (150/200hp) 2 stroke has let go and the innards of the engine have come out with a bang - not huge hours on them either.
I have to say I'd be very reluctant to go back to one.

[jeffstevens763@g]

I had a 6m commercially built ribquest 2.4m beam with a 90 Suzuki 4st 40 knots best I got out of her 70lts fuel 2 large adults and a dog usuall kit 100 miles 36 Lt of fuel used.

Cheers

[Xk59D]

jeff, are you sure about those numbers?

i had a 5.6m avon adventure, fair to say nothing commercial about it and would be substantially lighter than a "commercial" ribquest and a 100hp mariner 4 stroke EFI and the same size fuel tank. Also not as deep a V perhaps too........

correctly propped (6k max) i never seen remotely close to 40 knots, mph yes. I must admit it was brilliant on fuel, but not that good!

[Pikey Dave]

Originally Posted by Xk59D

jeff, are you sure about those numbers?

I must admit, I'm struggling with those numbers meself😏

DF140a on a RC 545. 38kn tops. 1.2nm/l if I'm gentle. Did Dave Rose mix fairy dust in with his gelcoat😄


.....sh1t happens.......

[jeffstevens763@g]

Positive 45 mph on the gauge spoke to Dave rose at ribquest who confirmed that was the best they got, I will say conditions were spot on 13.5 x 21 SS prop fuel consumption with out dought round Jura on RR started with full tank and topped up from jerry cans. Wishing I hadn't sold her now but got a sib with a lean burn Suzuki now that's even better, up to now run it for four hours on 3 Lt of fuel so the new bigger engines will be even better than mine 2009 90. The hull makes a lot of difference the new super sport ribquest is even better wider body more room to walk round.

[Xk59D]

a friend of mine has that same engine (2010 model) on a northcraft 6m and it will just shade 40mph with a downhill wind, it has 2 single seats/70l tank so a VERY light boat, that is not a "commercial spec" boat either, he is circa 0.6 litres per mile, which is similar to my old avon 5.6.

the gauge has to be over reading for speed. was it the water pickup in the engine you were using per chance?

i found a report on a ribquest 5.8 with 115 yam 4 stroke and it was 38 knots on GPS.

something doesn't add up, you shouldn't have sold it if those were the numbers!

[Teaspoon]

Would both of these motors pull a 90kg adult out of the water on a wakeboard?
I am assuming the 2 stroke will do it with less effort but if the 100 4 stroke will do it too I am leaning towards that just for the quiet factor.

[jonp]

I've just sold a 5m Haines signature with the latest 90 Suzuki 4stroke and while I really like that engine I doubt you will only see 40 knots towing it behind the car. With a 13.5x 17 pitch stainless prop I could only achieve 29kts.

The Suzuki's are great motors and with the weather we get most of the time a 90 may be enough pitched right as 20 knots is about all you will be able to achieve due to the weather.

I'm originally from the UK where I spent most of my life on boats commercially and recreational. After moving out here I soon found the conditions to be way different from back home (much rougher often big waves on very big ground swells as you would already know).

Out here we tend to power for the max hp so we can keep the power when doing those long trips home where you get bogged down among the swells. Smaller pitched props will help get that immediate power when you need it but you will often lose that top end speed when using them.

My currant rib has a 13x13.5 pitch prop to work bar crossings where up in QLD the same outfits are fitted with 13.5x17.

Maybe a Yam 100 or 115 would do the job. Are you after new or secondhand?

Suzuki and Yamaha have performance tests with many different Aus boats giving fuel and economy throughout the full rpm range.

Jon

[Phil M]

Who are your nearest dealers? All engines have their followers and loathers but are most new engines are pretty good. If you are looking at 115hp, the merc 4 stroke is the lightest at the moment. I have one and have been impressed. I make about 36kts in my 5.3m zodiac (heavy boat with 2.3m beam and weird 3 hull/chine design). I see 1.1 to 1.3 nm/litre when averaging 20ish kts.

Phil M

[jonp]

Over here Yamaha and Suzuki are top sellers. Our state alone has over 250,000 registered (licenced) boat owners. There was a short time when some of the modern 2 strokes were popular but soon lost their credebility especially within the commercial sector (police, maritime, fisheries, water taxis, oyster farmers etc etc). Do your home work on those if you get an offer you think good, they don't have the same reputation they get in the UK.

There are a few places in Sydney that sell ex government engines, mostly about 3 years old. These engine tend to of been serviced every 50hrs (I think Buxton marine was one).

When people talk about one engine being a few kg lighter than another I personally don't think this makes hardly any difference at all especially on a 5m+ rib. Nobody seems to consider we are different weight, you might have someone 50kg heavier than someone else. An extra 2 bags of ice in the esky or a couple of dive tanks in the back. I often have a 100lt tank at the back of mine for keeping live bait or collecting fish (under permit) for a south coast equarium.

[Xk59D]

The weight does matter within reason as said.

The boat may not have a lot of freeboard or already stern heavy etc so anything you can get off the transom weight wise will help.

However, if it is a few kilos, and it is still under rated specs then should be okay, it completely is down to the actual RIB though.

[jeffstevens763@g]

Originally Posted by Xk59D

a friend of mine has that same engine (2010 model) on a northcraft 6m and it will just shade 40mph with a downhill wind, it has 2 single seats/70l tank so a VERY light boat, that is not a "commercial spec" boat either, he is circa 0.6 litres per mile, which is similar to my old avon 5.6.

the gauge has to be over reading for speed. was it the water pickup in the engine you were using per chance?

i found a report on a ribquest 5.8 with 115 yam 4 stroke and it was 38 knots on GPS.

something doesn't add up, you shouldn't have sold it if those were the numbers!

yea water pick up/ pneumo gauge dont know how accurate suzuki guages are but we have pneumo's for reading diver depth which are + or- 0.1% i am only going by that certantly seemed that speed passing fixed objects if you give 10% discrepency thats 40.5 mph as they do with vehicle speedo's, i spent a lot of time setting it up balance,weight distribution,polished hull, proping & trim easy kept up with everyone at easedale RR with bigger boats & hp, i do know some with 150hp burnt nearly twice the fuel than me.fuel wise i never ran at WOT for long no point for me i want to see the area, on long distance i would keep it at 20-25 mph lot better fuel economy that i do know going round jura [conditions perfect] gps is not accurate all the time i have just spent 3 days looking for land based structures gps is 150m out on fixed coordinates at the minuet possibly due to the current climate.