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Old 14 November 2001, 13:40   #1
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Optimax & 4 Stroke Vs Diesel

Dear RIBsters,

I am the part owner and operator of Aqua Cab a fast water taxi in Falmouth bay. Not a RIB as yet.

This summer we used a Mercury Optimax, 200 HP outboard (Fitted by Rock Marine Services) on the taxi which we now have advertised with RIB Mag for £6,750.

We were very pleased with this engine, its reliability and fuel consumption. In three and abit months we clocked up over 4000 miles. The only problem we had was with getting a replacement part for the power trim from Sowester (Over 6 weeks) .

At the recent Southampton Boat Show I spoke to Yamaha, Mercury, Mariner and Evenrude with regard to re-engining the boat next year. We looked at Optimax and the new Four stroke and were hit with fuel consumption statements from all sides. Mercury were saying that there Optimax is more efficient than the four strokes, that it has extremely low emissions etc, Yamaha said the new four stroke was going to be considerably more efficient than their Optimaxs etc, etc. I came away from the show confused by the info I had been given.

We are now looking to re-engine the first boat and are looking at the possibility of acquiring a second boat, an 8.5m RIB to expand the fleet.

The reason I post this thread is two fold.

Firstly we are looking at using an Optimax again next year. Are the new large four strokes better fuel consumption han an Optimax. At an elevated cost the 4S needs to provide a mark able improved fuel consumption at up to mid rev range, rarely exceeding 4400 RPM. Cursing speed 22 – 28 knots. Before you all immediatly say, off course 4S is more efficent than 2S direct injection, the new Tohatsu 50HP is rumered to be more efficent than its 4S rivals. Will this also be the case for the larger engines.

Secondly, I am unsure as how to engine the new RIB (If we get one). Diesel (Quiet, better fuel cost, but expensive capital cost and takes up deck space) and a single Optimax (Cheap capital, Petrol only 32p per litre after tax back, easy to replace if mechanical failure but worse fuel consumption) Is a diesel inboard, with normal propeller or duo prop markedly more efficient than the new optimax engines?

I would be very great full to hear your opinions on these two matters and look forward to receiving your reply.

Regards,

Toby

Turn your speakers on and visit www.aquacab.co.uk and make sure you use us next time ur down this neck of the woods. www.aquacab.co.uk
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Old 15 November 2001, 03:29   #2
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Toby

Do you really need 200hp?

I know you run a "fast taxi" service, but how fast do you actually need to go?

Looking at the pictures of your boat, I would be surprised if the 4 cylinder Yamaha diesel (165hp) wouldn't get you up to 25 knots. "B4" the 10m cabin Ribtec built for Bernard Hayman a couple of years ago does a similar speed on that engine. The amount of torque produced is great for close quarters manoevering too.

It would be interesting to compare pricing for a 200hp Optimax with a 165hp Yamaha diesel (or even the new Mercruiser compact diesel perhaps?).

Even after claiming the tax back on petrol you'll get diesel cheaper. I'd be surprised if you need to pay more than 25p/litre for diesel, a saving of some 25% assuming equal consumption. How many litres of petrol do you use on average per hour?

I haven't got fuel consumption figures for any of the new breed outboards, but my get feeling is that you would do better with a diesel for this sort of application.

John
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Old 15 November 2001, 13:00   #3
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John,

Thanks for the reply.

Do I really need 200hp? OH yes. By the time you have loaded the boat up with 12 people and 150 litres of fuel and set out to cross the bay in a F6 you need that much power. It also means that we vary rarely have to push the engine harder than 75% of its maximum. This enables us to keep the fuel consumption down to 6-8 gallons per hour, as soon as you push the engine over that threshold your looking at 20 GPH, Expensive. Besides it’s bigger than mi mates, and goes faster to!

’’I know you run a "fast taxi" service, but how fast do you actually need to go?’’

20-25 knots. Except when I get some finer specimens of the female species onboard, then, with Top Gun/Danger Zone full blast on the stereo, she will comfortably jump up to 45+ knots.

Interesting thoughts on a diesel, very hard to convert hull for diesel due to bulkheads etc. I believe that the new compact engine, the Izuzu, is priced at about 8K with gbox and leg. But afraid I would need more power Scottie!

‘’Haven't got fuel consumption figures for any of the new breed outboards, but my get feeling is that you would do better with a diesel for this sort of application.’’

This is my gut feeling to, but diesels are more expensive to buy and install, take up deck space and don’t look as impressive as the monster Optimax. Plus the new breed of outboard engines are getting fantastically efficient. Are we not getting to the point that the difference is negligible (when your paying 30p per litre for petrol). Maybe Andy Clark would be a good man to ask.

Re: the thread about zapcating around Britain a while ago. Plans are afoot in Cornwall after enthused by participating in recent RB4.

Toby
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Old 15 November 2001, 13:22   #4
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zapcating...

Toby,
Please keep us up to date on the zap cat plans...would like to hear more...thanks
Neil
Feel free to send email...
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Old 15 November 2001, 13:28   #5
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.......vs Diesel

I have been up and down this road before and back again, many times. Although my application is different, it is still a decision I am facing .. After reading your first post, I was very interested to see the replies you would receive. Seems to me that you have subconsciously made the decision already, that is to go for the Optimax 200HP, and would like everyone to endorse your decision.
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Old 15 November 2001, 14:02   #6
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Have not made decision already, still undecided.

Two decisions to make, first 200 HP optimax or the new 200 yamy 4 stroke on the old taxi. Inviting thoughts on fuel consumption figures on these new generation engines. Will buy the more expensive four stroke if it is more efficient than the optimax but as yet do not know!

Second discussion with regard to fuel consumption between diesel and petrol for new taxi. As commercial operations can purchase petrol at diesel prices, is a diesel markably more efficient than petrol optmax or injection four stroke?

We burn 6 Gallons per hour on the taxi at 25 knots (7.5m loa and 2m beam with dry weight of 2 tones and payload 300 kilos. Does any one have figures for similar sized vessel and diesel?

Stealth after being up and down this road before what was your decision?
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Old 15 November 2001, 15:12   #7
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Toby,
After you add the cost of the 2 cycle oil mix to the figutres...wouldn't the 4 cycle engine win out economically?
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Old 15 November 2001, 16:17   #8
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OK Here we go.

I can give you very good figures on a similar boar and diesel power. Your figures of 300kg payload on a 7.5 metre boat indicates @ 6 gallons per hour would indicate that you are running the boat light and that you are burning more petrol than you would diesel. Most diesel units would only burn 1litre per nautical mile and would allow you a greater payload because of the greater torque.
For referance, Spirit which is 10 metres comes in @ 2.75 tonne + a fuel payload of 2500kg + 4 crew and @ 25 knots only burns 1.7 litres per nautical mile. If we run the boat "light" say 100 litres she will pull the best part of 35 knots and only burn 1 litre per nautical mile.
With a known payload of 2500kgs that means that if we were running a water taxi she isnt big enough to get all the people on board.
As far as maintanance goes the diesel wins hands down. In 1100 hours we have used 1 air filter, 1 x fuel filter, 4 x oil filters, 6 x fan belts,changed the engine oil 6 times and the outdrive 6 times. And it will rip the arms out of the strongest waterskier around.
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Old 16 November 2001, 05:52   #9
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After near on 4000 miles in 4 months we only refilled the oil tank, (12litres) a couple of times. An expense of maybe £100 or so. Worth considering, but not a major factor. The Optimax uses substantially less oil than the conventional two stroke.

Alan, thanks for the fuel consumption figures.
Very impressed that your 10m deep V RIB, a similar size to the new proposed taxi, can achieve 35 knots, running light with payload and fuel totalling 250ish kilos, at 1 litre per mile, 35 litters per hour (8Gph).

We are able to achieve the same consumption with the Optimax, and a compliment of say, 4 passengers and 75 litres of fuel, but only at 25, not 35 knots. Its when we push the boat faster than this, that the consumption starts to rise disproportionately to the speed and distance covered. If we were to run at 35 knots we would be looking at 11-12 gallons per hour (1.4 litres per mile). Our shallow V hull, similar to a riva and hard chines, which makes quite an efficient planning form, aids our case, we are also 2.5m shorter.

The diesel with better torque appears to hold much better fuel consumption at higher speeds and moderately better consumption at mid range. I did not think that the diesel would be that much more efficient!

My question with regard to the fuel consumption on petrol engines improving to the point that it is nearing that of a diesel, is fuelled somewhat by the car industry, where the gap appears to be closing.

Would you consider a water jet? Are they much less efficient?
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Old 16 November 2001, 08:00   #10
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Slightly off topic..........

Hi Toby, interesting to read that you had power-trim problems on your optimax - we had exactly the same on our Opti 150. Required two rebuilds of the trim system (under warranty) to fix! Other than that absolutely chuffed to bits with the thing. Powerful, economic & love the smart guages!

Out of interest what boat were you on for RB4?

Alan
(Cyanide!)
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