125 hp outboard fuel consumption 2 stroke vs optimax

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mabbs

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I have inherited my late fathers Shetland 2+2 boat (the one with a proper v hull) and am recommissioning her. As the transom was rotten I have had this replaced, and owing to water sitting in the transom well and the exhaust port being submerged, opted to have this made to 24 inches (extra long) - should have been 25" but thats another story.

The outboard is a Mercury 125hp 2 stroke from 1999 (bought new at Soton boat show), as it only ran for 50 hours and was then winterized she is according to the "mechanic as new" (albeit a 1970s design 2 of the cylinders cut out below 1,800 revs), she has not been run up yet as some Gypos tried to rob the engine. I have been told a leg extension from 20 to 25 inches is around £2k, plus other unknown costs, budgeting £2.5k.

Question boils down to how much fuel will I use in a season (assume 100hrs coastal) weighed against the cost of buying a new unit - looking at a cancelled order for an xl 115hp optimax which comes with a 5 yr warranty, in the low £7k region (less whatever I get for the old o/b). How much less fuel would an Optimax use by comparison - some say half but that must be an exaggeration?

Thoughts please?......
 
Initial thoughts is that 100 coastal hours in a Shetland is a serious amount. Are you really likely to do that many?
 
I'd like to, or be able to get the time! I used to trail to the broads for 2 weeks a year, and then used to get a week-10 days down in Cornwall, that was with a 535 with a Yam 2 cyl 2 stoke, and that used to burn the gas. I imagine likely to be more like 50-75hrs in reality. Don't know where the 100hr figure comes from. May be changing jobs so hopefully a bit more time. For a tight arsed farmer used to keeping machinery costs down the bill I'm looking at for this old but sound o/b is a bit frightening. Other concern is that if it was nicked I'd only get market value, not what I had just spent!
 
I think an optima. Will use a bit less, but not half. At fast cruise maybe 30%, but at slower speeds maybe less. You have to remember that of the total hours you do in a year, only a percentage will be at speed. What that percentage is will vary, but it's usually lower than you think.

Difficult one as to which way to go. The new engine brings not just better fuel consumption, but also warranty and implied reliability. Having sad that you will have to pay for dealer sevicing to keep the warranty.
If the Mercury really is like new, then I would be tempted to go that route. You still should get reliability, but you can save on service costs by Diy or going to an independent. It would take years to get back the circa £5k in saved fuel, especially when you add on the dealer sevice costs.
 
CLB has hit the nail on the head, getting back the 4 to 5 K in fuel
I bought a Mercury of that size brand new in 89/90 when that system of the carbs cutting out at 1800
I stuck it on the back of a similar boat to the Shetland and like your boat I had to have the blunt end strentghened
If the engine is good, stick with it they are great lumps.
 
Thanks for the advice, been away for a few days - had a chat with the bloke doing the repair, all of the bits have come over from the states for the extension, his colleague remarked he thought it had been moth-balled. He reckoned at full chat she'd burn 7 gallons/hr, whereas an opti would be 5 gallons/hr. If its more like a 30% saving I'm never going to get back the additional £3k spent on a new unit even if I keep her for the rest of my life! Other bonus is I like to do my own servicing, to get the benefit of the warranty this isn't an option.
 
Don't have a direct answer to your question, but I do know that most of the modern direct injection 2 stroke outboard have comparable figures to the 4 stroke and will certainly be better than the older 2 strokes on fuel.
My initial thoughts might be that a 125hp is a fair size for that boat...what is it rated at ? (Worth remembering that that age of boat will be rated for a 2 stroke, as you are thinking about anyway, but many make the mistake of putting a 4 stroke of equal hp on them and don't account for the massive extra weight...probably not relevant to you, but worth remembering)

Also, unless the boat has sentimental value to you or you plan to keep it forever, I'm not sure spending 7k on and engine (that isn't really required) is a sound financial investment. The boat is likely only worth around that, even with a brand new engine. Just a thought.
 
Thought I would return to this as having done two seasons with her, albeit only 50 hrs in total, to report back. Very pleased with performance when planning, maxing out at 32mph on the GPS with a clean hull, this is down to 28/29mph at the moment. Havent really looked at fuel consumption as its academic, probably around 35 ltrs/hr when planning at 4250rpm.

When trolling on 2 cylinders, such as when its been rough or the 1/2 hr trip down from Christchurch to Mudeford, she has a tendancy to foul up and stop, so I have changed to a premium oil in the premix and she does seem less reluctant to stop now.

Overall glad I stuck with the lump as I'd rather spend the money on petrol and have a light simple 2 stroke.
 
mabbs said:
Thought I would return to this as having done two seasons with her, albeit only 50 hrs in total, to report back. Very pleased with performance when planning, maxing out at 32mph on the GPS with a clean hull, this is down to 28/29mph at the moment. Havent really looked at fuel consumption as its academic, probably around 35 ltrs/hr when planning at 4250rpm.

When trolling on 2 cylinders, such as when its been rough or the 1/2 hr trip down from Christchurch to Mudeford, she has a tendancy to foul up and stop, so I have changed to a premium oil in the premix and she does seem less reluctant to stop now.

Overall glad I stuck with the lump as I'd rather spend the money on petrol and have a light simple 2 stroke.
Click to expand...

Many years ago when I had a 2 stroke we used hotter plugs to stop the fouling. This does mean though that full revs / power can only be used for a short period.
 
As a comparison, a Mariner 90hp 2-stroke on an Avon 5.4m Searider burns about the same as 150hp Mariner 2-stroke Optimax on a 6.5m Scorpion.

This is typically in the 25-30l per hour at 25-28 knots, for both boats.
 
Our Tohatsu 140hp 2 stroke - 2003 vintage but based on old tech obviously can use up to 70l/h at full chat but this is on a 6m aluminum cuddy cabin boat and does about 38kts with 4 on board.. we very rarely use it like that.. so i'd say the average would be around 4-5 hours from a 110l tank.. of ocean running.. at between 16-23kts.. (we cant go much faster here except on a few days of the year) We can normally get 2 and a bit trips to our local island (Rottnest) on a tank and its usually about 1:15 each way.. in the small boat we only went in favorable weather (less than 15kts sw wind and less than 3m swell) it may give you a rough idea of how many litres you may use over 100 hours..

Steve
 
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