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Old 15 September 2003, 03:53   #1
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Country: UK - England
Town: Wickford, Essex
Boat name: Seahorse V
Make: Avon Searider
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Engine: Mercury 50HP
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2 Stroke vs 4 Stroke

I am going to be in the market for a new engine this year. I have been looking through these forums for some opinions on what is better, either a 2 Stroke or a 4 Stroke. It was confusing to see the different opinions of one compared to the other.

This weekend I had the chance to find out for myself. My colleague has exactly the same RIB as me and had traded in his Tohatsu 40 2 Stroke and purchased a 2nd hand Mariner 50 4 Stroke. Since this was going to be the size of engine I would be looking at we decided to give them a head to head comparison test. The RIBS are almost identical ( both Searider 4m approximately the same age ). The level of equipment is also almost identical except I have fixed VHF and GPS and all his are handheld. The engines are both the same horsepower except his is power tilt / trim and mine is manual tilt / trim. Mine is a Mercury 50 2 Stroke and his is now a Mariner 50 4 Stroke. Both engines had recently been serviced, his cost a bit more than mine to do so even though they had been serviced at the same place ( Boating Mania in Upminster ). We did the test at Littlehampton, both of us carrying 2 divers and equipment out to the Mulbury harbour.

The first thing I noticed, was that his engine was a lot quieter than mine. Even when I was standing on his RIB, which was right next to mine, I could hear my engine over his even at tickover.

Also, we were both carrying 22litre fuel tanks, he made it there and back, with a little bit of cruising in the middle, where as I had to refuel in the middle ( I probably would have just made it if I had tilted the fuel tank ).

The only thing that I could say about my engine was that it was considerably faster than his. Every few minutes, I had to slow down to let him catch up. He claimed that the maximum speed he could make on the trip was 22 knots, I had no problems getting upto 27 knots before I had to slow down and let him catch up.

I am still not sure what engine to go for. I liked the quietness and fuel economy but I would definitely miss the extra power that I have. Also, my engine was considerable smaller in size and cheaper to service.

Is this the same with all 4 Strokes ? Opinions seem to differ on this topic. My colleague was a little dissapointed because he claims that he could definitely get more power out of his old Tohatsu 40 2 Stroke than he was getting out of his Mariner 50 4 Stroke.
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Old 15 September 2003, 04:29   #2
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Hi Slangley

The problem is with a question like this is that so much is down to personal opinion. It's a bit like asking what is your favourite colour - purely subjective. As you will see on other threads opinions are either sharply divided or in the middle!

I can only speak for my tiddler. I chose a 2s because of the weight, both onboard weight and weight when carrying it. The 4s was half as heavy again. Oh yes, and I also needed to be able to stow it in the back of my car and you shouldn't turn a 4s upside down. However that won't be a problem for you.

I guess that for a 4m rib weight would also be a major consideration. Perhaps your boat was faster because of the lighter weight.

So my humble opinion - get the 2s.

Keith (who cares what I think anyway) Hart
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Old 15 September 2003, 04:43   #3
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I agree, it is a very personal choice and different engines work better on different RIBs.

I am definitely buying a new engine ( 2nd Hand ) by the end of this year since there are some great deals at the moment and my engine is very old and starting to show it's age now.

It is a case that I want to get the best engine for me since it is a fairly big investment.

Looking at the plate on the transom, it says maximum 50HP. I spoke to Avon and they basically said that I could have a maximum Transom weight of 110KG. My engine weighs 100KG but the newer Mercury 50s weigh 93KG. I want one with power tilt / trim and looking at it, the newer Mercury / Mariner 60 2 Strokes only weigh 100KG.

Is there any reason why I cannot put a 60HP engine on there even though the plate says 50HP. There are a few well priced Mariner 60HP power tilt / trim engines on www.boatsandoutboards.co.uk at the moment. It still comes under the 110KG limit that Avon specify. Also, the previous owner had put bigger strengthening arms on the transom so it could probably take a little bit more than that.
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Old 15 September 2003, 04:56   #4
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You can't beat a Yammie 2 stroke in the 50 to 90 hp range, that's what I think after owning a few engines.

Merc/mariners are also good in my experience, but then there arem#t many unreliable engines out there nowadays.

I think you already have the answers to the pertinent questions

Pros for 4 strokes
New 4 stroke is quieter
New 4 stroke is more fuel economic

Pros for 2 stroke
Old clapped out 2 stroke is faster (that would do it for me!)
Old 2 stroke is nosier
if you are lucky old 2 stroke is smellier as well.

If you want a quiet life with more economy go for a four stroke
If you want a fast noisy life go for a 2 stroke

It depends on whether you are an Accountant or an Adventurer

Your call
Keep us posted
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Old 15 September 2003, 04:59   #5
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When you bring divers into the 2 vs 4-stroke argument, it seems to increase the problem. The extra weight added to by the dive gear means more power is needed. Our club boat has a two-stroke and it never fails to amaze me how little distance will empty the tank. A four-stroke would allow divers to go to dive sites further away. Otherwise its the two-stoke
and buckets of extra fuel. That pretty much cancels the engine block weight difference in my mind. As for power, take the hit and pick a smaller pitch prop.

The comparison is very interesting to read though.
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Old 15 September 2003, 05:53   #6
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I believe the Mariner and Merc 2 stroke 50/60 hp engines were designed by Yamaha, quite a common occurence with outboards. They first appeared in 91. Used one for a Round Scotland trip and it was very reliable.

If you plan to carry divers on a regular basis then the extra clout and lower weight of the 2 stroke would do it for me.

Have a think about how much fuel you are actually going to save with 4 stroke over the year, I suspect not that much and set this against the purchase costs.

Good time to go shopping for engines though, watch the prices tumble as dealers get desperate in Oct and Nov

Pete
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Old 15 September 2003, 08:28   #7
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60hp Mercury

On my last boat (5m Zodiac YL) I had a 2000 60hp Mercury 2-stroke, the engine was great, reliablity was never a problem, rarely smoked and pretty much started from first turn of the key, performance was also very impressive, upped the pitch to a 17 and the thing flew, was doing about 40 knots WOT.
The one thing I would say is that we used around 20litres a day just messing around (plus the oil), we now use around 25litres a day on a 5.6m Avon with a 115 4-stroke Yammy, abviously without the oil.

Mark
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Old 15 September 2003, 08:33   #8
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Not being an expert and also quite new to this. What is the pitch of the prop and how does it affect performance ? I often see people changing props for different reasons. I think my prop is just the standard one that came with the engine. It is also quite chipped. What sort sort of prop would I have to put on there ( Mercury 50 1985 ) to increase performance and where is the bext place to buy them ?
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Old 16 September 2003, 05:15   #9
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Explaining pitch is easier with a picture, and you might find one somewhere on the web (or in an engine manual).

But in words...pitch is given in inches, and is the distance travelled by the propeller in one full revolution.

Some people describe the idea of the propeller in a block of
wood. If you turn it one full turn, it will move through the wood a certain distance.

Beyond this, I think a picture is necessary.

Picking the right prop is important for several reasons...

A prop whose pitch is too large (long?) for the given engine
power may cause the boat to take a long time to plane (or not
at all if extra weight is present).

Petrol engines have their greatest power output at high revs.
A poorly selected prop can prevent an engine reaching higher revs.

The flip side is a prop whose pitch is too short. This means that the engine may climb to high revs, but the boat will not be travelling as fast as it might. And so on...

Lots of people rely on the engine dealer to pick a suitable prop for their setup. There are steel props (expensive), variable pitch props (user can set the pitch for the given circumstances...these are good also in that a damaged blade can simple be replaced), and the standard alu props.

Your prop should be undamaged in an ideal world. Fluid dynamics and all that. Any kink means the prop isn't pushing water as efficiently as it might. Depending on the condition of the prop, it can be repaired. Or you could buy a new one and keep the old as a spare.
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Old 16 September 2003, 06:10   #10
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Prop pitch is very important. Here are the crews of Cyanide and Magellan Alpha collecting the new prop (19" pitch) for my SIB Quicksilver.



Keith (spin doctor) Hart
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