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Old 02 May 2011, 15:28   #1
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single or twin???

hi,got a 5.4 searider,hopefully got an engine sorted, but just wondering whats best, single 90hp outboard or twin 40s, whats the performance differance? obvious safety benefits with twins, but a good reliable single is just as good? please give me your thoughts as i just dont know which route is best!! ta..
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Old 02 May 2011, 16:16   #2
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hi,got a 5.4 searider,hopefully got an engine sorted, but just wondering whats best, single 90hp outboard or twin 40s, whats the performance differance? obvious safety benefits with twins, but a good reliable single is just as good? please give me your thoughts as i just dont know which route is best!! ta..
It won't give you twice as much power compared to a single engine and make sure there is enough space between both outboards as otherwise you have LOADS of problems...

The single is a better a choice in my opinion. The look and feel of twin outboards are great but since your rib is small then it is really not about performance.... Twins give you extra manuverability But on a smaller rib the outboards are close together to give the rib what twins should. You also need dual battery system.

Consider long terms costs...two servcie per year instead of one!

One Single GOOD, Brand new, Shiny outboard with a small aux outboard will be more than enough for your rib.
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Old 02 May 2011, 16:55   #3
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You also need dual battery system.
In an ideal world yes, but you don't!
On a SR5.4 I'd go for a single Yam 90, I'm sticking twin Yam 75's on my Atlantic as I know how it performs and handles with them on.
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Old 02 May 2011, 17:06   #4
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Twins vs Single

Twin 40's will not give you the output of a 80 single, more like 60hp. What with the extra purchase cost, extra servicing as already mentioned, and the increased running costs I would go for a modern single every time.
Do a search on Ribnet, this has been covered quite a few times
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Old 03 May 2011, 03:01   #5
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Yeah, do a search. This generally gets quite heated!

As said twin 40s will give you about 60HP "in the water", with fuel prices rising all the time do you really want to be using twice the juice?

Even commercial operators who thought twins were essential are starting to think twice.
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Old 03 May 2011, 03:33   #6
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obvious safety benefits with twins, but a good reliable single is just as good?
I'd rather have a reliable single engine than temperamental pair of twins. But the pro's and con's depend where you are using it. If you are planning to make long trips far from shore with few sensible alternative landing places then chugging along at 4 knots on an aux might not be a pleasant prospect. If you are within easy reach of various ports then the difference between half an hour on the aux and 10 mins on half a pair of twins might not matter.

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Twin 40's will not give you the output of a 80 single, more like 60hp.
I'm sure 9d280 will be along shortly to highlight his experience that this is not always the case and depends on the size of the legs on the engines. If he's not then you'll find his views with this advice:

Quote:
Do a search on Ribnet, this has been covered quite a few times
Quote:
Originally Posted by vandad
You also need dual battery system
Most 40s will have a pull start option (permanent not emergency) so split battery not essential. Separate fuel more useful - but if you are refilling both tanks at the same place and time both are likely to get contaminated together?

Quote:
Consider long terms costs...two servcie per year instead of one!

One Single GOOD, Brand new, Shiny outboard with a small aux outboard will be more than enough for your rib.
Servicing two 40's at the same time shouldn't cost double the price of a 90. Don't forget that to be reliable, that neglected little aux also needs TLC.
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Old 03 May 2011, 04:14   #7
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You rang.......? I really should write a macro to auto type this lot......

Drag etc at the 25/30 knots you are talking about on an SR5.4 is nothing compared to the drag of pulling a 2/300HP gearbox through the water at 45 knots. Also the 'box on a 40 will be smaller than that on a 60/ 75. As a very rough guide drag goes up as the cube of the frontal area and the square of the speed. (it's a good chunk of the reason you drink fuel flat out but a modest throttling back gives a disproportionate fuel economy increase). Net result is this "-20%" or whatever the rule of thumb keeps getting quoted for the larger end is not really applicable down here.

As Pol says, you could get away with no batteries. Pretty much all 40s will run standalone, and have a readily accessible start string at the front.

As for twice the juice, bottom line is that if you are running flat out there isn't going to be a lot of difference betwween 80 total horses and a single 75. if however you do a lot of stuff like dinghy rescue at regattas, you can pottle about on a single 40, and fire up the other one as you see a sail turning into a centreboard!

As for the fuel contamination, the most likely source is seawater. You don't have the problem of underdeck tanks on a SR, and lets face it, apart from a couple of years ago, how many "poison fuel" eposodes have there been? Twin Hulks will mean if one ingests water you stand half a chance of getting back.

....and the other one that bizarrely hasn't been mentioned yet is weight. If you take any range of outboards and compare twin n Hp with a single 2n Hp, the total weight of the rig will alternate as you wander up the total horsepower. It's down to every manufacturer usung the same hardware and de- tuning for a smaller Hp to get two or three engines out of one set of components. (Keeps the manufacturing costs down) Net result is that at the Sub 150 HP as a genral rule if you pick a randon total HP, it's 50/50 as to whether the twins or the single are lighter. Then add 10-15kg of Aux and inevitaly the twins win on weight.


So, my advice would be to look at what you can find, and do a deep dive comparison. Ask to measure the gearbox / leg size at the bottom, you may be surprised just how much smaller the twins' boxes could be. As I guess you are not buying new it's even more important to compare, as you may find the twin "Engines'r'us" combo is the lardy option at that horsepower, whereas the single "fuelburner inc" may also be the heavy one so you are doomed either way. To put it in perspective, my Merc clamshell (60Hp) weighs 7Kg more than my old Suz DT25. It also has an identical diameter gearbox, unlikle the old Yam 55 that the merc replaced - that I got leg spares for off a 90!

There is no generic answer......
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