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chooky
01-07-2014, 06:24 PM
Which would have the best range A boat fitted with a single outboard which is on the marginal size for it and will have to run at higher revs to cruise or The same boat fitted with twins of the same motors that will be right down in the rev range to achieve the same cruise speed

bobbyb
01-07-2014, 06:37 PM
bigger is better, the thing that scares me about twins is the cost of servicing.
butt I am a tight bugger.

bonneville
01-07-2014, 07:34 PM
I recon that's a hard call to make !
twin engines will require more fuel, ( another tank or a larger one) which equals, more weight in the donks hanging off the back and probably double the weight in fuel and drag !.......so im thinking there probably wouldn't be much difference in range at all !
interesting question ! could be wrong ! im sure there are those here with fuel usage with the same hull or there abouts to get an idea, but I wouldn't have thought range would be considerably different ! ive not owned a twin, so I would be interested in others responses.
bonneville

Noelm
01-07-2014, 07:55 PM
Why would there be double the fuel? No one said anything about increasing fuel capacity!

Noelm
01-07-2014, 07:58 PM
To be somewhat scientific about it, to propel a certain boat at a set speed, requires X HP, now whether that HP is produced by 1 engine or two makes little difference, it still takes X HP, and to produce that said HP takes the same amount of fuel.

team_mongo
01-07-2014, 08:15 PM
Provided efficiencies are the same. Two transmissions, two skegs, but more prop area. A single will have the best range, twins are probably safer if you running long distances.

bonneville
01-07-2014, 09:01 PM
Why would there be double the fuel? No one said anything about increasing fuel capacity!

then the answer to the question is the single will have more range !

two motors, will not burn less fuel because of required H/P

HP comparisons to working less harder with two, is sort of right, BUT, there's a thing called extra weight/ drag/ and as already said, more fuel !!

The single would have the biggest range
on an apples for apples situation.
but lots of other advantages in a twin set up.


bonneville

tjotter
01-07-2014, 09:21 PM
is this a generic question or
do we specifically have a mono or a cat in mind ?

boboncc
03-07-2014, 07:43 AM
HP comparisons to working less harder with two, is sort of right, BUT, there's a thing called extra weight/ drag/ and as already said, more fuel !!

bonneville

I totally agree with bonneville's statement. May I ask, why not just get a more powerful outboard so that it provides more HP than one that is just "marginal"?

Cheers
Bob

scottar
03-07-2014, 08:40 AM
At a guess he has, in his opinion, an underpowered boat and was simply going to purchase a motor the same as he already has. Like I say though - just a guess

Dignity
03-07-2014, 08:44 AM
I know absolutely nothing about engines but I would have thought that any engine fitted that is basically struggling to cruise is not an option at all and you would be thoroughly disappointed in the outcome, I am sure if you did a search on this forum the same question and answer will be found in various guises. I would settle for a higher HP engine and surely it would be cheaper than buying 2 smaller units as well as servicing and maintenance costs. Once again is this question related to a purchase of possibly 2 similar boats already set up with what your asking or just a pub debate.

The Black Unicorn
03-07-2014, 08:56 AM
To be somewhat scientific about it, to propel a certain boat at a set speed, requires X HP, now whether that HP is produced by 1 engine or two makes little difference, it still takes X HP, and to produce that said HP takes the same amount of fuel.
Mmmmm. What if I chose to run a 90hp on my boat instead of the 115 it has on it? I reckon it would still be able to cruise at 20 knots but the 90 would probably be working harder and using more fuel than the 115 at the same speed. Kinda flaws your theory a little.

scottar
03-07-2014, 09:08 AM
Mmmmm. What if I chose to run a 90hp on my boat instead of the 115 it has on it? I reckon it would still be able to cruise at 20 knots but the 90 would probably be working harder and using more fuel than the 115 at the same speed. Kinda flaws your theory a little.

It's not that simple. If the 90 has passed the point in it's torque curve where efficency starts to drop to get your 20 Knots, but the 115 hasn't, then yes. If the 90 is at it's peak efficency at the 20 knot mark and the 115 hasn't reached it's optimum then it may go the other way. Chances are though that unless the 90 is running close to WOT to achieve the required speed, the difference would be very minimal unless the 115 is a heavier, larger capacity engine.

Noelm
03-07-2014, 09:49 AM
Yes, what he says is correct, there is a thousand variables of course, "IF" we are considering a certain speed, on the same boat, then the HP to achieve that speed is a constant value, that constant value equates to fuel used whether that be a 150 working at half throttle, a 90 running at ideal RPM, or anything in between (in a way) as I said, there is a lot of variables. People often up the HP, thinking they will save fuel (and sometimes they do) but a well powered boat, running at optimum RPM, will be every bit as efficient as a big HP motor running outside it optimum power band for the same cruise speed.

Fed
03-07-2014, 09:59 AM
More HP = Less Fuel, that's what I used to tell my old man 50 years ago.
:D

chooky
03-07-2014, 11:30 AM
This question arose out of a pub debate as Dignity stated.

stevej
04-07-2014, 08:43 PM
Mmmmm. What if I chose to run a 90hp on my boat instead of the 115 it has on it? I reckon it would still be able to cruise at 20 knots but the 90 would probably be working harder and using more fuel than the 115 at the same speed. Kinda flaws your theory a little.

my 90 is the same block as the 115 hp 1.7l v4
nothing would really change just less top end

stevemid
06-07-2014, 06:47 AM
I've come up with a new saying: "All things being equal never are."


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fisho64
06-07-2014, 10:33 AM
To be somewhat scientific about it, to propel a certain boat at a set speed, requires X HP, now whether that HP is produced by 1 engine or two makes little difference, it still takes X HP, and to produce that said HP takes the same amount of fuel.
thats way flawed Noel.

There are efficiencies in scale and two sets of gears transmitting 150hp each is less efficient than one with 300hp
Take it to extremes, how much do you reckon a containership would burn at 20 knots producing 50,000 hp?

However Chookys comparison of a poorly set up single doesnt give much to work on.

chooky
06-07-2014, 06:09 PM
the plot has been lost somewhat. Hypothetically a boat has a single 250Hp but requires a 300hp. You move the 250hp over and strap on another 250hp (500hp). You would be down in the rev range to achieve the same cruising speed that you would attain with the single.
Would there be little difference in fuel burn as the single 250hp would be working a lot harder.

Noelm
06-07-2014, 07:01 PM
We are not comparing one boat to a container ship, we are comparing the same boat cruising at a determined cruise speed, and whether 2 motors of equal HP to one big would use more or less fuel to maintain that cruise speed.

scottar
06-07-2014, 07:45 PM
In a perfect world - where the massive extra weight didn't upset the applecart and there were no losses either within the engine or due to hydrodynamics they would be about the same I suspect. In the real world I think you would totally upset the performance of the hull and due to this and the fact that there are losses the twins would use more fuel.

gofishin
06-07-2014, 10:32 PM
In a perfect world - where the massive extra weight didn't upset the applecart ....
....In the real world I think you would totally upset the performance of the hull and due to this and the fact that there are losses the twins would use more fuel.
x 2 for above!

Chooky, the problem with your 'hypothetical question' is that the 'twins' option is way above that supposedly required (over the top even), but the single is only just under.

In addition, these days a 300 sticker is the same weight as a 250 sticker (in one V6 colour) or marginally heavier in another V6 colour (but with ~double the actual extra HP from memory too). So being 50 'sticker HP' under may not really be that bad!

But hey, given perfect conditions, I know which one would win a race - on a dead-flat sea.

Maybe a more 'somewhat realistic' comparison would have been (if you are using the same motor choice) is:
Recommended power 200-250
Max power 300
Option 1 - 1 x 150
Option 2 - 2 x 150

Then the answer would be reversed. Funnily enough, on rare occasions you do see a boat which is way underpowered like above, and could have two of the donks which it has one of!

Getting back to reality, the best technical comparison of twins vs a single, that I can remember reading, was by F&B back in Aug/Sept 2008.

They did a detailed performance comparison on two identical 685 Explorers, one with an F250 and the other with 2 x F115's. A summary can be found here:

http://www.australianboatmags.com.au/pdf/previews/SURVEY31_prev.pdf

I have the mag somewhere, but it has been that long since I have read it the details are a bit vague.

From memory:

at cruise - similar performance, near identical fuel consumption
WOT - single of course had more top end (less drag & slightly more HP)
grip & 'zap' of the twins was understandably fantastic

At the time I was considering both options on my 685, but I went the single, and am very glad I did.

Cheers
Brendon







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Lovey80
07-07-2014, 02:05 AM
the plot has been lost somewhat. Hypothetically a boat has a single 250Hp but requires a 300hp. You move the 250hp over and strap on another 250hp (500hp). You would be down in the rev range to achieve the same cruising speed that you would attain with the single.
Would there be little difference in fuel burn as the single 250hp would be working a lot harder.

In that exact situation, unless the boat was already set up for twins with twin tanks etc and they were absolutely necessary... the solution would be to sell the original 250HP and put it towards the money being spent on the additional 250HP and get a single 300HP.

stevemid
07-07-2014, 10:08 AM
Brendon, the the F&B review only came with page 1 (or is it my setup?) Can you attach the whole article please? IMO this article comes closest to "all things being equal" in the One against two debate.
Steve

gofishin
07-07-2014, 10:26 AM
Hi Steve,
Not your setup or problems your end, only the first page is available as a free download. Members can view whole file - which I no longer am.

Will dig the article out for you at home, will take a few days though. PM me your email.
Cheers
Brendon


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LoungeLizard
07-07-2014, 11:05 AM
The simple answer is: "There is no simple answer"

Without further knowledge of the hull, motor characteristics, water conditions, RPMs @ cruise speed and so on it's simply not possible to say whether a single or twin setup will use more fuel.

From your example, when you say "boat has a single 250Hp but requires a 300hp" do you mean recommended HP? If so you could possibly still achieve cruise speed at (for argument's sake) 4500 rpm. With twins this might drop to 3000 rpm. If that's the case I would say the single will still be more fuel efficient.

However, if the boat is grossly underpowered and the single needs to revved to WOT (at which point it become VERY fuel inefficient) then the twins would win.

Beatsworkin2
07-07-2014, 05:02 PM
Next boat will have twins reason because it looks good reason2 safety reason 3 as u get older u can afford it

honda900
07-07-2014, 05:17 PM
worth a look.

http://www.youtube.com/watch?v=gG_-YmGN8SY

Regards
Honda.

caravellerob
07-07-2014, 06:05 PM
Wow thats suprising, I would have thought the single would have been a bit quicker top end. Looks like the twins beat the single in every way except for fuel consumption.

bonneville
07-07-2014, 08:07 PM
I don't think its just the fuel usages that the single had over the twin, what about hole shot !
quite suprising on how over all equall the test results were.
interesting as well, was the supposed rev drop with twins to cruise at the same speed !
this was "clinically" as close as you could get to all thing equall, of course, repowering or thinking of the option would be much harder to predict.
great post Honda cheers
bonneville

gofishin
07-07-2014, 08:29 PM
Interesting Honda!

Seems like a Yamaha sanctioned test too, so one would assume both have been propped with similar style props to get the most out of the donks.

Bit hard to read the tables on an iPhone though, hoping I can on a bigger screen.


Wow thats suprising, I would have thought the single would have been a bit quicker top end. Looks like the twins beat the single in every way except for fuel consumption.

Don't forget that the 150's are more than their sticker, and the 300 is less.

US EPA data have the 150 developing 160HP at a rated speed of 5500rpm, while the 300 puts out 288 at a rated speed of 6000rpm.

2 x 160 = 320HP vs 288HP

However... from what I can see, it looks like both wound out to 6000rpm, where the 150's would be punching out even more than 160HP too.

What this does clearly show is the additional drag that twins create and therefore have to 'overcome ' in comparison to a single.

In this case it takes an additional 32 'plus' HP to generate only an additional 1.4 mph.

Interesting hey?

Cheers
Brendon


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Noelm
08-07-2014, 07:44 PM
Where does all this business about certain motors putting out more or less HP than stated? The old Suzuki 140 being only 128HP, now the 150 Yamaha is 160HP and the 300 is a nice neat 288 HP.

caravellerob
08-07-2014, 08:14 PM
I agree Noelm, unless the motors are all put on on a test bench you would never really know what they make. Who knows the 140 suzi might make its claimed 128 hp at 4000 rpm. I wish the manufacturers would put out dyno figures / graphs showing the rpms / HP through the rev range. This would give a real idea of the usefulleness of a motor as oppossed to the 200 hp at 6000RPM which really means stuff all other then verifying the sticker on the side!!

gofishin
08-07-2014, 08:18 PM
Where does all this business about certain motors putting out more or less HP than stated? ...
Do you mean "where is..." Noelm? See below good quote incl. link which I stole from Honda's reply on a previous thread. (Saves typing :)) )


...The old Suzuki 140 being only 128HP, now the 150 Yamaha is 160HP and the 300 is a nice neat 288 HP.
The Suzy being 128 has long since been proven as utter BS. It has 138 HP as I assume you know/recall.

All data for power and torque figures for outboards are as below US EPA link.

What I have posted for the Yams was obtained the last time I downloaded the file, title incl 2007-2011data etc. And most likely still is relevant, except they will have added 'recent models' too. They do not retest outboards unless there is a model change, i.e. F300A... (V8) to F300B... (V6).

When I'm on my PC later tonight I will attach summary data from a spreadsheet I made.



So below is fact.

You can find the output of most outboards if you look hard enough, the 128hp suzi info is a myth. the actual tested figure is 138Hp.

the following link contains the american certification data for engines, so you should be able to find what you are looking for.

http://www.epa.gov/OMS/certdata.htm#marinesi (http://www.epa.gov/OMS/certdata.htm#marinesi)


Note the heading on the columns - ALL the numbers are in metric form. HP on the spreadsheet is in KW, torque is in NM - you need to convert it for HP and ft-lbs!
....
Regards
Honda.



Edit: Attached spread sheet of summary data for selected Yammy's. This is the raw data extracted by me a few years ago from the 2007-2011 US EPA Database. The columns in green are what I converted (to HP) and calculated from their 'Max Torque' at that particular RPM.

This data 'is real'. Anyone can check for themselves!

Cheers
Brendon

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gofishin
08-07-2014, 08:25 PM
I agree Noelm, unless the motors are all put on on a test bench ....
They are, by the relevant dept. of the US EPA. See above link.



...you would never really know what they make....
And that's why they are all tested by a totally independent body!

Cheers
Brendon


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tjotter
08-07-2014, 09:59 PM
re
http://www.australianboatmags.com.au...VEY31_prev.pdf
Not your setup or problems your end, only the first page is available as a free download. Members can view whole file...

I think anybody can buy, probably $5.
http://www.australianboatmags.com.au/shop/pdf_downloads.php

wayno60
08-07-2014, 10:31 PM
http://www.yamaha-motor.com.au/sites/yamaha-motor/files/StabiCraft_659SC_F100_x_2.pdf

http://www.yamaha-motor.com.au/sites/yamaha-motor/files/Bar_Crusher_640C_F200.pdf

gofishin
12-07-2014, 11:55 AM
Further from my previous post...


...Getting back to reality, the best technical comparison of twins vs a single, that I can remember reading, was by F&B back in Aug/Sept 2008.

They did a detailed performance comparison on two identical 685 Explorers, one with an F250 and the other with 2 x F115's. A summary can be found here:

http://www.australianboatmags.com.au/pdf/previews/SURVEY31_prev.pdf

I have the mag somewhere, but it has been that long since I have read it the details are a bit vague.

From memory:

at cruise - similar performance, near identical fuel consumption
WOT - single of course had more top end (less drag & slightly more HP)
grip & 'zap' of the twins was understandably fantastic


At the time I was considering both options on my 685, but I went the single, and am very glad I did...

For anyone that is interested, here is the summary page of the single vs twins comparo mentioned above...

PS. US EPA Data for both engines;
Model, Disp, HP, @ rated speed, Max Torque [N.m], @ rated speed, HP at max torque (calc)...

F115A, 1741cc, 118HP, 5500rpm, 151N.m, 4500rpm, 95HP
F250A/B, 3352cc, 240HP, 5500rpm, 303N.m, 4000rpm, 170HP

http://i929.photobucket.com/albums/ad140/BrendonTait/Misc%20photos/Comparosummary_zps79719c4f.jpg (http://s929.photobucket.com/user/BrendonTait/media/Misc%20photos/Comparosummary_zps79719c4f.jpg.html)

chooky
12-07-2014, 03:42 PM
My hypothetical question involved 1 single or two of the same single ie say 250hp or 500hp.

stevemid
29-07-2014, 09:09 AM
My hypothetical question involved 1 single or two of the same single ie say 250hp or 500hp.

This is why politicians never answer hypothetical questions.;D

fisho64
29-07-2014, 01:54 PM
Politicians rule #1-never ask a question unless you already know the answer:P