Are two twice as good as one?

[The Beard]

I haven't posted any of my "Gordon Bennet, I thought everyone knew that" questions for a while, so here goes:

I can understand that if you run a small boat, such as 3 or 4 metre, or you only use it on rivers, lakes or very close inshore, having a twin engine installation may be a little over the top. Likewise if you have a 7 or 8 metre vessel and sail a long way from shore then a twin motor set up would probably be a really good idea.

But, bearing in mind that two 75hp engines probably weigh more than one 150hp, or two 90s are almost certainly heavier than a single 175, are there any advantages to having a twin set up?

[chewy]

I'll say it before anyone else does, have a quick search and you'll find your answer.

Twins won't put out the same hp as a single, you lose something like 30% using twins, Ralph has formulas for drag etc.
Twins are good if you have two completely independent systems, fuel batteries etc.
You've got twice the running costs and servicing costs....

Originally Posted by The Beard

But, bearing in mind that two 75hp engines probably weigh more than one 150hp, or two 90s are almost certainly heavier than a single 175, are there any advantages to having a twin set up?

I know its just an example above but the Yam 75 two strokes are the same as the 90, so twin 75's would weigh the same as twin 90's.

[9D280]

Yep, the "twin X is heavier than a single 2X" is 50% b*llox. As chewy points out, most manufacturers use the same lump of hardware for two or three horsepowers, and two yam Xs may be heavier than a Yam 2X, but the Evenrude twin Xs may be light than the single. You need to look at each setup individually. (and remember the weight of the aux as well!)

The difference becomes more pronounced at bigger Hps because speeds (& therefore drag) goes up, and more Hps tend to share the same lump of metal.


Search for the previous threads, I have posted worked examples to illustrate that summary a bit better......

[The Beard]

Thanks both. I should have tried the search function. Good points about the weights of some motors being the same as more powerful versions. I looked on the Evinrude site and, sure enough a 75 weighs the same as a 90 and a 150 and a 175 are also the same. Thanks for pointing me in the right direction though.

Sometimes what seems a mystery to a novice is blindingly obvious to those with more experience.

[Channel Ribs]

It is a good question and it tends to become a marmite issue, some people love twins and would not want to be without others feel just as strongly about single engine installations.

Twins are great if you have unlimited cash or time for servicing, as Chewy says you also need to have totally separated systems in order to get any real benefit so your looking at twice the wiring/hoses.

Rescue boats often have twins, you get a bit of extra maneuverability and the redundancy is an obvious benefit. But resale values can suffer, most people want a cheap to run setup.

You can also get a false sense of security, believing that your boat will perform just as well with one of the two engines still working. This is only true in high end rigs (Martini's springs to mind).

[Stoo]

But, but, but...

Twins SOUND so cool and look awesome....

[9D280]

Originally Posted by malthouse

as Chewy says you also need to have totally separated systems in order to get any real benefit so your looking at twice the wiring/hoses.

Although at the smaller end of the scale, that only really applies to fuel, as most "small" engines are self sustaining once running and "pull start-able", so the whole twin battery / wiring argument is slightly irrelevant, and below about 50/60Hp, usually run from portable tanks anyway. Bottom line, can I start / run it without a battery?

Originally Posted by malthouse

Rescue boats often have twins, you get a bit of extra maneuverability and the redundancy is an obvious benefit.

...and on "calm" days you can pootle about on one and half the fuel bill...... (I am taking sailing club type rescue here, not RNLI out at 3am in a F9!)

Originally Posted by Stoo

Twins SOUND so cool and look awesome....

Yeah, the thought that ambled round my mind as I rebuilt the Clamshell..... two of the old Merc straight sixes...... or two of the 4 cyl 30s for a slightly less thirsty setup

[Channel Ribs]

Originally Posted by 9D280

Bottom line, can I start / run it without a battery?

Not usually the case with 75s or 90s.

[chewy]

Originally Posted by malthouse

Not usually the case with 75s or 90s.

We used to pull start the Evinrude 70's a piece of p!ss, haven't tried the Yam 75's although if you can start a 75 and the 90 is usuing the same block it shouldn't be a problem?

[Channel Ribs]

Originally Posted by chewy

We used to pull start the Evinrude 70's a piece of p!ss, haven't tried the Yam 75's although if you can start a 75 and the 90 is usuing the same block it shouldn't be a problem?

I would still prefer to have switchable batts and use the mighty key.

[two stroke mick]

I know that 2 engines only have the useable hp of approx 75 % of the combined. I am wondering if there is increased touque from using 2 engines as there are two props. This suitation is only important with heavy loads e.g. diving boat
and not top speed. i.e will 2*75hp be able to get a boat onto the plane quicker than a single 115 hp. The Single engine would probably be quicker though

TSM

[9D280]

Beware the 75% figure..... with smaller set-ups the top speed is less and the bits being dragged through the water are usually smaller. (therefore the absolute drag for the rig is substantially less proportionally to the Hp available to overcome it)

What I didn't mention above is that although the powerhead may be shared with another HP, the leg / gearbox may be shared with 5 or 6 other lumps! (e.g. my old Yam 55 I got spares for the bottom end off a 90!) So how overkill was my gearbox on that machine? - to put it in perspective, the 'box on my Clamshell 60 is about the same cross section as the 'box on my Suz DT25.

As for "more grip" you'd need to look at the blade areas, prop shapes etc etc. Some people here have got more "grip" by keeping the same pitch & changing to a prop with different blade shapes.... But I'll leave that discussion to the prop experts.

[Simon B]

The argument seems to fall into 2 distinct areas or requirements;

1. Redundancy is more highly valued than both performance and operating cost
2. An engine of the correct size cannot be found i.e. a single 400hp outboard = 2x200hp

However, few people who have auxiliary motors would say "yes I operate twins" but effectivley they do all be it in very un-equal twinning.

[Pikey Dave]

The whole 2 v 1 argument is VERY subjective, I've had RIBs with inboard, single outboard & twin outboard (current) all had advantages & dis-advantages. For sheer economy & out & out grunt, the Ribcraft 585 with the 175 Ficht was more fun than a barrel of frogs. However, when we started venturing further afield (20+ miles offshore) my missus insisted that we buy a new boat with 2 engines, who was I to argue The current boat with twin 115s on a 6.4m Ribcraft is very capable, it feels safe & stable on the water & is very surefooted in the rough. The trade off is higher fuel consumption & slightly lower top end speed when compared to the 5.85/175 combo. Personally if we were just pottering about & not diving offshore I would probably go for the single option. But for the added security I do like the twins. I have twin tanks that can be connected via a valve, twin Optima batteries with changeover switch. I have the option to run both engines from 1 tank or 1 engine from both tanks. Similarly I can start / charge either engine from either battery.
The choice is yours & depends on your approach / attitude to security v cost. There's no point going down the twin route unless you do it properly & that means adding around 50% to the cost of a single engine/hull setup.

Oh! & BTW they DO sound most excellent when you get the revs matched spot on

[GordyP]

Originally Posted by Pikey Dave

The whole 2 v 1 argument is VERY subjective, I've had RIBs with inboard, single outboard & twin outboard (current) all had advantages & dis-advantages. For sheer economy & out & out grunt, the Ribcraft 585 with the 175 Ficht was more fun than a barrel of frogs. However, when we started venturing further afield (20+ miles offshore) my missus insisted that we buy a new boat with 2 engines, who was I to argue The current boat with twin 115s on a 6.4m Ribcraft is very capable, it feels safe & stable on the water & is very surefooted in the rough. The trade off is higher fuel consumption & slightly lower top end speed when compared to the 5.85/175 combo. Personally if we were just pottering about & not diving offshore I would probably go for the single option. But for the added security I do like the twins. I have twin tanks that can be connected via a valve, twin Optima batteries with changeover switch. I have the option to run both engines from 1 tank or 1 engine from both tanks. Similarly I can start / charge either engine from either battery.
The choice is yours & depends on your approach / attitude to security v cost. There's no point going down the twin route unless you do it properly & that means adding around 50% to the cost of a single engine/hull setup.

Oh! & BTW they DO sound most excellent when you get the revs matched spot on

Agree with all of that, especially the sound!

I would also add that carrying a heavy load is easier, you get considerably more grip and lift from two props.

[Searider]

Just tried to find torque figures for outboards.

Doesn't seem to be mentioned in the brochures i've been able to find.

[Channel Ribs]

Originally Posted by Simon B

2. An engine of the correct size cannot be found i.e. a single 400hp outboard = 2x200hp

A desire for 400HP in the water would require something more like 2x 275HP.

[Simon B]

Originally Posted by malthouse

A desire for 400HP in the water would require something more like 2x 275HP.

Quite so Malthouse, I was trying to illustrate the point that above a certain level the engine is not made, however, I'd would enjoy gazing in awe at a 400hp and above outboard!

Pikey Dave's point on doing properly is very salient, if dont split everything and have the ability to cross fuel or cross charge then why bother, though a similar argument could also begin with a twin vs single inboard diesel set-up?

What do twins sound like when not matched exactly? I love the sound of a slight mismatch on merlin engines going past so you can hear the dopler effect more....I'll get my coat now.

[Pikey Dave]

Originally Posted by Simon B

What do twins sound like when not matched exactly? I love the sound of a slight mismatch on merlin engines going past so you can hear the dopler effect more....I'll get my coat now.

Twins with the revs mis-matched sound like...well, 2 engines. Get them in sync & they resonate, especially if you're in between high harbour walls or cliffs that gives the sound something to bounce off..mmmmmm

[The Beard]

Originally Posted by Searider

Just tried to find torque figures for outboards.

Doesn't seem to be mentioned in the brochures i've been able to find.

Mmm, I noticed that as well. I think that torque figures are generally (turbos possibly not included) simply a mathematical calculation based on the hp figure.