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Old 24 January 2011, 07:16   #11
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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

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Old 24 January 2011, 08:53   #12
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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.
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Old 24 January 2011, 10:41   #13
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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.
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Old 24 January 2011, 11:21   #14
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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
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Old 24 January 2011, 13:23   #15
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Quote:
Originally Posted by Pikey Dave View Post
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.
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Old 24 January 2011, 16:01   #16
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Torque

Just tried to find torque figures for outboards.

Doesn't seem to be mentioned in the brochures i've been able to find.
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Old 25 January 2011, 01:56   #17
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Originally Posted by Simon B View Post
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.
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Old 25 January 2011, 06:59   #18
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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.
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Old 25 January 2011, 13:44   #19
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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
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Old 03 February 2011, 22:38   #20
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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.
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