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Old 30 August 2007, 09:40   #1
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Engine Drag

Anyone any good at fluid mechanics?

I was thinking recently about the whole "twin vs single = more drag" arguments, and the thought crossed my mind - a small engine will have a smaller frontal wetted area than a big one, therefore less darg for a given speed.

Now, think of dragging your hand through the water - (an exaggerated example). If you go with your palm facing the direction of travel you get lots of bow wave & drag. Turn through 90 degrees so its acting in a rudder-esque manner and the drag drops considerably, as does the spray etc.

So, on that very very simple assumption I wonder, do you actiually get much more drag with two smaller engines versus one biggie, based on the fact that a smaller egine will have a lot less metal and therefore a slimmer leg, smaller gearcase, cav. plate etc in the water?

I will now duck & let the discusions commence!
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Old 30 August 2007, 10:34   #2
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I think if you look at measurements, two engines about 1/2 the size of a single large one will have more frontal area (that's conjecture, though...)

In addition, you have twice the little knobbly bits that add significantly to drag. It's not like you're putting a sheet of metal sideways in the water; you have anti-ventilation plates, trim tabs, gear casings, etc.

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Old 30 August 2007, 10:37   #3
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I'll start this one off then! Allright then I'll go second seeing as jyaski has beaten me to it whilst writing.

I would assume (but haven't calculated) that doubling the power of engine does not infact doube the size of the gearcase, ergo, no doubling of drag. One big engine presentls less drag than two half sized engines.

Furthermore, the extra weight of two engines would make the boat sit lower and create more hull drag, adding to the inefficency.

K
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Old 30 August 2007, 10:55   #4
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Assumptions?

I do think over time there is a lot of assumption in this area - I grab one random example I found a while ago:

Tohatsu 2- stroke 60 Vs twin 2-str 30s: 60=115kg , 30=52kg. so the twin 30s are 11kg lighter, (and that's before you hang another 20 odd Kg of aux on the transom of the boat with the 60!) Yeah, other makes & models the single wins the lard war, but the difference is usually +/- less than a spare tank of fuel in weight.

Granted the weight thing will vary from make to make and I suspect the difference will probably become a little less pronounced as the HP increases, but I was thinking simply about drag. Like one finger thru' the water vs two (I did say my example was a bit extreme!) Think the speed loss people report when they bolt / gain when they unbolt wings to / from the cav plate. - And they are relatively thin compared to the leg / gearbox and are designed to be hydrodynamic!

Also I bet the drag isn't linear vs speed at the kind of speeds we are dragging these things thru' the water.....

I'm not trying to argue this one way or the other, but merely bouncing a thought that crossed my mind around.


Anyone fancy measuring their engine and we can get some data?
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Old 30 August 2007, 11:08   #5
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My understanding is that if you take the combined power of 2 engines and then remove 20% of that figure then that is the relative performance of a single/larger outboard.

IE 2 x 175 hp outboards =350hp - 20% = 280hp single engine equivalent.
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Old 30 August 2007, 11:46   #6
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Most people reckon 2x outboard only give you 60 - 70% of the total power. So 2 x250 = 500hp but after the losses you get at most 350hp.

Using the example of 2x175 you would get 245hp at most.

With 3 engines they reckon the 3rd engine is only giving 20%.
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Old 30 August 2007, 14:59   #7
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Another factor is that with one outboard you generally have a 3 bladed prop and with two you're doubling the blades, this in it's self has an effect on performance and will lower top speed by quite a bit. Coupled with a larger surface area and you're on a looser.
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Old 30 August 2007, 16:05   #8
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Codprawn,

Yeah, I know that is a generally accepted figure, however I would imagine at the top end of the HP scale the difference in leg / gbx size is relatively small for a given HP doubling. Again to go to an extreme, think of the leg size of a wee 2hp aux vs a 4, or even a 4 vs an 8 - the legs seem to grow "fat" quite rapidly at that end of the scale... (that comment pure obbservation, I hasten to add!)

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more blade surely = more "grip" so smaller props could be used so less drag? (and being smaller they only pushing half the boat along, so can you up pitch a bit? Granted more drag from more blades, but being smaller possibly a smidgen more efficient? Another passing thought to add to that - the exhaust hole - The blade swept area will be minus this hole - and for a bigger engine you need a bigger hole to get rid of more exhaust.......

While perusing this earlier I found two main camps on a Boston Whaler website - "two is way better" and "two150s are cr@p compared to my 250 Opti / Etec / whatever". Nobody seemed indifferent.

I really am beginning to think some data might be a good call here. I'll go measure the Suz, anyone want to volunteer to go higher up the RPM range?

Ever regretted starting something?
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Old 30 August 2007, 18:14   #9
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Quote:
Originally Posted by codprawn View Post
Most people reckon 2x outboard only give you 60 - 70% of the total power. So 2 x250 = 500hp but after the losses you get at most 350hp.

Using the example of 2x175 you would get 245hp at most.

With 3 engines they reckon the 3rd engine is only giving 20%.
Sorry to be innaccurate with my last post however the experiance at Scorpion, who I know were unpleasent to you at Ribex 1968, is that the second engine has a 20% negative effect on performance. This is factual based on real world experiance, others maybe differant but on those boats it is 20%...
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Old 30 August 2007, 20:09   #10
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Ahh I was just quoting what a few engine makers had told me.
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Old 30 August 2007, 21:12   #11
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Question: Why does one run two engines to begin with, instead of one?

1. More Speed?

2. Greater Fuel Efficiency

3. Redundancy of propulsion?

If the answer is #3, my reply is that insurance policies never come for free. If the answer is either #1 or #2, I'd be very surprised.
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Old 31 August 2007, 01:53   #12
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I have driven two 5.5m ribs (from the same manufacturer) one with a 75hp and the other with twin 50hp (both 4 strokes) and yep there is a significant step up in size in the prop and gear box in the 75hp compared to the 50hp. However there is very little in it at WOT bar the twin rig is drinking more. So I would have to agree with the suggestion that you lose around 25% of the total power on a twin rig.

The advantages of the twin set up are the “hole shot” is impressive and increased manoeuvrability.
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Old 31 August 2007, 06:13   #13
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Quote:
Originally Posted by Tomas View Post
Question: Why does one run two engines to begin with, instead of one?

1. More Speed?

2. Greater Fuel Efficiency

3. Redundancy of propulsion?

If the answer is #3, my reply is that insurance policies never come for free. If the answer is either #1 or #2, I'd be very surprised.
4. Sailing centre has a fleet of small ribs & one big one - it commonises engines to reduce # spare parts held in stores & facilitate servicing?

5. Nobody makes a 400Hp outboard?

As you say, the main reason for going twin on a cruising rib I guess is redundancy, but that is also somewhat pointless having a "40 horse aux" if it can't get the thing on the plane - you might as well just go for the usual wee one on the side. I have experience of an Avon SR5.4 planing (just) on a single offset Yam 30 something like 5 up, and so I can see the RNLI having no problems with the twin 40 setups should one of them die. - a lifeboat that does 4.5 knots isn't going to be much use to anyone!

Jumping back to the weight discussion momentarily, I had a longer look at the Tohatsu brochure - the weight scenario I chucked in the pot yesterday seems to alternate as you increment up the total horsepower - and it all appears to be down to the "two or more HP variants out of one block" trick The way Tohatsoo have designed their variants means as you go up an engine size you end up comparing the bigger HP single to the smaller HP half size (single wins by miles) then next size up you have the lardy single cf. the higher power for the size twins. I.E. single 60 weighs more than two 30s, but a single 80 weighs less than two 40s, and carry on alternating up the scale from there..... (and if you add the aux weight, there's usually not a lot in it anyway even if the single wins on its own)
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Old 31 August 2007, 09:17   #14
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And then you get the likes of Suzuki who do a 200, 225 and 250 all weighing the same.

Taking it to extremes, a Honda 20 weighs the same as a Suzuki 9.9, give or take a little. So I'd rather have one Honda 20 on the back, and a good set of oars!
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Old 31 August 2007, 11:38   #15
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But If you had to choose on a 8 or 8.5m rib. Would you go for
- yamaha 350 HP = 350kg
- twin evinrude e-tec sb 200 HP = 2 * 190kg =380 kg

And according to prices I heard, the twin e-tec have +/- same price as the Yamaha.
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Old 31 August 2007, 14:26   #16
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Quote:
Originally Posted by 9D280 View Post
4. Sailing centre has a fleet of small ribs & one big one - it commonises engines to reduce # spare parts held in stores & facilitate servicing?

5. Nobody makes a 400Hp outboard?
Aye Aye
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Old 31 August 2007, 16:53   #17
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But If you had to choose on a 8 or 8.5m rib. Would you go for
- yamaha 350 HP = 350kg
- twin evinrude e-tec sb 200 HP = 2 * 190kg =380 kg

And according to prices I heard, the twin e-tec have +/- same price as the Yamaha.

By the time you have added an aux the weight is the same. No contest!!!

The only way that Yammy become attractive is when it hits 400hp. I wish Evinrude would hurry up with a 400hp V8 ETEC..............
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Old 31 August 2007, 19:25   #18
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Besides all that, you suffer worse fuel economy with two engines vs one engine when going the same speed. You also have to maintain two engines and also a better chance of an engine breaking down if you've got two. (double your chance of having to go to the local mechanic.... ).

There are from everything I've gathered, only three logical reasons to have twins.

1) You need more power than a single outboard can offer you. If a single 300hp outboard can't meet your needs, then you will need to go with at least twin 250hp engines to generate more power than the single 300hp. Twin 200hp engines wouldn't be worth it, because you'd be creating the same power almost.

2) You are doing some serious offshore boating and want the peace of mind. However you will pay a lot of money for that peace of mind.

3) Manueverability. However in a RIB this isn't really needed.......

There is also the reason that twins "look cool" but I don't really think that's a logical reason. Plus I think a big single engine looks just as cool, as long as your overall beam isn't too big! (more than 2.7m)
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Old 01 September 2007, 17:25   #19
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but jumping back to the original question.....

Yeah, the whole X is not equal to two X/2 HP engine argument seems reasonbably well proven, or at least most are singing in harmony over.

The weight issue seems to be a bit clouded by misconception (see above posts), and rephrase the question as On a 4.8m rib, would you rather twin 30s at a total of 104Kg or a 60 at 120 odd + an aux at another 20Kg?...... Argument turns on its head, but I suppose the weight of a 300H opti is about as use to me as the weight of a 40 to the owner of an 11m Redbay!


These are all well trodden discussions, but I am still wondering about the exact wetted surface & frontal areas of engines. I do seriously believe that arguments that may hold solidly at the 200+ horse end of the scale may be totally reversed down at the "diddy RIB" end. As a rough comparison, comparing a Tohatsu 50 to my Suz 25 most bits were approx 50% bigger in most directions.... I'd guess if I compared a 200 with a 300 Horse engine the difference would be proportionally a lot smaller.

There must be some science behind these "can't double for an exact match" arguments, and I want to find out why! This "drag" argument needs some data backup - if for no reason other than the weight argument has already been blown out the water for some comaprisons, and I'm now curious if it's true for leg dimension (related to drag).....
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Old 06 January 2008, 11:30   #20
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twin vs. single

I will add my own experiences, and I agree with 9d280 - sorry for the contradictors !

1st rib : a south-african catamaran 5.4 metre, Stingray make with either :
- 2x50 Tohatsu, total weight = 160kg speed was 44 knots @ 5900rpm with 15" SS props
- 1x90 Tohatsu, weight = 125kg, speed was 39/40 knots @ 5700rpm with 19" prop
overall behaviour was much better with the twin - holeshot, manoeuvrability, speed, safety (the boat was able to plane with 6 people on-board on a single engine on a flat water)

2nd rib : an italian Zar57 heavy (550kg bare weight) monohull :
- 2x50 Tohatsu = 36.5 knots with 13.5" props, 34 litres/hour measured at full speed
- 1x100 Mercury = 37 knots - l/h not measured, probably more


When the size of the rib - so the power of the engines, and the speed - it could be expected that the single motor becomes the winner - but not in term of safety !

What happens if you break your prop on a rough sea in a coastal area where you have rocks everywhere around you ...?
In this case, would you prefer a single or a twin ?
Should you have the chance to be able to give your answer one week later, I am sure it would be "twin".
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