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Old 30 August 2007, 10:40   #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, 11:34   #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, 11:37   #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, 11:55   #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, 12:08   #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, 12:46   #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, 15:59   #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, 17:05   #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, 19:14   #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, 21:09   #10
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Ahh I was just quoting what a few engine makers had told me.
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