I'm Puzzled

[Guest]

I\'m Puzzled

I have been reading about ancient Triremes (sad what?)

Their spec. was roughly 120 X 20 feet with a displacement up to 250 tons.

Under power ( 170 rowers ) not using the sail, they could maintain 7 knots for long periods and reach 10 at WOT.

Now as a man can only maintain about 1/10 hp over a period, so how could a ship of this size go so fast with the power of a 15-20 hp outboard?

Are we doing something wrong?

 

[BarryH]

Re: I\'m Puzzled

Because Torque and Power are to different things. The outbourd hasn't got the torque.

 

[jfm]

Re: useless 0.1hp crew

Not sure about that 1/10th hp per man. Seems a bit feeble. I would think 0.5hp at least, even without a good whipping. In which case, it had equiv of a 90hp outboard, which would do the 7 knots. imho

 

[jfm]

laws of physics

Eh barry? This is entirely a power question, torque doesn't come into it at all.

 

[BarryH]

Re: laws of physics

Well it sounded good when I typed it. To be honest, I can't really be bothered to use the grey matter at the moment, having a really shitty day at work. Roll on friday morning when I hitch up the boat and go.

 

[BarryD]

Re: I\'m Puzzled

Not sure of the HP per man - but is really a "prop pitch" question and water displacement issue. Assume that each oar has a blade of somewhere around 4 feet, then the 7hp is driving a propeller that has a working surface of 680 square feet.

Not sure where I'm going with this, so I'll stop here and let JFM rubbish my maths.

So you unscrew this, and put it safely here, then oopps...
All - IMHO, BTW, FWIW and NWGOI

 

[Dave_Snelson]

Re: laws of physics

Laws of fizziks eh!!

Torque and HP cannot be divorced as easily as that. 1HP is actually a rate of work done expressed as approx. 2000 ft/lbs per hour (old units, I know. But so is a "horsepower"). This means that if you can lift 2000 1 lb weights by one foot in one hour, then you have produced the workload of a "horse", or horse power. Conversly, you need only lift up one 2000lb weight by one foot in one hour, to produce the same net effect of one horse power. So it then becomes a question of gearing. Clear as mud yet??

Taking a turning shaft as an example. If a shaft turns at 1000rpm and at that speed has a sustainable rotational force of say, 100 ft/lbs (torque) then the shaft is producing X HP. If you can increase the speed of the shaft to 2000rpm and still maintain a torque of 100 ft/lbs, then your horse power double to 2X horsepower. You can, of course, do this the other way round and maintain the rpm and double the torque to get 2X horsepower.

To relate this back to boats, you need to start to factor in the difference in unrelated work efficiencies of purely moving or lifting a weight, to dragging it through water, the different forces and losses etc.

OK you can wake up now

Madoc Yacht Club
<A target="_blank" HREF=http://www.madocyachtclub.co.uk>http://www.madocyachtclub.co.uk</A>

 

[trev]

Re: I\'m Puzzled

An oar is a 'second order' type lever so their is some mechanical advantage, and the boat will also carry the effort forward (I know what I mean anyway!) so such speeds would be quite possible, depending on hull form - and these guys were pretty good boat builders !
It would also depend on a good whip round !!

Trev

 

[Robin2]

Re: I\'m Puzzled

This is a question of efficiency rather than power or torque. I suspect very little of the outboard's 15kw (20hp in old money) is actually converted into forward thrust - most of it just stirs up the water and wets your clothes.

Because the oars are large and move slowly through the water they convert more of the person-power into forward thrust.

Mind you, somebody who can work it out mathematically with some precision should treat himself to a nice gin and tonic!

 

[Guest]

Re: I\'m Puzzled

So why not powered oars?

 

[jfm]

Re: laws of physics

Eh, agree your maths, but what's your point? What I'm saying is that the speed of a boat depends on the power, not torque, of its engine

 

[hlb]

Re: I\'m Puzzled

A good point and computerised, could act as stablizers as well. The only problem I see is trying to get more than one boat at a time out of the marina and parking next to the ponton, less the oars could be turned flat side up and used as gang planks.

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Haydn

 

[tcm]

well i don\'t agree with his maths: error in line 1

This light fast saily warship boat seems bit heavy to me? Why on earth does it displace 250 tons? there's only bout 12 tons of people.

Very light and fluffy people too (cos they've been whipped, groan)

 

[DavidJ]

Re: I\'m Puzzled

Those Triremesians were prone to exaggeration, don't trust them.
David

 

[kingfisher]

Not that far off

Our crew manages an easy 4.5 kts under oars, 5.6kts during competition runs with our captains' gig. 10 rowers, 11m60 boat.

<A target="_blank" HREF=http://carolusquinto.tripod.com>http://carolusquinto.tripod.com</A>

Add a few feet to the waterline, and a few rowers. I can see it can be done.



Group of people on the pontoon: skipper is the one with the toolbox.
http://sirocco31.tripod.com

 

[wakeup]

Anway what did the Triremesians ever do for us?NM

.

I just can't be arsed...

 

[Guest]

Re: well i don\'t agree with his maths: error in line 1

I found the displacements on various web sites. This high value was given for Roman ones that had a bow ram so presumably were quite hefty. Various figures are given for Greek ones - typically 70 tons

 

[tcm]

Re: bet there\'s a sail too?

with a sail, and whiped oarsmen, I reckon 7 knots would do-able. However, mine would be even faster than 7knots cos i would buy a 2x v12 german-made whips and do the whipping myself, whereas learner would spend the whole summer with an abacus.

 

[castaway]

Re: I\'m Puzzled

I remember reading that when they were trying to work out how these boats were laid out, they found an ancient reference; quote "farting in the face of the oarsman below".. apparently this helped them work out the rowing positions.... But maybe thats were the extra power came from!!

Just though I'd lower the tone a bit.

Regards Nick

<A target="_blank" HREF=http://www.yachtsite.co.uk/fairweather>http://www.yachtsite.co.uk/fairweather</A>

 

[EME]

Re: bet there\'s a sail too?

The answer actually depends on the SG of the sea in question. Given that we are talking the Med , the answer is actually 5. 73124 knots. /forums/images/icons/smile.gif

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