It does'nt add up.

[gjgm]

Re: It does\'nt add up.

Have read that on a boat petrol engine only 15pct of the energy output results in moving the boat. For each revolution of the car tyre you move that distance, but not so with a prop pitch. My school physics crumbles at this point.

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[hlb]

Re: It does\'nt add up.

If you have noticed, boats dont need brakes. Just pull the throttles back and you stop dead. At least you come down to about 6 knots awfull fast. After that a boat is much more efficiant than a car. As when a horse used to pull hundreds of tons on a barge but only one or two tons on the road.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[ccscott49]

Re: It does\'nt add up.

No hills 0n the canals!!

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[hlb]

Re: It does\'nt add up.

Well I'd like to see you pull a couple of hundred tons, hills or no hills and your built like a horse!!...../forums/images/icons/laugh.gif

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[ccscott49]

Re: It does\'nt add up.

I've been told I smell like one aswell!!

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[hlb]

Re: It does\'nt add up.

You smell nothing like Harry anyway. Maybe he eats sweeter grass!!

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[ccscott49]

Re: It does\'nt add up.

Just wait till you get back down here!! Never been keelhauled have you!!....hmmm....no use asking Tutts for help, she'll be pulling on the line!

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[banus]

Re: It does\'nt add up.

The sierra cosworth engine give 225 bhp as standard. £150 worth of chip gives it 195 bhp standing on it,s head. swap the injectors and map valve 330 bhp. a bigger turbo and 450 bhp is easy. Enough bhp to pull the postman off your gran. enough torque to pull the front wall off your house. Spend about 10,000 and get 600 bhp as per european rally cross. The big problem is. Would you put all that petrol at 15 bar (hundreds of psi on your boat. NO THANKS

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[jon_bailey]

Re: It does\'nt add up.

Also, if you started to get salt water on all of those nice light-weight alloy components how long do you think they would last?

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[Bejasus]

Re: It does\'nt add up.

220 psi actually but bugger all below 3000rpm so totally friggin useless on yer average boat.

<hr width=100% size=1><A target="_blank" HREF=http://www.arweb.co.uk/argallery/h00>http://www.arweb.co.uk/argallery/h00</A>

 

[peterb]

Re: It does\'nt add up.

Top speed of my (petrol) car is about 140 mph. Most of the power is used in overcoming air drag, so that at high speeds power is proportional to the cube of the speed. At 70 mph I'm only running at one sixteenth of maximum power, with fuel economy improved equivalently. On boats similar relationships hold, but cruising speed is much closer to maximum speed. It's a matter of what power is actually required to drive the vehicle at the desired speed; boats are just draggier than cars.

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[ccscott49]

Re: It does\'nt add up.

Isn't that what I've been saying, I thought so, maybe I'm not explaining myself properly.

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[burgundyben]

Re: It does\'nt add up.

Well done, you've proved my arguement for not having egg whisk outdrive thingies ...

"Also, if you started to get salt water on all of those nice light-weight alloy components how long do you think they would last?

<hr width=100% size=1>Sod the Healey - I think I'll buy an E-Type.

 

[BarryH]

Re: It does\'nt add up.

A bit longer than your props! Oohh, slap me down for that one/forums/images/icons/laugh.gif

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[Steve_D]

Re: A Simple experiment

Try running 100 metres across the beach

Now try doing the same thing up to your knees in water

Now Swim 100 Metres

Compare, evaluate and summarize

Steve D

<hr width=100% size=1>Now I've got to think of another sig....

 

[burgundyben]

Re: A Simple experiment

great analogy, good point, cant beleive it took 35 posts to get here.

<hr width=100% size=1>Sod the Healey - I think I'll buy an E-Type.

 

[Guest]

Re: It does\'nt add up.

I was pondering this while listening to those ***!! birds in the early hours.

120hp car = 120mph (mine) , frontal area 4 X 5 = 20 sq feet

car at 30mph = 120 /4 ^ 3 = 2 bhp (ish)

boat area (mine) on plane 5 X sin 20 X 10 / 2 = (shallow V ,running on chines)

water = 700 times density of air

so boat needs 700 X 2 / (20 / 8) = 550 bhp , obviously rather high but if it lifts a little bit more, the frontal area would reduce enough to give a realistic figure

I guess the rapid braking when we shut down is caused by the reducing lift exponentially increasing the frontal area - or perhaps something else

Geoff

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[BarryH]

Re: It does\'nt add up.

All very tekkynical, but you forgot the bit thats not in the water and pushing all the air.

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[jfm]

Re: It does add up.

Just read this thread for first time. As regards the fuel consumption, this has absolutely nothing to do with torque, it's all about power. It also has nothing to do with drag thru water. The answer is that your original comparison is flawed. The engine in a cossie sierra (whether a 200hp or 600hp version) is developing about 50hp at 70 on the motorway, on a flat surface. Whereas your 200bhp diesel at 5 gph is developing say 175bhp (you dont actually say whether it is at WOT or whatever).

So, sierra is 50hp at 2gph, diesel is 175hp at 5 gph. Diesel wins massively

Agree about weight. Diesels always intrinsically heavier

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