Ford 6cyl 7210 engine.

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burgundyben said:
I'd be interested to read your further explanation of that.
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Literaly Displacement is a measure of the volume of water the shape of your hull below the waterline displaces. This is first worked out as a volume such as litres or m3. This is then transferred into weight (fresh water is 1kg/litre, sea water is 1.025kg/litre). Due to Archimedes principles of buoancy this is equal to the weight of the vessel, - well it is if it floats. However as a stated weight in naval architecture displacement is considered to include all fuel, ballast, stores etc. The actual weight of the vessel in air as it were on the quayside and empty is just the weight of the sum of its parts.
Because the boat is floating it can be assumed that the force acting upon the hull from below is at least equal to that acting down from above so the weight of the vessel in the water is neutral.
It is not so much the weight of the vessel that the engine must overcome to provide propulsion but the friction of the water acting upon the wetted area (the area of the hull skin under the water) and its air resistance.
At this point ( before I confuse myself any further and am forced to go up into the attic to search for my college textbooks on naval arc and propulsion) I will refer you to a neat little pdf published by MAN. I know what I'm trying to say but realise I'm failing to communicate it well.

http://www.mandieselturbo.com/files/news/filesof5405/5510_004_02 low.pdf
 
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longjohnsilver said:
Wrt the engine, we've got twin Ford lehman 135s and we normally run at about 1600rpm which produces approx 45hp each engine, so I'd guess that your 120hp engine should be fine.
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An average displacement boat will require 3.8 hp per ton to achieve hull speed, hence the old rule of powering at 4hp per ton. Anymore than that achieve very little gain in speed, you're just wasting energy (and money at $4 per gallon). The problem with too much power is under loading the diesel, which is about the worst thing for engine life (short of bad maintenance).

My own boat was considered underpowered, yet at normal cruising speed my engines were only loaded to 35%. Not good.

So if you subscribe to the above, then you'll want to run your engine at least 50% power.

Just my two cents' worth.
 
cryan said:
:) slip of the, "typing," tongue.
Anything constructive to add to the conversation?
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Yes, the boat weighs 18 te. Empty and 22 te. When loaded up. Simples. Never mind about weight in air, nor Archimedes nor Newton.
Re. Power, my awb Weighs 4 te. And normally trundles along using 6-7 bhp as judged by fuel consumption. I.e. Less than 2 bhp per te. That would suggest that the boat in question would need 40 bhp. To trundle, leaving plenty of surplus power for difficult conditions. One could nitpick about the extrapolation from 4 te. To 20 te. And about comparing an auxiliary cruiser with a trawler-style motorboat but I bet I'm right.., Anyway, if I've identified the boat correctly, the ad. Quotes a cruising fuel consumption which is consistent with 40 bhp.
 
SLC said:
6-8lt per hour?
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Actually the advert. ,if I've got the right one, quotes 5-6 litres per for cruising which equates to a measley 25 bhp. - I was including a "safety factor" to allow for different conceptions for cruising speed. I'm using the age-old figure of 20 bhp.hours per gallon which works well for old-style diesels.
 
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