A question of weight?

hobiecat

hobiecat

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Can someone explain to me this. What is relationship between weight and horse power on:

a) A displacement boat
b) a planing boat

If we were to load a displacement boat to double its weight would it need twice the hp to get it to say 9 knots. (i dont think so but interested in a simple explanation to understand the physics)

What happens if we do same thing to a SD or planing boat?

For the record I am commercially involved in catamarans.
 
As far as the double the weight double the HP question, probably not, but it depends more on the change that takes place to the wetted area, and therefore it's designed weight bearing capability. This instance also raises a major stability question.

On a displacement hull relatively low power is required to reach the 'hull speed' which is relative to the waterline length and a function of the speed that waves travel in free water. If increased load also increases the lwl then a small increase in hp may also increase the speed.

On a planing hull the way water breaks from the wetted surface of the hull is critical. The power required to achieve this is large, and can increase dramatically as load increases to the point that planing can very easily be NOT achieved due to excess loading. However, once planing is acheived, relatively low increments of power can yield large speed increases.

On a SD hull the rulles change somewhat, this is really a displacement hull form designed to leave the stern wave behind somewhat, but not with the extreme sections required for breakaway onto the plane, and for stability on the plane. These would in theory make better load carriers but as with all hulls, they can come in (relatively)good and bad shapes.
 
Andy_Wilson raises some good points.

The exact relationship between flow, power, speed etc is very difficult to model accurately, and so exact answers are virtually impossible. In general, the resistance, which the engines have to overcome, is generated by the viscosity of the water running over the hull, and the wave making resistance of the hull moving through the surface of the water. Depending on the speed and the length of the hull, the relative values of the viscous and wave resistances vary. At very low speed/length ratios, the viscous is the much larger value, whilst at higher speed/length ratios the wave making is the largest component. Generally the viscous resistance is related to the wetted surface area, and the wave making resistance the displacement, so increasing the loading of the vessel will change both, but not by the same proportion.

The planing hull generates dynamic lift, reducing both the wetted surface area and and the underwater volume, however as these drop in relation to lower speed/length ratios the speed increases, which still increases resistance, only at a slower rate than at speeds where the hull is entirely supported by buoyancy.

Displacement vessels do not generate significant dynamic lift (many actually create suction, even in deep water, and sink down slightly), and so at higher speed/length ratios do not provide a mechanism to control the increase in viscous and wave drag. As the wave system around the hull develops into one long wave along the vessel, any increase in speed tends to result in more wave drag, bigger waves, and an increasing trim by the stern, none of which do anything for the performance!

Cats can get around this to a certain extent, as the hulls, even in a displacement condition, have much better hydrodynamic profiles, and create smaller changes in pressure around them, generally resulting in reduced drag over a monohull, which is effectively wider than the cat hull of the same length. Care has to be taken with the spacing of the hulls - if they are too close, the wave and pressure systems interact and force the resistance up considerably. However, if they are too far apart, the bridge deck has to be stronger to cope, (and the marina manager gets more of your cash), the stronger deck increases the displacement, and so the resistance goes up.

This all makes it fairly tricky to detemine changes in power, speed etc. There are rules of thumb, and standard series to work from which will give you a good idea, but really you need to use a towing test tank (such as the one at DERA in Haslar, or Southampton Solent University) to get a good feel of the numbers, and even these are not 100% perfect!

Hope this helps!
 
So is there any direct correlation of weight and hp on a displacement boat? I guess added skin friction and air resistance play a role. Also guess that more power has to be applied to push the greater amount of water displaced. Do any manufacturers ever give graphs such a thing?

Understanding these issues are important to catamarans but have you ever tried finding anything written on this sort of thing?
 
Very roughly, and this works best for big vessels at low speed/length ratios, the shaft power required to push a displacement vessel is proportional to the displacement^(2/3) multiplied by the speed^3.

Planing hulls and cats are a lot, lot more complex!

Hope this helps..
 
Assuming towing test tank is commercially for hire how come one or two rather well known boat hulls came to behave so poorly in real life conditions.
Surely 10K spent in that direction would have been a wise investment.?
 
I just about understand what has been said so far. I find it an interesting topic particulary based on my catamaran experience. If tank tests and computer models struggle then what hope for people like me to try to understand the best hull forms? I know from experience that the lighter the cat the better it feels and performs . Some of the explanations above are not what I experience in the "real" world.
 
The container ships I served on did 18 knots empty and 18 knots with 35000 tonne on them. Same hp same rpm, only difference was the draught aft and trim,

Mike
 
If what you say is true then that just about crushes my thought processes. My logic said that the heavier the boat the more water it displaces and therefore explains the more power required. I guessed it was not proportional but............ /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif more than ever. Also crushes the formula above for "big boats"
 
Old time narrowboat 3mph empty, or with 25 tons of coal on board. Freeboard change (= draught change) about 2 feet. Power unchanged at one horse. Difference is felt in acceleration as there is a greater mass to move for the same available force. Top speed is so much a function of waterllne length that is is unaffected for practical purposes. The water is not mathematically "wide" or "deep" so there are surface effects to consider but otherwise the analogy hods true.

I notice a similar outcome on my boat. With SWMBO and smallest child on board on a day picnic trip, about the same engine revs for hull speed as all five of us with gear for a week's cruisng, plus tender on davits. SemiD hull.
 
If these examples hold true why cant we push a very large boat at reasonable speed with very small engines if the weight does impact much? If we build a heavy cat does it matter then? I know from experience different cat designs that weight does impact on fuel consumption - but unexpected things have more of an impact on the performance than I would expect. eg Put a different brand of antifoul on.
 
I must say this more direct form of subliminal advertising reminds me of how subtle and hillariously funny Saddam Hussien's propaganda mill used to seem /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

Sorry, but I still think small motorised cats are as ugly as sin, about as functional as the dome, and as stylish as Mr Bean's cloths. I couldn't care less about the fuel savings if I had to boat with my head hidden under a brown paper bag for fear of being recognised /forums/images/graemlins/shocked.gif

Otherwise the technical responses above by some were quite interesting. BTW, it is rumoured that some of the big cat fast ferries on the Dublin route will be taken out of commission at end of 2007 returning to conventional ships - pity, I liked the speed of the fast ferry.
 
[ QUOTE ]
I couldn't care less about the fuel savings if I had to boat with my head hidden under a brown paper bag for fear of being recognised

[/ QUOTE ]

It is this attitude that makes me think we dont deserve a chance of keeping red. The MBM campaign suggests a lot of people do care about red as they may be driven out of boating altogether. Design impacts massively on this issue and I thought it was worth discussing. My thread was interested in the impact of weight on fuel consumption comparing different boats and trying to promote a discussion. I genuinely do not completely understand it. Obviously you do. Its OK - I have had enough now and will let people like you get on with it. Last time I was told it I had to declare my interests which I have been careful to do each post. I will revisit the forum in the new year.
 
[ QUOTE ]
Just before LIBS? /forums/images/graemlins/laugh.gif

[/ QUOTE ]Brilliant Brendan - I'm on the floor /forums/images/graemlins/laugh.gif
 
You crease me up man! /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

Take a look at the post I did on CG1's post' Genoa'.... 'Cannes video', what do we see????? oh my good god, one of them, doesn't quite fit the scene /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif
 
To tidy this thread up I got this info from well know designer:

The number that is under consideration here is the Displacement/length [please do stop calling it weight!] ratio. Basically this describes the observed performance of hulls of varying displacements. A higher value for this ratio tells a designer that this boat will be slower than one with a lower displacement/length ratio. ie: more displacement on the same length = slower boat, or a boat that takes more power to push. The actual physics involved is immaterial it is an observed "fact".
 
FFS Victor - please stop the marketing. It's becoming tedious.

I have no particular problem with your products - but surely you can appreciate that your prescence STOPS most of the posters feeling positive about them.

You know those really, really annoying people who ring you at home/work relentlessly trying to get you to change your telephone supplier to them - cos they're <span style="color:green"> SO </span> much better than who you're currently using, and will save you <span style="color:green"> OODLES </span> of money on yer bills.............






That's you, that is. /forums/images/graemlins/mad.gif
 
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