Power-speed graph displacement catamaran

On another forum I am discussing the realities of electric only catamarans. One of the bigger issues is the guy I am debating is certain that regardless of the speed, to double speed you need 8 times the power. Which I can accept this at some speeds, I doubt it is a truism across all speeds and all boats.

Since the conversation is in reference to displacement cats I am hoping that someone here has a kw/kn graph of ideally a 30-35' displacement cat to use an an example.

 

Have a look. Mono hull displacement ship

 

That is a very gross over generalisation and not wholly correct.

Simply because it depends upon many factors. If the hull or short and fat and is heavy it shall behave very differently if the hull is long, narrow and light. Very differently!

That graph by Tanls, is a good example of incorrect data. That is clearly for a displacement type of monohull, not a catamaran, such as a seen with a classic fishing boat or even a landing craft. Neither of which can go fast, ie. at high Fn's beyond the prismatic hump.

To be able to go fast as a displacement catamaran hull form, eg a high Fn, you need slender hulls. If they are not then you shall suffer large increases in power for very minor increases in speed. Which would be "similar" to the graph, but not as extreme, as that posted by Tansl.

 

Ad Hoc,

That was pretty much my point. The hull in question appears to be a pretty narrow hull planning catamaran. http://www.syasperformance.com/south-pacific-cruiser-10m.html . His contention, and what I objected to. Is that if the range is 85nm at 6kn, then the range at 3kn would be infinite (power recovered by 3.5kw solar panels). He is basing this on the fact that his kayak at 5.5kn uses double the power it does at 4kn.

 

All I wanted to show is that the power / speed curve is a parabola, and although I said that my curve is for a single hull, catamaran shift away from the zone of possible planning also has a parabolic curve.
Do not know why, but I can be wrong, my curve is incorrect. The curve is for another boat type but, why is incorrect?. Hopefully, Ad Hoc, a reasoned explanation for what seems to me totally out of tune.
The curves can be traced back to wherever you like but are only valid, as we all know, within a certain range of values ​​of the Froude number.

 

If you do not understand why...you should not offer any suggestions/advice.

 

Ad Hoc, you may be right, it is possible that I should not give advice to anyone, but I'd like you to tell me why it is wrong curve. Maybe you can say that it is not appropriate to the case discussed in this thread, but still do not know how can you know that this is an incorrect curve. Would you be so kind as to explain it to me?. I do not pretend to know everything, and when I'm wrong, I really appreciate you showing my mistake.
Thanks, my friend.

 

Tansl,

Please read my post #3 above. If this does not make sense, then it is QED I am afraid, and in which case you need to take a major refresher in hydrodynamics.

 

Okay, I "need to take a major refresher in hydrodynamics", but why "graph by Tanls, is a good example of incorrect data"?. I can not understand.
How can you know the data and know that they are incorrect: I can not understand. Really, it is a question of hydrodynamics?, Hydrodynamics, does it allow you to know the data?. Amazing!

 

Obviously if doubling the speed is going to require 8 times the power (and fuel), it is not the right hull-form for the job. There are plenty of planing hulls where increasing speed through sections of the speed band actually improves mpg, which simply could not occur if the formula quoted applied.

 

Talking about displacement hulls : the power required at each speed depends on several factors, some of them are a function of the velocity squared. So that would be the only relationship, not direct, that could be set.
Saying that double speed means 8 times more power is not correct.

 

It would not be correct for a displacement cat, unless you went out of your way to design it that way !

 

Not sure why this thread is in the monohull sailboat section.....

It is unlikely that any boat is equally efficient at say 8 knots and 16 knots.

I'm not going to give graphs or equations, but just some anecdotal evidence - ie actual facts not theory. I have motored a 35ft cruising sailing catamaran at 8 knots with twin 9.9hp. I have motored another 35ft cruising sailing catamaran, with similar hulls, but about 30% heavier, at 16 knots with one 90hp outboard

I still think solar powered electric motor cruising boats smacks too much of perpetual motion to be credible

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

For small planing boats, being more efficient at 16 knots than 8 knots would be normal, but presumably the OP is only interested in displacement cats in this discussion.

 

Agreed, but a non planing cat might well do 8 knots or 16, as I illustrated above. Those are more likely speeds than 4 and 8 knots.

Of course it isn't just hull shape that is optimised for a certain speed range. It is unlikely that a propellor, unless it has controllable pitch, will be efficient over the speed range either

Richard Woods