Two hulls, which has least drag?

That would be nice, although not that simple, given all the obscure variables the ORCi VPP uses. My bet would also be that the added drag is larger for the wide & shallow hull... interesting, that heave response a factor there, did not come to think of it.

 

From the modelling ive done in michlet (of 2 identical displacement, identical length, typical slender hulls), the narrower hull always has less resistance upto a length to beam ratio of circa 25:1. Going any narrower increases the drag from increased WSA beyond 25:1... so for minimum drag, around about 25:1 is the sweet spot - and will look very similar to an americas cup racing catamaran hull form...

 

OK. It seems that modern cats are really flat in the bottom (very high B/T) - for planing? John's wider hull seems to have an L/B more like 10, and the narrower like 25?

 

I dont think its for planning, the surface area is just too small to give any real lift plus the angle of the sailing hull does not have a positive AoA like a powerboat does so i just dont think generating hull lift is worth going after, they have foils for this instead.

I think its more about maximizing displacement for a given waterline beam hull. A round bilge hull must be wider than a square high block coefficient hull, in order to displace the same volume when the length is constrained. The length is constrained in the racing rules, but if it wasnt -it would also be constrained by the ideal 25:1 L/B ratio.

Thus the minimum drag hulls, are around 25:1 L/B and have a high block coefficient body plan.

 

It doesn't have a transom stern, so it can't plan.

 

differ how?

Hi,

For your remark above on meaningful wave test from rc controlled model yachts, and that they differ substantially from full size counterparts. Do they differ also if the models are properly scaled? or is this differences mainly derived from scaling problems?
If so, for the layman, could you offer some insights to where it could originate from?

Kind regards

Patrik

 

How are you defining a transom stern? Are you claimng that any boat without a transom stern cannot plane?

 

How do you define a transom.

It has a very clear exact meaning in terms of hull shape at the stern. Thus what is your definition/understanding.

 

Ok, makes sense to me.

 

The IC-canoe, for instance, does not have a transom (?) but I think it certainly planes... modern cats do have sterns, even if John Perrys models don't.

 

Indeed those IC-canoes do not have transoms.

But they also do not plan in the correct sense of planing. They are ostensibly displacement boats: ULDB's.

 

I would have thought they are planing... they do about Fn= 1,25?
http://www.youtube.com/watch?v=zgdo4p90jHo

Catamarans may not plane since they are so narrow - on the other hand, water skis plane, don't they?

 

to Patrik111

This video shows why its fun to experiment

http://www.youtube.com/watch?v=sgwIBf4odCk

Ian Holt, originally from the UK, is a very good "real boat" dinghy and multihull sailor (he's sailed with me several times). He is also an experienced NA and has designed many successful boats

Over the years he has slowly developed his RC trimarans and had lots of control and stability problems early on. As others have found, he needed a big rudder foil to hold the stern down. You can see his boats don't look like any full sized one

Water skis, like catamaran hulls have totally the wrong AR to plane easily, they need lots of power to do so. So it isn't an efficient path to take on a sailing multihull. A true planing boat has to generate lift, so it needs to be wide and short, furthermore it has to have an angle of attack. Generally a IC10 is sailing too level to be said to "plane". Compare a canoe to a conventional dinghy at the same speed and you'll see it trims much more horizontally

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Mike

It is not Fn related in this sense. I design many catamarans that are running at similar Fn’s, but they are displacement, not planing at all.

The key is the ULDB; Ultra Light Displacement Boat.

These vessels have such little draft that their B/T ratio is very high. In other words they have hardly any draft when fully loaded. So when moving the water doesn’t have to go around the hull as in a normal ‘displacement’ type hull, since there is so little hull in the water, it naturally goes under the hull, promoted by a slight trim of the boat to begin with.

To plan requires hydrodynamic lift to dominate. In other words the hull, whatever its shape, is no longer supported by pure buoyancy, or hydrostatic forces. Since at rest, this is pure hydrostatic.

When planning a greater percentage if not all the boats weight is supported by hydrodynamic forces, i.e. lift. This has the effect of decreasing the draft from that when static, to when running. It literally “sits” on top of the water when running.

A ULBD has very little draft, they are “Ultra Light”, in that sense. So, for any dynamic forces to dominate, there is very little lift that is required to raise the hull. To start with. Since the forces cannot raise it beyond its weight, i.e. fly!

And since the water is already travelling predominantly under the flat hull, the flat area expose to the advancing flow will naturally generate some lift, just as any flat shape at low AoA to flow shall.

But here is the kicker.

If you increased the draft of the ULDB to that of a normal vessel, what would occur?...the flow of the water, with increasing speed shall have the usual lowering of pressure as you go aft, which causes the vessel to trim and thus sink at the stern. The water cannot flow cleanly away as it did when exhibiting very little draft, it is being forced to follow the hull shape.

Thus these canoes may appear to be planning, and in one sense there is some dynamics force being exerted on the lower facet, but they are not plan boats per se. Since if you just increase the draft the hull will trim and sink at the stern and eventually trim very badly and often with some loss all control. Whereas increasing the draft of a planning hull shall just require more power that is all, since the shape of the hull is encouraging the flow separation to create dynamic forces to support it.

 

OK, thanks Ad Hoc. I recall that at school the Professor would define planing (planing or planning?) as dynamic lift more than 50% of the static DSPL (but that was a long time ago), just as you suggest. At that definition, not many dinghies will plane, not at least upwind, as is suggested about most every trapeze dinghy.

Here's a planing simulation, at 3 m/s I think Fn is about 0,9 to the static WL - not quite planing yet, about at max. resistance. I should check how much lift there was in this situation.