Cat hull resistance change with heel?

This all depends upon the wave length/period its direction and your encounter frequency. Since it is appreciable to all boats. It would just occur at a different wave/period/heading etc from a harry to a cat for example, that is all. Thus what are the predominate waves in your locale, you wish to sail...and then compare. Thus try one!

But all poor motions can easily be mitigated by changing direction and/or speed and is applicable to every boat.

 

Yeah, certainly it would be slower with the small hull to leeward. I tried to demonstrate that earlier in the thread with the thought experiment of changing DLR in the extreme condition of either hull flying. Not that this would happen, but it shows the DLR, and hence total resistance would be greater with more weight on the shorter hull.

Now would not a similar thing occur by starting off with the majority of the displacement in the short hull to begin with? Sailing the wrong way around would just increase this effect rather than lessen it with increased heel.

I also reasoned that although this fact is pretty much indisputable, in practice it may not matter that much as the short hull will lose displacement while the longer hull gains it in conditions where a good DLR matters most, in stronger wind. As you say in light wind the WSA matters more and this will not be effected too much.

This is easy to take guesses at, but I see the HP team has been using Michlet. Has there been any effort to model the resistance of each hull, and total hull resistance at varying heel angles/speeds and then to compare that to expected heel angles of varying wind strengths and directions?

 

"harrys are considerably lighter than cats and somewhat lighter than weight to lee proas"

How are they being compared? Are you comparing the HP with a cat that is as long as the HP short hull or long hull? Or how?

I think the HP build method to be my favorite, with the best choice of materials (foam core with, VE or Epoxy), infused with less mess and fairing. If I ever build something I will want to do it in a similar way.

However I think you could build more than just a HP in this way. With the same engineering I cant see how a pacific proa would be heavier. I would think similar. Also how heavy would a cat be of similar accommodation, and how long would this cat be if you allowed for a lot of extra empty hull space fore an aft with the same material list? My guess is longer than the short hull on a HP but shorter than the long one?

 

Of course, except that changing course for comfort is not always desirable when sailing to windward. Assuming the waves are coming from roughly the same direction as the wind (~45 degrees off the bow when sailing to windward), the harryproa motion is less corkscrewy than the catamaran. When you sail a harryproa upwind in waves, it is very noticable. Aided, I suspect, by locating the crew in the middle of the ww hull rather than aft in the middle of the boat.

Dennis,
The added wetted surface, torque on the beams, weight and added unusable space all mitigate against a longer windward hull. Therefore, we start with the suitable B:L ratio, payload and hull weight and make the hull a suitable length. I guess for enough weight, this could mean equal length hulls, but the very low cost of lengthening the lee hull and the benefits from this means that equal length hulls on a shunting boat are not the most efficient way to proceed.

Comparing cats and proas is fraught. eg, the Visionnary Cruiser has 2 double berths, 2 singles, weighs 3 tonnes, 77 sq m sail area and large usable deck space. Maybe a 35' cat for accommodation, weight and build time. Maybe a 40'ter for sail area. And a 50'ter for length. Rank your parameters and choose which suits them best. eg, I doubt any of the above would cruise at wind speed as effortlessly under main and jib as the Visionnary does.

And the new Cruiser 60 http://harryproa.com/?page_id=1177 has 50% more sail area, is a little heavier and has a better layout so should sail even faster, while taking half the hours to build.

For given space, the cat will be heavier as it has the extra half hull, the mast supported in space with highly loaded attachments to the 4 corners and 4 water foils, plus their cases/strengthening. Obviously, some/all of these can be altered, but the more you do so, the closer you get to a harry. I know this as it is the path along which harrys were developed.

The weight differences between the harry and the weight to lee (the only thing they have in common with Pacific proas is low righting moment) types is caused by the following:

The higher the load imposed on a hull, the stronger and heavier it needs to be. A stayed rig imparts high loads to a larger surface than an unstayed one which imparts high loads to a small area.
A loaded hull with lots of cutouts (hatches, cockpits, pod, etc) needs more strengthening. The higher the load on the hull, the more strengthening required.
A stayed rig requires high bows for foredeck work, so the hull will be bigger.
The beams have to be designed for the worst case. In the harry this is normal sailing, where the beams have to lift ~60% of the boat weight. In the weight to lee, worst case is caught aback, when the beams have to lift ~80%. Consequently, the beams are heavier, maybe the mast as well. The exceptions to this case are if the little hull is so small that the boat capsizes the wrong way, or the mast falls down.
In hull rudders (weight to lee proas) need cases and considerable beefing up of the hull to withstand grounding loads. On hull rudders (harry) need neither. Small rudders (weight to lee type) need daggerboards, which are another case and more beefing up. The large kick up rudders on harrys mean no daggerboard is required.
The weight difference is exacerbated when the breeze gets up enough for the Pac type to pump water ballast.
The bigger the proa, the less these differences matter. But up to about 70' the harry is lighter. And more comfortable.
Again, these can be designed out, but the result is the same as for the cat. You end up with a harry.

There are also some major differences in handling and comfort related to small windward hulls. By small, I mean hulls which are designed for a particular load, then have more weight added, such as ballast. These are fine when the hull is loaded/flying, but in the lulls is draggy. Which is why the Pacific Islanders used easily movable human ballast, rather than supplies, rocks or water.

Intelligent Infusion is how all one offs will eventually be built. No reason it couldn't be used for cats, especially plywood ones.

 

Helm balance spreadsheet

Some valuable insights, thanks all, but I'm beginning to think the intuitions and rules of thumb are not applicable to a catamaran.

For example the rule about CE leading CLR by 5 or 10% is really all about compensating for the upwind torque a monohull experiences when it heels. In the case of a catamaran there is negligible heel so that torque does not exist and CE and CLR ought probably to coincide, when viewed side on.

But CLR and CE seem to be impricise indicators. Are they centroids or c/4 points or what.

So I propose to build a mother of a spreadsheet with the following calculations:

1. Lift and drag of centre board foils
2. Lift and drag of rudders
3. Lift and drag of windward hull at say 1 and 3 degrees of heel and 1 and 2 degrees of pitch.
4. Lift and drag (resistance) of lee hull at same angles
5. Rotational inertia of the boat about a vertical axis through the LCB. Energy is absorbed by the inertial mass when the boat is rotated.
6. Prop drag
7. And try to pin point the CE by summing the lift and drag of the wing mast, the fat head main and the tall blade jib. Not forgetting the CE moves forward and leeward.

But wait, there's more. All these items are forces, what's needed are moments.

Each force is associated with a radial distance from an arbitrary centre of rotation, probably near the LCB.

So this proposed catamaran helm balance spreadsheet should sum all the moments.

Be pleased to have your opinion.

Michael

PS Someone asked me to start a new thread, bit reluctant because I'm flat out building all day and designing into the night. And maybe the nautical term for auto-didactic catamaran designer is follie-a- deux.

Just for fun I thought you might like this picture of half the cabin top shell, ready for laminating. I just love plywood, epoxy and nice screws.