Balance considerations for Boardless Beachcat

Hello everyone,

I am generally aware of the interaction of rudder, daggerboard, sails and the hull for balanced behaviour when sailing.
If you aim for about 4-5% of the sail area as the lateral area of the daggerboards on small dinghies, on modern racing catamarans you are more likely to be in the region of 2% in total, i.e. more or less 1% of the sail area per daggerboard.

What about a boardless catamaran with a V-frame and possibly a skeg?
How large should the combined lateral area of the hulls and the skeg be in relation to the sail area?

Positioning of the skeg.
I assume that once a final hull shape has been determined, the skeg is more or less positioned to get the resulting centre of lateral resistance to the desired longitudinal position.

Are there any other considerations that need to be taken into account with a boardless beachcat / significant differences to boats with daggerboard?


I would be grateful for a few tips on this topic and statements on my assumption regarding the positioning of the skeg to achieve a good balance.

Many thanks and best regards
Nicolas

 

I suggest an empirical approach: get hold of profile drawings of boats with those features (Topcat, Dart, G-cat, several Hobie models, there was a Nacra model), measure the area, and work out its geometric centre relative to that of the sailplan. You can make some adjustments to that relationship through mast rake.

How much do you want to use the rudders for leeway resistance? If you can adjust the rake of rudder blades precisely, and if they are large enough, you can make them take about half the load, but still be able to steer with two fingers. Then your hull doesn't need to be that deep. If you can't balance the rudder that precisely, then the hull and fin will have to do most of the work or else you end up with such a load on the tiller that the boat won't be fun to sail.

 

Dear Robert,
Thank you for your reply.

I have already started on the approach you suggest. I have collected about 10 different plan views of various boats from 17 to 14 feet in length and will work out a list of different dimensions.

I would be able to design the rudder system to allow precise rake adjustment.
Are there any general recommendations or rules of thumb as to how much of the leeway resistance should be taken from the rudder, because of ... ?

I have already checked the plan view of a Topcat K4X and assumed a DWL so that the transom is slightly submerged. In this configuration the lateral projected area of the hull is about 1m² / hull with a sail area of 13.75m².
My design should be about 10m². Using the same ratio, I would need about 0.72 m² of lateral projected area for the hull including the skeg.

Below is a plan view of an early design stage. In this configuration the hull + skeg has about 0.7m² of lateral projected area.


At the moment, the centre of the lateral projected area is quite far forward and would probably need to move further back. But it can still be done.
The bow sections are very sharp at the moment, and I'm a bit worried that this is going to make the boat difficult to tack.


Best regards
Nicolas

 

If you want to use an existing rudder system, dotan.com has one model that is 845mm long from the lower edge of the rudder box to the tip of the rudder. And judging by this video, they are reasonably strong:

My experience with boats that rely mostly on the rudder are a Hobie 14 trimmed for racing with enough mast rake (mast tip about two thirds towards the rear beam when the boat sits on level ground) that I think the rudders took more than half the load, and Matt Layden's Paradox design. Layden estimated the rudder took 50 - 70% of the lateral load. On the Hobie, if I screwed up a tack, the rudders might stall and I would have to reverse to fall off. The Paradox has a rudder about twice the linear dimensions, and four times the area, of what you would generally expect on a boat that size, and I never had the same problem.

A second factor is that if you rely on large rudders for lateral resistance, but the water is too shallow to put them down, then you have quite extreme loads on the tiller, and you need a fair bit of speed to get enough rudder authority. So you would have to sail on a reach until the water gets deep enough to put the rudders down. If the rudders are very deep, they would also kick up in water that might be too deep to jump in and control the boat if there are some waves. And seeing that your design presumably should be able to sail through surf, that could be a problem.

I saw your Novacat 16 in the other thread. I guess from the lines that this new design is not intended for plywood, but to be built in composite in a mold.

If you want to broaden the range of shapes to inspire you, I drew from memory a catamaran I saw on a beach in France c. 1985. No idea what make. The inside (towards the centreline of the boat) of each hull resembled the Hobie Max, the outside seemed to be intended as a planing surface:


I never saw the boat sail, so I have no idea how well this worked.

The Hobie 14 has fairly large rudders for its size, and I could tack it in 8 - 9 seconds in 10 - 15 knots of wind, from first rudder deflection to hiking out and sheeted in on the other tack (dinghy sailors giggle uncontrollably at this point, or shake their heads in pity). Part of the technique is to shift weight aft, to shorten the waterline. I once had opportunity to sail a Dart 18, which had been described in print as tacking better than the Hobie (presumably Hobie 16, which doesn't tack as well as the Hobie 14). I tried a range of rudder deflections from more to less than I would use on my Hobie, and it was really slow to tack. I tried shifting weight back, which did nothing to shorten the waterline because of the wider stern. I tried shifting weight forward. Nothing I tried made that boat tack better than my Hobie 14. When I thought about it, I didn't understand why the Dart was supposed to tack well. I thought the rudders were shorter than on my Hobie, the waterline was longer, and the hull deeper. All of this should make the boat harder to tack. But sail with someone who races one of those boats, and then you will see what a sailor familiar with the boat can do.

I think it might be interesting to try a hull shape like on the Cirrus 901 F18 cat, and give it a low aspect ratio keel with wings. A discrete keel, say 30cm beyond the hull, and 60 - 80cm long. That shouldn't interfere with tacking too much. Then put a steel plate underneath the wing, so that when the boat is dragged over the beach, it doesn't leave microplastics behind. The Cirrus 901 seems to aim to combine the lift of a scow bow with the water shedding of a wave piercer. Sounds contradictory, but from what I read, from the admittedly biased designer, it might work. See for yourself:

 

Hi .. thanks for the reply and the thoughts.

I already use the Dotan rudders on my 16. They work pretty well.

The Paradox looks interesting. His so called "Bilge Runners" are actually normal bilge keels and acting as vortex generators in order to produce some lateral resistance. Interesting design.

I guess all kick up systems have this problem with high tiller loads when the blades are up. But the extreme comes with the length of the blade, thats true and needs to considered.

Also this design is intended to be built in plywood, using stitch and glue method. The plate curvature is not that much, as the buttock lines might imply. The forward most section already has a smooth transition between the side and keel plate.

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With regard to balance
I searched the web for some plane views of similar boats, traced them in 2D and did some measurements (assuming the views are more or less scale). I have six boats between 14 and 17 foot in length, all boardless (some also boomless).



Interesting thing ..
When talking about lead for a monohull, CE of the sails is normally located infront of LCR. Where the distance between these longitudinal positions is referred to be a positive lead.
For modern catamarans and also trimarans it is common sense to have 0 lead. So placing the pressure point of the daggerboards directly in line with the sails CE.
For most of the compared plan views the lead was negative, so the LCR (hull + rudder) was located infront of the CE of the sails.

I think it's a good starting point to use the average values from these designs and then adapt the own design further.