Scaling a Jarcat up by 4 percent?

There is an Australian trailer sailer catamaran called the Jarcat 6 which is 2.48 meters in width to meet Australian towing regulations.

Here in the US the maximum width is about 2.59 meters, which means an American Jarcat could be made about 4% wider than an Australian one. I have read that small increases in beam produce relatively large increases in stability, so it seems it would be desirable to build the boat a bit wider. Is this logical? Or is the extra 10cm not meaningful?

I can think of a couple of ways to increase the beam...

A. Add 10 cm to the bridgedeck. This is simple and the boat stays mostly the same.

B. Scale everything 4%. This is more complex because everything changes, including the rig. It also seems like more of an issue with scantlings.

The designer of the Jarcat has passed on, and the plans are now sold by his son, who is a structural engineer but not a naval architect. I asked and he thought widening the bridgedeck would be fine, but on the subject of scaling the whole craft he demurred. I heard there was a Jarcat that broke up - it had been scaled 30% in all directions without any increase in scantlings, so maybe that explains his recitence.

So is scaling this design up by 4% likely to be OK? I have seen similar scantlings on a cat that was 20% larger: 6mm ply, 20x20mm stringers, ~20x40mm chines and gunwales. That said, the Jarcat seems more lightly built with fewer stringers and frames. I have attached a photo of the framing as well drawing of one of the bulkheads (this drawing comes from the study plan in the files folder of the Jarcat group, so I assume it's OK to share here). The distance from the partial "V" shaped frame to the adjacent frames is about 1.2m. The distance from the frame behind that to the penultimate frame is almost 2 meters.

Scaling is appealing because the stability improves as well as the loading capacity. The volume goes up with the cube, but the shell weight goes up by the square. I calculate useful load at dwl to increase from 742lbs to 871lbs. Of course there is higher plywood utilization and higher outboard power requirements as well.

 

Hi @Juno7, welcome to the forum.
In my opinion, increasing the beam doesn't necessarily require a change in the hull scantlings. It would, however, require a check on the crossbeams.
The increase in load entails further changes as the draft also changes. The distance from the waterline to the wet deck, which will decrease, could be less than advisable.
On the other hand, I would significantly change the design of the junction of the crossbeams with the hulls because, in the drawings you show, it appears that the area with the greatest loads is the weakest of the entire beam.
In summary:

- Increasing the beam doesn't necessarily require changes to the scantlings. But it's better to check them.
- The other changes require a review of the scantlings.
- The embedding of the crossbeams in the hulls should be redesigned.​

 

Juno7. The increase in beam will only have a marginal effect on stability EG you may be able to sail in half a knot stronger winds before reefing. If you want an increase in internal space OK. The scantlings should be OK as a Jarcat 6 can carry EG 4 crew and the beam increase would be the equivalent of having all 4 crew sitting on the wind wood rail versus having the crew spread on both sides of the cockpit. The downside of widen the cat is the build allowed the use of 8 x 4 foot sheets of ply but at 8.5 foot you may have to scarf addition ply onto some ply sheets. If you are going to make it wider do not scale anything else. Jarcat 6's perform very well as they are designed and if well built with good materials last well. The guy who designed the Jarcat 5 and 6 had an excellent understanding of timber and ply, he designed a structure that could handle its intended task. These boats are excellent bay and near shore craft, they are not offshore boats. Hope you enjoy the build.

 

4% increase in beam will have approximately 4% increase in stability. You are getting 4 inches more deck space, but adding a lot of extra labor.

 

How do you quantify stability? How can you say one boat is 4% more stable than another? Frankly, I don't understand. Any explanation that would help me understand? Thanks in advance.

 

I assume the weak part is the area labeled junction "A" in the drawing? This looks less than ideal to me, however there are many Jarcats on the water and I haven't read of issues. Perhaps the skin carries the load? I have seen issues with the bulkhead cracking at hull bottom in several boats. How would you increase immersion?

 

Also:

Load would decrease relative to the increase in volume, so the "distance from the waterline to the wet deck" would actually increase slightly, right? At least for a given load?

 

"If you are going to make it wider do not scale anything else."

Ok

Yes, plywood utilization might be an issue. The added space of simply widening the bridge is attractive, and 4% improved stability is something.

 

I deleted my previous answer to this because I didn't understand the question. I guess what I really mean when I say "increased stability" is reduced risk of capsize.

Better initial stability and higher peak righting moment seem like good goals, although I know there's more to it than that. I do know model sailboats seem to have much lower stability than their full size counterparts.

 

Maybe I didn't understand you correctly, but if you increase the weight from 742 lbs to 871 lbs, the draft will increase, and therefore the air gap will decrease.
I would add something similar to the figure. But keep in mind that this is just an idea about the possible configuration of the beams and their connection to the hulls. The final shape would depend on the results of the hulls. This, I repeat, is just an idea.
Abandon the idea that stability will increase by 4%. That is completely incorrect. Whatever parameter you use to "measure stability," it will never be directly proportional to the beam.

 

Tansl,

That seems like a sensible modification. The frame in question is amidships and there is more space than is needed to get to the bunk below the cockpit. You can see in this picture Andrew Stokes repairing that frame in his slightly rotten Jarcat. Cracking of this ring frame at bottom center is common, but not at the hull-bridgedeck connection as far as I know.

Maybe I wasn't clear about loading, here is how I came to those numbers. Not saying this is a good idea or the numbers make sense.

Dry weight of the Jarcat is allegedly 858 lbs.

Displacement at DWL is 1600 lbs.

Assuming I scale the entire boat (xyz) to 104%, displacement at DWL becomes 1.04^3 x 1600 = 1800. And the structure, assuming it's just scaled up with the same 6mm ply, would be 1.04^2 x 858 = 928 lbs?

So initially we had (1600lbs at DWL) - (858 lbs dry weight) = 742 lbs load capacity.

And at 104% scale it's (1800lbs at DWL) - (928 lbs dry) = 872 lbs load capacity

I assume the 104% cat would have 104% clearance to the bridge at DWL.

 

I don't want to convince you to make any changes to your boat. I'm just putting forward ideas. It could be that the hull's skin panels are very strong and transmit the loads on the crossbeams to the bottom, and that's why the bottom cracks. I don't know, I can't say anything for sure. All I'm saying is that I would start studying the overall loads with a structure similar to what I've drawn, but I don't know the final result. There are many factors that influence the scantlings, and one of the main ones is how the structure was designed, which elements are considered primary and which secondary, and a long list of other considerations.
Giving advice without information, as is often requested, isn't very intelligent, so I'll keep quiet.
One thing is certain: breaks and cracks in the hull do occur, and we'll have to try to figure out why.

 

I think it's due to point loading on the trailer, but I don't know. The designer encouraged the use of the boat as a mobile caravan, which probably doesn't help. In the case of Andrew's boat there may be substandard materials (eg 3 ply instead of 5 ply) and construction techniques. The frame, for example, doesn't seem to be properly bonded to the hull. In other boats I've seen a fine vertical crack just above the keel. The recommendation of Don Nicholson, who is the support guy, is to add a second layer of ply in that area with the grain athwartships.

Regarding the load on the hull-bridgedeck interface, I assume the loads are taken up by other parts of the boat.

It sounds to me like the Jarcat structure is possibly marginal in some regards, but sufficient. As it has been proven over many years and many vessels it's probably safe as is but maybe not wise to scale up.

 

Do you have good discipline when it comes to keeping the boat decluttered and the weight light?
If no, I'd shy away from increasing the righting loads the structure and rig sees.

 

If you increase the boat on all dimensions, you are also increasing stresses and the structure has to be strengthened accordingly.