Horstman scaled

I agree with what Phil said.
"Making the rig larger and hulls wider means you don't like the design---so get another one."

 

If I do not scale up by %1.5 to %2 my hull will drop below 8.5 to 1 assumming they were designed at 8.5 to 1. I dont know if an inch in the dinette area or births will make much differance, but why not add an inch instead? I can trim down the frames to adjust for material but then I lose better than inch of width in the interior.If I scale about %2 L/overall B then I have the boat as per designed when adjusting for material, the boat I want.If I scale % 2 then why not %5 and use up some of my extra material.I dont see where streaching the over all beam, not the beam of the hulls from 19'1" to 20' would be any problem.Ed,s 31XR had a 20' beam, the 32XR had a 19'1" beam just like the 31'. I would feel all right scaling %2 in length only and keeping the over all beam at 19'1", this would keep me below 32'. My thought is by scaling both directions I will get the same boat with just slightly skinner hulls (8.8 to 1). My main concern with scaling is how much it will add to mast height and sail area. I am thinking that I would,nt have to add much to either using a sq top main and factoring in the skinner hulls. Rick

 

The Square cube rule

The reason large boats are thinner than small boats is because of the square cube rule. If you double every dimension of a boat it becomes 16 times more stable. So you need to increase the beam less. Also by increasing the length of a cantilever beam will increase the stress in the beam cubically (doubling the length increases the stress 9 times). It is almost impossible to capsize a cruising cat 60 foot or bigger as they are so stable. The rigs blow off before they lift a hull.

This is why I am shy of doing much with the beam. If you add extra to the beams - unis top and bottom that would be good but it would not be 5% extra. You have to engineer the beams again with the extra beam and recalculate. Not too hard but you should do it.

Going longer will almost always make a boat better to sail. Going fatter and wider is not necessarily so. Scaling effects are not linear. A 12ft Cherub is about 35% as wide as it is long. No 24 foot sports boat would do the same. A 70ft cruising cat will have a length to beam ration of 15:1 minimum. You can't do the same with a small cat.

5% is not much but I don't like making the hull fatter or the rig bigger.

cheers

Phil

 

The hulls will not be fatter, only spaced father apart.The tristar beams are I beam construction, or you could say a big box beam, very strong. As the tristar 31XR had a 20' beam and the same basic design I,m not worryed about going to 20' on the beam. I am more worryed about cost factor to my sail plan if I need to increase the mast height because of the scale. I can live with 5% added to my sail area if that is all it takes to keep the boat at designed speed. A longer boat with a wider beam and short mast should be a good thing for a cruising boat . Rick

 

Nothing a bowsprit and code 0 couldn't cure.

 

The lines plans on many boats are drawn to the outside of the planking. You adjust your mold/frame size inward to compensate for the planking thickness used.On some of his designs Dick Newick suggested the option of building with the lines shown used for the inside of the planking. That boat ha a 1/2- 5/8" skin, on the long slender shape that modification resulted in 1" more freeboard with an increase in payload to bring it to the load waterline. Your own pounds per inch immersion can be found through the area of the waterplane at the waterline. The amas have some area immersed too. In reality you won't be able to tell the difference from a 8.5 beam to length ratio from a 8.8.

 

Manitou is a beautiful boat and quite fast considering what she is. Gunnel to gunnel cabins and standing headroom are always nice. Her owner is a great guy as well.

 

Mine will be about an inch as well Cav, I will start by scaling length 5% and study on the overall beam. No change in mast height , may be some added S/A .Rick

 

Phil said:- " It is almost impossible to capsize a cruising cat 60 foot or bigger as they are so stable. The rigs blow off before they lift a hull. "

I don't think it is a matter of wind overturning a big catamaran.
It's more like a big sharp edged wave doing it.
Cats are less susceptable to doing this as the wave throws up the waveward hull and then quickly passes under the boat and lifts the downwave hull to correct the tipping. This results in an athwartship acceleration, or jolt, which is more unpleasant to the crew than risky to the boat.
Trimarans, particularly ones with low buoyancy floats, are more dangerous in this regard as the downwave float may have a tendancy to dig into the back of the previous wave if the frequency between the waves is too short.
We experienced such a night in a Summerville race on Lake Ontario.
The waves were 3M high and close pitched.
The boat (a Buccaneer 28) handled it much, much better than the crew.
I believe Richard Woods had such an experience in his cruising catamaran.