CNC Plans not Included

Maybe that's best left for the NA, he is going to charge me the same no matter lol.

The main thing is he is not starting from scratch he knows which parameters are most important to keep.

I bet, a too light boat is every designers dream. They can engineer in stronger heavier beams, make the hulls a little less full (V), adjust rocker for speed or seakeeping and so on.

I read that a decent ratio for cruiser hulls is between 1-10 to 1-13 and after 13 it starts to be race oriented. At 4.5 feet we are at 1-13.33, if he reduces the waterline to 4', for a ratio of 1-15, all he would need to do is add 1.5 inches to each chine and living space is not affected.

 

Yeah that is another reason I like the addition of the chines. Might need to warp a little more too, though the SIG and the Gunboats look pretty flat too.

Mostly outer, I did not compensate for thickness but I did note it in my mind as I drew so the interior will still work, it will just be custom.

For example, kitchen cabs won't need to be 24" deep but the counter will, and the queen can be 6'4 long instead of 6'-8, I wouldn't be using a std mattress.

Headroom in bridgedeck bunks is plenty so we can lose some and it doesn't hurt to raise the roof either but I wouldn't drop the bridgedeck. It could be cheated both ways with a good 3D software.

 

As to my question about using water for ballast, I found it's been done. http://hydroptere.com/en/the-sailing-boats/l-hydroptere-en---3d-view/

http://www.sandiegouniontribune.com/sdut-sai-americas-cup-073009-2009jul30-story,amp.html

http://www.pacificproa.com/articles/dave_culp_water_ballast.html

What a great way to make the boat more stable. I wonder if it's also been done to trim fore and aft. You could counter some of the force that drives the bows down and design them to be thinner. When you aren't dogging it, just pump the water to the forward tanks so as not to drag the transom.

Never mind, found this- On October 27, 2010, in the Multihull Anarchy forum, Ian Farrier said:

Next step is to add water ballast to stern, so as to keep main hull stern down when required,
and a stern ballast tank is standard on the F-85SR and F-32SR, with float sterns tanks optional.
This weight again can be got rid of for the 90% of the time when it is not required.

I want water ballast!!! lol

 

Your first discussion with the NA should be very very interesting.
Look closely in his eyes and you will see his bill increasing by the minute.

 

Ok ok scratch the water ballast ))

 

With 4' waterline it actually works out pretty nice, good call!

I made lower chine larger, it's 3 inches, nothing else was affected, and now we have 1-15 hulls!!!!!





Showing waterline only

 

looking much better

for a very small (incredibly small) increase in drag you gain a lot more stability. If you float high on your lines, add more toys and boys (crew).

 

I'm not following how floating high adds stability? If anything I would have thought raising the CG by floating higher would reduce stability.

 

hence "if you float high --"
if, God forbid, the boat turns out heavier than designed at the bow, its extremely difficult to rectify the problem - what DennisRB was warning about.
Sterns are much easier to fix.

 

I totally agree but not sure stability is the right term to describe that.

 

That was a different problem, buoyancy was strongly biased to the stern, having the cat at a constant 4.5 feet waterline might float her too high but adding weight would sink her and add stability. I read this in one of Wharram's papers on stability.

But then you would have to compensate with a bigger rig etc... and since I am going electric drives, that extra weight won't help.

This 4' waterline feels better, big boat with barely any fit out and no salon, she should be pretty light as boats go.

 

ok, balance then.


this boat seems somewhat similar in design targets
http://www.multihullcompany.com/Catamaran_for_Sale/LINDENBERG_65/FORCE

 

Thinking about it more, I guess having very fine forward sections coupled with too much weight forward would reduce dynamic stability and increase the likelihood of diagonal capsize.

Having had a boat that tended to go bow down was a pain. I actually planned bow extensions which is a big job. With no weight in the boat it floated OK but with full tanks and cruising gear it went more and more bow down which was irritating.

 

yes indeed. incline the hull bow down in 1 degree increments. there should be a dramatic increase in cross-sectional area with commensurate increase in
resistance at the bow - with the sails still driving forward from the mid section, not a good day.
This is just an example -- gray area indicates under the waterline
left: as designed cross sectional area under the fwd main bulkhead 0.1889 sq m
right: 5% bow down .5787 sq m

 

Different ball game here...

We have a 50% mid ship mast, long hulls for the displacement, Ie favourable DLR ratio, and no bridgedeck house. Should be easy to design this for minimal bury of the bows- any good sail boat designer will be all over this these days...