Catamaran Hull Design Question

[weatly28]

Deep thin hulls. Water slicers. I made a 20" long model cat r/c boat, 5" deep hulls,1/2" thick hull(side to side). Hull separation/beam was about 15". Compared to some other standard hulls tested on the same chop it handled very nicely,slicing through the waves. One thing I did notice was the hulls almost act as keels eliminating side to side movement/forces. I wonder how this would reflect in full size model. I have been wanted to go full size for further testing. Anyone have any info/experience on this? Thanks for any info in advance or pointing me in the right direction.

[DarthCluin]

A while back I found a copy of "Modern Sailing Catamarans" by Robert B. Harris, N.A., in Parker's Books in Sarasota. It is copyrighted 1960.
In the first chapter he describes his first catamaran design, Naramatac (spell it backwards). "...a 25 footer with little curvature on the outside, deep narrow sections terminating in flat bottoms, and a long flat keel profile...The attempt to obtain "lift" without the use of hydrofoils such as centerboards...very high length to beam ratio of 22:1...Her all up weight was 2,240 lbs. (with 4 crew), datum water line 22 feet, overall beam of 9'-2", and she was entirely closed in with 3/8" Douglas Fir plywood over 3/8" Douglas Fir plywood frames, with 3/4" x 3/4" spruce cleats."
In the final analysis Mr. Harris concluded "At all times the abundance of wetted area due to her fine lines added greatly to the total resistance. The flat bottoms pounded going to windward, especially the weather hull when close to the surface. She was slow in tacking due to the long flat keel, and partially to the bottom chines. the chines also created eddies along her entire length when rolling, which added to the resistance."
I have attached a couple of scans. The scan of the lines is a little rough. My current multifunction printer does not have a removable lid, so its tough to fit big odd shaped things like books in there.
If Naramatac had been lightly built with more beam, it might have performed a better, but wetted surface area would still be an issue.

[weatly28]

Thanks for the info.I will search more on that. I assumed you would have to give up something to attain the slicing effect through the waves without going with hydrofoils which is my dream but much harder project to build/compute. I would say the slicing of the waves would be worth the added resistance(wetted area). Its a powerboat anyways. I love the way it cuts/slices through chop. Getting on a plane is another story, I don't think this boat could do that without an immense amount of power. 10 knots is fine with me cutting through all that chop.

[themanshed]

Check out the G-Cat's they have symmetrical thin hulls. The G-Cat was once rated by Randy Smythe as a Cadillac ride when compared to other beach cats.

[Guest625101138]

Originally Posted by weatly28

Deep thin hulls. Water slicers. I made a 20" long model cat r/c boat, 5" deep hulls,1/2" thick hull(side to side). Hull separation/beam was about 15". Compared to some other standard hulls tested on the same chop it handled very nicely,slicing through the waves. One thing I did notice was the hulls almost act as keels eliminating side to side movement/forces. I wonder how this would reflect in full size model. I have been wanted to go full size for further testing. Anyone have any info/experience on this? Thanks for any info in advance or pointing me in the right direction.

Are you wanting to make a power cat or sailing cat?

If power cat what length and speed are you looking for?

An L/B of 40 is extreme. It is likely to have more drag than the lowest drag hulls for the job but it is worth comparing.

I expect it turns as if it is on rails. This may be a problem with higher centre of gravity and fast turns.

Rick W

[weatly28]

Thanks for the info, I checked out the Gcats. I would not make power cat at this time over 20 feet. So 20 feet would be the ideal size for the project.

There are some similar model hulls in the direction I am going. All leading to less and less hull area.

1. Deep Vee Cat
2. SWATH ( Very deep vee cat with foils for ride adjustment and leveling)
3. Hydrofoil

The deep Vee cat will fit my needs( budget ). As much as I would like to make a SWATH it would take a considerable increase in time and budget.

I will make some adjustments to the model with SWATH type foils and see how far I can get that deep vee out of the water and reduce drag. I will post video and results as they come.

Again thanks for all the info.