35' cat concept for the inside passage.

Not sure what an Arab horse is good for but if you hooked one up to a plow you'd surely have a wreck in about a split second. Its always been my favorite part about guest riders who bring Arabs to the show. Always a great wreck. Same for mules. Oh do I ever have some stories about all those trail rides back in the day. What a great time that was.

OK but if I'm going to build something and I'm not particularly concerned about the skill level involved then I'm likely to want to build something with the most favorable characteristics. I'm OK with Chines but if this rounded form is best and someone wants to model its COB and COM then I could work with it. As it is I don't have the programing to work that particular hull form into my numbers, so for now I'll just go with the simplest chine form so its easy to calculate by hand.

 

Groper: Can you explain the flatter aft section hydrodynamically?

I understand there is some speed increase associated with it, but do not understand why. Is there a little more displacement back there as well, for weight distribution reasons?

 

yah I could go for that little bit of extra buoyancy aft by that flat section. I think it was Steamer who I worked with to get my monohull design down. He mentioned that rounded sections aft of about the forward 1/3 made no appreciable difference in efficiency.

 

The flatter section is not restricted to just aft... Transitioning to rounded hulls for most modern multihulls, is about increasing the volume in the hull for living spaces NOT for hydrodynamic reasons. Sailboat racing is about extremes in hydrodynamic efficiency so the flat bottom carries on much farther forward you see...

Ive been trying to explain the hydrodynamics in my previous posts, my communication skills are obviously letting me down... but ill try one last time;

Wave drag is largely and directly proportional to the waterline beam, so keeping the waterline length/beam ratio as high as possible makes for an easily driven hull.

Viscous drag is directly proportional to the wetted surface area of the hull, so minimizing this area for a given displacement, again makes for an easily driven hull.

A flat bottomed hull, with vertical sides or as close to this as possible in cross section (such as a flat bottom U shape like the americas cup 45`s hulls)
has the least resistance because;

Firstly, it maximizes the displacement for a given length and beam hull because a square cross section will displace more water than any other shape when constrained by length and beam (and draft for that matter). But particularly the waterline beam is most interesting, because remember the beam is what largely affects the wave making resistance.

Secondly, you also get the minimum surface area for a given set of displacement, length and beam constraints. - The perfectly round bilge hull will have less wetted area, BUT in order to get the same displacement as the flat bottom U shape of the same hull length, its always wider in waterline beam.

Your constrained by length in most cases as people have financial limits to which they can afford in size, marina berths etc. And you cant make a hull very skinny but very deep, as this shape creates much more wetted area for less displacement, so what you gain in terms of wave drag - you loose in viscous drag.

Therefore, the flat bottomed with vertical sides shape allows you the best combination of minimum viscous and wave resistance by allowing the smallest waterline beam for a given displacement and wetted area.

Another example of this, and much slower speeds, are the HUGE oil tankers and bulk carriers... They have a very square cross section, flat bottom U shape. Now think about it; these boats need to be as efficient as possible as it directly relates to profit. So in order to carry as much "stuff" across oceans for the 25years theyre in service, every 1% in efficiency gain adds up to HUGE amounts of money in saved fuel costs. Why dont they use a pure round bilge hull? Because they could not carry as much "stuff" as efficiently...

Does this make sense now or...?

 

Ahhhh, now I see what you were driving at.... but I still contend that viscous drag is not significant in terms of a recreational cruising hull... The other points are VERY VALID...

 

Excellent! Thank you. Where you say "secondly" above really locked it in for me. I suppose we don't see this shape so often on cruising cats due to pounding/slamming issues, despite the efficiency gain... So round section bilges are the best compromise there?

 

Yes, thats the reason for the transition into the round bilge in the bow, slamming and god knows what other reasons i cant think of at this minute... In the midbody, where most of the accommodation is in a multi, a round cylinder provides the absolute most voluminous space inside than any other shape, much more than a vertical sided hull. I beleive this is the main driving reason (and best reason) to use a round bilge hull design in a multi.

Masalai, wetted area is very important regardless of the boats use; have a look at this graph below, its from a rough design i threw together for Bostons cat hulls modelled in Michlet.


You can see each component of resistance, and the top line is the sum of all components, Rt.

Rw = total wave resistance
Rwtrans = transverse wave resistance
Rwdiv = resistance of diverging wave system
Rv = viscous resistance
Rh = transom induced resistance

You can see that just below 7m/s (13.6kts) viscous drag becomes the largest component of drag, larger than the sum of all the wave resistances. The grapgh is divergent above this speed, so the faster you go from here up, the larger in percentage of total drag it becomes.

Around 10kts, the wave drag is the largest component by a considerable margin. Within this set of design constraints, wave interference drag is also greatest around 10kts (this isnt shown in the above graph, i have another one for that) which is unfortunate as this is the speed Boston wants to cruise at...its only a rather small component, but none the less its there...

Just for interests sake bos, the total displacement is 6.4tonnes and i assumed the waterline beam of each hull to be 840mm. 10.7kts (5.5m/s) requires 3kN of thrust which = 31.6 shaft horsepower assuming 70% prop efficiency and ignoring all other forms of drag including air resistance, appendage drag and more...

 

interesting stuff. so at 35' and considering for the round bilge form whats my optimal speed. I take it its not going to be derived by the standard hull speed formula. I'd think partly because I'm at such a low beam to length ration and also relatively shallow draft.

My hearts not set on cruising at any particular speed other than what leads to the least fuel consumption. Oh and on the latest forward view the waterline beam of each hull was ~42 inches or 1067mm and a rough draft of ~ 14" or 356mm. The numbers are highly subject to optimization tho and again nothing is set in stone. I'm just trying to find that middle ground were it looks like a cat and the numbers work out favorably.

Cheers and thanks for running some numbers.

I can only get so accurate with the weight distribution calculations without knowing what my hulls final configurations are. So getting that pinned down really helps to get a more realistic look at how the whole thing might come together.

 

The narrower you make the hulls, the more efficient they will become up until the point where the draft exceeds the waterline beam of each hull.

With a waterline beam of 1067mm and 356mm draft you only have a length/beam ratio of around 10:1 and you will have a high wave resistance - which from teh graph you can see is the largest component of drag at 10kts. So from an efficiency point of view, these dimensions are not good.

What you need to do boston, is decide what the absolute minimum width you can tolerate inside your hulls is going to be. So what spaces do you need in there to fit engines and bathrooms or whatever you want in there. Once you know the spaces you need, THEN we can look at hull shapes and so forth to see what will work most efficiently for you.

To provide an insight, if you decide you need a considerably wide hull for roomy accom spaces, the best shape for efficiency will be a round bilge hull - but you may not like the look of that...

On the other hand, if you only need a narrow space in your hulls, we can go for a very efficient design... Itll have almost vertical sided, narrow waterline beam hulls with some flair above the water line to squeeze a bit more volume into the inside of what will be still be a more cramped hull than the roomy one above.

I would assume you dont care whether the draft is 300mm or 700mm? unless you plan to operate in very shallow tidal flats areas that is...?

So, how much space do you need in your hulls to fit what you want in them?

 

I had them at a tad over 30 inches earlier ( in your 840mm range ) and liked them a lot but I seemed to be alone in that, so I fattened them up some. I'm completely happy with damn narrow hulls though.

issue is 2'6" is pretty inefficient use of ply but oh well, plenty of stuff I can do with the cut off.

I just fattened em up some according to all the folks telling me they were to thin, its funny actually cause I kinda liked it the way it was but changed trying to keep the peace, then once I had thought it kinda looked good a little fatter

I had them at 32' which is about a hairs breath away from 840mm in the first place. so ya, looks like I'll just go right back to it now eh. Kinda funny actually that the way I wanted them in the first place turns out to be just about right.

Ok well lets bust out that eraser again

going back to the 32" wide hulls. I'm OK with narrow hulls as I've got plenty of room up high. I'll cram the head into the hulls as well as the engines one way or another, storage, fuel, mechanical, once I know how wide the hulls are I can chose stuff to fit.

this is why I went with the hull shape first, once I figure out what works most efficiently in form then I'll jamb all the stuff into it.



slap a rounded bilge on that square form I have now and you've got that 840mm wide hull you mentioned and I've long ago done the drawing, only needs a few modifications to reflect the new sheer line

time to bust out the almighty eraser again I guess.

 

Good answers Groper.

The drag curve you show is very similar to the one I got in a test tank when, as a design student 35 years ago, I did some testing of 35ft catamaran hulls.

You can see my comments on hard chine hulls, and indeed on hulls in general on the FAQs page of my website.

It's usually optimum to get the BWL about twice the draft

I prefer to add flare to the hulls. Several reasons. Roughly in order, it adds interior space, it increases deck width, it reduces the slab sided look, it reduces wetness on deck, it reduces heaving in waves, it allows for better loading.

My first flat bottomed hull was the 22ft Janus designed in 1981. The 28ft Gypsy from the 1990's had the option of round bilge or flat bottomed. The vast majority of builders chose the latter option. Mind you it has a Veed hull forward to reduce any slamming, but in fact slamming was not a problem in practise

Cargo ships go so slowly I don't think you can compare them to small boats.

As I and others have said, power boats of whatever configuration, need more buoyancy aft to counteract the stern squatting and the weight of the engines, which in Bostons case is going to be considerable

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Oh boston, your still not getting it... now youve got a narrow hull WITH chines...

If you want maximum efficiency, use the narrowest hull you can tolerate, and put this bottom on it;


This shape is very similar to the model i created to run the resistance simulation in Michlet BTW... the numbers were pretty damn good considering no optimization was attempted...

 

That last drawing (by Groper) doesn't look right to me. I am not a designer and I have no hydrodynamic software (or time) to do any modeling but I can tell this hull would do one thing poorly. Sit at anchor in rough conditions.

It would be *very* loud inside as those hard sections pound up and down.

Thoughts?

 

The very front tip of the bow, should still be 4inches or so underwater... if you have significant waves at anchor, remember that the flat bottom is only the tip of a triangle up here and so has a VERY small flat surface anyway...

This is richard woods cat with a very similar type of hull;


I think the hull type, would be suited both in function and form to bostons general idea...

 

Thank you for posting a photo of my 28ft Gypsy design. That was a photo of my own boat in the Scilly Isles, UK taken about 15years ago

I think Catbuilder means slamming on the topsides rather than on the underside of the hull

Slamming like that is certainly a problem on those monohull-cut-in-half powercats that Moorings(?) Sunsail(?) offered

Another reason for having hull flare

Richard Woods of Woods Designs

www.sailingcatamarans.com