35' cat concept for the inside passage.

Wider beam = heavier bridge deck/cross beams = larger displacement = more HP.........

 

Everything I design involves an organic process. Its got to be able to evolve and the folks in on it gotta remain open minded. Specially when I'm working in a realm I'm not used to. I'll eventually include all the necessary design elements but thats no guarantee they assemble themselves in a generally recognizable form all the time.



300 gallon fish tank, that particular one was built as a terrarium, but I did about a half dozen as fish tanks, slightly different stand, much thicker glass, its got a glass shelf inside so there are sorta two bottoms on it, and one of them has even got a submerged light under the shelf to light the bottom better.

 

Bos, have you tried adding a bit more angle to the inside hull wall and gusset to bridge deck? You need some more volume in those hulls...

 

I haven't but there's a logic to that even if seems to not make much sense to the standard cat design.

vertical sides are more efficient, in terms of energy conservation. If you look at the latest of the America's Cup contenders both boats had vertical walls on there hulls. Bolger style hulls follow this same basic rule. A lot of folks have mentioned flaring the hulls to create a more usable space within but so far it looks like all the flair I need can be had in that 45° connection between the hulls and the tunnel ceiling. I was also thinking of not having just a few beams but a series of trusses joining the hulls. But I'm a long way off from figuring out the structure yet. Still just trying to hammer out the basics. I tried to make up for the lack of flair with more clearance in the tunnel

The displacement works out to a target weight of x and the reserve buoyancy is about 5x that and again Ive still got about 30 inches of clearance under the tunnel. Lots more than whats being recommended by folks with the stepped hull in mind. I went with the longest skinniest hulls I could trying to get that length to beam down as far as possible.

not sure if that mollifies any of the concerns but this thing is a long way from done so if its a universal thing that folks don't think the direction this thing is taking will work then of course I'd reconsider long before I ever got serious about it.

 

Some comments...

 

the latest incarnation, this ones got no skylight; a sore loss as it would have been a great centerpiece for the ceiling work. No roof area over the aft deck. Less clearance under the "bridge deck" 30" fully loaded 15,800lbs ~33" half load 36" at 10,250lbs. Flat bottom and less frills overall.

I'll throw in a shot of the previous design for comparison, one thing I didn't really like about the last one was the half arch under the roof extension, seemed kinda cheesy. I might extend the roof back out but loose the half arch thing. I need davits back there to lift the launch up onto the back deck so if I incorporate the davits as the supporting structure of the roof extension then I suppose I minimize my weight penalty. Might also add a rail around the roof area and access. kinda make use of that space better. Although that entails beefing up the roof which costs weight again. Hmmmmm long way to go yet. Again the new incarnation is 19' wide and 38' long







oh and this hull design ended up having more buoyancy than less for the same overall dimensions, so I'm pretty happy with reverting back to the Bolger style. Except for the slight flair as it did ad a lot to the aesthetic appeal, Bolger designs might be efficient but there also butt ugly.

The thing about having less clearance under the bridge deck is that it shortens the head room in the hulls, which leads to either a lot of dents in my noggin or gets into this whole spiral where there's no cat walk ( pun intended ) next to the deck house. Not something I particularly care for.

 

some logic on sharpies from one of our most illustrious members

 

From a resistance point of view, the "sharpie" style hulls you have drawn will be excellent - but this is not the issue im trying to raise with you. The issue is usable space inside and YOUR intended use of this boat - does not suit this shape you are trying to use.

High length to beam ratios are very easily driven, but they do not carry a heavy displacement well - you have already said its going to be a heavy boat with everything you want to put in it. So why? These hulls will pitch and heave badly in any sort of seas when used to carry a heavy displacement, they just dont have enough volume to dampen the motion well.

You have already said you dont need it to be fast, so again, why design it with such a narrow (high L/B) low volume hull when you will only be cruising slowly... you can still have your 12kts speed with lower L/B ratio AND get better load carrying capacity AND more usable space within them... If you were designing a different boat with higher speed requirements, then you would seriously consider compromising the interior volume for narrow hulls - but you dont need to do this!

You really need to figure out what you are going to put inside this boat, so instead of drawing front and side elevations, spend some time drawing up a better floor plan or GA, and remember to allow for all the stiffeners and stringers you are going to bang your head on... i think this will help you realize exactly how much space you really need in there...

Youve increased the size of this cat because you are not making efficient use of space within it... be careful here, as what is the point of building a 40' boat that only has the accomodation space of a 35' boat... you will end up spending more on building it, parking it and running it, just to get the same end result... thats the difference between a boat design and a good boat design...

 

Well its gotta look right and since its kinda 6 one way and a half dozen the other if you start on the outside and work your way in, or the inside, and work your way out, I just started where it seemed easiest. Besides the outsides not set in stone any more than the inside. The hulls certainly might change again but for now, as you suggest, sorta, until I get to the inside.

also I'm very concerned with efficiency, I want a boat that's economical to operate

I've got more space than I really need in this thing, I'm just trying to make it big enough to look like something other than a brick and to accommodate some day trippers who want to go check out some whales or whatever.

I've got a floor plan, don't think anyone likes it, but I got one. The latest incorporated that head space trick everyone seemed to like so much. Or at least it did in most places. Its coming up shortly, Just want to get some form of descent idea going as to what I would want the final product to look like.

Your so impatient

The latest incarnation with a few minor changes. One of which I'm sure you won't like but still its just playing with a pencil so no worries. I changed the hull aft from a deadrise to a pure sharpie form. Long before you mentioned it so don't get to irked about it, its just a concept so far. I also altered the lines aft a bit, seems to look a little better this way. I tried a rail and observation area over the cabin house but it didn't look very good at all, so I nixed it. I might consider one later but it would have to have a really svelt railing so its not as much a centerpiece as the nice wood one on the back 40.



the new displacement would be 13,440 fully loaded to get it to float on its lines with an 18" draft.

I'm not thinking of doing to many overnights with guests on board, or at least not charters. I just want to do day trips, eco tours out to see the marine wildlife and maybe some sightseeing. Thats why all the windows and the open floor space. Lots of nice seating forward and also enough extra seats to give folks a chance to pick there poison, so to speak.

I think you might have missed a post made earlier concerning the reserve buoyancy, and your right in that by going this time with a purely sharpie hull plan I lost a bit on the back end. I might just add that back, depends. Course I could always add back that 6" I knocked off the bridge deck clearance. The amount of ply doesn't change at all just the cut off. I'm not a scrap things together kinda guy.

I made a list of the ply I'd chose, again its probably got some flaws in it but its a start. I came up with almost 195 sheets averaging 1/2 in thickness for a weight of 7,215 in just ply. But that doesn't include cut offs or laps so I'm probably as much as ~35% high on that number. Conservatively I'm at least 20% high which gets me down to about 5,772 lbs of ply. If and I stress if this thing gets serious then maybe I'd pin that number down more accurately. But I'd be surprised if all the cut offs and laps didn't add up to at least 20%.

Not sure how that stacks up against most other 39' x 18' cats but thats my first stab at an actual number of sheets. I'm thinking I could build it for <10,000lbs all up, dry weight. If I was lucky. Probably more like that 6tons Mr Woods suggested. But for fun I'd aim for 5T and see how close I could get. I'd need a touch more than the 18" draft I've got now at 6T if I kept the sharpie hull plan, but its all just paper so far, so again, no worries.

One thing that was suggested ( thanks Tad ) was not to go wider. I added 2' to the width a while back so I'd have more room on the catwalk. But that two feet came with a pretty big weight penalty. I bet that ply would work out to <5T if I shrank it back down to 17.5' wide.

 

And supporting Tad's comment, this is a power cat, so a wide beam is not necessary.

 

Well post #231 shows the two widths I've explored so far. Both use the same width deck house but the base is wider by 2' in one of the two studies. I'm still working on the forward view, once I get that hammered out a little better maybe I'll be able to pin down that weight a little better. But just off the top of my head 2' off 3 surfaces each about 40' long ends up being about 8 sheets of ply. So its about 250+lbs in ply and whatever the associated finish is. Probably another 250lbs or so if you consider the structural benefit of a shorter span. But I loose that nice fat cat walk and super stable stance as well as begin to need a forward hatch, something I was really trying to avoid. OK so I didn't get it down under 5T either, but it would save ~500lbs finish weight

That forward hatch thing sucks, its either a hatch in my nice clean roof line or a jacuzzi pit in the deck, both of which are bound to leak. That or skinny forward while just waiting to slip on the seagull crap and end up dangling in the railing, granted you could push a pram down a 30" cat walk but 18" is getting about as skinny as you can and still have space to breath.

 

Although not quite what you have in mind you might find looking at this boat will give you some useful ideas

http://seattle.craigslist.org/kit/boa/2762356623.html

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

hmmmmm well it sure used the full amount of available deck space, No cat walk at all. Its forward access doesn't make itself very obvious, still trying to figure out how they might tie up.

another aspect of the weight issue is the different types of marine ply. This is all from the Boulter site which seems to be frighteningly expensive. For instance elsewhere a full bunk or Douglas ABX marine ply 1/2" full sheets can be had for about $35 each and I think there's 40 in a bunk, from http://www.google.com/url?q=http://...8QFjAA&usg=AFQjCNHE2zlwTHLeXlyDltthWukPDho4gA.
37lb per 1/2" 4x8 sheet of Okoume $135 retail cost A/B BS1088
56lb per 1/2" 4x8 sheet of Sapele $196 retail cost A/B BS1088
49lb per 1/2" 4x8 sheet of Merranti $95 retail cost A/B BS 1088
50lb per 1/2" 4x8 sheet of Douglas $74 retail cost A/B ( not specifies as B/S 1088 )

Still working out some aesthetic details but its pretty frightening when 200 sheets of ply ends up costing well over $20,000.

If I built out of the Douglas ( hate the stuff already ) I'd be ~25% hevier on the ply alone, all else being equal I'd end up being probably ~12.5% heavier overall. 12.5% of 6T = 1500 lbs, assuming no cut offs, minus about 35% for cut offs and it costs me 975lbs to save ~$12,000 in up front materials costs. $12,000 = 3000 gallons of fuel. So the question is if over the life span ( my life span in this case ) of use does it end up costing me more in fuel to move a 12% heavier boat down the bay than what I saved in the first place ~ $12,000 bucks up front. The rub, I'm going to run this thing on waste oils. So the answer would appear to be no, but, its a tricky question. Depends on what my best price is on Okoume, or what would I be willing to hemorrhage on this theoretical project.

 

As we have said before, you can build much of a 36ft powercat in 9mm ply

You should also look at Edensaw and Noahs. I have used Joubert plywood on several boats now and have been happy with it

Here is the photo of a displacement speed powercat I saw last year in the Bahamas. It looked good underway (sorry it took so long to post it)

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Bos: You could reduce your Okoume price a little bit. I have some extra 1/2", VERY high quality, Lloyd's approved (with certificate), BS1088 from Greece. No visible voids on the end.