Multihull cruiser - comments requested

[redou]

Hello. First post. Here is a jpeg of a design I've been working on.

Would the members mind sharing their thoughts on the attached simple drawings?
Please excuse drawing quality.

36 foot multihull, one large hull, one small hull - aluminum construction - one mast or two - meant to be a seakindly heavy multihull cruiser.

1) hulls are both 36 ft LOA
2) one hull is beam 7 ft. - the other hull is beam 3 ft.
3) total beam is 18 feet
4) large hull is 12,000 lbs loaded total weight of loaded boat is 16,000 lbs.
5) small hull is 4000 lbs loaded - has enough buoyancy to float entire vessel should large hull be holed
6) large hull has v bottom 45 degrees rise both sides
7) small hull has v bottom 45 degrees OR elliptical bottom to minimize wetted area
8) Draft large hull is 3 ft. Draft small hull is 1. 7 ft.
9) engine in large hull.
10) rudders on large hull, both hulls or 2 different sized rudders?
11) Boards to be lowered or pivoted from deck to aid in upwind and minimize
leeway.
12) mast(s) located 1/4 to 1/3 distance between hull centerlines - closer to
big hull further from little hull

My interest is in sailing characteristics of such a vessel. Would anyone care to comment on the following questions?
a) Hard to turn or manouvre in limited space? Compared to a 36 ft. monohull heavy cruiser?
b) upwind ability in degrees? with boards down? Suggestions on the shape and location of boards?
c) ability of vessel to 'heave to' or ride out a storm? modifications to make this possible?
d) suitability of one or two junk (jonque) sails? if masts not on centerline?
e) speed of boat - 34 ft waterline - compared to monohull cruiser
f) could one person dock and undock such a craft?

The inspiration is the Ndrua, although smaller hull is not shorter. I think shorter small hull will slow boat down with shortened waterline. Boat is not shunted like the Ndrua, but tacked like a catamaran. My logic is one larger hull to carry people, equipment and stores. Smaller hull provides righting moment for sails, deck space and buoyancy in case larger hull is holed. I calculated wetted surface of both hulls combined comparable to heavy monohull cruiser.
I am not sure about location of masts relative to different sized hulls but
think 1/4 distance from large hull centerline to small hull centerline is justified
by smaller hull carrying 1/4 weight. Any and all comments would be most appreciated as I do not know much about sailboats. Thank you for your time.
Redou@i-55.com

[Pericles]

http://www.harryproa.com/

Pericles

[ted655]

A few million Polynesians can't be wrong. Seems they settled a few islands using your design.
It's a proven design.

[masalai]

Why not build the two hulls the same size as this would make the build easier (duplicate the first one or double cut the stations etc)

[redou]

Originally Posted by Pericles

http://www.harryproa.com/

Pericles

I checked out the harryproa site. It seems the harryproa is a very light twin hulled, wide beamed craft made to go fast. It has a conventional sloop or aerorig in the lee hull and uses the weight of the upwind hull as a righting moment against the force of wind on the sails. Quite an interesting and unconventional design.
My concept is much more conventional. A metal multihull cruiser, 16,000 lbs. with full load. The safety of metal construction. Outperforms monohulls under most circumstances. Faster in both light air and heavy. Carries a true cruising load, the safety of two hulls. Can be singlehanded. I am not sure this is possible.

[redou]

Originally Posted by masalai

Why not build the two hulls the same size as this would make the build easier (duplicate the first one or double cut the stations etc)

I spent some time doing calculations on cats and tris at first. At 16,000 lbs design loaded weight limit - metal construction - I could not get the numbers to work. The tris were too heavy, too much wetted surface. Cats with similar hulls meant living in one or the other narrow hulls. If I put a house on the cats they got too heavy.
The Ndrua large and small hull meant I can get a reasonable living
space in one hull, metal construction, two hulls for safety, less wetted surface than a mono, more deck space and a faster boat in both light and heavy air. I could not get these results with either the cat or tri.
I agree two different hulls is more work to build. Regards, Redou.

[Pericles]

Redou,

Just out of interest, why ally?

The longer the hulls and the lighter the vessel, the faster it travels, hence the Gunboat 62 http://www.deltayachtsbrokerage.com/...unboat-en.html and others. http://www.thecoastalpassage.com.au/BobOram.html

Marine ply/epoxy composite construction weighs in at around 50 lbs per cubic foot, whilst ally is 165 lbs per cubic foot. The 16000 lbs would be cut to 6000 lbs or so. Innovation is to be encouraged, but you might just be building the equivalent of a Tiger Moth rather than a P51 Mustang.

Talk your ideas over with a NA to get another opinion.

Good luck,

Pericles

[masalai]

Redou, Persistant eh? I am NOT a N.A., my boat will be designed by a professional. It will be BUILT by a professional. Even if you work/weld aluminium professionally, DON'T do this. Help a boat-builder if you must but it is a professional task.

Aluminium as a material comes into its own above about 42 feet lwl (length waterline). Any shorter is too heavy. Use MARINE-plywood if you must. then use a Wharram design catamaran with equal size hulls. Cheap to build, proven cruiser if properly built EXACTLY according to the design & specifications & maintained in a seamanship like manner.
http://www.wharram.com/

This one should accommodate you nicely,
http://www.wharram.com/tiki38.html

Why re-invent the wheel? Just have a look around and build if you must from any proven design. If not get a professional design done and professionally built for your life's sake.

a) Having a straight keel-line will inhibit turning moment (have a look at the shape of a "hobicat" lots of what used to be called "banana" on those hulls.

b) Point as drawn, everywhere but high into wind. see a friendly NA and give him a laugh. The design process is not to be taken light-heartedly.

c) If either or both hulls are holed, possibly? to be cruel is to be kind here. I don't think you have sailed a cat let alone much else?

d) I would be reluctant to sail it cemented to the ground. if as per the sketch.

e) over Victoria falls, similar to a barrel, maybe a little slower.

f) easy don't go anywhere.

If you are still reading, then some comparative comment.
The Wharram tiki 38 ft weighs 6600 pounds fully laden for cruising
My 42 ft power cat weighs in full of fuel (2000kg) at 7500kg ready with provisions for a 30 day cruise.
as per your sketch the waterline beam would perform similarly to some racing bathtubs I have seen in "comic" events.
Think what would happen if one hull became non-buoyant - thats right, settle sideways or near upside down.

It is done on some designs for non-performance work (haven't seen it elsewhere yet) where there is one rudder and one centreboard. I am not keen on this idea...

Proa or cat designs work but not a trivial thing to set up or plan. Easy to draw. Not easy to get the engineering and millions of other criteria correct, safe, seaworthy and workable.

[rwatson]

Its fine to try to reproduce an idea from the past for nostalgia's sake, but the polynesians were limted by available materials and labour constraints.
They overcame these in the very best ways the could, but compared to modern designs, they just dont cut it.
Rightly pointed out, the Wharrams are probably the nearest modern 'relative' of the polynesion heritage, but you have to be a bit of a 'camper' and a traditionalist to love their lack of accomodation and relatively heavy construction.
Your design as envisaged won't even give you 1 Wharram of performance and seaworthiness, so I hope you are into nostalgia.

[redou]

Hello. I appreciate the responses I have received. I studied each one carefully and have more to think about. One fellow said his boat would be designed by a professional and built by a professional. I thought this was the Boat Design Forum, not the "Don't Design a Boat Forum." Another response described the concept as "nostalgic." What's nostalgic about diesel engines, aluminum hulls, and a MIG welding machine? Anyway, it appears most of the objections to my concept are materials based. What's wrong with metal if it achieves your objectives? Composite materials seem to be preferred. I do not understand the necessity for lightweight materials ( come to think of it, aluminum IS a lightweight material, I used to own an airplane made out of it!) in a cruising, not racing, design as long as the desired displacement is achieved and there is room for an adequate payload. I set out to design a 16,000 pound mid-thirty foot boat because it won't bounce around as much as a lighter boat. It will carry it's load without being below it's lines, it will have more way upwind, and windage won't be as much of an issue.
I proposed a 16,000 lb. at fully loaded for cruising weight, aluminum, one large and one smaller hull, 800 sq ft sail area boat. Mr. Malcolm Tennant, a well known yacht designer, is selling plans on his website for a 21,000 pound 38 foot cruising catamaran with a similar sail area. I assume his quoted weight is fully loaded for cruising. I could build the boat I have proposed in 3.5 mm STEEL and carry a 4000 lb payload and still weigh quite a bit less than Mr. Tennant's composite design. The difference is I do not have to have the amenities on board that a yacht designer's customers require. No bridge deck. No staterooms, one engine rather than two, and so forth.
Of course, I am in no way comparing myself to Mr. Tennant.
He is a yacht designer and I am merely a retired pipefitter. But a pound is still a pound, wetted surface is wetted surface, rocker is rocker, sa/d is sa/d, and aluminum makes it's own paint. Regards, Redou http://www.boatdesign.net/forums/ima...es/biggrin.gif

[masalai]

I am he, (posted by masalai) redou, and I use this forum to refine the design criteria and list of preferences by accessing the free expertise so generously made available on this forum. I am sure a child under 8 years old could more than match your effort. To present a design as a fait-accompli (spelling?) which would insult a bath-tub manufacturer.

MODERN Tennant designs are not USUALLY heavy, Ahhh, Sienna 38, still the underwater lines would indicate performance 200% or more better than your abortion. http://www.tennantdesign.co.nz/index.php?page=valencia is half the weight! With cats, WEIGHT is the enemy, If you want to build heavy, stick to traditional displacement designs in Monohulls. or even some of the ferro-cement designs which seem to keep on sailing forever.

Look more widely. rwatson, above, and many others are far more polite than I, but you are welcome to display your ignorance and stupidity. Few have applauded your sketch. Take the hint. A designer you are not! Do a lot of reading and learning with an open mind, not full of self righteous 5hit.

Twin engines offer redundancy, manouverability and weigh close to the weight of a single installation if you think realistically rather than pulling an isolated example to prove your idiotic point......

[masalai]

Read this::::
Good reasons NOT to design

[rasorinc]

hello, Find the hull you want, out of the material you want, that has been designed by a professional. Then design your floorplan and roof and your personal specifications. Than check your weights and mesurments. Then build it. Stan

[nero]

The more tinkering, design education, and model building & testing; the better you will be able to express your needs and wants.

Maybe consider getting TouchCad or FreeShip. It will help you to understand where to put the volume in your hulls. A flat plate down the center will help you out your design. It will also help you develop some rocker or bannana into the bottom. It may provide efiles for a NA.

Since you are not building a mold, 2 dissimular hulls are not that much longer to build than 2 that are the same.

Consider a light bulb hull if you want large interior width but want to keep a fine hull that will let the sea pass with less interuption. This may be of good advantage since you have no bridge deck.

Even if you set your hulls deeper into the water, you will still need to keep the cross beams well above the water.

As you design your hulls, you need to consider your interior layout. Cabin widths that will accomodate a certain size matress. How low you can make the bed fit. How much headroom is above the bed. How to get in and out of bed.

Lots of fun times ahead for you.

Order some study plans. The more you look at them the more you will learn. (good and bad ways)

[redou]

12 / 12 / 07
thank you for your interest.
I would love to find a hull designed by a professional that is made of metal approximately 6 or 7 feet wide with a 5 1/2 foot wide waterline beam mid 30 foot length, mated to a smaller, approximately 3 feet wide hull with an 2.5 foot waterline beam, with say, 18 to 22 feet total beam. I don't think anybody professional is doing that in any material. The reason for that could be 1) it won't sell 2) the physics are wrong 3) Nobody's interested? The next option is to do what the professionals are doing. Buy some plans and build someone else's idea. Well if people were all doing only what the professionals are doing, then what's the point of this forum? I remember one time I wanted to take all the paint off my airplane and make it shiny silver like Erhart's Lockheed. I was advised by lots of experienced people, some in very strong terms, that it would destroy the resale and the plane would deteriorate. I did it anyway. It gained several knots speed. Several years later, the plane sold for more than I paid for it. Later I saw it (or one just like it) on the cover of a magazine as an example of how to fix up a plane.
What I was hoping to find here was someone to hazard some educated guesses on the sailing performance of such a project under various conditions. It seems to me a trimaran is very similar, albeit much lighter, when flying an ama. I have the option of making the hull various shapes underwater, fins or not, boards or not, one rudder or two, flatter towards the rear to counteract pitching, elliptical hull, elliptical with a point, 45 degree, shorter waterline,longer waterline, more rocker, less rocker, whatever.
Anyhow, thanks for your time. Regards, Redou.