Converting multichine alu hull to radius?

[urbansailor]

I've read a few articles on radius chine hulls and how they come close in appearance to true round bilge hulls. I have seen a few examples online and I do like the clean look. Is it possible to take a multichine aluminum design and convert it to a radius chine. I think that an experienced NA would have to be involved in the conversion, obviously. But is this possible? I would very much appreciate any info on the subject

Thanks,
Urbansailor

 

[TANSL]

Naturally it is possible but you must keep in mind that the shapes of the cross sections change and also have to find how to get developable surfaces. That will produce more changes.
We will have to redesign the structure, distribution of reinforcements and, perhaps, plate thickness and reinforcement modules.
However, without knowing the boat, one can not talk seriously about the possibilities of your project.

 

[peterjoki]

Wouldn't this increase the displacement, and result in a very different boat of the same length and beam?

 

[philSweet]

I'm not sure there's any reason to start with a multichine hull as a parent. Just design a radius chine to suit. The chines of a multichine craft are usually chosen to minimize material waste and for ease of build. A radius chine is quite a bit different in that the radius has to run where it needs to. These are different design constraints and good examples of each type should not be expected to convert back and forth easily. Some single chine hulls will work okay though. By good examples, I mean multichine hulls that are economical to build. There are a few multichine boats that don't rely on economy as a selling point.

Which hull were you looking at as a parent? American's have a well-earned reputation of producing really crappy looking home built multichine hulls. I see you are in Brooklyn, so maybe you have in mind some of the better examples. The European's have some multichine builds which leave you nothing to convert - they are gorgeous.

 

[PAR]

A radius chine would generally decrease displacement, not increase it. It also doesn't have to be a developed shaped, as sheet aluminum is very easy to shape, given a skilled sheet worker.

You'd be better off converting a round bilge hull, to sheet aluminum with diagonal seams, than attempting to make a hard chine a soft chine.

 

[urbansailor]

Thank you for the reply's! PhilSweet, the boat in question is the Bruce Roberts designed Tom Thumb 28.
http://www.bruce-roberts.com/public/HTML/descriptions/TomThumb26_description.htm
Par, how much more difficult would it be to convert to a true round bilge? Will the labor costs increase substantially?

Thanks,
UrbanSailor

 

[PAR]

I'm pretty sure the "Tom Thumb" isn't really a Roberts design, just his version of it. It's a well burdened thing and in this size range, there are lots of designs to choose from, most with considerably better performance potential too.

If memory serves me, there's a version of the Tom Thumb is available as a round bilge (wood strip I think), so a conversion could be made to aluminum, though there are quite a few pilot/cutter designs available if interested in going slow and carrying a hefty load. Lyle C. Hess was influential in redoing the traditional pilot boat design for the late 20th century, examples include the Bristol Channel Cutter, her smaller sister the Falmouth Cutter, and of course Lin and Larry Pardy’s 24' "Seraffyn" and their subsequent 29' "Teleisin". These boats are considered wholesome, heavy for their length, by modern standards and not particularly fast or weatherly.

Is there a reason you're looking for such an antiquated design? It also seems silly to do an aluminum version of this design type, knowing it's not going to save anything, except increase it's ballast ratio. It's a bit like using carbon fiber to make pile driver plungers.

 

[urbansailor]

PAR, you are correct, the original Tom Thumb 24 was designed by Grahame Shannon and then he sold the rights to Bruce Roberts- I think? I corresponded with an owner of a Tom Thumb 24 a few years ago. He only had good things to say about the boat. But, I'm not married to the idea of having one built. As you say, there are other designs within this range to choose from. I'm drawn to long keels, outboard rudders and aluminum construction; so that does narrow my options a bit. But, there are at least two designs I've seen that appeal to me.
http://www.tedbrewer.com/sail_aluminum/bulldog.htm
http://dixdesign.com/hb30.htm
I'd love to hear your thoughts on these vessels as potential long distance family cruising boats. Thanks!

Best,

 

[philSweet]

You might want to look at the Baba 30 and give Bob Perry a call. I think that would probably go in aluminum. But I second PAR's comment about not seeming to gain much from the expense of aluminum unless you knock a few pounds off it. You save on the rig when you build a lighter boat, offsetting the addition expense of the ally build somewhat. I'm not talking about thousands of pounds, but maybe 600 - 800 pounds or so. Modern sails are much better and rigs can be lighter than they were.

 

[urbansailor]

Thanks, philSweet. I will check out the Baba 30.

 

[PAR]

The Dix design is easily a much better one than the others. It's faster, cheaper to build, more weatherly, will out handle and out run any of the pilot types.

Building a built down hull in aluminum is self defeating. The whole point to the use of light weight materials (aluminum, carbon, etc.) is so you can take advantage of these savings. If you build a pilot type of hull, even if ultra light in carbon/Kevlar, it's still got so much wetted surface and lack luster performance shapes, that you've just got a slow, yet costly hull.

The Dix design has the prop as well protected as you can get with the skeg hung rudder equally well protected. These two things offer way more in terms of performance and to a lesser degree ease in building than any built down hull form.

Lastly, Bruce Roberts has never bought the rights to anything, though has sold plenty of rights (his whole design drawer at one point) over the years. Tom Thumb was likely in the public domain, so he just re-did it. I don't have a lot of confidence in BR designs and most of them aren't really his work anyway, but someone else's. It's one thing to have partners and apprentices, but this isn't the way he's handled his design choices and offerings and I find his business techniques distasteful to say the least.

 

[Kailani]

Comments from the Designer Grahame Shannon
http://bluewaterboats.org/roberts-tom-thumb-24/

At 24 feet of length this little boat by conventional standards would be too heavy to work, but Shannon shed weight with clever use of steel as a skin over a frameless interior, stiffened by only interior furnishings much like the interior lining in modern fiberglass designs. And so the wee Tom Thumb 24 makes an appearance as the smallest of steel go anywhere cruisers.
…
I designed the Tom Thumb 24 in about 1983 as a bit of a joke. I had designed other steel boats and used to get a lot of people asking “why are there no small steel boat plans?”. I used to give the standard answers (too heavy, can’t make the plating thin enough without distortion, etc.). Then I had the idea that if we designed her like a GRP boat with a monocoque hull, frameless, using interior plywood furniture as stiffening, that the weight would be within reason. After a few calculations, I saw that this would work.
…
I drew up the plans over about 8 weeks with assistance from John Woods, who also had worked for Bruce Roberts when I did. Then I placed an advert in Cruising World offering plans for $99 with a small line drawing. Response was amazing and within a year we had sold 300+ sets of plans. Then I sold the rights to Bruce Roberts, and he has sold many hundreds more, I don’t know how many exactly.

 

[PAR]

Well, this means Bruce doesn't actually own the design he purchased from Shannon. He sold the rights to his designs in 1994 (I know, right) and since has showed the most atrocious business practices I've seen in the plans sales market.

This thread covered the details:

http://www.boatdesign.net/forums/all-things-boats-and-boating/bruce-roberts-lament-11805-6.html

One poster on this thread had difficulty understanding the realities of civil law practices and continued asking for different answers to the same set of questions repeatedly (a sign of insanity), eventually causing the thread to be closed, but the details are in my posts.

I wouldn't give Bruce the business, if his was the last set of plans on the face of the earth.

 

[bpw]

I spent the last 4 years cruising in a William atkin "Inga" heavy, full keel and slow. It took us a lot of great places but we are really exited to be moving to a fin and spade rudder boat. The performance of modern boats is so much better. Building a full keel boat seems crazy when so many used ones can be had for very little money. I know someone is going to get a lot of boat for a few thousand dollars when we sell our current boat.

Have you done any cruising before? If not I highly recommend getting out there for a few years in something cheap before committing the time and money to a build. Most likely your preferences will change a lot after a few years cruising.

I don't have time to right a whole lot now, but search my user name and you will see some thoughts on our boat. Happy to answer questions later.

Also, I wrote up a pretty long post about this subject on sailing anarchy, same user name.

 

[bpw]

And I second PAR on staying away from Roberts designs on a new build. Resale value is pretty crummy on his designs for a reason, and it doesn't cost extra to build a good design. I love Dudley Dix's boats and seriously considered building a Didi 34 before finding our new boat.