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Old 21-05-2013, 05:55   #1
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Build a Boat for The Great Loop?

I've been approached to build a boat for a friend specifically for the Great Loop. Over a few beers we decided on a powercat of modest dimensions. The overall design would be based on the Sunnydays boats here in KW


The departure from that design would be hinging the forward sections of the hulls to fold inward, saving LOA for locks and dockage. The hulls and deck would be either steel or aluminum and the house wood/fiberglass. Power would be small outboards just inboard of each hull on a liftable frame... up for skinny water, down for maximum performance. 10kt cruising speed target, 250 mile range, 5 day endurance.

Let the comments fly!
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Old 21-05-2013, 17:19   #2
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Re: Build a Boat for The Great Loop?

78 views and no comments?
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Old 21-05-2013, 17:46   #3
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Re: Build a Boat for The Great Loop?

I like the folding hulls idea. You could make them in such a way as to create a "breakwater" for the area between the hulls. It would make a nice "harbor" for the dink and a nice swimming hole while anchored in choppy areas.

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Old 21-05-2013, 18:04   #4
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Re: Build a Boat for The Great Loop?

Check out the Woods Skoota 28. Also has a 36, best to forgo the steel or alloy. Are you talking a big boat like the ferry or a modest cruiser? Hard to build a cat in metal much under 45' and have a decent boat.
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Old 21-05-2013, 18:44   #5
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Re: Build a Boat for The Great Loop?

Skoota is an interesting design! My buddy wanted metal for beaching.... "When you're grinding on rocks, nothing sounds better than steel!" (his words)

Also the folding concept is 100% easier out of metal.
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Old 21-05-2013, 20:10   #6
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Re: Build a Boat for The Great Loop?

With steel or alloy, it will be easy to do the folders! Might be a good place for hydraulics even Simple and no hands that way !! Wish ya were closer I mite love to to a little something on this idea !! have fun Or at least to me it sounds like fun !! LOL
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Old 21-05-2013, 21:59   #7
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Re: Build a Boat for The Great Loop?

Quote:
Originally Posted by capngeo View Post
Over a few beers we decided on a powercat of modest dimensions.
If you feel the boat in the photo is of "modest dimensions," I need to know who manufactured those beers.
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Old 22-05-2013, 05:16   #8
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Re: Build a Boat for The Great Loop?

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With steel or alloy, it will be easy to do the folders! Might be a good place for hydraulics even Simple and no hands that way !! Wish ya were closer I mite love to to a little something on this idea !! have fun Or at least to me it sounds like fun !! LOL
The internet makes for easy collaboration! I have a Bertram 28 that I'm stripping out next to the shop; as soon as it's done and gone, I'm going to get serious about this project

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If you feel the boat in the photo is of "modest dimensions," I need to know who manufactured those beers.
LOL! That boat is about 60'LOA and about 20' wide. The figures on the back of the napkin are about ~75% of that. The goal is to be just under the typical 30' minimum most marinas charge when folded and under 15' beam so it will fit in a slip and not need a T head berth.

I've been doing a bit of cyphering, and Corten steel which was the first material mentioned for the hulls, once painted is no more corrosion resistant than hot rolled mild steel and the approximate hull dimensions would yield nearly 4000# per EMPTY hull! Steel is out, looking at 5000 series aluminum (I have been wanting a bigger spool gun for my MIG anyhow) I'm worried about the modulus of elasticity for the 5000 series alloy as well as work hardening at stress points where the hulls fold. I am not an engineer, so typically I overbuild to compensate for perceived stress points.... Any of you engineer types care to opine?
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Old 22-05-2013, 05:57   #9
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Re: Build a Boat for The Great Loop?

Fairly small military boats fare pretty well in aluminium, including some landing craft, and it's a very common material for hard-bitten workboats down to small sizes (eg mussel farm tenders)

The engineering challenges are eminently doable, but rather than overbuilding (whereby you may introduce stiffness which is too localised to be helpful and can actually make things worse) it would pay to get someone involved who is competent to work out scantlings, especially in the folding vicinity, and to design the hinges.

It will take some careful and clever materials science to come up with the right materials for hinge pins, swinging hasps (which would probably be fitted with rollers, probably of a strong engineering plastic) ... etc etc. So whoever it is needs to be a marine engineer, not just a general engineer: the environment is so harsh in corrosion and galvanic terms.
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Old 22-05-2013, 06:06   #10
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Re: Build a Boat for The Great Loop?

The boat in post 1 is cold molded ply/glass, and has been in service about 10-15 years as I recall.... I wonder if that would be doable if it were revisited on a hybrid basis... Making the hinge/latch out of steel (316???) and bolting the connection?
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Old 22-05-2013, 06:07   #11
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Re: Build a Boat for The Great Loop?

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I am not an engineer, so typically I overbuild to compensate for perceived stress points.... Any of you engineer types care to opine?

If your not an engineer or naval architect don't waste your time or the clients money.
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Old 22-05-2013, 06:24   #12
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If your not an engineer or naval architect don't waste your time or the clients money.
Blah....some of the best engineers aren't!!!!
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Old 22-05-2013, 06:29   #13
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Re: Build a Boat for The Great Loop?

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Blah....some of the best engineers aren't!!!!
Name one!!!!!!!!!!!!
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Old 22-05-2013, 08:00   #14
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Re: Build a Boat for The Great Loop?

Samuel Seaward
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Old 22-05-2013, 08:08   #15
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Re: Build a Boat for The Great Loop?

Quote:
Originally Posted by capngeo View Post
The internet makes for easy collaboration! I have a Bertram 28 that I'm stripping out next to the shop; as soon as it's done and gone, I'm going to get serious about this project

LOL! That boat is about 60'LOA and about 20' wide. The figures on the back of the napkin are about ~75% of that. The goal is to be just under the typical 30' minimum most marinas charge when folded and under 15' beam so it will fit in a slip and not need a T head berth.

I've been doing a bit of cyphering, and Corten steel which was the first material mentioned for the hulls, once painted is no more corrosion resistant than hot rolled mild steel and the approximate hull dimensions would yield nearly 4000# per EMPTY hull! Steel is out, looking at 5000 series aluminum (I have been wanting a bigger spool gun for my MIG anyhow) I'm worried about the modulus of elasticity for the 5000 series alloy as well as work hardening at stress points where the hulls fold. I am not an engineer, so typically I overbuild to compensate for perceived stress points.... Any of you engineer types care to opine?
Capngeo, I'm an "engineer type", so I'll offer my 2 cents worth. The boat in post #1 looks like one of the surface piercer cats from Gold Coast Yachts in St. Croix, USVI. They are good clients of mine. Those boats are built in composite and/or wood epoxy. You could likely do such a boat in aluminum, but it should be designed and engineered accordingly with proper design and drawings. That style of hull may also have copyright protections, so you should check with Gold Coast Yachts at least as a courtesy before going further. Gold Coast's website is: Gold Coast Yachts Custom Boat Builder ? Catamarans, Power Cats, Luxury Custom Yachts. Talk to the owners Roger Hatfield or Rich Difede.

The best aluminum to use is 5083 or, less commonly, 5086. You can buy some framing stock in these alloys, but typically, 6061 alloys are of more variety in shape and size and so 6061 is commonly used for framing stock, such as flat bars and angles. One always designs and engineers metal boats to the welded strength of the material--it's possible to figure out all the details and scantlings with proper engineering. The devil is in the details, which can all be figured out "on paper" beforehand.

You are going to have a huge problem with the folding bows--the loads on the joints--the connections, hinges, and controls--would be enormous and extremely difficult to engineer well for all anticipated conditions, and at the same type be totally internal to the hull so that they don't protrude outside the hull surface. The joints and controls will be heavy and costly--in my opinion, not worth the effort.

That's about 2 cents worth, but I am happy to contribute further if you have other questions.

Eric
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