Hull forms and design theories

[michael pierzga]

Van de Stadt

[cthippo]

Originally Posted by michael pierzga

Sure...everything is a trade off. A narrow boat must be long to get any interior volume. Why are you limiting your length ? Long boats are fast under small power.

Because it has to go on a trailer to get from where it's built to the water. Also, as has been said before, costs go up exponentially with length.

Originally Posted by michael pierzga

Also consider how you are using interior space. No yacht is built like a box . Bring your freeboard down. A 40 footer would only have standing headroom down the centerline. Put some " deck camber in your design. Perhaps on half inch for every foot of beam. then use perhaps three quarters of an inch camber for every foot on the cabin top to gain headroom. . Some details like an " engine room " are not possible on a 40 footer...its an" engine box". No 40 footer has full double bunks, its a waste of space, use over under single bunks and put them outboard to use the non standing room interior volume.

That's exactly the design I'm trying to get away from.

I will put some camber in the deck, but with the program I'm using right now consistent curves are a challenge. It only needs enough to shed the water, not so much that it makes moving around a pain.

I know this is going to have the displacement of a brick, that's somewhat intentional to maximize both interior space and deck space. It doesn't need to go fast, 8 knots would be nice.

I have yet to see a 40 footer powered by outboards (though I'm sure they exist). The engine room is not just for the engine, most of the systems have to go in there as well. There needs to be room for a genset, fuel polishing equipment, the various pumps and fittings for the potable and black water systems, and a bunch of other stuff.

Likewise, plenty of 40 footers have a pair of twin bunks (see attached). It's just a matter of how you allocate space.

[cthippo]

What are the relative advantages and disadvantages of this kind of a hull form?

I know it's popular for tugs and AHTS designs.

[michael pierzga]

Disadvantages ? Look at the displacement !! Huge...its a motorized fuel tank.. A seagoing steam roller. No need to flatten to sea..best to gently glide over it. If you re thinking of a barge style then go Scow...

[Tad]

For a given structure and load, the more square the sections the higher (shallower) the vessel will float. Said another way the boxier your sections the more ballast you will have to add to get her not to float on top of the water.

Cold-molded construction (this can take various forms) is a relatively lightweight construction method with commensurate increases in cost to achieve high strength with low weight. This construction makes little sense for a extremely heavy displacement boat (as you have outlined so far). You build a lightweight and expensive hull then ballast it heavily and spend money hauling ballast everywhere. For this hull form (hard chine) steel makes perfect sense and will be far less expensive than cold-molded (laminated) wood.

Obviously the deep vee sections mean a narrower cabin sole but a lower VCG, (better high angle stability) It also means tanks could get pushed higher (outboard) so there's a tradeoff. The box sections mean the boat floats higher, reducing draft, increasing the quickness of motion in a sea, and raising the CG because all the weights are higher. The box section means your raised deck may be untenable in a big sea, the motion will tend to throw things around in the cabin. That's acceptable in a sheltered water/coastal cruiser, but not in the open ocean off California and Mexico.

This is what I did in the way of a flat bottomed house boat for inshore cruising......this boat is 40' by 16' and the hull draws about 22", construction is double plywood skin on sawn frames.

[ancient kayaker]

When I read the thread title “kayak as tender” I assumed a kayak was to be used as a tender for a larger craft. I thought I’d be able to help with the kayak design - my area of experience and all that ...

But this seems to be a tender for kayaks. Presumably to carry them to where they will be deployed. I did a quick web search and the only references to kayak tenders that came up are for the use in the para above, so I don’t know what we are discussing here. I have never heard of such a class of boat.

So I have questions. Under what circumstances do kayaks need a custom-designed supply tender, where the kayaks will drive the design? Or is this just a cruising boat that will carry a few kayaks along for recreational purposes?

What is the design driver here? Nobody has even mentioned - unless I missed it - what kind of kayaks or what the kayaks will be used for, and with the range of hull forms and sizes found in kayaks that is crucial. For ecample, in solo kayaks alone there is a 4:1 length range. If the kayaks are incidental to the cruising then the design of the kayaks should accommodate the needs of the primary vessel not the other way around.

Until this is clear, it seems a bit soon to talk of cost, construction hard chine vs rounded hull - or even the need for a custom design in the first place.

[cthippo]

Originally Posted by Tad

Cold-molded construction (this can take various forms) is a relatively lightweight construction method with commensurate increases in cost to achieve high strength with low weight. This construction makes little sense for a extremely heavy displacement boat (as you have outlined so far). You build a lightweight and expensive hull then ballast it heavily and spend money hauling ballast everywhere. For this hull form (hard chine) steel makes perfect sense and will be far less expensive than cold-molded (laminated) wood.

Why would cold molded be expensive? As I understand it it's strips of cove and beaded wood laid up in alternating directions over a removable plug. What am I missing?

I'd like to go steel, but it seems so cost prohibitive. a 4x8 sheet of steel is $360, vs $150 for two sheets of marine ply. I would also have to learn to weld, but that's not the end of the world.

Originally Posted by Tad

This is what I did in the way of a flat bottomed house boat for inshore cruising......this boat is 40' by 16' and the hull draws about 22", construction is double plywood skin on sawn frames.

This is why I love your designs, Tad!

That is almost exactly where I'm going conceptually. A little narrower and designed for somewhat bigger seas and we'd be in business.

Originally Posted by ancient kayaker

I have never heard of such a class of boat.

First time for everything

Originally Posted by ancient kayaker

So I have questions. Under what circumstances do kayaks need a custom-designed supply tender, where the kayaks will drive the design? Or is this just a cruising boat that will carry a few kayaks along for recreational purposes?

The boat is being built to carry the kayaks and their paddlers to remote places to go paddle, and also to provide a comfortable place to sleep at night. Many of the design elements, the large open deck and the davit, are to facilitate storing and handling the kayaks. Boarding will be accomplished by way of a fold down set of sea stairs with a floating pontoon at the end.

The kayaks themselves aren't anything special. In my "dream" design I wanted a 20' deck for the boats which would accommodate any of the single yaks and most of the doubles that are on the market. In the process of shrinking the design to a hopefully practical one this length has ended up around 16.5'.

[dcweed]

Do not forget that safe and functional access for entering and exiting the kayaks is critical. If you've ever done this off a conventional stern platform, you already know that is a poor arrangement. You've got the interior requirements down; work on this next. Regards, Weed www.dcweed.com

[liki]

Suppose you could also be able fit a "kayak carage" opening from transom and under saloon on a 40 feet hull.

[ancient kayaker]

Originally Posted by dcweed

Do not forget that safe and functional access for entering and exiting the kayaks is critical. If you've ever done this off a conventional stern platform, you already know that is a poor arrangement. You've got the interior requirements down; work on this next. Regards, Weed www.dcweed.com

Speaking from experience, the safest arrangement for entering and leaving a kayak in deep water is a platform close to water level and the kind of ladder you see at swimming pools. It is not an easy task, except for the young and athletic, to do this from a dock, without the ladder to hang on to. Easiest of all is in shallow water - a perfect custom arrangement for kayakers would be a level platform about 1 foot under the surface, but perfection is rare ...

Indian communities would often have a narrow ditch looping from a river and running under the palisade of their village and back to the river. this would pass between a pair of platforms so they could drop a canoe in and enter from either side. Very convenient. Of course, that was also for fresh water and security - the palisade had gates that could be dropped down into the water to prevent a water-born attack - but it was a neat system.

[Tad]

Originally Posted by cthippo

Why would cold molded be expensive? As I understand it it's strips of cove and beaded wood laid up in alternating directions over a removable plug. What am I missing?

I'd like to go steel, but it seems so cost prohibitive. a 4x8 sheet of steel is $360, vs $150 for two sheets of marine ply. I would also have to learn to weld, but that's not the end of the world.

Assuming good to excellent quality construction....

Cold-molded is expensive because epoxy and bronze fastenings are expensive, and good quality wood is not cheap. The big skiff shown above uses two layers of 3/4" ply on her bottom. Currently Lloyd's approved 3/4" mahogany marine ply is $205 a sheet, or there's lesser quality 3/4" mahogany at $170, or cabinet grade meranti for $55 a sheet. If all are epoxy coated they will last a long time. Plywood must be backed by framing in a boat this size, the framing timber (Douglas Fir) is at least $5 a board foot, probably closer to $7.......

If you order all your steel at once it's maybe $0.80-90 per pound, 1/4" plate (bottom only) is #10.2 per square foot so 326 per sheet and double that for framing and bulkheads....roughly $550 for 32 sq feet or $17.20 per square foot for raw materials.....

The mid grade plywood is $10.6 per square foot (1.5" total thickness), plus $5-6 in framing lumber, plus $10-15 in epoxy, plus fastenings and fiberglass sheathing........expensive......

Yes the steel must be cut and blasted and primed, but both must be painted.....

As the man hours in steel can be far less, total project hours are far less and the cost (rented buildings, etc) comes down and you get your boat sooner.

[cthippo]

Originally Posted by dcweed

Do not forget that safe and functional access for entering and exiting the kayaks is critical. If you've ever done this off a conventional stern platform, you already know that is a poor arrangement. You've got the interior requirements down; work on this next. Regards, Weed www.dcweed.com

I think I have the solution to this problem. Basically, it's a moveable boarding ladder with a float on the end. The ladder end of the float is larger to provide buoyancy for ther weight of the ladder which allows the float to remain level. When not in use the whole assembly os hoisted up so it is parallell to the deck and then flips up to vertical and is secured. Alternatly this could be done with hydraulics, but that sounds difficult and the stresses would be greater.

EDIT: Hey DC, it's such an incestious little town, I know some of your neighbors