Hull forms and design theories

Now that I know what I want to go in it, I'm looking for advice on hull forms for my Kayak Tender project.

I'm going to get a bit philosophical here in trying to explain why I'm making the design decisions I have, so please bear with me.

On of the basic issues in boat design is where to put the cabin. Fundamentally, somewhere you have to give people room to stand up, so there needs to be a space somewhere that is 6' 6" or so tall. Given some room for keel and such, that means that the minimum space from keel to cabin top needs to be in the neighborhood of 7' 6" to 8'. On a boat with an arbitrary 3' of draft this gives a freeboard of 4' 6" to 5', which is considered a bit much.

The way most recreational designs get around this is to only have the cabin be full height in the center and create a narrow walkway around the periphery to create a more manageable freeboard (See image 1). The trade off is you lose both cabin space and deck space to gain what is considered a more "nautical" appearance and it also tends to shift the CG downwards some.

Fig. 1



On many commercial bots deck space, either for working or cargo, is more important and so they have a continuous flat deck. On the other hand, while they may have holds, tankage, and engine spaces below the working deck, occupied spaces are less common.

On the kayak tender, deck space is important because of the need to securely store and handle up to 4 kayaks. On the other hand, I don't want to give up cabin space below decks, so I'm looking to the cabin section in figure 2. Having established the cabin dimensions I now need to build a hull around them. These dimensions leave 6" on each side of the cabin space for framing for a design beam of 12'. The tick marks in the lower corners represent available space to be lost to the shape of the hull.

Fig. 2



Another factor is the arrangement of the spaces and travel paths through the boat. Most designs are built like a tube, with a continuous path from one end to the other. Frequently the pilothouse is elevated to make room for the engine below, but fundamentally every occupied space is part of one big hallway (Figure 3). It also requires greater length for a given square footage of occupied space. Essentially this design trades extra length to minimize the height of the boat. It also means that the pilothouse is part of this "hallway", and since in conventional designs the only full height part of the cabin is in the center, so usually is the walkway. This pushes the helm to one side which is also not ideal. Both a centerline helm and not having the pilothouse be a walkway were items I identified as important to me in my SOR. The final result is that the engine space tends to be very cramped due to it's low height.

My original design which I presented in the thread "The boat I would build if reality were not an issue" was based on the hallway design except that the pilothouse was raised and the transition from below decks to the main deck took place in the salon.

Fig. 3



In my current working design, the spaces are stacked in such a way that one occupied space is above another. (Figure 4)This allows for a shorter hull for the same amount of space at the cost of a higher CG and total height of the vessel. in addition, it reduces the length of the engine room, but makes it tall enough to stand up in.

Fig. 4



So, given all that, how do I go about designing a hullform to fit all this into? Obviously it's going to be a very high displacement vessel for it's length and therefore require a larger engine and hence greater fuel consumption. Below is my current working design. Length is 39' 6" with a 12' beam.

Finally, for you NA's out there, what would be the ballpark cost to convert this kind of a design concept into buildable plans?

 

Your assumptions about ergonomics and designs concepts are full of flaws and lack understanding of common practices, industry trends and the "usual" engineering approaches. I mean no disrespect. As an example;

They may seem to be, but I can assure you they are not, for several reasons.

For discussion's sake lets say you can get this boat built for 100k (you can't but lets just say you can). Then a reasonable price might be 4k or 5k, depending on complexity. This would be an exceptionally good price for a custom design on a vessel of this class and configuration. The sad reality is you'll be very hard pressed to build this boat for 100k and 200k would be a more reasonable figure and still very reasonable for a vessel of this class. In which the design fee would be in the 9k range and more appropriate for the level of complexity in a yacht of this scale. The $9,000 design fee is still an exceptional price for what will likely be insured for well over a 1/4 of a million, probably quite a bit over.

 

Good way to start your design is to purchase a set of plans now. Stock plans are cheap as chips.
Have a look at a site like Selway Fisher....pick a set of plans ..pay your 200 or 500 bucks...log the plan data into your computer program ,then begin to play within the design to suite your Kayak program.... Once you finalize your concept run it by the naval architect for approval

No need to spend 10 grand for plans, no need to reinvent the wheel.

 

First....as we know nothing about the construction material, construction method, the voyages intended, the classification (if any), which systems must be included in the design, and most importantly who will build the vessel (interpret the plans).....thus nothing realistic can be said about the cost of the design plans.

Second......I can't see any reason that design cost would be associated with construction cost. The designer has zero control over the final construction cost.....but he or she does have control over how much design work is done, that can be a little or a lot....one designer can give you everything you need in four sheets of drawings, another can produce a confused mess in 50 sheets at vast expense.........

 

Many designers work on a percentage of final cost. I never trusted that system. In home construction it also promotes slow and expensive work.

 

That's the way custom designs work Gonzo....On the table right now I have SEVEN sheets of the yachts general layout plans...the designer redrew the boat... seven times... to accommodate the owners, the component suppliers and the builders input. If its a custom vessel by a first class designer, enormous man hours are devoted to the design. This attention to detail speeds construction and increases the value of the yacht. naturally the client must pay .

 

That's what I'm trying to do. I'd like to get as close as possible to the boat I want to build and then take it to a NA to review it' point out my more glaring faults and hopefully fill in some numbers I'm missing such as ballast, required engine power and stability before I start building.

I've looked at the Selway Fisher designs and while they come close, they still have that cabintop - walkway design. Granted, it's traditional, but not what I need. Granted, $200 is nothing in the total cost of the boat, but I'm concenred about paying that for a set of plan I'll never use. I'll have to give it some thought.

Construction material is cold molded wood

Intended voyages is inside passage with the possibility of near coastal to California or Mexico at some point. Expected endurance is a week to 10 days.

No classification, though I would like to use best practices wherever possible.

Built at home by me.

No question I lack understanding, that's why I'm here.

What is the thinking behind the standard "cabintop and walkway" design?

In the example you cited, are you talking about the fire and aft curve of the deck?

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Beyond just plan prices though, what is the process for coming up with a hull form for something like this? What are the considerations as far as stability?

 

Cthippo, I suggest you to browse through Michael Kasten's site at http://www.kastenmarine.com/

 

If you want a NA to design a custom vessel it will be very expensive...not worth the money and the NA might not take you seriously.
If you went to the design office of designer like Tad, almost certainly he would recommend suitable stock designs from his catalogue for your review. Purchase his study plans....then go back home with the study plans and once again refine your design needs.
I say purchase study plans, continue to work on your sketching...work carefully to accurately express your design needs . All great boats start off with this active customer involvement.
The designer will be able to identify what you want, propose details that you have not thought about and recommend cost effective building methods.

Its a huge mistake to waste your precious money on a custom design....far better to spend every penny you have on build quality and machinery specs.

 

Do you lay awake at night thinking up absurd comments like this, or are they just the whimsical exasperations, of a clueless, inexperienced mind?

 

OK, looking at hull forms...

based on what I've been reading on this site and others, I think I want a hard chine hull form. What are the relative advantages and disadvantages of these hull forms?

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#2



#3

 

Sure, a hard chine, plywood , epoxy sheathed hull form is easiest to build and design .

Personally for a work boat, utility craft, I wouldn't go with your hull form. To boxy, too heavy.

You need sea worthy craft with big deck space for handling small kayaks and a boat that is easily powered.

Id go for a hard chined narrow forward hull conceived like an aircraft carrier, build it lightweight and power it with a 4 stroke outboard to eliminate the wasted interior volume space of an engine room and big fuel tanks.

Use a stubby keel to generate stability and Perhaps water ballasted for on station duty, then light for transit duty.

Perhaps a hybrid tri

 

Every image you just posted is a custom design, so are you retracting the ridiculousness of your previous post

or just doing what you usually do and ignore reality.

 

The challenge is that I need a lot of hull volume since essentially the entire cabin has to go in it and so displacement is going to be high and it's going to take a lot to push it thought the water. If I went with a significantly narrower hull then the length to achieve similar interior volume would be prohibitive and I would be more concerned with top heavyness.

That said, I'm going to see if I can squeeze a couple more feet out of the beam to bring it down to 10'.

 

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.
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.

Van de Stadt designs a nice 40 footer. I would use this style hull design , re engineered for multichine plywood construction, then put " kayak wings" on the side decks by continuing the deck camber perhaps 1 foot, maybe more, past the sheer clamp to generate an aircraft carrier shape like the Kiwi sloop with wings. These mini stub wings would have large fold down tubular Kayak loading wings like a Libera class sailing yacht. Strap Yaks on lowered wings , climb aboard fold up to vertical for storage then hit the showers and chow down. Deck will be clear of Yaks and the yacht will be able to best utilize its volume for living space.
Would be great to figure out how to eliminate the inboard engine room and go 4 stroke outboard or a standard IO drive leg aft. Nothing worse than a big hot lump of steel in the interior accomdations.