A 39' "downeast" influenced design - Feedback Request

Price to build

Thanks, when you look at the exchange rate and add in shipping it works out about the same as a build in Australia.

 

Review of Dobson Lines......

LWL = 11.7m or 38.4'
Disp = 6100Kg or 13,400 pounds
Speed = 14 knots

S/L = 2.25
FNv = 1.70
Ideal CP = .70
Midsection area (Largest section) = 7.8 sq ft.
Optimum LCB for speed(and LCG) = 57 % LWL from Bow
Optimum immersed transom area = 45% largest section or 3.12 sq ft.

 

Tad,
Can I ask how you come up with those numbers?

 

Mat.....

We know the speed and the waterline length.....thus we know the Speed/Length ratio.

Optimum prismatic coefficient for a given S/L ratio has been published for at least 60 years, see Skene's.

You know the waterline length, the CP, and the displacement, thus you can calculate the maximum section.

CP = disp (cubic ft) / (mid sec * LWL)

Mid sec = disp (cubic feet) / (CP * LWL)

See my article for the ideal curves of areas and LCB location based on S/L.
http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf

 

Thanks Tad. Great article.
Amazing that the work of Skene and others of his era is still so readily used today. I guess if it's right, it's right. Has there been more recent work to verify (or refine) his findings, or are they just taken as a given?

 

I have to agree.... damned fine looking craft!
I also have to agree that the unusually curvaceous and upswept buttock lines are likely to result in some pretty extreme trim angles and far higher power consumption to reach the sort of speeds that you have quoted.
All that, however, is for your designer to figure out. I'm more interested in your adoption of the ducktail stern. As a styling feature I quite like it, but as an integral feature of a cruising design, I'd need more convincing. Easy access to and from a tender, or to and from the water, are essential to anyone who wants to cruise Tasmanian waters. As is the ability to stow a tender.
It also has a significant impact on the cockpit space available.

I know a number of well qualified builders in Tassie too, if you are looking for someone....

 

Thanks for comments re looks.

Trim angle at cruise speed (14 - 15 knots) seems to be a contentious issue – advice from the designer is that there will be some squatting and this is because he is concerned about “balancing” the behaviour the boat (i.e. in a following sea avoiding a tendency for the bow to dig in).

Access (egress and ingress) to the boat when away from marinas or piers etc is to be handled by concealed steps in the transom and a wide(ish) sponson. The tender will be a small inflatable (to be stored under aft seats) and I don’t see this as a significant issue.

Cockpit space is more than I need as I will not usually carry many guests and anticipate taking her out solo frequently (part of the original concept for this boat). I’m not after a boat crowded with accommodations and seating for larger numbers of people – this is a boat for me to enjoy in my retirement and I anticipate frequently my friends or partner will busy and I will go out alone.

What is important is that I get a boat that is optimised for efficient, seaworthy cruising at around 14 knots and is built well with timber. I also want a bit of speed in reserve (say 18-20 knots) for those times when I want to run from bad weather.

I have a preferred builder in Paynesville, Victoria and want to be involved in the build process so that rules out using a Tasmanian builder.

 

Tad, don't really understand FNv etc what I'm really trying to get is an increased level of confidence about this design in terms of the hull performance.

Critical aspects are getting the balance right between seaworthiness, efficiency (power requirements/fuel use) at intended cruise speeds (normal cruise 14 knots – low speed cruise 7-8 knots) and achieving low wake (for hull type) at low speed cruise. I’m also keen to avoid excessive changes in trim across the intended cruise range.

I’m very happy with the layout and the way she “looks on paper” and I’m sure with a few minor “tweeks” (like sheer, portholes, some more crown on house tops etc) I will have a great looking boat – to my eye at least.

This process has taught me how challenging balancing conflicting requirements (like house height vs headroom vs beam) are in completing the design. No doubt there will be some more “lessons” for me when I move into the build stage!

Thanks again for taking the time to provide comments/input into my retirement project. Please feel free to comment further.

Graeme

 

Hobart

Will,
Just checked out your web site and saw your in Hobart. I lived in Trumpeter Street opposite "Shippies" for almost 4 years while working at the smelter down on the river. Nice part of the world! Looking forward to cruising down when I get my new boat (sometime in early 2011 - 12-15 mth build).

Graeme

 

Have you sourced, and priced your Celery Top yet ?

I noted that Celery Top is not in the list of approved West Systems timbers (of course) when I was considering it for a project. Its not all that oily, so it should be OK, but it might be worth a few test panels.

 

Hi, Celery Top is one of Australia's best boat building timbers. Sourcing it can be difficult particularly for boat building but I have found a suitable supply. The following from the university of Tasmania may assist in developing your understanding of this timber.

Celery Top Pine
Phyllocladus asplenifolius
Colour
Celery Top Pine has a pale straw colour, darkening to a beautiful gold, with age. There is an occasional hint of red. The timber has a straight, fine and even grain with a close even texture. It is highly durable, resilient, and exhibits low shrinkage across the grain. In ground durability of heartwood is very good.
Use
A sturdy versatile and durable, workable timber admired and used for its stability, finishing qualities and its light base colour. Celery top pine has been used for boat building, benchtops, flooring, decking, joinery,indoors outdoor furniture, external cladding, wall panelling, beams and poles.
Workability
Celery Top Pine can be worked to a smooth flat surface. It planes well with the grain, drills cleanly to size, bends and turns well, and accepts paints, stains and varnishes readily.

 

Hi Dobsong

Thanks for the info, but I am an old tasmanian, and am very familiar with all the great timbers available. I have contact with a Celery Top Mill in the Huon Valley, but am always on the lookout for new sources.

The price is astronomical these days as is all timber.

If its not a secret, I would be interested in hearing about new sources and indicative prices.

Once again, I have little knowledge of Celery Top being used in epoxy work, as boats built with it rarely need any additional structural support until they are much much older.

Huon Pine was another major jewel of a Timber, but you would not be able to use Epoxy on it without a lot of treatment for the oils.

I suspect some sources of Celery Top could be a bit oilier than others, and may need to be treated accordingly berfore epoxying.

Were you going strip planking for the design ?

 

I suppose dobsong, you have considered your cruising grounds carefully and the range you will get... At a wild guess, 300 miles if you are lucky and have calm seas and stay in displacement mode? and 700litres to refuel? near Au$1400 at best and likely lots more later

I will admit that you have a very pretty vessel, but the question begs, for what and when???

 

Mas -
Setting aside any issues of squatting and any subsequent inefficiency, the boat would require approx 136 SHP to run at her semi-displacement cruise speed of 14 knots (SL = 2.26). Again setting aside any losses occured thru the transmission etc, the Yanmar 6LP 315 hp engine should consume around 25 lph at that speed, giving a 25 hr (or 350 nm range) with a 10% reserve.

 

Oh thanks my rough guess was 50 miles on the safe side of 10% reserve - PHEW - I thought I may be in for another bollocking with my big mouth - - saved (sort of) by a friendly expert - appreciation has been deposited