Long - Skinny Power Boats

[Boston]

my design limit is 60' and 4200 lbs disp
the calc Im playing with shows serious advantages to even the slightest weight savings

I like the look of the trihull configuration ( that big huge I could never afford it red thing I posted earlier )
but am curious about the box keel
something tells me the two are mutually exclusive

funny thing is when I consider elongated multihulls what I get is two cruise speeds
with neither being planing

for a 60' 14' beam 4200 lb vessel split into two 7' 2100 lb sponsons I get an initial extremely efficient movement up to 8 knots and then energy use skyrockets to about 140 hp in the 14 knot area
then falls again to about 110 hp at about 25 knots
you would think that would equate to planing but at no point does the wave resistance part of the graph show any disturbance
it just steadily rises
so I dont think this thing shows planing even at a hypothetical 25 knots

Add
so the gull wing kinda configuration of the sponsons kinda want to keep em balanced at speed
works for air planes should work for boats
ok
so whats that ITTC equation

[Ad Hoc]

18m weighing some 2 tonne all up....with a 250hp engine, fuel, outfitting etc etc...hmmm...have you done a real proper weight estimate yet?!!

[Boston]

so thats why the tri's are more efficient than the cats
the tri's concentrate the bulk of the displacement and ballance a long skinny hull form between two dynamically stabilized amas
and the cats have to have a greater proportion of surface area to displacement cause they are forced to sit on two hulls evenly
interesting
soo
how does the box keel get such good mileage

[Boston]

Originally Posted by Ad Hoc

18m weighing some 2 tonne all up....with a 250hp engine, fuel, outfitting etc etc...hmmm...have you done a real proper weight estimate yet?!!

I may have misquoted
my weight is 42000 lbs
thats 21 short tons

ya I just went back and saw I had left off a zero

[Ad Hoc]

ITTC...in a nut shell:
A standardised frictional correlation method was needed for estimating the powering between model data and ship. This was established in 1957 in Madrid. It is a simple formula

The frictional coefficient is thus 0.075/(logRn-2)^2

Rn = Reynolds number.

[Ad Hoc]

".,..so thats why the tri's are more efficient than the cats ..."

You need to be careful with carte blanch statements like this. Each design is different. The main influencing factor depends upon the length displacement ratio, which is affected by the speed you wish to travel at and the payload you want etc etc...design is a series of compromises. Whatever boat/configuration you choose.

Since at higher Fn, say in the 0.8~1.2 range a slender cat is more efficient than an equivalent mono....so, everything in its place. You must compare apples with apples.

[Boston]

there should be a direct comparison between length, displacement ratio, speed and beam

why is beam left out or are there actually a number of mitigating parameters and beam is just one of em
speaking of which some one some where has bound to have made a formula for that to

oh and I was thinking of vessels of the same length and disp

[Boston]

so back to the original question
which is going to be better ( faster ) in the 60',14'beam,42000lb range powered by a max 250 hp and 80 hp cruise power supply
a trihull or a box keel
in open water

this new calculator is a gas
ok so if I keep the fuel load to 4 tons my disp is 36000 lbs
on a 60' cat hull with 5 beam sponsons and a 14 beam overall Ild be burning 100 hp to get 23 knots cruising speed
with resistance leaping from zilch to that 100 ponies between 5 and 12 knots
I got to be doing something wrong
according to this that hull is capable of 50+ kts with 250 hp
cant be right

[Ad Hoc]

better and faster are two very different design parameters. Better is subjective and deals with cost, outfitting, quality, accessibility etc etc.

Faster is just a simple matter of power requirement for the hull. Based upon drag of the hull at varying speeds.

Yes, i know these sound like vague replies, but you need to pin down and begin to define in real terms what the principal dimensions and characteristics of your boat really is to fully answer. Since then a comparisons with equivalents and non-equivalents can be made. Then you'll see where the compromises have been made and which, if any, you are prepared to make.

Naval Architecture is not an exact science, despite what many believe. The main elements such as hydrodynamics, structures etc are pretty much an exact science, but the "whole design" is greater than the sum of each individual "part".

This si why you need to pin down your SOR into a spec, weight estimate and GA, then review and see if it is feasible.

Just talking "numbers" without anything 'concrete' is a waste of time and will tie you up in knots!

[Boston]

well ive limited it to 60' length 14' beam 4.5 draft 42000 lb and 250 hp with a cruise of 80 hp
in that range I would like to cruise at about 15 kts or there abouts
and Im just starting to look at hull forms
thought after speaking with several folks that I would take a look at trihulls and box keels first

[Guest625101138]

Boston
The attached shows what the lowest drag 5t, 15kts 18m max length hull looks like.

It has a draft of 460mm and a beam of 960mm. Power on the hull lifts to 16.2kW.

The idea with the faux-tri is to get a usefully wide cabin structure on an efficient underwater form. It means you reduce the required power by a significant amount. There is corresponding weight reductions. This reduces loads and you can build a lighter boat.

You have to iterate with the basic concept to meet your ultimate requirements.

Costs usually correlate to weight although if you need to build extremely light you can pay a premium. At 60ft and a target speed around 12kts you are in a good range to get the best from the idea but you have to think in terms of low weight. Weight and efficiency do not fit well.

Rick W

[Ad Hoc]

Well, you need to draw up a GA...then and only then can you establish what the weight will be. You cannot say 42,000lbs without drawing up how the boat will be arranged and certainly not without even knowing what hull form you have selected.

You need to start at the beginning..not the end. With respect, that is the amateurs way or the i have a computer prgram so i'll tap out numbers way of designing. They focus on the wrong bits and in detail too when you need to establish the basics first, not last.

[Boston]

Originally Posted by Rick Willoughby

Boston
The attached shows what the lowest drag 5t, 15kts 18m max length hull looks like.

It has a draft of 460mm and a beam of 960mm. Power on the hull lifts to 16.2kW.

The idea with the faux-tri is to get a usefully wide cabin structure on an efficient underwater form. It means you reduce the required power by a significant amount. There is corresponding weight reductions. This reduces loads and you can build a lighter boat.

You have to iterate with the basic concept to meet your ultimate requirements.

Costs usually correlate to weight although if you need to build extremely light you can pay a premium. At 60ft and a target speed around 12kts you are in a good range to get the best from the idea but you have to think in terms of low weight. Weight and efficiency do not fit well.

Rick W

does that 5t represent 5 short tons or what is that
42000 lbs = 19050 kilos if that helps

concept is basically to make it look like that old elco and still get through the water like I know we can today rather than a hundred years ago
that big red trimaran is really nice looking in its hull configuration
I could make that hull work for me I think
but I need to figure out its performance against say a box keel type hull
on the other hand if the tri has adequate performance I might just go with it as I just hate flat bottoms on a boat
my interior lay out is flexible to some degree

I thought Ild mentioned my design criteria over on another page
so was hesitant to repeat it
basically
I want a live aboard that is transoceanic
the hardest run Ill be making is across from Newfoundland to the Irish coast
I need to be able to have a reliable satellite hook up
and I want accommodations for 2 couples including myself and the girl
Im a huge fan of large enough spaces that Im not always cracking my head on something
and I hate cramped spaces so one large pilot house with an office area and kitchenette, one huge master berth with at least one smaller double berth a nice bath and tons of stowage
fuel is pellets 64 cu/ft pr ton at 350 miles per ton considering for the old elco hull
max fuel load is 7 tons to get me across the pond
wood build
vawt generators
electric auxiliary engine with three gorilla batteries
I know exactly how I want the rooms layed out but its not relevant yet

[Ad Hoc]

5t = 5 tonne or 5000kg

It has no relevance to your design SOR whatsoever..it is just a series of numbers plucked out of a program. Since your fuel load is 4t..!!..see my point, just meaningless numbers

As I've said above, you need to draw a GA then establish a real weight estimate based upon your GA...then and only then can you look at powering and see if you get what you thought...

[Boston]

Originally Posted by Ad Hoc

Well, you need to draw up a GA...then and only then can you establish what the weight will be. You cannot say 42,000lbs without drawing up how the boat will be arranged and certainly not without even knowing what hull form you have selected.

You need to start at the beginning..not the end. With respect, that is the amateurs way or the i have a computer prgram so i'll tap out numbers way of designing. They focus on the wrong bits and in detail too when you need to establish the basics first, not last.

the weight was based on the original hull configuration of the boat I started with
it may not apply once I get the hull reconfigured and is only good for a target weight max
so I been using it for the sake of argument

I sorta started all over the place having not tried my hand at boat design before
my plan is to take it as far as I can while I have time to play with it and then when I think Ive made some good decisions go find myself a competent NA and let him do a cost analysis
see were I am

original weight of that vessel was 42000 lbs so thats my max target weight
I calculated the weight based on the original scantlings and planking type and thickness decking bla bla bla and determined how much of that ttl was in the frame and how much was in the Finnish
I then increased the scantlings to bring the boat up to blue water standards although the exorcise was academic given that the configuration of the vessel was not blue water capable
it was however a place to start
or
one place to start

Im sure 42000 lbs fully loaded is on the heavy side of close enough for now
36000 with a 4 tone fuel load
28000 lbs empty weight
thing is once or twice a year Ill load it down with fuel
rest of the time it will be floating at the dock like most other boats