Long - Skinny Power Boats

[Chris Ostlind]

Originally Posted by charmc

Chris,

I like the concept, but 3' BWL doesn't seem enough for side by side seating, unless there is a lot of flare that I didn't see. I'm neither a giant nor a body builder, but my shoulder width is a good bit more than 18". I think 4' would be a minimum for comfortable seating.

The look is neat, very original for a power launch.

Thanks for the comments.

The overall beam of the vaka hull is 4.5', allowing for plenty of room for a side by side seating arrangement. There is a nice spray chine detail running from the bow to transom, that kick starts the small bit of flare in the upper hull panel. All this gives the boat a very slender hull in the water while still presenting a comfortable seating setup in the passenger compartment.

The fairly high Cp amas remain very slender, yet have sufficient buoyancy for the times when they will contact the water. During the design process, it came up that this design, while principally created for short hop coastal cruising, would also make for a very interesting harbor launch. Something along the lines of a classic car being used for double dates for dinner.

All we need now are 50's style diners at water's edge with carhops cruising out with the cheeseburgers and cherry cokes.

[charmc]

Originally Posted by Chris Ostlind

Thanks for the comments.

The overall beam of the vaka hull is 4.5', allowing for plenty of room for a side by side seating arrangement. There is a nice spray chine detail running from the bow to transom, that kick starts the small bit of flare in the upper hull panel. All this gives the boat a very slender hull in the water while still presenting a comfortable seating setup in the passenger compartment.

The fairly high Cp amas remain very slender, yet have sufficient buoyancy for the times when they will contact the water. During the design process, it came up that this design, while principally created for short hop coastal cruising, would also make for a very interesting harbor launch. Something along the lines of a classic car being used for double dates for dinner.

All we need now are 50's style diners at water's edge with carhops cruising out with the cheeseburgers and cherry cokes.

OK, the bow on view you added to #88 illustrates that nicely. She just might fit in with grilled salmon and a dry Chardonnay, too.

[big-boss]

Tad, that is beautiful. Will it be plans you have for sale? Company going to produce them? Very nice. Any more info would be appriciated.

[paulewill]

long and skinny with a flat flat bottom.

does it have trouble turning?

[dobsong]

[quote=Tad;195795]My latest "long and skinny" is 39' by 7'. Construction is plywood. Weight is less than 5000 pounds, power is a 50 or 60 Hp outboard in a well. D/L is about 52 thus she will eaily run up to S/L of 2.4, a little less than 15 knots. At 12 knots her S/L is 2, reqired power is 22HP and around 1.8 gph for 6.5 mpg.

Of course the accomodation is that of a boat 10-15' shorter, but the speed and motion in a sea will be far superior to shorter boats.

The form is based on Garden's Tlingit, I call it a Motor Canoe.

Hi Tad just wondering if this design got built or perhaps it has evolved?

I'm searching for a design around 40', hopefully an inboard that can cruise at 14-15 knots and has a range of more than 600 nm. I usually go out alone and sometimes with one or two friends so don't need a lot of berths etc. love the look of the downeast lobster boats especially the long lean ones with a nice simple pilot house....

Currently I have a 24'x9'x2'6" clinker Halverson built in 1961 in Sydney. Love it but it lacks range and the 318 Chrylser loves a drink. so looking to upgrade for my retirement.

Love to hear whats happened with the Motor Canoe..... any other suggested designs 9or designers or builders) welcomed

[Guest625101138]

I have been working on a retirement project for a solar boat. Something suitable for coastal hops. I started out with the idea of a long slender hull but it had certain limitations.

I have been working on a concept (provided by a forum member), which I have called a faux-tri. It is really a stabilised monohull. It behaves like a slender monohull from powering perspective, has the initial stability of a narrow trimaran, is self-righting and has the cabin of a decent size monohull.

I have done the hull shape to check power requirements and have a rough rendering of the proposed design per attached.

If I ever build it, it will be built very light with modern materials, probably carbon fibre on Corecell, in flatpack construction. With the target displacement of 500kg it will do 8kts with 900W in calm conditions. The electric motor I have is rated at 4.5kW on 48V and this should get it to 14kts in calm conditions.

I am really pushing the limits of efficiency to get the best from solar power but the concept could be applied to a conventionally powered vessel that would give good speed at very good economy.

I am in the process of building a 1/5th scale model to see if it has any vices in a seaway that are not obvious.

There are similar boats around so the idea is not new but it looks very good.

Rick W

[Boston]

so whats the magic formula to calculate long skinny hull speed
Im not after 1.34x the square root of lwl
that one is kinda broad for what Im interested in
Im after hull drag mostly and what it takes to get something up to a certain speed



this things got three long skinny hulls rather than one fat one
should go a lot faster but
whats the math to figure that out from
one of you folks is bound to have it right off the top of your head
or at least know where to find it

something tells me its more a mater of surface friction in a long skinny hull than wave resistance
so there must be an average coefficient of friction for say marine paint
resistance goes up as a square of speed if I remember
seems like it would have been mentioned in the thread somewhere but I didnt see it
thanks folks
B

never mind
I just found a hull resistance calculator
interesting thread though
was a good read
cheers

[Ad Hoc]

"...so whats the magic formula to calculate long skinny hull speed..."

There isn't one.

You'll get empirical data from 'similar' boats and their hp/ton giving certain S/L ratios..but that's it.

Just do like the rest of us, tank test to get proper results...otherwise, use lots of fudge factors from your 'basic' calculator!

[Guest625101138]

Originally Posted by Boston

so whats the magic formula to calculate long skinny hull speed
............
something tells me its more a mater of surface friction in a long skinny hull than wave resistance
...

Boston
Your gut feel is correct. The wave drag for the lowest drag hull for any given speed will be around 5% of the total drag if you do not have a length constraint.

The best examples are rowing sculls. Have a look at the wake they leave.

There is a friction formula based on empirical measurement termed the ITTC friction line. You will need to determine the Reynolds number to apply it.

Sadly Michlet is no longer freely available but it will give you drag within 2% of actual in calm water conditions. If you want the underwater shape for the lowest drag hull for your particular parameters then I can do it for you.

The faux-tri concept looks good to me. I have not been able to arrive at anything with lower drag with adequate stability.

I keep evolving the idea. My latest variation requires 750W to do 8kts. Should get to 15kts with the 4.5kW motor I have.

Rick W

[Boston]

750 watts is like one horse power
you could have the kids swim behind and push if you wanted
that pretty bloody incredible Rick

I been copying stuff like a mad man into a file called boat building tips and I got a whole section on calculating the Reynolds number
its slow going but Ill get a decent design outline eventually

I looked for MIchlet a while ago when I saw it mentioned in several places
was a futile endeavor
drove me nuts
thanks for the tip but I could have used it about three months ago :-)

Ive always kinda liked that thing Rick ( Faux Tri )
I think you may have a salable item there if you smooth it out some and it looks like your aiming in that direction

I found this groovy hull resistance calculator
http://www.google.com/url?sa=t&sourc...HI9vBRMp1D68eg
you guys probably all have it but I thought Ild throw it up just so you could see which one I found

its kinda odd but it does tell me the hull resistance in lbs pr knot
so its a good comparison tool for different weights and beams on my 60' project

I like wood
and Im loving steam
Im limiting power to 250 hp max 80 hp cruise
I want some of those Vawt generators for my passive power regeneration
( you can keep solar cells a simple cost analysis places VAWT generators way out ahead of the curve )
now I want a decent hull form
Ill worry about getting the look back once I establish the form

so whats better
box keel type or trimeran in the say 15 knot range or whatever I can get out of 80 hp and 1000 rpm

and whats that formula
ITTC friction line

Add man
Ill definitely be tank testing something eventually I just want to math out as much as possible
my skills are rusty but they're coming back little by little
been building to long I guess
and all the engineering that goes into a house is canned
no there was little opportunity to practice
this naval engineering is a bit overwhelming ( has this nasty intuition component to it that took some getting used to ) but I got time
the tank will come in handy eventually but for now
its all going to be done on paper till I can get to the point were I need to hire someone to make sure Im not just building an expensive coffin

[Ad Hoc]

"...The wave drag for the lowest drag hull for any given speed will be around 5% of the total drag.."

Yet another baseless statement from the none naval architect.

At slow speeds the resistance is wholly viscous, as speed increase it affects the total resistance, since the frictional resistance varies at the power of V^1.825. As the speed increases a series of 'humps' and hollows' in the energy system of the waves occur. This increases the overall resistance, and is known as residuary resistance. The total resistance is a summation of the two.

[Guest625101138]

Boston
The lines of my faux-tri are simply for the next stage before I front my friendly local NA to make it look pretty and determine constructability.

I will set GODZILLA to produce the the lowest drag monohull for 15kts and a displacement of 5t. It will be quite long. This forms the basis of a faux-tri because the amas just skim the water surface and offer little drag. It will give you an idea of the feasibility and a starting point for design iteration.

Rick W

[Boston]

nice
well make it happen cause on flat water that thing is going to be a step ahead in boating from what I can see so far

why is is more beneficial to not submerge the amas and make em carry a share of the weight
seems like
oh
wait dumb question
the volume is more efficiently held with the least surface area by one submerged and the others just balancing

how do you prevent sponson walking type instabilities at speed
wouldnt you have to submerge the sponsons at least enough to accommodate people moving around topsides

I still need to look up box keels

[Ad Hoc]

Boston

"...Ill definitely be tank testing something eventually.." Hope you have a cheap local place??

"..why is is more beneficial to not submerge the amas and make em carry a share of the weight.." Resistance!

"..how do you prevent sponson walking type instabilities at speed.."
The constant motion of the boat when being exposed to waves will cause a slight roll moment, to either port or stbd. Depending upon how you have designed your ama's, and where you have placed them, will dictate whether the restoring force of one ama is enough to right itself or not, and whether there is sufficient dampening to prevent oscillations.

"..wouldnt you have to submerge the sponsons at least enough to accommodate people moving around topsides.."
Basically, yes, you can design your ama's to be 'kissing' the water at DWL, rather than being fully submerged. For example, you can have your ama's "Y" shaped. So a thin section is constantly submerged. Only the fatter part of the Y comes into to play at larger heel angles..

[Guest625101138]

Boston
Attached shows the lowest drag hull for the set parameters of 5t and 15kts.

The hull is 29.5m long, 870mm beam and 365mm draft.

It requires 14kW on the hull to do 15kts.

Now if you think 100ft is too long you need to suggest a maximum length for the next iteration. Constraining the length to something around 80ft should not cause a huge increase in power. The other option is to set a lower cruising speed and accept the penalty running at higher speed. Normayll the drag would be minimised for the target cruising speed.

Rick W