Displacement Speed Question

Displacement Speed Question

I understand that when you have a long boat with a narrow beam your boat will go at a faster max displacement speed then a short boat with a wide beam...
I understand that adding length to a boat while keeping the same beam will increase the max displacement speed for the same boat....
What i dont understand is that there must be some length where there is a diminishing return for length increase vs speed increase....??
Thanks in advance for any ideas???
Cheers
Tug

 

True, look at some fast large ships like old destroyer, etc... The longer the boat the faster displacement speed. Look at container ship their displacement speed can be 30knots. The narrow beam lets you use less horsepower therefore higher speed. Narrow boat can also be easier in rough water not so steady at anchor.

 

Once you get over 5 or 6:1 length/beam ratio, things go out the window.

 

Depends on boat. Look at some ocean liners they had 13 to 1. My big canoe is 18 to 2.5 or a venetian gondola.

 

"...What i dont understand is that there must be some length where there is a diminishing return for length increase vs speed increase.."

It's a practical limitation, usually cost, or stability.

Since eveything just depends upon the length/displacement ratio and also the speed length ratio.

Going further back in history one can see this notion being tried by CM Ramus's hydroplane. in 1872 he was investigating a 360ft hull form with speeds up to 130knots!

 

It's a much more complex subject the beam/length ratios would suggest, though the rules do begin to change at 6:1. You also have to have the power, a prismatic coefficient suitable for the S/L desired, etc., etc., etc. Is there something specific you need, Tug?

 

PAR

Where do you get this 6:1 ratio from?...what is the criterion for this "change"?

 

Nothing really specific....just trying to balance my wants vs needs and costs vs comforts....
I wish to eventually build a boat to live on...
I have been reading as much and learning as much as i can from this wonderful forum so i could try to make an intelligent choice of which one to build....
I have learned that they dont have a boat that will do everything i want and an operating cost that i will be comfortable with.....
So from this tread i am trying to learn that if i can live with a certain beam what would be the length where the max displacement speed vs cost of construction would be an inefficient use of money..
Thanks
Tug

 

Tug
I started out constrained by this thinking but ASM set me on a new path. It is what I have termed a faux-tri. It is a trimaran only at the water surface. Above it is a conventional looking monohull and below it is a long slender hull.

It provides a good layout for accommodation. It requires very low power for speed. It has better initial stability than a conventional monohull of the same beam and can be made to be self-righting.

It is the extreme variation of somethink like the Atkins box hull.

You see a similar concept being used on some modern military craft.

There is discussion of the idea on this thread:
http://www.boatdesign.net/forums/boat-design/long-skinny-power-boats-5073-7.html

Rick W

 

Once the length gets excessive, surface friction increases more than wave resistance.

 

Tug

Best to draw up in terms of a list and also in terms of a general arrangement of what you want. A layout provides so much data intuitivity, such as a bunk that is 2m long, but the hull is only 1m long, as a simple example. In words,m it sounds great, have a bunk..but the reality once drawn up is very different. Then slowly you can tick off each one as a "yes" or a "no".
You can't get an ideal perfect solution, but you can get what you want and what you can afford and what is ultimately practical for your needs.

 

This is true but it size far easier for a longer boat to travel across any wave than a shorter boat. The bigger the boat the easier in scale, and real life it cuts thru the waves. Been on many a cruise ship doing 28knots, in 40 foot sea, just plowing along, with a little sideways actions, and up and down.

 

Went through same thing myself. Decide on 70 foot mono boat, 15 beam, 3 foot draft, 500 hp total engine power. Narrow hull less hp, easier to find dockage. Long enough for comfort and big engine room. Look at some boats like Maccgregor 68 way too narrow for comfort. Everything is compromise.

If I had to do again, I would get a little shorter boat, like 55 footer, same 15 foot beam.

Work on what you want boat for and then work backward.

 

"Once the length gets excessive, surface friction increases more than wave resistance."

True but the skin friction does not go up by a square or cube power, wave making does.

The skinny boat simply makes a small enough bow wave to power through , rather than climb up on, dragging the transom .

The huge hollow of a true displacement boat doesn't show up.

6-1 seems to be the minimum for a boat to create a small enough bow wave to sail or power thru.

This is NOT plaining.

FF

 

friction goes up by the power 1.825, nearly a squared!