View Full Version : Is there a max practical length/beam ratio

10-20-2000, 07:18 AM
How long can you practicaly build a boat compaired to its beam. would a 10 to one be about it? The narrow boats are about 72 by 6'10"
Thankyou for your comments

Tom Lathrop
10-20-2000, 07:37 AM
Don't know the full answer to that one but my old Destroyer was something over 11:1. I have seen the 10:1 ratio used to define the break between displacement hulls that are restricted by the "wavelength" formula for boatspeed and those that are free of this restriction.

If you are talking about construction problems, proper engineering should be able to make them longer than anyone would want. Maybe I don't understand your question, can you give details?

[This message has been edited by Tom Lathrop (edited 10-20-2000).]

Ian McColgin
10-20-2000, 09:24 AM
Much depends on the gross size, whether it's power or sail or human, and whether you're looking more at economy of operation or at sustained speed.

Like the old destroyers, kayaks and shell get to L:B 10:1 and thinner.

But other than specialty applications 5:1 like Granuaile or maybe 7:1 like some communter launches and if memory serves some of the 'plank on edge' english cutters is about as skinny as in consistant with any degree of load carrying.

There comes a time when the gain in better wave formation is less than the loss of boyancy.

10-20-2000, 05:27 PM
Thankyou. The question was about some mind musings. I live in Utah and have wondered if it were possable to build a craft with a side hinge in the middle and 4 feet wide so it could fold on a trailor and then straightened around on the water after launching build it si that the aft half was a self contained boat with engine controls ect and the front half a streamlined barge for fuil. yawl boat on deck were eachhalf 40 feet long it would give a hull speed on the order of 12 knots for a crossing to the 50th state of about 200 hours. perhapse twin almas for added stability and aux sail capasity and such is the dreams of this landsman
Thankyou Ian thinking of Bill Durhams writtings was thinking of a smallish 10 horse engine for fuil usage of one gallon per hour

Charlie J
10-20-2000, 07:10 PM
many catamarans out there approaching 20 to 1. Of course, it takes two hulls to balance the boat!!

[This message has been edited by c e jones (edited 10-20-2000).]

Jamie Hascall
10-20-2000, 08:14 PM
You might want to check out some of Wm. Gardens design books. He had a real penchant for the long and lean and built them from small to very long. When I get home I'll check his second design book and post you some particulars on craft such as Tlinget and Claymore.

Have fun


[This message has been edited by Jamie Hascall (edited 10-20-2000).]

Tom Lathrop
10-20-2000, 09:06 PM
Jeffery, Good thoughts. These things have and are being tried by some. The Pacific proa is one example and there was a couple of round the world power trimarans that used non planing skinny main hulls to set records. There is a mega rich guy in Malasia having a proa pleasure palace built that defies the imagination.

I think that the displacement multihull is probably the most efficient hull, especially for speeds to about 20kts and in rough water.

[This message has been edited by Tom Lathrop (edited 10-20-2000).]

Ian McColgin
10-21-2000, 07:05 AM
Love the musings. Check out Phil Bolger's stuff.

10-21-2000, 08:38 AM
Take a look at Nat Herreshoff's long boats from the late 1800's-early 1900's. they are long and fast. Also, take a look at the work of Nigel Irens (SP?) He designed and built an ungodly fast long slender tri which has been a record setter. A second also- somebody has a web site up where he build boats out of long lengths of plastic water pipe, these boats have L/B ratios of 40+. Other long slender boats are the human powered hydrofoil racers, See the Swedish boats in particular.

[This message has been edited by Henri (edited 10-21-2000).]

[This message has been edited by Henri (edited 10-21-2000).]

10-23-2000, 08:17 AM
Or Rowing Shells. Eights are what? Maybe a foot wide and 30' or 40' long at a guess? I don't know the real dimension. But they are not dealing with storms crossing the Pacific. Do you suppose you'd have engineering problems? If the hull has a hinge in the middle, hull failure would seem to be a problem.

10-23-2000, 12:57 PM
For the structure if the main hull I was thinking of two layers of 3/4th inch for the bottom and maybe two 3/8s or 1/2 inch for the sides, joints staggered. ( as per the scantlings published in Dave Gerrs nautical book. they are based on the length). as for the Henge is you axcept the added friction the henge could be several heavy strap type streaching a foot on either side of the joint. inside where the hulls join together a horozontal shelf would reinforce both ends of the hull. Bolts above the water line would join the hulls in the water and where the edges of hull bottom and the sides and deck and of the shelf would be filited and taped. As the bulkheads would act as a diaphragm (also 3/4th inch ply) to hold the sides in position I do not belive there would be structural problems.
In 80 I built a design called the Butterfly. It consisted of two dingys or John boats of 2 foot beam 8 foot length and 1 foot depth of hold ( height of sides) . the bow of each curved up and the stern was square. the sides and bottom where 1/4 inch ply the edges of the cockpit had 1x2 gunwales on the free edge of the sides. there was a decked area at the bow end. in the origional design it was a cubby hole open at the rear and if the boat swamped ut would trap enough air to keep it afloat. [ can anyone really build for themselves a boat with out changing it a little bit? :) ] in my version I lengthened one hull by 6 inches and the other by 9 inches. the decked area I turned into a top hatch area and filled it with styrofoam blocks. there were twin 1x2 keels and 1x2 chine logs but the thing that really gave it strength was the 3/4th decks and a mast partner/seat at deck level and two bulkheads of 3/4 ply at either end of the cockpit.

The two hulls could be bolted back to back to give a 2 man canoe. The origional design called for metal straps from keel to keel and another pair on the sides but it was just fine with the bolts through the transom above the water line.
the two hulls could be used seperately as individuals dinghys, or also with cross arms as a catamaram