Hello. Thoughts on building low friction towable cat barge

[Dick Hillside]

Hello forum.

I am about to start building a barge that shall be towed a normal family boat, option one a 50 hp outboard option two a 200hp diesel shaft driven thing.
Speads the barge should be able to cope with are around 7 to 8 knots wheather 1 to 5 buforts with chop to about 3 feet max. The load the barge schould be able to cary are from 1000 to 8000 lbs and was thinking of a catamaran hull.
I know the loads sound heavy for the engines mentioned but then again the weather is fear ant the speeads low, the hulls long and most of all easyly towable, ( i remeber towing 400 ton dreging barge with a 50 hp, the only wey we could stop after geting to spead of aprox ½ a knot was lowering the bucet but thats another story),

Question: enybody object ?
Thoughts: please let them rain
Coments: im all ears
Ideas: always welcome
My design: havent started designing yet, have till nex weak till start building, plenty of steel and the old Skene's elements of yacht design and C.AMarchajis sailing theory and practice both from the 60' beside me

Richard

[sorenfdk]

"havent started designing yet, have till next week till start building..."

Excuse me, but forget it unless your week has at least hundred hours, because that's how long it will take to design this barge! My guess is that you are not a naval architect, and if I'm right, it'll take you much longer - the books you mention are of no value in this case!

[Dick Hillside]

Actualy today was a good day and i managed to get the design started, your ques is wrong and the books are of utmost walue, along with a spline a nice pen and some paper. not beign a engineer helps you see.
From skene's i get in chapter resistance disp/lenght, wavemaking and prismatic coeficent walues, wich for a nice towing barge along with beutifull lines is of utmost importance, an experienced yaths man like myself dosent nead eny books or computer programs as it all lies in the beuty of the lines and experiance never designed a barge thoug, but why am i wasting time replying to a *****.
Thanks for the positive atitude.

NA from -35 Richard Hillside

[sorenfdk]

I may be a *****, but I've been designing small craft for 15 years now, and one thing I know for sure is that all does NOT lie in the beauty of the lines! How about weight and strength calculations? A beautiful barge won't be of much use, if it falls apiece!

[sorenfdk]

Oops - forgot something! Sorry!

I didn't mean to sound negative - just realistic!

[Dick Hillside]

I quit designing in -76, bean buildiung since, and we produse on the avarage 2 sets of plugs and molds a year in GRP for runabouts (smaal time hgh qualiy) while im a blue watter skiper when the weather suits me, now if i make the darn barge from 5mm steel frame it acordingly and have the deck welded watertight, the beams of a cat hul "plenty strong built" from say 5mm x 200 x 100, with the spead i imposed the strenght is no problem, what im looking foor if enyone here has some positive insights in the hull design, banana wersus straight, i will have to incorporate a sea anchor for slowing it down s this is a rental for island constructors, remote it as one cant have a tow bar and the dudes towing it, thoug the barge will be amater towd, the construction will be to par.


So best design of course would be a submarine, no time for that, floating cat, low speed, sturdy built, amaters using it will nead plenty boyansy aft and forward, cant restrict loading exept in manuals.

Sofar i have developpable surface fore and aft ends with 2 hard chines forsedd ino a senter section with 2 hard schines....and smooth beautifull lines with wake factor down to,,,,

[sorenfdk]

The scantlings you propose do not sound way off (depending on the length).

But there may be another problem: If it's for rent, you'll probably have to have it approved by the national maritime authority. I don't know where in Scandinavia you're situated, but if it's in Sweden, then it would be Sjöfartsverket, in Denmark it would be Søfartsstyrelsen. These authorities may very well want to see some calculations...

Regarding the shape, I guess a banana shape is not the way to go - it would probably not have enough bouyancy in the ends. A better shape would be a box-section midship with pointed ends (what we here in Scandinavia know as a "spidsgatter"). This would also be easier to build (but not as beautiful...)

Regarding the loads and hp, your problems will be acceleration and waves. Getting up to speed is probably not an issue here - it's after all not exactly a speedboat we're talking about. But maintaining a steady speed in the 3 ft. chop you mention will not be easy, and the loads imposed - even on a cat - will be big.