Looking for towing bit calculation

[jeastman]

Hey all

I'm new to this forumn and am currently designing a vessel that is 42m long and must be able to tow sister ship. The towing bit will of course be on the aft end. Anyone have a set of formula for doing this? Thanks.

[french44]

hello,
in your case ,you can use the equipment number to known the towline minimum required and after it depends of the classification for example on the DVN you have some table to calculate the design load on the bollards

[FAST FRED]

The SPEED of the required towing set is the critical part.

At 1 hp per ton of the combined displacements , the pair will move at the Sq RT of the LWL of one boat.

With 5 hp/ton good displacement speeds could be carried in poor weather.

A good prop will pull 25 lbs per hp, so the estimate for bollard pull is EZ to figure , if you have a desired speed.

FF

[safewalrus]

Don't put the towage point at the end, needs to be nearer the middle [but not in the middle - good chance of 'girting' like that (getting sideways on and being pulled over). You then put a 'gog' eye at the back end and use a variable length of wire to keep the tow wire close to the back end! gives better control of the tow, makes manouvering easier, and gets more power into the tow rope

[jeastman]

Hey thanks guys. I have the Equipment number but in this case am using GL rules and there's no table for the bollard or bits. I will check DNV rules. As for the location, the Navy in this case insists on it being at the aft end. It's not for regular towing but must have the capability.

[french44]

hello,
I suppose that you know some data about your boat. First it determines your bollard pull to known the traction. I send a xls file; it's in french but easy to understand. Second if you know the resistance of your boat it's easy to find the mini tension on your rope and after you apply the coefficient that you find on rules chap anchor,mooring,towing (BV,ABS...)
I 've made this calculation few years ago , I search in my paper data base but I'm not sure of the delay , but it's a good work to create a spreadsheet
best regards

[safewalrus]

Proves one thing like most Navies they don't know **** about towing! what your saying is that it's not a dedicated tug, but a vessel that can tow? Bit like a saloon car with a towing hitch on the back - it can do the job, but it ain't very good at it! Uses a different system for towing, which is not so critical! Forget bollard pull and all that stuff, just make sure your bitts at the stern are pretty damn strong (or use a combination - big strop around the after house fastened to as many bitts as possible). Attach your 'insurance wire' to one end, attach the other to the towed vessel's anchor chain (take the anchor off!) veer some half a shot into the water and pull the chain, the weight of the chain will pull the vessel forward and as long as you keep pulling the chain the ship will follow! Pretty easy huh! And don't need no fancy towing stuff! BUT if you want to be technical talk to the designers (these other guys)! Wish you luck with it either way

the Walrus

[french44]

dear walrus,
the first demand is : how does it make calculation for the towing bollard.
second the ship must be safe for that you must respect rules, regulations and classification. there are some difference between design and using a ship.May be you are a very good mariner but I if you can use any bollards on your boat for towing, may be at the begining the designer made good calculations with enough good safety coefficient
best regards

[safewalrus]

'44'

Yes in some ways you are right, but if you are unable to use the gear properly, no matter how good the designer is you are still going to have problems - it's know as seamanship! Secondly, as I was trying to point out there are times when you can over design! In this case it would appear that there is no need for a tugs configuration when the only towing is the occasional requirement to tow and injured partner! So why all the callculation and extra work (which takes time and money) when it could be better used on other aspects of the vessel - whatever the concept!

[chandler]

Do the french and the english really hate each other??

[french44]

hi chandler,
Make yourself easy, this exchange was friendly; the difference is only on the point of view between seaman and designer. It's good for all because discussing improves the knowledgment of each other and at the end we are all winners: seaman because the ship is safer and designer because his scantling is quite enough.I'm sure that WALRUS has same opinion

merry christmas

'44'