mast dimension

Hi (again) Is there any formula for calculating mast dimension?, planning a wooden gaffcutter rigg, what diameter, length of mast, boom etc.? is this asking to much purhapse?

 

What size boat?

 

Yes...Really depends on the boat size but equally as important is the type of sailing you want to do...a shorter mast can often be much stronger in heavy sea conditions...and can be great for running small storm sails in a heavy blow...while the same boat with a skinny tall mast might in some cases have much more stress on the mast and be susceptible to failure...
On the other hand..a tall mast can allow for a tall sail which will get your canvas up above the lighter winds in the ground affect or near-surface area of the water....the winds 20 feet or more above the water tend to be stronger...this is helpful in the mornings or other times when there is only light winds...your tall mast will catch more air in this case...rememeber...everything is a tradeoff in boating...

 

Thanks for answering! Boat size 8,3 meters x 3,2 meters draft 1,50 meters ca: 5-6 ton. I like: "shorter mast can often be much stronger in heavy sea conditions..." dimensions???

like pic' with a bowsprit...

 

Is this your design?

What is the weight allowance for the spars ?

What Stability info do you have? A full GZ curve?

The rig design depends very much on this information. The maximum compression in the mast is relative to the righting moment. Take it that the boat is knocked down by wind, the righting moment is equal to the heeling moment, the mast ends up in compression. Look up Euler.

Should be some good guides to help you. For a agaff rigger perhaps skenes ?

hope this helps

 

Germanisher Lloyd has 1922 regulations to downlod for free:

http://www.gl-group.com/infoServices/rules/pdfs/english/schiffst/teil-4/kap-1/englisch/inhalt.pdf

In my computer, for some reason this chapter is difficult to download and require to press "Reload" several times. After it all goes more or less smoothly.

In this regulations, it is only length of spar question, that is important. So in order to get good results, it is necessary to closely follow proportions of masts to hull.

For example (sequence of events, not so uncommon today, or 30 years ago):

1) Some old times boat (basis for new design) has main boom ~1.0-1.5m above deck.
2) Here come modern times guy : "I need a pilothouse!"
3) The boom is lifted to be 2.0-2.5m above deck
4) without second thought, all the sailplan in its entirety travel up ~1.0m = heeling arm goes up, as well as CG
5) dimensioning length of masts is now bigger (irrespective of Rules you design to) =masts have to be thicker and, consequently, heavier; CG once more travel up
6) Some esthetically concerned adviser comes by: "your masts are soooo short! Why not add a meter to them and why not make topmasts twice as long?" = weight of masts and rigging once more increase, heeling arm increase, CG travel up one more time

do we need to continue?


All this pessimistic stuff aside, for so long:
a) a sail area is kept the same relative to length, beam, displacement, ballast as in the olden days;
b) aspect ratio of sail plan is not increased;
c) sail plan is NOT HIGHER above LWL, than what could be seen in drawings of old time boats and ships;

GL Tall Ships Rules will provide excellent basis for design of masts.

As well, as Lloyds Register of Shipping, or any other.

GL is different in point, that other classification societies normally ask to pay read their Rules, while GL do not.

 

Full link to GL Tall ships Rules:

http//www.gl-group.com/infoServices/rules/pdfs/english/schiffst/teil-4/kap-1/englisch/inhalt.pdf

 

thanks Perm appricite it!
WHat do you say wood vs. steel pipe as mast, hwo about weight comparison? purhapse steel lighter???

 

It depend....
For your size of boat -~8.5m LOA - steel will never beat wood or aluminum.
Steel mast compare to wood at about 25m LWL and ~100t displacement.

 

I think that will depend on many other factors.
It also depends on the design weights too, you don't want to depart too far from the design gyradius.

I've designed several steel masts and even lattice masts in steel, it can be surprising how small a section in steel will meet the design requirements.

On a recent 60 footer the steel mast from available section was slightly lighter than the available alloy section.

The big problem with steel masts is ongoing maintenance. But by the same token they are very easy and cheap to fabricate, replace and repair.
On a gaffer I'd look at chafe strips to stop the luff attachments damaging the coatings.

 

Hi Mike & thanks for answer! Will you please explain: On a gaffer I'd look at "chafe strips" (?) to stop the "luff" (?) attachments damaging the coatings"
__________________
One good reason: "But by the same token they are very easy and cheap to fabricate, replace and repair"
Cpt. Heggster

 

The luff hoops or lacing tend to chafe the mast as do the gaff jaws, if you use steel the chafe will eventually wear through the paint and rust will follow.

If you can attach anti chafe strips you will make a gaffers steel mast a much lower maintenance IMO

However if you can find a decent baltic treeyou can probably use that to good advatage.

 

If you're building a somewhat traditional boat, the simplest thing to do is to look at similar traditional boats, and see what size masts they used for a given sailplan. Their rigs developed over time through trial and error, and are pretty safe to copy.

Do keep in mind though, that the old working boats had crews that weren't afraid to work. They were a lot more casual about being overcanvased to keep moving in light airs because they didn't have motors--and they expected to do a lot of reefing. So you might want to stay more to the conservative side overall for rig size, on a cruising boat.

 

I bought a great book on the subject