Converting Keel-Stepped to Deck-Stepped Mast

[Ben M-P]

Anyone heard of it being done?
How difficult would it be?
I'm asking mostly out of curiosity.

[ozskipper]

Cant be too hard. Cut the mast at deck height. Seal the deck and add a mast step. Insert a compression post. Re do electric cables through the deck and thats about it. Sounds like a five minute job lol. Ok it isnt. But it certainly isnt a hard one

Cheers
Oz

[Minggat]

Originally Posted by ozskipper

....... Cut the mast at deck height. ..... Insert a compression post.....

Use the section of the mast that you just cut for the compression post..?

[bob perry]

No, No, No!
A deck stepped mast requires a stronger section than a kel stepped mast. Anyone who has ever desifgned boats knows that. Unless maybe your keel stppped section is oversized ansd that is posible. But there is some engineering involved in making this change.

[Jim Cate]

So, yes, I too think it could be done given enough money and talent, but why in the world would anyone even consider such a move. Madness...

Jim

[ozskipper]

Originally Posted by bob perry

No, No, No!
A deck stepped mast requires a stronger section than a kel stepped mast. Anyone who has ever desifgned boats knows that. Unless maybe your keel stppped section is oversized ansd that is posible. But there is some engineering involved in making this change.

Bob, please bare with me here.. If I get a piece of 1in diam pipe and try to bend it with my hands 2 inches apart its much harder than if I try to bend it at 2ft apart. Leverage.

Why wouldnt a shorter mast be stronger than a longer mast? Assuming the compression post was taking the loads the previous section used to take I am confused how shorter is weaker.

Cheers
Oz

[Unicorn Dreams]

Just like Bob said.
A keel stepped mast is made out of thinner walled material as it h support part way up..
My ketch has both a deck and keel stepped mast. The main has a thinner wall thickness than the mizzen which is deck stepped.
Main mast is 63' from keel to mast head.
Mizzen is 35' from deck to mast head..
I believe I would listen to Bob Perry as this is his line of work and has designed many a beautiful yacht....

[bob perry]

Oz:
Your analogy is incorrect.
If you hold the mast at the keel step and then at the partners you effectively stiffen the lower part of the section. This is called "one fixed end and one pin end". If you just have the mast sitting on the top of the deck or coachroof you have "two pin ends" an you need higher moments of intertia to achieve the same stiffness.

Find a copy of Skene's Elements of Yacht Design. It's all in there.

[Mimsy]

This isn't meant to sound rude but I am afraid it might come across as such. Here goes anyway- why? Why would you want to convert your mast at all? If you want a deck stepped mast, buy a boat with that feature. If you prefer keel stepped, then go with that from the beginning.

I just remember looking at so many boats prior to purchase that had owner modifications that really left me scratching my head. If you are buying a boat, I would hope you've done some research into the design and designer. If you like a boat enough to buy it, you should trust that the designer used his/her best judgement as to the best type of structure for that particular design. If you don't trust the designers judgement, buy a different boat!