Gennaker mast head U bolt size

[Travelling Westerly]

I was re fitting my gennaker halyard mast head pully today and experienced a slight challenge whist up there.

The 2 holes drilled in the mast head sheave box plate cover were spaced apart by 24mm. The only U bolt I could find at 24mm spacing was a 6mm arrangement which has a WLL of 656kg. I'm not sure if this has enough strength to take the power of the gennaker when in use.

The old arrangement was an 8mm affair but it got lost when it was removed a few seasons ago.

The gennaker is for a 12m, 12t (ish) cruiser and has a 17m luff. Im not sure how to work out its size though.

I'm worried that the 6mm thick U bolt won't be strong enough up there..

Does anybody have any thoughts if it's suitable or not?

Thanks

 

[Frogmogman]

The formulae I have for sheet and halyard loads (for a Genoa, not a gennaker) are

Genoa sheet load in lbs = SA * V² * 0.00431.

Halyard load in lbs = (SA * .00256 * V²) + Sheet load

SA = Sail area in sqft
V = Wind speed in knots

 

[Chiara’s slave]

My guesses and calcs give you 1500lb at the halyard in 15kn of wind.

 

[Martin_J]

If you have a Selden mast, are you sure it wasn't a Selden offset/bent U-bolt..

A bit more expensive than a standard u-bolt but it might fit better..

 

[justanothersailboat]

Selden, or before that Kemp, masts sometimes (often?) have a U-bolt that is not a standard U-bolt size - it's narrow for its thread diameter, like you describe. So you're kind of stuck with the expensive Selden one I fear. At least that's what happened to me!

 

[Martin_J]

The Selden ones that I mentioned, I also seem to think, have small lumps on them that allow you to tighten then into place so they don't just hang there jangling around.

 

[Travelling Westerly]

Ah fantastic help, thanks team, much appreciated.

I do indeed have a Selden/Kemp mast. I had no idea it could be a Selden U bolt.

I wondered about the strange size and why it was slightly bent when I removed it!

I wish I hadn't put it in such a secure place so as I cant find it again.

Thanks again

 

[Martin_J]

I'm not on board at the moment to check my spare... but I really seem to think that it's the larger one of those Selden ones used at the masthead on my S38 .

Double check... Allspars or other Selden agent will know for sure.

 

[Frogmogman]

I wish I hadn't put it in such a secure place so as I cant find it again.

Don’t worry. As soon as you buy a new one you’ll find it !!