Lifting the mast 40 cm

[cal_d_44]

Hi all I am looking for some advice? I own a 40 foot cruising ketch and need to raise the height of the boom by 40 cm to improve headroom and also facilitate fitting a new coach house. I will be raising both the main and the mizzen booms by way of lifting both masts (see attachment). Can any one see any issues with this method? Yes I will need to change all the standing rigging and as it is due for replacement so I will do all now.

I know I could just lift the booms and then get the sail cut to suit however I would loose too much sail area.

Thanks in advance for the feedback

Cheers Cal

[philSweet]

I sure as heck wouldn't do it like that. How about a complete workup of what you want with pics of the old boat and sketches of the new house and mainsheet and vang arrangement. Then maybe somebody can suggest a good way to get there. Spec sheet for original rig and sailplan also.

[michael pierzga]

Very much compression load on a mast foot. Particularly a deck stepped mast. You would be better to correctly sleeve the mast.

Original mast extrusion plus an interior sleeve with 400 mm bury top and bottom.

[PAR]

I'd also be inclined to use aluminum to prevent corrosion issues. A sleeved extension wouldn't be difficult or very costly. It appears you have a deck stepped stick and wish to raise it ~16". Naturally all the rigging will suddenly become too short, but I'd imagine you've assumed this already.

The problem with this sort of thing (as you've envisioned) is keeping the pole in column. A deck stepped mast isn't the most secure thing in the first place. Introduce some joints and you have a higher potential to errors/issues.

A good welder/fabricator can weld up a set of flanges and a length of T-6, 6061 tubing (accept no other aluminum grade, regardless of what they use as an excuse) of appropriate diameter. The flanges should match what you have, so it can transmit the loads straight through to the compression post. If you have an oval shaped extrusion on your mast (likely) then again, the fabricator can cut a tube, add some side pieces and weld it all up. Make the tubing wall thickness of this extension about 20% thicker then the mast's walls, to cover welding, extension and fabrication weaknesses. Don't forget about weep holes to let out moisture.

[woudaboy]

People are alerady discussing the problems of short segment extension of a mast and the negative points of a deck stepped mast. It sounds like you are also committed to a major project and revision of standing rigging. Why not remove the compression post and sleeve in enough mast section down to make a new keel stepped mast of the desired height? I am in no way a qualified engineer to discuss details, but that might be an overall (albeit expensive) solution.

[cal_d_44]

Thanks Micheal and PAR, that is exactly the sort of feedback I was after, as I am in the early stages of planning I needed other perspectives on the job at hand and this gives me something to go off and by the sounds of it, it is do-able.

Cheers Guys

[michael pierzga]

It is do able.... lengthening a mast is professionally do able. Be wise, seek pro advice from a local NA architect or mast builder.

[PAR]

I disagree, but a welder familiar with aluminum should be talked to. The important considerations are extension wall thicknesses and alloy type. It's not a difficult engineering problem. Making a keel stepped mast is also less then desirable, for several obvious reasons.

[messabout]

Do not underestimate the horizontal thrust at the heel of the mast. Deck mounted masts need a secure socket of some sort to keep the bottom of the mast in the preferred location. An extension box of the kind in the drawing may not account for the side forces involved.