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Alan Wheeler 14-04-2008 02:09

Mast height
 
This maybe perhaps a too big a question to pose here and perhaps need to discuss with a designer. But seeing as it will be fun to discuss and the chance maybe someone will actually know the answer, I will ask anyway.
As some of you know, I am going through the process of replacing the mast on my boat. It's been a long slow process due to limited budget. But we now have a mast and are now working on the budget for stepping it.

However!!!
The mast we currently have on the boat is 46ft.
The new mast is 56ft. It is a used Seldon mast and has all the pre-cut holes for fittings and spreaders. So cutting 13ft off one or the other end is not going to be possible as the spreaders will be out of position.
The Mainsail will remain the same, at least for now. So when fully up, it will not reach the top. Nor the Headsail. But I am not worried by that. It thought maybe in the future when we need new sails, we could get them to fit the new mast.

Here are some of the specs from the blueprints to make things clear.
The Main Mast for the Ketch rig is supposed to be 43ft6". The actual is 46ft, which is the height specified for the sloop Rig.
The Mizzen mast is 26ft3".
The Keel is designed as either 5'9" or 6'9"
In our instance, We have the 6'9".

So!!
Are we going to have problems having the new mast another 10ft higher. Mainly in that weight extra weight way up there.
The angle of the forestay will change of course. Is that going to be any issue.
The Aspect in relation to the Mizzen mast is going to change. Is that going to be a problem to balance when under full canvas.
Fire away.

Steve Pope 14-04-2008 02:36

I don't know how many Kg's per metre your new mast section is but I would expect the design to be able to accomodate it weight wise. Your existing sails would work as per usual although it would look to be permanently reefed, the angle of the forestay wouldn't change all that much. The difference would be if you were to have new sails cut to fit the rig. Would be great in light airs but you would probably be reefing at 10 knots, 15 knots, 25 knots etc. I wonder if it would be all that difficult to reposition the spreaders and either plug / weld / or leave as is the old mounting holes.
The head would stay the same, you could even fit a solent stay and avoid having running backstays if you plan to have a staysail. Lots of food for thought.

Alan Wheeler 14-04-2008 02:57

Arrrr yes weight. I forgot that. The design calls for 7lbs/ft extrusion weight. I don't actually know the actual weight of the new mast. Apart from the shipping doco stated the weight as supposedly 280KG all up. But I can lift one end completely clear of the ground and I don't think I am that strong. I reckon two guys could lift it, if it wasn't so damn awkward. It came of an Elan 43. So I imagine the weights will be close to right, which means it's about 390lbs.
Its all anodised and in mint condition, so I am not too happy about cutting nor welding anything on it.

I should add, I think we are slightly underpowered in light airs. We can carry full sail to 20kts and then start reefing from then on up.

David_Old_Jersey 14-04-2008 03:32

I don't know the answer :)

But my gut feeling is that 20% is a fair bit, but given your original mast height was specced for both drafts of boat maybe you do have a bit of room to play with.....even if not the full 20%.

I can understand that you do not want to cut 10 foot off one end or the other (and have to re-drill), but what about cutting 5 foot off each end? (or even a couple one end and more the other?).......am trying to mentally visualize this (and failing!).........but maybe this would leave all your existing holes / fittings lined up mid mast and let you drill new ones at the ends?

Maybe also the result could be having the boom raised (or lowered) a bit from present?

GordMay 14-04-2008 04:08

Alan:
Did your new mast come with standing rigging?
Do you have access to a free or cheap mast crane?
If yes (both), why not step & rig the mast “as is”, and try it out, with your current, undersized, sails?

Joli 14-04-2008 04:31

Hi Wheels,

Our spar was raised by ~12 foot from 81 to 93 foot. The chainlplates were replaced to carry the larger load and the additional winches and blocks were added for checks and runners. Thats about it.

On the plus side, the boat easily sails with minimal overlap (many modern boats are now designed this way). It is easier to tack, the wind range is increased for the jib, and the taller mast reaches breeze a shorter rig does not get.

On the negative side you will need to be more aggressive with trim (ie lead positions) since the aspect is higher and when you crack the leach will open more quickly. Some times you will not be able to get under things (bridges and wires) We can't haul at a marine 2 minutes from the house since we cannot get under the wires (85 foot).

From a practical standpoint I doubt you can shorten the rig since panel stiffness is determined and supported by spreader position. I would set the spar just as it is designed and plan on using 100% jibs. Chances are you will be quicker, have a better motion, and it will be an easier boat to sail.

Joli

svTOTEM 14-04-2008 08:18

How much of the bottom can you chop off before the spreaders get out of place? Going from 46' to 56' is a 22% increase in height; whereas in Joli's case it is 14%. If the mast is 7lbs/ft, that's 70 lbs,; which equates to a hell of a lot more at the end of a long lever (mast). Heeling and hobby-horsing will be “enhanced”. How stable is the boat now? If she has a rolly tendency now…

If she is stiff now, she may tolerate some increase.
1. How much can you chop off the bottom before the spreaders are out of place? Even a couple feet would be helpful.
2. How long is the boom? Calculate how much area you would add to the main (luff x foot x ½ = area without roach). In a new sail you can make it a full hoist; while keeping the foot a little short and the roach flat or even hollow to lessen the sail area increase. Being a ketch rig will make it easier to balance the boat after the increase. You’ll have to play around with the headsail size and when to use the mizzen. I think this is more of an issue of weight aloft.
3. As Joli points out on the chainplates, this change impacts a lot more than reefing a little early. To increase by so much you’ll have to look at every component in the system: chainplates and bolts, is the boom from the old rig and if so will it handle the increase load, main sheet system, etc. In the end, I’ve got wonder if it would be cheaper to off the bottom to the old height, maybe add a 1’ or 2’, and move spreaders, gooseneck and lower halyard exits. This keeps all of the other masthead elements, sheaves, exits, crane, tangs, etc. intact.

SkiprJohn 14-04-2008 15:47

Aloha Alan,

I like the idea of cutting 5' off each end if you don't have standing rigging to go with the new mast.
I had a friend with a Cascade 42 sloop who didn't want to cut the extrusion and was about 12 feet longer than called for at 60 feet above deck. He couldn't go anywhere without a reef in the main and was always over powered. I know your boat weighs quite a bit more but I would be leary.
If you had all the rigging then maybe a test sail or two could help you decide.
Kind Regards,
JohnL

MidLandOne 14-04-2008 16:15

I would not like to say one way or the other except that it will likely look out of proportion to the rest of the boat.

Have you thought of having a brief chat to Tim Barnett at Barnett offshore Design in Blenheim just to sound out the possibility of using the full length? He may be able to make an instant (ie "Free" :)) dismissal of the use of the full length or say "Maybe", in either case there is a more definite starting point for how to go about using the mast.

John

sluissa 14-04-2008 17:28

Is it deck stepped or keel stepped now? If it is deck stepped why not cut a hole in the deck and make it keel stepped?

Just a crazy idea.

Sandero 14-04-2008 18:19

A taller stick means a larger main and headsail unless you do a fractional rig. Bigger sails means more loading at the chainplates. And the spreaders may be longer and or the tips positioned differently from the original rig. What about the boom length? Same? This is a higher aspect sail.

Assuming you can get all that worked out and new sails you will now have a moved the center of effort higher (most likely) and that sounds like the boat will be a bit tender (tippy as the wife calls it)... or over canvassed compared to the same wind conditions as the shorter rig and smaller sail area. This means you'll be reefed sooner or heel more or both!.

Am I missing something?

delmarrey 14-04-2008 18:28

Quote:

Originally Posted by Alan Wheeler (Post 152472)
However!!!
The mast we currently have on the boat is 46ft.
The new mast is 56ft. It is a used Seldon mast and has all the pre-cut holes for fittings and spreaders. So cutting 13ft off one or the other end is not going to be possible as the spreaders will be out of position.
The Mainsail will remain the same, at least for now. So when fully up, it will not reach the top. Nor the Headsail. But I am not worried by that. It thought maybe in the future when we need new sails, we could get them to fit the new mast.

You can cut some from both ends putting the spreaders in the right location or with maybe only having to move one, if it has a pair like I suspect.

As for being top heavy, mine is heavy duty for the boat and it's 52 feet off the deck and the boat only weights 14,000 pounds. :smiling:

Zach 14-04-2008 18:47

Sure would be nice to have the extra height in really light airs.

Faster wind speed the higher off the water you go! :D

Joli 14-04-2008 18:52

Del, you can't move the spreaders since the spreader bars are already cut through the spar. Where the spreaders are is where they are. Shave the top and bottom but then you end up with a long center panel and two short panels. I don't know what that would do to the spar loading but it can't be a good thing.



Quote:

Originally Posted by delmarrey (Post 152642)
You can cut some from both ends putting the spreaders in the right location or with maybe only having to move one, if it has a pair like I suspect.

As for being top heavy, mine is heavy duty for the boat and it's 52 feet off the deck and the boat only weights 14,000 pounds. :smiling:


Wotname 14-04-2008 18:56

Well I don't know either that I won't let that stop me from having an opinion :).

As you are keeping the same sails (at this stage), your sail balance, c/e etc won't change - as you have already stated; BUT I would be very concerned about the additional weight aloft until it was proven to be OK.

I believe you are ketch rigged so the designed height is really 43.5 ft and not the current 46 ft. The "new" mast being 56 ft makes for a considerable increase in the c/g of the whole main rig (remembering to factor in the additional height of the spreaders and lengths of rigging wire). This will certainly significantly increase the moment arm of the main system (can't do the figures as I don't have all the data) and isn't balanced by any other proposed changes.

Might be a silly(ish) idea, but trying hoisting say 60 to 80 lbs of water to the top of the existing mast and motor around in some choppy water to simulate effect of the new mast c/g.

If the change in motion is reasonably noticeable, rethink the proposal.

If there is only a small change in the motion, try motoring around in a decent swell and see what is happening.

Oh and if the boat falls over or the rig goes over, it wasn't a good plan :D:D


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