downwards load at mast base, how much is it?

bikedaft

bikedaft

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6 tonne hustler 35, keel stepped aluminium mast. some corrosion at base, so cut off 1", put base plate back in mast, and make up a 1"pad to put under mast. how much load will it take? i have heard of a rule of thumb of the same weight as the boat ie 6 tonnes, if so should i make sure it can take 12 tonnes, etc?

also what to make pad of? hardwood, s/s, or aluminium??

thanks
 
I can't help with an exact formula, but tension in standing rigging is supposed to be around a certain percentage of breaking strain which, due to a senior moment escapes me at the moment. (20%?)

Find the breaking strain of your wire size and the percentage to get the strain on one wire, count your stays & shrouds and you have the expected downforce from the rigging. Add a fudge factor for the weight of the spars, sails, etc, then double it to give a working safety margin. It should give you a rough idea of the likely force.

I'm no engineer, in case you haven't guess from the above, but I think I'd prefer an ally pad to a wooden one. It won't move with moisture or rot, nor will it corrode the mast. Also, I can't imagine a solid ally pad not being strong enough for your purpose.
 
bikedaft said:
6 tonne hustler 35, keel stepped aluminium mast. some corrosion at base, so cut off 1", put base plate back in mast, and make up a 1"pad to put under mast. how much load will it take? i have heard of a rule of thumb of the same weight as the boat ie 6 tonnes, if so should i make sure it can take 12 tonnes, etc?

also what to make pad of? hardwood, s/s, or aluminium??
Click to expand...

Provided the hew base is the same size as the old one it should be fine.

However, I would avoid the use of any timber in this area.
 
bikedaft said:
6 tonne hustler 35, keel stepped aluminium mast. some corrosion at base, so cut off 1", put base plate back in mast, and make up a 1"pad to put under mast. how much load will it take? i have heard of a rule of thumb of the same weight as the boat ie 6 tonnes, if so should i make sure it can take 12 tonnes, etc?

also what to make pad of? hardwood, s/s, or aluminium??

thanks
Click to expand...

The danger is that you will not have a secure base and any sideways movement could dislodge the mast. I am not sure how you would acheive this without a specially molded foot. Could you not loose 1 inch from the height of the mast - ie take up on the bottlescrews?
 
Statically and upright - Mast,sails and rigging combined weight plus the combined force of the vertical vectors of the shroud and stay tensions.

But DYNAMICALLY - who knows! depends on initial position (for example a 30 degree heel would change the static loading), rotational inertia of mast and/or the worst case angular and linear accellerations in any given axis of rotation or direction.

too difficult!

I suggest that if the pad was made of the same material (of the same or better quality) as the base plate is presently sitting on you can't go wrong - as long as it is adequately and positively located.
 
ok i'm not keen on wood either

rigging is 6 x 1 x 19, 7mm and 2 x 1 x 19, 8mm. trouble is i can only seem to find breaking strains up to 6mm on google, anyone got this info for 7 and 8mm?

cheers

(7mm = 1/4, 8mm=5/16 approx)

ps pad (and mast base) held in place by large tongue and groove
 
Last edited:
from http://www.s3i.co.uk/wiretehnical.php

breaking strain 7mm = 5000kg x 0.15 = 750kg X 6 = 4500kg

breaking strain 8mm = 6000kg x 0.15 = 900kg X 2 (fore and back stays)= 1800kg

total 6300kg. but this is only the static load, never mind bouncing about in a F8 etc. also there is the weight of the mast, rigging and sails. mmm. a lot then.

thanks
 
bikedaft said:
6 tonne hustler 35, keel stepped aluminium mast. some corrosion at base, so cut off 1", put base plate back in mast, and make up a 1"pad to put under mast. how much load will it take? i have heard of a rule of thumb of the same weight as the boat ie 6 tonnes, if so should i make sure it can take 12 tonnes, etc?
Click to expand...

The loads on the mast depend on 4 things:

- the static load when the righting moment is maximum,
- dynamic loads caused by the motion of the boat
- the width of the staying base
- extra loads imposed by mechanical backstay tensioners.

Here is a suggested solution to estimate the righting moment if you don't have a stability curve for the boat:

Take a point half way from waterline to gunwale amidships. Measure the vertical distance from there to the centre of the keel ballast. Multiply that distance by the weight of the keel to get a righting moment.

Divide it by the horizontal distance form the shroud chainplates on one side to the centre line of the mast to get the downward thrust of the mast (and the sum of the upward pulls on the shroud chainplates).

Now the unscientific bit - double it to allow for dynamic loads.

If you have a hydraulic backstay, add the tension of that as well.

Don't you just love precision engineering?

A timber pad is bound to compress a bit but will reach a final state and you can tweak the bottlescrews to take up any slack. A slab of 18mm Tuffnol would be even better.
 
Most of the above appraoches are overly complicated. Take the cross sectional area of the mast (ally) and compare the yeild stress of the ally to that of wood say. Typical somewhere in the range of 20:1. If you provide a pad of timber/ply that is 20 times the cross section of you mast, then you have nothing to worry about. The maximum load you cuold get at the mast base can only be that which the mast itself will take. If you have a mast say 150 x 100 oval with a 3 mm wall thickness, roughly 600 mm2 in cross section then a wood pad 200x60mm should be enough. Make a pad the same size as your mast base, and you have a large safety factor.

Any way what is wrong with wood, my mast is made of it, as is the mast step. At the foot my mast is about 6 inches diameter, sitting on a tenon which roughly halfs the contact loading area. That's good enough for me.
 
bikedaft said:
6 tonne hustler 35, keel stepped aluminium mast. some corrosion at base, so cut off 1", put base plate back in mast, and make up a 1"pad to put under mast. how much load will it take? i have heard of a rule of thumb of the same weight as the boat ie 6 tonnes, if so should i make sure it can take 12 tonnes, etc?

also what to make pad of? hardwood, s/s, or aluminium??

thanks
Click to expand...

There is little merit in a detailed calculation. All you need to do is ensure that the spacer will take the same compressive load as the aluminium mast section it replaced. You also want to make sure the material used will not degrade if it gets wet. Theoretically a teak block would do it but I would be inclined to replace like with like and use a piece of aluminium plate to avoid electrolysis. Probably totally OTT but, as long as the compressive load is greater than the original why would you care? If you want to avoid corrosion then grease it, paint it, or if you want to be clever alachrome it. Stainless will also do very nicely if you have a bit and I guess the weight would be insignificant so if you can lay your hands on a bit.....
 
I looked on boatdesign.net and the naval architect-type people on there don't know either. They all seem to add in a Buggeration Factor
 
thanks!

thanks everyone for replies.

the mast is about 12" x 9" oval section 5mm thick wall so will calculate area etc and go down that route,

cheers

ps the tufnol stuff sounds interesting
 
As said by one or two replies, weight of full rig plus total shroud tensions resolved into vertical load componants, then a finger in the air multiplier for effects of inertia as boat crashes about. This makes it almost impossible to calculate. My view would be for an aluminium plate as it's not the total area that's important but the actual contact area between the mast heel and mast step plate. The contact area can be quite small so compressive stress will be high and is likely to crush timber. (Wooden masts have full mast area contact so low compressive stress on mast step). You need to make sure your redisigned mast foot sits securely onto the mast step without any risk of jumping off forewards or sideways.
 
CLP said:
As said by one or two replies, weight of full rig plus total shroud tensions resolved into vertical load componants, then a finger in the air multiplier for effects of inertia as boat crashes about. This makes it almost impossible to calculate. My view would be for an aluminium plate as it's not the total area that's important but the actual contact area between the mast heel and mast step plate. The contact area can be quite small so compressive stress will be high and is likely to crush timber. (Wooden masts have full mast area contact so low compressive stress on mast step). You need to make sure your redisigned mast foot sits securely onto the mast step without any risk of jumping off forewards or sideways.
Click to expand...

I would point out that most masts that are deck stepped are actually sat on top of a GRP plywood sandwich that is often supported by a wooden prop inside the boat. There is no way that a hardwood block would not take the load, it might indent if the bottom is not exactly flat but would not compress overall. I did however say aluminium for preference...
 
How bad is this corrosion?
There isn't any twisting action on the mast at it's base just a compression load and unless there is evidence to suggest that the bottom inch is collapsing it'd leave it alone.....just my thought. Even if it got much much worse and started to show signs of compression I can't see any damage being done, just a loose shroad(s) as a tell tail?
 
I don't see any info re what the mast foot sits on. Is it an iron floor/keelplate or glassed in pad or what?
The idea being that the same material would suffice and meet the original design.
If there's a socket to match the foot, it must be raised and re-secured if you're not happy to lose the inch and howk in the rigging.
Main thing is, it must be secured to counter the thrust of the deck wedges.
 
You're quite right boatmike. A deck stepped mast itself sits on an aluminium mast step. This spreads the concentrated point load out over the contact area of the block or post which is underneath. The post itself has to be of sufficient area and limited slenderness to ensure it doesn't buckle as well.
 
the mast sits on a grp platform at top of keel. there is a s/s plate in the deep groove thet stops it moving about.

the corrosion is not going to get any better, and there is belling of the mast over the mast heel, up to 5mm at places. the worry is that it comes down over the heel suddenly under load, rigging slackens, and mast goes over. this would not happen in calm water, so would rather tackle problem now

will probably get an alum block, and machine in the groove for mast heel on top, and weld on the male equivalent on the base to suit groove in hull.

cheers everyone
 
gta said:
How bad is this corrosion?
There isn't any twisting action on the mast at it's base just a compression load and unless there is evidence to suggest that the bottom inch is collapsing it'd leave it alone.....just my thought. Even if it got much much worse and started to show signs of compression I can't see any damage being done, just a loose shroad(s) as a tell tail?
Click to expand...

If an inch or so of the mast concertinas, that is equivalent to lengthening all the stays by an inch. In a deck-stepped mast that would just make it floppy but in a keel-stepped mast it would put a lot of sideways force at the partners, possibly enough to cause the mast to kink and fold.

There can be a lot of twisting force on a mast via the gooseneck but more particularly from the thrust of a spinnaker pole.
 
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