Hi,

I wonder how to try a windsurf Carbon SDM mast on my Plume Kart, while still using my nice 4.7m2 land yacht sail. I'm not a "Mechanics guy" My mast step is 45mm inside when the SDM mast is about 50mm OD

Any experience to maring some "extension" from a 45mm tube or similar to a inside of a SDM windsurf mast?

Are 20cm overlap between the extension and the mast sufficient (on windsurf we commonly have 15-20cm overlap), or does it needs more?

Is the mast working in a similar way to a Windsurf or does the bottom of the mast needs some strengthening ?

Sailworks mini skeeter sails seem to work on a long Windsurf carbon mast without modification.

Why do I want to do this: the 4 pieces Plume kart alu mast does not seems to have an even bending curve, and the leech opens uneven/suddenly a lot above bottom 1/3. See pic below. I would like to try with a more even bending mast.


Alu mast construction is:
- ?l?ment 1:
- 1 tube D45 of 1150mm
- inside at bottom: 1 tube D40 about 550mm
- ?l?ment 2:
- 1 tube D40 of 1150mm,
- inside dottom 1 tube D35 about 400mm
- ?l?ment 3:
- 1 tube D35 of 1150mm,
- ?l?ment 4:
- 1 tube D35 of 550mm,
- with inner junction 190mm to go to bottom part and 190mm inside,
- ?l?ment 5
- 1 tube D25 of 1150mm

Thanks Hiko for the feedback. So worth a try and mast needs a reinforcement.

I have to find a 45mm OD strong tube that fits on my mast step (46mm ID). My idea is to 3D print a cylinder to fill the gap between the 45mm OD of the tube and the 48mm of the inside of the mast.

How the sail designed for the land yacht alu mast will fit on the windsurf mast? I agree with you that there is some black magic here. The only way to know is to try :-)

There are basically 3 types of windsurf mast bending curves: Constant Curve / Flex top / Hard top. I think that the mast I have is a Flex Top (Neil Pryde).

I will provide feedback here :-)

all windsurfing masts have an IMCS flex rating eg most 430 msts will be rated at 21 490 at 29. The higher the rating the stiffer the mast. To fit a land yacht type sail you will need (Usually ) to fabricate a tip extention (Don t extend at base) from setion of ally or any other material thin enough.This then reduces the stiffness (slightly but not enough to worry about) since you are moving the sail higher up the mast.

Most SDM masts will not fit down into a land yacht mast base (RDM masts will but whilst they are by far supierior in strength are very expensive even secondhand, I'm fortunate to have several rdms left from a lifetime of windsurfing ) I also use a 490 sdm on a 5.6 and 6.2 sail . My fitting for the SDM is as follows I bought a solid 35 by 700 mm stainless bar (Don't worry about weight dosent seem to make much difference, but it will never break or bend) I then by using ally tubing made it fit into the mast mase first then again with tubing widened the top to fit into the mast..Ive ended up with about 400mm into the mast. I then wound 2 fibre glass layers of matting and resin on the bottm 500mm of the mast,thereby moving the stress riser on the mast boyond the top of the base extension used it for about three years and not had a breakage,
Some have used the comercially available windsurfing mast base extensions ,they are realy not stong enough,they may look it but they will break sooner than later, the ones with holes for adjustment will break at the first serious use at the hole just above the mast step.
I've used the above sytem on my rdms and again on my 490 and up tp 7.5 sail not broken in two years.

The higher the carbon content of the mast the better (Mine are all 100% ) the quicker the recovery rate is .ie as a gust hit it bends but recovers back to normal quicker than an ally so power is regained sooner.

Only disadvantage ...two piece section so wont fit in boot of car.
Good luck and fair winds force 4bf please

Oh, and the feedback from Bruce Peterson from Sailworks on using SDM masts on Land Yachts: "Most standard diameter windsurfing masts have an ID of 1.875", which makes them easy to internally reinforce with a length of 1.875" OD aluminum tubing. The Mini Skeeter guys used to get 38-40" of tubing up inside a 520 cm windsurfing mast, until they ran into the taper. If you go that route, I would suggest that you make short lengthwise feather-cuts on 4 axis of the tubing end to relieve any internal hard spots from the end of the tubing edge. 1.875" OD tubing has an ID of 1.750", which is a very common size for your mast step stem."

This is consistent with some other infos I had found on Seabreeze: www.seabreeze.com.au/forums/Land-Yacht-Sailing/Construction/Mast-stiffeners?page=1

JMF

Measurements as explained by Kennatt showed that the mast curve was "smooth", and without kink. So I went back to the sail/mast fit (back to basics). In order to better understand the tensions in the sail, I removed battens. Clearly too much bend at second batten, which matches the problematic area when I use the sail. Similar issue, but less important at another joint above. I have read somewhere that this also makes the leach open, which is one of my issue. See first picture.

I also did similar tests using the windsurfing mast. Much better on the top part. A bit too much volume at the bottom. See second picture. I will try see the sail flying shape with the windsurf mast...

kennatt said..
most of the shape (in the lower sections of the sail) is created by the battons.If the batton is simply a flat level strip then the sail will be at its maximun draught at the centre of the battons. The ideal is a flat even batton except for the front section,which should taper from a point about 3/4 of the way down to the tip. Softer tips alow the draught to move back,stiffer ones hold the draught toward the front.The mast bend Dosen't have a great deal of effect on the draught but controls the leach,IE a softer mast with more bend will create a loose leech and alow the top to fall away alowing air flow to spill off the upper leach,(As in extra downhaul on a windsurfing rig when over powered You can downhaul a windsurf rig and reduce the effective size 6m down to 5.6 etc.) Stiffer mast less loose leach more power.
If you are trying to see the effects of mast bend then you need to rig the sail with the battons in place.
Most top Yand yacht racers have two sets of battons,to use depending on wind strength.

Hi Kennatt, there are some point where we have same understanding, and some others where we can have different one.

In my understanding, the battens are the "backbone of the draft. Their tapper helps control the fore-aft position of the draft. Their stiffness can help control its depth depending of the wing strength (a bit).

But the draft of the sail is first of all depending on the sail (cloth) volume. Bending the mast "eats" the luff curve, so the volume/draft. Can be achieved either by cuni (windsurf, catamarans...), or by the main sheet (mast bending + leach control).

Sure that for overall sail shape analysis, I need the battens in place. For diagnosis, removing the battens helped me see the tension path better.

JMF

I have found using this contraption helpful Put mast and boom etc in frame and crank it down with sheet and downhaul and lay it on top of the sail on the ground to find out if the mast and sail match as to luff curve .
With it cranked down hard with no battens I made my sails match the curve of the mast by recutting the luff to suit

With the frame up off the ground with sail in place with battens fitted on three chairs at Head Tack and Clew
gravity puts belly in the sail and the shape can be seen under various tensions of sheet and downhaul

Roach and leach are terms relating to the trailing edge of the sail in my experience
What you are dealing with is the luff curve and its adjustments
I am no expert but I have done six or eight sails like this over the years and they worked out quite well

Hiko said..
I am no expert but I have done six or eight sails like this over the years and they worked out quite well

Hiko, do you have tricks to unstich the sail to perform the modifications ? I find it somehow painfull, and would be happy to ease that process.

JMF

Hiko said..
A craft knife sometimes with a carpet hook blade but mostly a very sharp filleting knife between the layers of cloth I have found to be the fastest. A bit of practice with the fillet knife and it can be quite easy

Thanks Hiko for the tip. I would not have considered the filleting knife. Have to try a strong and flexible blade. And I hope a last question: what do you use to draw the curve ? Idelally it would be about 4.5m long, be flexible and have a constant flex. Looks not so easy to find.

I have a 2m fiberglass rod, which looks too short. Wood that I will find won't be regular and have a consistent flex. Ideas/tips welcomed.

JMF