TILLER/MAST RAKE, AND EBERSPACHER?

[Clyde_Wanderer]

Thanks to all who gave advice regarding tiller, one little question.
Is a laminated tiller stronger than a solid one, if so why.
And I only get a heavy tiller when the wind gusts up, under a constant (or thereabouts ) wind the tiller is in fact quite light and not a lot of feedback from it, is this normal.
Mast Rake, What should it be for a 30ft mast on 30ft boat with mast head rig?
And is there any site which gives any spec on this?
Eberspachers, Is it easy to convert a 24v truck unit to 12v to suit a boat, as my friend is getting me one for nothing.
Thanks again.

 

[gardenshed]

Laminated should be stronger because you glue together thin strips of wood with the grain running in the direction that you want. You are not leaving nature to determine what's going on "inside" the wood.
During a gust a number of things are happening:
The centre of pressure moves aft (sails changing shape due to the increased windspeed). Your centre of lateral resistance remains the same and so there is a greater "moment" (twisting force) trying to round you up into the wind. You react to this to keep the boat going straight hence the tiller "loading up". Also, depending on the shape of the hull, when heeled over this may also effect the relationship between COP & CLR causing your boat to try and round up.
Mast Rake: the distance that the masthead is ahead or or (more normally) behind the mast foot. More rake = mast tipped further back. Reason: if too far forward, you may get inversion (mid mast bending backwards usually followed by failure. Rake also helps position the centre of pressure of the sailplan in relation to the centre of later resistance of the hull. Too much rake = too much weather helm. Too little = lee elm or very little /neutral helm. Exactly what you need is a combination of the boat and the sails that you have. If a series production boat, the class association may give some advice. If not, a decent sailmaker can advise. Websites from Selden, North, UK sails etc all give good tips on rig tuning.
The whole COP/CLR relationship is most easily demonstrated by the way that a windsurfer is steered. Rake the rig forward (COP moves forward) and the board turns away from the wind. Rake it aft, (COP moves aft) and the board luffs up into the wind.
COP = the equivalent single point that represents the force produced by your sails. The way you trim the sails has the biggest impact on this (sheeting in/out) but the rig position is also a major factor.
CLR = Centre of lateral resistance = the point the you can push the boat sideways and it will not turn. i.e. push sideways at the bow and it turns one way, push at the sterm and it turns the other way. Push it in exactly the right place in the middle, and it doesn't turn.
Ebersbacher: dunno

 

[Sinbad2222]

Have to differ from you slightly I'm afraid.
The sails do not necessarily change shape in a gust. Even if they didn't heeling would still produce weather helm.
The actual explanation of why a yacht turns to windward in a gust is that while the the centre of hydrodynamic resistance moves slightly to leeward as the boat heels (due to the induced assymmetry in the hull shape presented to the water) at the same time the centre of effort moves even more to leeward and thus results in a turning moment into the wind.
This also explains why, contrary to intuition, a bigger genoa does not always reduce weather helm as it causes even more heeling with a resultant increased tendency for the yacht to round up.

 

[Pye_End]

Agree that the centre of bouyancy is the biggest driver for weather helm, and this moves as the boat heels.

Think poster would like to know how much though rather than the theory.

My 26 is about 12 to 18 inches, but would do not know if you keep this angle with a larger boat.

Worth bearing in mind the theory though as if you rake the mast forward it will reduce weather helm, if this is a problem.

 

[VicMallows]

FWIW, the normal rake on my Sadler 29 is around 1deg. (ie a plumb line dropped from the masthead would be about 7 inch rear of the rear face of the mast). However don't claim to be into sail/mast tuning!

Running a 24v Eber from 12v is going to be awkward. The most viable option would probably be to acquire a trashed 12v one (often available on ebay) and swap the motor. Not a five minute option though!. I think if I had a free 24v one I would configure my batteries to deliver 24v. Not something I would recommend though unless it's the sort of thing you'd have thought up yourself.

Edit: (sorry, got the 1deg and distance mixed up - said I was not into tuning)


Vic

 

[Sinbad2222]

Point taken.
I refer you to Ivor Dedekams superb book "Sail & Rig Tuning".
For mast rake he gives:
For Masthead Rig 0.5 - 1 degree ie 9-17.5 mm/m of mast height
For Fractional Rig 2-3 degrees ie 35-52.5 mm/m of mast height

 

[William_H]

All right I am tempted to show my ignorance by saying what I think...
Mast rake will have a lot to do with the relationship of the chainplates to the mast. On a mast head rig often the caps are square to the mast but the intermediate sidestay chain plates are aft of square to the mast. This distance aft provides a pull backwards of the centre of the mast against usually an intermdiate forestay whose distance forward of the mast provides pull forward to counteract the int. sidestay pull aft. Hopefully the effect being to hold the middle of the mast in place (in column) taking the downward pressure of forestay and sidestays etc. An extreme rake aft would see the top of the int. stays directly above the chain plates and so no aft pull.
I wouldn't want to rake the mast too far aft as you then reduce that aft pull distance of the intermediates.
On a fractional rig the aft of sqare position of the sidestays (caps) is even more critical as it often matches aft sweep of the spreaders tpo provide that push aft of the middle of the mast. (in lieu of int, forestay)
So I reckon it is important for the mast rake to be as little as possible but in any case relating to what the designer of the mast stay system had in mind.

Regarding rounding up (weather helm) if we look at a surfboard (no sail) it is steered entirely by varying the drag of the hull by varying the heel to left or right. So to my mind the greatest weather helm under pressure comes from the hull asymetry in the water when heeled. However of course this depends on the shape of the hull. But as said reduce jib size reduce weather helm even thogh COP moves aft the heeling is less so less weather helm.

So for my money mast vertical (it also gives more head clearance from boom and certainnly in smaller baots the mast rakes back as the stern goes down with crew on board. olewill

 

[Sinbad2222]

Good points William.
As you say depends very much on hull design which is the greater contributer to weather helm,sail plan, hull shape or the turning moment due to the forward component of wind pressure on the sail of the heeled boat.
To clarify with a ridiculous extreme, suppose the hull was cylindrical in shape like a log, then rolling of such a hull would have no effect on the hull shape presented to the water so you would not get the surfboard effect. If such a boat rolled however due to wind pressure on the sail then the mast and sail would lean over and there would thus be an induced moment twisting the hull into the wind ie weather helm. This is quite apart from weather helm produced by the difference in foresail and mainsail areas.
To take another ridiculous extreme case: of a catamaran with an enormous distance between the hulls, or an enormously wide surfboard. In this case heeling would be negligible so of the three causes of weather helm only the different areas of the two sails would cause weather helm but it would still be there.
Hope this is clear, it's so much easier to show with a diagram!
I would also like to point out that pre-bend is the best preventer of mast breakage on a fractional rig and rake is pretty well irrelevant.

EBERSPACHER I would go for an Airtronic D2 or D4 rather than the old D1LCs etc. The latter on eBay are usually clapped out ex Post Office vehicles. They also have the major disadvantage of needing 20 amps on start-up whereas the Airtronics need only 8 amps. This reduces the likelihood of voltage drop due to thin cables, dodgy fuses etc.

 

[Gordonmc]

As well as the motor you would need a 24v control box, glow plug and possibly a fuel pump.
24v ebers are cheaper on Ebay because of the hassle converting them. Take off all the 24v bits hanging off one and you're not left with much.
Doubling up two small 12v batteries is a possibility, but you then need some means of charging them.