where to place a mast on a new boat, without plans

[barnaclephill]

A non computer literate friend is building a boat as a copy of a Hartley 25 without plans. The only stated dimensions are: 7.5m mast, 8' bowsprit but cutter rigged, 25' hull LOD. Given the calculations for the Centre of Effort of both (all the) sails, what will be the formula for the placement of the mast? I understand that a line is drawn from the COE of both sails and somewhere along this line, is vertically above where the mast will/should be stepped.

Also, any rules of thumb for the placement of the centreboard in a shallow long skeg, or is it a matter of making a cardboard template and balancing that on a knitting needle type thing?

 

[kds]

"a copy of a Hartley 25 without plans"

Why - is intellectually challenged or just mean ?
Ken /forums/images/graemlins/shocked.gif

 

[Lakesailor]

As I understand it (I've been told by kitboat designers) the COE of the sail plan needs to be roughly above the pivot point of the hull. Probably above the centreboard. Moving it forwards give lee helm and aft give weather helm.

I made alternative steps for the mast on my trimaran so that I could experiment.

 

[Blueboatman]

Calculating the Lead
It is not an exact science,much will depend on the shape of the boat and how 'distorted'the underwater part becomes when heeled over-more significant on short fat boats..
By building a pivoting type centreboard your mate may well have a better chance of subtly balancing the sailing characteristics.
He would do well to talk to other Hartley owners,it takes as long to build a boat incorrectly balanced as one that is a joy to helm,if you see what I mean!

 

[Blueboatman]

Multihulls pose more of a problem cos when they really get going everything,the apparent wind ,coe and clr all change,also there are latteral issues depending on how much you are prepared to fly a hull..But you knew that..
Would you get booked for speeding under sail on the lakes by the way ? And when can the forum expect a timed lap 'sailpast' at the ferry crossing webcamera?

 

[barnaclephill]

[ QUOTE ]

Why - is intellectually challenged or just mean ?


[/ QUOTE ]
Yes Ken, he is cheap, in both the resourcefulness and frugal senses of the word. He is also a carpenter and is building it without buying the plans but making in timber what picture he sees, like earlier craftsmen.

Blueboatman: a very good article which explains it all - thanks a lot. The link has too many "http's" though. I didn't think of the underwater shape aspect also.Thanks.

Thanks LakeSailor, just like the alternative mast settings on the Brazilian 'jangads' (spelling?).
Will print out the GoodOldBoat article and comments to post to him. Thanks guys.

 

[Lakesailor]

That's why I went for lots of options. As I'm using a lee board instead of a centreboard that's another variable, tack to tack.
If I sail at more than 6 knots in the Bowness Bay area or the Lakeside or Waterhead areas at the top and bottom of the lake they can tell me off. If I get over 10 knots on the rest of the lake they can't touch me as the overall limit only applies to powered craft.

The web sail past is on the agenda but there are a couple of issues.

1: as it doesn't go to windward too well, and I'm not registering it for an outboard, I may have problems getting to, or back from, the webcam.

2: At the moment the smallish sail isn't letting me get much over 5 knots. However I have a full rig Laser sail on it's way (doubling my sail area) so the alternative mast step positions will come into their own.
Once I've got that right we should be on for a sail-past.

 

[William_H]

I don't think the location of the mast is nearly as critical as some would suggest.
The centre of lateral resistance of the keel/cb when down theoretically should be directly under of centre of effort of the sails.
For most boats then the keel centre is a little forward of centre of the length bow to stern with the mast stepped a little forward ie 1/2 metre forward of the centre of the keel.

A cutter rig should be really fine but obviously you would hope the boat will sail fairly easily with and without the jib on the bowsprit. This represents a huge shift in COA. of course he will also need to reef at some stage of wind strength.

Many boats up to this size have a mast step that can allow the mast to be moved forward and aft by 10cms or so. Although in my experience (with other peoples boats) it doesn't make a lot of difference. It is of course very important to have a mast support under the mast base and this, if it does not move will limit how far you can reasonably vary the mast base location.

The concept of weather helm/lee helm is usually completely overwhelmed by the weather helm that occurs when the boat heels over and the drag becomes asymetric. (more or less depending on hull shape) So to my mind it is more important to have a big partially balanced rudder to overcome the weather helm rather than try to get sails balanced. (there will be some howls of disagreement here)

So I applaud your mate for having a go. I have friend who designed his own hull. It is a bit like a cat with twin bows and twin rudders but single centre drop keel. 2 years back he took it home and added another 2 ft to make it a 22fter. it is strange to see but it really goes well. He has incrementally improved the performance over the years. I say every man to his own obsession. he even got a write up in an oz sailing magazine.

So yes let him learn by his own mistakes and enjoy the journey. olewill

 

[Blueboatman]

Interesting..I thoroughly agree with your mates tale of incrementally improving performance by changing things gradually ,it just takes time both on and off the water !
Regarding mast position not being so critical. One could argue that the actual position isnt so important because the sizes of the actual sails will vary according to the 'area' available in front of and behind the mast,thus offsetting or compensating for the mast foot position.
Remember all those 1970s designs with small high aspect ratio mains and big deckscraping genoas ?
Adjusting the mast rake can be used too for tweaking.
So yup,have a go but be prepared to be patient.

 

[Blueboatman]

Love it !
The laser rig should be very lively
I was joking about the speed limits but ,Crikey,maybe you should wear a disguise when doing the 'flybye'

 

[Sequoiah1]

The old fashioned way to deal with this is as follows.

Work out the flat plane centre of effort of the entire up wind sail plan .

Work out the centre of balance of the underwater profile (with board) and rudder.

The C of E of sail plan should be approx 6% of lwl forward of the CLR(centre of balance) of the underwater profile. You will find you can play with the position of the board to see what happens when you dont have it down.