Design Questions for 16-20 foot sailboats

I have two questions currently,

Is there a ratio range on where the -in this case- daggerboard is placed along the boat, or another way put, how far aft of the mast will the daggerboard go?

is there also a ratio between mast length and draft? and would it be valid to assume in general that a longer daggerboard would make a boat more stable if sailing with less crew and hence less displacement? conversly a shorter board with more displacement?

This does not pertain specifically to any certain boat but if there was somthing in mid to visualize I am building something with elements of a Finot OPen 5.70 and an RS K6 by LDC

Thank you

Alix

 

There are no hard and fast ratios as you describe. The area and center of the daggerboard are balanced against the area and center of the sailplan, with the area of the sailplan being ahead (forward) of the underwater area of the hull, including the daggerboard. This distance between the two centers is called "lead" (rhymes with "seed"). Different amounts of lead are appropriate to different designs, and there are various rules of thumb to use to adjust the "balance" between the two (as it is commonly referred to.)

In general, it is hard to have too much draft--draft almost always works to your favor for windward ability. Practical considerations usually limit draft (like the depth of water you are sailing in). Daggerboards generally are not weighted very much, so their weight, such as it is, adds little to the displacement and stability of the boat.

In general, I have found that for short keeled boats (meaning short in the fore/aft direction) where the keel or daggerboard is deeper than it is wide, then the center of area of the sail plan should fall right about in line with the leading edge of the keel or daggerboard.

Hope this helps. You should get some books on boat design to learn more. "Principles of Yacht Design" by Larsson and Eliasson is the current accepted standard. Ted Brewer's book "Ted Brewer Explains Sailboat Design" is another good reference for the amateur, which has an in-depth discussion of lead.

Eric

 

As Eric says above, there is no definite figure for the 'lead', but if you want a starting point, I would suggest 11% of waterline length.

If you are not quite sure about the balance of your design I would say put the daggerboard just a tiny bit forward of where the rules of thumb would have it placed then make the rudder rather larger than average. If the boat turns out to sail with the tiller slightly to leeward you can then trim the rudder down, which is not difficult with most small boats. If it turns out to be satisfactorily balanced then you will just have a slightly oversized rudder which is not such a bad thing, far more boats have rudders too small than too big.

John

 

I agree with Eric and John; just keep in mind that in dinghies the balance of the boat is heavily determined by the crew weight and position; so take a good rule of thumb in projecting the boat, and then try different positions and settings of the crew when you begin to sail your project, that will be the only way to achieve the best results on the water.

Mistral

 

Thanks for the advice.
In response to John Perry, If there was too much Lee helm from the Daggerboard position this could be decreased by increasing the Mast Rake and the crew displacement?

 

I'd give a bit less lead than the "standards" given for keel boat designs. Usually dingys are sailed more upright, so you don't get the weather helm induced from heeling. I've sailed numerous family daysailor type boats (not high performance dingys) that have an annoying lee helm, even in stronger winds.

 

Increasing mast rake is another possible remedial action which will slightly reduce lee helm. Simple calculations will show how much you can move the centres of pressure by raking the mast or by trimming the rudder size. The large separation between the daggerboard and rudder makes a change in rudder size quite an effective method of moving the underwater centre of pressure. In any case, I feel that a larger than average rudder is no bad thing, apart from improving manouverability and reducing leeway it will reduce the actual helm angle caused by any imbalance of hull and rig. The main downside is a bit of extra wetted area.

Longitudinal movement of crew weight generally has little effect on helm balance since the change it makes to the longitudinal position of the centre of underwater pressure is likely to be more or less cancelled by the change it makes to mast rake.

Helm balance is much affected by angle of heel and hence by lateral position of crew weight. The designer can only aim for ideal balance at a normal sailing angle of heel and accept some weather helm at extreme angles of heel and also perhaps a little lee helm at very small heel angles.

John

 

Excellent. Thanks John, Now once I get rid of the Virus on my home computer I can get some more progress done.

thanks all.

 

Virus

This is free and you get scanned with the latest virus signature. (No connection but I use their stuff)

http://housecall.trendmicro.com/

Their regular version of pccillin is excellent as well.