Daggerboard position

[miked12]

Lately I have been wondering about the affect of daggerboard position on performance. More specifically how does forward and aft positioning affect speed and pointing ability. Attached is a rough sketch of a design with the daggerboard forward of the mast. Is this going to have any affect on the weather helm? Any feedback would be appreciated.

Originally Posted by miked12

Lately I have been wondering about the affect of daggerboard position on performance. More specifically how does forward and aft positioning affect speed and pointing ability. Attached is a rough sketch of a design with the daggerboard forward of the mast. Is this going to have any affect on the weather helm? Any feedback would be appreciated.

A dinghy I assume?

Its all to do with the centre of effort of the sails compared to that of the foils. Weather helm is something people get confused about though. These days most of the development boats load up the rudder with sideforce, so that the lateral resistance is distributed between rudder and daggerboard. This is probably less drag than all the work on the centreboard, plus you can get away with a smaller centreboard.

This leads to a pull on the rudder from the side force (unless the blade is balanced) but if the boat can be sailed in a straight line without moving the rudder from dead central then I'd day the boat doesn't have weather helm.

If on the other hand you have the situation where you've got to pull the tiller say 5 degrees to windward to keep the boat on a straight track then that offset is causing drag, and I'd say that is slow.

If you move the daggerboard in front of the mast then the centre of effort will be midway between rudder and daggerboard. The daggerboard can be smaller, because its doing less work, the rudder will have to be much larger, because its doing more, and you'll always have a fair old pull on the rudder unless you have a balanced blade (ie with the pivot axis above the centre of the blade, not the front). It'd also be a nuisance to pull up and down!

It would be possible to build a boat to sail like that, but I doubt its a great idea, because the much bigger rudder would be more drag when used for steering. But I could be wrong!

[PAR]

Most day sailed craft have weather helm built in for other reasons, one being safety. If you dump the crew or it dumps you then the weather helm will bring the boat up into the wind and the boat stops, the sail(s) will flog and you can swim back and reboard her. Without this feature the boat will sail on and you're left with a long swim to shore.

Sailing by the lee, the reverse of weather helm, is difficult for most folks who learned on a conventional boat, and can be dangerous as is sailing with no helm feel. If you get caught with you pants down with a neutral helm (no feel) then you don't know if to come up or fall off to ease (ask any cat driver what happens when you guess wrong)

Not having a boat talk to you by way of feed back through the helm is difficult to keep in the groove. Therefore a couple of or a few degrees of helm to weather is necessary to provide the ability to steer and know what's going on. You may not be paying attention to the rig, sails or wind, but you'll feel the helm changes and with experience instinctively do what needs be.

To answer the questions, it depends on the hull design, placement and shape of the appendages and rig. You can't just move the board around and hope for the best. Moving one a few inches fore or aft can effect the boat quite dramatically, maybe for the good, but if a reasonable design then most likely for worse.

You can fool with sectional and profile shapes, area, proportion to rudder, lead, rig to appendage relationships, etc, but there is a bit of math involved and not something easily to work you though in an on line forum.

I didn't see a picture posted, what are you intrested in doing?

[human]

I understand the concept of balance (lead between Center of Effort of Sails and Center of Lateral Resistance of underbody). These two centres produce 'balance' (little bit of weather helm). But, I am still perplexed by the longitudinal positioning of keel; what impact does it have on the performance of the hull (midship, aft or forward)? One can move the rig around in concert with the keel (or daggerboard) to always produce the the correct 'lead'. Does a keel aft produce more directional stability?

[SailDesign]

But, I am still perplexed by the longitudinal positioning of keel;

To be honest, the positioning of the keel on most ballasted keel-boats has more to do with the longitudinal position of the Centre of Gravity.
Balance comes second, and explains why some keels are swept back and some aren't.
(That's the simple version, anyway...)

Steve

[human]

Originally Posted by SailDesign

To be honest, the positioning of the keel on most ballasted keel-boats has more to do with the longitudinal position of the Centre of Gravity.
Balance comes second, and explains why some keels are swept back and some aren't.
(That's the simple version, anyway...)

Steve

Thank you Steve

Now, The shifting of weights around the boat (to get LCF and CG aligned) normally leaves one to position the keel (as the largest single weight- 1400kg out of a total weight -displacement of 4500kg on the boat I am working on) close to CG. But, there are other weights that one can shift (engine, tanks, equipment, etc; that comes to 550kg on the boat that I am working on). I am still perplexed; and the thing is that the books/literature appears to be silent on this matter. Would shifting the keel around have an effect on performance (speed and behaviour)?

[CDBarry]

The keel, or the dagger board also produces wave drag due to lift. Thus it's longitudinal position will have an effect on the total drag of the hull, (like a bulb bow or stern flap) though what effect is probably a pretty difficult question to answer, and will vary with boat speed. There will also be an interaction between hull lift (to windward) and appendage lift. This could induce some interference losses or favorable or unfavorable interactions between the shed votices of the hull and the appendage. Again, a fairly difficult problem.