is the center board only about ballast?

[RichKrough]

I'm getting near the point of finally assembling my Hartley 16. I was laying out the componets to make the center board trunk last night when it occured to me that the center board trunk only comes up about 7" above the cockpit floor. If I reduced the width of the centerboard by 7" but increased the thickness from 3/8" to 5/8" the center board would weigh the same and give me a unobstructed floor in the cockpit. Any opinions on what impact this will have on the boat's handling under sail?
http://home.nycap.rr.com/richboat/centerboard1.jpg

[Bruce Hooke]

Does the board swing all the way down so that it is perpendicular to the bottom (or close to it) or does it just drop down until the top aft corner is near the bottom of the boat?

Either way you are going to loose quite a bit of lateral surface so it seems to me that the boat would likely not go into the wind nearly as well, plus you might well throw off the balance under sail...

[Bruce Hooke]

Following up...I just re-read the title of your thread, and NO, the centerboard is not just about ballast. It is also about keeping the boat from slipping sideways when you are on a beam reach or a close haul. And, it is also about the "balance" of the boat when under sail. A poorly balanced boat will either have a strong tendancy to head up into the wind, which is annoying and slows you down because you have to turn the rudder more, which adds drag; or the boat will want to fall off (turn away from the wind) in a gust, which is downright dangerous. A well-balanced boat should have a small tendancy to head up into the wind in a gust, which spills the wind and keeps you from healing over too far. A key part of the balance is the area and location of the centerboard. On many boats the centerboard is just heavy enough to get it to sink when it is lowered, so weight is probably the least important part of its job in many cases...

[Dan McCosh]

I once built a 16-footer with a wood centerboard, and later changed it to steel to help the stability. The weight of a 3/8 ins. steel board is substantial on a boat that size. Regardless, the main function is to provide lateral resistance. This in turn is mainly function of the length, rather than the width of the board, which means that making it narrower could even enhance the performance, by reducing drag. If the pivot point remained the same,it could also affect the balance of the boat, by changing the center of lateral resistance. The designer is supposed to think about these things. The change likely would work, but it would need some thought.

[JimD]

An old problem with centerboards and Bruce has just about covered all there is to say about it. Since the 'top' of the board in the up position is the after part of the board in the down position you would be moving the center of lateral resistance forward so the boat would probably have a tendency to turn into the wind more (weather helm), as well as being pushed sideways more due to less board over all. You'd have to do some serious redesigning to overcome all this. Best to just leave it alone and have the board case intrude in the cockpit.

[RichKrough]

Thanks for the replies fellas. In answer to Bruce's question. If I am interpeting the plans correctly the board looks like this when it is down. http://home.nycap.rr.com/richboat/centerboard2.jpg

(I think a couple of WB forumites have Hartleys so they are welcome to correct me)
Scaling from the plans there is about 7.75 square feet of board exposed in the water, if I were to cut the board down it would lose 1.2 square feet of exposed surface area(about 15%)
From the sound of things this might be something I best leave alone. I don't have enough knowledge about sailboats yet to redesign them. I did email Hartley I'll post thier reply when I get it.

[ 08-24-2005, 04:55 PM: Message edited by: RichKrough ]

[JimD]

If that's the correct lowered postion the square foot or so you'll lose probably won't make a huge difference. If you wanted to give yourself a crash course in basic design it might not be too hard to make the board a little longer, narrower, and let the whole thing sit lower, as in the lead edge of the board hanging down out the bottom of the boat a few inches when the board is retracted. Maybe. :D

Have a look at this:

http://www.tedbrewer.com/yachtdesign.html

[ 08-24-2005, 05:20 PM: Message edited by: JimD ]

[htom]

At a guess it would have to be a foot longer to get the area, and then lower to about 45 degrees (or even more) to move that area forward if you kept the same pivot point. This is a great example of how "you can't do just one thing".

[LLaver]

I am not familiar with this design, is the top of the centreboard case fully enclosed.

Lee

[Don Maurer]

If I reduced the width of the centerboard by 7" but increased the thickness from 3/8" to 5/8" the center board would weigh the same and give me a unobstructed floor in the cockpit. Any opinions on what impact this will have on the boat's handling under sail?
In my not so humble opinion, if you reduce the centerboard width by 7 inches to give you an unobstructed floor in the cockpit, you will spend all your time bailing and none of your time sailing because the top of your centerboard case will be below the waterline. Unless it is an external centerboard case, it must be high enough for the top to be above the waterline when the boat is swamped.

[JimJ]

The position of the board shown above is not the full down position. The projection (tongue) at the forward top of the board that appears to hit the top of the C/B case should in fact rotate up and aft until the board is more vertical.

Is the top of your C/B case sealed or does it have a slot running aft along about 2/3 of the top of the C/B case.

Here is the rebuilt C/B case from my TS18 with the board superimposed in red.

http://www.imagestation.com/picture/sraid183/p39b84fffb13914f30360e6fd0dda4e3a/f2ab0a88.jpg

In this photo I do not have the two compression posts installed at the forward end of the C/B case. The tongue of the board rotates up and forward and travels between the compression posts when raised.

Here is a TS16 cabin. The C/B should come aft to the stop through the slot in the top of the case.

http://www.imagestation.com/picture/sraid183/p8aa57768beb49454db633d98ac7dd736/f2aafffc.jpg

[Bruce Hooke]

Another thing to look out for when making this sort of change is that by narrowing the centerboard you greatly increase the forces on the board and centerboard trunk because the shorter trunk means the board has more leverage on the trunk. With a steel board the extra force on the board won't matter, but the extra force on the trunk is an issue.

[RichKrough]

JimJ has it right. I went back and made scaled cut outs on poster board of the center board and case It will be very nearly verticle when all the way down (40" below the keel) That being the situation I would lose over 40% of the boards surface area.

I am going to forget this idea, but I really appreciate everyone's input