Monday, February 20, 2017

______________ LENGTHENING A ZEPHYRS CENTERBOARD _____________

For the second time in a row I have purchased a boat with a centerboard that is woefully inadequate and shorter than is allowable in the class rules. You can see how I dealt to the smaller than allowable centerboard in my first purchase of a Starling Class sailing dinghy here:

My new purchase 'Slipstream' a Zephyr Class sailing dinghy and big sister to the Starling came with two centerboards. The first board is too short and compounds the lack of area by tapering at the bottom of the board - this board also has a pronounced warp in it which also compromises the boards performance. The second board is the correct width (without any taper) but is short of allowable length by a full 36 centimeters. It is this second board (without any warp) which today I began the process of lengthening.

This is the length of cedar timber that I am preparing for splicing into the centerboard. I am spiking the edges of the join in both the centerboard and the additional splice of cedar so the impregnated glue will act as a myriad of small nails. The centerboard has to support my weight if I happen to capsize 'Slipstream' so the join has to be strong.

This is the sort of building situation where sash cramps really come into there own. I only have two and both were put to good use.

The next job is to plane the additional length in sympathy with the existing aerofoil shape of the board and plane the timber to the correct width.

When I added additional length and width to the Starlings centerboard there was a noticeable improvement in windward performance. The speed of the boat through the water didn't increase but the boat pointed higher which meant I didn't have to sail as far as I had done previously and I was able to engage a lot more tactically with the other boats as the boat wasn't sagging off to leeward as it had done previously. I am hoping for a similar improvement with the Zephyrs windward performance although it will be a little difficult to measure as I don't have another Zephyr at the present time to compare performance with........

But this may change .......... tonight I had a phone call...... but my lips are sealed.... watch this space.


Ben said...

Hi Alden,
I can’t help thinking this is too coincidental. Aren’t you overlooking something? May be the design of a centreboard is optimized for different situations. I know that a longer centreboard of the same width creates more “lift”, so that you can sail closer to the wind. However it creates also more drag, that slows you down. You win perhaps at close hauled but loose at beam reach? Personally I would have liked to test at least two centreboards of different length and measure the performance. The outcome could be that you take different centreboards for different race conditions and routes. Although not a sailor, I like experiments :-), as you know.

Alden Smith said...

Ben, you are quite correct that with underwater areas where racing is concerned there will always be a balance to be found between 'lift' and 'drag'(friction).

This 'lift' and 'drag' compromise is more of a factor for the designer in a yacht with a fixed keel because the amount of keel area cannot be altered.

In a centerboard sailing dinghy the centerboard can be kept down when sailing to windward (into the wind) to make use of the full area of the centerboard and providing maximum lift - but - when sailing downwind the centerboard can be pulled up to reduce drag. This is what I do when racing 'Slipstream' :

- Centerboard fully down when sailing to windward.

- Halfway up when running downwind with the mains'l 90 degrees to the wind direction.

- Partially up when sailing on a beam reach.

Judgement that comes with experience is used when deciding how much to pull the centerboard up when sailing off the wind especially in fresh conditions because the amount of 'grip' in the water that the centerboard gives provides direction for steering and balance to keep the boat upright.

Ben said...

All right! Therefore you want to have the maximum allowed length of centreboard.
Did not know that you could use different positions. Thought that the centreboard would pop out when not fully downward.
Interesting. As a racer you not only have to trim the sails but also the centreboard.

Alden Smith said...

Yes, the maximum allowed length - which is 1370mm, which makes for quite a long centerboard.

As the centerboard is made of wood it has positive flotation which means it can have a tendency to float upwards a little when sailing. To stop that happening I have a length of elastic 'bungy cord' which I loop over the top centerboard to stop it floating up. When sailing downwind I quickly take the loop off so that I can raise the centerboard vertically and reduce drag.

Bursledon Blogger said...

Presumably it's a daggerboard, over here a centreboard is a pivoting board v's a daggerboard which slides up and down vertically (like a laser, optimist etc).

Whatever it's called looks like you did a good job.


Alden Smith said...

Max, you are quite right. It is a dagger board not a centerboard. Pivoting boards are not all that common in NZ - the OK Dinghy and Finn being the exception - so the terms seem to have become interchangeable over the years.

Personally I like the pivoting board as it is a hell of a lot more forgiving - dagger boards have a rather abrupt way of indicating you have hit the bottom.