The good news is that it not the fault of the boats, as some owners report how delightful their boats are on the helm. My boat, Sergeant Pepper, would be fine up to 18 knots but then life became hard work ... unless I paid a lot of attention to my sail trim. All of that is now in the past and she sails like a dream.
 
The story is that Van de Stadt first designed the Trintella II, but when it was built, they found out that it was a little bit rough on the rudder - too much weather helm. Therefore Van de Stadt shorted the keel, lengthened the back end of the boat and drew a rudder, with a small balancing part, on a skeg. This was the Trintella IIa and later the Seacracker 33. 
 
Before tackling the problem I had to understand the cause of weather helm in the first place. It must be said, is a desirable characteristic in moderation else you will watch your boat sail off without you if you go over the side when single-handing.

Weather helm is the tendency of the boat to round to windward. Let go of the helm and she'll point up and possibly just stop.  No broaching (as may happen with lee helm) and no great excitement. This is caused by the effort of the energy pushing the boat along being aft of the mast. Simply, the main is more powerful than the genoa. To reduce weather helm, this centre of effort needs to be moved forward; it's as simple as that.

So now to consider the primary factors over which we, as skippers, have control. In other words, how may we move this centre of effort (C of E) forward?

1. Shape of the mainsail
Most skippers keep the sail for well past it's designed life. Given the cost, we like to use them until they rot. But a tired sail which has stretched becomes baggier, and the bagginess is worse around the "sweet spot" where the most effort is being generated. The luff is held against the mast so the worst stretching is aft - towards the unsecured leech. As the baggy part becomes baggier, the draft of the sail moves aft and weather helm worsens. Take a look at your main and ask yourself if it's time for a new one.

Next time you secure the boat from a days sailing, slacken off the outhaul but when you go out, tighten the outhaul a little more than normal. This flattens the sail. Equally, many mains have a flattening reef placed perhaps one foot above the boom. Try it in calmer conditions and see if the flatter sail (which moves the C of E forward) helps.

2. Sail Trim
There's nothing like a good vang coupled with travelers with ample scope to travel. Some of the Seacracker's have pitiful travelers down in the cockpit well which prevent the skipper from doing much of anything. Others have the traveler fixed on the aft cockpit combing so are in a much better situation (but more on that later). Experiment with the traveler by staying on a steady course in steady winds and ease the traveler out. A lot. You may well find that the boat doesn't heel as much but the tiller may lighten up drastically.

3. Size of the mainsail
Ditchfield did it! Ditchfield was a designer who modified the sail plan and created drawings to extend the boom almost to the aft end of the cockpit. Now these aren't fractionally rigged boats intended for huge mainsails but fast boats with a keel configuration designed to be efficient. And to sail with an oversized main is asking for weather helm. Take a look and it may be that your main is just too big for the boat.

4. Mast angle
With a deck stepped mast, there's not a lot of choice, but what angle is your mast? If it's leaning forward, then something's wrong. If raked aft, then try easing the backstay. On Sergeant Pepper, the mast is vertical with a slight forward bend and this cured the problem once and for all.

5. Mainsail twist
It can be hard work to remember to use the vang, if you have one, but try tightening the vang - properly, with a winch - and see if reducing the twist helps. Of course if you still have roller furling on the boom, this won't help when you need it the most!

6. Install a traveler of a decent length
It's no use expecting a little traveler squeezed in the well of the cockpit to help you. When you need to de-power the main (which eases the helm) then you should be able to put the mainsheet purchase almost to the side of the cockpit. The equipment from Barton (with a Cruiser 3 series purchase) makes a great difference. 

7. Examine the rudder
A rudder which is too short will make the boat difficult to keep on course, but a rudder which is too long (measured fore and aft) can induce weather helm. There's no magic formula here, but it could be that removing as much as four inches from the trailing edge of your rudder could make all the difference. Don't fix this yourself, but be aware that if none of the above works, then this is the root of your problem.