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britinusa
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5404 Posts

Response Posted - 03/25/2012 : 17:31:51

Not Arlyn's 'someone' but we make a point of tightening our backstay hard. It has made a world of difference in the handling of the boat.

Paul

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Arlyn Stewart
Master Marine Consultant

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Response Posted - 03/25/2012 : 20:38:41

General sailplan theory says that moving the sailplan forward, ie raking the mast forward, reduces weather helm. And... that is true when the weather helm is caused by an aft sailplan yaw force or couple.

One of the clues that that is not the issue with the 250 is that the boat will actually suffer adverse weather helm under headsail alone if it heels too far. This means that with little doubt that the heeling issue is not the sailplan, which leaves something to do with the hull or keel.

The most significant clue to the cause came with the awareness that trimming the bow down brought some relief especially given that to go 180 deg to general theory that trimming the stern down relieves weather helm.

When exploring the possible reasons of why trimming the bow down could bring relief, the most likely explanation is that the center of the yaw force when heeling is moved. If the yaw force is forward of the boat balance or keel pivot shoving the bow to weather... the force center moves aft. If the force is aft shoving the stern leeward, it moves forward. Which ever, then lessens the lever arm of the force as the force is brought closer to the boat balance point.

Personally, I think the force is the asymmetrical footprint when the boat is heeled significantly. We know that the hull form for water ballast uses hard chines to increase righting arm and the wing keel inherited the hull form. We also know the 250 suffers severe leeway when heeling excessive, a sign that its hull form is lifting leeward... thus, the clues are that the stern is getting forced leeward by asymmetrical form lift.

When trimming the bow down, the center of that lifting foil moves forward closer to boat balance thus reducing the lever arm of the force.

I've argued for years that there is a solution but it would come at a cost of drag though the boat could then get rid of the oversize rudder that is required for it so the net drag might not be as bad as first thought. I've argued that the boat could use a pair of asymmetrical dagger boards well aft or possibly twin asymmetrical rudders. When the boat heeled, the windward board would clear the water and come out of play so its lift toward the center of the boat would be lost and the opposing board would then go vertical where its lift is maximum toward weather thus opposing the leeward hull form lift.

Just something to tease the mind.

Arlyn

Edited by - on

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 03/25/2012 : 20:59:41

Technically I'm not shortening the forestay, just eliminating a turnbuckle obstruction.

The suggested rake on the mast is 4". When I first got the boat the rake was 12", with pretty severe weather helm. Increased ballast at the bow brought the boat onto her lines and improved rake to about 8", with a noticeable improvement in weather helm. Last season I tried to adjust the forestay turnbuckle to go to 4" rake, and did all the trigonometric and thread pitch calculations to determine that I needed 10 turns to get there. However, the four washers under the drum on my CDI furler caused the turnbuckle to bottom out after only 4 turns. I did not want to disassemble the furler over water due to the risk of losing parts overboard, so I decided to wait since weather helm was OK.

Now that I'm on the hard and have it all apart to install a ball bearing upgrade, I'm going to remove 3 of the washers which should allow the last 6 turns to bring me up to 10 total. If I don't like the way the boat handles after that, I can back off some - even all the way back to where I was last season.

This really isn't about getting to some holy grail of zero weather helm. It's just about getting a fresh start using Catalina's recommended spec for rake, and making tweaks from there.

Edited by - TakeFive on 03/26/2012 04:38:34

Arlyn Stewart
Master Marine Consultant

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2980 Posts

Response Posted - 03/27/2012 : 04:24:39

Thanks for the explanation... yeah, It was necessary to counter bore the furler block to get all the turns on the turn buckle.

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Johnnybob
1st Mate

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32 Posts

Response Posted - 03/29/2012 : 16:32:11

Great looking table.I really like your choice of the cherry for the finish. Looks comfortable and easy to fold and unfold. My table remains stowed most of the time because it is so big. Thanks for sharing the process.

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TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 05/04/2012 : 19:21:08

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by britinusa
 Looks really good!!

When are the pics of it installed due release?

Paul
<hr height="1" noshade id="quote"></blockquote id="quote">
I splashed today, so I got the table down to the boat last night. Here are a few pics that I took as the sun was setting today:

<center>

</center>

Access the to v-berth and settee storage with the leaves down is SO MUCH better. I spent all of last season with the table stored in my basement because it was so intrusive. But there were times when I missed having something there. For instance, today, as I was tinkering in the cabin to get things ready for a short cruise this weekend, I pulled the leaf up a couple of times so I could have a work area. So even though I've never been a fan of drop leaves, in this application it's the perfect compromise. It will be nice having the table there whenever I want it, and out of the way when I don't.

I'd recommend this modification to anyone who has the courage to risk chopping his table in three. The cherry veneer was my own finishing touch, but not a necessary part of the project. Whatever finish you have (which varies with different vintage boats), all you really need to do is find a matching edge band for the part that you cut.

Edited by - TakeFive on 05/04/2012 19:57:32

britinusa
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Response Posted - 05/04/2012 : 20:25:22

11!

Paul

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Tradewind
Admiral

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531 Posts

Response Posted - 05/05/2012 : 06:07:29

Beautiful job! I'd love to try that mod, not sure my skills are up to it.

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TakeFive
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2272 Posts

Response Posted - 05/09/2012 : 20:10:34

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 General sailplan theory says that moving the sailplan forward, ie raking the mast forward, reduces weather helm. And... that is true when the weather helm is caused by an aft sailplan yaw force or couple.

One of the clues that that is not the issue with the 250 is that the boat will actually suffer adverse weather helm under headsail alone if it heels too far...

...Just something to tease the mind.

Arlyn
<hr height="1" noshade id="quote"></blockquote id="quote">
Just a quick comment. We did our first mini-cruise this weekend, and had several hours of close-hauled tacking on Sunday into a steady 10 kt breeze. Before launching, I had adjusted the mast rake to 4" with the waterline perfectly level on the hard, and actually haven't had a chance to recheck it since I've been on the water. However, I did note that in these steady breezes, the helm was perfectly balanced. I could just let go of the wheel and it stayed right in place. Realize that the wheel does have some inherent resistance, and I'm not sure a tiller would have been perfectly steady, but the wheel did not push back at all under these conditions. This was significant, because in past years I would often have to reef at around 10 kt (with full jib) to get good balance.

It's still too early in the season to make final conclusions, but so far it seems that the adjustment I made has taken an acceptable situation (I did not mind reefing at 10 kt) and made it better.

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