Bowtubes Design

I recently had to review the complete process of designing a catamaran bow tube with MSC (Marine Safety Center) at the USCG.
The boat in question had a Gold Coast style timber space frame bow tube that had failed.
Usually the GC units are pinned to the hulls to allow movement, but in this case it was bonded, and(or) bolted to the deck. It had cracked, water got in, and it began to rot.

As everybody at MSC is new now, the design assumptions that we have used in the past are no longer valid. In the past I used an aluminum tube section and A frame wires that matched the mast section and shroud diameter. That sounded like voodoo to the new MSC. I had to come up with complete calculations.
I assumed the span for the Eulers column bucking calculations to be between the forestay and the hull; half the overall length.
MSC disagreed. I had to use the whole length and assume no support. I argued that the A frame restrained upward buckling, and the forestay restrained it from downward buckling. I finally convinced them. I also noted that as far as I know, not one of my COI cats using my stock bow tube arrangement has ever failed. They were barely impressed.
I was surprised how close the calcs were though. And again, there had never been a failure. I expected generous safety factors.
I realized that several other things help us out.
If the tube was to deflect, it would have to pull the hulls inward. The hulls have huge section properties. The tube is restrained by that.
The load that MSC used was the breaking strength of the forestay. Load cell studies that Roger Hatfield did showed that the forestay is loaded less than the shrouds are. Typically they are the same diameter.
The tube itself resists bending some.
And finally, the weight of the tube, fittings and wires helps resist the forestay load. Not much but some.
All these compensating factors are hard to calculate easily however. But the evidence is that none have failed.

Then I remembered that there was a bow tube on one of my cats that fails regularly, though so far, not catastrophically. It was designed back in the hand drawing days and has a welded moment connection. The welds give out.
It might have still worked except for one thing. The builder freestyled the main beam.
Before designing it, I had just come back from my first MACM composites conference where Dr. Reichard had presented a paper on the structural design of the catamaran Fury. I had used that information to design a main beam that resisted twisting and polar loads also. The fairing I designed flowed into the sides of the hull. It would have been a strong and stiff moment connection.
The builder had heard that Fleury had broken up and he assumed it was the same boat. He told everyone that my design was thus flawed.
A guy wandering by the shop who had seen a Crowther cat in Australia a decade earlier was chosen to design the main beam. It had no twisting nor polar load resistance. So the bows were able to move way too much. And they keep welding it, to this day I assume.

Notice in the photos that the fairing looks like it goes to the far side of the hull in one picture. In the other one can see it lands on the middle of the deck. With no bulkhead under it.

Moral; follow the plans. And I had better make a mega spreadsheet for future submittals.  And we need load cell data.


Again, back.  I however see that no email will go out today. I’m most of the way through the 500 emails, but after seeing the morning news, I backed up everything before logging on.
That put me into ther time where the Eset anti-virus began uploading anti virus definitions. It slams the CPU up to 100% and it has stayed there since noon today. Nothing else is really possible until it decides it is finished. The email just keeps eternal hourglass going on.  They say that you can pause it but it never actually does.
All the email will have to wait until tomorrow.

More Thoughts on Brushing 2 Part Poly

There are many times when I don’t want to bother with the compressor and squirt gun to shoot 2 part polyurethane paint.
It could be too small a job; I don’t want to lug the compressor, or overspray will be a pain.
Awlgrip T0031 Reducer is amazing, but there are cautions.

First, it seems like magic.  I see my horrible brush marks, and in a moment, they are gone.  It’s smooth.   But, up to what seems like a half hour later, it could form a drip.  And, if you work outside, like I often do,  it will grab insects or chaff for that half hour.  Things to keep in mind.  Painting closet/cabinet doors.  Skythane 2 part poly.

45 Cruising Cat Update

I’m finally able to start updating the very successful 45 cruising cat plans. Some of the recent performance cruising cats that I have seen look like they intend that looking good is more important than being usable.
It will still have full headroom, excellent shorthanding ability and cruising usability.

It does have a hotter sail plan keeping with trending now. 

The plans are available at the old price until the study plan set of the update is available.

Shaw Powercat

This comes under the heading of stuff done way wrong. A fellow sent me these pictures and noted that the underside gets pounded in any waves. He wondered if I could help. When I saw the pictures, I remarked that it looked like something that John Shaw would do. He asked that I don’t hold it against him. 100,000 lbs. displacement.  71′ length, but looks like around 65′ of real boat. Draft 5 feet. Thinking some kind of wave splitter is the only possible solution.  If I could have only done an intervention before it came to this.

Squeege Gecko Ama

Chris Anderson is here squeeging the Rovelock laminate into the CM female mold. 1988 probably. I recall that this photo was also in the first or second issue of ProBoat magazine.  Laminates were done one side at a time, but the bag did the whole mold.  The rounded sheers were molded from sonotubes.

Racing 1981

I am doing more scans from the old photos. This is Smoholla the Shaman racing, probably in summer 1981. It would have had the board instead of the keel, but with the old amas still.
Its interesting that we are behind Seafire, a Brown 40, as we won every race that year. Except a second against Limelight at Hoggshead. The mug is the diabolically brilliant Bob Dean.  I’m not sure who the speedy cat on the left is.  Maybe it is Limelight.  This was a scan of an ancient Xerox.  You use what you got; goes for photos and sail wardrobes.

The Air Mold

Back in the 80’s I noticed that sometimes when bagging a CM panel on the mold, the part only touched at and near the front of the mold. Especially with stiffer plywood. We would put pipe or 2x4s on the top to press it down.

Eventually a lightbulb went off in my head.  Why have parts of the cylinder mold there that are not always being used?  I did one hull that way but about then Charles at Multihulls published my first design and I have been scrambling to keep up ever since.  It never got puttered-with nor published.

Its not that big a thing, it just saves plywood.  The hold-down gutter is the same.  A lumber runner along the bottom of each tiny station keeps the mold from tipping over.  And by ajusting the angle of the stations, more possible hull sections are possible.

With this mold system, the exact same type of plywood must be used on all of the panels in a particular hull.

First, a comparison with both types of CM.

Next, zooming into the Airmold.  Showing with both a panel on and off of it.

Tax Day Discount

It seems that I wasn’t paying attention to where the flow was going and the in-flow bumped me up to another bracket. I just noticed.
How does a 20% discount on all stock plans this week sound? And 10% this month?

The shopping cart does not allow discounts so I would have to refund you after.  Or get card number to me directly, paypal directly, or I can send wire information.

Kurt Hughes on Catamarans, Trimarans, and Boat Design