Do I need the mast support?

Yes you need one

There is alot of tension on this beam in way of the mast step. Some beach cat have been built without a dolphn striker (mast support beam) like the Hobie14 but they had a major weakness there and were limited.

There was at least one F18ht catamaran that had a reinforced front beam. The beam was made of thick carbon fibre.

The dolphin striker is the simplest solution to this situation

A heavily reinforced front beam might be another solution but I do not know how long a heavy walled aluminium tube woud last before cracking

Yes.

Its called a dolphin striker.

Grega,
What kind of cat is it? Do you know?

Grega,

If the boat did not have one before I don't think you would need one. It depends a lot one the front beam size and the space between the hulls and the mast and sail load. Was the wood platform supporting the front beam?

Yes, the wooden plateau was supporting the mast - it was attached beneath the beam. It was made of wood and attachet to the beam.

What do you think of reinforcing the alluminium beam with layers of carbon fibre?

I'll post the measurements later today; but the space between the hulls is about 4,5 feet.

The old wooden plateau can be seen in the back (see the arrow)

Thanks a lot forr the answers!

I've done the beam and dolphinstriker calc on for example my boat (Formula 16) and the Blade F16's.

If you give me the exact specs on your boat and beam I can tell you what the stresses and deflections are and tell you wether you need a dolphin striker or not.

But from the data give as of yet. 4 inch diameter beam (100 mm) and 4.5 ft bridges space and the describtion of the old wooden trampoline I would be very surprised if you need a dolphin striker for this boat.

Of course it all (now) dependents on the wall thickness of the beam and the sail area on the boat. But it is very diffucilt to get beams of this diameter with a thinner wall than 1/10 inch (2.5 mm) and it will probably by some 1/7 inch or thicker (3 mm or thicker). So the original builder probably used heavy beams. I would expect to find that they hold up just fine without a dolphinstriker. But of course I will not say anything definate due to US legal practices.

Give me the wall thickness of the beam, the mainsail area, jib area, mast height and distance between sidestays as well as distance rear beam to front beam and front beam to bridle points. The last measuered parallel to the hulls. You don't have trapezes I see is that right ?

There is also a easy trick to see wether you MAY need a dolphin striker. Rig the boat on land sit rotate the boat a little away front the wind sit on the hull and pull the mainsheet really tight Without flying a hull. If the beam flexes significantly than you'll need a dolphin striker. Most boats flex by some 5 mm to 10 mm in the middle of the beam (2/5 inch). Dolphin striker less beams can flex a just little more but handle the loads just fine.

Wouter

Grega,
I think that boat is a Sea Spray. It originally had a trampoline and did not have a dolphin striker. So unless you are putting a larger mast and sail area on it than came with the boat originally, you would not need to add a dolphin striker.

It appears that someone probably put that wooden platform on it when they added that outboard bracket and turned it into a motorboat.

There are some pictures of the Sea Spray at http://www.thebeachcats.com/modules... mp;file=index&include=view_album.php

Where do you sail? There are still quite a few Sea Sprays in the northwestern United States and Canada.

Of course, I could be wrong, and maybe it is not a Sea Spray at all, but it sure looks like one.

In the first pic it shows that this cat has three beams, on on the bow tips. IMHO, no dolphin striker needed.

@ Wouter
I'll post the measurements later today when I come home. Thxa lot for the effort.

@ Mary
It's not the Sea Spray; looking at these photos my cat is much larger. The cat was home built by the plans of some English designer about 15 years ago. I don't know the name of the original designer, since I've bought it used. I'm form Europe, Slovenia.

@ arbo06
Yes, I have 3 beams.

What a coincidence! -- a man came from Slovenia to take our catamaran seminar here in Key Largo this week. He sails an A-Cat over there.

That doesn't look like a Sea Spray...it has all that storage on the hulls and what sounds like used to be a wooden platform instead of a trampoline. However, the span between the hulls is relatively small and the diameter of that beam large that I too would suspect that you do not need the dolphin striker. If you were to add one, I would suggest using aluminum bar like most other catmarans have. You will have an extremely difficult time getting carbon fiber to bond to the existing aluminum. However, considering that your mast pin doesn't extend beyond the beam, I'm quite sure the boat was never intended to have the dophin striker (truss) support there. You should be fine like it is.

But the designer of the boat (which was homebuilt) obviously designed it based upon a certain mast weight and height and certain sail area.

So I was suggesting that if he is going to put more load on that beam than it was originally designed for, perhaps he would need additional support. Is that logical?

Certainly! There's no harm in adding a truss to help make the beam stiffer.

Hi,

I can't find the piece of paper with my measurements data. I'll go to the beach in the weekend and take the measurements again.

About the trapeses - I don't have them.
The beam tube diameter is 110 mm - so a little more than 4 inches.

Thx a lot for the answers!