22ft racing tri design

[henrikb]

Hello everyone!

A friend and I are designing a 22ft tri. The main hull is already built and we have now started to look closer at the floats!
Main data:
LOA: 6.5m
Beam: 5.2m
Disp: 400kg (loaded 600kg)
SA upwind: 32m2
Rudders and center boards on the floats.

The boat is built using carbon/epoxy/divinycell, vacuumbagging.
The main hull is completed as far as can be done without the rest of the parts, weighing in at 115kg.

The beams will have a centre piece of 2.6m that is a permanent part of the main hull, the floats will have the other part of the beam permanently attached. Assembly will be by mating the 3 parts and the water stay keep them together.

This weekend we were looking at the design of the floats, as we are planning to fly the main hull for sure, the displacement is critical, both for bouyancy and the longitudal stability. The loats are designed with 250% volume (1500 litres). Centre of bouyance is well forward (400mm I think) of the main hull CB.

Now to my questions...
I got some second thoughts about the displacement of the floats, is 250% too much??
The floats are 6.5m as the main hull, beam 55cm. Are they too wide?

Attaching some pictures!

Looks like a very interestng design. In my opinion- for a performance multihull- a length divided by beam of 12 is fairly wide; you might want to consider going toward 18 or 20(Tornado Cat). I don't think 250% is necessary with the length to beam ratio being more important for performance. I hope you're considering the use of lifting foils on the daggerboard-to provide some vertical lift like on the ORMA 60's- and perhaps rudder t-foils to enhance pitch stability. You wouldn't have to use curved foils like the ORMAS which are curved just because of the rule. The foils can make narrower ama's possible w/o too much wetted surface at speed and the foils will enhance pitch stability of the narrower amas. I think having the CB of the amas forward of the boat CG is a good idea as long as you don't go too far. In some model tests this was done to counteract the pitching moment of the rig(instead of foils) . The problem was that if this distance is too great when you initiate a tack while flying the main hull as soon as the rig loses power and before the main hull comes back down the boat will pitch up quite a lot. Again,in my opinion, having this offset is very good-particularly if combined with rudder t-foils.
Is it possible to consider increasing the overall beam of the boat? More power to carry sail at a relatively low cost in weight.
Good luck and please keep us posted!

[henrikb]

The design of the amas is ongoing! We have now analyzed the situation a little more in detail...
The advantage of increasing the beam of the boat is not that obvious! The "problem" with increased beam is that the driving force from sails will be increased also. To be able to withstand nosediving, the CB needs to change forward quite alot when flying the main hull. The change in CB needs to be the same as the actual change du to increased driving force. The crew should be able to remain seated at the same position even when starting to fly the main hulll.
At the same time you don't want a too fat ama, you also need some displacement aft to prevent pitching and exessive aft trim when sailing in light air...

Looks like we are staying with about 200% of total displacement (crew included), volumes distrubuted to get the correct CB at different conditions.

[Chris Ostlind]

Originally Posted by henrikb

The design of the amas is ongoing! We have now analyzed the situation a little more in detail... The advantage of increasing the beam of the boat is not that obvious!

Looks like we are staying with about 200% of total displacement (crew included), volumes distrubuted to get the correct CB at different conditions.

It looks like you have arrived at a very solid decision, Henrik. I'm looking forward to the build of the amas, the beams and your first sailing trials. If you approach the sorting-out process in the same way you have moved through the basic design/build elements, you are going to end-up with a very quick boat.

Chris Ostlind

[tspeer]

You ought to consider the effect of pitch trim on the ama design, too. If you consider the bow-down angle at the maximum diagonal loading, I think it makes sense to have the deck of the ama bow parallel to the water at that point. Then you retain the waterplane area all the way to the bow and have the reserve buoyancy located at the right height to be useful.

The level-deck-with-bow-down-trim criterion leads to sheer in the ama bows that is not currently fashionable. (Straight sheer being the vogue these days.) But I think it makes the boat far more robust to being driven hard. If you look at Newick designs, they usually have a very similar sheer.

[henrikb]

The frames has been cut already...
We paid very mych attention to the bow down angle, as you mentioned, the deck level is now parallel to the WL at maximum bow down trim, and also as you mentioned, the bows became quite full on the upper parts of the amas. I think we reached a robust design as was our initial target!

[henrikb]

The plug is finnished and now ready for the divinycell!
Now we need to get going with some detailed design on the beam attachments, dagger board trunks etc...

Speaking of daggerboards... What I have learnt is that the CE of the daggerboard should be aligned with the sailplan CE and not with a lead like on a single hull? Any other rules of thumb on this? Should the rudder be included when calculating the CE?

[Yacht Skipper]

Hello Henrik,

I like your project, very interesting.
Please keep me posted: yachtskipper(AT)gmail.com

Thanks

[henrikb]

Some progress! Divinycell now in place.
Need to sort out the CB placement and other details...
Now that most surface modelling is finnished, it's about time to switch to SolidWorks!

[rapscallion]

Originally Posted by henrikb

Some progress! Divinycell now in place.
Need to sort out the CB placement and other details...
Now that most surface modelling is finnished, it's about time to switch to SolidWorks!

Beautiful boat henri!!

[svfrolic]

I like this project.
You might want to take a look at that:
http://www.exploder.info/eng/content/view/19/125/
Multihull Mini Transat would be awesome .
Good Luck!
Maciej
maciejrynkiewicz@gmail.com
Annapolis, MD

[rapscallion]

Originally Posted by henrikb

The design of the amas is ongoing! We have now analyzed the situation a little more in detail...
The advantage of increasing the beam of the boat is not that obvious! The "problem" with increased beam is that the driving force from sails will be increased also. To be able to withstand nosediving, the CB needs to change forward quite alot when flying the main hull. The change in CB needs to be the same as the actual change du to increased driving force. The crew should be able to remain seated at the same position even when starting to fly the main hulll.
At the same time you don't want a too fat ama, you also need some displacement aft to prevent pitching and exessive aft trim when sailing in light air...

Looks like we are staying with about 200% of total displacement (crew included), volumes distrubuted to get the correct CB at different conditions.

what is the cant angle of the amas?

[pacice]

Hi Henrik,
Is there anywhere I can see photos of the finished tri.
It looks a great design and should perform well.

Thanks

[henrikb]

Hello!

The boat is actually not finnished yet, this is a garage project
Currently the mast (carbon), dagerboards and rudders are being built, rest of the boat pretty much completed. Final finnish and painting remains.
I'll try to get some pictures!
Launch is planned for this summer.

[henrikb]

Not much updating, but the project is moving forward!

Boat ready for final painting, unfortunately the painter have not been able to do it yet.
Carbon wing mast built, final painting ongoing. All Harken deck gear waiting to be fitted, North sails delivered, nets delivered.

Rudder and daggerboards still to be completed.

Some pictures attached!