22 - 24 trimaran

[bruceb]

I have been reading the planing or not arguments also, and of course, I have an opinion Around 1990, a friend of mine built a "planing" sort of tri about 20' long. The main hull was very flat aft, and the single cross bar looked a lot like an airplane wing with a downward dip at the ends creating very low aspect planing surfaces. He was a pilot and cat sailor- he called it a "wingmaran". It sort of worked, and definitely planed, but the main hull and the tips would plane at different times and not at all predictably. In just the right conditions, the craft would come up on top of the water and out-run the inflatable I was chasing it with- around 15kts plus. The small wakes that were generated looked like the wake of a fast flat-bottomed ski boat. The rest of the time, the main hull or the tips would start to dig a hole and the boat would round up or spin out. It was very easy to see which part was actually planing, and from the pictures I have seen of most bigger tris, they are not. Every planing dingy, cat, or powerboat I have seen rises at the bow as it comes on plane. If the larger tris were able to get enough lift from their hulls or amas, I don't think they would need lifting foils. Several beach cats have had flat bottom hulls and will plane under the right conditions, (even an Aqua-cat), but they give up more than it is worth the rest of the time. Bruce

[BWD]

Dry transoms happen for two possible reasons
1. trim
2. the boat has left before the water's inertia and viscosity allow it to close up around where the boat ain't no more

For most boats, at lower speeds, trim keeps transoms dry (or nearly). Once you sail past the speed at which inertia separates water from the transom, it can stay dry, while the hull is depressed. And it does depress, generally.

If it happens in a way that the water comes together behind the transom as it would around a streamlined body, it may be very efficient.
This happens in tugboats as well as multihulls (not the efficiency part), and it looks the case in the seacart pictures.

To the general planing question:

Historically, the Itzacat planed, the video convinced me.

And Parlier's seaplane-float shaped cat did, otherwise it could never have gotten up to 36kts. If I remember, the problem was more with consistency and average speeds.

I once thought some of those F tris might plane a little bit, but now realize (a) they probably don't and (b) who cares, it doesn't change the performance envelope from what you would expect.

The seacart doesn't "look" like "planing" but I say planing is as planing does. So. If you could find the actual ama volume immersed while flying a hull to be less than the total displacement, then couldn't you conclude it must have some dynamic lift?
I would consider that equivalent to planing or "might as well be planing."

It would be interesting to relate any "missing" volume to the size of the hole in the water behind the transom, and see if any interesting relationships turn up regarding that and drag..........

That Penelope cat pictured above looks like it might plane, but for some reason I disbelieve. It looks very light and everyone is in the back!

To me, the interesting part is shedding drag while adding power...

[tatoski]

Oldsailor,

I got the study plans. Thank you very much! I'll "study" the plans. Cheers!

Tatoski

[bruceb]

Chris, Do they ama's on both of Irens's designs really have 200% of the boat's displacement? I realize both boats are really light for their size, but the volume just looks too small. I have never seen one of these boats up close, I am just interested. Bruce

[oldsailor7]

Chris , with respect- you are twisting my whole argument.
You said "The planing thing for multihulls... I'd love to hear the collective wisdom on the matter and there's no time like the present"

Who's discussing "Planing for multihulls".

That old chesnut was laid to rest decades ago. It's only "Wannabe" designers who have little experience or who have not done their research in depth who keep trying to re-invent the wheel.

Let me make it clear. In the case of Seacart. GIVEN- the cylindrical shape gives the least wetted surface drag, and GIVEN -that due to the subtle rocker at the aft end, the flat circular transom DOES not drag in the water at low speeds since the heeling moment is low. At high speeds it is a Given that the lee hull will be pressed down in the water and the transom will be below the surrounding waterline. BUT, at the high speed that the boat is travelling the after part of the ama is PLANING. But I am sure not by design.
You said it yourself--"Yep, those sure are really fast semi-displacement indicators," Quote.

Also: Quote.
No fast moving trimaran is going to be living long with its leeward ama only immersed to the transom bottom. Any kind of wind, aggressive sheeting, or lifting of the main hull (which happens the moment that the ama absorbs the weight of the full displacement) or a combo of all three, will seriously depress the ama to, at least, its half-way immersed state on racing machines and to a much greater degree for les racy designs... untill the crew scrambles out on the windward tramp to give the boat additional righting moment.

If the turbulence is such an issue from a squared-off transom, then why are virtually all the modern designs moving forward as if they could care less about the drag potential of an immersed ama of the type mentioned? Quote.

Exactly. You have just made my point. As you have just explained above-- the water is cleanly cut off from the bottom and sides of the transom and is thus making NO TURBULANCE DRAG. IE:- Call it what you like, It's PLANING.

Under no circumstances should this be construed that Planing hulls on ANY type of multihull are a viable option.

Now all of the above is a quite different Kettle of Fish from a DIY small cruising design, which is brobably going to end up as a heavily loaded boat which sails most of the time with its leeward AMA well submerged at (relatively) slow speeds.

The turbulance drag of big transoms which remain submerged during 90% of the boats sailing time are a poor design feature.

IMHO.

[Chris Ostlind]

OS7,

I'll answer your post in the morning. It's late. I've been at the boat shop all evening and I need to get some shut-eye.

Later,

[oldsailor7]

Originally Posted by Chris Ostlind

OS7,

I'll answer your post in the morning. It's late. I've been at the boat shop all evening and I need to get some shut-eye.

Later,

LOL Chris. Thats OK.

Just remember this is a 22'-24" TRIMARAN thread and we really shouldn't be talking about bigger, faster, racing multihulls.

We have to remember that the effect of an item of drag has a much higher percentage of effect on a small boat compared to a larger one.

By the same token, as boats get smaller, so do the design parameters.
However the crew, their safety gear, personal clobber and food and water supplies don't change--and become an increasing percentage of the boats total displacement. This radically affects the design approach to small Tri's.

Again---Horses for Courses.

[franzship]

I need the sail plan for a Crowther Buccaneer 24 and information on other rigs from say, production monohulls which would work on it. Thanks!
Franz

[oldsailor7]

Franz.
We need to hear more about your boat etc:, as you need 5 posts on this thread before we can communicate by secure Private Messages.

[franzship]

The rig was walked off with.
franz

[franzship]

It isn't mine yet.

[franzship]

the owner spent about a month in the hospital. When he got out the rig was gone.

[franzship]

So now I might have a shot at it, but I need to see what fits or comes reasonably close.
franz

[franzship]

I read some of the ideas posted here about mods and they all sound pretty good!
Franz

[Samnz]

Originally Posted by franzship

I need the sail plan for a Crowther Buccaneer 24 and information on other rigs from say, production monohulls which would work on it. Thanks!
Franz

please move this to the Bucc 24 thread...