22 - 24 trimaran

[bruceb]

The boat looks interesting, where was that one? The stock hobie sails are too light of material and construction to last very long on that heavy of a boat. The mast would be marginal even with spreaders and jumper stays. I used to be a hobie dealer and know them well. Bruce

[tatoski]

The website address is: http://www.prcn.org/kismet/index.html
Plans are still being sold. Any firsthand info on how the boat Kismet k24 t sails? Thanks

[oldsailor7]

Why do so many wannabe designers draw their designs with flat transoms on the floats. Horrible drag when the float is depressed and it's when the boat wants to go fastest that they are most depressed.

[jamez]

The Hobie rig is an option there is also a larger rig designed for the boat. Kristofferson built the first one himself. It is briefly mentioned in an article in Multihulls J/A 1993 as quick to build and fast. The prototype unfortunately lost its mooring in a storm and was wrecked. Pretty sure thats the one on the website.

If you order a study print make sure you specify a materials list comes with it.

[oldsailor7]

Originally Posted by jamez

If you order a study print make sure you specify a materials list comes with it.

Thats a very good point you make Jamez.

I have been giving out a study sheet for free, but it only consists of a threeview and some information about the rigging details and sailplan.

I would really like to include a materials list and the building instructions so that the prospective builder can really see what he is in for.

However the printing cost and postage has to be recovered and to do that I am considering charging for a proper study set---but offering the price to be deducted from the cost of the plans if they are subsequently purchased.

Does that seem reasonable.

[jamez]

Hi OS7. the only reason I mention it is I ordered study prints from Bill for his 24'tri and cat and, as every other study print I've paid for (and I have lots) has had a material list, was astonished when they turned up minus these. Bill said the material list was in the full plans but promised - on 3 occasions - to email the lists to me. that was 5 years ago. Still waiting:-). Of course he may have lost my email addy or something :-)

I think you should charge for a comprehensive study set. You can always have a one pager to email people for free. The idea of taking the cost of study prints off the price of a plan order is more than reasonable IMO. After all the plans are not exactly expensive in the first place.

cheers

[oldsailor7]

Thanks Jamez.
I am not in the boat plans business as you know, but I just feel that this particular trimaran is so good that it should not be allowed to die. That is why I am making the plans available at a price anyone can afford, in the hope that some of them will be built again, albeit with some tweaking and appropriate mods, depending on the individual builders requirements.
I might just go ahead with that idea. US$10.00 would just about cover the costs.
What do you think.?

[jamez]

$10.00 is cheap - but as you say its to cover costs. I'd suggest sending the prints electronically though, save you the hassle of getting stuff printed and postage etc.

[Chris Ostlind]

Originally Posted by oldsailor7

Why do so many wannabe designers draw their designs with flat transoms on the floats. Horrible drag when the float is depressed and it's when the boat wants to go fastest that they are most depressed.

Perhaps a quick look at the images shown below will serve to further stir the pot?

The two black and white images show Jim Antrim's A30+, Erin and the all white tri is Antrim's 40' Zephyr, both sporting dead flat transoms on their amas and both extremely quick machines which have set records.

The red boat is, of course, Nigel Irens' wonder ship, IDEC at her mooring in La Trinite sur Mer, France. This is the boat that set the existing Round the World record for solo boats, as well as the just established Route of Discovery (Spain to Caribbean) under the experienced hands of Francis Joyon.

Another Irens design, Sodeb'O, also shows similar thinking.

[oldsailor7]

Four of those five Tris have TINY transoms. IDEC has the biggest, but all of those shown have a lot of deadrise toward the tail,which is designed to move the bouancy forward as the float is immersed.
Also Sodebo and Idec are huge boats and the drag effect will be a tiny percentage of the overall drag, compared to the same effect on a 24' tri.
For instance--on this one. 7point5m tri-1.pdf (268.9 KB, 37 views)

[Samnz]

Originally Posted by oldsailor7

Four of those five Tris have TINY transoms. IDEC has the biggest, but it also has the greatest deadrise in the tail.
Also Sodebo and Idec are huge boats and the drag effect will be a tiny percentage of the overall drag, compared to the same effect on a 24' tri.

Have you seen the Hughs 24? its got a big transom and the one racing here is very very quick.

[oldsailor7]

Yes. The Hughes 24 is a very elegant (and expensive) Tri. However if you look at the excellent video of it sailing, the deadrise on the aft end of the float keeps the transom out of the water almost all of the time. As in the Hughes 26, the transom is swept forward towards the deck, (like Miranda), which is a good way to save weight, without causing extra drag.
It's the floats which have a relatively flat keel and large vertical transoms like the SV 24 that I am concerned about.

Incidently the Hughes 26 looks like a modification of the Buc 24 with Miranda's features and a vertical stem.

[Chris Ostlind]

Originally Posted by oldsailor7

Four of those five Tris have TINY transoms. IDEC has the biggest, but all of those shown have a lot of deadrise toward the tail,which is designed to move the bouancy forward as the float is immersed.
Also Sodebo and Idec are huge boats and the drag effect will be a tiny percentage of the overall drag, compared to the same effect on a 24' tri.
For instance--on this one. 7point5m tri-1.pdf (268.9 KB, 37 views)

When you say deadrise, are you talking about the aft component of the rocker profile and not the vee, or lack of same, in the hull bottom when viewed from the bow, or stern?

The way I see it, the transom drag potential on any properly designed trimaran ama is a very small function of the total boat as a whole. You can get very slender transoms with very slender sections aft. The trouble with that approach is that it contributes to enhanced pitching moments for what is already (or should be) a collection of very slender hull forms. In an effort to dampen that effect and contribute to a more stable ride, you find the amas being drawn with fuller forms and resultant wider transoms.

Check out these images of the amazingly fast Seacart30 from Sweden. This is a boat that routinely flies its main hull and is regarded as pure state of the art in the genre. Full amas, large area transoms with squared-off ends.

The third photo is the transom on the absolute latest Hobie F18 cat being shown in Paris this last week. This is a full tilt, reverse, wave piercing bow slice and dice machine that is about to hit the market.

[oldsailor7]

Both of those are out and out speed machines and are most efficient when they are quite literally planing. The pic of the SC 30 shows the water cleanly separating from the transom--a clear indication that it is planing. In medium airs it might be a different matter.
It's interesting that the floats are quite literally cylindrical tubes with a bow at the front, the most efficient form there is for min wetted surface. There also appears to be little or no rocker in the stern sections.
Since the displacement of the floats are something like 200% and in very light airs they will barely be touching the water, the size of the transoms is irrelevant.
In the case of the F-18 I can't see the rocker, so I can't comment on that.

Stu Bloomfields "Raw to the Core" also has a flat transom with a flattened bottom edge and subtle planing strakes on it , and that Cat is probably the fastest 30' sailboat in Australia.
However it too has a reasonable rocker in the aft quarters, so it is unlikely to drag its transoms in light conditions.

BTW---my apologies for using the word "deadrise" when I should have said "rocker".
I should know better.

[Chris Ostlind]

Planing, huh? I know this has the potential to become a bad can of worms when it comes to multihulls... but just what, in your mind, constitutes planing for a multihull?

When I look at the photo of the Seacart flying its main hull, I see an ama which is immersed half way up the bow form, the full ama length is in the water and the transom, while leaving what many would consider a firm flow of water off its lower edge, is still rather vertically oriented to the water's surface. Yep, those sure are really fast semi-displacement indicators, but I do not see the typical indicators one would observe from a planing hull form.

Current design thinking for ama transom shapes does not include the pintail, or even radically reduced ama transom forms. Lots of older multihulls had ama transoms such as those you describe, but none of the more modern designs include that variation. At least, none of them with which I am immmediately familiar.

I've included some additional images of the Seacart30 with leeward ama fully depressed and windward ama flying. In these images, you can see the fully expressed form of the ama. Modern trimaran amas tend to look very much ike these shown with much fuller volumes well forward, a defined rocker arc sweeping aft and much smaller volumes at the transom.

You can also see, with the Seacart30, that the cylindrical forms as mentioned, only begin to dominate the form well aft of the midpoint. Forward, the shapes are more semi-elliptical, gradually concluding in a gentle taper as they move to the vertical orientation of the bow.

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?

The planing thing for multihulls... I'd love to hear the collective wisdom on the matter and there's no time like the present.