Multihull Structure Thoughts

Oldmulti, did you ever do an article on the Horstman Tristar 27-9 ? I have looked but couldn't find one.

 

Guzzi3, I don't think so but will chase it up for you, it may be tomorrow.

 

Guzzis3 , when I was doing my research on the Horstmans a thomwessels posted on Cruisers Forum oct 10th and 16th 2011 about his three Horstmans , I think there are other post from him as well . He was the one that scaled the 27'9 to 30'

 

Thank you for the replies. I was just wondering what it's payload is. I think I knew once but can't find it now. I found an entry for the 24 and the 31 but not the 27-9. They are all in this range similar boats but of course the numbers vary... As I recall 27-9s have crossed oceans...

 

You can find thomwessels commits on his scaled up 27'9 . There was also a 27'9 on our east coast that had a web site . You should also look at the 32xrc , it is about 500lbs lighter than the 31 and no single berths , so cheaper to build and faster . If you need more berths you can put a double in the stern giving you two kings and a full .

 

I have always found his model variants confusing. wouldn't build a horstman because it's far more work than a same size cat, but if a foam one came up for the right money...

 

I have to disagree with build time . A 30' Woods cat at 6000 lb displacement is 800 lbs heaver at total disp. than the Tristar 31 , so you have used more material and vol. which will take more build time . To some extent you will build by the pound in cost and time if you are comparing apples to apples or close . The tristar 32xr has a displacement of I think 4500 , that is 25% less than the 30' cat and 2' more water line , you will build the 32xr quicker and cheaper than the 30' cat , and it is already converted to foam from the get go .

 

Which Woods cat weighs 6000lbs ? The Scorpio is quoted at 1800kg - 3600lbs empty. 3.3t loaded so 1500kg payload.

The Tristar 31 is quoted at 5200lbs. I assume that is design displacement not empty weight. The 32XR is quoted at 4700 pounds and has a bigger rig I think. My computer does not read aloud his PDFs so it's hard for me to get information off his site.

The problem with empty weights is there is no standard. Different designers include different things.

 

Sagitta is the one I looked at 6000 lb loaded , the Tristar weights are also displacement loaded .

 

It would be good to know what they weigh empty. I'd think a Tristar 31 would have to weigh at least as much as Sagitta empty, so it must have limited load carrying.

Sagitta Javelin and Scorpio have the same hulls. Mr Woods quotes the same empty weight for the 3 but different loaded weights. I suppose Scorpio is pure cruising, Javelin racing and Sagitta a bit of both..

Scorpio and Javelin, 30' cruiser racers by Woods Designs (sailingcatamarans.com)

Regarding build times the only part of those cats that need to be molded is the hulls to the knuckle, and of course both are the same. The Tristars you are building 3 hulls one of which is different. Double diagonal is almost as slow as strip and I don't know how he gets that shape using foam. There is too much double curvature to tortur them in foam I think so it must involve a mold of some sort ? Obviously I've not built either but I'd be surprised if there was a HUGE difference.

 

From memory I think one of the reasons I went with the tri was greater load carrying ability , also from memory i think the useful load on the 31 was 2000 lb or 2200 lb . The load can be carried all in the center hull , all in the amas or split between as long as the load is balanced .

 

Gussi3 asked about the Horstman Tristar 27.9 cruising trimaran. Although this is an older design (in the 70’s) it had many good features such as a good rig, daggerboards and a strong structure. The 27.9 is 27.75 x 17.5 foot with a “displacement” of 3,250 lbs (I suspect this is the weight) and my guess is a payload capability of 1500 lbs plus. The Tristar 24 weighs 2,450 lbs and carries an 1,100 lbs payload. The 33 foot fixed aluminium mast carries a 200 square foot mainsail, 123 square foot jib, 300 square foot genoa and a 800 square foot light spinnaker. The length to beam on the main hull is 7.6 to 1 and on the floats length to beam is 10 to 1 in stronger winds. The draft varies from 1.4 foot to 3.2 foot over the daggerboards. The outboard can be 9.9 HP.

The accommodation is large for a 27.7 foot trimaran. It has 2 double wing berths, a forward single berth and a toilet area. The main cabin accommodation depends on if there is flare built into the main hull. If there is no flare then 2 sets of single seats and a small galley are beside the companion way. If there is flare built into the hull a dinette on 1 side and a bench seat on the other side with a larger galley area aft. The 27.9 has a more practical layout than many cats of a similar size. The only downside is the double berths have only 2.6 foot of vertical space over them, enough room if you don’t have claustrophobia. The advantage of this layout is the large nearly flat deck space for sail handling etc. Minimal nets.

The construction is on the main hull and floats round bilge double diagonal bottom (probably 2 x 3 mm ply) with stringers and 12 mm ply bulkheads. The bulkheads have top and bottom timber flanges and are deep which increases strength. The decks are 9 mm plywood or 6 mm ply 12 mm foam 3 mm plywood with stringers in between. Now we come to an interesting statement from the Ed Horstman design web site. I quote: “Trimaran and Catamaran construction may be either: Cold Molded Double diagonal plywood or Foam Fiberglass Sandwich Construction. Both construction methods are fully covered in the Designers books: Trimaran and Catamaran Construction and Foam Fiber glass Construction. BOTH books are part of plans for all sizes over 21 ft. Plans are leased to build ONE boat, NO time limit . Person or persons, by leasing either a TRI-STAR TRIMARAN or TRI-STAR CATAMARAN set of “stock plans” agrees to build one Trimaran or one Catamaran from said leased “stock plans” and not to Copy or Sell any of the drawings Leaser receives. Free consultation is provided to the original non-professional builder till he or she is sailing.” Interesting, Ed was still selling and supporting plans at 85 years of age but he would be 90 now I do not know his current situation.

From my readings of Horstman designs they sail well but depend on the daggerboards for upwind work. They go to windward at about 50 degrees. They are 60% to 70% of windspeed type of tris in light to moderate conditions and will put up good all round averages. Peak speeds for the Tristar 27.9 would be 15 knots judging from some of the comments I found.

The jpegs give part of the story, hope this helps Guzzi3.

 

I looked at the material list for the 31 , main hull takes 320 sq ft foam , amas take 450 sqft , I figured 27 sheets of foam for all other 1/2 to 3/8" ply for decks , bulkheads ect , foam is listed as 1/2" but scantlings state 5/8" , so 55 sheets of foam and I bought an extra 6 . You might be able to come up with a close weight based on # of foam sheets .

 

How many square feet/meters if you build everything in foam ?

The Waller 880 needs about 200 sqm 2150 sqf in round numbers.

The HP EX40 says 116 sqm foam ex waste.

Sagitta is about 180 sqm.

I would think to build a Tristar 31 you would have to be close to 200sqm of skin bulkheads furniture etc ?

Oldmulti: Thank you for the information. It's a pity there isn't a stated empty and loaded weight.

I realise it's a very different boat but RK's Scarab 32 has a significant load carrying capacity. It's a really viable long distance cruiser...

 

There is a stated empty weight somewhere , I will have to get the plans out and dig . I figured 55 sheets total to replace all plywood , that's furniture as well , it,s a simple layout . 55 sheets is 1760 sq ft .