Modern hirondelle

Manfred. Your metric figures need a decimal point, not a comma. Very confusing for those still using the old system.

 

Sorry, redreuben, I took this figures from MULTIHULLS (USA) as an example.
What is wrong? Where should be the decimal points instead of commas? Please, show me.

 

No big deal Manfred, I just think it would have been clearer to write 22.76m2 rather than 22,7605m2. At some point some extra decimals were added and the comma is confusing rather than a decimal point as in my example. With a decimal point you are clearly dealing with a decimal measurement.

 

Who uses metric anyway?

 

Ha!

Manfred.pech:

You mean the AERORIG ( or Swing Rig or Balestron Rig).

Yes. I'm sorry sometimes my mind does not cooperate. I could not for the life of me remember the name.

I was thinking something like a naca 0009 but as you say at 8 knots it probably does not matter, and at some point the sideways strength starts to get compromised.

I really don't know how heavy to make the keels and how to attach them to the hulls. The hull bottoms I propose are 3/8 80 kg sqm divinycell or similar with 600 gsm 45/45. I had thought to fillet around the keel and tape them to the hull, maybe 3 layers gradually down to one layer to spread the force. As for the keel no idea. Maybe 3 layers of the 600 gsm no foam ? I don't know if that would be rugged enough to too heavy ? Too light ?

The problem as always is while I can calculate material strengths I don't understand the loads properly. The hull proper should not twist, that's the critical part. If you make it strong enough not to twist it won't break in bending in this small size. What I don't understand is the worst case scenario for the keels. Holding the weight of the hull while still is no problem, but that isn't strong enough to survive an impact, especially from the side.

As always thank you all for the replies.

 

What does 3/8 mean ?

Stub keels get good reinforcement from having frames and side to side ribs glassed in internally. It stops the big hole in the hull warping and twisting.

 

From Ed Horstmans foam fiberglass construction . boat , 20' - 30' foam 3/8" , FRP thickness .110 , app. weight / ft / sq 1.00 to 1.25 . Boat , 31' -35 ' 1/2 " foam . As a note the tristar 31 calls for 5/8" foam . Using only only one layer 45/45 glass is most likely not enough glass , triaxal may be enough . Keel area is three times hull thickness o/s to 18" above keel , inside keel can be built up a number of ways with foam or glass only . Ed,s book is good , as well as the how to on bateau web site . To get any near thickness on glass i will have to use mulitple layers of 45/45 and 0/90 , I will still be under the weight of 24 oz woven and two layers 1.5 oz mat .

 

I don't think you should put a big hole in the hull Red. The keel should be able to get chewed up without sinking the boat.

Very helpful Rick. I could tell him what to do in wood which is in fact what I'd use on the keel for its combination of strength, weight and resilience but I'm into natural fibers.

 

True that, but I had kinda assumed you might want to use that added displacement for tankage or batteries, hence the top would be open or partially open. So whatevers

 

I think the tank trick is to have the pipe and vent exit above the waterline but don't breach the keel before researching. I actually use bottles instead of tanks so the entire water supply can't get compromised, then the empties are flotation.

 

Wow ok lots of replies.

Redreuben 3/8 means 3/8 inches, or about 9mm.

I had planned to leave the hull sealed but put a screw in inspection hatch in, and also make part of it a sump with the bilge pump in there. Either way I'd planned to leave it mostly isolated from the main hull and the hull floor in tact.

I had planned to use 3/8" on vertical and curved surfaces and 1/2" on horizontal surfaces like cockpit floor and cabin roof, extra stiffness to walk on. The Farrier F82 (and other boats I know of in this size range) uses a single layer of 600 gsm each side but I guess there is a lot of curvature and shorter spans. ? Dunno. I thought it was adequate...it is only 23' long and 9'6" wide.

rberrey FRP thickness .110 , app. weight / ft / sq 1.00 to 1.25. 0.110" ? 1 - 1.25 pounds a square foot ? Sorry to be a bother, I think everyone can see some of us are used to metric and others inches and pounds and the terms can be confusing... "the how to on bateau web site ", may I trouble you for a link to this ? I've not seen it.

I've smoothed the cabin and tweaked a few things but I'm not satisfied. You will see a dip between the bow frame and the next one, that is an artifact of the smoothing my package has done and I'll want to get it out later. Also I'm thinking I've gone a bit overboard on prismatic, I think that second frame needs to be a bit slimmer...anyway still very much a work in progress..By the way it's orange because it's easier to see the curves in that colour, in case anyone was wondering...

 

Too heavy mate - a boat this size can be built in 1/4in ply no worries. It all comes down to unsupported span distances. You can build lighter weight structure by using lots of stringers and frames then a thin skin doesnt have to span far. Or you can span further and use thicker plating but it will end up heavier at the end of the day.

For the really large spans that need to be stiff - such as the bridgdeck floor for example (nothing feels cheaper than a springy floor) you can investigate the use of ply sandwich panels. Personally - i think that 1in XPS foam with 1/4in ply skins would be upto the task. Ive made a table like this and it was stiff as hell and felt very tough - i doubt it would have had delamination issues as it was all bonded with thickened epoxy. If its good enough for truck trailers, caravans, structural insulated panels etc i reckon it should be good for small boats...

Mini keels on bob oram cruising cats were made from 4 transverse frames of 1/2in ply. Then wrapped around these with 9mm ply to get the foil shape bringing the skins togeather at the trailing edge. The front of the foil could not bend that tight so curve was laid up in fiberglass then glassed over the whole lot joining to the ply skins. Alternately you could use bending ply for the leading edge radius and glass over it once you have the shape. Internal transverse ply frame to ply skin junctions all filleted and taped from above before bonding the entire minikeel to the bottom of hull, again filleted and taped to hull and using bonding angles where you cant access on the inside when you offer it up. Good enough for a 6 tonne cruising cat...

 

Bateau.com is the website , go to Tutorial,s , How To , Foam sandwish constrution . I am in the inches and pounds camp , as are Ed Horstmans books , the weight of your hull should be between 1 pound to 1 and 1/4 pound per square foot . In stich and glue 45/45 is used on plywood that has fibers for the most part in 0/90 direction , foam lacks any strength in a given direction , I have noticed a lot of foam builds are in triax , so I am assuming you need more than just 45/45 direction wise . You will also need gel coat or a 6oz layer of glass for a finish on the outside of hull . You have to keep in mind that strength is not the only concern in your hull fiberglass layup , punture resistance has to be addressed . In my case I have bought Kevlar and 6 oz S glass for a finish glass , woven and mat would have been more cost friendly glass wise , but maybe not epoxy or weight wise , it all depends on what you want . Ed has his non foam sandwish decks designed as a plywood PVC foam sandwish construction , 1/4 " ply / foam / 1/4" ply . The couple that built the tristar 31 in the UK said the only regret they had was in using cheap plywood on the decks , this should be someone else,s lession learned . Rick

 

groper: Thank you for the input but I'm not building in ply.

rberrey:

Bateu: Thank you for that, I'll have a look.

I have emailed Mr Horstman twice this year and no reply. I bought the MT26 study plans years ago but decided it's not the right boat for me. I have to say that both Mr Horstman's plans and Mr Farrier's become difficult reading when deciphering what materials are required for a particular build. Mr Farrier is logical in his approach but it did take me a lot of re-reading before I finally figured out all the layups for an all foam build, then the layups for a strip build on the F82. Mr Horstman's study plans only provide a basic materials list, which again I did not find entirely clear.

I am of the generation in Australia which went through the transition from proper measurements to metric. I can do most simple conversions in my head but when you're talking rates like weight per unit area or volume I bog down, and when people truncate the description I can get lost, so thank you for the clarification.

Are the builds you are referring to in this size range ? The tristar 31 for example is a MUCH bigger boat. In fact the tristar 24 is a lot bigger than what I'm looking to build. It may be only 1' longer but it's 17' wide as opposed to 9'6" this cat will be. Mr Horstman's site quotes 800 kg in one place and 1100 kg in another. Either way I'm thinking this boat will have a sail away weight more like 600 - 700 kg (1350 - 1550 lbs).

The tristar 24 deck is unsupported for about 2 meters (6'6") along it's length but also right across nearly the whole beam of the boat. It's got a bit of compound curvature to it but it's a huge expanse of unsupported deck that has to take people walking on it.

As another example Mr Woods has a materials list on his website for gypsy (28' catamaran). The solid glass or foam sandwich options use 600 gsm biaxial. Obviously the solid glass uses layers to build a thickness. I believe the foam option for eagle (his 24' catamaran) uses 600 either side of 3/8" foam. These are open deck cats of course, but they have solid bridgedecks.

The foam is purely a stiffening agent, like the web in an I beam, so yes the glass provides all the strength in tension and compression. The foam keeps them apart so as the panel tries to bend the glass is in tension and compression where it does a lot better. Solid glass is poor in bending. 1 - 1.25 pounds per square foot is about 5 - 6 kg/m^2. I was working on 4 before fairing filleting taping and painting etc. Just the bare laminate. I was planning to resin infuse the flat panels first and see if I could manage it on the double curved hull sides once I'd got a feel for it. Puncture resistance and stiffness were considerations when I opted for double curved (fully rounded) hulls. I didn't do that for wetted surface as the difference is negligible.

Anyway, as always thank you for the comments.

 

So your building in pvc foam sandwich- great.

For a boat this size you would be looking at 10mm foam core with 600gsm either side. For the bridgedeck you would go thicker dependant on the span between stiffeners, i used 20mm on my last cat. For areas of localised compression loads you should increase the laminate thickness purely for toughness as the foam core is easily dented with thin laminates - i went upto 1000gsm on my floor for this reason but still damage when someone slipped and dropped a scuba tank down hard on it...