Modern hirondelle

Hi again,

The cockpit floor is 1.7 x 1.8 meters, 5'7"x5'11", and supported on all 4 edges. The cabin span is bigger but has intermediate support from the mast compression post pretty much in the middle of it. It will also have some single curvature to it. I had thought 1/2" or 12mm core would be enough. Maybe not.

I had planned to use higher density core around fittings. The mast will sit atop a compression post from cabin top down to a load spreader across the bridgedeck. The only other area I can think of for localised load is the forestay. I'd planned to mount a 4hp 4 stroke outboard at the back to the cockpit to a moulded solid glass "sub-transom" with a cover over it. I'd prefer to avoid a nacelle keeping the tunnel as clear as possible. I was just going to make a minimal braced structure to carry the outboard loads. This sort of detail would become clear as I complete the model.

There is other stuff aswell I'm thinking about. One area I'd thought to make unconventional is the sterns. Normally on larger cats with sugar scoop style sterns you see steps, but I was thinking because the boat is so small a swim step type of thing 200mm 8" above the waterline then a vertical panel at the back of the cockpit with a ladder or individual rungs to climb up into the cockpit. The tillers will be in the way but still it should be a convenient place to re-board the boat after a swim or man overboard. The swim step would be about 700 mm 2'4" long and 400mm 16' up to 500mm 20" wide. I would leave the combing shown above on the outside of each stern.

 

That floor span done in 20mm is what i had. It still needed a longitudinal stiffening beam down the center or it felt springy under foot. I made a 200mm depth triangle section bonded to the underside of the floor. It was then satisfactory.
Your cabin roof will be better due to curvature, its also not as critical in terms of how it feels under foot as its not normally walked upon on a regular basis so its feel isnt really important.

 

You should be looking at a lower weight than 5-6 kg / m ^ 2 because these numbers should be based on woven and mat . Ed is up in age , I've not talked to him in a few years , so I hope his health is ok . His study plans as most designers plans are not enough to build a boat from . His plans are very detailed and most people with general construction exsperience will have no problem following . The problem with baseing your fiberglass layup off study plans is they do not show all the places the designer changes the layup . On Ed,s plans he shows places markes as 2x outside hull layup , 3x o/s layup , 2x i/s layup exct. Places like bulkheads or keel area,s will be differant than the hull . On his decks they are all supported , top side of decks are as groper describes . One more thing from Ed,s foam construction book . Hull Conversion . Cold molded plywood to foam fiberglass conversion .Cold mold = cm , Single fiberglass skin = SFGS , SFGS , Single skin refers to hulls that are built up of fiberglass laminations only , no foam . Formulas , cold molded plywood to foam fiberglass construction . A: Ratio 1cm to .66 SFGS . B:FSGS x2 = core thickness. C: .7 of SFGS = total FG laminate . Example using two 1/4" cm plywood skins , equaling 1/2 " plus the glue and fiberglass hull coverof say 1/16 " , which gives .25 + .25 + .062 = .562' of cm .
1. (A) Ratio 1 to .66 or .562 x .66 = .371 SFGS
2 (B) .371 x 2 = .742 , or 5/8" or 3/4" core thickness .
3 (C) .7 x .371 = .259 " total laminations thickness
4 .259 dividedinto two surfaces = .129" thickness of FG laminate per side . note for # 4 you might add 10% to outer laminate to compensate for the extra abuse it receives .
I took Ed,s layup of woven and mat and used it as a base for my material knowing as he writes in his book it will be a thinner skin . This formulas might give you something you can work with to back check yourself , or tweek your material . Rick

 

thanks for the details, Rick. Very interesting analysis. What weight (density) and type of foam is assumed in the conversion?

 

I would assume it would be a closed cell foam like H80 .

 

my 11m cat was built from 15mm and 20mm h80 foam and e-glass skins.
Typical laminates were - 800gsm triax per side for hulls on 15mm foam- glass doubled over below waterline for 1600gsm outer skin only, doubled over again on the hull bottoms for beaching in case of random rocks in the sand etc 3200gsm bottom only.
Deck 15mm - 800gsm triax per side.
Bridgedeck floor 20mm - 1000gsm triax per side
Bulkheads 15mm - 600gsm dbias per side.
Cabin roof 20mm - 800gsm triax per side.

This boat was capable of 27kts powered by twin 115hp outboards and was very stiff, for a smaller boat with lower speeds these laminates could be reduced.

 

just for reference
35ft open bridgedeck Shuttleworth Malihini home built key led "The hulls were built on a male plug and by using vacuum bagging techniques. Each hull weighed 500 pounds when it came off the plug."

 

Again thank you for the replies. A couple of things:

I would never try to build from study plans, my point was that both they and full plans vary considerably. I have full plans for the F82 and went through them extensively. I am prepared to believe I've got it wrong but from what I can see the foam hull and floats mostly used either 1/2" or 3/8" H80 or equivalent and 600 gsm 45/45 with appropriate overlap. Strip used uni and a lighter weight. Can't remember but maybe 300 gsm ? When I've looked over boats in this sort of size I try to convert the materials across to alternatives, but I've not reduced it to formulas as rberry has above. That's very helpful. What I do find is similarities. Where you have foam and 45/45 it is usually about 600gsm and 3/8 - 1/2 with localised reinforcing. Kevlar used to be popular round the keel but apparently it's hard to work with.

I'm a metalworker by trade and mech eng by profession. I've done a lot of building in metal and plastic but I haven't played with glass since I was young. I have a good "feel" for the materials I'm familiar with but until I've made some panels I don't have much feel for this. That is why I've used other designs as a guide. The examples above are a tremendous help, more data points to work from. Obviously I don't want the boat to be flimsy but I also don't want to build a battleship. I'm not building a racer but I do want it to sail...and it's got to go on a trailer ad that's another level of complication.

Yeah if you look how Shuttleworth's are built that figure is for the lower hull skins only not the whole hulls. I had a decent look at cheetah, which I love, but while it should not cost a LOT more than similar boats to build in my estimates it would take about 50% more time. Lots of moulds to build and some exotic materials, but boy what a beautiful boat...Check youtube for videos of Shuttleworths being built. Some very interesting stuff there...

The tristar 24 is so close to what I want but I could not get past the headroom over the bunks, and with Mr Horstman not responding... His plan prices are good (24 - $550) but he charges $110 shipping to Australia and that's 20% of the cost of the 24. That would be ok if I could be confident he would support during the build but atm it's a bit uncertain. Likewise I hope he is well and happy.

Also please bear in mind the difference in loads between a very small boat like this and even a 26'/8m cat with say a 16'/5m beam are pretty substantial. Again looking at the Woods site the 26' elf is quoted at under a ton while the 28' gypsy is 1800 kg, almost twice as heavy. There are other variables there of course but we all know boat weights go up by the square or cube of the length and for good reason. Springy floors will be consistent but this boat is less likely to hit an obstacle with the same force as a 2 ton boat...

Thank you all for your interest. Very interesting input.

 

Decreasing weight while increasing strength , or in my case increasing strength for the same weight was why I deviated from Ed,s chorce of material . I could have done this strength wise by useing triax only , and excepting a much thinner skin . My only question in your chorce of bias 45/45 only would be why not go with a Triax layup ? I have had no problem getting support from Ed in the past , I am not sure of the future but took this into consideration before I bought my plan,s . I do know one member of this forum had problems with the MT 26 , but as the MT 26 has been built from these plans before I feel help could have been provided from forum members or Ed to complete the build , just my opinion . It would also be my opinion given your profession that you could build the 24 from Ed,s prints , and support from this forum if any was needed . The only questions to Ed have been concerning changes I have wanted to make , such as scaleing up 6% . I know you have heard that cost of plans are a small part of the boat , make sure you build the boat you want , it can be a long process and one time shot . Rick

 

When choosing triax vs biax vs others it ones again comes down to unsupported panel sizes and load paths. The strength and more importantly stiffness of each unsupported panel is influenced by the fiber orientation and ideally you want quadraxial fibers across all panel areas for panel flexure. Biax works, so does triax but quad is better and it will be stiffer for the same laminate thickness if a load is applied in the center of the panel. If you have a deck and floor which form the tension/compression caps of an I beam, and the mast bulkhead forming the shear web - then you can use triax athwartship in the deck and floor to put the fibers where theyre aligned with the stresses of the mast compression and again achieve a lightweight/very stiff structure. The shear web should ideally be double bias to align with the stress in the shear web.... and of course a local column of sorts directly under the mast to transmit the mast compression right through the web so it wont buckle under the localized pressure.

Your hull topsides are shear webs and so dbias is all thats really needed for global stress- however again if you wish to achieve best stiffness for each unsupported panel (local stress) you would use a quad or make your own via 1 layer dbias +45-45 and 1 layer biax 0-90.

All this is rather acedemic for a small boat like this however - reality is even extremely light dbias laminates of 450gsm per side will suffice for all global and local loads in a boat of this size provided the stiffeners are not spaced unreasonably far apart. The extra glass you put down beyond this is purely for durabilty/toughness as foam core boats with thin laminates are very fragile in terms of being knocked around...

 

Ok. Those are both some meaty comments. I need to think about all that.

Anyway in the mean time I need to get some more details sorted on the model. I don't need to commit to layups until the build starts anyway. I've been really lazy these last few days and I'll be busy until friday but hopefully I'll get back into it then.

Regarding the tristar 24 there are really only 2 things that concern me. First I'd like to raise the whole deck 4"/100mm. This would give headroom over the bunks and adequate standing headroom for me (I'm 5'6"). The second is how much timber is still in the foam build. It looks like he substitutes foam for the skins and the panels for the bulkheads, but it looks like the frames of the athwartships bulkheads still have timber edges and some infill. It looks like other parts are still timber. Without a plan set I can't be sure how much timber is left. I know timber is a viable building material but I remain scarred by my previous ply boat experiences

This cat would fulfill my wants every bit as well as the Tristar, and it should be lighter and quicker to build, although obviously there is a lot of hours in designing it. The only issue is the beam, making it a compromise. It would be nice to have 14' in a boat that size, or at least 12', but the towing envelope is 9'6" so I have to work with that. The mini bridgedecks like the jarcats, little barrier, red barron etc are not racers, but they are not terrible sailing boats, and this proposal has the same length to beam ratio albeit scaled up a tad. If it sails ok I will be very happy.

Thank you very much for the comments. When I get a chance to sit down and focus properly I'll have a good think.

 

Biax 0/90 is better for fore and aft layups to resist rig tensions than dbias 45/45/

 

There are plans for both foam and plywood bulkheads for the tristars , no wood needed except over the bulkhead for the mast . I bought foam for the bulkheads but may use some 1/2 " baltic birch form ply I have instead as wood bulkheads will be easer materal wise . I took a picture of a Tristar 27'9" that was modified that came up the river looking sharp , and came back down with an ugly dogger added . One of the mod,s pre dogger was the cabin deck , it looked good , and I don't think it would add much windage . There was a door in place of a hatch , the bulkhead was raised may be 18" with a flat spot running forward in a pie or wedge shape . They had then filled the area between the pie and raidus areas of deck with a pennate shape with windows . This might give you the head room you would need if you modified the 24 in such a way . Rick

 

Thank you for that.I'm thinking maybe 3/4" foam plus heavier skins on both cabin top and bridgedeck might be a good idea. The bridgedeck especially as the boat will be dragged onto and carried on the trailer by that. I need to calculate how much weight it will add.

I'm getting better at this new quoting function the forum offers. SO you are building a Tristar 31 ? I don't suppose you would consider posting the picture of the 27-9 would you ? There is so little on Horstmans online... Thank you for the clarification on the bulkheads. Is there much timber in other areas in a foam build ? or can you eliminate most/all of it ?

The 24 has the option of a"bubble cabin" I call it, just through the center. It gives 6' headroom. The problem though as I see it is the low headroom over the bunks so I'd thought raising the whole deck 4" would sort everything, no bubble cabin, just raise the outside float sides, the cowling at the front of the "wings" and the top of the rear bulkhead. The boat would look the same only 4" higher. The deck would be flush and offer 5'9" headroom down the middle which is enough for me to stand up and take a shower .

I don't need 2 doubles, and the 729 cat has 1 double and 2 singles. I did battle with the computer this morning but I've hit something I don't know how to do so I'd left it for the moment.

 

I have found two pictures of the 27'9" , one is a print which shows the side view which is what you need to see . The only problem is I don't know how to post it , low computer skills , or no skills . The other is in an email where our Engineer sent it to me , if you send me your email add. I can forward it to you . I think I emailed it to Cavelier , if he has it he maybe able to post it . That picture is a forward view , because the boat is white you mostly see the ugly dogger . But if you blow it up some you can see the window on the modified cabin top . I don't see where you would have to use any wood if you don't want to on a Tristar . There are spots that it is easer to use wood , like the mast step , bow stim , interior cabnets and tables might be other area,s . I also have some concern about rot , but the form ply wont rot , and if it is used as a core for the bulkheads , glassed , and shimmed up a couple of inches with foam off the keel area it should be good . If you were to lay your aft cabin bulkhead out on the 24 , from the center of the bulkhead raise it say 16", extend it say 2 1/2 foot to each side of center line , think of an almost pie shape with no point slooping to the bow , fill in the side between the part you have rasied and the original radius deck , and that is what the 27'9" would resemble .