Lee wants a new trimaran design...

[Todd Bradshaw]

That's still really a question for a boat designer, not a sailmaker. The size and volume of your amas is going to be a serious factor in determining how much heeling force the boat can stand up to, so before designing the rig, you really need to design the hull package that it will sit on. That hull really doesn't thrill me though, whether stretched or not. In a package in that size range you can do so much more. On one end of the spectrum would be something light like some of Gary's outriggers, done as a tri if desired. At the other end would be something roomier (though certainly heavier and more complex) like our old Farrier trimaran, which could seat up to six people comfortably in the open cockpit. The reasonably narrow main hull flared out, well above the water where that part never got wet, and all that cockpit room, plus the wingnets was wonderful for a boat under 20' long.

[Lee.007]

Hmmm. Flared hull. I never thought of that. Excellent idea. That's why I am a Soldier and not a Naval Architect. I can see how a narrow hull can be fast, with the flared portion providing a lot of living space. Almost like cheating. We always say, "if you aint cheatin', you and try'in. But if you get caught, you aint' try'in hard enough!"

Does anybody know of a home-buildable design that has the flared hull?

[NeilMB]

Does anybody know of a home-buildable design that has the flared hull?

Besides the Seaclipper 20 you mean? Richard Woods’s Strike series from 15-20 feet have the same sort of widened cockpit on a slender hull. So does the W17.

One of the things that I haven’t been able to figure out just from looking at plans is how slender hulls with wooden seats supported on the akas compare to the more complete cockpits of the seaclipper/farrier/strike/w17.

Gary Dierkings picture earlier in the thread seems to illustrate the former, with sideways seating but not a full cockpit.

[Todd Bradshaw]

This is a photo of the main hull of a Farrier tri like our boat being restored with the amas and other stuff removed. You can see the flare. It does wonders for cockpit space.

tramp-trimaran-15.jpg

[JimD]

And generally speaking, there are good reasons why the main hull of tris have narrow as possible waterline beams. Probably would hurt the performance and comfort to have a flat bottom 18 footer fat enough in the middle for two people to sit side by side.

Edited to add: I see others have mentioned the need for a narrow waterline beam. Somehow I did not see those comments when I posted. As for flared hull plans, most of the small simple day boats have dory style widely flared single chine or a double chine, apart from the Strike. If it were me I would have no problem designing a 'hiking flare' myself, if a more enclosed, sheltered platform to hike onto - in practice the place where you sit and sail from - were what I wanted, keeping the hull as narrow as possible at the waterline, acting as more of a foot well than where you actually sit.

[Lee.007]

This is starting to sound like a lot more work than I want. I could design up a flared drifter. But, I aint no Naval Architect, and I learned the hard way from designing my kayak - free boat designing software from the internet aint all its cracked up to be. Especially when I don't know how much rocker is enough or too much, how much volume to give me bouyancy is enough, etc. So I will stick to a design that someone else has already proved will float, and actually sail and tack easily. My kayak with the sailrig tacks like trying to turn an oil tanker around. If I don't give it some paddle, I will almost always end up in irons, no matter how slow or fast I turn the rudder. It will turn fast, but it just won't go through the eye of the wind very easily at all. I don't think I have enough rocker. Anyway, I think I will just build a drifter, maybe stretch it but keep the same beam, add some "wing nets" made out of cedar which could be alternate seating while sailing, and the wife can sit on the wing nets, in front or behind in the hull. I want to do this to get another boat quickly. Oh, and Yes, I still plan on building a Sea Clipper 20.

[Todd Bradshaw]

It is not at all unusual for it to take an entire season of sailing to learn to reliably tack a trimaran, but that is one place that a jib helps. You leave it cleated on what will soon be the "wrong" side and let the backwinding jib help to push the bow over onto the new tack. As mentioned before though, a jib really needs shrouds or backstays (or both) to maintain decent shape and keep the luff tight.

[tri-guy]

The Seaclipper 20 sounds like a good choice. I'm not crazy about those akas, but as a builder of a Marples CC23 I can highly recommend the quality of the design work and plans for anything John has done.

[JimD]

This is starting to sound like a lot more work than I want. I could design up a flared drifter. But, I aint no Naval Architect, and I learned the hard way from designing my kayak - free boat designing software from the internet aint all its cracked up to be. Especially when I don't know how much rocker is enough or too much, how much volume to give me bouyancy is enough, etc. So I will stick to a design that someone else has already proved will float, and actually sail and tack easily. My kayak with the sailrig tacks like trying to turn an oil tanker around. If I don't give it some paddle, I will almost always end up in irons, no matter how slow or fast I turn the rudder. It will turn fast, but it just won't go through the eye of the wind very easily at all. I don't think I have enough rocker. Anyway, I think I will just build a drifter, maybe stretch it but keep the same beam, add some "wing nets" made out of cedar which could be alternate seating while sailing, and the wife can sit on the wing nets, in front or behind in the hull. I want to do this to get another boat quickly. Oh, and Yes, I still plan on building a Sea Clipper 20.

I agree, just build a drifter. for something to park your butt on over the side a simple plank to sit on would do.

[Gary Dierking]

What makes trimarans fast. Read the PDF here: https://drive.google.com/file/d/0Bx_...uyBIyGIgxqq9Jw

[Lee.007]

Thanks for the link Gary! That is a very interesting read. Good info to know so if I modify a design I won't screw it up.

[Lee.007]

Ok guys, riddle me this, What the heck happened to Mark Gumprecht? I don't see any plans or blog posts on any of the normal small trimaran sites after 2012. I was hoping to find a 20 foot or 18 foot Drifter plans. I am thinking of just splitting the Drifter vaka in the middle, inserting a 4 foot hull section, do the same to the amas, and give it a try. I think I need to curve the bottom of it to match the side profile so I don't have too little rocker, but I don't know how to eye ball that. I like having numbers. I saw where there was a 17 footer, but I can't find plans for it anywhere. The good news is Duckworks has the 12, 14, and 16 footer plans available for free!! Sweet. I hope that didn't put Mr. Gumprecht in debtor's prison.

Does anybody know if he designed/built a 18 footer or 20 footer? Or if he is still around? Sure would be interested in some info.

[NeilMB]

I have seen pictures somewhere (but I can’t remember where right now) of a Mark Gumprecht Drifter 20 prototype that had a cabin, but like you I think that the 12/14/16 that are on Duckworks are the only plans available.

People get into other hobbies or all number of things could have happened to him. This page https://www.duckworksmagazine.com/08...kiff/index.htm has an email address and you could write and ask. My experience is that boat designers appreciate people being interested n their work, especially interested enough to ask questions.

[upchurchmr]

I sent a message to Gumprect several years ago about that boat.
No response.
Good luck.

But that is not a simple quick boat.

You ought to look at the Cross 18.
One problem, no active plan sales.
I have an incomplete set.

Again, quit looking for super simple sail rig.
It will handicap your boat, tremendously.
And your development as a sailor.

[Lee.007]

Again, quit looking for super simple sail rig.
It will handicap your boat, tremendously.
And your development as a sailor.

Yes Sir upchurchmr! I have decided to use stayed masts from now on with my trimaran boats. I really want to add a jib to my current boat. But I don't think the sail rig amas give me enough buoyancy to carry 100 sq feet of sail. Last time I tried, well, you guys can read about that disaster on my "newbie needs help..." thread. Soooo, I might just build me some amas using the drifter 16 plans, and adapt them to my current sailrig and kayak boat. Maybe attach the sailrig amas to one of my family plastic kayaks with a 40 to 50 sq foot sail. I have a 13' plastic tandem pelican I have been itching to put a sail rig on. If I do that, and build a Drifter 16, then I would have 3 sailboats to begin my pirate armada. I am going to have to start sewing some sails if I do that!!

But yes, you are right, I gotta stop being intimidated by the complexity and start learning how to manage multiple sails.

OH, I almost forgot, I searched for the cross 18, and could not find plans other than a reference to the "My Boat Plans" software. I went to their website and they had a $20 off coupon, bringing the price down to $27 for the whole deluxe package. The $20 coupon ends tonight at midnight tonight if y'all are interested. At first look, some of the plans look pretty old, but the videos look pretty nice, and there are several instructional books from the Navy and Coast Guard as well. Also includes some boat design software. I haven't search through all of the 500+ plans yet, but I am hoping to find a larger drifter, and maybe the Cross 18 in there too. I figured it was worth $27 to give it a try. Only pain was downloading all the files. They have a cd option, but that was $20 more. Took me about 30 min to download all the software.

[upchurchmr]

Sorry if I'm being too pushy.

Please let us know if you actually find the Cross 18 plans in that 500 plan set.
My understanding is that you might find "study plans" which would not give you enough to build a boat.
I'm really interested if you got something worthwhile.

The cross 18 is significantly old style, but a very useable boat from what I have talked to people.

[Lee.007]

Sorry if I'm being too pushy.

No, you are not pushy at all. You speak the truth. I would rather someone give it to me straight.

Well, I was not totally disappointed by the "my boat plans" deal. But, almost every single one of the plans was a pdf copy of plans that look like they came from a 1940's boat magazine. There were a few that will take a lot of math and lofting to build a boat. Not that it was a total waste, but I think it certainly did not live up to the hype. I think the free Drifter 12, 14, and 16 plans were much higher quality, and they were hand drawn. Oh, and the thing that really irritated me was that there was not one single trimaran in the bunch, and only a small amount of sailboats. 90% of it was power boats. I do like the instructional books that were included, and the videos that you get access to on the website.

But hey, you get what you pay for.

I am working on stretching the Drifter 16 out to 18 feet in my autocad. All I am doing is inserting a 2 foot section between frames 2 and 3, making it a total of six feet between these two forms. Then I am fairing the curve, and creating a fifth frame to sit equidistant from form 2 and 3(three feet to each frame). I am generating the fifth frame from the geometry formed by the faired curves that I drew where the stringers are supposed to go. I will post a pic when I finish drawing. I am debating if the 5th frame will stay, or if I will just use it as a form and remove it after the hull is glued up and dried. I think a good compromise is to keep it, but cut the center cross piece out of the top. I was worried about the long stretch of hull walls with no support, but Gumprecht has about a 8 foot stretch with no inside frame because he says to remove frame 2. I will let y'all know how it works out.

[Lee.007]

Alright fellow multihull fans,

Here is a drifter 16 (the vaka), stretched to 18 feet. Max beam works out to around 33 inches, max waterline beam is about 21 inches fully loaded. Makes the length to wl beam work out to almost exactly 10:1. The paper Gary sent me about "Why Trimarans Go Fast", says 9:1 is about as low as you want to go for a cruiser/racer hull, so I should be good on that aspect.

So here is another question I have for y'all what is the benefit, if any, of a flat bottom boat vs a v-shaped hull? Does the v-shaped resist lateral pressure from the wind when going to windward? Is a flat bottom easier to tack? I was thinking about making a slight v-shaped hull on the bottom. Not much, just a little so its not flat. But I am worried it may affect performance in a bad way, especially tacking.

So, can y'all think of any reason why the vaka in these pictures won't float or perform nicely??

StretchedDrifter.jpg

[upchurchmr]

Will this have a centerboard/ daggerboard/ leeboard?
Don't count on the square edges of the hull to resist leeway.

Looks OK if you have a board of some kind, big enough.

How much volume will you have in the amas? When you draw them.

[Woxbox]

Rocker improves tacking, but can reduce speed. Leeway is controlled by the foil, not much by hull shape. Also, hard chines create turbulence and need to be rounded out as much as is practical. There are tradeoffs everywhere that need to be balanced against each other.

[Lee.007]

Rocker improves tacking, but can reduce speed

Hmmm. So how much rocker to get nice tacking without hurting speed too much? Is there a hard and fast calculation somewhere, is it just kind of a trial and error kind of thing?

Leeway is controlled by the foil, not much by hull shape.

Yup, I knew the leeboard controlled the leeward side slip, but I was worried a boxy hull would cause a lot of lateral resistance, making it hard to turn and tack.

Also, hard chines create turbulence and need to be rounded out as much as is practical.

Ok, I think I am going to round off the corners of the chine as it gets close to the bow, but maybe leave it a little hard beginning about 1/3 of the way up from the stern. I watched the videos listed in the "my boat plans" and they are basically Northrop Grumman training videos for a high school design contest back around 2010. That also said the hard chines cause turbulent drag, but better planing ability. I don't know if I will ever get fast enough on a small trimaran to plane, and the design training video also said hard chines make for a rough ride. I might just round them off all the way back. The video also said for sailing you want efficient hulls as opposed to planing hulls.

By the way, I am not at all happy with the "My Boat Plans". The more I get familiar with it, the more I am realizing this guy did nothing but collate a bunch of free plans and free videos from all across the internet and from old magazines. I am not mad about $27. But if I ad paid the normal $47 or more, I would be furious. And I still have not found a single trimaran in the bunch.

[Woxbox]

Lee -- Different designers have taken different tacks over the years to work with all the competing issues involved in trimaran design. Dick Newick kept his boats as light and skinny as possible with extremely slippery hull designs. They go fast without needing excessive sail area but have very minimal carrying capacity for their width and length. On the other end of the cruising boat spectrum, for example, Ian Farrier designed boats with a remarkable amount of main hull volume. But his boats are much heavier for a given length and have tall rigs to drive them.

But here's another interesting detail. The Newick main hulls are quite slender, I think 10:1 to 12:1 waterline length to beam ratio. Farrier hulls run more like 8:1. This much width should kill speed but it doesn't because the run -- the after section of the main hull -- is quite wide and flat for a tri. So the tall rig levers the boat up onto the leeward ama, and the main hull lifts and goes into a semi-plane across the surface. It works. (I had an F-27. You can feel this happing. My boat never went 10 knots. It would hang in there at 9.5 until there was enough wind to get the main hull on top of the water, at which point it would immediately accelerate to 11 or more.)

The chine turbulence issue is not as severe in a narrow hull as it is in a wider one, but still shouldn't be ignored. As to how much rocker, there's no formula. It is critical that the transom is not submerged when the boat is moving, however. So you need at least enough rocker to make that happen.

Your image looks like a conventional tri vaka. As I think Todd mentioned, tacking trimarans comes with practice and getting a feel for your particular boat. I think it would be a mistake to focus on that one feature too much when working out or choosing a design.



One other thought -- the "rough ride" issue with hard chines must come from discussion of monohulls. Trimarans give the best ride of any type. Hardly any heel but much more forgiving in the rough stuff than the typical catamaran.

[Gary Dierking]

Here's the profile of my flat bottomed 24' Wa'apa hull. It tacks fine as long as you have the center of effort of the sail in the right place. So I think yours will be fine.

[Gary Dierking]

Check out the stern rocker on this F22 tri. It is not a slow boat. One of the things this much stern rocker does besides easy tacking is to prevent nose diving with a big spinnaker up. The suction at the stern prevents the bow from diving.

[Lee.007]

The chine turbulence issue is not as severe in a narrow hull as it is in a wider one, but still shouldn't be ignored.

Roger that. I plan on just rounding off the corners so it is not a sharp corner, and call it good.

As to how much rocker, there's no formula. It is critical that the transom is not submerged when the boat is moving, however. So you need at least enough rocker to make that happen.

So according to the original design, the transom sits may 1" out of the water. Stretching it out two more feet, I think I am adding more a lot more volume than wait, but I have not calculated the displacement yet. I need to do that so I know where the water line is going to end up, and the sweep the transom up if I need to.

As I think Todd mentioned, tacking trimarans comes with practice and getting a feel for your particular boat. I think it would be a mistake to focus on that one feature too much when working out or choosing a design.

Yes indeed. But I swear I think my kayak sailrig for some reason does not want to go through the eye of the wind nearly as easily as it should. I will keep practicing. And you are right, focusing too much on one design aspect will force sacrifices on others. I think somebody said something about trade-offs??

One other thought -- the "rough ride" issue with hard chines must come from discussion of monohulls.

Yes, they were talking about designing monohull ships. I was just worried that waves slapping the sides of the amas might make it rough, but now that I think about it, I remember watching monohull power boats at the ramp get bounced all over the place while they were hauling out, and I just kind of rolled over the broadside waves.

[Lee.007]

The suction at the stern prevents the bow from diving.

Say what? Suction? You're gonna have to explain that one in red-neck-ese for me to understand that. Are you talking about the Bernoulli principle? Somehow the water running along the hull sucks it down in the water??

[Gary Dierking]

Say what? Suction? You're gonna have to explain that one in red-neck-ese for me to understand that. Are you talking about the Bernoulli principle? Somehow the water running along the hull sucks it down in the water??

I first heard about this from Sir Tom Davies of the Cook Islands who designed and built some traditional style Polynesian double hull canoes. They generally have a low wave piercing bow with an up-swept stern. This is not just for style as I found when I built a smaller replica. In very steep waves where my low bow should have been under water at times, it refused to submerge. Principles are only valid when you can duplicate it with an experiment and that is what I did. The photo below shows a 75'er modeled after Sir Tom's work and some smaller ones. One of mine is in the middle.

[upchurchmr]

Lee,

The issue of not having the transom in the water is important (Woxbox comment), but there is another way to make this happen.
Typically daysailing catamarans do not have a great deal of rocker.
Going upwind, crew weight is shifted forward to raise the stern. Care must be taken to not sink the bows too much for the gustyness of the wind.
On a reach, the bow is driven down by sail forces. In lighter wind crew weight is moved back as much as possible to keep the stern just touching the water - to be safe when a gust drives the bow down.
In heavier winds, where the boat is sailing fast, the crew weight is moved far back to prevent the bow from diving in. In this case the stern is under the water surface, but the speed is so great that the water comes off the stern cleanly, making a "trough" similar to when a power boat planes.
When going down wind, crew weight should be farther back to prevent nose diving.

Crew weight shift is critical in a small, light weight boat for good performance.
This is why fixed seating in the main hull will automatically hinder performance in higher wind, and seating on a bench fore and aft can provide for better performance, safety, and control.

You can always choose in hull fixed seating provided you don't try to push the boat too hard. Or you can limit the winds you sail in - most of the time.

Either way you can still have fun.
But the fun/terror of sailing on the very edge of disaster are some of the times I remember best, IMHO.

[mattglynn]

Hey Lee,

It sounds like you're interested in building a Seaclipper 20. My wife and I built one last summer, and briefly cruised it in Croatia this fall(we live in Germany). The plans from John Marples are excellent, and his support could not have been better, usually responding to emails within 24 hours. The boat has proven to be fast, dry and comfortable for its size, and strong. There is an option in the plans for what John calls the "sport cockpit" which we built, and that makes it very similar cockpit wise to the farrier tramp/eagle? that Todd posted above. The only issue I see in regards to your stated list of requirements is paddling at 2-3 knots. We don't have an engine so I built a couple of extra long SUP paddles, and with my wife at the bow and me at the stern with the tiller between my legs we can make about that in flat water with little wind. Anything above 10 knots and paddling upwind becomes impossible. If you have any questions about the boat or building process feel free to ask.

IMG-20211111-WA0000.jpgIMG-20211116-WA0000.jpg

[Lee.007]

That's a sweet paint job Matt! Thanks for the advice. I gotta ask about that cockpit and the tent. Do you have some sort of platform that slides out to cover the foot area? Or did you just cut a hole in the tent, and sleep on the "tramps"? If you have pics of the inside of the cockpit with your sleeping gear setup that would be great for inspiration!

I do want to build a Seaclipper, but I need to focus on finishing my master's degree, and a lot of workstuff too right now. So, in the interim, I thought I would build something smaller. And Yeah, for the Seaclipper I think I will put a small outboard on it, like 5 HP or less or something to get me back to shore when the wind craps out.

[Lee.007]

And speaking of a 5hp or less outboard(Ive seen some 2.5 ~ 4hp on amazon), what is everybody's favorite? I don't care about looks or speed, my biggest requirement is reliability and ease of use. EDIT: Oh, and low price too. Suggestions?

[NeilMB]

You’re not the only one dreaming of small trimarans, smaller than the Seaclipper 20. This page http://www.tackingoutrigger.com/index5.html has been exciting me recently with all of the designs collected on one page.

Lots of the boats that have been mentioned on this page, but some that I hadn’t seen before. They’re all pretty big projects though. The Slingshot and Drifter might be the simplest of everything there though. Is 16 far enough from 20 to matter here? (20/16)^3 = 1.95 so about twice the boat?

[mattglynn]

We bought a cheap tent and cut the bottom out of it, put grommets in the bottom, laced shock cord through them, and hook the cord around hooks on the underside of the gunwhale. The tent is quite secure, we rode out some 60+ knot Bura winds at anchor with no issues. The cockpit is roughly 7'x7' so there is plenty of space, I'm 6'3" and almost have standing headroom with the tent up. No platform that slides across, it would be nice a queen size air mattress would fit. The side decks/seats are plywood and we made 4" thick cushions that we use as sleeping pads, and sleep one on each side. We shim up the daggerboard as a stanchion and bolt a ply tabletop to it, needs some refinement but works ok. I couldn't find any good photos with the tent up so I took some screenshots from a video, please excuse the mess but its tricky living on an open boat in november. There are some pretty cool looking electric outboards these days, although a powerful trolling motor would probably be enough for the drifter.Screenshot_20220218-092235_Photos.jpgScreenshot_20220218-091716_Photos.jpgPXL_20211011_075348408.jpg

[Woxbox]

Matt -- That's a really nice job you've done with your tri.

Lee -- I agree that you'd be a lot happier with a simple electric motor. Plus, the battery can do double duty to recharge cell phones, etc.

[Lee.007]

Hmm. I hadn't really given the electrics much thought Dave. I have never used a trolling motor. How big a battery(weight?) do you need, and how long does it last? If I get becalmed 5 miles off shore, will it get me back? I'm sure it all depends on boat weight, batter size, motor size, etc., but on the average what have most of y'all seen?