Multihull Structure Thoughts

[montero]

I understand the pitfalls of building your own boat. But we now have an incredible number of products that aren't boat-specific and therefore cheaper, and we can use them with caution.
I plan to use prefabricated components from another industry.
I spoke with AI today, and sometimes she point out problems I don't notice.
The basic problems remain: the cost of a heated room of the right size, high material costs, and limited time available to work on the boat.

Resale value? A hole in the water.

 

[oldmulti]

Drifter trimarans are designed by Mark Gumprecht. The Drifter 17 is a small trailable cabin trimaran for bay and river cruising etc. The tri is 17 x 12 foot with a 22 foot plywood and timber wing mast that can be home built. The main is 115 square foot with a 60 square foot fore triangle. I do not know displacement figures. Mark designs light boats that require good building practice, but these boats structurally are very good. Follow the plan and you will have a good boat.

This is a lead into an update on the Drifter 17 trimaran. Due to some good work by a tread regular, he obtained a set of Drifter 17 plans and asked Mark if details could be spoken about, Mark agreed. Mark has effectively retired from design work and we need to respect his privacy. We will provide more details over the next few days of the design, build materials and plan details. Some of the information is on the web with a few build jpegs but the plans have more detailed.

A few words from Mark when he sent the plans. “She (Drifter 17) is very easy to beach with the centerboard and kick up rudder. It is nice to have the cabin for that kind of trip and have protection in rainy weather. I have mostly sailed in protected waters, but I had one day going to weather in 18 to 20 knots and bumpy seas, and she did very well, and was amazingly dry. Make sure you overhang the deck in the bow as shown on the plans. That’s what keeps you dry.”

A few jpegs to remind people of the good looking tri.

 

[oldmulti]

The Drifter 17 is 17 x 12 foot with an all up towing weight of 1,900 lbs including trailer. This indicates a boat weight of about 1100 to 1200 lbs. The draft over the swing up centreboard is 3.5 foot.

The Drifter 17 is a ply and timber design with glass on the exterior. We will start on the rounded floats, there are 7 frames of 6.4 mm plywood with a width of 48 mm sides and up to 125 mm across the top where the cross beams are supported. There are 16 x 16 mm timber stringers (4 per side including the gunnel) and a 19 x 31 mm deck stringer. There is a 6.4 mm plywood stem is inserted for the stringers to attach to at the bow. The float side plywood is 3 mm. The float bottom is strip planked with 6 mm spruce or WRC with 120 gsm cloth on the inside. The inside of the float should have a WEST type system done. Finally, a 6.4 mm plywood panel chain plate panel is attached for the cap shrouds. The 6.4 mm plywood deck is then attached. The angular transom is 6.4 mm plywood with an addition layer of 3 mm plywood to help provide thickness for rounded edges. The exterior of the float then has a 120 gsm glass cloth with epoxy all over with an additional glass tape along the keel line.

This is the first of 4 detailed descriptions for the hull and cabin structure of the Drifter 17. You will get detailed drawings when I have done some conversions but for today a few general jpegs. A lot more to come. Thank you Mark and a thread member.

There is a video of the Drifter being set up and sailing at: drifter 17 trimaran - Search Videos https://www.bing.com/videos/riverview/relatedvideo?q=drifter+17+trimaran&refig=6a6d3369c04d4f389f09cb198ff4dd27&pc=ASTS&ru=%2fsearch%3fq%3ddrifter%2b17%2btrimaran%26form%3dANNTH1%26refig%3d6a6d3369c04d4f389f09cb198ff4dd27%26pc%3dASTS&mmscn=vwrc&mid=D59C812B91092AA6633BD59C812B91092AA6633B&FORM=WRVORC&ntb=1&msockid=8775f23e8e3511f1a190f00b848f94cb

 

[oldmulti]

The next instalment of the Drifter 17 design is about the main hull. The Drifter 17 is a ply and timber design with glass on the exterior. There are 7 frames of 6.4 mm plywood with a width of 48 mm sides and up to 125 mm across the top where the cross beams are supported. Frame 2 has 12 mm ply sides (cross beam support frame). Frame 1 (crash bulkhead) and 5 (end of centreboard case) are full width. The stem is 9 mm plywood. Gunnels are 16 x 19 next stringer down is 16 x 23 mm (crossbeams rest on this level) next 2 stringers are 16 x 19 followed by the transition stringer to the 9 x 18 mm WRC strip plank curve to the flat 9 mm bottom plywood keel plank. The interior strip plank is glassed (and 9 mm ply keel plate) with 200 gsm glass cloth with epoxy. The hull sides are 3 mm or 4 mm plywood. The transom is shown as 6.4 mm plywood. The exterior of the shell is glassed with a 120 gsm glass cloth with epoxy and an additional 120 gsm glass cloth and epoxy over the strip plank and keel plate.

The deck structure is complicated and is best worked through with the drawings. General information is deck stringers for the cabin roof to the sides is 25 x 50 mm, deck beams 32 x 19 mm and cabin hatch support beams are 19 x 42 mm. Cabin structure, fore deck and cockpit floor seats are 6.4 mm ply all covered by 120 gsm cloth in epoxy.

Hopefully tomorrow I will be able to show you some of the plans to help with the verbal description. Some further main hull build jpegs attached.

 

[tane]

how is the transition from stripplanked portion to ply made? Cannot make it out on the pics

 

[fallguy]

how is the transition from stripplanked portion to ply made? Cannot make it out on the pics

I thought I knew, but the ‘stringer’ looks angled outward on the bottom. So I posted and then rethought it.

It looks like instead of a simple rabbet for each; they made the ‘stringer’ the first strip.

This picture is what I see. They taper the stringer to allow for the inside glass.

Took me some edits to get to it. Good question.

Not sure why they don’t taper a 9mm timber back down to zero and glass up onto the plywood. I suppose the timber gets too weak.

Not sure I’d build this way. I would want 50mm of glass onto the plywood and so I would need to return the 14mm step back to zero for glassing; unless I missed something.. Oh. I did. OM has more details to post; sorry.

 

[tropostudio]

how is the transition from stripplanked portion to ply made? Cannot make it out on the pics

It's noted 9mm ply bottom and 9mm strips at bilge. That's a flush fit, and appears so in the photos. The hull sides and WRC bilge strips land on a common stringer. If the hull side ply is 6mm instead of 9mm, I'd presume the stringer is rebated .

Ah, fallguy has looked closely. I just assumed a rectangular stringer with a rabbet to accomadates the thicker WRC layup.

 

[oldmulti]

Drifter 17 plans. To help simplify some of the discussions I have managed to download some individual plan sheets in a JPEG format. Sorry for the landscape mode but it easier for printing. Use the rotate option to get the orientation you want. The plans are the main hull, the float and deck plan. At the end of the of these items, I will send the full plan PDF.

 

[tane]

I wonder, if the strip portion is really worth the extra effort over a simple chine. Sure it has less wetted surface & there is (maybe) less turbulence when sailing at a slight angle, but we do not have a high tech all-carbon no-effort-spared speed record machine here after all...

 

[tropostudio]

@tane - Seems like the Drifter 17 is from an accomplished woodworker who designed and built boats, and is now retired. Sure, it could have been hard-chined. Or it it could have been a soft chine from ply, filleted and gorund and glassed around. Lots of ways to skin the cat that probab;y won't affect performance or structure much.

What is pretty cool is to find the videos of how the boat is has been used:

https://www.youtube.com/@RusinVanDyke/videos

I leave it to you to cull through his channel. My take: Cool little boat; good for 1 person to camp-cruise; the top hinge on crossbeams was sketchy and needed to be re-built to remove play, and I still don't know how the bottom of the cross beams are secured. Methinks engineering there is dicey.

 

[fallguy]

I wonder, if the strip portion is really worth the extra effort over a simple chine. Sure it has less wetted surface & there is (maybe) less turbulence when sailing at a slight angle, but we do not have a high tech all-carbon no-effort-spared speed record machine here after all...

Since I’m not a sailor, I typically avoid responding, but as a non-sailor, I would be wondering how the hard chine performs at any high degree of heel. If it is not straight, then it may cause some handling issue that makes rudder work harder. Basically a banana shaped strake, etc.

But I qualified my reply, and just offering my dime and to be able to participate in this magnificent index.

My boat is getting splashrails added and been avoiding my 100F garage for the finish. I also added fixed trim tabs courtesy of baeckmo that took my boat from odd looking running to great plus about a knot and a half top end.

Carry on, I will continue to gaze in wild wonder and try to keep quiet!

 

[oldmulti]

Drifter 17 cross beam structure. Mark designed the cross-beam structures from timber and plywood. The folding system works well if you have very light floats on smaller size trimarans. Simple cheap and effective. If you have heavy floats or are older alternative folding systems may be better, but we will talk about the Drifter design as it stands.

The crossbeams are a box section, 105 mm wide by 125 mm high at maximum depth, with 6.4 mm plywood sides. The top and bottom are 2 layers of 6.4 mm plywood or 2 layers of 6.4 mm spruce or mahogany. In each internal corner is a 12 x 12 mm cleat. The bulkheads are 18 mm plywood approximately 600 mm apart. At the hinge points there are 18 mm bulkheads and on the top and bottom internal flanges is a short 18 mm plywood block to hold the screws for the external stainless-steel hinges. The hinges on top of the beam are 3 mm stainless steel 90 mm wide. The joint plates on the bottom of the cross beam are 3 mm stainless steel, 64 x 210 mm They are screwed in on one half of the folding beam and with a metal plate on the other half “bolted” together to hold the beam in position. This allows the beams to be folded or held in position. This is an important structure as the cap shrouds go to the floats transmitting loads through the cross beams.

The entire box section is a WEST system internally. Externally a layer of 300 gsm glass cloth in epoxy is done.

The plan below gives more detail.

 

[HelmutSheina]

Since I’m not a sailor, I typically avoid responding, but as a non-sailor, I would be wondering how the hard chine performs at any high degree of heel. If it is not straight, then it may cause some handling issue that makes rudder work harder. Basically a banana shaped strake, etc.

But I qualified my reply, and just offering my dime and to be able to participate in this magnificent index.

My boat is getting splashrails added and been avoiding my 100F garage for the finish. I also added fixed trim tabs courtesy of baeckmo that took my boat from odd looking running to great plus about a knot and a half top end.

Carry on, I will continue to gaze in wild wonder and try to keep quiet!

It's a multihull, high heeling angles tighten the sphincter. Richard Woods has been designing hard chined multis for a very long time.

 

[oldmulti]

This Drifter 17 item will be about the wing mast that is a ply timber build and the centre board which is a spruce or WRC build with glass around the exterior.

A mast builder describes the build as: “Drifter 17 mast is relatively easy to build, if you have good woodworking skills. I would not recommend it for those who do not know how to make scarfs. It is built in 3 mm and the beams in spruce, laminated on the outside with a layer of 170 gsm glass and epoxy. I use PVC foam to make a rounded leading edge. There is a 13 mm fiberglass tube on the trailing edge, split, for the sail headline. You can add carbon if you want to make it stronger, but I think the base mast with glass cloth is strong enough for a boat of this size. The basic section without rigging weighs 27lbs for 22 foot long. The profile cord is 165 mm, before adding the mainsail rail, which ultimately gives 178 mm long, and 66.7 mm thick (external dimensions). The rigging is stainless steel cable 3mm 1 × 19.” There is a fiberglass mast head and the plastic/metal ball mast foot. The diagram of the mast helps understanding. A web site describes the mast build, pivot base and mast head details. The site is: Self-Built Mast for Small Trimaran https://smalltrimaran.co.uk/self-built-mast-for-small-trimaran/

The mast is supported on a mast beam in the cabin roof of the cabin.

The centreboard is a shaped 25 mm thick spruce or Western Red Cedar board that is 292 mm chord x 1155 mm long that is wrapped in a 170 gsm cloth with epoxy. The daggerboard case has 9 mm sides with 27.5 x 22 mm timber surround. There is a grove in the top of the board for uplift or downhaul of the board.

Again, all of the above is relatively easy to build if you are careful and follow the plan. It takes time and patience. An alternative is to get a small beach catamaran rig with an aluminum mast but be careful. The righting moment of the Drifter tri will be higher than the 8 foot wide beech cat and the mast may be heavier. Both could impact performance or require more maintenance.

The jpegs below will help understand.

 

[oldmulti]

This is the final item on the Drifter 17 trimaran for now. First, will be Mark’s comment when he first sailed the trimaran and some thoughts. Second, are the final jpegs and the full plan PDF (4.4 meg).

I launched and sailed on Gypsy Wind for the first time! The boat seems to be sailing very well, I spent the night anchored in the river, and it was quite comfortable in the cabin. The boat has been in strong gusts several times.

I tried to keep as much buoyancy at the back as possible, with the aft cockpit. With no one on board, the stern floats high on the water, which is what I wanted. But he seems to be right in his lines with me on board.

I had some great sailing moments yesterday and really learned a lot more about the boat. I went out to sea for the first time, with a steady wind, and I'm really happy with the way it behaves. The ocean was pretty smooth, with a few small waves, and it was very comfortable. The boat feels very dry: the overhanging deck at the front really helps. It tacks effortlessly, even with the mainsail alone, which is nice, because it's an easy way to reduce the sail in windy conditions, just by rolling the jib.

Trampolines are perfect. I used polyester net from "Challenge Sailcloth", with a 5/16" (8mm) boltrope. I used aluminum awning rail on the cabin and floats, and the ends are laced on the arms. It takes about 10 minutes to set them up, and maybe 5 minutes to disassemble. With the boat's folding mode, you can't leave the nets, and I don't really want to leave them mounted while towing.

I use an electric trolling motor, which I really like: much more manoeuvrable than a small speedboat, and incredibly quiet. A 45 to 50 lbs thrust motor, with a 36" (92cm) shaft is perfect. I'm going to install a pair of solar panels to keep the battery charged.

The mast works perfectly and has remained completely straight in all conditions. The rotation control arm works perfectly, once adjusted, I never touched it. The mast simply follows the sail when you tack or gybe, and downwind it rotates almost 90 degrees.

I think the timber construction is very suitable for a mast of this size, without using carbon. An advantage of a wooden pole is the larger size for the same weight, which really adds to the rigidity of the pole. The wide base for rigging really reduces loads and compression on the mast. I used Vectran 3/16 (4.8 mm) for the main props, which fits very well, and is easy to splice. The horn is made of 1/8" (3mm) 1×19 stainless steel cable. It is very important that the guy wires of the horn do not stretch.