Sailing Dinghy Design

[Ian]

Hi, it does seem we are working on somewhat simular boats. I have the same length and beam but a smaller sail area (13 sq.m.) My research would tend to confirm your sail area choice as being at the very top of the cruising dinghy range which makes sense if your are to pursue those racing dinghys. The bottom of that range for this size boat was about 10 sq.m.

I chose to go with a yawl rig. Leg of mutton mizzen, standing-lug main and a jib on a bowsprit. She will ballance with just mizzen and jib. My vertical center of effort is so low that I have wondered if I should go for a larger sail area but decided against it.

Have you considered having two mast steps so that you can use just the main for cruising, or could you perhaps locate the alterative mast step so that she would ballance with smaller jib? Possibly she could convert from a masthead rig to 3/4 rig for cruising? I'm just throwing out ideas--I don't know your situation.

I am going to locate my self-draining sole quite low, so rather than have a open transom and take water on board from following waves, I am going to have a well located just forward of the aft deck bulkhead with one large dinghy baler in it (or possibly a well on each side, each with a baler). This should suck the water out quite quickly when under way and, if left open, will drain out rain water when she is on the mooring.

I am going to have a swing centerboard (or centerboards) with a aspect ration of 2.0 and a eliptical planform. I like the idea of two boards located side by side on either side of the cockpit. This would get the case out of the way, reduce the draft by 30cm and raise the center of lateral restance about 15cm which should reduce the heeling moment somewhat. The only difficulty I can see with the idea is that the root of the windward board may ventilate when the boat is heeled right over. In a static condition as simulated on my computer this will occur at about 20 degrees of heel. I have no idea how this will translate in wave making conditions. Also, I have no idea what angle of heel she will normaly sail at. I know dinghys sail flatter than keel boats--racing dinghys particularly--but can't come at a number. It sounds like you have done enough dinghy sailing to advise me on this and on the righting considerations that I mentioned in my last post.

Construction is basicaly plywood lapstrake, except that the bottom is almost racing dinghy flat and the garboard planks are quite wide and will be stitch and taped at that first seam. The bottom will be 3/8ths inch ply glassed both sides and the rest of the planking will be 1/4 inch and only epoxy coated. There will be a shallow keel with a bronze rubbing trip down the center-line and short bilge runners on either side to protect her against concrete launching ramps and rocky beaches (soft sand beaches are not common around here).

Your idea of changing over from a centerboard to a ballast keel for cruising sounds interesting. Would you be using a dagger-board configuration? I would imagine that you could build the case to take a wider lower aspect ratio ballast keel and insert blocking into the case to convert to a higher aspect ration foil. Is that what you have in mind?

My spars will be of local spruce (cheaper than sitka) and hollow. I have used the birds mouth technique before on large spars and have found it to be a quick and easy way to make hollow tapered spars. The rig is intended to be free-standing but I will make provision on the mast for running back-stays on the main in case she needs them. My jib is largeish for a free-standing rig (1.6sq.m.), though the considerable rake of the mast will help to oppose the pull.

I will have a folding dodger fwd (I have about 4m between the masts) which will provide enough shelter for a lunch time nap or for cooking. This will be suplemented with a hoop tent at night. I'd like to set it up so it is still possible to be outside the tent in the very aft end of the boat, so the length of the tent will have some adjustment. Boards between the side benches will provide a wide sleeping area.

A cruising dinghy thread sounds like a great idea. I am new to forums and am a bit vague as how these things are set up. Is spoedvraat afrikaans for 'speed freak'?

[spoedvraat]

Hi Ian,

I see there is a new thread called cruising/racing dinhghy design, which is what we discussed, and I think we can continue the discussion there.

[niclouw]

Does anybody know of a dinghy design that allows for sailing in really shallow water?

[TaSSie_deVil]

Well... those hydrofoiling moths (provided that they are completely up on the foils and stay there, so the boat is about a metre in the air, which is unrealistic) have a draught of less than 20mm.... is that shallow enough? Also, if you still consider ice to be water, the iceyachts cut less than 20 mm into the "water" also...

Most sailing dinghies really depend upon their centreboards and rudders for directional stability, so in reality in order to sail in really shallow water, depends upon how shallow the water is that you are talking about.

[niclouw]

Has the following idea been tried / or might it not work?: Instead of having one deep centreboard in the middle to prevent heeling , why not have three longitudinal running protrudences at the bottom of the hull . One down the middle , two on each side . This should have a combined surface area of the centreboard but at a fraction of the depth.Also to reduce rudder depth, a twin rudder could be used.

[tspeer]

Yeah, that's just the problem - it's a fraction of the depth. The purpose of a centerboard is not to prevent heeling - in fact it promotes heeling. The purpose of a centerboard is to oppose the side force from the sails.

The lift from the keel/board/strakes has to equal the applied load from the sail rig. So the lift is independent of the area. Yes, the lift is independent of the area - think about upwind vs downwind; same area, different lift, because of the sail trim. The leeway angle is inversely proportional to the area, however. But, as long as the boat isn't stalled, the board design is not a factor with regard to the lift.

What is sensitive to the board design is drag. The drag due to lift is inversely proportional to the depth squared, and is independent of the area. Since all the board designs have the same lift, this puts a huge premium on making the board deep.

The drag due to lift is also inversely proportional to the speed squared. So you can get away with a shallow keel if you are fast. Think Hobie 16. Not a good choice if you have a fat hull that can't go fast to cut the drag.

The drag due to wetted area is proportional to the area and proprotional to speed squared. So you don't want to carry around a lot of excess area. When you make the board deep to reduce drag due to lift, the only way to cut the area is to shorten the chord. Hence high aspect ratio.

Substituting strakes for a centerboard is going in the wrong direction.

[dpoliver]

hey all. thats awesome to see a design in progress. i can relate to the autocad frusterations as i am an engineering student learning the program myself. im working on a preject to refit a laser II with an i14 rig. but im missing the asymetrical spinnaker part. no one has one kicking around eh? if so let me know.. allnaturalhigh@hotmail.com

david

ps. any tips or words of advice on the project are welcome too... thanks!

dp, just out of curiosity why are you doing this?
Are you going to add racks to the Laser to bring the beam up? What's the Laser hull weight? A 60% increase in SA is going to require all the help you can get but I'm sure you know that. Are you going to change the board and rudder? Have you considered hydrofoils?
I'm curious about your reasons why and it would be interesting to hear how it works out.
The I14 site( http://www.I14.org ) has a "list" you can subscribe to and post your spinnaker requirement-you might get lucky!

[dpoliver]

i think the rated hull weight is about 160 pounds. yes racks will be in order to imcrease the leveling power. there is an interseting article on i14.org about adding a nice wide set of racks. the addition of a double trapeze will also increase leveling power drastically. as for the foils... i hadnt given those much thought yet. it seems most skiffs use longer narrow rudderblades and daggerboards, as i presume this is faster? hmm, maybe this might be in order...