To give you the particulars here are some Numbers for my design/build. It’s a Single Diagonal Boat with an absolute max depth draft.
LOA = 329mm
LWL = 329mm
BEAM = 90mm
DSPM = 335 g
Ballast = 220 g
Ballast Ratio = 65.6%
As built VCG from the top of the box ~ 190mm (with the “A” rig)
I’ve met my design intent and my build goals concerning the weights. BUT! I’m wondering if I’ve gone too far. Here is my rationale/thesis. Light things, when subjected to the same force as heavier things, accelerate faster. I’m not quibbling about boats and their weights, but rather the individual pieces of the boat. For this discussion lets break the boat into three parts. The Bulb/keel, The hull/rudder (including all internals and deck) and the rig/sail(s).
Bulb/Keel weigh together about 230g
Rig is about 25g
Hull/rudder = 75g
So here is where I am going with this. Going down-wind, I’m suspecting the rig, being the lightest object is attempting to accelerate the fastest. The hull is a slower second, and the elephant is the keel bulb, taking slightly longer still to get up to speed. I’ve observed my boat being hit by the puff and trimming nose down at first and then trimming more level as the rest of the boat gets up to speed with the puff. In the heaviest of breezes we all know that the boats generally cannot get up to the wind speed and the resultant trim moment causes severe bow down attitude (nose dive). The other corollary to this observation is that the boat can carry much more sail area upwind than down. However, on a race course boats go both up wind and down wind and thus the sail selection is decided upon the ability to control the boat down wind.
This makes me wonder. Why are we giving these boats such deep keels and high righting moments? The extra bit of draft is not needed for lift, and any further we place the bulb away from the hull the lower the center of drag becomes which will also cause nose down attitude down wind. I know there are 3D boats with less draft, but has anyone really explored the concept of reducing the draft or the weight of the ballast, and then still been competitive?
The short answer, Nathan, is Yes, and No.
I tried shorter fins on 2 boats…didn’t seem to help. Of course, that’s hardly definitive. It would be neat if someone who has a removable keel Footy would do some comparison sailing with long and short fins to give us better data.
Your boat sounds good…hope to see it in Wolfeboro.
I believe you may be attributing the nose down effect to inertia when it probably should be attributed to the resistance of the water (drag).
A lot of wind from astern drives the bow lower in the water (and lifts the stern) because of the effort required to push the boat through the water. The taller the rig (COE further above waterline), the longer the lever pushing the front under.
Or another approach - while not practical to do, if you were to fit a keel fin twice the length of your existing fin, you would only require half the bulb weight for the same cross wind performance. While the longer fin would increase drag, the hull would sit higher in the water due to lower all-up weight, more than compensating for the increased fin drag. (In reality the greatly reduced weight would make such a boat really hard to tack)
Hope this helps…
Since you know I’ve registered three of my designs and I’ve already built 2, with the third planned to be completed by the weekend, I might have to explore this design progression. The boats do have removable keels, and for how I’m making the keels this experiment isn’t too difficult to develop. My plan would be to make 3 keels initially.
- The design Draft & Weight
- 25.4 mm shorter ( basically just cutting the last 1" off the bottom of the design keel) using same ballast weight
- Design Draft with 105g bulb.
as I said before I have “trial horses” ready to evaluate the changes against.
With respect to measurement of the shorter keel. If I read the rules correctly I would have to adjust the height of my “boom” so that it still swings freely above the box right? or is there compensation for boats that have reduced draft?
I am planning on Wolfeboro, just trying to firm up how many boats/skippers I’m coming with
I encourage your testing, Nathan, and look forward to your results. I also envy your being able to do some sailing this time of year. My friend Mark and I are developing a couple of simple free-sailing Footys for kids, and we tested at a dock that has bubblers last week. Had to tie a fishing line to the boat so it could be retrieved if it got stuck on the ice 20 feet out, and I almost fell into the lake when I slipped on the snow-covered dock!
From my experience, a one inch change won’t be enough to really notice (maybe try 2 inches less) and I agree with MrP that the very light boat will have tacking issues. You don’t have to change your rig on a short-fin boat…you are allowed to put stuff in the bottom of the measurement box to bring the boat up to the desired level in the box.
Bring friends on May 4…dress them warmly…see you then.
p.s. That’s a really nice-looking boat and rig. Please post more pics.
Thanks for the comments Bill. That’s rig version 2. Rig 3 is lighter, neater and more precise. I’m posting a link to an album I’ve created that I’ll keep uploading pictures to as I take them. Someday soon I’ll get my good camera out and take some higher quality photos and add them to the album.
The boat in the already posted picture is the first one I built and has had quite a few modifications, it’s carrying the second boats rig/ larger(est) sail, while the real 435 is using rig 3.0 with a B(ish) sail (no numbers).
My keel fin in the picture is built using flat High Impact Polystyrene that has been scored along the leading edge and then folded around a solid 3mm carbon rod. The plastic is then glued to the rod and itself creating the “foil” shape. The whole thing gets covered in a layer of clear packing tape. This is faster to produce than carving cedar, and this shape has better characteristics than the cedar fin of the first boat. With only a single internal spar and the CG of the bulb slightly behind the spar axis, the fin twists a little when the boat heels providing a slight increase in angle of attack.
In this video you can see the two boats sailing. I’m not driving. The trailing boat with numbers on the sail has the ultra high aspect ratio fin (cedar), that and a new skipper I think account for it’s twitchy behavior. My apologies for the blurry video.
MR. P. Thank you for your comments as well. Although I’m suspecting your alternative approach with the 2x draft at half the bulb weight would most likely be worse for the bow down trim in puffs. The projected frontal area of the keel fin would increase by twofold and the centroid of that area would be further down the fin creating a greater moment. So if it really is hydrodynamic friction causing the trim issues this could very well be worse. Although a lot with the behaviour of these little boats doesn’t make sense to me, so who knows!