Dick a couple of considerations : 1st, the bulb weight in my designs is always above 60% of the total weight, very happy when I reach 70-72%. For the nARROW the ratio is >62% 2nd the righting moment with 300mm fin is 0.30 x 0.675 = 0.2025 kg/mt 3rd the righting moment with 275mm fin is 0.675 x 0.275 = 0.1856 kg/mt 4th the righting moment with 300mm and 650g = 0.195 kg/mt As you can see, a slightly longer fin with a lighter bulb you get a better righting moment of the 275mm fin, not only but the boat will be lighter and the LWL shorter compensating partially the vet area. Are all small considerations, but a bit here and a bit there are always helping to be ahead of 1cm !!. heheh!!
Hi Claudio: Always a pleasure to read you down-to-earth and well analyzed thought. Thanks for sharing! I'm considering a shorter fin for light air myself now... Thomas
Results are in ......... now what to do? Stopped by my local UPS Store and asked the young lady if she would weigh the bulb from my DF65 - so unless they have their scales set up to measure heavy I was quite surprised with the DF65 Bulb that came with the boat. As noted above it hangs from a keel that is about 300 +/- mm long and the bulb itself came in at 535.24 grams (1.18 lbs). So Claudio, when time permits, can you calculate and let me know if you think I should go with the shorter keel of about 240mm and this bulb.? As I recall, your original Esterel narrow deck RG65 called for a bulb weight of 675 gr. and the drawings for the nARROW specified 609 gr. Will those 74 grams make that much of a difference, or should I just not worry about it and go with the long (standard) keel from the DF65 ? I can always postpone decision until I get hull off the building board, add the deck and throw some of the radio gear into the hull to get a full weight.
First build the whole boat, including rig and electronics... weight it and compare with the designed displacement. The difference is how much you can put in the bulb! Now you have that part solved. Now, time to decide how long you want the fin: it's a compromise between righting moment and the drag created by the fin. Look at this fin design created for maximum righting moment and less drag (for an IOM in this case):
Hi Dick I resume the nArrow design models: nArrow 1 = narrow beam, rounded shadows, PC 0.54, flush transom, 300 mm draft including bulb, DSPL 1009g, bulb 609, ratio 60% from nArrow 1 to nArrow 3 increased Prismatic coefficient and deeper transom nArrow 3 = narrow beam, rounded shadows, PC 0.57, deeper transom, 300 mm draft including bulb, DSPL 1083g , bulb 675g, ratio 62% nArrow 5 = same as nArrow 3 , but with narrower deck Why this change from 1 to 3 and 5 ? A deeper transom produce an artificial elongation of the Water line and thus adding a potential higher speed capacity ! As you can see the major change is the PC and transom under water. Because of that, the DSPL is different and the bulb weight also. Generally my designs have a bulb that has ratio of minimum 60%. Now, if you wish to use a shorter Fin and a smaller Bulb, you will loose lateral stability. This loss can be partially recovered with a shorter mast that induce a lower CE position. Hi Tom I'm surprised by the fin shape of the picture. Hydro-dynamically is not the best choice, would be better to have a narrow top and wider bottom if mechanically acceptable to conserve also the aspect ratio for better efficiency. Why ? When the boat is heeling the top of the fin may protrude above the water level and generate disturbing drags at the Water-Air interface causing loss of speed ! A trick is the one used on Class M where the widest part of the fin is positioned some 10-12cm below the water level. This solution allow to get the Fin area required without going too deep while keeping an anti-drift effect. BTW, as "rule of thumb" the FIN area+ Rudder area should be in the range 5.5% to 6.5% of Sail Area where the rudder is 1/3 of Fin area. For faster boat it is suggested to stay on the lower % range. Catamarans, being very fast, they can go below 4% of SA for Fin + Rudder area Cheers
Holy Smokes ..... is this thing NARROW !!!!!! I gave in to my temptation to lift the hull (and stations) from the building board and turned it over. Yikes !! Not sure if can get my hands inside to do any work, add the winch and servos, etc. The DF65 is about 111 mm and this rocket is less than 85 mm at the deck. Starting to get excited. Back on the building board so I can sand the balsa then time to paint in some epoxy inside and get my graphics ready for the exterior of the hull. Also need to fabricate the deck. Wish we weren't so close to the end of summer.
No, No - if I am going to be on the leading edge, it has to be the "bleeding edge". Fabricated and roughed out the foam bow piece and slid into place just for photos. Now I have to remove and shape with more care and keep fitting. With the bow on, I really like the look of the hull. Still want to leave templates until I get exterior sanded. Waterline cedar strip fit very well - good planning or just dumb luck ?
That looks fenomenal! Can you take some pictures from above, stern, bow, and birds-eye view (diagonal)? Just for our enjoyment... :-D
Thanks Thomas - Will try to get them and post tomorrow. Right now, the foam plug is driving me crazy, so I want to go back and pull it out to trim it, and give it a shape a bit closer to final. Looking down on it, it seems to bulge out a bit - almost like if you hit your thumb with a hammer! Also need to bore some large hols through it to remove some of the foam. Re-cut and re-set the waterline strip and extend the upper hull plank (gunwale) forward. Tonight I also want to see if my drum servo will fit. Claudio and I talked about it and my first plan is to mount it with the drum on it's side and see if I extend the sides of the drum with some styrene sheet stock and maybe eliminate the tensioning rubber band on the main sheet. Not much weight, but can be a pain in the butt if it breaks and you have to fun a new one. Maybe will consider jut a short rubber band with a large plastic bead for the mainsheet to run through. I do have some blocks coming that could also be mounted for tension but little if no friction. I want to build a set of sails using Claudio's Gadget but I think I will purchase a set of sails from Radio Sailing Shop in Australia. I've always like the stuff that Andrew Cook puts out under the "Go Spectre.com" logo. So will get a set from him, and then play around with a homemade set. I really wish I could find a jam or cam cleat small enough to use on the boat. I really hate trying to tie stuff. Would be so nice to have a row of the cleats and just run lines to them and snub off. Maybe I could find someone with a laser CNC and have him make me some. Just think - no bowsies to fool around with when you have cold, fat fingers. Dick
Dick, in my opinion the servo-arm installation should be much simpler. Here below the static power calculations that suggest a minimum servo power of 3.6 kgcm for 15knots of wind The proposed servo of GWS offer 7.2 kgcm for 46g. Drawing explained : The main boom is acting as a lever anchored on the pivot point. The attached weight is representing the wind force exerted on the Center of Effort - CE . This force is applied at 33mm from the pivot point The sheet anchoring point is where the pull force of the servo is applied. This force is applied at 135mm from the pivot point. Considering the ratio 33/135 and the security factor of 2 is calculated the servo power required. see drawing. The sheet length is also calculated. PS: Drawing the Servo-Arm setting with the Hitec HS645MG very similar in performance to the GWS, but more compact. The only element that do not move longitudinally is the Fin-Box that is linked to the LCB position. The servos set, Mast and Fair-lead are positioned longitudinally in accordance with the position of the CE that depends from the Sail plan.
Was running with the grandkids most of today - so didn't get much accomplished. On the positive side, I looked in my "electronics" drawer and found that I still have one Hitec HS 645 MG arm winch left. I used this same winch on the series of JIF65 that I had previously built and which held up without problems. Still need to see if I can get it to swing horizontal, or if I need to go with a vertical swing. I have also included a bunch of photos from various angles, and you will see, I need to do a lot of work with the foam bow piece, which after a rough cut, didn't fair in very well. Left Top: Not a heck of a lot of interior room. No place to store my beer keg after I get winch and batteries in. Left #2 Top: Side view and you can see the ugly bow piece I need to shape. Middle #3 Top" an angled view of the hull Top #4 Right: Inside with templates still in place Top #5 Far Right: View from the stern Bottom Row Left: Interior view approximately middle of the hull fore/aft Bottom Row, Right: A closer look at the ugly foam bow piece. You can see where I cut a groove for the waterline cedar strip as well as some marked trim lines for shaping. Still seems like a lot of line to move with a winch arm though.
Dick, not easy to put the fingers inside !! I checked the horrizontal arm, but it will be almost impossible to fit the servo in that position. The top and side views do not show the real context until you watch the cross section where you can see how much the servo is squeezed in. Further the servo mass is put to one side while for the balance purpose the battery set shall be positioned on the opposite side.
Claudio - I came to the conclusion, that dropping the winch in with arm on top wouldn't provide me with an arm long enough to move that much line you recommend. In the photo the winch is setting on the bottom of the hull. When templates are removed, it "might' have enough space for a longer arm, but turning it on it's side and having the arm go from horizontal to vertical will give a lot more room. I still want to see if the GWS with multiple turns will provide the needed length of mainsheet movement. I am assuming you calculate the arm length as being about 6 cm and it turns around 60 degrees which is common for most winches. I could also buy a new winch and have the supplier change the arm to 90 degrees or 180 degrees as alternative. Anyway it will still be a rather tight fit. I also found a bit of time tonight to clean up and modify the foam bow blank. It now doesn't look like a tumor any longer, although can still use a bit of touch-up with file and sandpaper. I think I can also bring the bow bottom up a slight amount as it still resembles a chin, which I don't care for at all.
Thanks for the photos Dick!! @Claudio: I like your solution as the board where the servo is mounted adds to the structure of the whole boat. The rudder servo could be secured at an extension of the same board, aft the sheeting post. But I would put the hoist point for the sheet much farther away of the servo. Maybe at the transom or the rudder post. That way there are no forces at the bottom of the sheeting post. Just my 2c. In any case, I will use the GWS winch (that is, when I have time to resume my own nArrow build). In your original plans the winch is sitting just behind the find. What is your take on sheeting routing? Thomas
Hi Thomas, here below a shematic assembly for a winch. The servo used is the Eurgle. Others servos shall be controlled dimensionally. First, from previous drawing is indicated that the Main Sheet travel is around 166mm. For that purpose I consider a Travel of 175mm, this will give room for installation tollerances. Important is that the travel sheet is mowing tangetially to the Fair-lead; this option allow the same pulling angle with the fair-lead for the full sheet lenght. For that purpose the pulley A is mounted on one corner at the transom, the Winch sligtly shifted as well the pulley B from center line in order to keep the strings equally distants from the Mast. Rudder servo not drawn to avoid complicated drawing, nevertheless also the rudder servo as well the rudder stock and arm can be installed inside. Since the Winch is positioned partailly behind the Fin Box and sligtly shifted laterally, it will be necessary to check the longitudinal and trasversal balance before closing the deck by positioning the rudder servo, the winch, the battery and representative rig weight and observe the balance in the water (bathroom). Servo winch and Rig position are supposed to stay as drawn therefore any balance shall be found first with the rudder servo and battery position. In case of need the bulb position is the most effective choice.
Had some time today, so took the foam bow block and did some sanding and fitting. Wanted it to be slightly under size, so I could apply more balsa strips to cover the block. I got the block located and glued in place and was able to get some leftover balsa strips glued in place. Will take more time to do the bottom of bow block - from waterline, under bow and up to waterline on other side. These will probably be very narrows strips, that ill have to be individually cut. In photos below .... Left photo - Holy Pink Swiss Cheese ! Nope just pink insulation foam with a couple of holes to remove weight and fitted to the front hull template Middle Photo - Start of adding bow strips of balsa Right Photo - Almost completed Starboard side of bow block. In hind sight, I suppose I could have shaped and added the bow block prior to starting to add strips, so will have to live with the decision made. If you build one of these narrow hulls, add the bow block on the building board and just strip the entire hull with one piece strips.
Dick: I think your bow should be just a solid balsa block. Easier to sand, no planking needed, and stronger result (think collisions!) Just my 2c.
Had some big pieces of 2 inch thick foam left over (or waiting to be used) for more multihulls. Didn't have to drive to get some balsa blocks.