__Displacement & Hull Design__

The next key dimension is the displacement which, of course, equals the total weight. While it is clearly desirable to maximise waterline length to minimise wave drag, deciding the optimum weight is more difficult. As the hull moves through the water, larger displacement means more water has to be pushed out of the way, bigger bow and stern waves are produced and there is more wave drag. In addition the larger the volume of the hull the faster the water has to move to get round it as it passes by, so the more skin friction there is over a larger area. So, all the main drag components get bigger as the displacement increases. The Delft series of tank tests on a range of hull models, described in both yacht design books (see design tools), show that wave drag is mainly directly proportional to displacement, with other hull shape parameters having a relatively minor influence.

However it does not follow that reducing weight and displacement to an absolute minimum will be an optimum. The boat on the beat has to balance heeling moment from the sails with righting moment from the lead and the shape of the hull. A light, and probably narrow, hull and a small lead will give only a small righting moment and hence a large heel angle. The drag may be less with a light boat but so will be the ability to carry sail. The sail drive varies as the cosine of the heel angle, roughly, so excess heel will slow the boat. Somewhere in there is an optimum weight which is linked to the weight of some relatively fixed items, the electrical gear, servos, battery, receiver, etc., to be used and the minimum weight hull, fin, sail rig etc., that can be constructed before they become too flimsy to use.

As with any yacht of any size the weights of all the parts have to be estimated, a desirable lead weight chosen and the total weight decided using previous design experience, a weighing machine and a few calculations. The previous Iambus design had a total weight of about 500gm but used heavier servos and battery initially. Lighter gear became available and was used as time went on and the lead was increased to 275 gm. Other designs like the SuperBug weighed 475gm with a 275gm lead. There were some lighter designs like the Ranger which was nominally 300gm total. However the conventional balsa construction left too little weight over for the lead and my version of it had excessive heel and very poor performance. The only Ranger I have seen go well to date is the Scott Wallis version that used a very light hull made of an expanded plastic sheet material, Depron. However, this is quite fragile and prone to leaking. Also it could only be used for single curvature hard chine designs although a round bilge version the Half pint2 is available the original design is not suitable for the round bilge design to be used on Iambus2. Something between 475 and 300gm seemed probably the best way to go.

With lighter electrical gear becoming available it was decided the Iambus2 would have a 400 gm total weight target, with the lead weight being whatever was left over when the rest was built and its weight subtracted from 400gm. Reducing weight is probably a good direction to go, but not too far, to reduce drag but not reduce too much the ability to carry sail. Later VPP sums indicated that even with a smaller lead than the heavier SuperBug types this weight should give higher speed on both the beat and the run in all wind strengths. The fin, rudder and lead, from previous designs and from estimates were expected to displace about 25gm so the required hull displacement was 400 – 25 = 375gm.

However it does not follow that reducing weight and displacement to an absolute minimum will be an optimum. The boat on the beat has to balance heeling moment from the sails with righting moment from the lead and the shape of the hull. A light, and probably narrow, hull and a small lead will give only a small righting moment and hence a large heel angle. The drag may be less with a light boat but so will be the ability to carry sail. The sail drive varies as the cosine of the heel angle, roughly, so excess heel will slow the boat. Somewhere in there is an optimum weight which is linked to the weight of some relatively fixed items, the electrical gear, servos, battery, receiver, etc., to be used and the minimum weight hull, fin, sail rig etc., that can be constructed before they become too flimsy to use.

As with any yacht of any size the weights of all the parts have to be estimated, a desirable lead weight chosen and the total weight decided using previous design experience, a weighing machine and a few calculations. The previous Iambus design had a total weight of about 500gm but used heavier servos and battery initially. Lighter gear became available and was used as time went on and the lead was increased to 275 gm. Other designs like the SuperBug weighed 475gm with a 275gm lead. There were some lighter designs like the Ranger which was nominally 300gm total. However the conventional balsa construction left too little weight over for the lead and my version of it had excessive heel and very poor performance. The only Ranger I have seen go well to date is the Scott Wallis version that used a very light hull made of an expanded plastic sheet material, Depron. However, this is quite fragile and prone to leaking. Also it could only be used for single curvature hard chine designs although a round bilge version the Half pint2 is available the original design is not suitable for the round bilge design to be used on Iambus2. Something between 475 and 300gm seemed probably the best way to go.

With lighter electrical gear becoming available it was decided the Iambus2 would have a 400 gm total weight target, with the lead weight being whatever was left over when the rest was built and its weight subtracted from 400gm. Reducing weight is probably a good direction to go, but not too far, to reduce drag but not reduce too much the ability to carry sail. Later VPP sums indicated that even with a smaller lead than the heavier SuperBug types this weight should give higher speed on both the beat and the run in all wind strengths. The fin, rudder and lead, from previous designs and from estimates were expected to displace about 25gm so the required hull displacement was 400 – 25 = 375gm.

displacememt__hull_design.pdf | |

File Size: | 47 kb |

File Type: |