John Perry
Senior Member
- Joined
- Nov 27, 2003
- Posts
- 308
- Reaction score
- 54
- Location
- South West UK
Doug, I agree that the increase in take off speed attributable to the lead ballast is not huge, but obviously it will make some difference. From your figures the lead is 30% of total craft weight, so thats roughly 15% higher take off speed if foil sizes, angles of attack etc. are all the same, which they dont have to be of course. To achieve that extra speed for take off, again with same foils etc., you would need to overcome about 30% more drag. It may mean optimising your craft more for the take off speed range at the expense of top speed. One of the problems of designing sailing hydrofoils is that to get the thing to take off it helps to have big foils and big sails, but once you have managed to get foil borne you probably want to go very fast which requires tiny little foils and fairly small sails. Surface piercing hydrofoils help, but have their own problems of course. Once you are going fast on the foils, the lead ballast may not make a lot of odds, indeed it is conceivable that it might even increase your top speed by increasing the maximum available righting moment. Hydroptere takes on ballast for that reason, but I think they have their ballast more favourably located than you are proposing, at least from the speed point of view.
I am not saying that your design is not going to work, I would never say that after the experiences I had with my own bifoilers back in the pre Moth foiling days. Whenever people who knew about sailing saw my boats I would get a pittying look and be told, purely in my own interests of course, that I needed to go away and rebuild the craft with three or four hydrofoils spread as widely apart as possible, in order not to be simply blown over sideways. I still have bits of the second prototype lying around and the irony is that if I now put it together and sailed it somewhere like Chichester harbour (which now has Moth bifoilers wizzing all over it) people would come up and advise me to stop messing about with a crude home made boat and just buy myself an off the peg Moth hydrofoil!
Anyway, as I say, am not saying your boat wont work, just trying to get a better idea of what it is you are proposing. It sounds like it will be basically a bifoiler with a wand system like the Moth but you will add wide cross beams and small amas and there will also be a hydrofoil (fully immersed type?) under each ama. I also went for small amas for my second prototype, these were only for a bit of low speed stability and they did help. The moth sailors have now shown that amas are only for wimps! So, you will have a hydrofoil under each ama and since you say the cross beams have a large dihedral I assume that only the leeward one of these ama hydrofoils will be immersed. Does that mean that the craft will sail heeling to leeward? A nice thing about the bifoiler configuration is that heeling to windward rather than to leeward helps to balance the forces on the craft without the struts of the foils taking a huge amount of side load. This means the foils dont ventilate, the struts dont bend and the lift is mostly coming from well immersed surfaces. From the moment that I made a first hop off the surface I realised that heeling slightly to windward just feels right for this kind of craft. Perhaps if you incline the ama foils (i.e. give them lots of dihedral) you could again balance the craft with minimal hydrodynamic forces on the foil struts, maybe pivoting trailing struts. But then, what are your central foils doing - yes, I know, they are giving you pitch control. Hmm, I wonder, if you take off the main centre hull foil, could you get the pitch control from just the ama foils and a stern foil? If you used surface piercing ama foils that would give you the Hydroptere configuration - it does work, but perhaps you want to experiment with all fully immersed foils. So, fully immersed foils with wands on each ama plus a stern foil. Just thoughts, you can go round and round in circles with these ideas, sometime you probably want to stop and build something. I can see that other things being equal, your wide cross arms and ama hydrofoils will potentially give you several times as much righting moment as a bifoiler, so you have some potential there - a boat 'to be more accessible to people with physical infirmities' !! Good luck with it anyway.
I am not saying that your design is not going to work, I would never say that after the experiences I had with my own bifoilers back in the pre Moth foiling days. Whenever people who knew about sailing saw my boats I would get a pittying look and be told, purely in my own interests of course, that I needed to go away and rebuild the craft with three or four hydrofoils spread as widely apart as possible, in order not to be simply blown over sideways. I still have bits of the second prototype lying around and the irony is that if I now put it together and sailed it somewhere like Chichester harbour (which now has Moth bifoilers wizzing all over it) people would come up and advise me to stop messing about with a crude home made boat and just buy myself an off the peg Moth hydrofoil!
Anyway, as I say, am not saying your boat wont work, just trying to get a better idea of what it is you are proposing. It sounds like it will be basically a bifoiler with a wand system like the Moth but you will add wide cross beams and small amas and there will also be a hydrofoil (fully immersed type?) under each ama. I also went for small amas for my second prototype, these were only for a bit of low speed stability and they did help. The moth sailors have now shown that amas are only for wimps! So, you will have a hydrofoil under each ama and since you say the cross beams have a large dihedral I assume that only the leeward one of these ama hydrofoils will be immersed. Does that mean that the craft will sail heeling to leeward? A nice thing about the bifoiler configuration is that heeling to windward rather than to leeward helps to balance the forces on the craft without the struts of the foils taking a huge amount of side load. This means the foils dont ventilate, the struts dont bend and the lift is mostly coming from well immersed surfaces. From the moment that I made a first hop off the surface I realised that heeling slightly to windward just feels right for this kind of craft. Perhaps if you incline the ama foils (i.e. give them lots of dihedral) you could again balance the craft with minimal hydrodynamic forces on the foil struts, maybe pivoting trailing struts. But then, what are your central foils doing - yes, I know, they are giving you pitch control. Hmm, I wonder, if you take off the main centre hull foil, could you get the pitch control from just the ama foils and a stern foil? If you used surface piercing ama foils that would give you the Hydroptere configuration - it does work, but perhaps you want to experiment with all fully immersed foils. So, fully immersed foils with wands on each ama plus a stern foil. Just thoughts, you can go round and round in circles with these ideas, sometime you probably want to stop and build something. I can see that other things being equal, your wide cross arms and ama hydrofoils will potentially give you several times as much righting moment as a bifoiler, so you have some potential there - a boat 'to be more accessible to people with physical infirmities' !! Good luck with it anyway.
