paulncc
New Member
is it ok to convert a fin keel to bilge? are there any stabily problems? any advice welcome
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fin to bilge keel
- Thread starter paulncc
- Start date
Avocet
Well-Known Member
You're nothing if not ambitious!
I think it's not a task to be undertaken lightly. The biggest problem I can forsee is the strength of the hull. I guess bilge keelers are very strong either side of the centreline whilst fin keelers are strong on the centreline. Fin keelers won't be designed to take the ground as often as bilge keelers and on occasions, the shock loadings fed into the hull can be very large as the boat settles on the bottom when the tide recedes. For the hour or so that the waves are just enough to lift the boat and bang it down on the sea bed they can really take a pounding. Also, drying out on soft mud can put very big flexing loads into the hull if the keels are angled outwards because the boat tries to trap a "wedge" of mud between its keels as it settles. You don't say what sort of boat it is. If it's one that was commonly available with bilge or fin keel configurations it might be an idea to have a look at one of each and see how different the structure is in the relevant areas. Also look at the tickness of the laminate if possible.
Finally, there will be all the usual hydrodynamic areas to consider - getting the centre of lateral resistance in the right place and getting a reasonable righting moment. I'm not a naval architect but I would have thought it was quite a big job!
Good luck!
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I think it's not a task to be undertaken lightly. The biggest problem I can forsee is the strength of the hull. I guess bilge keelers are very strong either side of the centreline whilst fin keelers are strong on the centreline. Fin keelers won't be designed to take the ground as often as bilge keelers and on occasions, the shock loadings fed into the hull can be very large as the boat settles on the bottom when the tide recedes. For the hour or so that the waves are just enough to lift the boat and bang it down on the sea bed they can really take a pounding. Also, drying out on soft mud can put very big flexing loads into the hull if the keels are angled outwards because the boat tries to trap a "wedge" of mud between its keels as it settles. You don't say what sort of boat it is. If it's one that was commonly available with bilge or fin keel configurations it might be an idea to have a look at one of each and see how different the structure is in the relevant areas. Also look at the tickness of the laminate if possible.
Finally, there will be all the usual hydrodynamic areas to consider - getting the centre of lateral resistance in the right place and getting a reasonable righting moment. I'm not a naval architect but I would have thought it was quite a big job!
Good luck!
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jimi
Well-Known Member
Why?
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snowleopard
Well-Known Member
firstly- are you able to provide the structural strength in the right place for the new keels - hate to hear of them breaking off!
if you keep to the same total weight so your waterline stays in the same place, and assuming the twin keels are shallower than the fin, the centre of gravity will be higher so you will have lost stability.
one way to get round this is to have the keels made so that most of their weight is at the bottom, if you look at pictures of hunters you'll see what i mean.
this is dangerous territory, i'd suggest talking to a designer before taking the risk.
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if you keep to the same total weight so your waterline stays in the same place, and assuming the twin keels are shallower than the fin, the centre of gravity will be higher so you will have lost stability.
one way to get round this is to have the keels made so that most of their weight is at the bottom, if you look at pictures of hunters you'll see what i mean.
this is dangerous territory, i'd suggest talking to a designer before taking the risk.
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Ivy
Well-Known Member
I've recently built two boats, one fin and one builge, the builge keeler had a lot of extra stiffening around the whole of the keel area and much higher up the topsides than the fin keel. if you can get at the entire centre section of the hull ( assuming it is GRP) and add a lot of extra matt, then try it, but you will probably find it a more financialy viable prospect to sell your boat and buy a builge keeler.
there may be a small difference in weight and stability, but if you have a high ballast ratio, it shouldn't make much difference. You can allways add more ballast to the builges later. You will notice a lack off pointing ability when beeting to windward.
Ask yourself the following questions:-
Are builge keels available for your boat?
Have you got the expertise?
Have you got the time and patience?
Is it financialy viable?
Or, should I just buy a boat that fits the bill?
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there may be a small difference in weight and stability, but if you have a high ballast ratio, it shouldn't make much difference. You can allways add more ballast to the builges later. You will notice a lack off pointing ability when beeting to windward.
Ask yourself the following questions:-
Are builge keels available for your boat?
Have you got the expertise?
Have you got the time and patience?
Is it financialy viable?
Or, should I just buy a boat that fits the bill?
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oldsaltoz
Well-Known Member
I would have to agree with Ivy, sell the fin and buy another boat, you will never recover the cost on resale, in fact your existing boat will probably loose market value if converted.
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
paulncc
New Member
thanks to all that have replied you have given me plenty to think about. the boat is a flare 25 there were'nt many built but the designer was john westell whom i belive to be well known and has a good reputation. the hull appears to be 19mm thick at the bottom and 15mm or so at the sides.
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gjeffery
Well-Known Member
I dont know the boat. There are obvious issues of existing hull geometry and structure; not to mention making good the hull after you remove the keel. I would not consider such a major adaptation, without taking the advice of the designer, who will be aware of the design optimisations that were originally made. The opinion of your insurer might be determining!
Any redistribution of ballast will affect the stress throughout the hull. This is complicated because it is not a static condition. You should also model the dynamic stability of the revised design.
I would be very surprised if such a fundamental change could be justified financially, or even in practical benefit.
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Any redistribution of ballast will affect the stress throughout the hull. This is complicated because it is not a static condition. You should also model the dynamic stability of the revised design.
I would be very surprised if such a fundamental change could be justified financially, or even in practical benefit.
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SimonD
Well-Known Member
As an earlier poster said "why"? If it's so that you can take to the ground, get some beaching legs. Got to be cheaper. If it's to improve the sailing ability......!
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