BobWilliamson
New member
Another question from a nosey old b&%£"r. Would raising the cabin by say two feet make a craft seriously unstable?????
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structural changes
- Thread starter BobWilliamson
- Start date
snowleopard
Active member
provided you kept the hatches shut it would actually improve the AVS (angle of vanishing stability). on the other hand it's almost certain to increase the EUC (extreme ugliness coefficient).
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BobWilliamson
New member
I was actually being serious, but thanks anyway!
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snowleopard
Active member
... so was i
if you look at stability curves in mag articles you'll often see a small jump in stability around 90deg heel which is caused by the cabin top starting to immerse and providing righting buoyancy. this of course only works if you prevent water getting in.
to prove what i'm talking about, imagine you built the cabin top 20ft high, once flat on the water there'd be no way of tipping it futher.
of course there would be a small negative effect on the initial stability by raising the weight of the cabin top but you would definitely increase ultimate stability.
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if you look at stability curves in mag articles you'll often see a small jump in stability around 90deg heel which is caused by the cabin top starting to immerse and providing righting buoyancy. this of course only works if you prevent water getting in.
to prove what i'm talking about, imagine you built the cabin top 20ft high, once flat on the water there'd be no way of tipping it futher.
of course there would be a small negative effect on the initial stability by raising the weight of the cabin top but you would definitely increase ultimate stability.
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BobWilliamson
New member
Seems feasable looking at it from that perspective but I was rather hoping to keep the craft upright at all time and would prefer not to test the theoretical righting effect from a increased cabin size.
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Ivy
New member
On a boat of 150ft LOA it probably won't make much difference but on a Topper it could be disasterous. what kind of boat have you got?
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BobWilliamson
New member
Ah a completely different view from another reader. Its a 16.5' Wave Crest cruiser/sailer. Think maybe sombody is trying to kill me??
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Ivy
New member
I don't know the boat but I woukd imagine it is a bit short for that amount of extra cabin top, you might get away with it if you add some extra ballast either in the builges or om the end of the keel. I would try to get some information from a designer if I was you.
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oldsaltoz
New member
G'day Bob,
You can get a pretty good idea how the finished article will look if you take a couple of pictures, head and side on, then draw or paint the changes on the photo using the boat length as a scale to get the proportions right; take care to good and accurate job, or the results will not reflect the finished article.
If you don't like what you see, you have saved a lot of time and money; on the other hand, if you like the new look, get some advice on how to go about it and what effects it will have on handling and stability.
The above method sorts out most ugly changes at the photo stage, but some make it to the planning stage and get dropped when the cost or effect of changes are detailed.
I hope this helps. . . . . .
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
You can get a pretty good idea how the finished article will look if you take a couple of pictures, head and side on, then draw or paint the changes on the photo using the boat length as a scale to get the proportions right; take care to good and accurate job, or the results will not reflect the finished article.
If you don't like what you see, you have saved a lot of time and money; on the other hand, if you like the new look, get some advice on how to go about it and what effects it will have on handling and stability.
The above method sorts out most ugly changes at the photo stage, but some make it to the planning stage and get dropped when the cost or effect of changes are detailed.
I hope this helps. . . . . .
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
BobWilliamson
New member
Yeah! best be safe than sorry, I was not convinced by the other readers information. I could be considered a complete novice but I'm not a complete idiot.
Ta for the info anyway, I'll publish what happens in case anybody might be interested...........Thanks again.......BW.
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Ta for the info anyway, I'll publish what happens in case anybody might be interested...........Thanks again.......BW.
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BobWilliamson
New member
Thanks Brian. The design is already considered in fair detail my major problem is how much will the stability of the craft be affected by the changes that I make.....BW
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oldsaltoz
New member
G'day Bob,
Just how much the stability will be effected will depend not only the height, but how much the extra material weighs and at what point it's added.
Try to estimate the total weight of the structural change.
If we talking less than say 10% of the hull weight (excluding the keel) then the effect would be minimal, anything over this may require much closer examination.
You could lighten the structure with the use of composite materials such as foam core, and not have to worry about rot or a boxy look; you can make some very nice shapes using foam without loosing strength.
On the other hand, think about ways to add this weight in open buckets to your boat in the location and mid height of the proposed modification, take her for a run, if anything drastic happens the water will flow from the buckets and not tip you.
But it sounds more like you are planning to add 2 feet in hight to the cabin sides and some extra width, say 3 feet at most, I'm not familiar with the boat type so don't know what length is involved, therefore it's difficult for me to estimate the total weight involved.
But I hope this helps. . . . . . .
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
Just how much the stability will be effected will depend not only the height, but how much the extra material weighs and at what point it's added.
Try to estimate the total weight of the structural change.
If we talking less than say 10% of the hull weight (excluding the keel) then the effect would be minimal, anything over this may require much closer examination.
You could lighten the structure with the use of composite materials such as foam core, and not have to worry about rot or a boxy look; you can make some very nice shapes using foam without loosing strength.
On the other hand, think about ways to add this weight in open buckets to your boat in the location and mid height of the proposed modification, take her for a run, if anything drastic happens the water will flow from the buckets and not tip you.
But it sounds more like you are planning to add 2 feet in hight to the cabin sides and some extra width, say 3 feet at most, I'm not familiar with the boat type so don't know what length is involved, therefore it's difficult for me to estimate the total weight involved.
But I hope this helps. . . . . . .
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
BobWilliamson
New member
There are some possible good ideas to try out Brian. Dont know about the foam though, my spare time is rather limited and as I am a DIY enthusiast, the foam bit might cause me some problems, with regard to time I mean. But whatever, you have injected some good ideas in what you have said, Thanks.....BW
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oldsaltoz
New member
G'day Bob,
Have you considered pre forming foam panels, to be final cut and fitted, they never need painting, are very light and very strong, easy to work with and you can conceal cables, fixing blocks etc.
All you need is sheet of class, wax it, apply gelcoat, cut your foam to a rough shape and include any cables or fixing blocks or frames, glass it, apply a coat of resin to the gelcoat, lay the foam on and add a sheet of ply with some weight on it, next day you have a panel ready to trim and fit with a glass smooth finish; make all panels required before you cut the boat.
I have used this system many times with great success for items such as Hard tops with cables for lights and speakers, engine box covers, anchor winch covers, hatch covers etc.
Tip: After you apply the release wax, put in the sun for a while or run a heat gun over it, then let cool completely before you apply the gelcoat; this will produce a very high gloss finish.
Hope this helps. . . . . .
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
Have you considered pre forming foam panels, to be final cut and fitted, they never need painting, are very light and very strong, easy to work with and you can conceal cables, fixing blocks etc.
All you need is sheet of class, wax it, apply gelcoat, cut your foam to a rough shape and include any cables or fixing blocks or frames, glass it, apply a coat of resin to the gelcoat, lay the foam on and add a sheet of ply with some weight on it, next day you have a panel ready to trim and fit with a glass smooth finish; make all panels required before you cut the boat.
I have used this system many times with great success for items such as Hard tops with cables for lights and speakers, engine box covers, anchor winch covers, hatch covers etc.
Tip: After you apply the release wax, put in the sun for a while or run a heat gun over it, then let cool completely before you apply the gelcoat; this will produce a very high gloss finish.
Hope this helps. . . . . .
<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.
Jacket
New member
Thats a pity, because all the other readers information was good. Yes, the raised cabin top will raise the AVS, and provided you keep it light, shouldn't have too serious affect on stability at normal angles of heel.
However, Snowleopards comment on ugliness shouldn't be ignored- a 2 foot high cabin top on a 16 foot boat is likely to make the boat impossible to sell in the future, so you're sacrifising a lot of money.
But the bit that would really worry me would be the increased windage. A cabintop of the size you're talking about is likely to seriously impair windward performance, and make manouvering in a crosswind tricky.
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However, Snowleopards comment on ugliness shouldn't be ignored- a 2 foot high cabin top on a 16 foot boat is likely to make the boat impossible to sell in the future, so you're sacrifising a lot of money.
But the bit that would really worry me would be the increased windage. A cabintop of the size you're talking about is likely to seriously impair windward performance, and make manouvering in a crosswind tricky.
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snowleopard
Active member
what do you mean by <the foam bit might cause me some problems>?
do you mean you wouldn't trust it or you think it would be too difficult? if either of those, let me reassure you. i have built 3 boats using these methods and done 3 atlantic crossings in them. it's neither dangerous nor difficult! always happy to provide more info.
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do you mean you wouldn't trust it or you think it would be too difficult? if either of those, let me reassure you. i have built 3 boats using these methods and done 3 atlantic crossings in them. it's neither dangerous nor difficult! always happy to provide more info.
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Avocet
Well-known member
Remember the weight of the occupants of the "wheelhouse" as well as the weight of the materials it is made from! Presumably the whole reason for having it is to put people in it and these will probably be the heaviest part.
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BobWilliamson
New member
Comments noted, will reply in more depth another time. thanks BW
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Jools_of_Top_Cat
New member
obviously only answers the the questioner likes and that agree with are suitable.
I have yet to see a home made job of this type look good. Small boats have less headroom because small boats are in essence small. I have a little more than 5ft across my bridgedeck, she already has way too much windage.
Yes more would be nice, but alas, she would look damn ugly for it!
My advice would be don't do it, as has been said, selling will be niegh impossible, windage will cause problems, and she will probably lose all her looks; but you don't want to hear that do you.
<hr width=100% size=1>Julian
<A target="_blank" HREF=http://www.topcatsail.co.uk/TC_minicruise_2003_01.html>Cardigan Bay Cruise</A>
I have yet to see a home made job of this type look good. Small boats have less headroom because small boats are in essence small. I have a little more than 5ft across my bridgedeck, she already has way too much windage.
Yes more would be nice, but alas, she would look damn ugly for it!
My advice would be don't do it, as has been said, selling will be niegh impossible, windage will cause problems, and she will probably lose all her looks; but you don't want to hear that do you.
<hr width=100% size=1>Julian
<A target="_blank" HREF=http://www.topcatsail.co.uk/TC_minicruise_2003_01.html>Cardigan Bay Cruise</A>
BobWilliamson
New member
Would like to talk more but tight for time at the moment, catch you later Thanks BW
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