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Old 25-03-2020, 07:11   #1
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Structure mods

I have a 40í Lagoon Cat, 2008, with a 4í draft. Iím wanting to know if thereís any reason why I canít cut my keels down 14Ē to give me a 2í10Ē draft? Are there any reasons why I canít do this? Iíve contacted the builder but they said if they havenít tried it they just wonít give a response. Nothing from them yet. I operate a day charter doing tours in Belize and wonít be making any major crossings other than the Rio Dulce or southern YucatŠn. Iíve seen this done on another Cat so I know it can be done but I wanted to know if I shouldnít. I realize I will have to shorten my rudders but I intend to also make them 6-8Ē wider to compensate for shortening. Any thoughts performance wise or other?
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Old 25-03-2020, 07:32   #2
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Re: Structure mods

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I have a 40í Lagoon Cat, 2008, with a 4í draft. Iím wanting to know if thereís any reason why I canít cut my keels down 14Ē to give me a 2í10Ē draft? Are there any reasons why I canít do this? Iíve contacted the builder but they said if they havenít tried it they just wonít give a response. Nothing from them yet. I operate a day charter doing tours in Belize and wonít be making any major crossings other than the Rio Dulce or southern YucatŠn. Iíve seen this done on another Cat so I know it can be done but I wanted to know if I shouldnít. I realize I will have to shorten my rudders but I intend to also make them 6-8Ē wider to compensate for shortening. Any thoughts performance wise or other?

In fact, I extended the keels on a similar cat to get better pointing and tacking. It really helped, which gives me an idea of what shortening them would do to a boat that doesn't point very high.



I can't say on a structural basis, but I'm pretty sure she'll point like sh__ and only a non-sailor would buy a boat that was modified in that way. On the other hand, if it suits your business model, go for it. But expect that she won't do much above a beam reach.
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Old 25-03-2020, 07:41   #3
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Re: Structure mods

I was hoping that widening the rudders would compensate for this and possibly give me better performance at slower speeds such as docking, anchoring or crossing shallows.
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Old 25-03-2020, 07:54   #4
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Re: Structure mods

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I was hoping that widening the rudders would compensate for this and possibly give me better performance at slower speeds such as docking, anchoring or crossing shallows.
In addition to seriously increasing leeway while close hauled, you also risk seriously messing up the balance of the boat. You are taking lateral area away from the keels, and moving it back to the rudder. This will have the effect of moving the center of lateral resistance aft pretty dramatically, which will increase weather helm.

Also, don't forget, you will need to keep the rudders "balanced" and add area to the front and back of them. If you just add area to the trailing edge of the rudders, you will GREATLY increase steering loads.

A boat is a "system" and chopping it up can have serious consequences, that might not be obvious beforehand to the "novice" navel architect.

But, again, in your charter business you might not care because I suspect you don't spend a lot of time sailing hard on the wind.

If the resale value isn't important, then it's your boat!
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Old 25-03-2020, 08:09   #5
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Re: Structure mods

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I was hoping that widening the rudders would compensate for this and possibly give me better performance at slower speeds such as docking, anchoring or crossing shallows.

No, that will just create lee helm, which will make pointing even worse (lee helm pulls the boat to leeward).


Remember when widening rudders you must maintain balance (add to the front and back). Otherwise the helm will be massive forward and worse backing. You will also be changing the profile (they will need to be MUCH thicker).



You could lengthen the keels considerably, fore and aft. Since this requires creating a new profile, it's kinna major. It will slow turn in a bit. You will also need to reduce the sail area (keel profile must be proportional to sail area) and move the sail area aft (increasing mast rake will help).


I suppose a complete re-work in balance could be accomplished, but you will need to think like a navel architect. My background is engineering, so I think that way.


Also, when it gets rough, shallow keels don't work as well as deeper keels.
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Old 25-03-2020, 08:24   #6
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Re: Structure mods

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No, that will just create lee helm, which will make pointing even worse (lee helm pulls the boat to leeward).

Damn! That's what I get for typing BC (before coffee!). You are right of course.
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Old 25-03-2020, 08:46   #7
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Re: Structure mods

Besides the obvious re-engineering issues, you state that you are a charter business.

If you modify the boat without going through a naval architect and the boat manufacturer, and if some mishap were to occur you could be 100% liable. Also, you may also not be insurable, or insurance could fail to pay any claims.
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Old 26-03-2020, 08:43   #8
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Re: Structure mods

Dont do it!
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