Reducing the draft of a long keel

Hi to all,

New on the forum. I am hoping to find some advice on vessel keel design modifications.

I am wondering about reducing the draft of a long keel on a motorsailer.
The vessel is 85 feet long and with a draft of 11 feet.
She has a full keel and I am wondering if it would be feasible to reduce the draft to 8 or 9 feet...
It seems like a heck of a lot to cut off the existing keel & I am imagining that a lead box / bulb would need to be bolted on either side of the remaining keel to compensate for the removal of all this weight.
I wonder if there would be added or reduced drag?

I am expecting that sailing abilities upwind would be decreased & leeway increase but then being able to access many shallower anchorages around Bahamas & the world would be a definite benefit. Perhaps a minor increase in speed off the wind due to less drag? Also vessel in question has short masts so imagine motors are ticking over most of the time.

So my main question is : Is it really feasible to reduce the draft on a vessel of such dimensions by 3' and maintain a good seaworthy vessel?
Modern vessels of similar size often have a fixed draft of 8 or 9' so I would have thought it possible... but they are generally purely sailing vessels and not with a steel superstructure.

Has anyone seen this done before on similar sized steel vessels?

How about cutting through a thick steel keel... torch may not work... what other methods...

thanks for any ideas inputs and thoughts on deep drafted full keel motorsailers.

 

The picture leaves much to be desired in order to make any qualified observations.
It appears to have some bilge keel hull projections, which indicates it may already have problems with roll.
I would expect that shortening the keel would require shortening of the mast to maintain balance.
This will come with performance penalties.

 

Please do not try to cut a few feet of the keel of this fine vessel - just accept her for what and who she is, and acknowledge that a few extra feet of draft might limit you in some anchorages and / or marinas.
What is the primary reason why you want to reduce the draft?
I presume that you are the owner of this motor sailer - or you are maybe thinking about buying her?

It is very possible and easy to do if you design it like this from the beginning - and it is very difficult and complicated to do as a retrofit later on.

 

captain crunch mccormack will do it for you, you will both get on so well

 

Don't even think about it.
Really.

 

Feasible? No.

 

Given it is a steel boat, I don't see reason why it couldn't be done. Just add some extra ballast to compensate shallower draft.

 

Extra ballast may be required to give it the same righting moment, but removing below water volume, will reduce the ballast required for it to sit on its original WL.
Given the amount of boats out there for sale, this is one idea i would walk away from, though from a technical prospective, quite possible. Does not mean you should though......

 

Is it really feasible to reduce the draft on a vessel of such dimensions by 3' and maintain a good seaworthy vessel?
ANS: Yes, it is technically feasible, and extra mass is needed to duplicate the righting moment of the original design, this will give the same stability of the original. Your predictions about performance loss are correct.


How about cutting through a thick steel keel... torch may not work... what other methods...
ANS: Torch cutting of steel over 12 inches thick is common in industry, not a technical problem.

SKARABORACRAFT's comment above is the most reasonable in my opinion.

 

With all that deadrise there is not much salient keel to sacrifice.
As mentioned above, it is, IMO, the wrong boat.

 

As has been stated by other posters, it is probably technically feasible to reduce the draft of that vessel by 3 feet and maintain the point of maximum righting arm/moment. But, it would most likely be a money hole where you could have built a new hull cheaper and quicker.
On the other hand, stability is only one side of "seaworthy". The other side of seaworthy is "seakindly". Reducing the draft will in most probability totally destroy the sea kindness inherent to that hull shape. In SF Bay vernacular; you will have to paint it pink and name it "PIGDOG"....which means "contemptible or worthless".

 

Hey! So, cutting off 3 feet from a long keel – that’s a big deal. Reducing draft from 11 to 8-9 feet will definitely need some lead bulb or box added on sides to balance the weight loss. Stability and seaworthiness could get tricky after such changes.

And yeah, your upwind sailing ability will drop. But, access to shallower anchorages is tempting, right? Tough choice!

 

Cutting 3 feet off a keel will require that the bulb/wing be of more weight than what was removed to keep the same stability. However, increasing the displacement will also increase the draft and change the stability curve. In short, the behavior of the boat, maximum stability, roll damping, etc. will change and probably in a negative way. It is not a straightforward modification.

 

What is "same stability"?
The same hull, with more displacement, can have greater initial stability. As for whatever the "stability curve" is, whether it is the curve of the GZ values or the dynamic stability for that new displacement, it does not have to be worse, whatever system is used to judge whether a curve is better or worse.
Indeed, "It is not a straightforward modification" and, therefore, knowing whether the modification will change the stability in a "negative" way (whatever "negative" is) is not easy. In these matters one should not give an opinion, because they could lead to the adoption of unfortunate measures; one must speak with concrete data and calculations.

 

X amount of ballast at 8' underwater will not give the same stability as X amount at 11'.
To maintain the same stability curve you'll need X plus an amount that will need to be calculated.