Reasons for the stern to be lower in water

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K-ron

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Hi people,

Seeker shall be coming out the water soon for a much needed bottom clean and paint. Last year we noticed how low the stern was sitting in the water and the subsequent growth on the topsides which are now a few inches below the waterline. The boat is a Nicholson 32 and we carry a lot of extra cruising gear and initially I thought this was the reason for the change. Now I'm not so sure, could a soggy core in the rudder cause our stern to drop some 3/4 inches?

Thanks in advance

Kieran
 
Nicholson LWL 7.3m I believe and beam 2.8m. That gives 20.44 sq m. Use a complete finger in the air factor of 0.7 gives a waterplane area of 14.3 sq m (143,000 sq cm). I cu cm of water weighs 1g (forgetting allowance for salty water). To make the boat sink 1cm would require 143 Kg.

Bit more onerous to calculate the effect on trim as you'd need the shape of the waterplane area, but I'd say it's obvious that there's more weight involved than could be contained in your rudder.

Surprising how much cruising gear you accumulate. I've six toolboxes in the stern cabin and it does have an effect.
 
lpdsn said:
Nicholson LWL 7.3m I believe and beam 2.8m. That gives 20.44 sq m. Use a complete finger in the air factor of 0.7 gives a waterplane area of 14.3 sq m (143,000 sq cm). I cu cm of water weighs 1g (forgetting allowance for salty water). To make the boat sink 1cm would require 143 Kg.

Bit more onerous to calculate the effect on trim as you'd need the shape of the waterplane area, but I'd say it's obvious that there's more weight involved than could be contained in your rudder.

Surprising how much cruising gear you accumulate. I've six toolboxes in the stern cabin and it does have an effect.
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Thanks for doing the maths. The space between the water line and top edge of antifoul has always been much wider near the bow than the stern, so I suppose as we have just added more and more weight it became noticeable at the stern first.
 
K-ron said:
Thanks for doing the maths. The space between the water line and top edge of antifoul has always been much wider near the bow than the stern, so I suppose as we have just added more and more weight it became noticeable at the stern first.
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It is traditional to have the waterline boot top marking significantly higher at the bow than midships, and slightly higher at the stern - ie not a straight line parallel to the water. https://www.boatdesign.net/threads/sheered-waterline.35840/ shows an example.
 
As said it is unlikely that a soggy rudder could affect the balance of the boat in fore and aft plane. I suggest you try to get photograph of the boat from the side while you are sailing it with typical crew compliment in place. This will show if the stern is excessively low when sailing. As said this will slow the boat if stern is too low. Also indicated by turbulence in the water at the bottom of the transom where it drags.
Only weight in the boat can change the trim. In almost every case a boat sails better with weight forward or at least not in the stern. Once the centre of gravity is corrected then you should redo the water line boot top and a/f level to suit the trim when on the mooring. olewill
 
jwilson said:
It is traditional to have the waterline boot top marking significantly higher at the bow than midships, and slightly higher at the stern - ie not a straight line parallel to the water. https://www.boatdesign.net/threads/sheered-waterline.35840/ shows an example.
Click to expand...
From my highly competitive dinghy sailing days this is true when going to windward weight forward will support an increase in boat speed. When running or broad reach getting the stern in can reduce drag and increase directional stability. But use your crew for that. In reality in most boats this will be hardly noticeable unless it’s built as a racing snake, then a few tenths of a knot can be the difference to winning or losing.
 
You mention you are going to lift the boat soon - If you have a soggy rudder - you will find out when you lift her as if the rudder is full of water - it will drip, fairly continuously (unless its flooding from the stock).

You can always drill a hole in the bottom of the rudder, which (if soggy) will then drip and its very easy to re-seal, though if it does drip - you need to find the source of the leak.

And that might need a new thread.

I'd suggest - start the new thread, now - and then you can anticipate what might need done when you do lift her.

In the meantime - follow previous posts.

Jonathan
 
Simple solution: Get a bigger and heavier anchor. Ah, but what kind, I hear you say? There now, that'll keep everybody busy for a wee while. :D
 
I find the liferaft stowed aft makes a big difference to longitudinal trim. This especially true on older boate which tend to narrow considerably at the stern.
 
paulieraw said:
From my highly competitive dinghy sailing days this is true when going to windward weight forward will support an increase in boat speed. When running or broad reach getting the stern in can reduce drag and increase directional stability. But use your crew for that. In reality in most boats this will be hardly noticeable unless it’s built as a racing snake, then a few tenths of a knot can be the difference to winning or losing.
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I would have thought;
Stern down = turbulence at transom = drag
Forefoot down = reduced drag + increase in directional stability
i.e. when sailing rudderless, a tack will be more achievable with crew aft as the Centre of Lateral Resistance will be further aft.
 
Spirit (of Glenans) said:
I would have thought;
Stern down = turbulence at transom = drag
Forefoot down = reduced drag + increase in directional stability
i.e. when sailing rudderless, a tack will be more achievable with crew aft as the Centre of Lateral Resistance will be further aft.
Click to expand...

You are correct, the reason for having weight back is to improve directional stability. The bow tends to push in on a run too far and you start to to dig lifting the rear so reducing the drag, so is weight back to help with stability and to prevent the front digging in. Though to be honest on a big boat you would may not notice it so much.
 
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