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Old 29-03-2019, 01:09   #1
UFO
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Transoms Under Water

Looking at a video of a catamaran motoring with sails down and the Transoms at the rear are submerged by around 6-12 inches.



Would motoring lower the transoms at all? or would it be the same if sailing and not motoring?


Picture at the marina looked like they were out of the water by maybe 5 or so inches.
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Old 29-03-2019, 02:12   #2
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Re: Transoms Under Water

Motoring almost always causes stern squat. Our monohull will keep the transom out of the water sailing, but under power it sinks quite a lot.
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Old 29-03-2019, 02:37   #3
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Re: Transoms Under Water

It is caused by the screw pushing the boat forward being situated under the boat. The hull being retained because of the water resistance, the screw will move more forward, pulling the transom lower.
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Old 29-03-2019, 05:28   #4
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Re: Transoms Under Water

Yeah, the force from the sails is well above the resistance , the force from the propeller is below.
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Old 29-03-2019, 05:29   #5
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Re: Transoms Under Water

And especially if you are trying to motor quickly -- pushing close to, or past, the theoretical hull speed -- then the bow will be trying to climb the bow wave, which pushes the stern even further down.
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Old 29-03-2019, 05:40   #6
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Re: Transoms Under Water

Squat is also caused by hull shape. The underside of the hull acts like an inverted wing, with water rushing faster along the bottom of the hull creating a "vacuum" from reduced water pressure, causing the hull to squat. Large vessels squat more in shallow water than they do in deeper water.
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Old 29-03-2019, 06:01   #7
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Re: Transoms Under Water

Common enough that is made it into music lyrics somewhere back near WW2:

Quote:
The stern was down from the turn of the screws, as on through the waves we flew and flew
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Old 29-03-2019, 13:35   #8
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Re: Transoms Under Water

When designing a sailboat you have to cater for the fact that the force of the rig is coming from way up high. On my 11.6 m cat, the centre of effort of the rig is about 5-6 metres above the deck, which is about 8 metres above the centre of bouyancy of the hulls. This difference between where the force is applied by the rig and the drag of the hull causes a torque.

So designers have given us hull shapes with nice upsweep at the aft sections. This is good for light winds but also produces a torque that pulls the sterns down at speed. This counters the effects of the torque (bow down) produced by the rig.

My cat goes stern down at speed too (under motor). Its outboard motors are almost at the same height as the CB so there should be next to no effect of the motors rotating the hull due a distance between two forces. So it must be down to the hull shape rotating the boat at speed. Under sail, she stays flat when going fast.

The effect of putting rocker into keel lines is well known and down very carefully and minimally in fast planing boats like sailboards and dinghies. Powercats and ski boats have almost straight hull (buttock) lines aft. This reduces the tendency to squat. There have even been superyachts with hydraulic sections aft that pivot down at speed to reduce keel rocker at speed.

So squatting is all normal for a good sailing boat.

cheers

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Old 29-03-2019, 19:46   #9
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Re: Transoms Under Water

Thanks Everyone
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