Cruisers Forum

  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
Thread Tools Search this Thread Rate Thread Display Modes
Old 29-03-2019, 01:09   #1
Registered User
UFO's Avatar

Join Date: Oct 2017
Location: Somewhere on the Ocean
Boat: Lagoon 440
Posts: 1,456
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.
UFO is offline   Reply With Quote
Old 29-03-2019, 02:12   #2
Registered User
Sailmonkey's Avatar

Join Date: Sep 2009
Location: Houston
Boat: Ď01 Catana 401
Posts: 9,626
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.
Sailmonkey is offline   Reply With Quote
Old 29-03-2019, 02:37   #3
Registered User

Join Date: Nov 2013
Location: Nice, France
Boat: Hunter Marine 38
Posts: 1,342
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.
sailormed is offline   Reply With Quote
Old 29-03-2019, 05:28   #4
Registered User

Join Date: Mar 2009
Location: Southern Chesapeake Bay
Boat: Norseman 430, Jabberwock
Posts: 1,438
Re: Transoms Under Water

Yeah, the force from the sails is well above the resistance , the force from the propeller is below.
ggray is offline   Reply With Quote
Old 29-03-2019, 05:29   #5
Registered User
denverd0n's Avatar

Join Date: Jun 2008
Location: Tampa, FL
Posts: 5,020
Images: 6
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.
denverd0n is offline   Reply With Quote
Old 29-03-2019, 05:40   #6
Registered User
Reefmagnet's Avatar

Join Date: May 2008
Location: puɐןsuǝǝnb 'ʎɐʞɔɐɯ
Boat: Nantucket Island 33
Posts: 4,865
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.
Reefmagnet is offline   Reply With Quote
Old 29-03-2019, 06:01   #7
Registered User
Dsanduril's Avatar

Join Date: Aug 2011
Location: Petersburg, AK
Boat: Outremer 50S
Posts: 4,229
Re: Transoms Under Water

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

The stern was down from the turn of the screws, as on through the waves we flew and flew
Dsanduril is offline   Reply With Quote
Old 29-03-2019, 13:35   #8
Registered User

Join Date: Aug 2009
Location: Lake Macquarie NSW
Boat: Chamberlin 11.6 catamaran
Posts: 920
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.


catsketcher is offline   Reply With Quote
Old 29-03-2019, 19:46   #9
Registered User
UFO's Avatar

Join Date: Oct 2017
Location: Somewhere on the Ocean
Boat: Lagoon 440
Posts: 1,456
Re: Transoms Under Water

Thanks Everyone
UFO is offline   Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Coolmatic cu 55 under performing under way. unbusted67 Electrical: Batteries, Generators & Solar 1 16-12-2014 13:22
How to Extend Transoms thinwater Multihull Sailboats 5 11-08-2013 10:01
Retroussť stern? (sugar scoop transoms) captpierre Monohull Sailboats 0 07-01-2012 13:37
35' boats with walk-thru transoms ? Aquah0lic Monohull Sailboats 7 28-01-2008 18:21
Water Bladder under Aft Bunk? Lynx Plumbing Systems and Fixtures 18 13-12-2006 23:33

Advertise Here

All times are GMT -7. The time now is 03:03.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.