Do Stern Extensions Make Much Difference ?

[Joli]

The Schock 35 is the old Santana 35 (mono). The builder used all the molds but plumbed the bow, adding 2 foot or so of waterline. They also added a bigger rig, the outcome was a boat that was 1 minute per mile faster per its PHRF rating. Big change for essentially the same boat.

Laidback basicly built a bigger boat and I would guess has added considerable speed. Adding waterline and power yields speed.

[Laidback]

Originally Posted by cchesley

The one thing hull extensions do that is counterproductive is add wetted surface. I would not expect significant improvement in light airs.

Can't remember the research that showed if the sterns' extremities were out of the water at payload design BUT if payload was exceeded and the sterns were then below the water line - significant drag would be the result. The extensions
although providing additional whetted area, countered by reducing drag thereby improving SOG.

[GordMay]

Adding length to the stern (behind the rudder), effectively moves the rudder effort forwards (towards midship), reducing steering capability. I’ve seen a 4 ft. cockpit extension virtually ruin a (formerly 43') Chris Craft (twin screw power boat).

[Laidback]

Gord,
In my modifications that issue was dealt with simply by also moving the rudders aft.
Many designs of modern catamarans have gone for spade rudders which are relatively easy to move if necessary.

[markpj23]

Manta catamarans started out as a 38ft model, then went to 40, then 42. I believe they just extended the pontoons and worked with the original mold.

[colemj]

Originally Posted by markpj23

Manta catamarans started out as a 38ft model, then went to 40, then 42. I believe they just extended the pontoons and worked with the original mold.

Hi Mark,
That's correct They also moved the rudders aft when extending the sterns.

Mark

[Stein]

As I understand it, Laidback extended stern and bow. The stern previously ended below the surface, which will create serious drag, especially at relatively low speeds. (At high speed, the flow will release and drag drops a lot.) Even if he had not increased the rig power and moved the rudder aft, there cannot be any doubt that the boat would be notably faster, and behave better in most situations. The increased wetted area is insignificant compared to the removed stern drag, the increased lenght to width ratio and the resulting improved wave pattern. It's possible to get really nerdy on this, but I'll stay on the main points for now.

The added stern lenght may have affected steering a bit, but as most such sterns are rather shallow and have a round profile, I'd guess the effect would be close to nothing. Moving the rudder aft, the steering would be better than before. The bow extention will normally be sharper and have a better "grip". Thus this extention will cancel out the steering advantage mentioned, and the boat will probably be very similar to what it was before, but faster and generally better. It was overloaded, but now will have extra load capacity. It's a bigger boat with less resistance and more power. It will be a rather much faster boat. Such changes are much more work than they seem, but may be worth it, and may also not be. Depends upon the boat and the needs.

[Laidback]

Originally Posted by Stein

The added stern length may have affected steering a bit, but as most such sterns are rather shallow and have a round profile, I'd guess the effect would be close to nothing. Moving the rudder aft, the steering would be better than before. The bow extension will normally be sharper and have a better "grip". Thus this extension will cancel out the steering advantage mentioned, and the boat will probably be very similar to what it was before, but faster and generally better.

Stein, Thanks for the insight.
As far as steering was concerned - little or no difference at very low speed - in fact fairly typical of cruising catamarans over 25ft, ie the catamaran was/is like a train on tracks : the rudders not much use for example in maneuvering in a marina. It was found that it was best to set the rudders fore and aft and use the drives to steer the boat. Once the boat was out and doing 3kts or more, then the rudders could be fully functional.
Definitely cruising catamarans with a relatively high freeboard, without deep ballasted keels need someone at the helm with patience and experience to enter a prescribed berth in a busy marina.

[Abaco]

Quite a few production cats have been extended by extending the sugar scoops. Does anyone know if the rudders where moved in any of these examples:

Manta 38 to 40 to 42
Leopard 42 to 43
Leopard 45 to 47
Seawind example mentioned before

I'm sure there is more.

[Laidback]

The Robertson & Caines / Simonis - Voogdg designers of the Leopards mentioned do not appear to have moved the rudders as the LOAs were only increased 1 0r 2 feet , more likely that engines being moved might have been a reason to also move the rudders aft, rather than extending the sterns being the reason.

[colemj]

Originally Posted by Abaco

Quite a few production cats have been extended by extending the sugar scoops. Does anyone know if the rudders where moved in any of these examples:

Manta 38 to 40 to 42
Leopard 42 to 43
Leopard 45 to 47
Seawind example mentioned before

I'm sure there is more.

The rudders were moved aft for the Manta 40 to 42. Not sure about the 38 to 40.

Mark