Simpson Inspiration 10.5 vs 11 vs 11.5

[gerrytv]

I'm working on a wee project converting a 43' Tennant performance cruiser with a flat deck (accomodation was all inside the hulls) to a 50' bridgedeck cat. The lengthening is all at the back, but half scoops with a central vertical web. From the outside the hull lines continue on, from the inside its just a boarding platform and steps going up.
The displacement doubles, so the whole thing will sit 80 mm deeper at the stern. The hull beam of course increase proportionally as well.
Just another reason for doing the same thing.
Two more years to see how it floats and performs. :-)

[HPM]

Originally Posted by angus logan

Hi HPM,
My wife and I own an extended inspiration. Can't help you much with improvement in performance as it was extended before we bought her. We have slightly extended and sharpened the bows and will be interested to see if this gains us anything. Be more than happy to talk anytime.
Cheers
Angus.

I'll let you know
I'll exten from 10.5 to 11m meaning I end up with a water length of 10.5.
Peter

[angus logan]

Our waterline length now measures 11.5 metres.

[sb1]

Originally Posted by HPM

Hi
I am wondering whether "sugar scoops" to extend my Simpson Inspiration 10.5 is a good idea and by how much and who could help...

Hi there, I just wondered whether you decided to extend your SI, and how you got on. We have one (unextended), and it would be good to improve performance a little and perhaps reduce the hobby-horsing in certain conditions.

To bad it won't solve the shocking visibility from the helm...

I love the boat though - not complaining.

Cheers

Damo

[HPM]

I can tell you in 4 weeks. I hope

[Factor]

It will be worth it. I have sailed the 10.5 and a 10.5 extended on a number of boats). (I have also sailed Roar Ege in its configuration before Angus Logan bought it,) strong improvement in motion and quite discernible improvement in performance.

[catsketcher]

I think there is a reason cats get extended about 1m - its a simple question of waterlines and buttocks.

The simplest way to extend a cat is to just continue the sweep of the keel line (maybe straighten it somewhat) and to do the same with the hull sides. With most cats of between 30-45ft this will give you a length of about 1 metre before the sterns get very narrow. As this is the easiest way to do it is the most common.

(Note - when an owner wanted to extend the main hull of his round bilge Twiggy, Lock Crowther - it was a long time ago - suggested NOT continuing the keel line but inverting the sweep. Otherwise most of the extension would have been out of the water. The owner did and it seemed fine. Plenty of old IOR monos have kinks in their aft sections and went fast - Waverider and some Whiting designs)

You can extend by chopping the boat in the middle and putting some new middle in. This has been done twice in Australian mults - one was Kindara and the other was Rushour which has only recently been re-launched.

If you are seriously looking for more buoyancy, rather than just a cleaner wake, you can foam up the sections forward of the stern and flatten out the keel upsweep. This will give you the maximum flotation compared to an add on. Friends did this on their Fastback 30 that is now 36ft. You also get better top speed but may lose some in light winds with more wetted surface.

If all you are trying to do is clean up the wake, the add on is easiest. If you are trying to get the sterns higher and get the boat to handle serious weight aft consider foaming up in front of the sterns and adding onto this. I don't think you need to do a cut in half job. Don't worry about the hydrodynamics - more waterline is better but if you get alongside and under your boat you will see how adding, say 2 metres, is pretty silly if you want don't want pointy transoms.

As an aside, when my mother's Crowther 10 got the extra metre added on I could hardly recognise it. And it sailed much better too.

cheers

Phil

[Boatguy30]

I always say I asked my designer to make my boat the right length the first time.

I'm confused on the 8 knot thing. In how much wind? My boat sorta seems to drag the transoms at around 7-8 knots when loaded in say 18-22 knots of wind. As soon as you have 25 knots of wind you're doing 10-12 knots and transoms are lifting nicely. Is this not how all cats sail?