Simpson Inspiration 10.5 vs 11 vs 11.5

[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.
Currently my boat has a hull to water length ration of just over 1:8. and seams to have problems to go faster than 8kt or so.
With an extension to 11m the ratio would be 1:8.5
and an extension to 11.5 would mean 1;8.9 or so.

I'd like to hear advice on the difference in length.

Thanks

[UNCIVILIZED]

The most common length of extensions tends to be 1m/3'. Occasionally you'll see 5', especially if it's a smaller cat, & large twin outboards have been fitted in place of a pair of 10hp ones. For example, there was a 35' Shuttleworth that had 5' added to the hulls, & she had a big outboard on the end of each hull.

As to why 1m, I couldn't say. Except perhaps that it's probably not worth the time & effort to only add 1/2m to the hulls. Maybe a longer extension negatively affects handling, or rather the boat's motion in waves? But most of the accounts I've read where boats get stretched by 1m or so, state that they markedly improve the boat's ride. In addition to making them less prone to dragging transoms, even when loaded down with cruisng gear.

I gather that you don't have kickup rudders?

[HPM]

Sounds reasonable and encouraging. So 1m it is.
Dragging the transom is an issue for me.
And no I don't have kick up rudders
Thanks

[UNCIVILIZED]

Just to confirm this for your own peace of mind, you can do a bit of looking around online at catamarans that are for sale, & you'll run across a humber of them that have been extended. Plus, most designers would probably be happy to discuss such things with you. Basically I'm saying that it would be wise to cross check what I've stated by viewing several other sources of info. Not that I'm misstating anything about hull extensions.


About a month ago I ran across a Schionning 10m +/- online that had been extended by about 1m. Can't recall which website though.

[44'cruisingcat]

You'd normally continue the same rise of the hull at the stern, so the amount of extension that would be of benefit would depend on how steeply it rises, and by how much the transoms are immersed, when fully loaded.

[smj]

Once the stern clears the water you can place a hook on the extension to gain length.

[old pirate]

Many of the large cat builders found with hindsight the extension did improve performance. 2 to 3 foot seems adequate. They could also charge extra for the boat as it was now "bigger" Look at Voyage Mayotte, the originals were 47 then stretched to 50 with a bigger scoop. likewise the Privalege 43 to 45 and the Leopard 45 to 47 the same boats, bigger scoops. Sometimes minor adjustment to the bow as our Simonis 51 was stretched from 51 to 55 foot from both ends.

[smj]

Manta from 38-40-42 and the Island Spirit from 35-37-40. Multiple extensions with the beam staying the same.

[HPM]

How important is the water length to beam ratio?
I read that if this ratio is greater than 8:1, the max hull speed laws of displacement hulls would not apply. The reason given was because the bow wave is too small.
Is that true?
HPM

[HPM]

I Does anybody know of a stretched Simpson Inspection 11?
I'd like to get in contact with skipper Ta HPM.

[44'cruisingcat]

Originally Posted by HPM

How important is the water length to beam ratio?
I read that if this ratio is greater than 8:1, the max hull speed laws of displacement hulls would not apply. The reason given was because the bow wave is too small.
Is that true?
HPM

Yes, that's true. Obviously it's not a "line in the sand" between generating a huge bow wave and none at all. But tank testing has found that at around 8:1 bow wave propagation has diminished sufficiently to eliminate the hull speed limit.


Multihull Dynamics, Inc. - News Article

[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.

[angus logan]

[angus logan]

Hopefully these photos come through..

[UNCIVILIZED]

When calculating this, I hope you guys are taking into account that WL beam is a good bit less then overall hull beam. So the L/B will work out differently if this isn't factored in.