MBY - Curate's Egg

[jfm]

Nah, Hugo was one of Steve Cropley's protégée's at The Autocar.

Ah, my mistake, and I apologise to Hugo for my completely spurious caravan connection.
If Hugo did have a caravan, it would be an Airstream

That reminds me: I've got a caravanny boatie picture to post. On my home PC; will do later.

 

[jfm]

Not sure to see well, did they remove winglets from curved fins? And if yes, why?
The 4% better MPG sounds amazing.
Did you have a chance to compare also the bare hull results, or just straight vs. curved fins?

The winglets are still there, but slightly unclear in that picture. Here's another...

4% not that surprising a result. They are big things and if you induce an AoA hydraulically they sure lift the boat. You gotta find a balance between the drag loss from the lift and the energy needed to drive the fin with it's AoA of course, which is why your net answer is small %. You can play with the whole thing on my boat: go deep into the password protected set up menu, induce AoA hydraulically, feel the thing lift, and watch the LPM real time display as you do it.

I've never had the chance to test the same boat bare hull with no fins. In my previous Sleipner installations , and I think with all competitors, if you removed the fins you are left with a thick shaft (65mm dia iirc) sticking down quite a long way, and the drag of a round shaft is possibly worse than with the fin in place, so the data would be useless. With these new generation sleipner stabs there there is a much shorter shaft but I still think it is long enough to destroy the data. Pics below show the new design (keyless taper, locked hydraulically) to show you what I mean - the stub would induce material drag at 23 knots, I think

 

[jfm]

FWIW my Newtonian physics comment was pushed by a sentence something like: "your body - or more important your G&T - won't fall into the middle of a circle when turning or for that matter be pushed outwards". Falling rather than being pushed being the unfortunate choice of word which provoked me...

Ah ok I understand. I think DM is right: the athwartships component of the steering vector is located at the rudders, well below the boat's roll centre, so it induces the boat to roll into the turn. That does indeed make the G+T "fall" down hill. The fall has nothing to do with an effect of the rotation of the boat, as in the opposite of centripetal force or something, and everything to do with simple gravity once you've induced roll/heel

 

[MapisM]

4% not that surprising a result. They are big things and if you induce an AoA hydraulically they sure lift the boat.

Yep, understood, but I assumed your comparison was AOTBE. I mean, IIRC you could fiddle with the control also in the original setup, so I guessed that the 4% difference was strictly meant as the improvement introduced by the curved fins vs. the straight ones.
If that is the case, even if I understand that in principle the first are working a bit more as hydrofoils than the latter, 4% is impressive, imho.

PS: agreed re. the test with the shafts sticking out of the hull, that would be meaningless.
And not just in terms of drag: I suppose they could disturb the water flow towards the props even more than the fins.

 

[benjenbav]

Ah ok I understand. I think DM is right: the athwartships component of the steering vector is located at the rudders, well below the boat's roll centre, so it induces the boat to roll into the turn. That does indeed make the G+T "fall" down hill. The fall has nothing to do with an effect of the rotation of the boat, as in the opposite of centripetal force or something, and everything to do with simple gravity once you've induced roll/heel

Ah yes, re-read the falling down bit and now see what was meant.