MapisM
Well-Known Member
I never tried that boat, but you're making me curious.
What does "too flat" mean? A low AoA is normally a plus for a planning hull.
American trailerable sport fishers
- Thread starter Nick_H
- Start date
What does "too flat" mean? A low AoA is normally a plus for a planning hull.
wakeup
Well-Known Member
I mean as level all the time no matter what almost, the trim tabs had little or no effect and even tilting the drives made not much difference on the AoA. I was trying to get the speed up a little at 3/4 throttle by trimming the drives out but the hull almost seemed to stick flat to the water line and AoA didn't seem to change which I have never experienced on other boats, normally they are too sensitive to TT or drive trimming and it takes a little practice to find the sweet spot. She did look the part though as you say JtB.
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julians
Well-Known Member
All that you describe (ie riding flat, not requiring trim etc) is normally the sign of a good hull and a well balanced boat.
MapisM
Well-Known Member
+1. If anything, the fact that trimming out did not increase the AoA just means that the hull could have used even more power.
Flaps OTOH are only meant to reduce, not to increase the AoA (if and when necessary), so it's not surprising that they didn't do a lot on an already flat attitude.
Unless of course they were malfunctioning - e.g. they actually stayed lowered regardless of any command.
Wakeup, did you also check visually if they moved correctly?
jfm
Well-Known Member
Yes in general, but I think Wakeup means this particular boat was 1 degree too flat though julians. And probably uunderpowered, as MapisM says above, and as Wakeup was saying in the first place. By getting a degree of bow lift he would lose a lot of wetted surface without much change in AoA
Wakeup, your actual boat is here, being driven by a miserable looking Frenchman. It must have been late in the day - 4.30pm- past his going home time
The throttle sticks are hard down and the thing isn't exactly fast
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MapisM
Well-Known Member
Yeah, a speed demon she definitely ain't, judging by the video: I'd be surprised if that bimini could withstand more than 45/50 mph, if that.
Hard to evaluate the hull attitude, though.
Surely the O/Bs don't seem as trimmed out as they should be at WOT, anyway.
julians
Well-Known Member
Could the props be wrong for the boat, so not reaching max revs on the engines?
jfm
Well-Known Member
Yup to all that. The bimini was definitely a "use at anchor only" job
. And yes you cannot see hull attitude in the vid, except that at 1:18 there is kind of a hint of plenty of bow in the water - but that is just wild speculation and it was turning to stbd a bit tooAnyway, my thought to Nick_H remains the same, and I'm sure you agree - if you want the convenience and smoothness of a modern 4T o/b, make sure to get a b-i-g one if you want to go even mildly fast (and then, erk, you have a lot of kilos slowing you down...!)
EME and I were treated to a ride in a Wahoo LX600 with Verado 300s at Cannes boat show 2013. One of the Wahoo company owners/founders (Julian) was at the helm, and it was nice and very fast, being a lot lighter than the Sessa key Largo of course and with nicer motors than those Suzis, but it was not a rocket ship in your books MapisM
D) Alas Nick_H, it had no loo
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Nick_H
Well-Known Member
As we've drifted onto handling a bit more generally, I had a trip out in a Hysucat 8.5m rib at SIBS, with the test driver for Powerboat & Rib magazine, and the MD of Humber Ribs.
It's different not only because it's a catamaran, but it also has hydrofoils bridging between the two keels front and aft.
I have to say it was mighty impressive, like riding on a cushion of air, albeit in the calm conditions of Southampton Water. We did cross the wake of the Princess 62 try-a-boat at 40-ish knots though, and the RIB didn't bounce or slam at all, it was just a "thud,thud" just like driving over an electrical cable in your car.
Cornering was weird as you'd expect, the RIB stayed completely level and the lateral forces were amazing. Without a firm hold on the handrails you'd be over the side.
MD of Humber Ribs was also very impressed, though of course was keen to point out just how good his own Ribs are.
This is the same boat, but taken in Marseille at another time.
It's different not only because it's a catamaran, but it also has hydrofoils bridging between the two keels front and aft.
I have to say it was mighty impressive, like riding on a cushion of air, albeit in the calm conditions of Southampton Water. We did cross the wake of the Princess 62 try-a-boat at 40-ish knots though, and the RIB didn't bounce or slam at all, it was just a "thud,thud" just like driving over an electrical cable in your car.
Cornering was weird as you'd expect, the RIB stayed completely level and the lateral forces were amazing. Without a firm hold on the handrails you'd be over the side.
MD of Humber Ribs was also very impressed, though of course was keen to point out just how good his own Ribs are.
This is the same boat, but taken in Marseille at another time.
MapisM
Well-Known Member
Yeah, agreed. Sadly, emission regulations killed the O/Bs which were really designed for speed, like the epic Merc 2.5 EFI.
No 4T will ever go anywhere near the power to weight ratio of those screaming animals, not to mention the experience of being kicked by a couple of them on a small catamaran, barely touching the water at 3 digits speed.
That said, there's more to going fast than sheer power alone.
Hull design is one (big) thing of course, but for any given O/B setup another neat trick is fitting a hydraulic jack plate.
When we are talking of 50+ knots, as at a guess should be true for that Wahoo, that might be a game changer.
It does add yet more weight in the wrong place, but by shifting the prop further aft (thus increasing the bow lift-inducing lever arm), and raising the X dimension on the fly, the net result can be a BIG speed increase.
Such setup does require also a surface lower unit and prop, but as they say, the only sure thing about speed is that it costs money...
MapisM
Well-Known Member
That's a possibility, but impossible to tell from that video alone.
MapisM
Well-Known Member
Blimey, that thing seems to have just one O/B in that pic, is that for real?
Re. foils, they are indeed supposed to increase efficiency, but only only up to a point.
I mean, when the air entrapment effect begins to create enough lift (which varies with each hull, but as a rule of thumb above 50mph or so), hydrofoils only add drag.
That's the reason why you will never see them in VERY fast boats.
They might well be effective in the 20 to 40 kts range though, which TBH is the more realistic one, for a RIB like that..
jfm
Well-Known Member
Check out this thread on Hysucat. He is claiming 72 knots from 250hp, on the 8-point-something metre RIB. Bearing in mind the MRC/JFM Capelli 8.5m RIB does 30-35kts with a 300hp Yammie, that's mighty fast http://www.boatdesign.net/forums/boat-design/hydrofoil-supported-catamaran-designs-21189.html
z1ppy
Well-Known Member
jfm
Well-Known Member
Here she is, photo-ed in Ocean Village S'ton before transport to the Med. http://moorebrokerage.net/pdf/2011_CAPELLI_TEMPEST_850_MIB-511.pdf?241012200859 Feel free to suggest performance improvements!!! MRC will be interested too. The engine is already jacked up (+ X dimension) as far as feasible - prop can lose grip/ventilate in corners. Engine is nicely run in - 90 hours I think
colhel
Well-Known Member
I think the front bit wants to be more pointy
Scarron
Well-Known Member
Looked at the spec, looks a bit tubby at 2100kg for only 300hp, only one thing for it - More power! take off the 300 four stroke Yammy and clip a pair of 250HO 2 stroke Evinrude E-tecs on the back.
Griffin Marine
Well-Known Member
Does she obtain max revs as seems slow she is a heavy boat though.
Tim
Tim
wakeup
Well-Known Member
Hi MM,
I bow to your experience here as mine is limited to a 5.0 v8 single 21 footer and a 26 footer single volvo. Both boats I have driven over the last 10 years and both 'jumped' out of the hole without problem and increasing the angle of attach always seemed to get quite a bit more speed up to the moment of porpoise. So it was a surprise to me that this Sessa didn't go like a bat out of hell. It was sedate and uninteresting to drive.
I bow to your experience here as mine is limited to a 5.0 v8 single 21 footer and a 26 footer single volvo. Both boats I have driven over the last 10 years and both 'jumped' out of the hole without problem and increasing the angle of attach always seemed to get quite a bit more speed up to the moment of porpoise. So it was a surprise to me that this Sessa didn't go like a bat out of hell. It was sedate and uninteresting to drive.
wakeup
Well-Known Member
Yes JFM, you have hit the nail on the head as always. Lifting the nose to reduce the wetted area just didn't seem possible on this boat and resulted in a sedate and boring drive. Felt like I was driving a big, slow flat thing.