hull extension

[Deleted User YDKXO]

Well, not difficult of course, but 2 x new pumps, 4 x new actuators, 2 x new tabs plus ancilliaries and fitting (at SoF prices) is still going to be '000's I would guess, and more hassle to organise, for something I don't need (ie. more variability than existing tabs can provide). Fixed tab is free to me as I employ fabricators, and i'd just fit it myself next time boat is out the water. Simples.

OK point taken, fair enough. Out of interest, how are you going to decide what shape and size this thing is going to be and how are you going to adjust it's size and shape if it doesn't do what you think it's going to do?

 

[MapisM]

No it doesn't mean that at all.

Well, I'm afraid we'll have to agree to disagree.
In my books, if the MAX ("optimum" being a tad subjective) speed is achieved with zero (meant as neutral) tabs, that's one of the best indications of a proper hull behaviour.
There's an obvious method to increase the bow up attitude in really BIG following seas - slow down. If you don't, you're looking for troubles anyway.
Otoh, on long and soft following ocean swell, good for some really fast cruising, it's not a bow up attitude that you need, it's rather a firm and responsive hand on the throttle.
Tried and tested at 80+ mph along the US Pacific coast, with 40' or so jumps between one wave and another.

 

[MapisM]

for something I don't need (ie. more variability than existing tabs can provide).

Nope, it's not more variability that you need, it's more stern lift.
Hence my previous suggestion of longer tabs.

 

[Nick_H]

Nope, it's not more variability that you need, it's more stern lift..

That's what I was trying to say, but badly worded I think.

 

[Deleted User YDKXO]

Well, I'm afraid we'll have to agree to disagree.

Yup, agreed
.

Tried and tested at 80+ mph along the US Pacific coast, with 40' or so jumps between one wave and another.

Can 57ft flybridge boats do that?

 

[jfm]

MapisM, i may be ignorant here, but assuming the same surface area why are you saying long/thin tabs better than short/wide ones?

I can see that fast offshore boats have long thin tabs but I don't know why. Normally with a foil you want it short and wide, like an aeroplane wing and like the steps in the hulls of...guess what...those very boats that have long tabs!!??

 

[jfm]

how are you going to adjust it's size and shape if it doesn't do what you think it's going to do?

I'd think Nick can easily get them made articulated with lengths of chunky studding (in lieu of hydraulics) to adjust the angle of attack (with spanner, using his diving kit). This would also allow easy removal of the whole tab if needed, by withdrawing the hinge pin, leaving just a flat plate on the transom

Alternatively (but less preferably imho) they could be fixed and have big wedges of plastic (machined from block) attached to the underside by bolts/machine screws. Swapping the plastic blocks for different ones would allow adjustment to angle of attack (in discrete steps).

 

[MapisM]

Can 57ft flybridge boats do that?

LOL, nope, none I'm aware of, at least.
But the principle is still the same: in a big following sea, where fast cruising puts the boat at risk of stuffing the bow into a big wave, my first choice would be to slow down, and if for any reason it's necessary to go fast(ish), I'd rather want a helluva good helmsman than a "bow-up" boat...

 

[MapisM]

like the steps in the hulls of...guess what...those very boats that have long tabs!!??

Actually, steps are a different matter altogether. Their shape and height is mainly meant to ventilate the hull behind them, hence reducing drag. But if you look at them from an hydrodinamic standpoint, their main purpose is to split the hull in different sections, all of which with a positive AoA.
In a sense, it could be said that each of these sections act as huge trim tabs, as wide as the whole hull and as long as the hull sections which they are splitting.

Re. long tabs on very fast boats, also space constraints force such choice, since outdrives can interfere with tabs, particularly with twin, non-staggered engines. There are other reasons, though: the increased leverage makes them more reactive, they are also meant to stabilize the boat crosswise (e.g. when crossing other boats wakes or upon a jump re-entry), at very fast speed they actually make the hull longer...

All that said, coming to think of it, you're correct: on the boat we're talking about, making them wider could work equally well, if that's feasible. None of the above reasons are relevant for a P57, in fact.
Anyhow, I'd still prefer larger (either wider and/or longer) tabs, than a central fixed extension - though I understand that there's quite a difference costwise.