Underwater stern scoop on large displacement powerboats?

[BIGBOATBUILDER]

Hi, back again with another question.

I noted recently that more and more non planing powerboats have a scooped underwater shape at the transom (=buttocks run up towards the waterline and then take a modest bent towards horizontal or even back from the water surface.)

1. Does anyone have experience with this?
2. What are the advantages? One I guess is to reduce trim angles at max speed.
3. What risks do they pose?
4. Is it sensible to use something like this on a vessel acting at a maximum speed of Fn=0.39?
5. Design guideline?

Thanks for your input as usual.

Best regards, Charlie

[KCook]

I believe it's done mostly to relieve drag from the stern wave. But I could be wrong ...

Kelly Cook

[CTMD]

Without seeing a photo of what you're talking about I'd guess what you are seeing is.

1. A rise in keel area aft to reduce transom drag.
2. A flattening out to stop the boat trimming or squatting at speed due to the rise of keel.

[PAR]

I think he's referring to a built in hook, which was a common way to keep the bow down in some hull forms.

[tom28571]

Par,

This is the usual case of trying to answer an incomplete question. Do non-planing boats have hooks? I don't recall ever seeing one. Maybe he is talking about the much discussed tunnel as per Atkin. Maybe not. Who knows

[Willallison]

A little difficult when the original poster doesn't seem to be about to clarify things... but I suspect that PAR is on the right track.... you do often see a downturn in the otherwise upsweeping buttock lines of displacement craft. It is an attempt to reduce squatting as the boat approaches displacement speed.
As to how it might effect seakeeping, I think this would vary a great deal from vessel to vessel. In some cases I could imagine that you would be increasing the volume aft to such an extent that directional stability could be effected when running down-sea.
It could also have a small effect on the efficiency of the boat as you would now be presenting a blunt immersed transom.

[PAR]

I think Will has it right. I've also seen hooks developed into the buttocks of displacement craft. I haven't seen it used in a reasonably modern craft, but have seen it in powerboats as recent as the 1960's.

I do know it's a tricky thing to play with. Getting the right amount of "hook" for the expected S/L's the yacht will travel in, is problematic at best. It's not going to help anything at low to moderate speeds, but will help a burdened hull as it approaches displacement speed.

Many of the Atkins displacement hulls had hooks. I've drive a double ended 39' box keel motor yacht reasonably frequently and it does work. The wake is hardly noticeable at displacement speed and she doesn't seem to squat much. In a follow sea she's a kitten. This particular box keeler was John Atkins favorite boat, having personally logged many miles in her. It was also his last box keel design, so the development of the type was as refined as it got.

I'm also old enough to remember nailing cedar shakes onto the butts of power boats to get their bows down, though a different application, as these were semi and full plane craft. I've spent a whole day nailing and removing different thickness and taper shakes, to get the boat to "flatten out".

I think you're right Tom, it's difficult to tell what the original poster intended, especially when not followed up.

[Willallison]

Even 'die-hard displacement officianardo's' are doing it. Nordhavn, for instance tacked such things onto a number of their boats in recent years

Well - have a nice weekend guys - I'mm off for a couple of days on the boat...

[FAST FRED]

"Nordhavn, for instance tacked such things onto a number of their boats in recent years"

Probably to compensate for engine selections that are 200% or 300% oversized .

FF

[Guest625101138]

Godzilla will sometimes optimise to a hooked stern as shown for a heavy displacement vessel designed for 10kts:
http://www.boatdesign.net/forums/att...0&d=1185958001

So one answer is that it produces the lowest drag hull under certain design constraints.

Rick W.

[tom28571]

Ah, the mention of nailing shingles on the transom reminds me that some do use tabs sticking down from the transom in exactly that fashion. The are just moved up or down with screws in slots to get the desired stern lift. The claim was that they are as effective as trim tabs and no more drag. I was skeptical of the drag claim but can see that they would be effective.

If a displacement hull is driven into the upper region of its "hull speed" this hook or tab or reverse deadrise or whatever it's called can help prevent stern squat. I use this effect for that purpose to ease the transition onto plane. It works.

[Willallison]

Originally Posted by FAST FRED

"Nordhavn, for instance tacked such things onto a number of their boats in recent years"

Probably to compensate for engine selections that are 200% or 300% oversized .

FF

Nordhavn are not usually associated with that kind of behaviour FF - Grand Banks and others have most certainly been guilty of it over the years though...

[BIGBOATBUILDER]

Hi everybody,

thanks for the replies, I was out of the office for few days. Sorry.

PAR & Williallison are right, non-planing means displacement boats in this case.
And yes some of them have scoops. I was referring to hooked buttocks on the last 10% of the vessels length of these boats.

Regards & thanks for the input.

Charlie