A Tale of Two Hulls....

I've got a question about planing and semi planing hulls that has bothered me for many years.

In the illustration attached below I've shown two V hulls of similar shape other than for the lower rear run.

The first is certainly a displacement only hull. I've noticed in pictures and real life that this sort of rear shape seems to pull up a very small stern wave or wake as it slides through the water.

The second would seem to be more of a planing hull or perhaps a semi planing hull. I've also seen first hand this shape in use on many boats that have no chance at all of planing. In operation these hulls pull up a massive stern wave that froths the water up marvelously and leave strong wake waves behind. And all at some 6 to 8 knots.

So, now to the question. Am I missing some element here that seems to promote a planing hull shape for boats that will never plane? Or is there a semi planing speed that favours the B shape over the A? And if so then what should the semi planing wake look like?

For an identical length and beam how much faster and how much more power does the semi planing hull need to outdo the displacement type?

If it helps with any calculations required to show the differences the hulls in the sketch are intended as 22 foot by 8 foot small cruisers that are supposed to look classical.

I'm playing with a tradeoff between trying to achieve the best fuel mileage but I'd be willing to trade it off a little for some extra speed.

 

Can you try attaching the image again?
(Sorry - for some reason a couple people have been experiencing problems with attachments this week, but I haven't been able to figure out a common issue yet and all my test attachments worked fine using IE/Opera/Mozilla...)

 

Ah well, that's what happens when you get called away for "duties" while typing....

The picture...

 

According to Thomas Firth Jones, about one horsepower per 50 pounds displacement is needed to plane a properly designed hull. Only about 1/10 that much power is needed for a displacement hull, and a semi-planing hull, attaining 2-3X hull speed, would have an intermediate power requirement (with appropriate hull design).

Your "B" design looks like a crude planing hull to me. I am assuming that the transom is immersed at rest. I have seen situations like you describe, a planing hull which will never actually plane in use. I always figured that such a hull, with its straight aft sections, would be a familiar shape, easier for a yard to build and, thus, cheaper to buy for those not interested in maximum efficiency.

 

Your figure looks like a typical displacement hull on top and a typical semi-displacement hull on the bottom to me. The displacement hull will require slightly less power at speed/square root of length (S/L) equal to one, but the power to speed curves of the two will cross over between 1.3 and 1.35. At S/L of 1.34, where most displacement hulls overate most of the time, the two hull forms require about the same power.

The other benefit of the semi-displacement hull is with about 4 times as much power, you can achieve S/L of about 1.75 and the wake will look like what you described. With much more power, it can achieve S/L of 2 to 3. This gives the semi-displacement hull a little bit of a sprint capability with modest power and without most the detrimental side effects attributable to a true planning hull. For example, the semi-displacement boat can carry as much payload as the displacement hull (although if overloaded it will not achieve S/L = 1.34) and seakeeping will be comparable to the displacement hull and much more comfortable than a planning hull.

One benefit of the semi-displacement hull is for coastal cruising. Typically, coastal cruisers like to make daylight passages, especially in the PNW where there are floating logs and crab pots to watch out for. At S/L = 1.75 you get 30% greater daylight passage range than at S/L of 1.34. Likewise, S/L=2 provides 49% greater daylight passage range.

Additionally, semi-displacement hulls operating at S/L of 1.75 to 2 tend to smash through choppy seas and react much less than planning hulls and a little less than displacement hulls.

In very heavy seas the semi-displacement hull must slow down just like the full displacement boat, however, since it can exceed S/L = 1.34, it can surf down wave fronts when running with the seas and has better directional stability than a full displacement hull.

Regards;
Mike Schooley

 

Mike, thanks so much for your reply. I was away for a few days over the weekend and came home to your comprehensive but confusing answer. I'm new to this marine design nonsense so be assured that it's me and not your answer that's confused...

The Speed/ Length Squared bit... what are the units of length to speed? That would let me use your figures to appreciate the speeds involved for a given length of boat.

At the cross over point of 1.3 or so you say that the power requirements would be roughly equal. For a 24 footer of about 8 foot beam can you offer any ball park power figure in HP for this? You then mention the bit about requiring 4 times that power. I'm just trying to get a handle on some figures for such an arrangement. 4 times the power seems like a LOT of extra fuel use and hardly seems worth the extra tankage in a smaller cruising boat but I need to see the speed figures before I can wrap my mind around the aspects that such speed would bring. Bumping THAT up by "a lot" to get up to an S/L of 2 sounds totally out of the question to my mind and would indicate the desireability of a true planing hull at that point. Not so?

I just always assumed that the stern wake lifting up so high and frothing up like that was a huge waste of petrochemical. But from what you're indicating here it's not THAT bad. It's not great but it's not that bad. Right?

But I still like the idea of the displacement hull shape tied to a rounded tugboat like stern. The drawings were done from Carlson's Hulls program and I have yet to figure out how to add the extra points that I see in some design files that let you form the hull to a closer to real shape. But if there's a real advantage I suppose I could be talked into a proper semi displacement shape.

I've read over the transportable cruising thread and see that the design of that is very much a semi displacement so I figure that you guys know what you're doing as you all seem quite concerned with fuel mileage (knottage? ).

 

The length is in feet and speed in knots. For a 24 ft boat the displacement speed would be about 1.34 * (24)^.5 = 6.56 knots. The beam will affect the power required, but not the displacement speed.

I don’t have the equations handy or the time to do it, but maybe someone else on the list could. If you have a copy of “Voyaging Under Power,” Beebe provides curves that allow you to estimate power based on weight and displacement.

Yes, but it is small compared to the 10X power that a planning boat would require to go 3 to 4 time S/L. There is a saying regarding boats, You can go fast or you can go far but you can’t go fast very far. Portager will have a range of 2,600 NMi at 8 knots or 760 NMi at 12 knots. That is a big reduction in range, but if I am trying to get from one Harbor to another before dark or bad weather it might be nice to have the option.

I guess I would agree with that. It will cost more to go faster, but sometimes it might be worth it.

I do to, but until I retire I only get 5 weeks of vacation a year. If I can go a little faster I can stretch my vacation a little.

All I know is, in the power boat ranks, there are many more planning boats than semi-displacement and more semi-displacement and displacement. I don’t know if that is a statement on boat types or a condemnation of our way of life.

Regards;
Mike Schooley