Displacement boat - speed

My foil is a horizontal blade on two vertical pylons. It's just forward of the lower leg.
I thought Shannon Yachts is simply using an inverted V and hook on their runs, to keep the bow down and trim reasonable. This would seem to pressurize the chines (or at least delay bleed off), much like you over size chine flats Tom.

 

Ok, thanks, I thought it was that, just not clear from the illustration how wide it is.

 

That is close Paul, although my chines don't just get wider aft but deeper also. Whether that is the whole reason Bluejackets have zero stern sinkage at all speeds is not completely known but that was the intent. http://bluejacketboats.com/

 

I'm not completely familiar with your design Tom, though have some experience with wide chine flats. So the BlueJacket series is a warped bottom? This would make sense, given the pressure wave will progressively increase, as flow moves aft. If chine flats are made wider, it has a similar effect as decreasing deadrise, so plane efficiency goes up to a degree, within certain speed ranges, but particularly at lower speeds. At least this is my take on it.

The wing on Floom is nearly full width of the stern section, which was necessary because of buttock angle and hull volume aft. It allows displacement and semi displacement speeds, in a hull that without the wing, would be restricted to displacement speeds only (she'd squat way too much once attempting to push through 1.5 S/L). After quite a few tank pulls with different configurations, I arrived at this shape and gained more than I expected with available HP. It also has a unique affect on the wake, especially at speed.

 

http://www.ayd.it/press/Superyacht-38_autunno-2013/Super-Yacht_MYTIC-30m_sett-2013.pdf

 

Asymmetric is a mistake: in rough seas stalls

 

Duco84's vessel has a very slim & nipped in stern. PAR's hydrofoil is fitted to a relatively wide stern & the Mytic has a flat bottom aft. Tom's Bluejacket is rightly acclaimed as a very successful design incorporating those clever chines. Would such a hydrofoil prevent stern squat at around 7 to 10 knots? https://www.youtube.com/watch?v=AyFrls0mg-M

OTOH, trying to graft a bustle to the stern of the OP's boat, would be as exquisite as the rear end of a Willendorf Venus.

http://www.ancient-origins.net/site...igurines-europe-paleolithic.jpg?itok=K5UWbq-r

 

Good to have some others interested in getting past that distracting "hull speed" issue. Paul, Perry and windraf, thanks for the discussion and other links. My basic hull is a monohedron with 10 degrees aft deadrise. The chines are added and do somewhat resemble a warped bottom. Most warped hulls hold the chines level and raise the keel. Mine is opposite to that. (I'm waiting on comments about the deep aft transom creating drag) The idea was to get the boat up as quickly as possible with minimal squatting. The chines were to act like low drag trim tabs and have positive trim angle of one degree relative to the monohedron bottom. Kind of stumbled into several other benefits and the boats do better than I might have hoped. If you look at the sequence of photos of all speeds from slow to WOT, there is no stern sink whatsoever and max trim is about 3 degrees at WOT.

Maybe this resulted because I started with a full planing hull and attempted to make it run well at slow speed rather than starting with a displacement hull and trying to speed it up. That was a conscious decision probably because most of my experience was with planing boats. I think it is only possible if hull weight and therefore bottom loading is closely controlled. I have had interest from a mfg in making the Bluejacket is fiberglass but I'm not really interested since I'm sure the result would probably be disappointing.

The C Dory 22 is a case in point. It weighs the same as my BJ24 but is much smaller and requires almost double the power to plane, mainly because the cute dory form has a very small bottom and consequent high bottom loading. With a 90hp it is about 7mph faster at WOT. It also has the usual hump getting on plane and does not have the manners of BJ in the low cruising range of 11- 17mph. This is not a bash of what is actually a decent little boat but just a comparison of what works well in that speed range and what does not.

Walt Schultz has a patent on reverse deadrise but I don't know what it actually claims and his boat looks a bit like the one Perry offered. Paul's boat looks less extreme.

 

Seems you and I came to similar assumptions along different paths Tom. Chine design is tricky, really tricky. In moderate speed craft, just about anything can be asked to suffice, but when you start to push the envelop a bit and hedge toward longitudinal instability, drying out the chines, drag reduction, just to squeeze another 2 MPH out of it's top speed, you start to look at stuff, previously scoffed at. This is when testing and observation come to play. It takes a lot of passes and playing around with abysmal ignorance, to realize something. Maybe you'll get lucky, but most of the time it's simply an incremental set of adjustments, that result in improvements, often quite small by themselves, eventually resulting in tangible, maybe noteworthy differences that count. My foil development was over a pretty long time frame and started with a very small powerboat, with twin foils (fore and aft) and playing with incidence angles to get it to run level. This lead me down other paths, eventually to take Floom's warped bottom displacement hull, but with a long clean run over the theoretical speed threshold. It would have been nice to just get lucky, but it took several versions of the hull and wing combo, to get it to work. Now, it looks simple, but you didn't see the ones that drove the bow under.

 

I do not expect my design to be suitable for high speed. Just what high speed is, I don't know but caution builders not to power them beyond recommendations. My chine is definitely not of the non-tripping variety so hot dogging with a big power plant is not recommended. I think 30mph would be the limit for best use and 25mph is more practical. The 24 footer with a Yamaha 50 has done 25mph on a few occasions but 22-23mph is more usual. This is pretty good speed for a cruiser anyway. Fuel use in the mid range is about 5mpg and longer cruises always work out to a bit less than 2gph. Got to hold the weight down near specs to get that performance though.

 

Duco84, I don't know if this will help but here's some advice from William Atkin to a man whose fantail yacht was having a problem similar to yours. I just now happened upon it while reading through an old annual of The Motor Boat magazine, http://books.google.com/books?id=cf...ce=gbs_ge_summary_r&cad=0#v=onepage&q&f=false , on page 949. He recommends "squad boards" and includes a design for same for the 24' boat in question with his reply.

Of course the pros around here will have their own opinion of this device but if I may it could have this in its favor: it would be a period correct modification, one that could look right.

 

I'm not sure how much faith you'd like to put into "Billy's" treatment on the subject, seeing as his career was essentially over in the 50's, dieing in the early 60's, before the '63 series tests where conducted. The annual you where reading was written in 1922, so I'd have to guess we've learned one or two things, in the nearly a century since.

Tom, high speed is when a St. Christopher medal isn't enough and you want St. Christopher himself, sitting in your lap while underway. I consider high speed the point any particular hull form/power combination pushes the boat to the edges of stability. This is where you need to know what you're doing, as a small mistake can result in a catastrophe.

 

What I meant by high speed was in relation to instability or other unsafe conditions of my boats which fits well with your definition. My solution is to stay well away from that point with the bluejackets. I found some instabilities with warped hulls in my testing in the 1990's. Some instability is putting it mildly since one model actually yawed so badly that it swapped ends and broke the towing rig. My rig did not tow from the bow but from the CG and actually allowed yawing to show such instabilities.

 

Oh, I'm not here claiming that they would be some panacea. Only that they were something "period correct" that had some net positive benefit (or else Atkin wouldn't have recommended them).

Modern takes on the idea add buoyancy out past the stern, not just boards placed to try to provide a physical counter force surfing the water as the stern goes down.

Though if one were to build a board that tried to look old school but had a cored construction like they use with modern surf boards, hmmmmm....

Would probably require somewhat stronger to-hull struts.

 

We're on the same page Tom, there's a very broad line, usually drawn with a well used crayon that defines it, but most stay well below with cruisers. I don't have any designs that come close, except for racers, which edge along it hoping for the best in some cases. I'll also bet we've found similar results with testing, as well.