Long & Skinny boats - British Seagull racing

Thanks PAR for the dignified response. Apologies for resurrecting an old thread. As an exponent of these (top-fuel dragsters of the kayak world), I would have been remiss if I hadn't taken the opportunity to throw in my tuppence. If only one viewer is more familiar with sprint kayak hullforms/speed potential as a result of my rejoinder then it served it's purpose. These boats are definitely outliers in the small displacement boat world and your assertion that they don't represent the preponderance of paddlecraft is, of course correct; much in the same way that Sailrocket a (boat that can only reach in ONE direction haha) is so far removed from a weekend cruiser in his Beneteau as to be a different animal altogether. Although, I suppose the same could be said for Seagulls as well in the context of small powerboats. My comments weren't entirely immaterial to the thread, to the extent that I honestly believe the Seagull could become a much faster boat if it appropriated some of the parabolic/elliptical sectional shapes and volume distribution of these specialized kayaks. So in that respect I would say we are both correct. And yes I might have given ply construction short shrift, as evidenced by the aforementioned sprint kayaks built by the Danish concern Struer, whose mahogany boats are quite simply works of art and successfully pull off tightly radiused curves, although the ply is probably tortured half to death in the process.

 

If you really, truly had 4 hp to work with, I think planing boats would already dominate the class. So something isn't right. You need to give us an accurate weight-of-everything at the start line, the distance of the course, and the fuel consumption over the course.

You also need to provide a table of the motor's WOT performance on a couple of your boats. Need rpm, speed, conditions, and if you have to ability to do so, fuel consumption and manifold vacuum. Do this at your starting weight - 50 pounds and again at starting weight +50 pounds.

How much effort have you put into propeller development? What are the prop's specs? What is the exact gear ratio (tooth counts)? Can you post a closeup of the prop and lower unit below the plate?

If you are stuck with a stock prop, the motor may not be able to push itself through the water at 20 knots, let alone a boat. One thing a long boat does do is provide better inlet conditions to the prop. If the motor is tapped out rpm-wise, this may be all you can do to get a little extra. Just trying to think of reasons that long, skinny boats seem to be doing well in the class. I think it would be worth a try to run the prop partially in a tunnel to improve inflow, and shorten the boat to save weight and windage, not to mention the convenience of storing the thing.

Another test I would do is to tow the boat with a bigger one and see what speed causes the seagull (running WOT) to produce zero thrust and kick up. You can find this by just pulling on the top to see how much thrust is being created. Measure the rpm at this point also.

 

Okay I've re-read and ruminated on your replies to the O.P. PAR and it has left me confused and needing further clarification, if you would be so kind as to indulge me. It would seem that you are suggesting the boat actually have flatter sections amidships in the interest of encouraging early planing? The OP seems unsure as to whether the boat is beginning to transition to planing mode or not. At 15mph with a 3-4 hp powerplant, (displacement unknown) can planing be initiated with flatter sections/more waterplane or is an even more efficient displacement shape the way forward for additional speed? (which was my supposition) Is it possible the boat is just trimming up (squatting)? and not really initiating planing mode? Would moving the CG forward be of benefit here? I honestly can't tell from the pics if the powerplant is outboard or some kind of novel stern drive. If greater speed can be achieved in displacement mode, then my previous comments re. sectional shapes may have some validity, but if it needs to plane in order to go faster then I guess the hard chines for flow separation aft and even flatter bottom might be better, as you seem to suggest. Given that premise, seems like the flatter (less dihedral) bottom would have to graduate into sort of short-chord Rhombiod sections forward to maintain fair curvature, and have a reasonably (but not quite so radically) fine entry. The wedge shape in planform also makes sense from a planing standpoint but if lower resistance is the goal it seems the occupant would sit further forward with an outdrive throttle extension and the stern would taper without much of a transom. . I quess the meat of the question (and I will defer to your expertise here) is whether the boat is on the verge of planing and could achieve this at 3.5hp with just a bit more encouragement from the hullform, or whether it would benefit more at this juncture from lower resistance (ie high prismatic, sections optimized for lowered wetted surface),.. Are their other design considerations in play here that I'm missing? I'm kind of a dilettante and have never seen a Seagull in person so I may be barking up the wrong tree, There may even be one-design restrictions I not aware of re. construction LWL etc. Is the only requisite concern single occupant and wimpy motor?