Spray patterns, 21 and 25

[Bushwacker]

Originally Posted by cdavisdb

I've always been puzzled about why my 25 is so extremely dry riding. It sure doesn't look like it should be, with such a sharp and vertical sided nose, hardly any flare. Watching Moose's videos of Dave's 25 and Terry's 21(Gathering thread, page 10), I got a few clues(thanks Moose). I'm not real well versed in the technical aspects of this stuff, so would appreciate some feed back.

Looks like chine position and shape is key. Watching the 25 run in small chop, the hull throws spay out and back. In relatively flat conditions of the video, it exits the hull way back from the bow. I've watched it running in rougher condition and that doesn't seem to change,at least until its too rough to watch. The chine must be at just the right angle to deflect spay outward as it comes up the hull side. Since the chine rises fairly high at the bow and goes essentially all the way forward, it must continue to do the same deflection as the waves get bigger. That deflection keeps the spray away from the windshield, even in a moderate cross wind.

The 21 doesn't have that chine forward. In low chop, it looks like the 25, the front of the bow is riding above the waters surface, spray is thrown back, out, almost down. In a little bigger chop the bow starts to spit the wave and the spray pattern changes, getting much more vertical. Bushwacker said the spray went up vertically when the 21 hit a big wave. That's consistent with what I could see.

Another fascinating question is comparative rides of the two hulls. The the 25's chine has to provide some lift and make for a harder landing than without it. With no chine and a super sharp bow(sharper than the 25, believe it or not) will the 21 provide a softer head sea ride in the nasty's, even though its smaller and lighter?

The 25 was Moesly's last design before he sold the company to Potter, so it's the latest evolution of the VDH design. I believe it was preceded by the 27' Seamaster, and I've noticed that the middle panels on both of these hulls flare out to a conventional lifting strake or "chine" about 2" wide up forward at the vertical step between the middle and outer panels. This strake disappears back about amidships, so I think Moesly added it to create a drier ride, just like he added the ~1" wide flat at the chine up forward when he modified the 19' Bowrider mold to create the 20. This flat also disappears about amidships. I don't remember if the 27 and 25 also have this conventional lifting strake at the step between the center and inner panels, but I suspect that Carl had learned by the time he designed the 27 and 25 that the deadrise up front was so high that a narrow flat spray deflecting strake up forward didn't significantly affect the ride, provided it disappeared by the time most of the hull was in the water at amidships.

The spray on Blue Heron's 25 pictures looks like it's being deflected horizontally by a flat strake, while the water coming off the inner and center panels on my 20 in the picture below, which has no strakes ala the 21, is simply following the angle of the inner panels. It depends on the outer panels and flat at the outer chine to knock down the spray, and the spray at the outer chine does look like it's being thrown out flatter than it is on the inner panels. My experience with the 20 is that if you get wet, it's because you're going too slow! It seems to depend on the flatter outer panels being high enough out of the water to deflect the water coming off the inner panel. It definitely rides drier than the 21 (but not as softly!) and it's wetter than the 25. The 21 has no flats anywhere, and as Terry said, it was designed to be raced and staying dry was not a consideration! Carl said that he intentionally designed it for very little dynamic lift, which I interpret as no lifting strakes and minimal flare in the bow. He wanted it to slice through the waves instead of lifting over them.

I think the more sloping bow on the 20 and 25 may also be responsible for their excellent handling in following seas, because as you run into the back side of a wave, the displacement and resulting lift as the bow is immersed would not increase as abrubtly it does in the 21 and 19 hulls that have a more vertical stem and less flare.

BTW, Carl loved that shot of my 20 when he saw it, and said they had spent HOURS at Lake X back in the early 60's trying to get a shot like that of the 21!