Spray patterns, 21 and 25

[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?

[kmoose]

I think it depends on where the wave is hitting the hull at the time. I also think the 25 stands a way better chance of bridging the gap on stuff like we were in Sunday. I plan on mounting points on the side of my rig to get more stable video like that taken from the windshield mount the next time some of us can get out together.

[kmoose]

I edited the original to show just the Seafari running along side.http://youtu.be/K4E1yGq6q50

[Blue_Heron]

The bow of the 25 is pointier above the chine than the 21 and hangs way over the point of entry. More flare too. The windshield stays pretty dry unless you trim her up too far in the slop and she falls into the next wave. Keep her nose down and you'll never stuff the bow. Some pics off Sebastian from the Long Point gathering:

[gofastsandman]

I got to see the 25 Seafari on the ramp. Really interesting. Sick deadrise. I can see why she lands so softly. Very steep all the way up.

The 21 is really steep on the pointy end as well but she blends back a bit more to her stern.
The steps are also higher, which may help with surface tension.

I think that is the secret of the 21. And she is really deep.

Really.

[Islandtrader]

A couple of things...

At one time Mr. Moesly once said, a bigger boat will normally ride better than a small boat.

The 25 was designed after the 21, so there should be some improvements build in.

Really the 21 was designed to go racing, and punching through head seas was important and getting wet was not.

The other thing I noticed from the pictures and I think Sandy and Denny can attest to it...Blue Heron and I think Connor are always standing up when they are punching thru the waves. On the 21 when I was driving and when Sandy was driving we never felt the need to stand.

Last of all this information below comes from a guy who has tested the 25 and 21 very throughly.

Quote:

Cleaning up my home messages and wasn't sure if I answered you. The boat is up and running and we have ventilation problems with the outboards that Jamie positioned somewhat between the keel and mid hull panels. Next week we're modifying the outboard bracket to move the engines furthr apart like I've seen on some of the old photos. We think if we clear the riser between the keel and mid hull panels we will see cleaner water. To counter the weight of the Armstrong bracket and twin Suzuki 140s, we've had to put 500 pounds of chain in the bow to get her to balance right. She runs great in a head sea, plunges a little too much to my liking in a following sea. Overall, very good forebody but not as good as the SeaCraft 25'. It probably could use more flare in the bow and a big spray rail forward between the chine and keel. What realy surprised me is how softly she landed when launching off 3' foot chop on top of 4' long swells. Considering the average deadrise is about 20 degrees compared to the 26 degrees of the SeaCraft 25', she is almost as soft!

The 21' is quite impressive and with the mods we've made to the 25', including a production 30' based on a geosim of it, it is obvious Moesly was a great designer. We are working on a high seakeeping 24-26' RIB with high displacement of ~ 8500 lbs. Based on our tests on the 25', she simply tops out at a displacement of 7500 lbs so it does not make a good parent craft. I was thinking of stretching the 21' to about 24.5' and was curious about the differences between the 21' and the Potter 23'. I have limited experience from riding the 23' over 30 years ago and as best I remember it wasn't that impressed with the ride and remember getting soaked. I need to know more about the 23' to see if stretching the 21' will detract from her seakeeping.

We have used a digital camera and 3d software to create a very accurate solid model of the 21 and 25 and the differences/similarities are amazing. If you keep the keel panel of the 21' as the baseline parallel to the static waterline, the mid and outer panels slope down as they move aft, about 1.5" over 6'.

Anyway, didn't mean to go on so long BTW, the serial # of my 21' is 111.

[parrott]

The old 19's are also the same way as the old 21's. No reverse chine. Looks like it would slice thru the water. Spray rails are gonna be a must.

[cdavisdb]

Terry, thanks for sharing those comments. I had not seen that info, great stuff.

I think it was you first posted the comparison of the lines of the 21 and 25. Looking at that, the idea of stretching a 21 (I'd make the bow more like the 25) sounded like a great idea. Too bad I don't have the dinero.

On standing vs sitting, at least for me, its more a function of how well I want to see. I usually stand up for transition times and when I need to see real well. Maybe one addition to that. When crossing a steep wake at a sharp angle, its a lot easier to control the boats lean if I'm standing up. The boat's head sea ride is fine sitting, at least up to a Gulf Stream 4 ft at 20 knots.

[kmoose]

Connor, I wish I would have gotten out offshore in your boat to feel some comparison on how my 23 rides compared to the 25 in similar seas on the GOM. I am really starting the the thought process on how I could modify a 25 to match the layout I currently have and whether the benefit would be worth the expense and time. What are your thoughts on comparitive sea handling between the two?

[Blue_Heron]

Originally Posted by cdavisdb

On standing vs sitting, at least for me, its more a function of how well I want to see...

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