Hydrodynamics of inboards
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Category: General Correct Craft Discussion
Forum Name: Common Questions
Forum Discription: Visit here first for common questions regarding your Correct Craft
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=32935
Printed Date: November-28-2024 at 9:37am
Topic: Hydrodynamics of inboards
Posted By: john b
Subject: Hydrodynamics of inboards
Date Posted: February-28-2014 at 6:32pm
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Can anyone point me to a technical paper that discusses the physics of inboard boats such as our CCs. Until I saw Tim and Joe's boats at Green Lake I mistakenly believed that DD inboard ski boats were limited to speeds <60mph. There are so many forces that seem to be in conflict with this design. I can't get my head around some of them. Thanx. ------------- 1970 Mustang "Theseus' paradox" If everyone else is doing it, you're too late! |
Replies:
Posted By: GlassSeeker
Date Posted: February-28-2014 at 7:27pm
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more importantly...whats the fastest Tims boat will pull a barefooter? ------------- This is the life |
Posted By: desertskier
Date Posted: February-28-2014 at 7:37pm
| Flat bottom V-drives will easily go over 100mph. A flat bottom DD should be capable of the same. Just put a bigger motor in it and some dry stacks. |
Posted By: JoeinNY
Date Posted: February-28-2014 at 7:51pm
Ya might want to do a little more than that, less angle on the drive shaft would be a good place to start, steel running gear/prop, high speed rudder, and less hook on the back would be a minimum if you really want to push it. ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1477 - 1983 Ski Nautique 2001 1967 Mustang 302 "Decoy" http://www.youtube.com/watch?v=cO5MkcBXBBs - Holeshot Video |
desertskier wrote:Posted By: TRBenj
Date Posted: February-28-2014 at 8:20pm
Faster than just about anybody I know is willing to go! Low 60's, I'd guess. The boat is capable of mid 60's under the right conditions. This is the fastest I've clocked it, though I think its capable of a bit more with some carb tuning and hull tweaks.  John, I'd be interested in reading some articles too. Everything I know (which isnt much) has been gleamed from various sources on the web. There isnt a lot of widely shared hull theory, but some of the go-fast websites (screamandfly, offshoreonly) provide some clues. Lots of tribal knowledge out there. I think some of the principles that apply to go-fast outboards and i/o's are applicable, though inboards have more constraints and tricks surrounding the running gear, like Joe touched on. |
Posted By: john b
Date Posted: February-28-2014 at 8:41pm
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If I remember correctly Tim, you did 64.X on my iPhone on Green lake with me in the back seat. It was not a real speed run, just a quick roll on with no attempt to shift weight or try different water surfaces. There was some chop so that probably loosened it up a bit. A couple of the hard to imagine things going on are asymmetric thrust, prop shaft turbulence, and possible rudder stall from being at least partially shielded by the slip stream of the prop shaft and the unequal pressure from the p factor. Sometimes when I try to picture it I envision a condition where one side of the prop is actually producing drag while the other produces thrust. I know it is possible since my friends V drive Cole runs over 100mph in the 1/8 mile at Blarney Island. It is quite a different animal and very dangerous even by my standards. ------------- 1970 Mustang "Theseus' paradox" If everyone else is doing it, you're too late! |
Posted By: 8122pbrainard
Date Posted: March-01-2014 at 2:39am
NG, Besides some HP gain, what do the dry stacks contribute to the hydrodynamics? ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: SNobsessed
Date Posted: March-01-2014 at 2:31pm
They sound faster  ------------- “Beer is proof that God loves us and wants us to be happy.” Ben Franklin |
Posted By: TX Foilhead
Date Posted: March-01-2014 at 11:37pm
| To alter the strut angle very much you have to move the engine forward or down or go to a v drive. Lower strut agle would make it faster, but then you get more bow rise. Everything's a trade off, how fast do you want to go and how much handling and ski ability do you want to give up? |
Posted By: 80SN Barnfind
Date Posted: March-02-2014 at 12:52pm
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John, Take a look at anything by Savitsky, he goes well beyond the principles of planing hulls. http://www.westlawn.edu/ReferenceInfo/SavitskyPlaningHulls1964.pdf" rel="nofollow - Savitsky 1 http://academic.amc.edu.au/~psahoo/Research/Int.Workshop-AMC-2003/PSahoo/Hydrodynamic%20Evaluation%20of%20Planing%20Hulls.pdf" rel="nofollow - Savitsky 2 Noel ------------- Knowledge comes by taking things apart: analysis. But wisdom comes by putting things together. |
Posted By: SNobsessed
Date Posted: March-02-2014 at 3:42pm
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Wow, I wonder how many boat manufacturers did this amount of math. ------------- “Beer is proof that God loves us and wants us to be happy.” Ben Franklin |
Posted By: 8122pbrainard
Date Posted: March-02-2014 at 4:05pm
 I'd say not many especially in the early days. Gar Wood for example would design his hulls from his knowledge and experience. It was more of "seat of the pants" engineering.John, I've been trying without success to find the patent that was used to design the hull for my X55 Dunphy with the steps. The principal of getting more hull out of the water was an old one but for the X55 it worked. The 55 designation was for them wanting the boat to go 55. Mines close as it's been radar'd at 54. Steps in the hulls go way back to the gentleman's racers and then carried over to to the modern hydros. I remember one comment Reid said way back that he was going to take the chain saw to the bottom of one of his hulls and create steps!! I know he never went that far!------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: Gary S
Date Posted: March-02-2014 at 8:16pm
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Yet ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: 8122pbrainard
Date Posted: March-02-2014 at 8:33pm
Pending and I'm sure still in the back of his mind! ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: Waterdog
Date Posted: March-04-2014 at 7:25pm
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The Problem Child top fuel hydro set the world drag race record at 262 mph in a 1000 feet. THAT was a pass! ------------- - waterdog - http://www.correctcraftfan.com/diaries/details.asp?ID=3896&sort=&pagenum=2&yrstart=1978&yrend=1978" rel="nofollow - 78 Ski Tique |
Posted By: TX Foilhead
Date Posted: March-04-2014 at 7:46pm
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Problem Child isn't a flat bottom or a v hull, it's a hydro so totally different animal. It's also got a W drive 2 props with 1 motor. Oh and the 1000ft record is 3.39@258, I was there for that one.  They seem to have that boat dialed in now to run 3.50's at about 250mph, it's very reliable and they still have something left. Spirit of Texas can get into the 2.40's, but it they break a lot of stuff some time though they break Problem Child and get a win.
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Posted By: Waterdog
Date Posted: March-04-2014 at 10:50pm
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Hydrodynamics of inboards is the name of the thread so I though it out there. The Problem Child face book page says 265.52 is there best MPH If the Spirit of Texas can hit on the combination before they run out of money they CAN win the championship but it'll be a while before there in the 2.40s (I know ya meant 3.40s) Texas has a LOT of hot rods (how bout those Texas Fuel Altereds) ------------- - waterdog - http://www.correctcraftfan.com/diaries/details.asp?ID=3896&sort=&pagenum=2&yrstart=1978&yrend=1978" rel="nofollow - 78 Ski Tique |
Posted By: TX Foilhead
Date Posted: March-05-2014 at 12:11am
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It is kind of funny how almost every Fuel boat is either based in TX or has a driver from TX. It's a pretty big deal locally, I know at least one person who races in every class all the way up to Alcohol Flat, and mr Erlich lives just up the road in Round Rock. I was trying to keep things pointed in the hull differences that we have, flat and V. The W drive is a whole mother ball game and as far as I know that is a custom piece. Last summer Eddie Knox was going to rebuild one and had every thing ready to crack it open, but unfortunately a guy I work with was on the rope so I had to leave the pits to watch him run. I really want to know what's inside that box, and the next time I get a chance I won't leave. I think they may have made some quicker and faster runs, but haven't backed them up for an official record. As for the Spirit of TX, I've seen some absolutely amazing driving by Scotty over the years and I know they are the only ones that seem to be able to push Problem Child, unfortunately last year they were so far back all they could do is try to take out PC. They purposely sand bagged qualifying in San Angelo to meet PC in round 1 and ran a quicker time but red lighted. They came back in Marble Falls and qualified 1 and then all hell broke loose on Sunday with the broken drive shaft. Can't wait for things to get started again, another month or so till it's game on. |
Posted By: Waterdog
Date Posted: March-05-2014 at 8:47pm
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Back to tow boats. All of them are designed to wet a lot of the hull to track straight. While the guy at the end of the rope is doing his level best to knock it off course. Joe and Tim pretty much over power the friction of the water to get to 60mph. Most will do 50 with a warmed up engine. 300 hp should get ya to 50, 400+ to 60 is pretty much common knowledge. Pretty soon after that the stock underwater hardware would be seeing its stress limits. When I worked on the F-4 Phantom we would say with enough horse power you can make a brick fly ------------- - waterdog - http://www.correctcraftfan.com/diaries/details.asp?ID=3896&sort=&pagenum=2&yrstart=1978&yrend=1978" rel="nofollow - 78 Ski Tique |
Posted By: TRBenj
Date Posted: March-05-2014 at 9:37pm
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Not only is a larger wetted surface advantageous for tracking, but spreading the weight of the boat over the largest possible surface area makes the smallest wake. That sort of design methodology makes for plenty to overcome when it comes to pushing these tugboats faster. Thats one of the reasons the hydrogated 196 (and possibly 206 and 216) offered in the late 2000's have pretty good speed potential- they essentially have removable hook, allowing them to lift the bow at high speeds. They arent small boats and require a lot of hp to push them faster, but there is speed potential there. If youre using a 2nd gen SN (70-81) as your benchmark hull, 300hp will get you over 50mph, but Im not sure that 400hp will guarantee you see 60mph. Alan's '81 and Hotboat's Martinique were in the 380hp range and were 56-57mph boats. Newer/larger hulls, starting with the BFN and 2001 will require higher power levels to hit those speed numbers. Both of those boats start to reach hull limitations in the mid-upper 50's that require even more hp to push through. Joe's '83 probably is pushing a tad north of 500hp to touch 60mph, and I'm not far behind... though the v-hull BFN does a better job of getting up and out of the water than the flat bottomed 2001. Bill Burgess shared a few secrets on here as to how he got his '80 SN to run fast (allegedly 72mph on the radar). It included turning a steel speed prop (11x15ish) a lot of RPM, as well as a shallower shaft angle. Not sure what he had for hp, but he did have nitrous on the boat. |
Posted By: TX Foilhead
Date Posted: March-06-2014 at 1:27am
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Here you go, roughly same size boat as ours 20ft and an slight V bottom. Notice the expected Hp and the expected speeds. Since it's a GN mold I suspect it will handle and not just go in a strait line. If they don't get really carried away with the rigging my guess is the price would be about the same as a 200. http://www.riverdavesplace.com/forums/showthread.php?106665-Connolly-21-V-Drive" rel="nofollow - http://www.riverdavesplace.com/forums/showthread.php?106665-Connolly-21-V-Drive |
Posted By: john b
Date Posted: March-06-2014 at 4:07am
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I'm still here, I have been watching this thread and have nothing to add since I really don't know anything. I'm also still trying to muddle through the Savitski papers. Thanks everyone! ------------- 1970 Mustang "Theseus' paradox" If everyone else is doing it, you're too late! |
Posted By: TRBenj
Date Posted: March-06-2014 at 9:25am
Im surprised that they only expect 75-80mph out of 700hp. |
Posted By: TX Foilhead
Date Posted: March-06-2014 at 9:56am
| Those guys will run an automatic transmission which probably takes more Hp to turn and then add in a drive shaft and a drive and a very long prop shaft and you loose a lot more Hp by the time you get to the prop. Also figure that's in some very nasty water where you can't stay on the throttle all the time. |
Posted By: 8122pbrainard
Date Posted: March-08-2014 at 8:22pm
NG, I'd still like to know about the dry stacks. ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: TX Foilhead
Date Posted: March-10-2014 at 3:11am
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Not an inboard, but I think everyone would be happy with about half what this thing does. %20" rel="nofollow - http://m.youtube.com/watch?v=HZ5sLPcWsDY&sns=fb |