What causes hull porpoising?
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Category: General Correct Craft Discussion
Forum Name: General Discussion
Forum Discription: Anything Correct Craft
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=4823
Printed Date: May-15-2024 at 7:27am
Topic: What causes hull porpoising?
Posted By: 65 'cuda
Subject: What causes hull porpoising?
Date Posted: September-21-2006 at 7:06pm
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My barracuda is a porpoising hull, and I would like to minimize or eliminate it.
Below 33-34 mph the boat behaves very well. A couple of miles an hour above that and a small swell will start it bouncing. Wide open running (42 plus), and it will porpoise all by itself on gla$$y water. No, it does not have water soaked foam. It does not have any foam at all. The previous owner put two small tabs on the stern and said that improved behaviour some. I'm thinking about manual (turnbuckle style) trim tabs or "smart tabs". I just don't want to chase my tail here and wind up with no solution and holes in the boat to repair. Is the down angle of the prop partly to blame? At high speeds does this try to push the bow down more than the ideal planing angle for the hull? Does this start the process? Would providing lift at the stern with tabs balance this tendency or make it worse? Any insight would be helpful. Thanks! ------------- Gary http://www.correctcraftfan.com/diaries/details.asp?ID=941" rel="nofollow - 1965 Barracuda SS |
Replies:
Posted By: 79nautique
Date Posted: September-21-2006 at 7:28pm
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I believe you want to but a little more down angle on the tabs to bring the nose down, If you bring rhe bow up it's going to make it worse I believe. Throw some weight in the bow and try that. If it gets worse then I'm wrong but I doubt it.
What prop do you have? when I went to the ACME it helped mine out compared to the MW that was on it. ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique |
Posted By: 05 210
Date Posted: September-21-2006 at 7:40pm
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I'm pretty sure bow rise compounds that problem significantly.I would think you'd want the bow to ride lower to lessen this,but not so low that you encounter bow"steer".I'm a newbie to ski boats,but on a PWC trimming the pump nozzle down in effect lowering the nose corrects this situation,and shorter hulls have more tendency to experience this.
Mike |
Posted By: Munday
Date Posted: September-21-2006 at 7:48pm
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My drag boat experience says your right about the down angle,what I think happens is;your boat trys to climb up on an angle the same as your shaft.But lacking the horse power to carry the nose that high it falls forward which slows the hull down a bit then the process repeats.If you trim the nose down with tabs or ballast what your doing is slowing the hull down which will stop the porpoising.
Also running into a light wind chop will free up the hull enough to limit or stop the porpoising sometimes. Be very careful as I've seen boats swap ends if you don't have center fins and the boat gets high enough the prop can walk around the boat on the surface of very near it. just my .02 good luck Munday |
Posted By: scott8370
Date Posted: September-21-2006 at 7:50pm
I think you should try the smart tabs. Did you try wet sanding the hull? , maybe that will help------------- Scott |
Posted By: 65 'cuda
Date Posted: September-21-2006 at 8:01pm
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I've already added weight to the bow. I moved the battery from under the pa$$enger seat to in front of the lifting ring support, it is a series 31 so its pretty heavy. This did not make it better and may have made it slightly worse ( I coudn't say for sure, there was too many month's between the before and after).
------------- Gary http://www.correctcraftfan.com/diaries/details.asp?ID=941" rel="nofollow - 1965 Barracuda SS |
Posted By: 79nautique
Date Posted: September-21-2006 at 8:18pm
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gary
What RPM's are you seeing with the current prop? you might be able to add some cup to it to help out, seems pretty low speed wise when it kicks in. The adjustable tabs are the way to go, either the expensive ones or the manual turnbuckel style is the route to go. I would try add additional weight even if it's only temporary to verify your heading in the right direction. ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique |
Posted By: stang72
Date Posted: September-21-2006 at 8:36pm
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My mustang will do the same @higher speeds...it is worse with riders in the rear seat...better when it's just me and one rider up front...wieght towards to bow seems better for me. I do suspect some soaked foam...and will find out this winter!
Also...I have heard changing the engine angle can solve the problem...thrust is changed. Of course that is tricky and more involved...I'm not sure how things are aligned...might be worth investigating! ------------- stang Face plants are not that funny when it's you face! http://www.correctcraftfan.com/diaries/images/3720/photo1.jpg" rel="nofollow - The Super Air |
Posted By: Tim D
Date Posted: September-21-2006 at 9:58pm
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I always thought porpoising was caused by too many rpms, causing the rear of the boat to lift out of the water above its plane angle (which is determined by hull length) and causing the bow to dip down in the water and then bounce back out. We all have fixed drive shafts which point downward, when spun fast enough, it would eventually lift the back out of the water too high. I know ReidP has a Mustang that will hit the high 50's which will porpoise. ------------- Tim D |
Posted By: Hollywood
Date Posted: September-21-2006 at 10:10pm
Come on people, add more weight to the bow and you won't bounce as much. It's not that hard. ------------- |
Tim D wrote:Posted By: scott8370
Date Posted: September-21-2006 at 10:10pm
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I had a 71 Skier that did it only when there was someone in the back seat. ------------- Scott |
Posted By: stang72
Date Posted: September-21-2006 at 10:16pm
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I do think wieght way up in the bow would help a lot...fat sack with about 100lbs as far up as it will go!
I will give it a try! Prop can make a bit of diff too...when I switched to a 13 x 13 it became noticably better. ------------- stang Face plants are not that funny when it's you face! http://www.correctcraftfan.com/diaries/images/3720/photo1.jpg" rel="nofollow - The Super Air |
Posted By: boat dr
Date Posted: September-21-2006 at 11:39pm
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after installing the velvet drive,durbumatic is all almuminum,i added 50 lbs to the stern.above 35 mph it to hopped. added a set of the s/s smart tabs.cant remember the guys name but he had all the right ans.do not install them on the port and starb. edge,move them closer to the center line.several gas cyl. are available to fine tune the hopping,and not get the bow steer. took a little adj. and fine tuning but this rabbit dont hop no mo........... boat dr ------------- boat dr /diaries/details.asp?ID=4631 - 1949 Dart /diaries/details.asp?ID=1533 - 1964 American Skier |
Posted By: scott8370
Date Posted: September-22-2006 at 12:11am
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Gary, try the smart tabs. If they don't work, maybe I'll use them on my Sidewinder. We can patch the holes when you bring your boat in for the gelcote work. ------------- Scott |
Posted By: skicat
Date Posted: September-22-2006 at 8:22am
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I think the trim tabs should help out as well. On my crownline if I have the motor trimmed up (bow is higher)& get into some rougher water, it will start to bounce as well. If I trim it down the bounce will go away (bow is down more). Trim tabs should act similar to that. Now if I can just get the skier project done! ------------- Greg http://www.correctcraftfan.com/diaries/details.asp?ID=2427&yrstart=1981&yrend=1985" rel="nofollow - 86 BFN |
Posted By: TRBenj
Date Posted: September-22-2006 at 9:16am
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Before you go drilling holes, I would try a new prop. Ive read that porpoising can be caused by not having enough pitch. A quick search on this site for "porpoise" or "porpoising" should give you some good info. Looks like several members have reported that the new Acmes lower the bow/raise the stern compared to the original props. Some members claim that the Acme stopped the porpoising altogether.
At the very least, youd end up with a superior prop when its all said and done! ------------- |
Posted By: 77stang
Date Posted: September-22-2006 at 9:20am
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my boat never did porpois until i moved the battery under the bow for space considerations. After that it porpoised pretty bad. moved it back to factory location and it stopped.
its been my understanding that as your speed increases your planing platform decreases and becomes less able to sustain bow weight without slapping back down. that stated you'll hear several folks tell you to fix porpoising by adding bow weight. i cant argue with that - ive heard too often where it solved someones problem. i guess bottom line is your boat is out of balance either by design, weight distribution, prop angle, flexing hull, or permanent hook. your job is to identify and correct. ------------- http://correctcraftfan.com/diaries/details.asp?ID=1360&yrstart=1971&yrend=1975 - '77 Mustang 17 |
Posted By: David F
Date Posted: September-22-2006 at 9:53am
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77Stang:
What you say makes a bit of sense to me. I am really clueless to what cuases porpoising and how to fix it. With that said and with what you said, could it be that adding weight to the front simply forces the bow to stay in the water (i.e large planning platform as you say). Now, if the boat is balanced properly, the boat can get on plane and support the bow out of the water no problem. Upset this balance by moving the battery way forward of windsheild and maybe you cause the problem. I also think that moving a battery is a bigger weight transfer than just the actual weight of the battery because you are moving balancing weight from one place to another which sort of has a compounding effect if that makes sense. So, maybe I would try moving the batter back to original location and then adding a bit of weight in the rear to keep that nose up. Again, just my thoughts and I will admit I am clueless on this subject. ------------- |
Posted By: 79 Mustang
Date Posted: September-22-2006 at 10:04am
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I recently traded my 79 mustang for an open bow ski supreme...if you find my diary entry ( A red and white Mustang from Indiana) you will get the whole story on the trade....and also will learn that my hope is to find a sport someday (hoping for a deal like Randy-in-Ohio received).....in the mean time....a story about porpoise in my off brand open bow:
After buying the Supreme we were happy as far as size and space, but the boat had a terrible tendency to porpoise at 33+ mph....it was impossible to reach WOT. After considerable research I purchased a Bennett "WAKE TAB". This item is available from Overtons (edit of location). The Wake Tab is an hydraulic adjustable single trim tab that is mounted in the center of the boat. It is made for inboard applications. I spoke with a Bennett engineer about my issues and discussed the advantages / disadvantages of twin tabs mounted on each side of the boat vs. the single mounted tab in the center.....the single tab has less of a tendency to disturb the wake pattern, however, it also does not allow you to "level" the boat side to side if your boat has a list to one side or the other depending on your load. I have been thrilled with the center mounted "wake tab". I also installed a dash mounted indicator gage for position of the tab. At 25% deployment this tab takes all of the porpoise out of my boat and the boat will now reach WOT and almost 50 mph. Prior to the wake tab speeds above 35 would cause severe porpoise to the point of making the boat dangerous to drive. The wake tab did not disturb the wake much...but at higher angles of deployment it does cause a minor rooster tail to form. We have also noted an improved barefoot table as the tab breaks up the prop turbulence a little. Installation of this tab has also improved fuel usage as the boat now planes quicker and at lower speeds, with less throttle. The addition of the tab has also improved rough water ride as I can now force the bow V into the waves using the tab. Driver / rider comfort has also been improved as the tendency for the bow to run "high" has also been eliminated. Here are a few pictures of the TAB mounted on my (I am sorry it is an off brand) Ski Supreme. I too had a marina owner tell me to start tweaking the pitch in the prop and to add a little cup to it. His comment was that maybe I did not have enough prop to carry the nose of the boat with the 300 HP 360 engine. Prop adjusements are something I have yet to play with....however, these adjustments will cause a loss to top end speed but maybe a little better hole shot is what the marina guy has said. I can not yet speak to the prop, however, I can speak for the wake tab and my satisfaction with it. I have attached a few pictures of before the tab and then after the tab on my boat. Hope this helps. Scott 
http://i86.photobucket.com/albums/k118/sshepher6630/tab2.jpg 
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Posted By: 77stang
Date Posted: September-22-2006 at 10:07am
That makes a lot of sense and i do now that the shorter the boat and narrower the transom the more delicate balance is maintain ------------- http://correctcraftfan.com/diaries/details.asp?ID=1360&yrstart=1971&yrend=1975 - '77 Mustang 17 |
Posted By: Hollywood
Date Posted: September-22-2006 at 10:09am
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How far down is that last picture? I'm guessing 100% deployment? I bet that barefoot table is nice! ------------- |
Posted By: scott8370
Date Posted: September-22-2006 at 10:12am
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That tab even looks good on your boat. ------------- Scott |
Posted By: 79 Mustang
Date Posted: September-22-2006 at 10:18am
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Hollywood....Yes, The last pic. is 100% down. Rarely get to that setting....at 100% it begins to act a little like a rudder and also starts to cause bow steer to occur. Yes the barefoot table is nice.
It also gives the wake a little different shape at wake board speed and at slalom speed the stern is lifted enough that the wake flattens out very nice. Barefoot table is flat and smooth. Some change in turbulance also. Scott 8370...yes...I too was pleased with the look. |
Posted By: 79 Mustang
Date Posted: September-22-2006 at 11:13am
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Overtons is the purchase place...here is a link to the item:
http://www.overtons.com/modperl/overtons/detail/pdetail2.cgi?r=detail_view&item_num=71185 - http://www.overtons.com/modperl/overtons/detail/pdetail2.cgi?r=detail_view&item_num=71185 |
Posted By: 67nautique312
Date Posted: September-22-2006 at 12:45pm
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Gary,
My hull design is alot like yours and i had the same problem..... I bought a drive shaft that was an inch longer than the factory one. This in turn moved the prop closer to the rudder pulling the bow more level and giving the boat more steering control 
I dont know if mine is at 100% performance but it's alot better than what it was before! With the old school hull design like we have i think we are limited on Performance but Damn those old boats look good. Nice boat Gary!! 
Paul, ------------- 1963 Classic (handed down to my daughter) 67 Riviera 68 barracuda 1971 Ski Nautique Promo 86 Silver Nautique 1995 Ski Nautique |
Posted By: 65 'cuda
Date Posted: September-22-2006 at 5:24pm
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OK, it looks like I may have moved too much weight forward by putting the battery up front, however I'm stubborn, and really don't want to move it back. Maybe a lighter battery? Oddysey or Optima dry cell? Any body want a lightly used high capacity AC Delco Marine/RV Battery?
What are the effects of a longer prop shaft and an Acme? Right now I'm running a federal 13 x 13 Cupped. I'm really thinking about the smart tabs. The bennett wake tab looks interesting but pricey and a pain to install. I've tried to post a link below for much better pics. (after the resto). http://s96.photobucket.com/albums/l192/garyandliz/?action=view¤t=DSC01065.jpg&slideshow=true&interval=3 ------------- Gary http://www.correctcraftfan.com/diaries/details.asp?ID=941" rel="nofollow - 1965 Barracuda SS |
Posted By: skicat
Date Posted: September-22-2006 at 6:00pm
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Nice looking cla$$ic Gary! ------------- Greg http://www.correctcraftfan.com/diaries/details.asp?ID=2427&yrstart=1981&yrend=1985" rel="nofollow - 86 BFN |
Posted By: 65 'cuda
Date Posted: September-23-2006 at 5:01pm
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Better link?
<A HREF=http://s96.photobucket.com/albums/l192/garyandliz/"pics"</A> ------------- Gary http://www.correctcraftfan.com/diaries/details.asp?ID=941" rel="nofollow - 1965 Barracuda SS |
Posted By: The Dude
Date Posted: September-23-2006 at 6:06pm
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I had a tab on my 87 Supra Saltare. I'm pretty sure it was original equipment. It was a manual tab...I could only make adjustments manually when it was out of the water. I played around with it a lot. I never had porpoising problems, but with tab adjustments there were significant changes in the wake. With it down the table got smaller and softer. It wasn't quite as big as your bennet tab, but mounted in the same place.
There's a lot of trivial info for you! ------------- Mullet Free since 93 http://www.correctcraftfan.com/diaries/details.asp?ID=717&sort=&pagenum=1&yrstart=1991&yrend=1995 - 95 Sport |
Posted By: reidp
Date Posted: November-12-2006 at 2:11pm
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It appears from posting activity that I may have nothing better to do today, as it's windy and somewhat cool for NC, but this is another subject that's haunted me for years in the quest for speed, so I've researched a bit over the years. It might get lengthy.
Munday was pretty much right on from everything I've ever read as far as cause; the reoccuring oscillations of bow lift and then not having the abilily to maintain the angle so the hull drops back down and starts all over again. See these two explanations: http://www.glen-l.com/weblettr/webletters-7/webletter58.html#notebook http://www.ba$$boatcentral.com/bbcdictionary.htm But shaft angle in a inboard like ours actually serves to push the bow "down", and changing that angle won't affect the problem. The angle of the shaft in regards to hull angle has to to with that "for every action there's an opposite and equal..." saying. Direct the flow down the bow goes down, direct it upwards, the bow comes up, basically the premise behind a trimmable system. Straight inboards aren't as efficient speed-wise as outboards and I/O's in part because of this downward thrust angle. If you've ever seen the jackshaft setup used mostly by sportfish boats, they take advantage of the typical inboard engine placement for balancing, then run a shaft back to the transom to an outdrive. This setup will outrun and burn less fuel than a typical inboard drive system, but has it's cost and drawbacks as well. But back to the porpoising and stopping this motion, if you can't fix it with weight transfer, then tab(s) is the only the way to go, unless you want to redesign the hull. Correct Craft, like others, attempted to design the older hulls with a sensible compromise between comfortable ride (no bouncing) and efficiency (best speed per rpm). They could have designed them with even more hook in the very aft bottom portion, which some apparently need, but with a significant tradeoff in light load speed and other handling nuances. Guys who are NOT concerned with how fast your boat runs, you can stop reading here: Personally, because I want her to run faster, I'll take the loaded boat porpoising and off-set it with tabs, and then have the best speed while lightly loaded. As the Glen-L designer noted, some porpoising is desirable in a higher speed boat. I know some of you guys are familiar with the Jersey Speed Skiffs. Well if you put the tab down and stop the bouncing, the boat slows down. Along that line, the CC hulls that porpoise a bit have the highest speed potential. Same reason that most of these older ones will run faster with a full tank or body or two in the back seat than when riding by yourself. Less wetted surface as the nose is lifted. And don't ask me why, but it's widely known that two hulls popped out of the same mold can end up with different ride characteristics in regards to porpoising. So in essence, while this thread is/was focused on the problem of porpoising, I've spent several years on several hulls (Mustangs) trying to GET them to porpoise, or more specifically to lift the bow and run faster. Basically, I'd like to have a CC Speed Skiff. The '63 Cla$$ic we picked up last year has a uniquely designed full width tab on it the size of a swim platform. See pict below. My guess is that very early in its existence, maybe in tandem with or a result of the 390 engine install, they encountered the porpoising. So while I had an idea why someone had installed it, I had no idea just how well it worked. Upon first running her, she planed off immediately with minimal bow rise, and laid out a table so flat there was hardly no wake at all, but plowwed like crazy and when you went to turn you almost had spray in your face at the helm. I've adjusted it up as high as it's rigidity will allow attempting to bend in 2 directions, but you could place half of our members in the back seat and still not get it to bounce. I'm at a crossroads with it.  ------------- ReidP /diaries/details.asp?ID=231&yrstart=1971&yrend=1975 - 1973 Mustang |
Posted By: 65 'cuda
Date Posted: November-12-2006 at 5:13pm
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Reid, thanks for the input, I've made some changes to get the weight back to where it belongs, and I'll get a few more lbs out of the ront of the boat this winter. I'm also going to do a tab of some kind, manual or hydraulic. Unfortunately testing will have to wait until spring. On your other topic, YES I would like the boat to run faster! I have the M318 polyhead in her. Where are some areas I can get some HP out of that motor?
Thanks ------------- Gary http://www.correctcraftfan.com/diaries/details.asp?ID=941" rel="nofollow - 1965 Barracuda SS |
Posted By: 79nautique
Date Posted: November-12-2006 at 6:07pm
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Man I woud love to sit down with you and bill and kick back a few coldies and talk about this subject, hull design/lift ,,,,,if there ever was a GOD on this site ,,,,,,,Mr Burgess is it and you would be a close second if not equal put he has forgot more than.... you know the saying
but you new comers have no clue, Doc don't even try. go to orlando and check out the 70 something dog and that's as close as you will ever come. Bill is still the man, Mr SS I would love to hear how his season went glad to see you still enjoying these cla$$ic and wish I knew a fraction of what you have forgot over the years. Hopefully I'll make to orlando this year, somebitch by the time i got done you edited yours, reid, and now i have to go back and start over well not really, Reid what do you think??? could we really get Bill to come over to your place and kick back with his busy schedule and shoot the sh*t over coldies? ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique |
Posted By: reidp
Date Posted: November-12-2006 at 8:11pm
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Chris,
Don't know how/when you're dividing your time btwn IN and FL, but I'm planning to stop by and see jbear around Dec 26. I've spoken to Bill Burgess several times on the phone and would love to drain him of some additional knowledge while down. But I only drink coldies once in a while. Once in the morning and quite a while at night. Gary, While that engine is a fantastic one, you're probably already aware that few of the parts interchange with the later LA engines. Thus performance mods get in the pocketbook pretty deep. From time to time you see alum intakes on ebay. But the next upgrade would be a camshaft and those aren't available unless you get the existing one re-ground. After market or hi-po heads, not out there. I found a really neat guy and his company in VA (check out Mopar Mecca) which specializes in Polys and other Mopars, but it comes with a price that was up there. He has Poly stroker kits that can take it to 360-380 CI and 300-400 HP. If you were planning to do a rebuild, you could make certain changes like the cam regrind, higher compression pistons, a little head work, an intake, and you wouldn't be talking all that much more than the standard rebuild and get probably to the 275 HP range, but as for simple bolt-ons, you're quite limited. I'm big on the engine because I like originality and it's a neat looking engine with those valve covers and all. (See these below currently from Ebay)I'm game to help you in any way I can.  ------------- ReidP /diaries/details.asp?ID=231&yrstart=1971&yrend=1975 - 1973 Mustang |
Posted By: MaddMarxx
Date Posted: November-14-2006 at 12:07am
79...you are so right. I live right by the ramp, he parks his truck on ramp and blasts across the lake and back in like 30 seconds, drives back on the trailer and goes home, All that is left is the smell of his rocket fuel..
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Posted By: Munday
Date Posted: November-14-2006 at 9:31pm
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Boy now that cav plate is old school.Sure eliminates rudder cavitation I bet.Somebody spent a good deal of time fabbing that.I saw a guy once took a Ranger I/O ba$$boat back to the factory because of hull porpoising.When I next saw the boat I looked under it and I swear it had a wood shingle gla$$ed under each side at the transom line.And the guy loved it.
Course he had trim too. Munday |
Posted By: jbear
Date Posted: November-14-2006 at 9:38pm
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Mark: Another nice pic from Orlando. I met Mr. Burgess there and really enjoyed talking to him.
john ------------- "Loud pipes save lives" AdamT sez "I'm Canadian and a beaver lover myself"... |
Posted By: skiinxs
Date Posted: November-15-2006 at 10:36pm
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Pretty simple to stop, just need to put the bow down. As has been mentioned, add on tabs, either fixed or adjustable will do the trick, although not the norm. Typically tournament ski boats are fine tuned by adding or decreasing the hook on the bottom. If the boat runs level now, an equal amount of hook on both sides will keep it level while putting the nose down a little. What you really want to find out is how much to add before you actually do the gla$$ work. A really easy quick way to find out how much you need is to add a small bead of silicone sealer on the bottom at the very rear edge on each side. After the silicone drys, run the boat and decide if you need more, less, or if you have it right. (Still porpoising try more, no porpoising, try less until you find just the right amount.) Believe it or not, the bead of silicone will exactly duplicate what an equal amount of hook added with gla$$ will do and is really easy to experiment with until you find the right amount. Once you have it right, measure how much hook you need and add the same amount of gla$$.
Dave |