What causes hull porpoising?

Mark: Another nice pic from Orlando. I met Mr. Burgess there and really enjoyed talking to him.

john

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..

   

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?

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.

     

Better link?

<A HREF=http://s96.photobucket.com/albums/l192/garyandliz/"pics"</A>

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

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

That tab even looks good on your boat.

David F wrote: 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.

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

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.

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!

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.

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.

Tim D wrote: causing the rear of the boat to lift out of the water above its plane angle

Come on people, add more weight to the bow and you won't bounce as much. It's not that hard.

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!