thsorens

I have an old Webcraft 23 ft Cuddy with a 383. It has the sport style seating with the big sundeck/hydralic engine hatch which puts the rear couch well forward of the engine, but still this thing drives like it is incredibly ass heavy and porpoises teribly at WOT unless I stab the outdrive almost all the way down. Would Tabs help me, and if so what size tabs should I use on a 23' boat? I have no trouble getting on plane, or with side to side balancing, just porpoising. If I put two adults down in the cuddy, and 2 adults in the captains chairs, it rides like a dream, but hard to find volunteers for that!

Sydwayz

Tabs will not fix porpoising, but they will help you control it. I would suggest a system from Bennett Trim Tabs for your application. When you contact them, you will probably talk to MJ Thomas, who is extremely knowledgable, and Bennett sets the standard for excellent customer service in the boating industry. I imagine they will suggest the ST-12 or ST-16 tabs. ST line:
http://bennetttrimtabs.com/sport.htm

I had the ST-12s on my 26' Sonic. I upgraded to the ST-16s when I had the opportunity since I had to remove them once. They both worked great, but the 16s obviously had more effect than the 12s. Some folks here know the 26' Sonic can porpoise depending on load and fuel. Knowing how to drive the boat, and set your drive and tab trim controls it. It won't fix it, as someone else without seat time in that boat, won't know how to control it until they play with the settings.

Contact info:
http://www.bennetttrimtabs.com/help.htm

throttleup

Sometimes the right prop will reduce or eliminate porpoising. The fact that the boat is stern heavy and putting people in the bow helps to reduce the porpoise indicates that a stern-lifting type of prop may help your situation. The prop you have now could be worked to increase the stern lift which may reduce the porpoising.

Matt.

thsorens

I Havn't played with props much. I had a nice Mirage 19" pitch that worked pretty well with the old 260, and actually didn't popoise AS bad, but that wasn't near enough prop for the 383. I have a generic 3 tab SS 21 on now, that runs about 5000 RPM with the new motor, but porpoises like a Mo-Fo! What type/brand of prop do you consider a stern lifting prop? Sounds like maybe a new prop and a set of trim tabs would "help" my problem. Fact is, the shape / design of this boat is the real problem, but kinda hard to fix that without poppin for a new Baja!
Tom

1BIGJIM

Hey THORENS what river is that? Looks like my boat in the background

tcelano

Porpoising is caused by having the boat's apparent center of gravity, after all moments induced by propellers, trim tabs, and the hull itself, aft of where the center of lift pressure due to stagnation of the flow under the boat wants to be. You can make it stop by either shifting weight forward or deflecting the trim tabs. For shiphandling reasons, often a forward weight shift is not desirable, as you don't want to come down nose-first into a wave trough.

I have done extensive research work and testing on this exact subject, including the effects of the tabs. One very interesting thing I learned is that when computing the edge of the stability envelope, based on hull loading, speed, and deadrise angle, deflecting the trim tabs has the compound effect of not only pushing the "virtual CG" forward, but it unloads the hull as well, pushing you further into the stable operation regime. This sounds simple, but I was able to prove that it works exactly as expected. I did that by instrumenting a model K-380 plane to determine its exact lifting characteristics operating in the wake of the boat.

I don't want to push the envelope of advertising on this forum, but if you go to www.speedboatperformance.com, the online calculator there is actually based on determining the critical running trim angle just on the stable side of the threshold of porposing. After adding in the aerodynamic lift and drag forces, hydrodynamic drag due to the drive units and propellers, then accounting for the transmission losses from the motors out through the propellers, you can come up with a very accurate estimate of a boat's performance.

For more interesting reading, check out the 1998 editiion of SNAME (Society of Naval Architects and Marine Engineers) Transactions, volume 106. My article on the above subjects starts on page 269. That work, as well as more recent things I've done predict behavior observed in most boats where instability occurs at midrange speeds, but magically goes away at the higher speeds.

fund razor

Dam good answers.

tcelano

I couldn't get an electronic copy of the paper to post, so here is a scanned one:

tcelano

And the rest:

thsorens

OK TCELANO, you are into this WAY to much man! I hope you are making some money in boating, ie manufacturing/selling performance parts or you are a column writer for a performance boating mag! WOW! Very impressive knowledge, but give me the skinny. Will tabs help my boat or not? I know, I probably need to do the math, but give me your gut........yes or no, and how big?

Also, BIGJIM, its the Muddy Mo through Omaha NE.
L8r
Thanks guys!