mcrsr

on my boat, a 32 pachanga, the trim tabs are mounted even w/ the bottom of the hull, when the tabs are fully up, they are dead parallel to the bottom of the hull(imagine extending the hull straight back w/ the tabs- is this costing me performance? the boat runs 73 mph now, and i would think when you start to trim the boat out and the bow rises the tabs are contacting the water- is this the case?? do i need to raise the tab rams to get the back of the tabs up, or is this insignificant?? help!!!!

zt260

What kind of tabs? They should not be level with the hull. Normally they are an inch or so up, and they retract a few degrees beyond the hull line. As for speed loss I would guess you could be losing a couple mph. If you can remove the tabs temporarily you will know for sure.

mcrsr

unfortunately they are bennets w/ twin rams- iguess it could be worse and i could have boat leveler!!! anyway, why did they put the tabs so low anyway??

zt260

Just an improper install, happens far too much.

tcelano

Yes, like ZT260 says, they are normally mounted 1" above the running surface, and can retract to 3-4 degrees above parallel. On some boats, however, including several Fountain models, the boats are set up with the Center of Gravity very far aft, which is good for top speeds, but bad for stability. These boats would porpoise like crazy at midrange speeds, and probably right up to near WOT without any tab deflection. Anyway, where I'm going with this is that some mfg's install a collar on the trim ram to prevent retraction of the tabs above the baseline, and ensure that the tabs are always deflected at least enough to avoid a dangerous condition. It also helps shield their reputation from inexperienced drivers...

At any rate, it is better to have the tabs mounted above the running surface so that A) they will have some angle of attack and be producing some lift when they first make contact with the water, B) to allow the tab to be dry, and avoid the speed robbing viscous skin friction associated with the tab area, and C) to avoid creating bow lift due to flow attachement which actually can "suck" water upwards when the tab is retracted.

If you want to do some reading, search through the archive posts. I uploaded a research paper on this topic based on scale model testing at the U.S. Naval Academy's Hydromechanics Lab. I was doing some work with the Navy's Special Warfare assault boat at the time.

zt260

As you know, the bennets are not very good. I would replace them with better tabs. Obviously you would mount them correctly and this problem should be gone.

I bent a set of bennets 3 years ago, I replaced them with 380 K Planes, but they run big $$$. The difference is amazing.

mcrsr

thanx for the help, i was considering taking the tabs off-leaving all screws in place and leaving rams attached and trying boat to see if there is any difference-does this sound like a worthwhile test???