A couple hundred pounds of sand in the bow or a water ballast tank works too.

I think Fountain has a water tank in the bow of the new Poker Run boat for that reason..but don't quote me...

So whats better? Trim tabs bottom flush or set up 1" from bottom?

Also, BIGJIM, its the Muddy Mo through Omaha NE.
L8r
Thanks guys![/QUOTE]
I thought that looked like the Muddy MO... I frequently boat on the MO. What's the latest on river levels this year?

As far as your original question; I'd start by changing your prop or by having Throttle-Up adjust it. It might fix your problem without adding tabs.

Any thoughts on having the TABS follow the "V" of the hull or have the TABS be mounted parallel to the water at the height of say an upper strake ,but not as high as the chine??? Most of the Fountains I saw at the NYC boat show in '03 had TABS mounted that way,parallel to the water. DO you think a 26' scarab with a lightweight HIGH H.P. S/B and an Arneson #6 would need 1/2" of rocker over the last 3' before the stern to make this hull work ??? What about continuing the lower strake, which now ends about mid-ship, all the way to the stern??? Thanks obads 31

Most definitely the only way to mount a high-performance trim tab is with a vertical offset above the running surface. If you don't you have the ability to generate zero lift at zero angle of attack, with the entire surface (no joke on a K-380) wetted. Although you are a ways back from the leading edge of the hull, there is still considerable momentum loss to the boundary layer in contact with the tab surface. This translates to increased drag and slower speeds. Even if you pull the trim tabs up past zero degrees deflection, water flow will stay attached to them and make the turn to run up the tab. Mounting them 1" above the surface is what I have seen on everything, including the military craft I have worked with, and gives you a step to enforce a clean separation at low deflection angles, and allow just the tip of the tab to drag when you want.

I don't think there is much debate on mounting horizontal to the vs. parallel to the running surface. In fact, I wish I'd thought of that first.... When you mount the tabs parallel to a deep-v hull, you automatically throw away the component of lift acting horizontally (Lift force acts perpendicular to the tab surface). Cosine of 24 degrees is 0.91, so you pitch away 9% of the lift for the same amount of drag. Additionally, you get wierd steering and non-intuitive roll effects, especially on narrow boats. My own boat is only 5'10" wide with a 24 degree deadrise and has K-380 planes. I can pretty much steer with the tabs, and they produce the opposite rolling moment as what you would expect since their lift vector is aimed pretty much below the vertical center of gravity of the boat. That would all go away with mounting them horizontally. If hacking apart my transom to remount them wasn't going to be such a pain, I'd do it. The other argument I've heard is that horizontal mounting minimizes chine-walking. The folks that realized this run keel pad boats which are very prone to chine-walking.

Hope all this makes sense and is somewhat useful. Sometimes it's hard to make generalizations since every boat reacts slighly differently. When I wrote the integrated code which makes the performance predictions for SpeedboatPerformance.com, I left off the effect of trim tabs. I did this not just to save processor power, but mainly because I think the optimum setups don't need the tabs except in the midrange. If you go run the program at the above mentioned website, you will likely see that your engine RPM is predicted pretty much dead on in the top end, and it may be slightly different in the midrange, but right on again below 35 mph. This is because we are simulating what your boat would do without the tabs down, and without excessive trimming from the drives. The code is already so horrendously complex, but we have a very powerful server that can run it in a matter of seconds now. The initial versions took several minutes on a PC. Adding the extra layer of iterations to find the optimum trim tab setting for each of the thousands of iterations required to come up with the prediction for even one speed would bog the server down, make the user wait, and for what? To tell the user how he could go slower with his trim tabs down?

Anyway, have a good night,

Tcelano

Sorry, didn't see the other parts to the last question.

An ASD on a 26' scarab would probably work, but it depends on what is being done to the boat. I would expect you to have to shift weight aft to make this work. If you are using one of those big transom-mounted drop boxes Arneson makes, it might just do the trick. If you are moving the motor forward in the boat to add a transmission or something (bravo conversion kit??), seriously consider shifting some other weight aft if you can. I'd install everything first and run it before doing anything radical, expensive and irreversible to the hull. A highly raked prop might do what you need without getting into big-time bottom modifications. You are in a better situation making the conversion with a single than with twins, as your prop can be positioned relatively low.

Lower strakes: Depends on how low they are. If they run near the keel, there isn't enough cross-flow to make them effective near the transom. Most of the lift of your boat is generated right where the hull meets the water (the stagnation point). In fact, some of the spray root actually gets pushed back forward, causing stagnation pressures even higher than you expect from simply converting flow energy to pressure using Bernoulli's principle. This is why you can run with so much of the bow out of the water and balance on the center of pressure. Also, low strakes will likely just disrupt the inflow to your propeller and make it run worse.

wow!! that made me dizzy !!

tcelano,
To confirm what you were saving in post #25, if you were to remount your tabs today, you would mount them parallel to the water, not parallel to the vee...correct?

Yes indeed, I would mount them parallel to the waterline, especially on narrower boats or keelpad hulls. Be sure to check drive clearances, because without the tabs pushing outward, they are more likely to interfere with the propeller(s) mounted this way. Keep that 1" offset as a minimum at the outer edge to prevent the effects stated above, and don't worry about the 4 or 5" gap on the inside. The tab will drop down and the tip will hit the surface of the water just fine.

I can't recall seeing K-280's mounted this way. I'm not sure if you'd have enough chord (length) to make it work effectively. Somebody on here can probably comment on it.

TCELANO Thanks for your reply!! My '81 scarab 26' is one 0f 10 built with a factory Arneson set up. When I bought it the boat was all original with lots of extras for this size boat. Fresh water tank and sink in a tiny galley ,a frig, a potty ,a mirrored foreward bulkhead, stereo w/ speakers that weighed 10 lbs each, etc, etc. I ripped /stripped out all that crap ,stripped out every wire hyd. line control cable,gauge,rub rail, cleat,ANYTHING that would move is GONE !! The MTR was a High PO PLeasure Craft 280 HP 350 S/B ,mounted BACKWARDS ,mated to a "72 C" Velvet Drive "V DRIVE" with a drive shaft going to the coupler on the ASD#6 which is direct drive and mounted VERY low in the "V". The transom had the remnants of trim plates that went from chine to the "V" and then the same on the other side. I sent the drive out for a rebuild and the same for the trans. I bought a new VORTEC 330 HP 350" and then STainless exhaust,an AIR GAP manifold & carb, MSD distributor,etc.I'm also building a ALUM. 427 S/B with the intent on keeping the weight down as much as possible. I am a little unsure of the ultimate potential of my hull. IF you put a 6' straight edge at the stern, along the keel ("V") it is gapless the whole length. IF you go 1' up from the "V" on either side ( 1'up fore & aft) there is about a 3/16" hook in the last 12 to 18" before the stern. When you do the same type of measurement at the one (and ONLY) strake that runs about a 1' in or down from the chine there is a similar amount of hook. ALSO that full length strake stays parallel to the keel for only about 3' .It then starts to follow the warp of the ever increasing "V" shape going toward the bow. It seems to me that a lot of the "ULTRA" performance "STRAIGHT V's" have a lot of the bottom, the strakes,all parallel to the keel for a lot more of their hull length than my hull has(or dosen't have) WOULD a lighter rig have less wetted surface and then need less of the bottom to be parallel??? Would'nt an Arneson work better on any hull with some ROCKER built in so that the prop 2 or 3' off the stern could control the amount of BOW LIFT you wanted for different conditions ??? Lots of different questions here!! Any thing you can Explain would HELP!! THANKS obads31