Smart Tabs question

[jim dozier]

I decided to start another post instead of hijacking a previous one. Regarding Smart Tabs I have a couple of questions. They sound like they would be a useful tool for slow to medium speeds, especially for getting on a plane, and I have and still do consider them an option for my 17.5' semi-V pad hull with a 115 hp Evinrude. I can believe that they would not slow a boat's top speed down. But if you have a boat that needs bow lift and isn't porpoiseing, how would they increase top speed. It seems that they might either lift the stern causing the bow to sink increasing wetted area and slow the top speed or they might do nothing if they were adjusted to back off at full speed. This isn't meant to be a criticism just a question on the mechanism for increasing top speed.

 

[JB]

Re: Smart Tabs question

Boat topic.

 

[Jdeagro]

Re: Smart Tabs question

jimd;<br /><br />You will need to explain your situation a bit more. The last thing you want to do is drive the bow down too far when running at higher speeds, and this is exactly why we provide the ability to adjust the lift pressure to the individual boat.<br /><br />It is also why we have more travel in the actuator than required to bring the tabs to level. By allowing the tabs to come up beyond level, we can insure that the pressure never exceeds what it is adjusted for.<br /><br />Are you saying that your boat runs too flat at high speeds and you can not bring the bow up with the motor trim?

 

[umblecumbuz]

Re: Smart Tabs question

When a boat's sitting in the water, it has a 'footprint'. This is the surface area of the hull in the water. But the 'footprint' is not the only thing that slows a boat. The volume of the hull in the water also plays its part.<br /><br />Clearly, the more you can reduce this footprint, along with the immersed volume, the less water resistance will need to be overcome by the boat.<br /><br /> As the boat begins to move forward, the shape and size of the footprint alters, with the bows rising and the stern sinking to compensate. Because the stern sinks, the immersed volume is still substantial. <br /><br />Properly adjusted trim tabs keep the stern higher in the water without allowing 'bow-plough'.<br /><br /> Although the bows lower a little, the footprint of the lowered bows of a typical v-hull is less than the equivalent footprint at the stern without trim tabs. The immersed volume is also significantly less. <br /><br />These two factors - a smaller overall footprint and less immersed volume - are what allow a boat with properly adjusted trim tabs to accelerate faster and reach a higher top speed.

 

[jim dozier]

Re: Smart Tabs question

OK, I am still testing props and engine height. I posted the speed results in another post and Nautijohn responded that Smart Tabs might improve top speed.<br /><br />Intial run with the 18 pitch 3 blade with the AV plate at 1.75" above pad top speed 5700 rpm 47 mph. Reset jack plate incrementally so AV plate 3.25 inches above pad. Top speed 18pitch 3 blade stainless 5900 rpm - 49mph (GPS avg upwind and downwind).<br /><br />My boat is an 1987 semi-V hull with a pad bottom. My impression is that the boat needs bow lift. This may be due to some combination of hull design, age, or hook. The boat does not porpoise and currently I have to trim out quite a bit to get maximum speed. That is, the speed seems to increase the further I trim until I get blowout. It may be that I need to test some other props. I have been able to test 2 through the good graces of some experts I have met here.<br />The boat is doing well now outside of some high speed torque steer which I may correct with some Torque Tamers. I am not going to set any records but its fun and 10 mph faster than the old DT85 I had on it.<br /><br />My guess is that Smart Tabs would improve my hole shot, low and mid range cruising, and ability to pull water skiers, but I am wondering if they would actually add any top speed. Still fishing's explanation makes some sense but I still wonder if you push up the stern won't the boat go down on the bow. This is an advantage on takeoff, its an advantage if the bow is too high, but is it an advantage on a boat whose bow is not too high at top speed?

 

[biloxiriver]

Re: Smart Tabs question

Although I don't fully understand the science, the affects they had on my Tiger Craft which is a 21' CC inboard 302 shaft drive (weight about 3600 # loaded) are:<br />1. It gets up on plane faster (less fuel);<br />2. It stays on plane at lower rpms (less fuel; and,<br />3. For whatever the reason (science) it runs a little bit quicker at WOT (went from 32 mph at 4200 rpms to 35). But I sure don't run WOT very much because that 4bbl Holley loves gas at WOT.

 

[Jdeagro]

Re: Smart Tabs question

Jimd;<br /><br />I hate to say this but if the bow is downat this speed and you can not get it up enough with the trim before you blow out the prop, then I am suspecting that something i the hull design, balnce of the boat, or a hydrofoil is the cause.<br />If you do not have a hydrofoil on the motor then put a streight edge on the bottom of the hull near the stern to see if there may be a hook (concaved). <br /><br />Also Raising the motor too much can minimize the ability to bring the bow up, not to mention the water pickup issues. <br /><br />As for the tabs, at top speed they are retracted and although they are under pressure it is far less than when deployed at slow speeds. <br /><br />How does the boat steer at high speeds? Does it dig the bow in and lean hard?

 

[jim dozier]

Re: Smart Tabs question

I wouldn't say that the bow is down at speed so much as it isn't way up. I believe there may be a little hook at the rear, some of which appears to be designed in, particularly at the outside (left and right) edges of the hull semi-V. The pad is flat and the hull is straight near the center. The boat handles well at speed and there aren't any hook type steering problems. If I trim way down past where I normally would the hull veers a little to the right but I don't usually go that far. I guess my question regarding Smart Tabs was this:<br />If a boat is handling well at speed and NOT suffering from too much bow lift and NOT displaying related handling problems like porpoising, would Smart Tabs have any affect good or bad on top speed. My intuition says that they would have no effect on this kind of boat if properly adjusted. This is not to say they wouldn't benefit this boat in the holeshot and lower to mid-speed cruising. And I can certainly see a benefit for Smart Tabs at top speed if a hull was too high in the bow, was porpoising, and requiring excessive downward trim for speed and stability.

 

[Jdeagro]

Re: Smart Tabs question

jimd;<br /><br />Since the Smart Tabs run at less effective lift pressure when cruising, and since this is adjustable the added control should give you more speed. If the hook in the hull is enough to keep the bown down more than it should then any additional lift may restrict speed. When running at WOT the boat should ride on the rear quarter. If the side splash indicates that the boat is ridiing low in the fron the splash will move forward. <br /><br />By reducing the pressure on the Smart Tabs for WOT speeds, the low end performance ( towing and accelerating to plane) should still be significantly better with the tabs