Re: Making progress- got it to 70mph
Originally posted by LubeDude:<br /> no way in He*l youre going that fast with the engine at that trim angle in that chop with only that amount of roostertail, Period. I wernt born yesterday.
Thanks.<br /><br /><br />My first impression was that even if you were born at nite--- it probably wasn't last nite.<br /> LOL.<br /><br /><br />Last week, the lake was so darn rough, I couldn't hold it for long-- even hanging on with both hands after letting go of the steering wheel, but that's what the GPS says. On previous runs with the same trim settings, it did 63.3 at 5200 rpm. <br /><br />It still needs lots of trim/prop work, the eng. is also sitting too deep but its getting there at 5800rpm/25p prop/1.87 gears. At any speed and rpm over about 4500, it always computes out on the prop calculator programs at 2% prop slip--- and backs up the GPS readings, but takes them out to a few more decimal points.<br /><br />With that small amount of slip rate (anything better than 10% is good), the prop is almost going through the water like a screw goes through wood rather than thrashing and flailing its way along. <br /><br /><br />You're right, there isn't hardly any wake or rooster tail---- neither one of which translates into "more (of either) = more speed". At other trim settings, it got plenty of rooster tail, but couldn't hardly take off--- we're talking cavitation forever, or get much over 50mph even if it tached up over 6000. But, the water blowing up in the air about 10 feet looked pretty cool. <br /><br />Another thing to look at is the wake generated by the sponsons. I've found that it takes going at least 60 to get the rear 6" of the sponsons up & out of the water with me-- at 260 pounds in this thing.<br /><br /><br />As we work on the trim, the engine will be raised up higher on the jackplate/transom, possibly even with the prop not running totally submerged- the prop tips running out of the water will allow more slippage, but combined with the drop in resistance from the lower unit, the rpms will come up-- equalling more speed. <br /><br />Perhaps more than enough to compensate for the added slip rate. <br /><br /><br />If you can take the pounding, (several impacts per second) light boats will go faster in rough water than smooth. You're only hitting the wave tops rather than dragging the hull through water constantly. It works sort of like The ancient Egyptians moving them big stone blocks across the desert using rollers instead of dragging them across the sand*.<br /><br /><br />At that speed--60+, the airfoil along the sides of POCO LOCO should be generating about 300 pounds or a little more of lift for the rear of the hull--the same way an airplane works. <br /><br />Pulling the rear end up out of the water with air currents changes the way the engine has to sit compared to a V or other hull designs. The boat lifts itself with the airfoils, and the air compressed up under the tunnel running between the sponsons. The engine doesn't have be at a steep angle to force the hull up & out of the water to overcome 'wet' resistance. The hull is trying to fly on the 'ground effect' or cushion of compressed air under it between the water and the top of the tunnel. <br /><br />If the trim angle of the engine raises the nose too much more than about 3 degrees--- these things flip, or blow over real easily at speed.<br /><br />Hydros running a race course are in fact set up this way. They give up top speed to gain acceleration on the short triangle shapped courses they run. The distance between the bouys is MUCH shorter than they need to develope top speed. So, acceleration is more important to them. <br /><br />To do that, they have to overcome water resistance by getting as much boat up and out of it as fast as they can with horsepower instead of air pressure........<br /><br />...... which calls for engine trim angles that tend to generate tall roostertails. And that's why you see the (often fatal) 'blow overs' during these races. <br /><br />I don't care about acceleration. At this point, I'm trying to see how fast a 150hp Merc. XR4 can get this 1300-1400 pound boat up to on a open lake. <br /><br />*<br />A good example of how they work is to compare them to a prop driven airplane. Look at a single engine Cessna, or even a B-17---<br /><br />The axis of the propshaft is pretty much on plane or parrallel to the centerline of the fuselage. The engine of an airplane isn't angled up to make a airplane lift or gain altitude--- they only pull it straight ahead, the wings make it rise or lift. The engine & prop. only gives it forward speed....... A hydroplane works almost exactly the same way.<br /><br /><br />Along with another interesting Hydro picture:<br />I've posted on the webshots site (page 2) a picture of a kid doing a little over 120 in a experimental 4 point hydro-- mine is a old style 3 point. There is about 2-3 inches of air under the boat-- even at the rear. The only thing in the water is the prop...... and there's no roostertail behind it.<br /><br />Most of what you see is steam* from the engine's exausted water expanded by the vortex behind the cowling from speed & the engine turning up over 10,000rpm.<br /><br /><br />*<br />I think this picture was taken in Canada,,, and not exactly in the heat of their Summer. So the cool air will amplify the visual effect of hot water vapor compared to what most of us normally would see under the same circumstances.
