High Flying Hydros

[Bill Huson]

You could try shaving (planing) the air traps, my usual quick fix to adjust the attitude of the boat. On one boat which was dragging tail I added to the air trap with a 1/8" x 1/14" strip of mahogany. If the boat is trapping a ton of air, you could attach a small (1/2") angle aluminum across the bottom a few inches aft of the front edge. This is a spoiler, and inboards have an array of spoilers that they attach for different race courses and conditions.

As others mentioned, engine height and trim is a key factor, and prop design as well. A good prop man could eyeball the boat running and suggest a better wheel.

[dwestby]

Where are you planning to test. WE should maybe ready by then also. I would set the motor 1/4 deep and netural. to start with fin about 3.5 to 4 inches deep then figure where to go from there up of down under or what every your plan is. Before I go I have 2 or 3 ways I want to go if the boat does this or that. It is easier than upping around on the beach trying to figure what and why the boat is doing what its doing. I would make all changes in 1/8 of a inch and have a plan. Hopefully my DMH will been ready for testing if parts are in????????????? Bring lots and lots of props grinders and patience. You will move in a laydown remember csh vs DMH is a lot different in speed and handling If you get that Caddy ride you are probably going to get lapped. been there done that

[ram95]

The boat between the sponsons is essentially an airfoil. It has a cordline which equates to an angle of attack, usually positive. Some refer to the curve a hull has from its leading edge to the underneath as 'the amount of lift' the hull has built into it. Okay.

The faster you go, the more lift is generated. Hopefully, enougjh lift will be made to lift the sponsons off the water to reduce water-friction-drag. Lift creates drag but not as much as the water on the bottom of the sponsons.

You keep the throttle wide open. You go faster. Because your hull still has a positive angle of attack, and even tho the AOA has't increased, more lift is generated. Of course the generation of more lift most likely will cauise the bow to raise...increasing the angle of attack.. so you either move forward, sit tight!!, or reduce power.

If you are running a prop which lifts the back end, it serves to hold down an increase in the AOA by raising the back end. But, due to the higher speed you are going, the hull is generating more lift without an noticeable increase in AOA (ie the boat is still at about the same angle) because the prop is also working harder to raise the back end. You're on a teeter-totter. The airspeed is trying to lift the front, the prop is trying to lift the back.

Suddenly, you encounter a 4 knot gust of wind which causes an instant increase in lift.. the bow raises.. AOA increases... more lift happens... you've already moved your weight forward a little... It would be nice if the prop would counter the upward movement of the bow. But the prop is already lifting as much as it can. The prop is doing 'water-speed'. The boat hull is doing airspeed. If you were lucky your weight shift was done in time. If not.. the back of the boat drops down when the bow comes up and the AIR which was exiting out the airtrap at the back of the boat now packs up under the boat and must dump to the sides. Now we have the full force of the air at 65mph under the hull of the boat - trying to create a blow-over.... you move forward.. too late!!??

Some hulls are much more tolerant to increases in airapeed... some become flighty. Make simple changes first, one at a time.and try to test them out under similar conditions. Just remember speed thru the air is different than speed over the water.

[blueskyracer]

Where are you planning to test. WE should maybe ready by then also. I would set the motor 1/4 deep and netural. to start with fin about 3.5 to 4 inches deep then figure where to go from there up of down under or what every your plan is. Before I go I have 2 or 3 ways I want to go if the boat does this or that. It is easier than upping around on the beach trying to figure what and why the boat is doing what its doing. I would make all changes in 1/8 of a inch and have a plan. Hopefully my DMH will been ready for testing if parts are in????????????? Bring lots and lots of props grinders and patience. You will move in a laydown remember csh vs DMH is a lot different in speed and handling If you get that Caddy ride you are probably going to get lapped. been there done that

Dennis we will be testing about ten miles south and east of the race site in Montana if you want to join us the last weekend of April see photos from last year http://www.hydroracer.net/forums/showthread.php?t=18788 We camp there all three days and have chase boats and pit crew all set up.

[Dave M]

Question

As some asked, are you thinking about blow over backwards, or sponson tapping down the strights, or just losing the rear end while on the straights ?

In my opinion, Bill Rosado nailed it on one point. Add some air trap to the rear. That should counter some of the lift. Remember, you want near the edge, not at the edge of out of control. At the edge results in flips where you don't know what happened. Near the edge results in a known factor of why you flipped, most of the time.

For it breaking loos in the rear, you air traps are probably to low in the rear and there is nothing holding the boat in the corner either. You could add a small fin in the back or raise the traps. Might work.

Rememebr, these tips are not taking into consideration the design of prop you are running. Some designs, as some eluded too, will overcome the boat design and produce a too loose set-up. not much you can do about it, change props.

I am sure you will get a lot of tips on how to fix it. Just know your design, and the intent of your experiment. This will answer your questions the best once you test.

[blueskyracer]

Should be a fun experiment since everyone has told me this will not even compare to a Yamato 302. Kind of like riding a very fast horse. Gap the mane and hold on!!

[Dave M]

Prediction

I don't think you are going to have to many issues with your new DMH. From the pics it looks like it will simply be a matter of putting your weight where the boat likes it. Remember, no one boat is the same on where you are positioned to handle the best, and achieve max speeds.

For rigs that require you to add weight, get some portable weights, like those tube weights and put them in the cockpit with you, go test and move them around and you can find where your rig likes the weight. Make a pass, change the location, or do it while underway. Just be cautious. Every boat has a sweet spot.