Need help with new boat porpoising problem

[mackid068]

By the way, how do these boats do in saltwater (ie long island sound...)

[Raggi_Thor]

What about gluing a trangular wedge to the bottom close to the transom?
Don't know what you call it, but it has been discussed here before.

[ferrazz]

I took powertechs advise and bought a dolphin fin. I will try it this weekend to see what happens. I was told by someone outside of this forum that this is a very common problem with these type of boats. (flat bottom, race type monoplanes) He said that the way it is always solved is to put a wedge on the bottom of the boat like Raggi Thor said. I just dont know how long, thick wide etc. Any input would help. He told me he has lots of experience with these same boat powered up to 35 hp and they often build these wedge into the hull. I guess it will slow down top speed but solve the problem. This is the third suggestion so I will try one at a time . I guess in this order
1 dolphin fin
2 wedge ????
3 trim tabs
As far as these boats in salt water. They are really only for smooth water. As you can see they are touchy enough in smooth water.

[mackid068]

Darn, they'd be fun going off a wave...

[Raggi_Thor]

Hmm. I read in another thread that the problem often is that the weight is to far forward. Maybe you (also) should try to sit closer to the transom....

[atahawaii]

Generally porpoising instability is handled in one of two ways. Both you have mentioned one is by adding weight forward and the other is by using trim tabs. You can also install adjustable fixed trim tabs that you can adjust manually after each run and they won't need power to do it this way. Generally increasing the weight, deadrise or trim angle also help reduce porpoising. When Savitsky's Long Form is used the last step is checking for porpoising. I know you added weight up forward, try adding more weight up forward. Your on the right track, sure is a nice boat, hope you get this resolved.

[kmorin]

ferrazz, its always been my understanding that porpoising was a result of imbalance of the CG and the running waterline's forward edge. As you get on plane (this mini looks like is planes on the trailer) the water line moves aft toward the transom until some stability is reached. As the after bottom looks nice and flat (not bulged) the waterline moves aft pretty fast until its past the CG. Then the boat tips over the waterline as the waterplane becomes shorter than at rest. In this condition the bow has risen until the waterline's forward edge moved aft, then the bow fell and that is the cycle of pitch in porpoising. Unstable cyclic pitch is what you've described.

Once the boat is stable again fore and aft, with the longer water line, the power takes over and you rise up on step again. To break the cycle folks have advised you of all the obvious balancing vectors of force.

Add "lift" to the stern so the trim angle (bow up pitch) is limited and the running waterline lead edge is forced to 'stay' forward of, or at least near to the CG. The anti-cav plate surfaces are upward lift aft, the trim tabs are upward lift aft and the wedge along the after trailing edge is lift aft: all of the advice in these departments are to lift the stern to keep the bow from running too far up- so that the balance between the aft lift and the lift at the foreward edge of the waterline are in balance and you could avoid unstable cyclic pitching.

In a boat this light, with a horse power to wt ratio as tight as your's, balance is more and more subtle. If you use a wedge it will most likely have different degrees of effect with different wt of drivers. If you install trim tabs they're most likely more adjustable for a wider range of wt.

One method that will work TEMPORARILY as a 'test wedge' is to screw a 4" wide pc of sheet aluimum (0.060" or so) transversely to the bottom so the after edge is common to the hull's. Using some bondo or duct tape to fair the forward edge and use counter sunk flat head screws. If you use tape to fair the leading edge warm it with a heat gun and use a linoelum roller or veneer roller to really press it to the hull. It won't last long but will get you a trial run of two.

Now install various shims between the hull and the aluminum -which being flexible will bend in a slight cup downward. The wood shim -say 3/8" laying between the aluminum and the hull will force the after end of the sheet aluminum into the wedge or hook. If you try different thicknesses you can most likely find a balance between the wedge's trim forces and the boat'c trim attitude as it runs before you install the real thing.

Another way to do this same thing is to put a 90 bend of material attached to the transom and bent downward at the after end with an adjustable wrench aft each run to determine the amount of trim needed. These are just fixed trim tabs -but in each case the screw holes can be drilled out when you're done and filled, without permanently dinging this very pretty hull.

Incidently, if you lay a straightedge along the bottom 4' of hull and there is the slightest bulge downward that will cause porpoising in many cases. By 'slightest' I mean 1/4" or more out of flat in the after planing surface. If the hull is cupped (upward) aft- the hull often plows- but that takes a fairly deep bubble in the center 1/3 of the planing area.

When you finally win, and you will, you'll enjoy it more for the effort.

Cheers,
kmorin

[Tin Man]

Another thought, along the lines of those mentioned previously.

You mentioned that you tried running with the motor at various trim angles, including the lowest pin hole. In the pictures, your transom appears to be a little more vertical than some. I wonder if the lowest trim setting still isn't low enough because of that. You could easily experiment by loosening the motor mounting bolts and inserting a thick shim under the top bolts, thereby creating the equivalent of a more angled (less vertical) transom.

Just my two cents.

Tin Man

[kmorin]

I think that Tin Man is onto something, the *.pdf of the plans shows a 15deg aft transom which might not be enough to balance the boat in a running attitude. His idea about shimming out the top motor mount bolts works great to test, and a wedge could be glued to the after surface once the outboard was trimmed under far enough to trim for more flat running.

Cheers,
kmorin

[ferrazz]

Turns out the boat was just too heavy in the back like everyone described., probably due to a little too big of a motor. The perfect fix was to install a stingray Jr hydrofoil fin. It worked amazing. It leveled the boat right out, stopped any cavitation and makes the boat plane up immediately with no bow rise on start up. Its amazing. It also gave me more speed than I theoretically should have after running the numbers on my prop and rpms. The boat hit 31 mPH with a 9 pitch prop !!! The numbers say it should only do 25 with that prop and RPM. The gear ratio is a 1 to 1.85 and the prop diameter is 9.2. I must not get any prop slip. If I go up several degrees with the prop it should do 40 easy. Looking for a custom prop now. It runs as smooth as glass and handles amazing at all speeds. I checked the bottom of the boat and it is dead flat. Thanks for all the help. This is a talented group. You were all correct in your assumptions. Thanks for saving my many hours of hard work from being for nothing. Scott

[rxcomposite]

Ferazz,

It seems from the photo that your cavitation plate (the large horizontal plate) is in line with the flat on the transom. It should at least sit one inch below the flat of transom.

It also appears that your motor is mounted vertically while you still have a negative rake on the transom. That means you are not getting the full range of trim from the outboard.

If the flap solved your problem, then i think the motor trim is not getting the full range of motion. Normally your bow should go up first, then you trim the motor so that it raises the stern slightly until you get optimum running trim angle. then you can reduce power as the boat gains more speed without adding power. The motor trim should be able to handle this. Flaps are used for fixed inboard drives with no means of adjusting thrust angle.

I think it is the wrong thrust line. Planing boats are very sensitive to thrust line.

Rx

[Thin water]

I know this is an old post but in case any one else builds one of these the above boat is missing the "After Plane" The bottom should extend past the transom several inches and the transom angle is supposed to be 75 degrees. The after plane acts like a giant trim tab that extends the full with of the boat. That is the best looking minimost I have ever seen. It could be retro fitted with the extension but that nice paint would have to be redone. I am building one now.

[otseg]

Most 13 year olds get driven to baseball practice on Saturdays, my Mom drove me to the lumber yard to buy wood, screws and weldwood glue to build mini max's. That was 42years ago. I had Mercury KG-7's that were about 15 hp.

Your boat is a beauty. The trim planes for the cavitation plate do remarkable things but your goal will be to get every last mph. You need to jack that engine up as hgh as you can. I had a set of yardsticks and would shim the engine up by 1/8 inch tillI could not get on a plane, then remove one.

The bottom needs to be absolutely straight with no rocker. You also need to be sure that the transom angle allows the engine to be square to the bottom.

rock on.