Noob Needs Help with Stepped Hull

[mtgoat]

Hello All,
I've put together an MHZ Speed Star and it isn't running the way I'd hoped. When the boat reaches a certain speed, it starts hopping and won't go any faster.

It's a 21" stepped mono. I'm running it with a Feigao 540 16S (2218 kV), 3s Lipo, and a Graupner K33 prop (46mm pitch). It has a 4mm straight shaft. CG is at 33% of hull length from transom. I had it at 29% at first and the problem was much worse.

I don't have any way of measuring the speed, but my buddy's BBY Oval Master 21 blows it's doors with a 540S motor and 2s lipo.

Here is a 25 sec. video of the boat in action.

As you can see from the photos, there is very little room for trim tabs. Although I could probably fabricate something that would work, I'm not sure that would help. Maybe ditching the straight shaft for an adjustable strut or stinger and flex shaft would do the trick.

Opinions anyone?

[Jeff Wohlt]

Your prop is pushing the boat forward because of angle and the step is just causing problems. It is hopping bad.

It should ride on both edge surfaces...front step and rear. You also have a bunch of chine walk and tabs can help that as well. Most of this is because of the V design and narrowness of the hull. You actually may need two sets...inner and outter to get that to run well. Look at some of the sites in German and see some set ups.

That is rough water but it is certainly hopping.

Did you try to bring the CG forward?

[mtgoat]

I did bring the CG forward as you suggest, but not past 33%. It has been an improvement, but it would be difficult to move it farther than that without adding weight.
Thanks, I will try to find some examples.

[Fluid]

That is a tiny boat for 3S power. We are now using 24"-27" boats for 2S. The main problem is that you are overdriving the hull. It was designed for lower speeds and less power, so it is unstable when driven too fast. Moving the CG further forward might help, but eventually you will unload the prop and things will get worse. Flattening the drive shaft angle will help, but that's hard to do without rebuilding the boat. Really, there is not much you can do other than reduce the power. Try it on 2S, it may even be faster with a slightly larger prop.


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[mtgoat]

Fluid,
Thank you for your input. That is some hard news to bear. It is against my nature to try and make the boat go slower. I am more inclined to keep all my hardware and replace the hull with something more suitable. I've spent more on batteries than on the hull.

Would anyone recommend a mono hull that would be suitable for the equipment listed above? I sure appreciate any input.