Findings

Upon launching - I discovered a serious problem with the boat. The balance was off.

When I tried to power up, instead of getting up on plane, the back of the boat dug in and the bow rose alarmingly high. At slow (no wake) speed, with three of us in the boat, it was very stable, handled well, and didn't seem to sit too low in the water - though it was definitely too heavy in the back end.

Part of this problem was my fault. When I decked the boat, I wanted a nice, large, pretty, varnished deck that would be eye-catching. I decked the boat about 1 to 2 feet further back than the original plans called for. This put the seats further back than they were supposed to be. And rather than building light wood benches, I installed the back-to-back fold-out seats (more weight).

I did several things to help try to solve the balance problem:

1) The motor was on the second tilt setting - this was brought all the way in.

2) The motor was raised about 1.5".

3) I ultimately added about 120 pounds of weight in the front. This was done by putting 60 pounds of concrete into each of two 6-gallon gas cans, and stowing the gas cans (now filled with concrete) under the front deck.

4) A Stingray Jr. Hydrofoil was added to the motor.

The above tactics helped. I can now get the boat up on plane with 2 people on board. Three people is just too much weight in the back half of the boat to get on plane, but at least I can do it with 2.

The only other problem now is that once I get to about 20/21mph, the boat starts porpoising.

This is still possibly due to the balance being off.

However, I'm also thinking it might be due to the skeg that I added to the bottom.

The skeg wasn't in the plans. When I was building the boat... it was one of those things that "seemed like a good idea at the time"....

I had owned a boat that had nothing on the bottom, and gave you a REALLY funny feeling and loss of control if you went broadside across a wave. I didn't want that to happen with this boat - so I added a nice, big, FAT skeg - one that was solid enough that the weight of the entire boat and engine could rest on it if grounded, without any worries.

Unfortunately, I'm starting to wonder if the skeg isn't what's causing the porpoising.

Here's a picture of the skeg:

I was also concerned about whether the height of the motor was causing me problems.

The motor is a 30hp Nissan LONG Shaft motor, 4 stroke, with tilt / trim. So yes - it's also HEAVY (~189 pounds - probably 40-60? pounds heavier than the old 2 stroke of the design era, and possibly a little larger than the designed engine for this boat too...

The design was originally for a short shaft motor. There was a cutout in the transom that I omitted, as well as just generally raising the transom about an inch. As I mentioned before, the motor was raised about 1.5 inches higher than it's normal resting position on the transom - so at this point, I think the level of the cavitation plate is approximately close. (OK, some people will call it the "anti-ventilation" plate. But the rest of us won't worry about that argument...) The intake of the engine is in the water. The cavitation plate is below the bottom line of the boat. And the prop gets plenty of water. Here's a pic of the back end of the boat:

If you can zoom in on the pic, you can see the hydrofoil is just below the level of the boat. The hydrofoil isn't below the level of the skeg, but I think it's close enough for the motor.

I'm still wondering if the skeg is the primary culprit for the porpoising. I've seen a LOT of designs (several from the same designer) that show the front decked, and the driver very far back. If that was a HUGE problem, people would have known about it... right??

So... On to the NEXT project...

The next boat will:

1) NOT have a BIG HONKIN' SKEG. Maybe two little ones to give me a little bite at slow speed, but not a big one with that front ramp on it!

2) The deck will only go back 4-5 feet at most, so that the seats can be moved up further.

3) The transom will be a little higher to handle the long shaft motor.

With improved balance, I can get rid of the 120 pounds of ballast in the front. Hopefully THEN (and without the drag from the skeg?), I'll be able to get up on plane with 3 people on board.

UPDATE: 8/14/13

I just installed a pair of Nauticus Smart Tabs SX, SX9510-60 (I ordered sx9510-40, but got 60's, which is for the next size larger boat - may be ok in my case).

We tried these out on 8/13/13, and it solved the porpoising problem!!

Now I will need to experiment with removing the hydrofoil on the engine, and tuning in the settings on the trim tabs.

I didn't bring a GPS with me on the first run, and it "seemed" that the boat was definitely dragging a bit more. But the ride was very stable - so the trim tabs clearly made a big difference.

Update 6/24/20

See section on converting Mischief to electric power.

While slow (cruising speed of 4-5mph), Mischief makes an excellent cruising boat under electric power with a Torqeedo electric outboard and lithium eBike batteries. Current configuration is with the Torqeedo 4R outboard, a 48V, 50Ah eBike Battery for main power, and a 48V, 20Ah eBike battery for backup.

With an electric motor, the boat will only achieve slightly better than hull speed (~4.8mph) at reasonable power.

For reasonable electric cruising, max power needed is around 1500W - certainly within the range of the Torqeedo 2R. However, the Torqeedo 2R is a 24V system - which would be harder to power with eBike batteries. Ironically, going to the lower power system may ultimately require more battery boxes, since it most likely be powered by two series 12V batteries, unless building a custom battery.

The 48V eBike batteries have demonstrated that they are a very cost effective package to work with. It's wonderful to get a single battery package that fits in a single group 31 battery box (for the 50Ah battery), and is light weight. The 20Ah battery fits in a group 24 battery box.