It's All About Attitude...

Ocean skiffs are often heavily loaded, and can have a hard time getting on, or staying on plane. It seems like this question, or one like it, pops up on Internet boating forums every month or so:

Toy Boat 2 is the perfect platform for trying to answer this question. At 17'6" LOA, an estimated weight of 3,200 lbs fully loaded (see our Boat HP Estimator page for a convenient boat weight calculator), and only 90 HP, it seems like TB2 would be a total slug. Here's what we did:

The first thing we did was to make sure we had the right prop. Having the right prop meant that the boat was running as well as could be expected.

The usual way to determine this is to adjust the prop diameter and pitch so that with your normal load, at wide-open throttle, the motor will turn within the manufacturer's recommended maximum RPM range. For most motors nowadays, that number is somewhere between 5,000 - 6,000 RPM (consult your owner's manual or the manufacturer's web site).

In the case of Toy Boat 2, our 1991 Johnson 2-stroke was rated for 4,500 - 5,500 RPM. 

After some trial and error, we arrived at a ComProp 4 blade, 13.5" diameter, 15 inch pitch propeller. With this wheel, we could turn 5,000 RPM at wide-open-throttle (WOT), with a maximum speed of 31 MPH.

After we had the right prop, we went out and did a series of tests with no trim tabs, trim tabs, a hydrofoil (a Marine Dynamics Stingray), and both. We measured time to get on plane, then backed off the throttle to find the minimum speed that we could run and still maintain plane. We made 5 runs for each set of test conditions, alternating directions after each run, in order to help compensate for the effects of tide and wind. The numbers shown below are the average of the values recorded.

• Both hydrofoils and trim tabs clearly improve time-to-plane. In the case of the trim tabs, with the proper engine trim, time-to-plane was reduced by almost 30%
  
  
• Trim tabs alone helped more than a hydrofoil alone.
  
  
• Engine trim appears to contribute more to low-speed planing than either the trim tabs or hydrofoil.
  

We settled on using the trim tabs by themselves. They had proportionally more effect than the hydrofoil, and provide the ability to correct listing as well. We pulled the hydrofoil, and patched the holes in the antiventilation plate with Marine-Tex epoxy.

Of course, we already had the trim tabs installed. If we had to start from scratch, would we still go with the trim tabs, or would we start with the hydrofoil, which is roughly 7 times less expensive than the trim tabs? The answer is "we would go with the trim tabs, unless money was a major issue".

The ability to correct listing is a real plus, and it turns out you can also control spray in a crosswind to some extent by tilting the hull slightly up on the windward side. On Toy Boat 2, lifting the chines on the windward side really helps to knock down spray that is being blown into the boat by a crosswind.