Make a Hydroplane Turn Left and Right

Terry Heddin · Sep 23, 2024

What's the best setup to make a hydroplane turn right and left? Not racing, just recreational running.

larcor69 · Sep 23, 2024

Remove the turn fin but it won't turn very well

John Beardslee · Sep 23, 2024

Mount a straight fin at the CG in the middle (left to right) of the hull. Like you might see on a flat bottom drag boat.

Hydro Junkie · Oct 2, 2024

John, the issue with that is that water will be thrown up against the bottom of the hull. Unlike a drag boat, a hydroplane has "airtraps" that hold air under the hull which, in turn, supports the hull. Unlike air, water will be forced up against the bottom of the boat on one side of the fin, giving the boat an unbalanced force on the bottom. If you have ever watched a stock built Dumas sport 40 1977 Atlas? They tend to roll toward the left due to the turn fin being located on the inside face of the right sponson, throwing water up against the right side bottom of the hull.
I know someone is going to say "What about the round nosed boats from the 50s and 60s? Their turn fins were on the inside of the sponson as well". And while that is true, the fin on those boats were usually no deeper than 6 to 8 inches below the bottom of the sponson. The results of that very shallow fin was that, with the boats jumping around like they did, the fins weren't really that effective until the boats were slowed way down, negating a lot of the water being thrown against the bottom of the boat.

John Beardslee · Oct 3, 2024

Hydro Junkie said:
John, the issue with that is that water will be thrown up against the bottom of the hull. Unlike a drag boat, a hydroplane has "airtraps" that hold air under the hull which, in turn, supports the hull. Unlike air, water will be forced up against the bottom of the boat on one side of the fin, giving the boat an unbalanced force on the bottom. If you have ever watched a stock built Dumas sport 40 1977 Atlas? They tend to roll toward the left due to the turn fin being located on the inside face of the right sponson, throwing water up against the right side bottom of the hull.
I know someone is going to say "What about the round nosed boats from the 50s and 60s? Their turn fins were on the inside of the sponson as well". And while that is true, the fin on those boats were usually no deeper than 6 to 8 inches below the bottom of the sponson. The results of that very shallow fin was that, with the boats jumping around like they did, the fins weren't really that effective until the boats were slowed way down, negating a lot of the water being thrown against the bottom of the boat.

He’s not racing and wants it to turn left and right. With the fin on the CG much of the spray effect is reduced. It will work.

Hydro Junkie · Oct 4, 2024

Not really. The water will still be thrown up into one side or the other of the hull bottom in the turns unless he slows the boat down to where it's falling off plane. If he's got to slow down that much, he might as well leave the fin on the back of the right sponson to make the boat still able to run a hot lap or two.
I've seen this subject come up many times over the years, in one of the forums I'm a member of or at the pond. The guy asking is almost always someone new to the boats that says he wants a play boat that can turn both ways, a few times even has a completed boat that he wants to test. The results are always the same, he's not happy and we never hear from him again. If someone wants a play boat that will turn both directions, they need to look at a vee, shallow vee, a flat bottom or a tunnel.