Whale-tail hydrofoils on small runabouts

[marshmat]

Anyone know any good plans/sources for whale-tail hydrofoils?

The boat in question is a 15' x 5' open runabout with a 1994 Johnson J30 longshaft. The hull is a 2'-wide flatbottom on centre and a 24-degree vee to the outside of that. Prop is an OMC 3-blade aluminum, 10x13. Hull weight is 115kg and the motor another 50-60kg or so. The motor is set with the antiventilation plate about 1/2" below the bottom (any higher and it vents in turns) and it runs best at about 5-degrees positive trim.

Since putting this motor on (upgrading from a 15) the boat now has a couple of quirks:

1) When running solo or with one passenger sitting aft (CG about 4' forward of transom) a steady porpoise develops at about 18-20mph. Negative trim does not help at all. Putting the passenger in the bow (CG about 5-6' forward of transom) solves the problem and she is then good to 30mph.

2) When running slightly stern-heavy, there is excessive bow rise (up to 15 degrees) when climbing to plane. Negative trim helps a bit but screws up high-speed handling. Adding weight in the bow solves this.

Both problems seem to point to a 'whale-tail' hydrofoil bolted to the antiventilation plate. Trouble is, cash is a tad short for this job. So I'm looking for any do-it-yourself plans for such a gadget.... having built the boat myself, I'm comfortable doing the modifications but would rather work from something proven than something I drew up myself... Any help, folks?

[yokebutt]

Matt,

How about trim-tabs? I'm thinking used piano-hinges, some stainless sheet-metal, a few pop-rivets, a couple of discarded turnbuckles from the nearest rig-shop and you'll be in business.

Yoke.

[Corpus Skipper]

I'm with Yoke on this one. Whale tails are better left to someone with more letters after their name as they employ a foil shape that is probably easy to screw up.

[marshmat]

Hmm... that might work, ya. Crank 'em down when running tail-heavy, and back to level when the bow's loaded..... how big do you think they'd need to be? (i've never seen tabs on such a small boat before...) 10" wide by 8" long or something? This does seem like the kind of thing I could hack together in the garage and have it work!

[gonzo]

If it is a tiller motor, a piece of PVC pipe extension will let you steer from farther forward.

[marshmat]

It's a tiller motor, ya... but it's got a few controls in the end of the tiller handle other than the throttle (engine stop, emrg lanyard, throttle friction) and I can't find an extension that doesn't block those.

[gonzo]

Could you fit a PVC pipe with a hole for the emergency lanyard?

[Aquatek]

[quote=marshmat]Anyone know any good plans/sources for whale-tail hydrofoils?
[\QUOTE]

Sounds like your looking for a "Doel fin" hydrofoil. It actually comes in two halfs and is usually fitted onto outboard motors and sterndrives on either side of the anti cavitation plates but if your good at DIY you can make your own underwater mounting for it.
I bought one from Chertsy Marine and fitted it to the anti-cavitation plate of a Cresent Marine 50hp 3cyl outboard on the back of a 14' x 6' Plancraft Sabre III.
Once it was fitted the difference to the boats perfomance was very noticable and the boat could get onto the plane almost instantly!

Regards

Aquatek

[marshmat]

Yeah, that's kind of what I was thinking of, Aquatek. Too much $$ for this project though, unfortunately, at least from the dealers around here who would have to order it in from halfway around the world.

I'm kind of liking the scrap-metal-trim-tabs idea, believe it or not. I've never seen them on a boat this small though, anyone tried this and can comment on how well it works? Any idea what size the things would have to be? (Boat data in post #1.) Thanks for the ideas everyone.

[lewisboats]

Ebay!

Steve