Hydrofoil

[Tangles]

G'Day all, I am now the proud owner of a 3.8M tinnie with a 15HP on the back. Its a sole man boat (mostly) and a mate reckons that it might be worth fitting hydrofoils to the engine.

Anyone have any wisdom on the subject?

Cheers
Tangles

[jimi]

Hi Tangles, hydrofoils on motors seems to be a fairly contentious issue, with lots of arguments for and against. Personally, I have one fitted to my outboard and so am in the for camp .

Fitting one will generally help you boats performance in a few areas. You will definitely get on the plane faster, especially if your boat is a bit heavy in the back end. You will probably also eliminate any porpoising if that was a problem before.

You may notice a drop in top end speed, due to the increased water resistance from the foil itself. I haven't noticed much difference in this regard.

They are good on boats that may be slightly underpowered (that's why I got one) as the motor just doesn't have enough oomph to get on the plane when the boat is loaded up. The foil gives you a little bit of extra lift that is generally enough to get you over the hump. They are certainly a cheap way to boost performance.

I think foils are great, but you should remember that there are other things you can do to improve you motors performance that don't involve fitting a foil. For example, most motors come with a prop that is designed to be a one size fits all solution. Since no two boats are the same, changing it will often boost performance.

Choosing a prop with less pitch will help the motor spin it in the proper rev range to develop maximum torque which will get you on the plane faster as well as give much better low speed performance. You will loose top end speed though. Conversely, a high pitch prop will give you more top end speed, but the motor may struggle to get you on the plane, as it can't get into the right rev range to produce enough torque to spin the prop.

Stainless props will give you better performance than alloy because they flex less, but if you hit something in the river, they have no give so may damage your motor worse than with an alloy prop. Plus they cost a lot more.

Also, changing the height of the motor may help. The cavitation plate of your motor should be close to lining up with the keel of your boat. I've heard up to 1" below is good . If it is much lower than than it will drag in the water too much and reduce speed. Too high and you will get cavitation.

Hope that helps.

Cheers
James.

[jimi]

Tangles, looking back on my post, I forgot to mention one important thing..... Hydrofoils are just another tool that may help you to fix a specific problem that you are having with your boat.

You don't mention that you are having any problems in your post. So, if you are happy with your boat the way it is, my advice would be not to fit one. If it ain't broke don't fix it.

Cheers
James

[Tangles]

Thnaks for that, very much appreciate it.

Given its a new boat (we are talking weeks), hard to say Im having problems but concern I had is that being one bloke up the back, the bow rides quite high so I was wondering whether the hydrofoil would lift the stern.

Suppose thats actually a different question now I think about it.

Dont think there is any issue with being underpowered but being a boating novice, happy to be enlightened. Seems to be planing to me.

[jimi]

Tangles, one thing you could try if the bow is riding high, is to make sure the motor leg is trimmed in.

On the bracket that bolts the motor to the transom, there should be around 5 holes that have a bolt that slides through them. This bolt holds the motor leg out a various different positions. If the motor is on one of the holes furthest out from the transom, it will cause the front of the boat to lift up.

The pin/bolt that holds the motor is easily removable. You could try pulling it out and changing positions to see how the boat rides. If you put it in the first or second hole out from the transom, that generally gives the best ride.

You may end up with the opposite problem (ie. the bow too low, ploughing into waves etc) if you trim it in too far, so play around with it until you find what suits you.

If the motor is already trimmed in and it is still riding high, a foil will probably help you.

Cheers
James

[Tangles]

Thanks James, dont recall which hole is being used (maybe that's why its high) so I will check and then experiment.

[Jocool]

Hey tangles...if your in Sydney..let me know. I got a couple of sets of foils in the garage you more than welcome to try before buying! One set is plastic the other alloy. I dont recall the brands right now.

You can screw them on, and if you take em of all your left with is a couple of little holes on the cav plate which wont matter much !

[Tangles]

Thanks Jocool, let me play around a bit more with the current set-up but if this fails I might take you up on the offer. Cheers.

[Tangles]

Just to complete the story, spent a good few hours experimenting over the weekend and sorted the problem - answer was in the trim, had to move it out four holes from where it was and now it flies. Amazingly, nominal difference in ride between one or two blokes.

Anyway, will shelve the hydrofoils idea for a while - thanks for your help, really appreciate it.

Cheers
Tangles