Hydro fins, what do they do are they worth it on a 6hp engine ?

Lukey · 08 January 2007, 15:54

After my feeble little engine allowed me to stand still on the thames the other night ( read jesus nearly didn't make it on the thames, thread ) I have doubled the power and bought an other outboard ( 6hp ). This should make the world of difference on a 2.7m sib. At least I think it should.

Thing is I have seen and heard of hydro foils fins, what do they do and would they be of any benfit on my engine ?

jyasaki · 08 January 2007, 16:43

In theory, they do a few things: they isolate the prop from the surface, thus reducing propr ventilation (aeration.) They also (reportedly) add a small amount of lift as they are driven through the water, thus getting the boat on plane easier, and allowing you to keep it there at a slower speed. They supposedly have an effect on stabilization as well, preventing porpoising at higher trimmed out settings.

Do they work? I don't know; but both boats I've run have had them. I put one on my Achilles 14/Honda 40 soon after getting it, and can honestly say that I don't recall any difference. My current Polaris came with it on, so I haven't had any experience that would lend any kind of comparison.

My theory (read: "wild-assed guess") is that they *may* help smaller planing boats, especially those that exhibit poor handling tendencies. I don't think they'll have much effect at all on heavier boats (Not sure any RIBS would fall into this category, though. I'm thinking of the 26'+ CC fishing boats here.)

A boat that handles well to begin with will probably not improve much; the added drag may actually hinder performance.

At 6hp, I'm not sure you have enough power to actually generate the speed you'd need for them to start generating lift. You probably don't have much ventilation problem as it is, so it's not going to help there. Then again, it also probably wouldn't hurt much, either.

jky

PeterR · 08 January 2007, 19:33

I fitted them to a 30 hp yam on a 3m rib,25hp on a 3m rino rider rib bought both boat to the plane a lot quicker so i was able to tow knee boarding sons better.I have never had them on a 6hp the odds are good that you would benifit but thats only based on a gut feeling and the marked improvement i found on the prementioned boats.good luck.If funds are the issue i would be happy to lend you a pair to try that i,ve got in the garage

hamster · 08 January 2007, 22:35

Although not on a rib I have fitted these on a 13' dory which was used as a safety boat at my sailing club many years ago. The hull was very damp and the 15hp Suzuki took ages to get the boat on the plane with two people on board. I fitted a set of fins and the boat would plane easily with three onboard after that. Made a massive improvement. It also planed at a lower speed with just one or two onboard so i assume would be more economical at these speeds.
A few years ago I borrowed a 4m searider from the RYA which had a set of (what I thought looked small fins) on its 25hp 4-stroke Yamaha. I found it planed just as easily as the 4m riders I use at work with 30hp 2-stroke yams which don't have fins. Even with a couple of people on board.
Would highly recommend trying them. On a small relatively light boat I should imagine they will help make the boat more stable, especially at higher speed.

Is the cavitation plate on your engine big enough to fit the fins on?

jyasaki · 09 January 2007, 00:14

Well, I stand corrected. I'll defer to those with experience with smaller boats and motors.

jky

Lukey · 09 January 2007, 10:57

Your a star thanks, they aren't that expensive from what i can see, unless you want to sell yours, seems a bit pointless lending them to me ( appreciate the offer though ).

If you do want to sell them give me a shout.

As for the question will they fit on, no idea, but necessity is the mother of invention

Quote:

Originally Posted by no fear

I fitted them to a 30 hp yam on a 3m rib,25hp on a 3m rino rider rib bought both boat to the plane a lot quicker so i was able to tow knee boarding sons better.I have never had them on a 6hp the odds are good that you would benifit but thats only based on a gut feeling and the marked improvement i found on the prementioned boats.good luck.If funds are the issue i would be happy to lend you a pair to try that i,ve got in the garage

Nick Hearne · 09 January 2007, 11:15

I have used them on a 5.75m rib & they worked for me giving a heavy stern some extra lift!
But if you have just got the new out board then why do you think you need them?

Lukey · 09 January 2007, 11:40

Quote:

Originally Posted by Nick Hearne

But if you have just got the new out board then why do you think you need them?

I got stuck on the thames a few nights ago with a underpowered engine. So I got a bigger one. This one should push me along better but being a sib it isn't that directionally stable so I wondered if these would help, also any extra speed is always accepted as well as saving fuel and getting on to plane quicker or at all ( haven't tryed the new engine ). The hp limit of the boat is about 5.5hp so I can't get a bigger more powerfull engine, so i would like to wring as much out of this one as I can.

There was never any chance they would go on the 3hp engine but there might be a chance they will fit on this one

Nick Hearne · 09 January 2007, 12:05

Have a look at this! http://www.compass24.com/cgi-bin/abn...l&ARTNR=959400
Says 3hp to 50, but I would still see how the new engine work B4 getting them as your problem the other day was lack of power which you now have!

cameronke · 09 January 2007, 14:50

Looks like a Yamaha Malta in Nicks link there and that is just under 4hp so should be good for your 6.

Cameron

badbaws · 09 January 2007, 21:41

I used my Avon 3.10 over the summer with and without fins.. I only used a 10HP mariner and yes the fins allowed it to get ont he plane quicker with especially with 3 people onboard.. I even towed a Knee boarder and just about managed to do it..
I did try it with a 6 HP Johnson.. but the engine would not rev to full power unless the hood was off and the fingers were in near the flywheel!! But again with fins on it did get on the plane quicker than a 4 stroke 9.9hp Mercury I used on the same boat!!
$ strokes should be banned they give will give a hernia trying to carry them!!

Poly · 10 January 2007, 19:40

if they work so well at this range of engine size - why dont the makers just mould them into the design in the first place (or as OEM optional extra at time of purchase)?

Lukey · 11 January 2007, 10:55

Quote:

Originally Posted by Polwart

if they work so well at this range of engine size - why dont the makers just mould them into the design in the first place (or as OEM optional extra at time of purchase)?

Simple maths, more you stick on the enigine the more it costs. Suprised it isn't offer as a extra though.

Poly · 11 January 2007, 13:46

Quote:

Originally Posted by Lukey

Simple maths, more you stick on the enigine the more it costs. Suprised it isn't offer as a extra though.

But if I understand the concept correctly it is just a larger anti-ventilation plate - the materials cost is minimal and would give them an "added selling point" over the competition.

Nick Hearne · 11 January 2007, 16:26

They will get in the way on a small outboard that is cared & they are not needed on all boats! So why fit them as standard!