Hydrofoils for outboards, good idea?

Wesley · 11 February 2013, 07:14

Hello..

I wanted to pose a question and see what the general response is from those who may have experience..

I have a small 3.5m rib with a 20hp outboard I use as a PWC for leisure etc.. I was thinking of attaching a hydrofoil to the outboard as recommended by a friend.

Anyone have any thoughs on what improvements this will make and is it worth doing?

Any advice / views are appreciated..

Thanks in advance and happy boating

IanH · 11 February 2013, 07:35

Do you ever take the engine off and lie it on its side? If you do, the hydrofoil will interfere and will need to be removed each time you do that.

Wesley · 11 February 2013, 07:40

Not a problem I never take it off the boat. Just wandering about any performance boosts or the opposite... etc..

colcreate · 11 February 2013, 07:41

They are cheap and easy to fit so best way is just go ahead and try them to see what happens. Worse case you will chuck them and be left with the four holes drilled in your engine Anti-cavitation plate, like me.... Result vary considerably and can increase stability, plane quicker and therefore increase fuel economy.... Or they can totally ruin the handling and planing attitude of your rib. So don't fix what isn't broke, or if it is broke then make sure you fix it properly before adding a further "improvement" or complication like adding hydrofoils. What are you trying to fix?

Wesley · 11 February 2013, 07:45

Nothing to fix, just thinking of maximizing performance, fuel efficiency etc.. I heard you can get foils that clip on so perhaps if I can find one of those, as you said it might be worth seeing what the difference is with and without. before drilling holes etc.

Budgie1 · 11 February 2013, 07:49

I have them fitted to 60hp Merc and previously on a 25hp Mariner and the difference is incredible, Planes so much quicker and at lower speeds, also easier to maintain constant speed when wakeboarding, ringos etc. think they are a bit like Marmite your either love or hate them, from what I've been told they don't offer much performance improvement (if any) on larger outboards

9D280 · 11 February 2013, 11:06

Aye, they will either astound you with the performance increase or make you wonder why you bothered.....

In a total opposite to Budjie1, I bought two ribs with them fitted - one a 25Hp, the other a 55 - (Close enough for a comparison!) The 25 all it did was knock 2.5 knots off my top speed & make it nose dive at anything above about half throttle. On the 55 it did absolutely nothing ecept make the boat less responsive to helm & more uncomfortable in waves.

Do a search of the forums - it's pretty much 50/50 in the love / hate ratio stakes!

I have a couple of pairs lying in a box if anyone is interested.....

Nasher · 11 February 2013, 15:29

My opinion and experience is that they can be used to fix a problem, treating the symptom instead of the disease.
If the boat is set up correctly in the first place they won’t have any benefit.

They did completely sort out the handling issues I had on a Flatacraft Force 4 years ago, in that the huge chine walk problem at WOT completely disappeared.

The ‘Disease' in that case was me putting a DT55 triple on a boat that wasn’t rated for that much power or weight. Bloody fast though

Nasher.

Locozodiac · 11 February 2013, 16:21

Before buying : Check/test this, the decide...

1-Inflate well rib to at least 3.0 psi once on water.

2-Set trim angle for engine so sit perpendicular to water level, gnerally 2-3 hole out from transom.

3-Distribute weight evenly on deck, passengers, cargo. If boating solo move your weight forward. If with 2 one up front, other at driving position.

4-Go at least to 3/4 throttle to plane fast, once on plane can reduce throttle a bit for rib to be maintained on plane.

Boaters likes to place trim trabs, doel fins to plane fast without testing the above, but doesn't realize that by distributing weight accordingly and throttling fast will have near same effect as fins, tabs, etc.

Happy Boating

jyasaki · 11 February 2013, 16:58

Quote:

Originally Posted by Locozodiac

Boaters likes to place trim trabs, doel fins to plane fast without testing the above, but doesn't realize that by distributing weight accordingly and throttling fast will have near same effect as fins, tabs, etc.

Kind of a blanket statement. Probably true for many cases, but doesn''t take in the entire range of problems that might be present.

Hydrofoils have a couple of effects, the trim/planing one is pretty well recognized. Some people say that it's the wing effect of the water flowing by the foil; some say it's the deflection of water driven upwards by the prop. Either way, the theory is that it gives lift to the back of the boat that wouldn't be present without the foil.

The other is the enlargement of the area covered by the anti-ventilation plate, which essentially increases isolation of the surface from the prop. If you have problems with ventilation with the motor trimmed where you need it to be, then a foil *may* help, at the expense of a bit of additional drag.

Do they work? Personally, I don't know. The one on my current boat has been there since day one, so I don't know if it affects performance or not. On my smaller boat, I added a pair of Doel Fins, and didn't notice a dramatic difference (that I can recall, anyway.) A friend of mine put a foil on his 15.5' Zodiac/Merc 60, and clains a huge difference with getting on plane, and much better handling at speed. YMMV.

jky

hamster · 11 February 2013, 21:16

I've used then on various boats used mainly as safety boats and can only report good things. Usually boats upto 5m which aren't at the higher end if their permitted engine rating. The fins allow the boat to plane a lot easier and at lower speeds even when loaded.
The also allow the boat to plane with out first pointing the bow at the sky if the boat isn't/can't be loaded properly as stated above.

I used a 4.5m rib recently for coaching in Chichester harbour which had them fitted. I could get on the plane easily with little power required and still stay below the 8kt speed limit which meant the boat handled a lot better when chasing dinghies with spinnakers up and used far less fuel than when I used the same boats a couple of years ago with out the fins fitted for a similar purpose.

Sent from my iPhone using Rib.net

JamesF · 11 February 2013, 23:31

Fins did help a little on one of our boats, but really they were an only-slightly-effective workaround for the real problem, which was that a 25hp Mercury Bigfoot four-stroke is the wrong engine for an SR4.

Without an obvious problem to fix or work around, I'm not sure if I'd be inclined to start drilling holes. If there is such a thing as a clip-on set, they might be worth trying, if only to satisfy curiosity.

Locozodiac · 12 February 2013, 03:20

In this particular case a 3.5 mtr rib using a 20 HP engine which matches perfectly that size rib should perform as expected without doel fins, trim tabs. If being underpowered, doel fins can help plane faster, anyway will need to have a well ballanced boat, engine perpendicular to water level and match perfect engine transom height to start with.

My actual 420 rib with a Tohatsu 2 strokes 18 HP rated for a 30 HP jumps on plane at only 3/4 throttle with me driving next to engine. As stated before try your combo as it is before going for doel fins, by the way are garbage, plastic bags, kelp collectors...

Happy Boating

RAZZER · 14 February 2013, 22:47

I have been offered a set to test in a scientific trial this spring.
I intend to fit them to a 4.8 -5 m RC with a 60hp engine fitted.
The RIB is superb but we want to see if the fuel saving claims and extended planing speeds are true.
Keep you posted of our findings...

gridge · 24 June 2013, 07:16

Hi Razzer,

did they help?