Fins/Hydrofoils, do they work?

[amityslave]

I want to fit a Piranha hydrofoil kit to my anti-cav plate but have not found much on the 'tinterweb' about success or failure of said hydrofoil.
Anybody use one?
It's going on a Tohatsu 50hp on my 5.2m deep V.
These things are supposed to give upto 30% fuel savings, get you up faster (ooer) and up for less revs.
For the £38 it costs for one of these, it seems a bit of a good (and cheap) idea to me.
Any input gratefully recieved.

[captnjack]

"up to 30% less fuel" translates to you might notice lol

I had one years ago. It did help lower the planing speed by lifting the stern a bit. Top speed was about the same. I don't think it did anything for fuel consumption because I still ran the boat WOT all the time

[Starovich]

I've had most success with underpowered boats, planing earlier and at lower RPM, still as the capt says, you run it at WOT you-ll save no fuel at all.

[arkamelis]

hi amityslave
im in the same position has you when it comes to hydro foils, i have brought the doel finn ones, the two piece ones, i have not fitted them yet due to fitting instructions and lack of support from manufacture. i have read positive things on the net about getting on the plane faster and can improve turning,would be nice to speak to some one that has fitted them on the site and get there feed back, but doesn't seem to be many on the site

[PeterM]

I have !

I have a 4m Avon with a heavy 50hp 4stroke ( as in my pic on ther left), that while fine once on the plane was not so fine in the transition ...very bow high .....

I fitted them (doelfins) easily in less than an hour & they transformed my boat. Nice gentle small bow lift when getting going , planes within the local 10knt limits .

I lost a knot or two of top speed, but I dont do that very often so its fine.

I have power trim which makes them very affective , not sure I'd want them without, or on a tiller steered boat.

It was never under powered, just stern heavy ....

I'm glad I did it . ( though drilling the holes is always unpleasant) . Fitting really is line them up with the back of the anti cav plate, mark the holes , drill them ( which may need the prop off) slide them back on and bolt in place.

I recall eplainin it to someone on here with the same boat at the time who I think may have fitted them (Lakelandterrier maybe ? ) so a PM to him may shed more light on his views.

A search on here should uncover some posts about them I have made ( and others) - search Doelfins

Pete

[TRevor Lawson]

Hi have some doel fins fitted to my 25hp mariner on back of honwave 4 mtr sib, it does seem to get on plane at lower speeds , as peter m said can make tiller steer a bit of hard work in certain conds, not sure if it helps my fuel ecconomy as usually wot most of time. Easy to fit , just have to be brave with the drill.

Trevor

[thomas]

I had them on my 4.5m Valiant, I couldn't work out if it was better with or without them - they were there as the engine was on the heavy side and it was stern heavy so the fins helped it onto the plane without the bow pointing at the sky.

It was better at low speed and onto the plane with them but once on the plane I guess you need to be trimmed perfectly else they'll cause a heap of drag.

If they did live unto the manufactures figures then wouldn't the outboard makers have cottoned on and redesigned their legs?

[mister p]

Investigate hydraulic planing/trim tabs that fit to transom. Work a treat on my SR4. Planing is immediate, stability great, loss of 'flightyness' when on WOT which can be translated as previously mentioned "stability". They are adjustable which is a big pro over the DOELFINS.

[amityslave]

Well thanks everyone for the input! to be honest it's all pretty much what I expected.
The drag problem if not trimmed just right might be the only disadvantage but I like the idea of a lower bow on take off. I did take the 30% savings bit with a large pinch of salt, WOT always rules out anything sensible!
On ballance, getting on the plane a bit earlier might give up a few extra MPG.
I'll keep a note on how it works out and update in the next couple of months.
The boat, motor, trailer etc. etc. was second hand and all came in under £6k so spending on hydraulic trim tabs is not really an option on this boat but will seriously consider it on my new one.
Thanks all

Originally Posted by amityslave

I want to fit a Piranha hydrofoil kit to my anti-cav plate but have not found much on the 'tinterweb' about success or failure of said hydrofoil.
Anybody use one?
It's going on a Tohatsu 50hp on my 5.2m deep V.
These things are supposed to give upto 30% fuel savings, get you up faster (ooer) and up for less revs.
For the £38 it costs for one of these, it seems a bit of a good (and cheap) idea to me.
Any input gratefully recieved.

[mister p]

Sorry chum, the "hydraulic" bit just means that they have a pair of shocks acting on them, the adjustment is manual. They cost under a £100 a pair.

[Nick Hearne]

I have had them on 3 boats SR4, 5.75 Viper & a 6.5m hard boat, I found the them very stern heavy with too many people wanting to sit at the back the 6.5m hada 220hp inboard so plenty of power but you get 10 people all sitting too far back & it would really struggle to get up and go but ones fitted the fins did a great job making low spead estuary work muck better being able to trim in & putting the bow down!
I bought my SR4 with them fitted & it has a 50hp 2st so again not under powered & again I can trim in & keep the bow well down, I did try I with out the other day as it was just me going out & I gained about2 knots & it chime walked terribly without them so I put them straight back on!

[longjohn]

I didn't want to drill the can plate simple fitting so I fitted this beginning of last season:

Force 4 Marine Dynamics Stingray Stealth Hydrofoil - Only £89.95 - Force 4 Chandlery

- just slips over the anti can plate and then held in place with 6 grub screws horizontally tightening to the sharp edge of the can plate - there is a gripper bar between the screws and the plate - so no damage. You can see the 3 grub screw holes from the underneath shot attached.

Still firmly in place after more than a year - still good for well over 30knots - possibly a knot or two reduction in top speed not sure. On the fuel consumption side don't think it makes much difference except at low speeds as I can now plane within the harbour speed limit and its much much easier to maintain at just on the plane speed. Jumps immediately onto the plane with almost no bow lift. Much less pitching action and more stable in rough seas. Boat was never under powered (70Hp on a 4.7m) but was probably a bit stern heavy.

All in all thoroughly recommendable.

John

[amityslave]

Thanks Longjohn, looked at the Force 4 option but decided the construction/design of the Piranha two part version should be just as good for less than half the price (already on order anyway). Not a problem to fit for me as I fitout boats all day long!!

Getting it up faster for less energy without poking myself in the eye is the goal....... just like a viagra advert really.

Cheers

[9D280]

Getting on the plane a few seconds earlier is all very well, but once there do you cruise for miles and miles on the plane? The plus points for these only really kick in when:
- you have a modern engine that doesn't drink at low RPM
- you have a long run of 8 knot speed limit down the river to get to the open sea
- you are doing a lot of stop - start (e.g. Rescue boat)

Bottom line is these will create drag whilst being pulled through the water. Any wing does. Other than reducing your speed to "just on the plane" for long perioods will likely cancel any savings you see by getting on the plane quicker. If you have power trim, tucking it in to get on the plan has much the same effect.

I have removed them from 2 secondhand ribs so far as they reduced the handling to atrocious levels, and lost me 2 knots off the top speed!

Which reminds me, anyone want a pair of cheap fins?.......

[amityslave]

Thanks 9D280, I take your points. The most relevant to me will be your last two, I will be doing both a lot of distance when just on the plane, and there is also going to be a lot of stop start as a support boat (inshore rag and stick types needing someone to hold their hand).
It's the distance bit: cruising at around 10-15knots that I really would like to improve, at present this is a grievous mix of on and off plane with the type of swell and short chop common around my cruising grounds. There's no river issues as I'm straight out to sea. At present it's balls out (hense the name BamBam) or dribble along, hopefully I can iron out some of the extremes of conditions and hit a happy medium.

[longjohn]

Originally Posted by 9D280

Bottom line is these will create drag whilst being pulled through the water. Any wing does. Other than reducing your speed to "just on the plane" for long perioods will likely cancel any savings you see by getting on the plane quicker. If you have power trim, tucking it in to get on the plan has much the same effect.

Having just fitted a bigger built in ss/s fuel tank amidships, which will have affected the boat balance/trim to some degree. I thought I would do some testing with an without the hydrofoil/wing fitted.
tested:

• Top speed
• min plane speed
• WOT max RPM
• and general handling

finding were that:

• Top speed - identical with and without 'foil
• Min plane speed - at least 3 knots lower and much easier to hold just on plane speed, with the hydrofoil fitted. (Easily within the harbour 10 knot limit)
• WOT max RPM - identical (i.e. recommended max RPM for engine @WOT achieved)
• Better handling with a smoother & more stable ride with hydrofoil fitted

So don't agree with the quote above - for my boat it's a great improvement at all speeds. And given that top speed and WOT RPM are identical with and without, I would suggest that there is no overall increase in drag caused by the hydrofoil. In fact, at high speed, most of the 'foil appears to be clear of the water. And 'tucking the engine in' to get on the plane quicker in much more effective with the 'foil, producing a nice level, more or less instant transition to plane with no interim high bow attitude.


Appreciate it's horses for courses and not all boats respond, benefit or require to the same degree. I see 'foils as a cheaper, simpler alternative to trim tabs (with effectively automatic operation) on smaller boats.

[9D280]

Originally Posted by longjohn

In fact, at high speed, most of the 'foil appears to be clear of the water.

And there's your answer - with your setup you get the lift, but then the foils are clear of the draggy stuff when moving.

Best of both worlds!


As my SR4 / 25Hp was "underpowered" the foils never lifted clear - all they did was nose dive the bow! The current boat made it incredibly difficulty to steer at anything abouve about 18 knots, and made the ride very uncomfortable in a short swell. No PT in either case so it was always going to be a compromise.

[amityslave]

thanks Longjohn and 9D280, the next couple of weeks will be the tester, I'll do a report when I get back. Weather and work stopped play before I had a chance, this time I should get at least a full week of cruising. I think with a fairly common layout of 5.2m boat, 50HP 2-stroke, 2x 25ltr fuel tanks at the rear, it should be of use to a few people.
I finally went for the Piranha foils, price was right and composite material is bomb proof.
I'll be in touch!

[amityslave]

Well well well that was fun. The new fins worked better than expected!
With 2 crew and a full fuel load in the stern (50lt) not to mention the packed lunches, we got on the plane almost instantly at 6-7 knots. The fins remain below at this speed, but from 10ish they surface and we are flying along.
We had a total of two weeks of play in various conditions, but in each case we rose above it.
Detail:
Piranha Fins (two part port/stbd)
Shaft tucked well under on a stock Tohatsu 50 2-stroke
No towing

The basic positive results are:
Nose stays much lower at very slow speeds and stays lower on take off
Acceleration from just on the plane is much improved
Gets up on the plane much much faster (almost instantly)
Ride quality is noticeably smoother
Handling is twice as responsive with extremely hair raising turns

Although the fuel burn seemed to be about the same, I take the point that anything that potentially drags is gonna cost more. This said the fins seemed to be riding at 'water level' at speed, our need to make distance at 8-10 knots (fins just submerged) might be the real burner. We were having too much fun flat out to try distance at slower speeds!

Hope this helps others, I'll update as the year progresses. Thanks to everyone who has inputted, it just goes to show each experience is different, luckily mines a goodun!!

[DJS]

[QUOTE=longjohn;456104]I didn't want to drill the can plate simple fitting so I fitted this beginning of last season:

I am thinking of one of these for my speedboat which is a bit stern heavy with a 90 4 stroke and amongst other things e.g. bow in the air, it also tends to porpoise if I trim up! I am also looking at the Cobra stainless version and just help with comparisons do you have the dimensions available for the stingray?

Thanks in advance!