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Author Topic:   Doel-Fin Disadvantages
mustang7nh posted 03-27-2003 08:16 AM ET (US)   Profile for mustang7nh   Send Email to mustang7nh  
Are there any downsides to having a hydrofoil? And do the different brands make alot of difference? What if you run a jackplate so the engine is up higher and a prop like Stilletto, will a hydrofoil effect this setup in a negative fashion?


Bigshot posted 03-27-2003 09:27 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
What size boat and engine? How do you know you need one? On 13's I think they are pretty helpful for porpoising and heavy loads. On 15'+ Not necessary unless underpowered or live in an area with rough seas and planing at a slow speed is necessary a lot of the time.
jimh posted 03-27-2003 09:56 AM ET (US)     Profile for jimh  Send Email to jimh     
It seems to me that the downside of installing a cavitation plate extension is:

--you have to drill holes in the original cavitation plate, thus weakening it;

--you increase the wetted surface you are dragging through the water, thus increasing drag;

--you increase the risk of striking an underwater object, and if such a strike occurs you may fracture the cavitation plate of your lower unit.

jimh posted 03-27-2003 10:06 AM ET (US)     Profile for jimh  Send Email to jimh     
"And do the different brands make [a lot] of difference?"

I always get back to the fundamental issue, which to me is roughly "why don't outboard engine makers include larger cavitation plates on their engines?"

The notion that a $2 piece of plastic will somehow correct or improve the entire operation of a boat and motor combination that might have cost $50,000 seems like a stretch to me.

Let's say I am a boat builder and an engine maker, like Brunswick. I sell boats and motors in pre-rigged packages. I was smart enough to build the motor--a manufactured good that is millions of times more complex than the plastic cavitation plate. I was smart enough to design the hull, to mold it, and to induce people to buy it.

But I am not smart enough to add a little piece of plastic to the cavitation plate so that my entire product will perform better.

My boat will go faster.

My boat will use less gas.

My boat will plane better.

But I am blind to all this and I just can't figure out how to do this by adding a piece of plastic to the cavitation plate. The plastic would probably increase my costs about $2, but I don't add it.

Perhaps there is a global conspiracy among all boat makers and engine makers to not provide this performance enhancing piece of platic on their boats and motors.

triblet posted 03-27-2003 10:10 AM ET (US)     Profile for triblet  Send Email to triblet     
They catch kelp, if you live in kelp kountry.


Bigshot posted 03-27-2003 10:49 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Like I said on 11&13's they work well for planing and porpoising issues. On my 15 & 17's I have NEVER seen the need and actually removed a few. Sure I could plane at 2000 rpms but when & why would I want to do so. Holeshot on 15 & 17's is great and porpoising is not evident so I do not see the need. I do run one on my 20 Hydra-Sports and it works well(porpoising), but slows me down a tad. I will obviously remove this once I open my wallet up for the proper cure....trim tabs.
JBCornwell posted 03-27-2003 11:05 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Ahoy, Mustang.

I use a SE Sport300. They claim to be "the fastest" hydrofoil. It is on a '80 Montauk/Suzi DF70 with 4" setback. The cav plate is at keel level. Prop is a Suzi SS 18".

I was told to expect it to reduce WOT speed and increase fuel usage.

I am able to plane more quickly and at lower speed. My WOT speed is unaffected. I have not noticed any change in fuel usage.

I do not agree that it increases wetted surface or drag. Nor do I agree that the foil is intended to change engine performance. It changes hull performance.

The wetted surface of the hull on plane is noticably reduced and the boat rides very noticably smoother than without the foil.

Downside? Transition into porpoising on over-trimming becomes quite sharp. You get some ridicule from people who have theories that don't necessarily affect on-the-water performance.

Red sky at night. . .

andygere posted 03-27-2003 05:28 PM ET (US)     Profile for andygere  Send Email to andygere     
I suppose the same arguments could be made for trim tabs, yet they are regularly added to many top-of-the-line boats to improve performance and handling. My Montauk has a Whale Tail aluminum hydrofoil installed by the previous owner, and I am convinced that it helps the boat plane faster and stay on plane at lower speeds. I think in a lot of ways, the hydrofoils act like a single, centralized trim tab that allows an outboard motor's existing trim mechanism to be more effective. My theory on why Mercury, Yamaha, Evinrude, Johnson and others are not building them into their engines: They look funny and the aftermarket companies all have patents that they defend vigorously. If one of the outboard manufacturers had invented the Doel Fin, I'd bet they would be built into many of the motors made by that firm. As I mentioned in another post, I believe my Dad's Dauntless 16/Mercury 90 would benefit from a Doel Fin to help reduce bow rise and get it on plane a bit faster. Others seem to agree that Brunswick didn't quite get it right on this combo but that the $2 piece of plastic helps quite a bit.

Chuck is right about the kelp, but for me it is a minor drawback.

roofer posted 03-27-2003 05:36 PM ET (US)     Profile for roofer  Send Email to roofer     
I am adding one to my 2000 Outrage that has a 135 opti on it. I use my boat for water sking sometimes and am hoping that it will help to get the skier up faster, we will see. I will let you know if I see an improvement in the performance. Also if it does lose some top end speed it wont afect me that much since I use the boat on the great lakes where due to choppy water it is hard to find a time when I can go WOT.
Hoop posted 03-27-2003 06:39 PM ET (US)     Profile for Hoop  Send Email to Hoop     
I have a Doel-Fin on my 2000 Montauk, and had one on my 15 Dauntless before that. I figure that I lost 2 mph top end speed with the Doel-Fin, but it planes at lower speeds and has improved the handling of the boat. Overall I like it.

On the Dauntless there was lots of bow-rise, and the Doel-Fin stopped that, improving visibility from the console when coming up on plane. And it porpoised, too, a problem the Doel-Fin also solved. I can't comment on the 'improved mileage' claims; I don't measure gas consumption to that precision.

There have been previous threads on the Doel-Fin and other cavitation plate extensions. My loss of top end speed experience was corroborated in those threads.

Best regards,

captbone posted 03-27-2003 07:50 PM ET (US)     Profile for captbone  Send Email to captbone     
The loss of speed depends on your boat and how high your motor is mounted and trimed. On my old 22 hydrosport the doelfin would come out of the water completely when on a plane so there was no effect. When running on a plane and trimed correctly, have someone go aft carefully and look to see if the cavatation plate on the motor is above the water. If it is then you can almost bet you will have no loss of top end. As far as the plate breaking, I do not beleive under normal situations that it will be a problem. Prolonged speed at subplaning (plowing) then I can see a threat. My only problem was picking up sea weed now and then.
witsendfl posted 03-27-2003 08:06 PM ET (US)     Profile for witsendfl  Send Email to witsendfl     

I have had good sucess w/ different hydrofoils. I prefer the DOELFIN over the others. They have a lifetime warranty as I recall. I have used them on differnt configurations. The first was on a 120 HP Merc I/O. Helped out tremendiously (sp). I have used them on 3 dofferent O/Bs.

70 HP Johnson on a 15 foot Sport. The motor has p/t&t. Bigshot is correct about planning faster. It also helps in cornering. The hull remains in a flatter postion while turning vs. a slight tip to one side. The product does work. I have seen them used on much larger motors also, and users have professed the same. I bought my first Doelfin at a boat show for $59.95 boat show special. Mount as far back as you can on the cavitation plate.

I am sure it will work for you

Good Luck !

witsendfl JimK

mustang7nh posted 03-28-2003 07:33 AM ET (US)     Profile for mustang7nh  Send Email to mustang7nh     
Thanks for all the info.

When you say "mount as far back" as you can, are you referring moving it toward the transom or away?

I'm running a 1988 20 Outrage with 200 johnson 1988 and would like to plane the boat on slower speeds because of the rough water when heading into the waves (chop). I like the idea of the trim tabs but have heard about the wood not going across the whole transom and hence creating some mounting issues.

If I went with tabs I think I would go with Bennetts, but how difficult is the mounting and how do you know the best size to pick for optimum performance? Thanks

frostbite posted 03-28-2003 08:51 AM ET (US)     Profile for frostbite  Send Email to frostbite     
My '87 Montauk came with Doelfins on a 90 HP Yamaha. I took them off. Try holding the tip of an oar in the water at 30 knots.
RCS posted 03-28-2003 09:10 AM ET (US)     Profile for RCS  Send Email to RCS     
My 20'(also 1988) came with Lenco tabs and a Doel-fin. the tabs are mounted within the limits of the wood backing at the transom and work great. I'm not sure of the size but I will measure them and get back to you. This is my first experience with tabs so I can't compare to Bennetts. I can say the Lencos move very fast and the hull reacts with very litte movement of the tabs. The Doel-fin has no drawbacks I can put my finger on. With the tabs down I get a top speed of about 40mph with a 200hp outboard. I guess that's 2-4 mph slower than what the chart shows on the reference page on this forum. The boat planes very fast with the tabs down. I'm not sure if the Doel-fins alone would help much with the time to plane. I'll try to send you an email photo of the transom and tabs on my boat if you'll send an email address.
tully_mars posted 03-28-2003 11:48 PM ET (US)     Profile for tully_mars  Send Email to tully_mars     
The obvious bad thing about doel fins are they create a lot of stress on your cavitation plate thus weakening it. One of my oldest friends broke the entire starboard side of his cavitation plate off of his 140 evinrude by just leaning on it a little one day when the boat was on a trailer and he was changing the gear fluid.

Tully Mars

ducktwin posted 03-29-2003 12:19 AM ET (US)     Profile for ducktwin  Send Email to ducktwin     

I agree, the fins are an extension of the hull, not the engine. It just so happens that they are sometimes needed, as an aid, to planing efficiency

lakeman posted 03-29-2003 08:29 AM ET (US)     Profile for lakeman  Send Email to lakeman     
I have run the sting ray on my Grady 206-g for years never though much about it. I was broken by a boater running into the motor, I took it off and went fishing, came back and bought a new one immediatily.
Have a friend with a Bayliner big Cabin 23' or 24' with a 350 I/O, boat would hardly get up on plane or took a long time. We/he installed a dolphin on the boat and it now alomost jumps up on plane.
I think it helps some boats.
djahncke posted 03-29-2003 10:09 AM ET (US)     Profile for djahncke  Send Email to djahncke     

There is a pair of doel fins for sale on eBay. This could be a cheap way to get a set with which to experiment.

I had a pair on my Dauntless 17 until the powerhead blew on my motor (unrelated to the doel fins). I am going to put doel fins back on as soon as the warrenty on my new motor runs out.

Steve Leone posted 03-30-2003 12:57 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
On some outboard applications the addition of fins will weaken and/or damage the lower mounts. I have seen it many times over. I agree with Jimh that if they were that great they would have casted them at the factory that way. Actually O.M.C. did on some of the comercial models (40hp)but that only lasted a couple of years I believe. Steve
mustang7nh posted 03-30-2003 06:10 AM ET (US)     Profile for mustang7nh  Send Email to mustang7nh     
thanks for the great info everyone.
litnin posted 03-30-2003 06:37 AM ET (US)     Profile for litnin  Send Email to litnin     
I scrape my leg on mine in the garage.
SSCH posted 03-30-2003 08:57 AM ET (US)     Profile for SSCH  Send Email to SSCH     
Trim tabs are a much more efficient way to control hull attitude than Doel fins or any other engine mounted device. Tabs cannot raise the bow as can the engine mounted devices, but to raise the bow in a materiel way you have to add a lot of drag with your fin. Normal engine trim is all I've every needed in my Whalers to raise the bow plenty. Trim tabs have an added advantage of controlling roll since you can adjust either side independantly, which is a problem with the narrow classic Whalers.

What do you suppose would happen if one side of your Doel fin broke off while you were running? If its really doing anything, I'd bet you'd have an interesting experience.

jimh posted 03-30-2003 10:09 AM ET (US)     Profile for jimh  Send Email to jimh     
On many boats the transom is constructed with an angle, perhaps 10-13 degrees, so that the transom is a greater distance from the bow at the gunwale than it is at the keel. An outboard engine mounted directly to the transom would be able to be trimmed all the way in against the transom, and at this angle the thrust of the propeller would tend to lower the bow.

It has often been mentioned that on some OMC model outboard engines, the arrangement of the power trim was such that the engine could not be trimmed in (or down) sufficiently to produce thrust at an angle that would keep the bow down, especially during the transition from displacement to plane.

One solution for this is to insert a wedge-shaped shim between the engine and the transom (with the thicker portion of the wedge on the upper end). This has the effect of allowing the engine to be trimmed in at a greater angle and permitting the propeller to work at an angle where it can develop thrust that will lower the bow.

Of course, installing such a shim requires remounting the engine on the transom, a significant amount of work and something that not every boat owner can perform in his driveway on a Saturday afternoon.

I don't know if there is an agreed upon standard for the amount of transom angle usually designed into a boat. It may be entirely optional and vary widely from boat to boat.

On small and relatively light boats, which is a good description of a Boston Whaler, the distribution of weight can be very different from boat to boat. The weight of the outboard engine can vary over a wide range, perhaps as low as 200# or as high as over 400#. Some owners like dual batteries; that adds more weight which can be located in a variety of places. Use of larger and multiple fuel tanks also adds significant weight and allows various locations to be used.

And there is the weight of the operator, which in the case of a young boy might be less than 100# or, in the case of an older gentleman of leisure, might be approaching 300#.

It is not difficult to imagine a situation where two identical boats have very significant differences in their weights and the distribution of that weight, as well as the angle of trim available to them for their outboard engines.

On a boat with plenty of extra weight at the aft end of the boat and an outboard engine that is heavy and does not have full trim range, it may occur that the transition from displacement mode to plane results in a great deal of bow rise.

It does not seem farfetched that the same model of boat, with a different engine and different accessories, driven by a different skipper, might be able to transition from displacement to plane without any signifcant bow rise.

doobee posted 04-02-2003 11:52 PM ET (US)     Profile for doobee  Send Email to doobee     
They will negatively affect resale value.
priceless91 posted 03-12-2007 04:30 PM ET (US)     Profile for priceless91  Send Email to priceless91     
leave holes in engine when you take fins out
Hoosier posted 03-13-2007 12:25 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
The new Stingray XPRIII hydrofoil is a no holes model. I saw it in Bass Pro Shops catalogue.
WT posted 03-13-2007 01:27 PM ET (US)     Profile for WT  Send Email to WT     
You don't need to drill holes to install the Turbo Lift. It is built like a "tank". ?action=view¤t=DSCN1293.jpg


Bensjoy posted 03-13-2007 04:51 PM ET (US)     Profile for Bensjoy  Send Email to Bensjoy     
Marine Dynamics, manufacturer of "Stingray" stabilizers, is introducing their "XR3" model which requires no drilling to install.
Hoosier posted 03-13-2007 07:39 PM ET (US)     Profile for Hoosier  Send Email to Hoosier ProductDisplay?storeId=10151&catalogId=10001&langId=-1&partNumber=91124& hvarTarget=search&cmCat=SearchResults

This is all I could find about this new foil. Big question is: will it stay on?

jimh posted 03-13-2007 08:06 PM ET (US)     Profile for jimh  Send Email to jimh     

It belongs in PERFORMANCE. If you want to talk about this topic, move to PERFORMANCE.

[Discussion closed to further comments]

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