Doel fins?

[valvebounce]

I am running a 1963 18hp Evinrude,the cavitation plate is 2" below the hull,would I be wasting my time fitting Doel fins because of the depth of the cavity plate?
The boat gets out of the hole pretty quick at the moment.

 

[jrs_diesel]

Is there a specific issue that you are trying to address?

 

[Faztbullet]

Save your $$$....

 

[oldboat1]

^^^agree. You could raise the motor easily enough with a piece of one inch stock on top of the transom -- see if it makes any difference.

 

[ondarvr]

The real purpose of a fin, foil, or whatever you want to call it is to enlarge the current anti-ventilation plate so the motor can be mounted higher and not suck air from the surface during hard acceleration. They will not magically improve every aspect of boating when you bolt one on (although the marketing depeartment may say this is their purpose), this limited effectiveness and the misunderstanding of what they do leads to many odd uses and inaccurate opinions. There are situations where they get used to do different things, but they aren't the best way to achieve those results, and may cause other unwanted side effects.

​If this is what you feel you want to do (raise the motor), it may or may not help, but it requires repeated test runs to adjust the motor height until you find out whether it helped a little bit, or it was a waste of money.

 

[bruceb58]

I had a Doel Fin on a boat that I bought. It would cause my boat's stern to wash out in tight turns when picking up a skier. It was a very dangerous add on the boat I had so I took it off. It also slowed me down because it added so much drag.

If they were so great, the manufacturer would be putting them on in the first place or offer them as an accessory. They don't! Does that tell you something?

 

[ondarvr]

If they were so great, the manufacturer would be putting them on in the first place or offer them as an accessory. They don't! Does that tell you something?

​This is one of the misunderstandings, virtually every motor does come with one, it's just an enlarged AV plate.

Obviously in your situation it was used incorrectly or never adjusted to see if it would help in any way, and they are of no help in what most people want improved about their boat.

 

[bruceb58]

​This is one of the misunderstandings, virtually every motor does come with one, it's just an enlarged AV plate.

Obviously in your situation it was used incorrectly or never adjusted to see if it would help in any way, and they are of no help in what most people want improved about their boat.

There are no adjustments for these. They just bolt on. It's shaped like a wing to "supposedly" lift up your drive while getting up on plane.

My boat got up on plane equally as well when I took it off.

 

[ondarvr]

There are no adjustments for these. They just bolt on. It's shaped like a wing to "supposedly" lift up your drive while getting up on plane.

My boat got up on plane equally as well when I took it off.

​Again, more misunderstandings.

​You raise and lower the motor on the transom until you find the sweet spot for your boat/motor combo along with how you want to use the boat, you may see an improvement, or it may do nothing, or it may even make things worse. After doing all this you may need to change the prop, then do more adjusting and testing.

​The biggest hit they take in their reputation is people that just bolt it on and head for the water, they see little or no improvement in some aspects and possible negative effects in others, then just claim they don't work.

​If you don't do the work of adjusting and testing you will never find out if one helps, most of the time the results aren't really worth it though, but sometimes you find out it helps a great deal.

Forgot to add, they should not be dragging in the water at speed, this would an improper adjustment, and where most of the negative comments come from.

 

[bruceb58]

​Again, more misunderstandings.

​You raise and lower the motor on the transom until you find the sweet spot

That would have been pretty much impossible for me on my OMC Cobra sterndrive.

 

[aspeck]

I HAD one on my boat when I bought it. Figured there was a reason. It was the worst handling boat at slow speeds I have ever captained (and I have captained everything from from 12 foot jonboats to a 58 foot commercial tour boat on a lake). End of last summer my engine blew up (19 foot DC Renkin Seamaster with 115hp Johnson). Just splashed the "new" motor ... a 140 hp Johnny that I rebuilt to replace the 115. No fin on this one. Could not believe the improvement of slow speed control! Truly amazing! Not sure what made the difference ... was it the fin? :noidea: But I am one happy camper with the current set up ... and no fin. Take that for what it is worth ... which isn't very much. I never tested or adjusted with the old set-up ... just assumed it was there for a purpose so I didn't mess ... and you know what happens when you ass-u-me ...

 

[flyingscott]

Underpowered boats or boats that struggle to get on plane will see the most benefits from a fin. Boats that are adequately powered and do not struggle with hole shot generally will not see any benefits. They have no effect on top speed at all because they should be running out of the water when the boat is on plane. If the motor is set to low on the motor the fin will drag and cause potentially dangerous handling problems.

 

[bruceb58]

Better to spend money on trim tabs.

BTW, Mercury says not to bolt anything onto your cavitation plate.

 

[ondarvr]

That would have been pretty much impossible for me on my OMC Cobra sterndrive.

​Correct, that's why they have very limited success on I/O's, people frequently find that they do more harm than good, or if there are some benefits they need to decide if they outweigh negatives.

 

[ondarvr]

Better to spend money on trim tabs.

​99% of the time trim tabs are what someone should be using when they bolt on a fin, a fin just isn't the right tool for what most people want to fix.

​And for liability purposes what would you expect Merc to say, when improperly used they can create some odd and scary handling on some boats.

 

[ondarvr]

I HAD one on my boat when I bought it. Figured there was a reason. It was the worst handling boat at slow speeds I have ever captained (and I have captained everything from from 12 foot jonboats to a 58 foot commercial tour boat on a lake). End of last summer my engine blew up (19 foot DC Renkin Seamaster with 115hp Johnson). Just splashed the "new" motor ... a 140 hp Johnny that I rebuilt to replace the 115. No fin on this one. Could not believe the improvement of slow speed control! Truly amazing! Not sure what made the difference ... was it the fin? :noidea: But I am one happy camper with the current set up ... and no fin. Take that for what it is worth ... which isn't very much. I never tested or adjusted with the old set-up ... just assumed it was there for a purpose so I didn't mess ... and you know what happens when you ***-u-me ...

This is a very common response or opinion, It was on the boat when I got, took it off and things got better.

​The prior owner probably read some advertising hype and bolted it on thinking life would be great with this thing installed, most of the time there were no issues that needed to be fixed to begin with, and no testing was done to see if any gains could be made. The next person gets the boat and thinks this odd handling is normal, and then at some point removes the fin and life gets better. Again, there was no problem that needed fixing to start with, and no testing done by either owner to see if adjustments could result in improvements. The conclusion is that these things are terrible in every application and shouldn't be allowed to exist.

 

[bruceb58]

And for liability purposes what would you expect Merc to say, when improperly used they can create some odd and scary handling on some boats.

They actually state the reason is because they can break off the cavitation plate

 

[Chinewalker]

The AV plates on the old Johnrude 18s are almost paper thin - okay, cardboard, but still thinner than most seen today. Bolting anything on it will likely lead to breakage.

 

[aspeck]

The conclusion is that these things are terrible in every application and shouldn't be allowed to exist.

. Ummm, I never made that conclusion ... never said they were terrible and should not exist ... had a boat years ago ... 16 footer with a 40hp merc ... the fin in that added greatly in planing and on plane stability. No issues at slow speed maneuvering either. Which is why I assumed on this one ... not all boats or applications are equal.

 

[ondarvr]

. Ummm, I never made that conclusion ... never said they were terrible and should not exist ... had a boat years ago ... 16 footer with a 40hp merc ... the fin in that added greatly in planing and on plane stability. No issues at slow speed maneuvering either. Which is why I assumed on this one ... not all boats or applications are equal.

​I was speaking to the general situation, not exactly your experience.