Adding hydrofoil to Dinghy?

[Marty]

Looking for feedback as to if adding a hydrofoil finn will improve ability to plane on my 11' Dinghy (Walker Bay 310)? At times when I have a couple passengers the 15hp Yamaha 2 stroke struggles to plain at all and was wondering if adding one will help? Seems to be mixed reviews online from what I have been reading.

Hope all is well and take care,

[bud37]

Heh Marty.......does that 310 have a hard or soft bottom ?.... a 15hp two stroke should be more than enough you would think. I see a lot of people with those foils on their outboards, I don't really know first hand, never used one....but we had an inflatable keel dinghy and it was hard to get up but once it did, it flew.....now with the aluminum hull the same 9.9 gets up easier although I did reprop it.

Have you tried a different prop yet ?

[bud37]

It just occurred to me, do you mean add a fin on the outboard or those spring loaded tabs you put on the dinghy hull/ transom ?

[Tireless]

When I purchased my new 12' dinghy last spring with a 30 on it the dealer put a fin on the motor. Most folks have them where I am.

I was advised that they do assist.

Greg

[Marty]

No I was referring to the dolphin finn you can attach to the outboard. Boat is hard bottom and the motor is a Muskoka 9.9 BTW. I have not tried a new prop yet, but that may be the avenue I go down. It has the stock aluminum and perhaps I look at a SS one, although the kids may beat it up.

[buster53]

No I was referring to the dolphin finn you can attach to the outboard. Boat is hard bottom and the motor is a Muskoka 9.9 BTW. I have not tried a new prop yet, but that may be the avenue I go down. It has the stock aluminum and perhaps I look at a SS one, although the kids may beat it up.

Your original post, you said you had a 15 hp Yamaha. I agree with others, you should easily plane running a 15 hp engine and 3 average people. 9.9 hp is a whole different story. 2 people, yes. 3, you will struggle

[bud37]

I see now......Did you get much from the mod ?

In any case there are some props now with bigger blade surface areas to push harder.....I dropped ours down in pitch, works better, just can't run to full throttle as it revs out much earlier, but really at that point the thing is flying. We have much the same dink with a 9.9, just aluminum hull.

Go see Les at CO2....maybe the whale tail would be cheaper than a new prop....but a spare prop is always good to have so you could get one with a different pitch to try and use whatever works.

[Viper]

If you're talking about a stabilizer/hydrofoil, they help to get up on plane faster providing everything else is properly set up; prop, outboard height and length, etc. The problem with most I've seen is that you have to drill holes in the cavitation plate. This is not the best thing to do on an aluminum casing for a few reasons.

Drilling holes will weaken the cav plate and diminish its ability to remain intact in an impact. An add-on fin also increases your footprint and chances of hitting something in the water. I've seen cav plates get ripped off because of this. When that happens, it sometimes means replacing the lower gearcase if a new section can't be welded on. Some places won't weld without removing the guts so either way it can be an expensive fix.

The other thing to consider is that drilling holes in the casing breaks the protective paint layer and exposes the metal. This now increases the potential for corrosion, and it won't be limited to the drill sites, it'll migrate from there. In addition, these kits usually come with stainless steel screws which will also contribute to corrosion from dissimilar metals in contact with each other and add to the presence of more stainless steel in the water around the casing. This also increases the potential for corrosion.

Stainless steel props are great for performance and are a must in some higher HP applications as they don't flex under load. Flexing results in lost efficiency. The problem with this beneficial quality is that they are not very forgiving in an impact. While an aluminum prop's blades may simply bend or break away with no further damage to the rest of the drive train, an impact with a stainless steel prop can easily result in a bent prop shaft and broken gears, very common. Adding a SS prop also increases the potential for corrosion as described above.

What ever you do, please don't bolt on fancy bling like a stainless steel skeg, even to cover up a partial break due to an impact. Again, you're adding stainless steel in the vicinity of an aluminum casing, you're drilling more holes, and the SS is not forgiving in an impact. The aluminum skegs are meant to bend or break away so the rest of the gearcase remains intact. Adding a SS skeg results in the entire skeg being lost and possibly the part of the gearcase, I've seen it happen a few times. They look great though!

[Marty]

Source of the post

No I was referring to the dolphin finn you can attach to the outboard. Boat is hard bottom and the motor is a Muskoka 9.9 BTW. I have not tried a new prop yet, but that may be the avenue I go down. It has the stock aluminum and perhaps I look at a SS one, although the kids may beat it up.

Your original post, you said you had a 15 hp Yamaha. I agree with others, you should easily plane running a 15 hp engine and 3 average people. 9.9 hp is a whole different story. 2 people, yes. 3, you will struggle

Around here we refer to 9.9 motors bored out to get closer to 15HP Muskoka 9.9's

[Midnightsun]

You cannot assume by modding an engine you are getting the "equivalent" of factory 15HP engines. I do have lots of experience with 15hp Yamaha 2 strokes and can tell you a hard bottom dingy with this motor on the back is excellent and would not have any issues planing with 2 passengers on board unless they were extremely over weight to the tune of 250+ pounds each.

That being said, I believe your issue is you have a 9.9 and the theoretical 15hp is just not happening. Shifting your passengers forward until up on plane is a far better solution IMHO. If this does not work then no modification of fins or other measure will function, just need more juice for the application. As for changing the prop from aluminum to stainless, I seriously doubt there would be any noticeable difference except in your wallet