Adding hydrofoil to Dinghy?

[buster53]

Just as a reference...
My brother used to have a Logic, maybe 9-10 feet, not a lightweight boat, with a Honda 15 on the back. With 4 of us, about 700 lbs of people, it would plane. Talk about a struggle. It would take a good minute, maybe longer, but it would get up on plane. I don't recall if we ever tried with 3, but would imagine it would have been a fairly easy plane.
With just one person, it was dangerously fast.

[Marty]

Source of the post 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

Thanks for the feedback and yes if weight is shifted we have no problem plaining. I was thinking the SS route might help as on my previous boat 245 Bayliner the engine was under powered and struggled to plain with anything over 5 passengers with an aluminum prop and I converted to SS and worked like a charm.

Cheers,

[Tireless]

My previous dinghy was a Brig 330, 11', with a 9.9 Merc. I am 6'4" and go 265, pre Covid. I could get that on plane with the Admiral and I and it would clip along pretty good. With 3 people, I was dead in the water and restricted to putting along.

In my experience, I have learned that the longer the dinghy the better your planing ability. I had a friend with a 9' dinghy with a 15 Yamaha on it and he struggled to get it on plane with him and his wife in it. With him alone that thing was scary fast for a dinghy.

Greg

[Midnightsun]

The only advantage from aluminum to stainless is stainless is less pliable therefor will not bend under load. When aluminum flexes the pitch obviously changes a bit when the highest load is applied such as when attempting to get on plane. Once up and running on plane the prop tends to regain its original pitch or most of it. There is a second advantage which is the stainless version can be made thinner because it is less pliable and therefor offer a little less resistance on the leading edge. These differences are very small and any obvious benefits can simply be accounted for by change of diameter/pitch from one manufacturer/model to another. All this is more for bigger boats, on a 15HP the prop seems like a toy in comparison and I doubt there is any flex in these due to the small size.

The biggest disadvantage with stainless is the weight difference which is probably 3 fold. Then there is the internal damage the much more rugged stainless will cause when you eventually do hit that rock. Aluminum on the other hand self destructs so you simply end up changing the prop as opposed to shafts and gears and whatever else inside which could take the impact.

Now for the big important question. Did you change the original prop when you modified the engine to pump out 15hp?

The way we do it here is we buy a 15 and put 9.9 decals on it.

Here is a link to 15hp Yamaha props. As you can see there is only 1 choice for diameter so you are stuck with this. Pitch varies so those could be played with a little. https://www.rubexprops.com/boat-propell ... 8%20Spline)

If you look for 9.9 Yamaha 2 stroke props you will see the list is identical to the 15hp selection.

[Cooler]

If it was me, I would just add a big cooler filled with ice and beverages in the bow of the dinghy, and keep my speed to something less than rocket. The beverages will keep you hydrated for the extra 5 min your travel will take. WAY less than a SS prop that will eventually take out your shaft and/or gears if the kids are really bent on bending some props. er

[Marty]

Source of the post If it was me, I would just add a big cooler filled with ice and beverages in the bow of the dinghy, and keep my speed to something less than rocket. The beverages will keep you hydrated for the extra 5 min your travel will take. WAY less than a SS prop that will eventually take out your shaft and/or gears if the kids are really bent on bending some props. er

Best advice yet!! I'll take you up on this one!

Cheers,

[Midnightsun]

Well if that's the route you want to go may I suggest you add one of these also to do it in style! Only the best will do. https://www.burnewiin.com/accessories/b ... tle-opener

[ColRon]

Hans with the money he saves by NOT getting that stainless prop , he should be able to get at least two of those high dollar bottle openers. One for the boat and the other for the dingy. and remember a lot of late night testing went into thier design.