Hydrofoil

[Valiant Knight]

I used "Doelfin" hydrofoils on all my RIBs.
The result on the 3.5 m with a 25 hp engine was outstanding.
The result on the 4.5 m with a 40 hp was still impressive
I now use them on a 5.2 m with a 90 hp. Without them, going into plane (driver only) meant a long weelie, before the bow came down. With them, the rise of the bow is limited (in height aand time).
The influence of the hydrofoil decreases with boat length. For a 7 m long boat the impact may be minor.

Antonio Manuel Carrapatoso

[9D280]

Originally Posted by Nos4r2

So far, every boat I've helmed that wasn't massively underpowered and had doelfins/hydrofoils on has had some pretty scary handling issues at WOT when trimming up.

By you guys' standards most of my boats are / were "underpowered"

When I got the Humber with a yam 55 on the transom it had those fins.... it handled like a pregnant whale. took them off, tweaked the trim, handled fine. (and also got up on the plane Ok with the correct prop, once changed!)

Rewind to the SR4 / Suz 25. Same story, although it did take a bit longer to get up. Not surprising considering the ballast chamber, but keeping them on not only made it handle really sluggishly, it also knocked about 2.5 knots off the top speed, so they don't always "ride above the waves".....

I'm not knocking them for every situation, however I have had 100% "better without" experience with them. Personally, I'd them in "last resort" territory and tweak the trim etc. first.

[Nos4r2]

By underpowered, I mean motors that'll only just make them plane.

The last boat I helmed that had them which wasn't underpowered would at higher throttle openings in the rough in a following sea was massively oversensitive to trim and would randomly feel as though it was trying to pull the transom under then pivot around the motor and broach.
Certainly not what you need when you're trying to keep the bow up.

[jyasaki]

Originally Posted by ajstars

Ok, I will go and play with it and drop it down to see exactly where it sits.
In relation to the trim tabs, I have seen electric and automatic ones. I will be honest in that having never seen them in action I don't understand how they work. I guess they keep the water down at the back rather than allowing it to rise and hence give lift as well?

They deflect water downward, essentially lifting the transom. Adjustable ones allow the amount of "lift" to be tuned to suit your needs. Down a bunch when getting up on plane, less down when cruising, and asymmetrical when countering list to one side or the other.

Auto ones are primarily just for getting out of the hole, I think. I'm sure someone who has more experience with them will chime in.

Caveat: I don't have tabs, and have never played with them.

jky

[jeffstevens763@g]

i had auto tabs on my quicksilver 500 some years ago to cure a slight list to starboard they sorted the list and made the ride a lot better on the plane, but the quicksilver has a planing pad at the stern which tends to make them suseptable to movement i found but the tabs made it better.
bennett trimtabs for info.

[cptsideways]

Do not under estimate the lift force of fins in the water, a really good design could easily give 100kg of lift even at lower speeds, even better if they were designed to cavitate at higher speeds & reduce their lift massively but I've not seen any that do yet.. well not deliberately anyway.

[9D280]

Originally Posted by jeffstevens763@g

i had auto tabs on my quicksilver 500 some years ago to cure a slight list to starboard they sorted the list and made the ride a lot better on the plane, but the quicksilver has a planing pad at the stern which tends to make them suseptable to movement i found but the tabs made it better.
bennett trimtabs for info.

Hee hee. My Humber had a horrendous list to Port. After the antifoul was removed I found a repair.... presumably carried out at speed when it was owned by a sailing club.....which meant the rear 2' of the hull was somewhat asymmetric! One trip to the boatbuilder and a good time upside down in his workshop later, and she rides perfectly level now.