Fixed Trim Tabs (not doel fins or foils)

[Si Crowe]

So,

my little sib (seago 240 airdeck with 5hp 2 stroke) that i use as a tender and pissing about in has an all or nothing attitude about it. As in, squatting stern heavy in displacement speed or, up over the hump and planing along at 12kts plus.

because of the short waterline length you physically cannot sit at a steady 6 to 10kts as it surges on and off the plane.

with that in mind, i'm thinking of making my own fixed but adjustable trim tabs. help keep the bow down at lower speed and maintain low speed planing (before you ask, yes i have moved weight about, altered engine angle and height).

i cannot bolt on foils to the leg as the outboard lives either in a locker on the boat or on the aft deck rail so fins will stick out and get in the way.

below is the kind of thing I'm gonna make with a turnbuckle, plate ally and stainless hinges. the turnbuckle will be removable with qr clevis pins so when the sib is stowed the tabs will sit flush folded up to the transom.

my question is how long should the tabs be? i thought 20cm long 15cm wide (width is restricted due to sponsons and outboard leg but length... up to 43cm (height of the transom for stowage).

these are huge but you get the idea

quick release clevis pins to remove the turn buckle for stowage.

[Martin]

That’s a difficult one Si, makes complete sense what you’re trying to do, just not sure if it’ll work without getting them deeper in the water ?

It’ll be interesting to play about with and find a resolve.
Maybe knock up a foil that slips on the cav plate with easy add/remove fastening instead ?
It’s not like you’ll be hitting sound breaking speeds, just minimal lift required.

Martin

[Si Crowe]

Because I'll be using turnbuckles I will be able to lower and raise the tabs (not on the go obviously) but there will be fair bit of adjustment so i do think i'll get some lift at the back one way or another. no doubt a fair bit of fine tuning but once I've found the sweet spot I'll lock of the length.

As it is, I've clocked 16kts on gps with the little sib, not bad for only 5hp but i just want to be able to get a steady cruise at 8 or 9kts without it dropping off the plane all the time.

I want to leave the engine alone as far as possible so no foils or fins but i am gonna get a tiny-tach to see what revs i'm getting at wot... perhaps, because the boat is so light and the engine came off an XP class rnli inflatable which is far heavier the prop could potentially go up a pitch too...

In some cases, lengthening the planing area can in fact increase top speed because its a more efficient by sitting more level and in my case far less prone to porpoising which mine does allot of.

Either way, a fun little project that wont cost the earth to do and is easily reversible if it has no effect at all.

[Andy B]

I did exactly that on an over powered Dory i once had.
My problem was that it would sit in a big hole at slow speeds and heel over slightly at full throttle.
Not as good as smart tabs, but with a bit of fiddling i got them to work....
Unfortunately i don't have and good pictures of them just this!

[Si Crowe]

Here is a basic mock up of the trim tabs, obviously it's a cardboard template but I have a sheet of 10mm thick white HDPE which is the perfect medium, light, flexible but rigid enough and glass smooth. at 160mm wide and 300mm deep will create a fair bit of lift which i can dial in to perfection. as my transom is only 600mm wide i need to leave enough space for the outboard to rotate so length is preferable to girth.... fnar fnar!!

I'll screw the ally hinges to the transom and seal with sikaflex and through bolt the upper end of the turnbuckle clevis with an m6 bolt with 40mm penny washers either side, again, sealed with sikaflex.

The tab will be screwed to the lower clevis with a 20mm m6 button head bolt to minimise drag. I'm not fitting a penny washer below the tab as all the force will be upwards so no pressure but there will be one on top on the tab to spread load.

With the turnbuckles partially extended when the tab is 100% in line with the bottom of the Sib i'll have adjustment up and down to fine tune. and when the sib needs rolling up to stow away the turn buckle will be removed (quick release clevis pins) and the tabs will fold up to the transom so not in the way.

[Si Crowe]

Just on the off chance anyone is interested....

the trim tabs WORK!!!

the 2.4m sib now has far less of a hump to climb over to get on the plane, planes at lower speeds and zero porpoising which it did allot of before... when the sib is rolled up and deflated the fold up out of the way no problem. weight distribution isn't as crucial and not having to stretch all the way forward to get on the plane is a real bonus, i just sit there and up she pops.

i just set them to 0 degrees trim so not deflected down at all and i didn't fiddle with the trim angle, just bolted on and away, no doubt i can be improved and i need to raise the engine about an inch but other than that...

very happy.

[ChrisParish]

That’s excellent news. I was tempted to try something like that on my old zodiac, but it was that or launch wheels and I went with wheels.

[Si Crowe]

That’s excellent news. I was tempted to try something like that on my old zodiac, but it was that or launch wheels and I went with wheels.

Mines used as a tender so lives rolled up in a cockpit locker... wheels ain't needed...

But i'm seriously impressed, with a total spend of £20 I've got a set of real home brew tabs that work... as in exceed my expectations...

I did a test with them folded up so not having any affect and difference is night and day... takes ages to get on the plane, once up it's bouncing up and down, you can't move for fear of upsetting the trim.

Put them back down and pop in the clevis pins, give her 1/2 throttle and it pops up on the plane, flat and true, no bouncing, I can move about and whats even better, still getting 15kts out of a 5hp outboard!!! with a 14 stone fatty in it!!

[harrison]

Nice one Si, and some serious performance gain to boot.

Reckon there’s a market out there for some quality sib size tabs...go for it.