Trim tab questions

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snowbird30ds

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On my princess 30ds I have 20" x 8" trim tabs and fixed tabs over the prop area and I always needs full tab to get anywhere near planning (TMD41's on shaft @ 150hp each), according to bennetts sizing guide I should have at least 30" x 9" but no where near enough room for them as it is so.....
What is the purpose of the fixed tabs in the middle?
Would it make sense to remove the fixed tabs and fit much wider tabs?
How much extra drag does longer tabs make? (24" x 12" is the other option according to bennetts)
The tabs are spaced back from the mouldings on the transom at the moment.

transom.jpg

No doubt more daft questions to follow.
Thanks in advance for any useful info.
 
The fixed tabs are usually to correct a stuff up by the designer.

The Manhattan 56 had a huge one. It lefts the stern when it is too heavy.

The manufacturer probably came up with the best compromise of lift vs stability - but someone on here will come along in a moment who knows more.
 
Agree with the above “ stuff up -.... “
It’s too heavy for the lift generated by the Hp available.
Actually it’s not managing it stern suction very well ,that’s a more accurate description of what’s going on when you try to plane , pop up , the stern sinks too much requiring max tab down .
Extending the WL L with the add on help .

Centre of Gravity too far back away from the centre of lift is not helpful .
Ideally the holy grail is to get the CoG and CoL as near as poss .
Moving weight sternwards does not help .

Problem these days ( I know yours is an old boat ) not many if any place the ER in the centre .
Those that do split the accommodation in two ,with an aft cabin complex , think mine Itama ans older 90 ,s Pershings , Sarnicos some Rivas check out the Cossaro (sp ? ) 20 M .

Agian ideally the gold standard is tabs useage for only to adjust the bow .Down in a head sea , up in a following fast sea .
Not many theses days can do this .

When you plane , it’s widely accepted that as the speed is increases the Angle of attack AoA decreases with speed .
They say optimal AoA for a planing hull is LESS than 4 degrees .
Fortunately my boats old school design and needs no tab to plane and the AoA does decrease from 18 to 38 knots with zero tabs ,

Your best bet for a fix could be bigger fixed tabs .I would leave the movable as you are aiming for the gold std .
Next time it’s out get some 1 inch marine ply and double the size esp the L of those fixed tabs .
Experiment a bit size wise ,If the waters warm rig up a temp fixation bolts through the exsisting fixed flaps , try a few different sizes ,When happy then permanently fix , glass over etc .
That’s cheaper that a repower adding Hp
Assume you have de cluttered the Kg,s cruising stores too .
 
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I do run fairly bum heavy as there's nowhere else to stow things, I also have the wrong rope cutters fitted at the moment ( 2 blade spurz) which I plan to take off to see if makes much difference to efficiency, they were on it when I got it, may eventually replace with 3 blade but they are not cheap. (lots of pot markers on east coast)
 
Are you pulling the correct revs, bottom clean etc . Make sure there is nothing else wrong before worrying about tabs
 
Doing a very quick Google I think your engines should max at 3300 rpm under load which would suggest you are under propped, has your boat been on a river maybe previous owner put props on for river use? Neil
 
Should have added if you are dropping off the plane once on it fiddling with tabs is not going to make a lot of difference, I would guess you are not getting the speed you should either. Good idea to try and find a boat test report to give you an idea of what boat did when new.
 
Props have been fettled by clements engineering to their suggested size and pitch and at full throttle just reaches the 21 knots they expected from it but I'm not about to travel at full rpm all the time, it's around 16-17 knots I'd like it to sit nicer.
Maybe just the way it will always be.
 
The only way you are likely to avoid below the water line engineering is by moving weight forward.

If you try this in a small under powered tender the results can be dramatic.

Boat I have had tend to be fully on the plan at around 14 knots or so and fall off it round and about 12 using full tab.

You can try this to some degree by shifting people forward and see what happens. If may run flatter and stay on the plane longer.

If that works things you could move (relatively) simply would be batteries and water. Each have their issues ( cables / tanks) but the only other thing left is fuel and that will be a major change!

Finally you can shift " stuff" either by throwing it out or just moving the junk forwards to under the forward berth for example.
 
Your 150hp shaft drive engines are not ideal for the size, weight and hull deadrise of your boat, Hence the fixed hull extensions fixed/trim tabbs.
In the past it was not uncommon for boat manufacturers to add a wedge to the aft end of the hull mould to give the boat more lift if they found the performance below the hoped for figures.
Dont know how they modded mould but have seen quite a few boats with this wedge shape at aft end of the hull
It would certainly be a cost effective way for you to simulate this method by having some wedges made up full size for attachment to your fixed tabbs . Go max angle first and you can then you can always reduce.
 
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