PLANING BOAT PECULIAR USE QUESTION

[kolyo]

Or would 24"x24" be even better - my logic being you are effectively increasing hull length, plus if you mount the rams in the same place, for a given deflection at the tip (3" say) you only need half the trim angle? Or is my logic badly flawed there?

All I know is that a hugely experienced boater has swapped the OEM 12" x 12" trim tabs with 12" x 24" ones on a Bayliner 2556 and the result has proved to be well worth the effort. After installing the larger tabs on his boat he observed a noticeable increase in top speed and, more importantly, in lift, particularly at lower speeds allowing the boat to STAY ON PLANE at as low as 7 kts.

 

[kolyo]

Dream on. A doubling of the trim tab size is not going to turn a boat which likely planes at about 12-15kts currently into a boat that planes at 7kts... QUOTE]

I never said that just by enlarging the surface area of the trim tabs a heavy boat like mine would be able to GET ON PLANE at 7kts.

 

[ontheplane]

No I realise that but what effect WOULD it have?

I am sure there is a reason, but never understood why the tabs are relatively small surely bigger is better, or do I not understand how they work?

 

[gordmac]

This chart seems to indicate that your 7.4 is producing about 60hp at idle (800rpm). If you try going everywhere at idle, that will give you an idea of what 60hp can do for you, but if you add a 60hp engine you will need to be at full throttle to produce 60hp, and that won't be much fun or very kind to the engine. Realistically, if you put a 60hp engine in the back, you will only really have 40-50hp to use unless you go everywhere flat out.View attachment 103860

It is not as simple as that! These graphs show the maximum power available at a particular engine speed. The engine is run at full throttle and the brake on the dynamometer adjusted to give each rpm to produce the graph. In use at tickover the throttle will be almost closed and power will be very low.

 

[ontheplane]

But to move a 25' ish boat at displacement speeds takes very little power - and it's propped and geared for the huge torque of that V8.

Not sure, but my guess is that it tickover it will do 3-4 knts - and with the throttle cracked open a fraction, that will go up to 6-7knts.

The OP will only need to "properly" open the throttle to get near or over hull speed - so to reach 9knts might use twice as much fuel as 7 knts