Trim with less than full fuel load

[Vincentc]

With my new found appreciation that it is OK to maintian fuel tanks at less than full (see fuel prices thread) I am now pondering the choice of were to keep / lighten the fuel ballast. With a 283 gal tank below the aft berth and 92 gallons in the engine room, will weight forward or aft be better?

My 43 DC with twin 390 671ti's seems to struggle to get on plane and I assume that more weight forward and less aft would be beneficial; however I note that with a full fuel tank forward, 1/2 full aft fuel, full water 1/2 full holding tank the boat floats a bit bow down at the dock.

Has anyone studied the question of which fuel tank lighten, or whether it would even make a difference?
Vincent

 

[REBrueckner]

Can't answer your specific question, but if you do decide to shift weight/fuel forward, you may notice rain water will no longer drain properly from decks and bridge deck area....another helpful piece: fuel weighs rought 7 lbs per gallon, so if you move say 200 gallons around, its about 1,400 lbs shifted, enough to make a small difference in plane attitude/angle....most boats benefit from a bit more wight forward for mor elevel running on plane...

 

[Passages]

Too many varibles to answer the question. Differs between models and even between same model under differing arrangements. Do you have dingy on deck? heavy furniture on one side of the salon? what about your water tanks?

Experiment and see what works best for you.

 

[REBrueckner]

" My 43 DC with twin 390 671ti's seems to struggle to get on plane ..."

I find that surprising....do you turn up rated RPM's at WOT?? or maybe we have different ideas of what "on plane" is...What speed do you make at WOT?? 20kts or so?? I would think 390 HP would be plenty to get you moving nicely...

 

[Finalee]

With full water tanks, 3/4 full fuel tanks, my 43 DC planes at 1700 rpm. (14 knots) WOT is 2350 rpm and 24 knots. Trimming the bow down shortens the time and lessens the rpm to plane. I maintain that trim, bow down most of the time. Engines are 435 hp 6v92s.

 

[Vincentc]

Thanks for the comments.

I am not accustomed to a planing hull this heavy and that may play a role in my perceptions. 390 x 2 hp seems like plenty hp to me. 1.5 gears and 24 x 23 x4 wheels give 8.5 kts at 1200 rpm (usual cruise). Tachs at top indicate 2200 port and 2400 stbd at top. This is with the Glenndinning turned on. ( either the tachs are off or the sync is not working, though it sounds like it is. At slow cruise, the tachs indicate 1200 port and 1225 stbd) 1900 - 2000 rpm yields about 14.5 kts GPS reading which feels like it is struggling to maintain plane.

I do not think we can match Finalee's 14 kts at 1700. What are your gears and props?

The pre-purchase engine survey bore scope indicated a chipped ring on the #6 cylinder port engine and sleave wear on the #5 stbd. The engines have 2500 hrs and the surveyor recommended overhauls. We made 500+ miles without incident, we want to enjoy the "new boat" for a while and I am waiting and watching before undertaking the expense of the recommended overhauls. This condition and the cost of fuel discourages much running in the planing range.

Fuel consumption does not appear too bad and appears to be in the 1.5 to 1.8 nmpg range at 8.5 kts after you take out the fuel used by the generator.

Comments regarding water draining resonate, rain water does not readily flow aft. I think I need the bow up a bit more while at the dock.

The inflatable is on the bow. It weighs 105# and I plan on moving it to the swim platform resting on pivots to allow it to rest on its transom with its bow against the stern rail.

Regards,
Vincent

 

[Finalee]

They are Allison gears 1.5. I believe the wheels are 24x24x4. The boat was redone in the 90's by the PO. Those speeds are in calm conditions and no current with a fresh bottom. Did not realize when I purchased the boat how unusual it is. Has stabs too.