Worse fuel economy: tabs or higher rpm?

[Z-Worthy]

This may be a dumb question, but I would like to hear some opinions on the subject.

Our 1997 400DA is new to us this summer, and I'm still trying to figure out the optimal cruising setup. The engines are 3116 CATs and they really seem happy around 2200-2250rpm, but I think the bow is about a foot too high (anchor is directly on the horizon line when I'm sitting at the helm). I can bring the bow down to a more comfortable level either by increasing the tabs slightly, or by running the engines up to 2400rpm and planing out more. I don't have any sort of FloScan or fuel monitoring system and without making multiple long trips to calculate it I'm stuck playing a guessing game.

So which one is a bigger drain on the fuel economy? Higher rpms on the diesels or the increased drag from the tabs? I know a lot of people talk about throwing a few hundred lbs of chain in the anchor locker, and I'm still considering that option. But for now, which method is preferable for bringing the bow down?

 

[fwebster]

It is actually a very good question………..

There is no general or set rule for the tabs vs. increased rpm question because every hull is different. The variables are power type, load on the boat, size of the tabs, and the speed at which you run. The theory behind this is the less wetted hull you have the lower the drag and the higher the efficiency you achieve. Tabs tend to raise the stern, which decreased wetted hull, but that also increases drag. So, your overall efficiency obviously varies with engine load and rpm's….the faster you run the lower the efficiency of the fuel burned, but if it achieves a higher speed then your NMPG might be higher.

On a Caterpillar 3116 powered boat, the difference in fuel burn between rated cruise of 2400 rpm vs. the usual sweet spot of 2200-2250 is the difference between 22.6 gph and about 19 gph…..i.e. not much. On a 400DA, lower the tabs all the way, set a comfortable rpm, then gradually raise the tabs until you get no more speed increase. That is your most economical setting. I've got a heavier boat than yours, but my 450DA likes full tabs until I get to higher rpm settings..at 2250 rpm, I get a small increase by slightly raising the tabs. With a perfectly clean bottom (rare on the Gulf coast), running 2400 rpm, I'm almost at full up on the tabs.

Enjoy your Cat powered 400DA……its one of Sea Ray's best efforts.

 

[Z-Worthy]

Frank, I used this advice last summer and played around a LOT with it. You're right; she likes a lot more tabs than I expected.

Which leads to another discussion. It seems that a lot of the 35-45ft pleasure boats in our area run really bow-high. Judging purely by waterline, some of these guys are cruising at a 10-15% angle (Carvers come to mind). I try to get my 400DA to about a 5% angle for both visibility and comfort. Is that all boat design, or are a lot of skippers running their boats without enough tabs?