Planing my 2000 2800scr

[George K]

Got a question for someone out there:
On a full tank of gas and load for bear, my single engine 28’ 7.4 mpi comes up on plane no problem. Normally, I keep the tabs all the way down along with the drive down all the way too.
Question:
Is it normal to leave the tabs and drive all the way down when up on plane?
When cruising, I know I’m suppose to pull-up the tabs and bring the drive up a little however, the boat starts to fall off if I attempt to bring up the tabs. Also, the drive likes to stay all the way down for maximum speed.
On plane, I rev about 3700 - 3800 rpm to get around 35klms
Thanks for any input on all you veteran boaters out there.
George

[yahoo82]

If I did my math right, that’s about 22 mph which I found in my 2900 will hold plane but barely. The sweet spot I found is about 26 mph (40kph) and at 29 (46 kph) it’s even better. When I’m at 26 I can back off the tabs a little and start trimming the drive out. I also found my 2900 with twin 4.3’s tends to porpoise easily (a lot of engine weight). I found the best configuration was trimming the drive out (puts the prop thrust in line with the boat, instead of angled down). Then I would back off the tabs until it just started to porpoise. My speed would climb and I’d make another adjustment to throttle, drive and tabs. I’d mostly use tabs to control porpoising. It ran more efficiently than controlling it with the drive.

Another consideration would be the type of prop. Not sure what you have but when I upgraded from Aluminum to stainless it also added cup and the additional “bite” helped prevent ventilation as I trimmed the drive up. It really helped speed performance and the “grip” helped keep on plane at lower speeds.


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[George K]

Thanks yahoo82,
It seems that the 454 (7.4mpi) is plenty big enough of an engine. It’s a stock bravo 3 prop and like I said, it comes up on plane without much effort. I wonder though if maintaining 3700-4000 rpm for hours is advisable? It seems a bit high and the boat uses a lot of gas!
George

[yahoo82]

Maybe we can get some comments from others on their normal cruising speed/RPM. 3800-4000 rpm is typical for me as the boat just feels happier there.

One thing you could try is to see if you are actually at (or near) the same throttle position when running at the higher end as the lower end. Since fuel usage is a function of boat speed, RPM and load, you may find your burning the same amount of fuel because the boat isn’t dragging through the water as much, and you end up with less load in the engine but at higher speed. I put together the attached chart for my boat when I was changing props. The most efficient place was ~3750 RPM but going +/- 250 TPM from there only impacted fuel about 0.05 Gal/Mile … like 7%. It was worth it for a more comfortable ride. No-one buys a boat to save money lol .

(Note those are my old speeds from before I changed from AL to stainless props)




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[George K]

Impressive response (graph and all) - thank you kindly! Here in Southern Ontario, I just payed $2.60 per litre of gas. My tank holds 380 litres (about 100 US gallons). That’s just short of $1000 for a fill from bone dry - but hey, I can go most of the afternoon on that (sarcasm). I really didn’t worry much about gas prices till lately. I can afford the cost but still - ouch. Hence, my trim fussing. Thanks again. Happy boating!