RPM's and Cruising speed on 2120 PH

[ThePeterParker]

Hello Everyone,

Had my 3rd trip out last weekend and played with the tabs and engine and at full plane with 4 guys I got 32 mph and 4700 rpm's and was wondering is this good or bad? engine felt like it was handling well but I don't know what I should look for on speed vs rpm's?

 

[DaleH]

Can't tell that way ...

Try to clean the bottom before the 'test' and find out the rated maximum RPMs. Ideally you want to be within 200-300 RPMs off the max RPMs with 1/3rd fuel, 2 people, (or you and 1/2-tank fuel) and ALL typical gear carried on board.

I myself would NEVER run a boat with a prop that couldn't achieve max RPMs to 200 off. Tweak the prop if needed. Then and only then can we comment on your run w/ a heavier load.

But, if your OB is 500 RPMs off max (on test load) or more and then you load her up ... you could be lugging the motor, which is THE FASTEST way to burn up and kill a powerhead, less running no oil.

 

[ThePeterParker]

Thanks Dale will try in the spring time

 

[rloomis]

What size engine, prop, and what height is your engine mounted?
If you're getting 32mph @4700 rpm, that's pretty good if you still have enough throttle left
to wind up to 5800-6k rpm and then you should be hitting pretty close to 40+mph.
If that was your max at WOT, then you have some tuning to do next spring
Like Dale said, you want to make sure you can get within a few hundred RPM of max at WOT
so you're not overpropped and stressing your engine.

Here's what I get on my 2120 with a F225:
viewtopic.php?f=19&t=10103

Since that data was taken, I raised the motor 1 additional hole to highest position and
added a Bob's Machine Shop wedge to give a little extra negative trim.

 

[warthog5]

A fuel flow meter will tell you where everything needs to be....Motor trim, tab adjustment, motor RPM.

 

[DaleH]

... added a Bob's Machine Shop wedge to give a little extra negative trim.

Why would you ever want/need more trim IN?

 

[rloomis]

... added a Bob's Machine Shop wedge to give a little extra negative trim.

Why would you ever want/need more trim IN?

This is what it looks like (except the caulking is cleaned up now). I always had way more positive trim range than I could use. But with a heavy F225, 48gal bait tank, and full 100gal gas tank, I was often bottomed out looking for enough negative trim to push the nose over to stay on plane at lower speeds while making way into choppy headseas. The wedge gives me that extra push I need to make a little better time in those conditions and beat up my bait a little less. When planing smooth seas at high speed my trim is right at the midrange or sometimes a little higher, but I still have plenty of positive trim range to spare.

 

[DaleH]

Bigger (wider) trim tabs would be MUCH more fuel efficient ...

 

[sydngoose]

Need more information to provide accurate answer.
But, my 2120 w/ no bottom paint, powered by a f150 spinning a Yamaha Reliance 15m 14 /12 prop w/ 1/2 tank fuel (50 gallons) with me and my dog (66 pound golden retriever) hit 38 mph at 5800 rpm's yesterday morning on the river. Just for a point of reference.

 

[Mokee]

My 2120 with a F-200XB spinning a 3x14.5x15 at 6000 rpm hits 43 mph with a full tank and 4 people. Cruise at 4,000rpm clocks 27-28 mph SOG according to GPS.