1972 commander 31 sedan …. Getting on plane .

Trim tabs?

@Whitney Pillsbury yes factory and they work also .

I suspect the tachs are in error. What rpm is idle, does rpm and motor sound seem to track linearly or does sound and speed track linearly? I ran across a tach that had connections for 4, 6 and 8 cylinder engines on the rear.

@Timothy Miller i will pull the tach when I’m at the boat next . They idle at 500 rpm , which seems a bit low . I may idle them up a hair to around 600-650 too

Very tired engines? I'd do a compression check, original spec is 160psi.

Are the carbs original? Could the linkage be only opening them part way?

Max cruising RPM should be 3600rpm, and the engines are screaming at that speed. My '74 flybridge sportfish version 31' runs 35mph at full-throttle, same hull and engines.

Why do you say the boat is rear heavy? Have things been added? The flat stern on my boat keeps the bow low on a full-throttle standing start, the bow never really rises out of the water, even at max-wake speed. With 900-lbs of fuel and a dozen people on the back, it still lifts up on a plane without lifting the bow much.

Scot can you tell us what RPM / speed you pop up on a plane? 900lbs of fuel? What size tanks do you have and where are they located? My 31 has 2 original round steel tanks 50 gallons each aft. My 31 also sat stern down. I replaced the old scaled up cast iron mufflers and exhaust pipes with custom fiberglass which helped. No genset… my boat is a Sedan with no flybridge. 25 gallon steel water tank under the v berth. Just getting the newly rebuilt engines broken in and tuned, but have not run up beyond 2200 rpm yet.

Optional 75-gallon fuel tanks, never had water in the freshwater tank in the bow. Stock fiberglass exhaust and iron mufflers. My engines do sit close to the front of the engine room, my buddy's '69 has the 327Qs much farther back. I have thin wedges permanently in the trailing edge of the hull for trim, factory original, my buddy has trim tabs he has never used. Both our boats run the same, never really lifting the bow out of the water at cruising speed. The flat stern only rises about 6" from the keel, out 42" to the chine, fairly flat bottom. That creates a lot of lift even at modest speeds, and a lot of drag.

There is no discernable transition to "on a plane", my boat lifts a bit, but the bow is still cutting water. The wake flattens out as the boat lifts. This is my only pic of my boat at speed, probably 18-22mph? It feels on top at lower speeds, 18mph feels like a good plane with the wake flattening out. 25mph feels like a fast plane. 35mph feels like it is flying, but I feel a bit of bow-steer if I hit waves, so the bow is still cutting water.

I'm not sure about RPMs, just got it back in the water yesterday, so been a while. I usually try to keep the engines under 3,000RPM, just to preserve them.

The '69

These are of a 38, mine and a stock photo. I was probably doing 12 there, on way up to plane. The stock photo was probabaly 30, guessing. Just for reference.

My experience is that by watching the GPS speed and trimming bow down above, perhaps 2000-2500, significant increase in speed will be noted.

You will need to reverse this when slowing to keep the nose dry

19mph GPS no current, 1/4 tanks:

25 mph, about 3200 rpm:

15mph:

I'm guessing the presence of people onboard will also have an effect, plus the presence of chop. For your boat, I do think 3200 is high for just 19mph. You might try faster-pitched props. Keep in mind, if you want to run at 25-30 all day, she's probably going to disappoint you. If you're good with 20, that's pretty cool! You might do a fuel consumption analysis. Gosh they used to have a program (flow scan?) Not sure what's new in that regard these days. Nice pillow, btw!

Do you have an express or sedan? I have a 31 sedan flybridge with 327 Q's. Generally at 2000 rpm I'm doing 8 knots and still at displacement speed. Between that and 26-700 she is squatted making a huge wake. After that the nose comes down and runs at 15 knots at 2800 and 18-20 knots at 3000. I'm running a 15x16 prop with a super cup. These props have been great. I keep the original mickey mouse props as spares. I also put in new fuel tanks a few years ago increasing from 50 gals each to 88 gals each. I do not have a gen-set.

Don't be afraid to run the boat, she'll run great at 28-3200 rpms. For 350Q engines, 3000 rpms are a walk in the park

Thanks guys for all the info. The WOT and plane info was more to confirm proper engine tuning. Here on Puget Sound there is just too much to stuff to hit, submerged logs, shoals etc to run around at 20 or 30 kts. But to plane for fuel efficency is really important givin the price of fuel.

I'm running about 15 gals per hour at 3000 rps 16-18 knots. The biggest savings came from changing carburators. I took off the AFB carbs. and put on quadrajets. Big difference!

David did the quadrajets bolt roght on the old intakes? What model #s

Not sure on models but you do need a conversion plate that you can get at any speed shop. It raises the carb about 1", still plenty of room under hatch. I will look at model #s this weekend

I found theses numbers on the side 1705 9280

3363 w hope this helps

I replaced my original Carter carbs with Edelbrock 1409 carbs. The engine guys at CD's Engine Service in Hudsonville said these are the same carbs, but much more modern, better than rebuilding my original Carters.

The throttle cable attachment hit the intake, had to cut off a part I didn't need. Really didn't need to, because I ended up adding risers, which should help keep them cooler and help the fuel atomize a little better. 3/4" risers gave me more space around for the throttle cables, and a place to add nipples for the valve cover hoses, otherwise have to drill out the blanked ports on the carbs, which is about the same effort. I bought a third 1409 from CD's for my Century and had them drill and tap that port, and I put a nipple from Amazon on it. Square-bore risers are available from almost any engine place, NAPA or Autozone, I ordered taller risers to add the hose inlets.

The throttle cables attached without changes, the carbs are already tuned for a small-block V8, they run great.