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.

Great info thanks

@Scott Miller the boat is all original in terms of carbs , engines , fuel tanks etc. it has a heavy original kohler Gen set That sets in between the engines. I went full throttle this last weekend . It went now up and wouldn’t break over . I was on the original tabs , they were moving but still didn’t make andofference . Could the marina may have put the props on the incorrect sides ? Right on the left and left on the right ? I couldn’t get it to move more than about 12 knots no matter how much gas I gave it . Engines ran good , other than sucking down a quart of oil each when i did that and a lot of gas . Maybe I do have worn out engines :( the boat is original with 1200 hours on the hour meter

1200 HRS IS nothing. there has to be another factor. my port engine has over 3000 hrs. The oil usage is alarming, I go thru a quart, maybe 2 all year. You should have no trouble cruising at 3000 rpm's. I have a flybridge sedan and have cruised from Mays Landing Nj to various points in the chesapeake bay running any where between 28 and 3200 rpm's (depending on chop and Delaware Bay) for hours at a time. Those beasts are made to run, Tune them well, feed them fuel, oil & coolant and they will last a long time.

@David Bernard it’s weird. It runs really well , it just won’t go any faster . The transmissions don’t seem to be slipping either. Mechanically the boat seems to be in good shape . They start well , idle smooth and power up smooth , but it’s just a dog. The props are brand new 15x16 (cupped to make 17 pitch ) which the oem props are 3 blade bronze 15x17 so that shouldn’t be an issue . This boat was idled around for the better part of 20 years on lake texoma in north Texas and never ran hard at all so maybe it’s just carboned up ?

2,100 rpm and 9 knots sounds about right with the correct props. Burning that much oil hints at tired engine. A compression test is required, probably can rent a tester for free from Autozone or other parts stores. I prefer one that threads into the spark plug hole, not the rubber stopper you have to hold into the hole. Simple process, watch some videos. Put the spark plugs into an egg crate and label which cylinder, look up how to read spark plugs.

Lots of possible issues, but compression test will really tell you the health of the mechanical side of your engine. No point looking further at spark and fuel systems, until you know the health of the mechanicals.

Matt, Where are you located. Something just does not sound right. You have 15x16 props with cup. (like mine) You have 350's (I have 327's as of last year Port 327 stbd 350) but still won't plane properly. When it is in full throttle, how many rpm's are you getting? Did I here 4000? You should be doing 33 knots at 4000 rpm's. That genset is between the engines? That should be far enough forward not to make her butt heavy.

It almost sounds like the boat is dragging something??? What are the symptoms of the rudders being out of sync??? Not both aligned forward at the same time?

Buy an inexpensive hand tach from Amazon and see if the prop shafts are turning the proper speed in relation to engine rpm. I picked one up you put a small reflective tape on the shaft hub and take some readings

@David Bernard I’m in north Texas on lake texoma. On the ok /Tx border. Yes I was full till at 4000rpm and it just chugs . Trans both seem tight , engines run strong too. Could the do dos at the Marina put my new props on backwards maybe ? On the wrong side ?

No you would go backwards when you put it in forward. Certainly does not make sense. You should be flying at 4000r's. Can't imagine the generator being that much of an issue. A friend of mine had a 31 Sports Express with 350 Q's and no tabs. It ran comfortably at 3500r's. Below that it seemed to lug even though it was on plane. The sports express has to be so much lighter than my sedan fly bridge. What model Commander is Yours?

@David Bernard its a sedan model. Has the salon with the sliding back door. No flybridge. Inside helm station

@David Bernard I’m lead to believe now , that the transmissions are slipping after further investigation . Engines will produce the rpm , but it won’t push the boat any harder . Think the old paragons have seen better days . So for the time being she’s just a dinner cruiser at a barn burning 12 knots ……

Have you checked the fluid in the tranny's? I would also check to see if the tachs are reading correctly. If they were reading 4000 R's, those engines would be screaming. My port tach over reads sometimes.

Same as mine except I have the fly bridge. Those paragons are good strong tranny's I would look at tach's. An old tach & dwell machine would be good, if you can find one.

.

Captain Mat ...... Purchase or borrow a digital dial back timing / tach gun and install on engine then run up to full throttle to verify rpm under way . Before that test remove throttle linkage at carb and CONFIRM you have correct full throttle at carb & helm match . You must also haul out your 31 and have the props removed and sent to a prop service shop that does Prop Scan or Hale MRI scanning to CONFIRM the props are correct pitch & dia . That is the only way to clear up your issues for prop size .You are going nowhere without a positive prop scan report at this time . Adding a 1 inch carb spacer which has been mentioned here will not help your boat to plane off issues . The best / only carb spacer for your engines is a 4 hole phenolic spacer 3/8 size and correct 4 hole base gasket from Dashman.net with no modifications needed for Pcv hose which is much better for heat soak reduction / performance . How did your 31 pass a survey / sea trial with these issues you are having ??? Do NOT replace carbs for any new carbs . Keep it simple .

Tim

-----

@tim toth well I went against my better judgement and didn’t do the survey. I should have but the boat is still solid , just projects to get it in tip top shape.

Yesterday we went out with the newly adjusted packing gland on the starboard engine. The good news we popped up on a plane at 2800 rpm… hit 15 kts at 3000. The bad news my packing gland shot up to 300° I have the alledged dripless green packing putty and had it broke in and set and dripping about 1 drip every ten seconds stopped in the slip. I will loosen the gland backup and try again but not convinced this stuff really works. My port shaft does not drip at all ever and runs about 15° above water temp. It is packed with what ever was in itcwhen I purchased the boat

That sounds about right. I have the old standard flax packing that drips every 10-20 seconds. It does not fill the bilge. I think it evaporates. Dave

My port stuffing gland doesn’t drip or overheat??? I wish I knew what was in it…. It aint broke so I am not messing with it. It is an undersized packing gland with a hose within a hose at the shaft log end…. A little decerning but it does not leak. When time comes to replace the hose , I will be faced with a decision.

I have used the Teflon coated flax and found almost no weeping and no significant, >10F rise. Emphasis on almost.

Getting that coupling off can be a real bear

I purchased a couple 1” shaft pieces I can place between the flanges and use the 4 bolts to press the 1-1/4” shaft out. The big issue is on my boat, hull #1, the forward bulk head in the engine room is further aft leaving no room …. Literally 1” between the packing gland and the end of the shaft coulpler and I cut my shaft log down another 1” and an inch off the tail of the Buck Algonquin packing gland!!

I might have to hual out and remove the putty and go back to old school.

I like new technology but the tried & true is more comforting

To press the shaft off the coupler I use longer bolts and place a socket on the shaft and tighten until it comes out.

Exactly