1972 Uniflite Sport Sedan 31 performance issues


New member
Hello all

I've been going through the forum about the old Uniflites with Chrysler 318's. It seems that quite a few of you cruise in the high 20's, someone even said he was getting up to 40 mph.

I bought my boat 2 years ago. One engine overheated and stalled on the way to my new slip. For that entire summer I couldn't bring it over 2000 RPM without it overheating. Through trial and error I replaced the raw water pump, fresh water pump, manifolds, risers, cleaned heat exchanger, new carb and fuel pump.

Last summer there were no overheating issues. That said, the previous owner kept a fairly comprehensive journal. When they bought the boat 20 years ago they claimed that it would do about 22 knots at 3300 RPM. At that RPM I am lucky to get 17 knots. I usually try to cruise at 2700 RPM and I get about 14 knots.

I have fuel flow meters, working trim tabs, and my speed I am getting from my GPS.

Does this sound normal? Others seem to be getting a lot more speed from the same boat. Could the engines just be getting worn out? Should I check the compression? It has an updated point system, it seems like everything is working as it should, just real slow.

I doubt the 40 MPH claim is legit....

I would start by checking the spark advance...if that's not working properly, your engines won't make rated power...
Thank you for the quick response. Is that just checking the timing? I guess I'd have to use a dwell meter for that?
The engines are both just under 700 hours and were rebuilt once already
it is checking the timing as a function of RPM...and a clip on (inductive) timing light is usually best...

The Dwell Meter measures the point gap indirectly...and that setting has to be correct before checking the timing.

Hours really don't matter unless you know for sure that the distributors were overhauled when the engines were rebuild...sad to say but its very likely the distributors didn't get anything more than a set of points, the condenser, a rotor and a new cap...
Ayuh,..... How long has this hull been in the water,..??
How long has it been since the bottom was cleaned,..??
Keys to speed (if that's your goal) are clean hull with fresh paint, tuned propellers, properly functioning engine and transmission, and unloading unnecessary weight. If you have made certain of all those factors and still have difficulty making speed then it must be something radically wrong somewhere.
Thanks for the responses guys. I boat in the Pacific North West. Ocean boating, but I keep the boat in a boathouse on a river, so the water is brackish.
I pull it out every year and clean the hull and add a new coat of anti-fouling.
Its just my wife and I on the boat, it is fairly well balanced.
I don't mind going slow, its just that if I'm only doing 14knots but am supposed to be doing 22 at the same RPM I'd like to figure out why. If I can make it to my destination 80% quicker for the same amount of fuel that would be amazing!
Aft of one engine is the starting battery, aft of the other is the holding tank, so there is very little room for a timing light, or is there another place to aim it?
I've attached a photo of the engine room.


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Uni's are heavy boats. They're way overbuilt, which is good, but using those two tired-out Mopars to push it is asking a lot. I think you're about at the edge of your performance envelope with those ponies.
No way that boat has ever seen 40 mph or even 30.
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If you REALLY LOVE this thing and plan to keep it until you are buried at sea, then consider a pair of big blocks. You have the room for them and they can turn a fatter wheel than what you have. This will wake that thing up. Your fuel costs will go through the roof but you'll certainly be able to get there in less time.
I am a cheap fart, so I would probably send out the heads for reconditioning, put in a new camshaft/lifters, full tune up kit and be happy with life in the slow lane. I'd have a real close look at the lump that overheated to see if the rings scored the bores or some other dismal failure occurred.
Slower speeds than before with the same RPM has nothing to do with performance.... Something has changed with the hull/props/load.

If the bottom has not been blasted in the 50 years this hull has been in service, even 15 years... that is a lot of weight in paint. About 4 knots worth.
Maybe ill try grinding off the paint below the water line and doing a repaint next year. I'll also budget for a tune up/.rebuild. It is running fine, just seems slow. I'll do the timing right away and see if that helps at all.

Thanks for the input guys.IMG_3022.jpgIMG_3023.jpg
40 mph was not a myth for this hull although I must admit it won't do more than low 30s for a short period of time before it slips back into the 20s, since I had the transmission rebuilt. I am going to check the pressure relief valve spring, and cable adjustments to make sure the transmission is fully in the forward detent position this year after it goes into the water. I wish I would have taken a video of the 40 mph speed at 4000 rpm the day that I did it to convince the skeptics, but I didn't and I certainly don't cruise at that speed.
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There are many contributing factors to getting the best speed out of your boat. These are the ones I find important.
Engine condition:

  1. My engines were rebuilt about 300 hours ago. An engine that lacks good compression will not produce the torque required for higher speeds.
  2. Keep your motors in good tune. Keep the plugs and wires in good condition and the timing in spec. I set my timing with a vacuum gage as it makes setting the reverse rotation engine easy.
  3. I run Pertronix electronic ignition and a flamethrower coil for good strong spark.
  4. I have newer Edelbrock 4-barrel carburetors.
Boat weight:

  1. Keep your vessel as light as you can and don’t carry unnecessary items which contribute to overall vessel weight. A heavier boat won’t run as fast as a lighter one.
Hull condition:

  1. Keep the hull free from loose paint chips and algae as this will increase drag and reduce overall speed. Your boat bottom looks nice and clean so I don’t see any issue there.
Weight distribution:

  1. When my boat sits at the dock it sits bow down. That is to say the bow sits lower in the water that the stern. When I power up onto plane the engine thrust lifts the bow onto an even keel without the use of the trim tabs. Using the trim tabs would reduce overall speed if used.
  2. I use the amount of freshwater in the forward mounted tank, fuel and wastewater levels in the back tanks, and placement of items within the boat to fine tune the weight distribution.
  3. A boat that sits stern or bow low on plane, will significantly rob from your overall speed.
Running gear:

  1. Make sure your engine and drive shaft are aligned so that there is no drag in the drive shaft cutlass bearings.
  2. Make sure your stuffing box gland nut is not too tight. There should be about 4-6 drops a minute when the drive shaft is stopped. This water leakage is used for lubrication. If it is too tight the stuffing box will heat up at higher rpms and produce drag on the engine.
Transmission condition:

  1. Make sure your Paragon transmissions are right full of transmission fluid. Even a cup low will cause slippage. A little too full is OK.
  2. Make sure the transmissions shift properly into the detent positions when in forward and reverse. A transmission that is not fully into the detent position will slip at high speed. Adjust your cables if the shift positions are not correct.
  3. Check the hydraulic pressure produced by the transmission. It should be about 60 psi at 500 rpm and 100 to 140 psi at higher rpms. If the pressure is low check the pressure relief valve for burs and gum and the pressure relief valve spring for condition. If the pressure is too low the transmission will slip.
  4. If all of the transmission steps above are good and the transmission still slips then the clutch plates may need to be replaced.
These Uniflite 31’ boat hulls were designed for speed and perform amazingly when in good condition. I have had mine at 40 mph on GPS when the boat ran well. Running at 40 mph is possible in short runs but not recommended as these engines are not well suited to burn ethanol fuel. I had to replace the heads on my starboard engine after my last high-speed run because I was using unleaded fuel in a 1972 motor with unhardened valve seats and one valve seat failed. Use premium fuel if you can on these older engines. I have since replaced the heads and used a set of newer heads (1974) witch have hardened valve seats.
My starboard transmission was rebuilt but it still slips at high speed. There are very few used parts around for these old Paragon transmissions and none could be found to rebuild the hydraulic pump in my rebuilt Paragon transmission. I am hoping the pressure relief valve is sticky or the pressure relief valve spring is weak which I can fix.
Good luck with yours.
There is another item that needs to be checked and that is the propeller. These Uniflite 31' sport sedans are shipped from the factory with a 16 X 15 propellers and you will need to check that someone hasn't installed different propellers as too high or too low a diameter or pitch will produce unsatisfactory results.
Uniflite Specs.png
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