73 F-31 only goes 15kts

roninva · Post by **roninva** » Sat Aug 01, 2009 9:48 pm

Hey all. I bought a 1973 F-31 with 2 Chrysler 318's Rebuilt by Jasper in 1990 and they have some hours on them. They go up to 31-3200 RPM's on a good day, normally 3000 rpm is the max I can get it to. But even when the Chesapeake bay is like glass, the boat max speed is 14-15 knots. Why? It goes up on plane. The bottom is clean.

Everything I see about the boat says it should go about 27kts. The props looked fine when it was out of the water on a survey as I bought it. Could they be the wrong props? The transmissions seem fine. both sides seem to work identical so I am discounting it being a weak engine or tranny.

guglielmo6160 · Post by **guglielmo6160** » Sat Aug 01, 2009 10:30 pm

I have a 71 F31 and I have never been able to go 27 mph, I have ford 302's in perfect shape,, the best Ive done is 20, with a lite load and a not so clean bottom

prowlersfish · Post by **prowlersfish** » Sat Aug 01, 2009 10:44 pm

My F30 wth 351 ferds would top out at 27 cruse 20-21 Clean bottom

LSP · Post by **LSP** » Sun Aug 02, 2009 8:46 am

Ron ...how are you checking your speed?

roninva · Post by **roninva** » Sun Aug 02, 2009 9:45 am

My GPS is where I am getting my speed from. It feels incredibly slow and It takes me about an hour & 15 minutes to go 18 miles.

prowlersfish · Post by **prowlersfish** » Sun Aug 02, 2009 9:52 am

You say the bottom is clean , are the wheels and shafts clean ? and I do mean clean . how long ago was it out of the water . Are you in Va ? in the lower bay it won't take long for growth to slow you down

every thing points to a dirty bottom or running gear Bills 31 is running faster and we know his boat's bottom is not clean .

You may also want to make sure your secondary are opening on the carb .

I would say 90% of all speed issues are do to dirty bottom or running gear , yet people spend $$$$ working on the engines try to fix it

Running a dirty bottom on a boat is like driving with a flat tire on a car

roninva · Post by **roninva** » Sun Aug 02, 2009 10:26 am

I don't know anything about wheels and shafts but I had the stuffing boxes repacked a couple months ago. We take it out every couple weeks. With 94 degree days, yes, things grow FAST! But even the day it was clean it was a bit slow.

prowlersfish · Post by **prowlersfish** » Sun Aug 02, 2009 10:38 am

"Even the day it was clean it was a bit slow " Do a quik haul or get a diver I will bet you will be happy .

I run out hamton where are you boating from ?

roninva · Post by **roninva** » Sun Aug 02, 2009 12:11 pm

I'm at Cobb's Marina in Norfolk

guglielmo6160 · Post by **guglielmo6160** » Sun Aug 02, 2009 1:43 pm

I agree, with all ,,

prowlersfish · Post by **prowlersfish** » Sun Aug 02, 2009 5:10 pm

Your maybe 10 miles from me

rossjo · Post by **rossjo** » Sun Aug 02, 2009 9:36 pm

By jove, you must have a Dirty Bum !

No problems ... a bit of cleaning and off you go ...

Seriously - a pair of 318's might go a bit faster than that when she's perfectly clean. 3,100 RPM is a bit slow - if you find no issues (clean and nothing hanging on the shafts, etc. and the secondaries opening - does it have a 2 or a 4-barrel carb?) why not prop DOWN a bit (less pitch) and wrap those engines up to 3,600?

A friend of mine had a beautiful new 36 Luhrs with a pair of 502's. These are very powerful engines, yet tis big boat would still only run about 17 knots. We would leave at 3:30-4 am to run 50 miles offshore, and burn about 350 gallons of fuel.

Anyway - the F31 isn't light (10,450 lbs), and offered power options of "Twin to 560HP", http://www.trojanboats.net/images/1973%20Trojans.jpg so you may be peaked ...

The 318 was rated at 150HP @ 4,000RPM in 1973 with the reduced (from 1968) 8.6:1 compression and the torque was < 300ft-lbs @ 2,400RPM (a good cruise speed).

The boat is fairly heavy though, so who knows ...

Here are the 1997 318 HP & Torque curves (better than 1973) ...

prowlersfish · Post by **prowlersfish** » Sun Aug 02, 2009 9:59 pm

Ross , I believe the marine 318 was rated at 225hp (4bl)If I remember correctly , the ford 302 was the low power option at 188 hp on the F30 I had and the 318 was the high power at 225hp (1977) .( 351 were put in ) the car engines where lower rated .

LSP · Post by **LSP** » Sun Aug 02, 2009 10:03 pm

Paul ... you're right. I have the 318s and they're rated at 225.

prowlersfish · Post by **prowlersfish** » Sun Aug 02, 2009 10:07 pm

The old brain remebers old stuf like that ,But I can't rember what I was doing 2 minites ago . Now what were we talking about ??