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prop slip - very high? 36%!!

FstaRockr Burns

FstaRockr Burns

Regular Contributor
Took boat out today for first time - 24ft searay 220 1974 with ford 351. Currently have a 17p prop on.

at 2500 rpms on a 1.3 ration (mc-1?) we were doing about 18-19 mph - I entered that into some online calculators and it told me slip was 36%?!!

That seems incredible - only a cruiser / fish boat - nothing manic. There is a 18" prop under the seat - or shd i go for 19?

According to this i need a 26p prop for a 15% slip?!!

http://www.mercuryracing.com/prop-slip-calculator/

thanks!

fyi was a bit choppy out - this was with light breeze from behind.. chop was a bit rough to go on plane, plus the boat enjoyed wandering around a bit so i wasnt too comfortable!
 
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I agree your making it way too complicated. First thing is check max timing advance by putting a timing ligh on it and slowly throttle up until the timing stops advancing. Note the rpm and total timing (all in). If that is within spec then give it WOT and note RPM flat out.
If your running points set the dwell angle at 31 degrees and then adjust base timing at 10 degrees.
All in or total advance should happen at around 36 degrees at 4000 rpm
Size your prop to achieve 4200 rpm max
If the engine revs up past max rpm just keep it 4200 or below until you can get a prop with more pitch
 
Hi - thanks for the responses. Was a bit rough to open her up last week. I did set dwell at 29, and timing rose steady but as far as I could tell was all in around 3200 rpm and around 30 degrees. This is a brand new distributor btw, and everything is mint inside. Base timing is at 10 already.

Ill do a WOT if we get some flat water and report back. The boat had ALOT of counter steer to the left (on the wheel), and on 2 occasions she bit hard left and right - with the hard steering it did not make me feel safe.. so will retorque the gimbal, try replace the trim rubbers and see what happens.

For a 75 model im sure the shaft has worn - id consider installing a steering damper on the back. Ive read alot of success stories regarding this. One chap broke his gimbal and said because of this was able to limp home..

Not up for pulling the motor out at all - not yet anyway!
 
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Docksidemarineservices said:
Really? Slip?

What is the WOT RPM? That is the only number you should be concerned with.
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Hi - so was out this week, max rpms i could reach was 3500, carb butterflies fully open.. speed was 31mph. this boat is supposed to reach 4200 - i need to recheck as Ive had odd issues - after running about 45 at 2600 ( cruising) it starts shutting down. Instantly restarts tho - last thing i did was change points yesterday, the old (new) ones were quite worn! literally idled home. Everything else is brand new.

Also suspecting a carb gasket leak when hotter.. at cool idle i have about 19-20 vac, when shes hot it drops to 16 and hunts quite a bit more.

So perhaps thats affecting my wot rpms, but i did this test before having run too long -

anyways - hope these numbers help..
 
My guess here is it is having issues with fuel supply? Connect it to a portable tank and see if it holds a steady WOT rpm. Before you can determine the proper prop you must get the engine close to 100% as possible.
 
using the firing order stamped on the intake - 13726548.. sure its a 351 but ill have to double check.. looked at some comparison pics from 302 and 351 esp the water pump hole location to the head.

Will have to measure and get back -
 
"""last thing i did was change points yesterday, the old (new) ones were quite worn! """


That's an issue!!

1. did you lubricate the point cam with some grease?

2. are you sure you have a ballast resister in line to the coil? Coil should be around 8-11 volts when running. (that number is debatable but in the ball park)
If you are seeing 13.5/14 volts at coil when running at 2500 rpms then you have an issue and will burn out the points and the coil. Also burning out of points can be caused by a bad condenser (capacitor).........Maybe get new points and condenser or covert to electronic conversion if available (pertronix) and never worry about it again!!

When water testing a boat the MAX rpms (within specification) is how a prop pitch and diameter is determined.
Typically the boat manufacturer has done this but considering the age of your boat that doesn't matter anymore.

3. You need to confirm the gear ratio of the drive.

see how many revolutions the prop will spin with one turn 360 degree turn of the engine.
 
re" see how many revolutions the prop will spin with one turn 360 degree turn of the engine."

An easier way, although it requires 2 people...one to watch the prop , the other to crank and watch engine pulley mark.

Quick and dirty way...
Mark engine crankcase pulley and the prop with tape or a dab of paint.
With hot wire from coil removed from distributor end and grounded...
Crank engine until prop makes exactly one rotation.
The engine will have made either 1.3 rotations or 1.5... That's your ratio

OR.... more accurate way...(given the engine's propensity to not stop quite instantly when you stop cranking.)
Crank engine until prop makes 10 complete revolutions while counting the number of compete engine revolutions. Divide the number of crankcase revolutions by 10.... that's your ratio...i.e. 13 or 15 = 1.3 or 1.5 reduction.
 
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Find it hard to believe 233Hp will spin up a 1.32 drive ratio
 
thanks guys -

kg yes lubed the cam gear, I was running the new/old prestolite. New condenser too. I measured ohms of coil and ballast for a total of around 3.2 which is in the ballpark for a v8 after doing alot of reading. I set dwell at 29, initial 8 and total 28 at 2800 rpms - out of the box it seemed to work very well.

Ill re-measure the volts at those rpms and see. Will confirm the gear ratio and get back. I believe the tank pickup needs to be checked as stated above (possibly why the stalling happens at 1/2 tank and gets worse) - i suspect the fuel sloshing around creates bubbles assuming a cracked pickup, and I want to check for a vac leak at carb base too - since its pretty ok but when hot hunts and sometimes stalls when throttling down from a quick rev.


Ill go and measure the intake bolts and take some snaps of the intake to confirm a 351w.

fyi at idle when warming up i get 19.5 inches vac - after shes run about 45 mins at 2600 the issues start - funny enough that was AFTER 1 day of trolling about and a 30 mile run included -
 
Havent poked a temp gun at it - but the gauge seems to work well. Idling brings it up gradually.. once hot it stays at 12 o clock for a while, if i run it for over 15 mins it slowly creeps up - at about 1/4" before the red i back down and let it cool (only under load that is) - at idle it stays around 11pm.

Im suspecting my base gasket has hardened - with this kind of slow heating & hunting after being run for a while and the idle vac dropping from 20 to 15-16 im suspecting an intermittent lean condition definitely caused by the engine heating up.
 
You really should get an accurate reading.

Typical temp should be 150 -170......only under high load and warm water should it get at or above 180

190+ is too hot for a 140/143 degree thermostat.


So maybe find an analogue gauge??????
 
fstarockr burns said:
........... The boat had alot of counter steer to the left (on the wheel), and on 2 occasions she bit hard left and right - with the hard steering it did not make me feel safe.. So will retorque the gimbal, try replace the trim rubbers and see what happens.
is the drive equipped with a trim fin/torque tab?
If not...... Install one and adjust it correctly!
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fstarockr burns said:
hi - so was out this week, max rpms i could reach was 3500, carb butterflies fully open.. Speed was 31mph. This boat is supposed to reach 4200 - i need to recheck as ive had odd issues -
if the engine will not make the oem specified wot rpm, it will be over-burdened, and not only over-burdened, but will be so at all rpm.

after running about 45 at 2600 ( cruising) it starts shutting down.
if the progressive ignition advance is not spot on, 2,600 rpm can be a very detonation prone rpm.
Mild detonation can cause what you describe.
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when water testing a boat the max rpms (within specification) is how a prop pitch and diameter is determined.
not quite true!
The engineers have given us a suggested wot rpm (a test rpm only by the way) as to determine if the propeller and the final drive ratio are correct!

and by the way...... Since "revolutions" is plural, it's rpm...... Not rpms!
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sandkicker said:
re;
" see how many revolutions the prop will spin with one turn 360 degree turn of the engine."
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bob, i agree. That is actually backwards from how it is to be done!
The reduction numbers refer to crankshaft revolutions per prop shaft revolution.

quick and dirty way...
Crank engine until prop makes exactly one rotation.
The engine will have made either 1.3 rotations or 1.5... That's your ratio
yes..... That is correct!

or.... More accurate way...(given the engine's propensity to not stop quite instantly when you stop cranking.)
crank engine until prop makes 10 complete revolutions while counting the number of compete engine revolutions. Divide the number of crankcase revolutions by 10.... That's your ratio...i.e. 13 or 15 = 1.3 or 1.5 reduction.
that would work also!
Click to expand...

fstarockr burns said:
............... I believe the tank pickup needs to be checked as stated above (possibly why the stalling happens at 1/2 tank and gets worse)
rather than spending your time connecting up an external fuel tank ........... Spend this same time pulling the dip tube and checking the screen and replacing the fuel filter cartridge.
Both of these would need to be done anyway..... So why not get it out of the way now?
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fstarockr burns said:
measured it now - 9 1/4" between the heads from bolt center to bolt center -
if memory serves me, there is an approximate 2-3/4" +/- difference between the bolts (center to center) re; 5.0l -vs- 5.8l w .
The 5.0l will measure approx 8.5", whereas the 5.8l will measure more like 11.2".
If you are measuring near 9-1/4", you likely have a 5.0l.
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Bt Doctur said:
Disagree 8.5 approx for 5.0 and 9.5 approx for 5.7
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I agree BT...A 351W has a taller deck heigth therefore, 351would be 9.5" inch vs. 8.2" inch for the 302..



And as usual too much unneeded info from you know who.....

Engine manufacturers set max RPM (S) and boat manufacturers/OEM powertrain manufactures set gear ratio and prop pitch/diameter to achive ~80% of that max rpm rating. As we know almost all small block V8's typically have a rpm redline of ~ 5200-5500 rpms based on many factors but no need to get into that.
So at ~ 80% of say 5200 rpm that would put you in the ball park of 4200 +/- rpms. All about reliability and longetivity. So as with SBC with a red line of ~5500 rpm that would translate to ~ 4400. Thus the typical spec of many marine SBC engine of 4400-4800 rpmsssssssssssss.
 
That old 351 Winsor was a pretty fine engine, and it had a reasonable stroke compared to the new 5.4 lite Triton. Have one of those in my RV and, though it runs great, its long stroke knocking sound at 4 grand is a bit unnerving. Dumped that idiot 5 W 20 oil crap and put 10 W 40 with Lucas Oil Stabilzer. That calmed the knocking sound down a bit, but with its 4.12 inch stroke it has to be knocky.

Jeff
 
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