For Matthew A--Cont. from prop forum

[Frank Acampora]

OK Matt: As I said in the prop forum, the Merc prop you bought has a diameter larger than the stock prop, it is also cupped, and it is killing your RPM.

However, I would expect your newly rebuilt engine with 140 PSI compression to turn a stock 17 pitch prop a little faster than the RPMs you are getting on a 17 foot boat.

So: First thing to check (or double check if you will) is the timing. Be sure that at WOT you are getting 30 degrees advance. Stranger things have happened. I once set an engine to 42 degrees advance instead of 32.

Second: be sure that your throttles are opening fully at WOT. Also check all gaskets (especially the bypass covers and cylinder drain system) to be certain that you do not have an air leak. Be certain that the reed V blocks are tight against the gaskets and those same gaskets are fully sealing the manifold to the block. Small air leaks add up and will hamper production of full power. Be certain that the center bolts are fully tight and you are not bleeding air between crankcases of each cylinder. Also, you did remember to put the center bearing seals around the crankshaft, didn't you?

Third: Be sure you do not have a partially clogged fuel filter. With restriction of a partially clogged filter, the engine will start well, run well, but just not produce its maximum power. Example: My 140 on my cuddy runs 38 MPH. When the filter starts to restrict fuel flow, the engine sounds the same and runs smoothly and well, but only at 33 MPH. Clean the filter and speed goes back up to 38.

Fourth: Be sure your lower unit has 2 to 1 gears. It makes a big difference. Set the flywheel to ) degrees. Mark one blade on the prop. Turn the flywheel 2 full revolutions. If the prop has only turned 1 rev, then gears are 2 to 1. If it has turned about 1 1/4 rev, the gears are 1.78 to 1.

Fifth: Your engine should have been mounted on the transom with the bolts through the top holes on the clamps . the bottom holes would then give 1 inch lift if the engine was raised to them. So: if the wrong holes were used, drill the top holes and raise the engine. seal the unused holes with silicone RTV. Raising the engine an inch will reduce exhaust backpressure some and will also reduce lower unit drag a little. It will however, increase your ventilation problem in the turns. A cupped prop will help this.

Now, I'm going to say something very controversial: If you have an old stock 13 3/8 diameter prop and you reduce the diameter by 1/2 to 3/4 inch your RPM will rise by about 500 and you will also gain about 2-3 MPH. If you have a really bad ventilation problem reduce the diameter a little more and cup the prop with an aggressive cup. The reduced load of the smaller diameter will be offset by the cup, and RPM will remain about the same. The cup will help to reduce ventilation/cavitaion. If you work carefully, you can do this modification to an aluminum prop yourself.

 

[Matthew A.]

Re: For Matthew A--Cont. from prop forum

Thank you very much for all your help Frank. It is greatly appreciated.

Bearing seals!!!! You're kidding right!...The guy at the machine shop told me I didn't need them because I was installing new pistons and rings!!!


Just kidding. No. I installed them. I'll be sure to go thru your check list. I run a full size canister type fuel filter/seperator as well as running the in-line, see-thru glass filter between the fuel pump and carbs as you once advised incase of fuel pump failure. Could using the two filters be causing to much restriction? I'll go ahead and replace the canister filter with a new one.
The size I quoted regarding prop size for both props was incorrect. The semi-clever non-cupped prop is a 13.49" x 17" the cupped prop is a 13.25" x 17". Unfortuneately, at the time I made the new stern bracket, I failed to take heed that the ventilation plate is sitting 1/2" to 3/4" below the keel line. As well as the fact that the motor was installed using the upper most top mounting hole and the bottom most lower mounting hole of the original stern brackets. Ooops! Live and learn. lol! Prop ventilation in turns is not a problem unless I turn hard or turn so hard as to break the stern loose. The original stern bracket installed on the right side means simply drilling a new hole in the transom for the lower mounting bolt. In my ignorance, when I fabricated the new bracket I drilled just two locations for the bolt holes. To make it worse, I used the mounting bolt pattern of the original installation as a referrence for the mounting holes of the new bracket. Fortuneately, the new bracket can be drilled. I don't mind having to take apart the bracket assembly since I want to drill a hole in the bracket to install a grease fitting where the trim unit thru-bolts to the stern bracket to help prevent galling. Something I am glad I did for the upper pivot bolt location.

I have the timing set to 28 degrees instead of the old 32 degrees. Do you recommend setting it to 30 degrees over 28?

 

[Matthew A.]

Re: For Matthew A--Cont. from prop forum

I went looking for props today...mostly just to see what sort of options maybe available. Mercury [which I already have a hub kit for] have several aluminum 3 bladed cupped props. Any thoughts or recommendations. Lower unit gear ratio 2 to 1
They are as follows:

13.25 x 17 [which I have and wish to return]
13.25 x 15
12.75 x 21
12.25 x 15
11.75 x 17
14 x 13 extra cupped for pontoon boats 25 to 28 mph.

Michigan have these props in both cupped and non cupped as follows for 85 hp Force 88 thru 89 single exhaust.

12.625 x 21 cupped
13.5 x 17 cupped and non-cupped
13.5 x 15 cupped and non-cupped

When I replaced all the seals in my lower unit I couldn't help notice the lower unit housing appeared to be somewhat designed as a dual exhaust unit but then...not. The exhaust passages appear to have been cast into the housing to do so. However, the water intakes were not. I don't understand why they would go as far as implementing the exhaust passages into the design of the lower unit along only to place the H20 intakes in such a location and manner so as not to. "Oh well...things that make ya go Huhmmm"

 

[Frank Acampora]

Re: For Matthew A--Cont. from prop forum

Basically, you can't go fast without pitch. Racing engines usually use a relatively small diameter, small blade area, large pitch prop. Of course, these racing boats are very light.

On a non racing all around boat you also need some area to minimise slippage. You can't have all pitch and no area.

So you need to run a compromise prop to push the heavier boat and have good performance at all speeds.

Now, in your case, the 85 is a 3 cylinder engine versus the 4 cylinder Mercs and Evinrudes. As such, it has less torque to swing a big prop. This is one reason why the 2 to 1 gears help it. They multiply the engine torque a little more than the 1.78 to 1 gears.

As far as timing, unless you have reason to need the less spark advance, leave it at 30. Just be sure you don't run it lean. Try opening the carbs by hand witout advancing the timing. You will see that the engine will not rev up well at all. Timing is the most important factor in engine speed.

Cupping a prop has no effect on pitch; it helps the prop "grab" the water better so it can be run higher without ventilation/cavitation. Running it higher allows the boat to go faster. However, cupping has the effect of increasing the load the same amount as about a 2 pitch increase.

Since your engine has lower torque you want to reduce the demand for it yet you still want speed (within reason) So since you want speed, you want pitch which also increases demand for torque. SO, then you would reduce blade area by reducing diameter. THUS: The prop you would want to try would be the uncupped 11.75 X 17 pitch. Second choice would be 12.75 X 17 pitch. This hopefully would let your engine rev up and deliver reasonable speed. Note that a larger diameter 15 pitch prop would let the engine rev up to the same RPM, but theoretically, at the same RPM, 15 pitch will give less forward speed than 17 pitch. MAYBE!

Now, all that said, there is no guarantee and you may still need to play around a bit. But I really think that the smaller diameter prop will help since your 17 footer is not a heavy boat.

 

[Matthew A.]

Re: For Matthew A--Cont. from prop forum

Both the 12.25 x 15 and the 11.75 x 17 are cupped props. The boat has a maximum load weight rating of 625 lbs. My fishing buddy weighs around 270 lbs. I weigh around 160 lbs. Our gear probably adds another 150 lbs. for a total of 580 lbs. I am not certain if it's load rating includes the weight of fuel. I am guessing that it doesn't. We removed the original 20 gallon fuel tank and replaced it with 3 seperate tanks. All three tanks sit in-line along the centerline of the boat. The rear most tank is an 12 gallon tank that sits under the splash well and is empty most of the time. The second tank holds 14 gallons and the third holds 8 and half gallons. The 14 and 8 gallon tanks when full is roughly 170 lbs. 20 lbs more then the stock 20 gallon tank.
Top speed is not really an issue since water conditions rarely allow us to go faster then 25 mph. comfortably. 16 to 20 mph is probably average cruise speed for average conditions. Considering the up and rising fuel costs, selecting a prop that will consume less fuel at our cruising speed would be ideal. Would this change your recommendations on selecting a prop?

 

[Frank Acampora]

Re: For Matthew A--Cont. from prop forum

I always carry 1 extra prop in the boat. SO: When I intend cruising at partial throttle for extended periods, I always change my 19 pitch for a 21 inch pitch. I find that it does act a bit like an overdrive and I do get a little better economy. Little being the operative word and acceleration sucks. BUT--a penny saved---


So: maybe you would want to try a 19 pitch--But then, why do you want to return the 17 pitch prop you have now? I thought you were dissatisfied because of the low WOT RPMs

You do know that you can't have it both ways. There is no magic prop--everything is a compromise of some sort.

--although years ago one prop maker did offer a prop that could be shifted from lower to higher pitch by nailing the throttle, then backing off and re-applying throttle. Then, when you stopped, it automatically defaulted to lower pitch setting. Never caught on though---expensive both to buy initially and to repair and I don't know about reliability.

 

[Matthew A.]

Re: For Matthew A--Cont. from prop forum

Edited via research