RPM Question

marvin the minnow · Jul 7, 2011

16' pontoon
Honda BF 40
Michigan Wheel 992403 VORTEX 12d x 10p 4 blade Prop

At WOT im only reading 4500 RPM's. My motor seems to be running fine and the tach is measuring the correct RPM's while the motor is idleing. Could my prop be causing my motor to not reach the potential WOT RPM range?

Bob_VT · Jul 7, 2011

Re: RPM Question

Sounds like that...... your rpm range should be 4600-5600 .......... another possible way is to check your mounting depth of the motor. If you have the ability and clearance (W/O putting the water pick up in jeopardy) raise the motor up on the transom mount possibly one hole. That may allow your rpm's to come up.

marvin the minnow · Jul 7, 2011

Re: RPM Question

So your saying my motor may be sitting too low on the transom? Another thing to note is that I am not seeing much if any variation in the RPM's when I adjust the trim.

Bob_VT · Jul 7, 2011

Re: RPM Question

Yes, the mounting of the motor itself. My V-4 on my starcraft is mounted on the 2d hole and the actual motor sits above the transom itself by about 3/4"

Years ago I had a 115 Mariner on a bass boat and changed to a 4 blade prop. I had to raise the motor which improved the performance and allowed me to reach optimum rpms

marvin the minnow · Jul 8, 2011

Re: RPM Question

Can someone else chime in and give me their opinion about the current prop?

anubix00 · Jul 8, 2011

Re: RPM Question

my bad. Original statement was true but not pertinent to this posting. Sorry for any confusion

marvin the minnow · Jul 8, 2011

Re: RPM Question

The motor is sitting on the lowest hole. I guess Ill start moving it up one hole at a time and watch the results. Sorry propably basic boating knowledge, but how do you raise the pitch?

SparkieBoat · Jul 8, 2011

Re: RPM Question

yes try raising the motor, the higher the better, if it is too high it will cavitate. (higher RPMs with no increase in speed) once this is set right, you may consider a lower pitch prop to get your RPMs up about 500 or so. make SURE your RPM guage is working correctly and giving accurate RPMs.

1980Coronado · Jul 8, 2011

Re: RPM Question

anubix00 said:
If you have access, try raising the pitch a little more. It'll give you less low end power, but give you more top end speed (rpm)

Why would you recommend increasing the prop pitch on a motor that isn't reaching the max WOT range? Counter intuitive! Increasing pitch would lower the WOT rpm even more.

marvin the minnow · Jul 8, 2011

Re: RPM Question

I was a little confused myself as "raising pitch" to me is incresing the pitch of my prop, which puts me in the wrong direction for desired RPM's.

marvin the minnow · Jul 8, 2011

Re: RPM Question

Looks like Anubix00 ment lower pitch. We all make mistakes. thanks for the input Anubix. Im far from knowledgeable in the boat world and appreciate all the advice I can get.

1980Coronado · Jul 8, 2011

Re: RPM Question

marvin the minnow said:
Looks like Anubix00 ment lower pitch. We all make mistakes. thanks for the input Anubix. Im far from knowledgeable in the boat world and appreciate all the advice I can get.

The post was about less low end and more top speed....That's increasing/raising pitch. I'm not sure he understands the problem you're having.

Silvertip · Jul 8, 2011

Re: RPM Question

anubix00 said:
If you have access, try lowering the pitch a little more. It'll give you less low end power, but give you more top end speed (rpm)

Absolutely untrue. Less pitch increases RPM, increases hole shot, but will lose some top speed UNLESS the engine is lugging severely. RPM increases because the prop is taking less bite on the water. Therefore it takes more revs to move the boat the same distance as before.

steelespike · Jul 9, 2011

Re: RPM Question

Not sure but I think some of us may have missed this is a 16ft pontoon.
If the motor is extremely low raising it might help a little but at pontoon speeds it will be limited.Trimming has little effect on a toon mostly to just find the position with the least venting (cavitation)issues.It would help a lot to know the speed at max rpm.Is 4500 with a light load?
Depending on the year of the motor my info indicates a max rpm of 6,000. My thoughts
are there is a tach issue or a motor issue or a serious setup issue. Even if the motor could make up 300 rpm per inch it would take a 5" pitch change I don't think they make a 5" prop.

marvin the minnow · Jul 11, 2011

Re: RPM Question

So I raised the motor a couple slots on the transom and saw very little change in RPM's (increase 200-300). I have no way of determing speed but my guess is 20-25MPH at WOT with 2 people in the boat. I do notice a slight miss or hichup while crusing at very low RPM's, but this goes away once the motor is running over 2000 RPM's

steelespike · Jul 11, 2011

Re: RPM Question

200-300 is an improvement If there are no venting problems you could go higher.Just test in gentle turns and diferent weight placement.
Trying to guess speed is like the doctor saying I don't know whats wrong but Ill give you this shot.
In my opinion if your reaching 20 mph more than likely your turning more than 4500.
Don't have a friend with a GPS? You could try an 8" pitch but if your actually near max rpm you could end up over reving.

1980Coronado · Jul 11, 2011

Re: RPM Question

Someone needs to give this a sanity check, since I'm not a pontoon guy. My in-laws have a toon at the lake house, but I don't take it out much. I just keep thinking that this is a pontoon and hence is a displacement type hull or (hulls). You may be able to get more rpm out of the motor, but at some point won't you just be burning more gas and not really increasing speed? Being a displacement hull, I'd think there is some point at which more rpm doesn't do any good. I think "ours" is a 24' toon with a 90 hp Yamaha. The times I have driven it, it seems as if there is a point at which more throttle doesn't do much to increase speed because you're pushing so much water.

Just thinking "out-loud" here, wondering if this setup is basically maxed out already being it's only a 40 hp motor pushing basically a 16' barge. Even if you dropped the pitch to increase rpm, what would be gained? I think it's fine to try to optimize the setup as is...moving the motor up...etc. ....but even if you were to get the rpm up, would there be any real gain?

I know I've seen some screamer pontoons but they have huge motors on really large diameter tri-toons.....this isn't that.

SparkieBoat · Jul 11, 2011

Re: RPM Question

you just do not want to lug the engine with too low of RPMs, he needs his RPMs closer to 5K, he said he gained 300 by raising the motor, this was a definite help and good thing to do, maybe drop to a 9 pitch now and he should be good.

1980Coronado · Jul 12, 2011

Re: RPM Question

SparkieBoat said:
you just do not want to lug the engine with too low of RPMs, he needs his RPMs closer to 5K, he said he gained 300 by raising the motor, this was a definite help and good thing to do, maybe drop to a 9 pitch now and he should be good.

I was thinking, since he's running a 4-blade prop, why not just keep the same diameter and pitch and go to a 3-blade....if that's possible? If he gained 200-300 rpm by changing the engine height he should be at 4700-4800 rpm based on the original post. Dropping to a 3-blade could gain another 200-300 rpm, which is probably a bigger change than just dropping the pitch 1". If the max WOT rpm is 4600-5600 as posted by Bob-VT, the change in number of blades should get the rpm to something like 4900-5100.

Dropping to a 3-blade shouldn't drop the top speed like changing the pitch. It may actually increase speed since the rpm increased with the same diameter and pitch. Just a thought.

Something like this maybe: http://boatpropellers.iboats.com/Honda/40_HP_%281995Newer%29_10.00-Pitch_Vortex_XHS_992602_Propeller/205/34420/?*******=628743709

steelespike · Jul 12, 2011

Re: RPM Question

1980C, it's not trying to get more speed so much as gettng to motor closer to its rated wot rpm.
for the health of the motor.The motor will use about the same gph(gallons per hour) at wot wheather its turning 4800 or 6000 the key is running within it's wot rpm while producing a good speed.Speed at a given rpm determines mpg gph devided into miles per hour.

RPM Question

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