Does Boat load impact max RPM's?

[H2ofria]

I need help understanding what if any effect does the total load in the boat have on maximum RPM's. The following is my situation:

Phase 1:

At WOT (5300) RPM with just me and some gear in the boat (maybe 550 lbs total) I get a top end of 22 mph.

With three people and a dog as well as motors, gear - fishing and crabbing (maybe 900 lbs in a boat rated for 1,160) I maxed out RPM at 4400, top speed of 10 mph, could not get on plane.


Phase 2:

I installed Smart Tabs SX.

With about 800 lbs of load, boat got on plane at about 4000 RPM and topped out at 4400 rpm, 15 mph.

Left most of the load on the dock so, with just me and gear in the boat (again about 550 lbs) went to WOT and maxed RPM out at 4,750, 18 mph. (This could be caused by the Smart Tabs adjustment)

I know I can adjust the Smart Tabs to increase top end but the decrease in max RPM when loaded showed up before the Smart Tab installation.

So does the increased load have that big of an impact on maximum RPM or do I potentially have an engine problem.

I hope this makes sense.

 

[david_r]

Re: Does Boat load impact max RPM's?

your weight or lack there of will put you deeper or shallower in the water. i have a 115 johnson on a 16.5' bass boat, never had to trim it out to rum max rpm, i bought a hydroplane for the motor and the nose seemed to try to dig into the water and slowed me down a lot. i hit the trim button and she came back to life, so you may have to adjust your trim tabs as you change your weight. just a thought.

 

[tashasdaddy]

Re: Does Boat load impact max RPM's?

what motor and boat are you running.

 

[kenmyfam]

Re: Does Boat load impact max RPM's?

Yes it does. More weight = more to push and more weight = more boat under the water = more resistance.
As Tashasdaddy says what is your outfit. You may get some really helpful suggestions if it is known.

 

[JB]

Re: Does Boat load impact max RPM's?

Sounds to me like you are slightly overpropped for a light load and very overpropped for the heavy load. That's just a guess, as you didn't tell us what engine you have on what boat, how high the AV plate is, how you are trimming and what prop you are turning.

 

[H2ofria]

Re: Does Boat load impact max RPM's?

The boat is a 16 foot dual consul aluminum Smokercraft with a 28 hp Evenrude. (Pictured under my username.)

I have already had the prop discussion in another forum, the boat is properly propped. It is underpowered but right now a larger motor is financially out of the question which is why I added the Smart Tabs.

This is more of a theoretical question. I really just want to understand the impact of boat loading on maximum rpm for any motor and boat set up

I know that weight will impact "speed". And that "speed" and "rpm" are related but are not the same thing.

Basic logic would seem to indicate that the maximum rpm of any given engine would not change as the load changed. I would expect maximum "speed" to change with load but not maximum "rpm".

Of course logic was never my strong suit.

 

[JB]

Re: Does Boat load impact max RPM's?

If you are overpropped the speed is irrelevant because the engine will not reach its peak rpm. Sort of like climbing a steep hill at low speed in 5th gear.

Someone may think you are correctly propped. That someone is not JB.

While prop slip may increase some with increased load, the engine must reach its operating rpm range to reach top speed for that load.

Going from minimum load to max load on your boat would be roughly equivalent to putting a max weight trailer behind your car. Doing it with too much prop pitch would be the equivalent to trying to drive the car, with trailer, up steep hills in overdrive.

 

[dajohnson53]

Re: Does Boat load impact max RPM's?

The boat is a 16 foot dual consul aluminum Smokercraft with a 28 hp Evenrude. (Pictured under my username.)

I have already had the prop discussion in another forum, the boat is properly propped. It is underpowered but right now a larger motor is financially out of the question which is why I added the Smart Tabs.

This is more of a theoretical question. I really just want to understand the impact of boat loading on maximum rpm for any motor and boat set up

I know that weight will impact "speed". And that "speed" and "rpm" are related but are not the same thing.

Basic logic would seem to indicate that the maximum rpm of any given engine would not change as the load changed. I would expect maximum "speed" to change with load but not maximum "rpm".

Of course logic was never my strong suit.

That logic doesn't hold. Your prop is like the gears on your car - except you only have one available in a boat, while you have multiple in your car. If your car only had one gear, you can imagine that the RPMs and speed would be different if it was heavily loaded and/or going uphill vs if it were lightly loaded and/or going downhill.

Just like a car - assuming one gear - your boat's speed is totally dependent on the RPMs. In a car, in any given gear, the faster the engine is spinning (rpms), the faster the wheels will turn and vice versa. Therefore when the heavier load and/or uphill road causes the engine to labor and spin slower (lower RPMs), this causes the speed to go down. This is also puts more load and stress on the engine. Like you trying to walk up a steep hill with inappropriately long, (and resulting slower) strides vs, shortening and quickening your strides. Your knees will feel the difference in stress.

Same is true for a boat: Selecting a prop that is inappropriate for the heavy load you're carrying is like trying to drive your car (or walk or bicycle up hill) in too high a gear. It will decrease RPMs, therefore decreasing speed and increasing stress on engine.

Hope this makes sense. There is also a factor called prop slip which might change depending on load and RPMs, but the above is the essense of the issue, I think.

It sounds to me - and I could be wrong - that you decided your correct prop is the one that will get you to recommended WOT rpms with just you and your basic gear in the boat. In my opinion, from what you've written, it might be more appropriate to prop for the heavier load. You won't get as much top end when your boat is light, but you'll have better performance when it's heavier. You'll also have to be careful not to over-rev when you're running light. But, if your boat is underpowered, all you can do is try to prop to get it up on plane at the proper RPMs with the heavy load and accept whatever speed that gets you... until you can get into a more powerful engine.

What is the max HP recommended for your hull?

 

[j_martin]

Re: Does Boat load impact max RPM's?

When I had a 16 ' flat bottom bass bote with a 20 merc on it, I carried 3 props. One was for lonesome fishing, one was for with a partner, and the other was for the big-mac-lover partner you occasionally got. I pulled a houseboat off the rocks in the backwaters of Mississippi pool 8 with that one once.

If you're underpowered, you either have to gear low (pitch low) and back off when the load's light, or change gears (pitches)

If you hold down the rpm of your motor and run it hard, you'll hurt it in the long run.

Also, as you may have noticed, the add on "helpers" just compensate for bad setup. They tend to have a much higher power cost than they would have you believe.

hope it helps
John

 

[H2ofria]

Re: Does Boat load impact max RPM's?

Thanks for the replys!

While I am properly propped for just myself it seems I will need to go to a lower pitch if I want to take the wife and dog along.

I like the idea of having more than one prop, they are easy enough to change.

My boat is rated for a 60 hp engine but right now I really can't afford to buy a newer outboard let alone a new one.

 

[JoLin]

Re: Does Boat load impact max RPM's?

With just you and basic gear aboard, you dropped 550 rpm and 4 mph after installing the tabs. That's a lot. Play with the Smart Tabs adjustments before you go chasing down a new prop.

Agree with JB that you were slightly overpropped to begin with, and you probably still need a different one, but get the boat running at best performance with what you have, first.

 

[H2ofria]

Re: Does Boat load impact max RPM's?

Thanks for all of the advice.

I adjusted the Smart Tabs until the cows came home and it had absolutely no effect on getting the performance back to where I was before the Smart Tabs. I even tried raising to motor, as suggested in another thread I had going, still no effect.

So I bit the bullet, called my bank, and bought a new 50 horse. It should be on the boat next week. I lost a summer messing with this, hopefully next summer I will actually get some fishing in, if I don't have to get a job to pay for the motor.

Thanks again for everything!

 

[kenmyfam]

Re: Does Boat load impact max RPM's?

Good luck with the 50 !!!!
Let us all know how you make out.

 

[CATransplant]

Re: Does Boat load impact max RPM's?

You'll be a lot happier with the 50. You were seriously underpowered with that 28 on that boat...less than 50% of the rated hp. Now, you'll plane easily when loaded and fly when you're alone.