Engines,props, and wide open throttle

[Delray]

We recently purchased a boat, when looking at the printout I saw that the engines had NEVER been over 5000 rpm. Took it for a spin ( two people, half tank ) ran fine, on plane quick, wot never got above 5000.
Took it to my mechanic who was surprised as well. Now, I’m not a go fast guy, I like 25-30 mph, so at first look it doesn’t really bother me. Mechanic says they should be around 54-55 hundred if loaded like we were,( which we be 99% of the time). I realize that one doesn’t want to lug the engines, but as I said, 99% of the time it will be the two of us, and I will not be running into the 5000‘s.
So my question is
1: just how important is it have “ideal” rpm’s at wot if your never going to reach that anyway?
im sure other questions will pop up.
BTW. Engines are 150 yammies 600 ish hours
thanks.

 

[EagleCraftCustomBoats]

Can we have some more information, boat specs and engines weight etc

 

[Foxsea]

The WOT number is so the top end and bearing loading are within the design spec.'s. It's especially important at the typical cruising rpm's because that may well be in the upper range of the torque curve. If you are that far out at WOT in typical operation, I'd change your prop. You likely need 4" off the pitch. You'll have better performance and you may also find better economy results.

 

[Delray]

Boat is a campion 682, weight is 4750 dry, tank is 106 gal (650-700lbs) full, so somewhere in the 6-6500 lb range loaded ( that’s just a guess) twin 150 Yamahas turning 19 p ss props.
I have spoken to a few with the same setup (boat, motors) that are running the same props, numbers seem to be close.
The day we had it out, we were running around 48- 4900. If one gets about 400 rpm for two degrees of pitch I’d need 17 p props, or more as Foxsea suggests, just seems like a lot of pitch to take off, considering others are running the same props. I don’t mind doing it, just want to get it right without a lot of swapping around.
Thanks

 

[scott craven]

You should get at least 5500 rpm and I think the prop change will do it.
As far as running WOT, the older 2 strokes were built for it and could endure.
I wouldn't be running a modern 4-stroke at WOT for any more than a couple minutes
at a time, too hard on the internals.
just my opinion.

 

[Aquaholic]

Are you trimming out the engines and lifting the trim tabs while trying to reach maximum rpm’s? A correct propeller choice for a boat should not reach maximum RPM at wide open throttle with the bow down and the tabs down etc. You should have to work it a bit to get maximum rpm.

 

[SpringVelocity]

Wow must have correct prop. I was told same as close to 6000 rpm.

My Suzuki 150 has a 19 inch stainless prop and I get 5700. Fully loaded. I would switch out you prop.

 

[RELIC]

Lots of good threads on this site if you search for "prop selection" information includes:
- making sure the leg height is correct
- pitch change vs rpm (about 200rpm/ deg of pitch)
- 3 blade vs 4 blade
- and differences between prop models
Not an exact science but the best results are found by testing the props but hopefully the advice on this site will help with the process.

 

[MikePA]

Wow must have correct prop. I was told same as close to 6000 rpm.

My Suzuki 150 has a 19 inch stainless prop and I get 5700. Fully loaded. I would switch out you prop.

19 inch? Or pitch? That's a big prop for a 150!

 

[SpringVelocity]

19 inch? Or pitch? That's a big prop for a 150!

pitch

 

[Squire]

The US version of the Yamaha Outboards site has a performance bulletin section that shows actual performance of different boats and configurations. When you find a similar weight boat rigged with twin F150s it gives you real world expectations.

PB_SHT_Gamefish25_T_F150XCA_6-19-2018_OCC | Yamaha Outboards

yamahaoutboards.com

(Edited to show a boat closer to the 682’s specs)

As someone already explained, you need to compare to your boat at optimum trim and balanced laterally so you can avoid tabs. If your motors are mounted at the proper height, optimum trim for speed is usually as high as you can go before the prop starts to ventilate. You are trying to find the sweet spot for your combination so sea conditions should be pretty much flat calm for the exercise as well. Achieving best performance in best conditions sets you up properly for the range you normally run in and should give you best fuel economy and performance in all conditions.

Good luck; it takes some tinkering but it’s satisfying when you know you have it dialled in.

 

[Delray]

Thanks for the replies.
I agree with not wanting to run high rpm’s on a regular basis, which I don’t. Typically I like to stay at 38- 4200 rpm.
Yes boat is trimmed up, could probably get a bit more rpm but not close to another 1000.
I’m primarily interested in the reason/ rationale for wot rpm’s and whether not reaching xx number is causing undue wear or stresses on the engine during “ normal “ operation, again Foxsea has alluded to this.
I will add that the engines have been wedged back, I’m assuming to alleviate some sort of performance issue.
I know it’s not an exact science, well it probably is, just not to me. I will do some searching on this site.
thanks again

 

[Brando]

When your on plane at cruise have someone else hold the wheel and check to see that your cavitation plates are visible, just barely but should be out of the water. If they aren’t you need to raise the motors. Are the motors on a bracket of some kind or on the transom?

 

[CBsqrd]

Thanks for the replies.
I agree with not wanting to run high rpm’s on a regular basis, which I don’t. Typically I like to stay at 38- 4200 rpm.
Yes boat is trimmed up, could probably get a bit more rpm but not close to another 1000.
I’m primarily interested in the reason/ rationale for wot rpm’s and whether not reaching xx number is causing undue wear or stresses on the engine during “ normal “ operation, again Foxsea has alluded to this.
I will add that the engines have been wedged back, I’m assuming to alleviate some sort of performance issue.
I know it’s not an exact science, well it probably is, just not to me. I will do some searching on this site.
thanks again

Hitting the right RPM range at WOT is so you are running the right size/pitch prop across the whole RPM range. If you are that far below the recommended range, you engine is working harder at lower rpms - cruise, getting on plane, etc. that what the manufacturer considers optimal. I expect if you stepped down in prop pitch your speed at a given rpm would drop (maybe less than you think if the decrease in pitch is offset by lower prop slip), while your fuel burn would improve with the end result a better miles per gallon. Your boat should get up on step much easier as well.

Boat manufactures tend to overprop their boats to make the performance look better. Most performance bulletins are run with the boat relatively lightly loaded with fuel / people / supplies compared to what the average user (around here) will have in the boat. Things like bottom paint will also affect speed so it is best to run your own tests to find the right prop for you.

When I repowered a few years ago I tried a few different props to find the right set. My mechanic recommended that we aim for the top end of the range all things being equal as it would mean the engines wouldn't be working as hard overall.

 

[Squire]

Thanks for the replies.
I agree with not wanting to run high rpm’s on a regular basis, which I don’t. Typically I like to stay at 38- 4200 rpm.
Yes boat is trimmed up, could probably get a bit more rpm but not close to another 1000.
I’m primarily interested in the reason/ rationale for wot rpm’s and whether not reaching xx number is causing undue wear or stresses on the engine during “ normal “ operation, again Foxsea has alluded to this.
I will add that the engines have been wedged back, I’m assuming to alleviate some sort of performance issue.
I know it’s not an exact science, well it probably is, just not to me. I will do some searching on this site.
thanks again

The wedges were likely added to address some porpoising at lower speeds. I have the same boat but with the single F300 and I added wedges for this reason. I also raised the engine one hole because the cavitation plate wasn’t visible at cruise.
As you are not the first owner and wedges have been added it’s possible the mounting height was tinkered with as well.
That would just leave props to play with and I’d recommend going with a four-blade as it will likely improve hole shot and mid-range performance as well as getting you in the optimum rpm range.

 

[bpsuls]

In my experience WOT is not the only deciding factor on proper prop. I have two props, once that goes to 5700 and another that goes to 5400. The 5700 burns way more fuel at cruise than the 5400 (i cruise between 4200-4400). Like others have said, experimentation is key. So many people think you just need to nail the high end of the WOT range and then that's it job done. I have a Yamaha F200 and the specs are that WOT just needs to be between 5000-6000. It says no where in there that it needs to be close to 6000. Holeshot can be a useful indicator as well, my 5400 has a great holeshot as well.

 

[Corey_lax]

I put a 4-blade prop on to get some stern lift, and I lost top speed and RPM, but it sure got on plane quick. So just keep in mind that if you re-prop to get to a higher RPM, you might negatively affect how quick you get on plane.

There's always trade offs and you just got to keep trying until you find one that you like. It would help if you can borrow numerous props and take a day to trial run them before you buy.

 

[brutus]

I put a 4-blade prop on to get some stern lift, and I lost top speed and RPM, but it sure got on plane quick. So just keep in mind that if you re-prop to get to a higher RPM, you might negatively affect how quick you get on plane.

There's always trade offs and you just got to keep trying until you find one that you like. It would help if you can borrow numerous props and take a day to trial run them before you buy.

I also went to 4 blade, will never run a 3 blade again,hole shot and fuel savings are night and day. A friend of mine has a 682 and a single 250 yam, also running 3 blade 19pitch, slow as hell getting on plane and staying there, he runs it at 4800rpm doing like 32 mph and burns a lot of fuel, loaded he needs to do the shuffle to the front to get on plane lol, told him to change his pitch to 17, won’t do it stuburn old fart lol

 

[Rain City]

The best indicator of a good prop fit IMO is holding on plane at lower speeds. More control, better for rough seas. I lost a touch of top end speed and ran at a slightly higher RPM at cruise with a prop change I made on my old Bayliner. But the performance and handling of the boat was night and day. 4 blade all the way.

 

[Explorer682]

We recently purchased a boat, when looking at the printout I saw that the engines had NEVER been over 5000 rpm. Took it for a spin ( two people, half tank ) ran fine, on plane quick, wot never got above 5000.
Took it to my mechanic who was surprised as well. Now, I’m not a go fast guy, I like 25-30 mph, so at first look it doesn’t really bother me. Mechanic says they should be around 54-55 hundred if loaded like we were,( which we be 99% of the time). I realize that one doesn’t want to lug the engines, but as I said, 99% of the time it will be the two of us, and I will not be running into the 5000‘s.
So my question is
1: just how important is it have “ideal” rpm’s at wot if your never going to reach that anyway?
im sure other questions will pop up.
BTW. Engines are 150 yammies 600 ish hours
thanks.

I had this same boat, 2004 682 Explorer with Twin 150’s , ideal RPM was 3800 and that would be about 50kms/hr, WOT was around 5200-5400 ( dependant on sea conditions load etc) at that rpm it was moving on flat water close around 70km/hr but the burn rate went WAY up so never went there often ,with 15.25X19M s/s props. The bracket had wedges but I never adjusted anything as that was the set up from factory