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Counter-intuitive Trim Result

sunedog

sunedog

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Had a chance to do a long cruise on smooth water and played with the trim some. 2017 23RSB with ESP and a Yamaha 250 VMAX. SS "Turbo 1" prop but I can't find a part number or the size and pitch on it. Toons are pretty clean (as clean as you can get them with a scotch bite pad in the water.). About 3/4 tank of fuel (52 gallon tank).

I generally keep the trim about one tick above the middle on the gauge. Result: about 20.8 MPH at 4550 RPM
IMG_1617.jpg

I moved the trim down significantly to well below half. RPM's dropped and speed increased more than 1 MPH? A hair under 22 MPH at 4200 RPM. I was not expecting that result.
IMG_1616.jpg

And here's the prop I can't identify. I'll take it off when I do my annual maintenance and will probably find the part number and pitch.
IMG_1520.jpg
 
I am guess that you were experiencing slight slippage until you trimmed down.

WOT results are needed to advise on any pitch changes.
Are there any other stampings on that prop? Would be small numbers between the blades or inside the hub on the rear.
 
Bill N said:
I am guess that you were experiencing slight slippage until you trimmed down.

WOT results are needed to advise on any pitch changes.
Are there any other stampings on that prop? Would be small numbers between the blades or inside the hub on the rear.
Click to expand...
Agreed on the slippage. I'll have to go out and run it to verify WOT. I don't run it wide open often. And nothing stamped on the prop. I bet there's something on the hub that I can't see until I pull the prop.
 
I’ve got Mercury gauges and my trim has a lot of movement at each level (0 to 26). Except when I’m in real shallow water, I’m always somewhere in the zero range. When I first got our boat, it took me a minute or two to understand how it registered the amount of trim.
 
sunedog said:
Had a chance to do a long cruise on smooth water and played with the trim some. 2017 23RSB with ESP and a Yamaha 250 VMAX. SS "Turbo 1" prop but I can't find a part number or the size and pitch on it. Toons are pretty clean (as clean as you can get them with a scotch bite pad in the water.). About 3/4 tank of fuel (52 gallon tank).

I generally keep the trim about one tick above the middle on the gauge. Result: about 20.8 MPH at 4550 RPM
View attachment 34716

I moved the trim down significantly to well below half. RPM's dropped and speed increased more than 1 MPH? A hair under 22 MPH at 4200 RPM. I was not expecting that result.
View attachment 34719

And here's the prop I can't identify. I'll take it off when I do my annual maintenance and will probably find the part number and pitch.
View attachment 34720
Click to expand...
Sorry if I missed something, but are these your numbers at WOT?
 
BulldogsCadillac said:
Sorry if I missed something, but are these your numbers at WOT?
Click to expand...
No sir. Just a comfortable cruising speed on a relatively long run. I still haven't had an opportunity to do a wide open throttle test since I posted that.
 
You may be reducing slip with lower trim, but also consider that deck angle for minimum drag matters. At those modest speeds drag is generally going to increase with deck angle (think plowing). The amount of trim needed for best fuel economy is going to vary with load distribution too. As you may have noticed, miles-per-gallon will be significantly better with the lower trim in the situations stated.

Best trim for speed/economy can be very different between pontoon designs and operating conditions. I know our boats are very different, but speed/economy for us is no trim until above 4000 RPM unless there is at least two adults up front (and that's with about 2 degrees of extra tuck under, negative trim, provided by a transom wedge).

BTW: Trim gauges are not calibrated. One half-scale is not the same as another.
 
I'm going to say you had the trim up too high so your performance increase was due to achieving a proper trim after lowering it. First off, don't you start with it all the way down? People have a tendency to over trim boats. You can get by on a pontoon without ever trimming up, it's not like an I/O. If you do want to trim it up, a cumulative total of 3 to 5 seconds over full down should be the most you'll ever need. It's a very small adjustment. You'll learn to "feel" the right spot and going above that will only be bad. Those gauges aren't very accurate even when they're working as they should, just use them as a reference point AFTER you figure out the good spots.
 
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Nautical said:
First off, don't you start with it all the way down?
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No sir. Generally leave it alone. We don't often hammer our way across the water. Fast enough to beat the heat but not spill my beverage.
Nautical said:
You'll learn to "feel" the right spot and going above that will only be bad.
Click to expand...
Been pontooning for 33 years. On my 5th boat right now. Have only had it two seasons so still learning what it likes. But thanks for the tips.
 
Love my Active Trim.....just saying....start and go!
 
Hey, I was just trying to answer your concerns and explain why you are experiencing the issue you posted about. I've only been boating since 2015 but I'm fairly certain you are starting off trimmed up enough that your prop, while still in the water, is sucking down enough air to create the EXACT scenario you are describing. When you lower it, you're grabbing more water which explains why your RPM decreases and speed increases. I seriously doubt there are any other explanations based on what you posted. If your trim gauge is anywhere near calibrated, half way up is WAAAAYYY too high. You might reach that half way spot at full speed and WOT while finding the sweet spot but you're real close to where it will start to cavitate. I've loaded literally hundreds of boats onto trailers (I was a boat broker from 2016 until recently) and either the gauges are broken and don't move at all or they work but either way I've yet to see one reading that high unless I put it there when loading it on the trailer.
 
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Time for either a new prop or tune that bad boy!

If cruising is your primary goal there are better, more efficient props than a Turbo 1. From what I understand a Turbo prop is designed more for speed and bow lift, maybe a poor-man's Enertia. With that VF250 you swing a bigger blade and get better numbers in the midrange for sure.

That skeg could probably tell some stories too!
 
Bill N said:
I am guess that you were experiencing slight slippage until you trimmed down.

WOT results are needed to advise on any pitch changes.
Are there any other stampings on that prop? Would be small numbers between the blades or inside the hub on the rear.
Click to expand...

Resurrecting this thread because I finally figured out my prop is a 14 1/4 x 15P. The number cast into the hub is ET-14215-R. Previous owner (Hi Tripp) got the boat up to 47 MPH. I haven't gotten it quite that fast, but in all honesty, I rarely wind it out for a top speed run.

According to the slip calculators, 22 MPH at 4200 RPM's with a 15P prop is over 35% slip. That's horrible.
 
Your slip % is not surprising given how little diameter you have. If you want to improve that number increasing D is necessary.

That being said, most of the time when you increase D you lose P (pitch) so that's probably not desired with your VF250, therefore an entirely different prop is what I would recommend.

Yamaha SWS2 or Merc Enertia FTW!
 
Potomacbassin’ said:
Your slip % is not surprising given how little diameter you have. If you want to improve that number increasing D is necessary.

That being said, most of the time when you increase D you lose P (pitch) so that's probably not desired with your VF250, therefore an entirely different prop is what I would recommend.

Yamaha SWS2 or Merc Enertia FTW!
Click to expand...
I'm Ok with the performance. But would consider a new prop if the payback was there. I run around 4000 RPM almost all of the time and it hurts to know how inefficient this prop is at that speed. Do you think one of the ones you suggested would be a good midrange prop?
 
sunedog said:
I'm Ok with the performance. But would consider a new prop if the payback was there. I run around 4000 RPM almost all of the time and it hurts to know how inefficient this prop is at that speed. Do you think one of the ones you suggested would be a good midrange prop?
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Yes 100%.

My boat from the dealer came underpropped with a 14.25 Reliance, when I switched to a better prop I gained around 8-10mph and 20% lower slip at 4500 RPM. Top speed I gained around 4mph so the most significant gains were in the low and midrange. Absolute night and day.

Having said that, that's in ideal conditions with a perfectly running outboard, bimini stowed and clean toons. And a Reliance is significantly different than a Turbo. But the correct prop can make a huge difference.
 
Just put the boat in storage today. I need to pick this subject up in the Spring. Thanks for your suggestions!
 
sunedog said:
Resurrecting this thread because I finally figured out my prop is a 14 1/4 x 15P. The number cast into the hub is ET-14215-R. Previous owner (Hi Tripp) got the boat up to 47 MPH. I haven't gotten it quite that fast, but in all honesty, I rarely wind it out for a top speed run.

According to the slip calculators, 22 MPH at 4200 RPM's with a 15P prop is over 35% slip. That's horrible.
Click to expand...
I never played with the prop or did these calculations - the boat perfomed good enough for us, and would top out above 45 when we needed the speed. I assumed bennington knew the best prop for the boat and equipped it correctly.
My new boat has a 300, and it performs much differently than that one did. I get around 24 mph at 3,100 rpms. I find I get the best gas efficieny around 3,300 rpms. This one tops out right around 45mph but is a bigger and heavier boat.
 
Trippernsc said:
I assumed bennington knew the best prop for the boat and equipped it correctly.
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Keep in mind, Bennington doesn’t prop the boat. The sales dealerships props the boat. They often prop for general purposes, although I think with bigger HP motors they tend to “try” to prop it a little better than general useage/purposes.

Anyway, pointing this out as many don’t pay attention to prop at pruchase assuming it’s been propped appropriately. That’s not always the case. Sometimes dealerships prop nicely, and sometimes they slap something cost effective vs efficient or performance oriented.

Anyway, just putting this out there FWIW.
 
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