2006 DF 250 RPM issue at WOT

[cboise]

I have read many threads on this forum trying to figure out why my df250 w/800 hrs will only reach 5K rpms. It is mounted on a 24.5' Seafox Bayfisher weighing in dry at 2750lbs.

I have done the following:

1) made sure fuel flow to low pressure filter is unrestricted
2) replaced the low pressure filter
3) replaced the filter inside the High Pressure pump housing and the filter on the outbound side of the HP pump. VST had collected some light brown residue which I assume is ethanol related
4) new plugs
5) adjusted throttle plate opening from the control on the helm
6) compression reads 160psi steady across all 6
7) trimmed it at various heights

when I peg the throttle the motor runs great up to 4800-4900 rpms then makes a distinct noise change and sounds "throaty" like its sucking more air. then levels out at 5K rpms. Hard to explain but that is the best i can do.

Now i know some of the issue maybe the large plexiglass wind screen and possible dirt on the hull. At WOT when then the head wind died briefly I saw it jump to 5400 rpm and speed went from 42.8 to 45 MPH.

Should I be concerned or just run it and fish ?

[winb377]

What size prop are you running on it ? That would be my first port of call

[cboise]

thanks for the reply. It a Suzuki 3x16x23R

[formula233]

I had the same engine and I could get 49 mph at 6000 rpms
with a 3x16x21.5. I think you may be over proped. A dirty bottom will also drag you down.

[Moonlighter]

One of the unknowns right now, is did this engine ever rev past the current 5000 in the past - in other words, is the problem of not revving out a recent think, or, has it only ever revved to around 5000rpm??

If the OP could clarify that very important issue, then we can then offer more considered help.

Now, right now, in the absence of any indication from the OP that this is a new problem, if I assume that the engine has never (apart from the one time he mentioned where it hit 5400) then the engine is clearly over-propped.

I am not familiar with the boat he has, but I fish regularly out of a friends Striper 2301 W/A (24 ft LOA) and it has a DF300 Suzuki, and runs a 16x20 Suzuki 3 blade stainless prop. Revs out to 6000rpm, which is perfect.

So that to me indicates that since you're running a 23" pitch prop, on a 24.5 ft boat with only a 250, that, straight up, you are over propped. These Suzuki engines NEED TO BE ABLE TO HIT 6000rpm at full throttle, otherwise you are lugging them and your top spend speed, hole shot and fuel economy will all suffer, and, worse still, you are reducing the life of your engine.

So, again assuming that the motor has never revved past about 5000rpm at WOT, you need to change props and go to something like a 16x20, or possibly even a 16 X 18.5. Only a test will show which one gets you info the correct WOT rpm range.

We see lots of US boats that come to Australia that are badly over propped. I don't know what it is, one can only assume that people think that they will get better cruise or top speed by fitting a bigger prop, but that of course is totally wrong, false, and incorrect. It is critical that even though you may never run at WOT, the engine must be able to get to somewhere very close to its max stated rpm if it is to perform to the optimum.

[formula233]

The 2006 df250 has a 2.29 gear ratio. The newer DF250/300AP has a 2.08 gear ratio. there will be a difference on props between these 2 engines. I had a 2.29 and went to a new Df250ap with the 2.08 GR. I had to drop down 3" of pitch to make about the same WOT RPM. I agree need to know if this boat/engine ever went above 5K rpm. Justoff the cuff I would say a 3x16x23 prop hitting proper WOT RPM would be a 50 MPH boat.

[cboise]

thanks for the replies folks. 5400 RPM is the max that I have seen (for a brief moment). Bought the boat used in February. Previous owner just installed this new prop. Did not trial the boat - lake frozen.

The manual does confirm the overall reduction gear ratio is 2.29. If i use an online slip calculator with the following data:

5000 RPM
2.29 Gear Ratio
23 Prop Pitch
43 MPH

It comes back with a .095 proper slip number.

If I use the Gear Ratio without reduction of 1.83 it gives me a .27 prop slip figure

Which of the Gear Ration numbers should i be using ?

1st reduction gear ratio
(Crankshaft drive gear: Driven gear)
32 : 40 (1.25)
2nd reduction gear ratio (Lower unit gear) 12 : 22 (1.83)
Total reduction gear ratio 2.29 (40/32 × 22/12)

[Moonlighter]

OK, what we now know is that you appear to be over-propped. Assuming that there is not excessive growth on the hull!

The gear ratio doesn't really help much in solving the over-propped issue you have.

Why? The prop slip calculator includes one major assumption: that the stated prop pitch is actually the prop pitch! It is not at all uncommon to find a prop that is supposed to be, say a 23" pitch, when measured it turns out to be an inch or even more different. So this throws the calculations right out the window. Suzuki props in the USA seem to generally und state their pitch - there was a recent thread here about a smaller hp Suzuki where I recall this was the case.

So, if you stick with the same brand and model prop as you have now, the general rule of thumb is that 1" of pitch = 150-200rpm change.

So let's assume that under ideal conditions you can currently achieve 5200rpm. That means you need to find another 800rpm to get you up to the optimum 6000rpm. So that is somewhere around 4" of pitch, therefore you need to reduce the current 23" to around 19". So that's your starting point.

The best thing to do is if you can establish a good relationship with your local Suzuki dealer or a prop shop, who will let you trial a couple of props until you find one that performs to the optimum on your boat.

I've mentioned this before so I will set out again how we test props here - I'm a member of a couple of boat and fishing clubs and we often have new members who don't understand too much about props and how to test them for optimum performance. So this is what we do. Hope This helps. There really is no substitute to having facts and data when it comes to props!

The first thing, before anything else, is to check that the engine height is set correctly. This DOES NOT involve rulers under the hull etc.

It DOES involve a water test where we put the boat onto the plane and reach a good cruise speed, trim set to normal position, and someone goes to the transom and checks where the anti-vent plate is running compared to the water surface. It SHOULD be skimming the water surface. It SHOULD NOT be submerged under water and creating unnecessary drag.

Before messing with props we get the engine height set right. This is important because an engine set too low can badly affect engine performance, we have seen gains of 300-400rpm and 10-20% improved economy and speed just by getting the engine up to where it belongs! I am not joking!

Once the engine height is set right, we then test the current prop by finding a smooth bit of water, using the standard load the boat usually carries, and at every 500rpm from idle to wot (trimmed out) we record Speed and Fuel economy. Like this:

RPM Speed (water). Fuel ec (mpg)
1000. 6knt. 7.2mpg
And so on.

We then do the following tests:
1. a "hole shot" test where we time how long it takes to hit 5000rpm when we hit the throttle hard from idle speed.

2. We also record the minimum speed and rpm that the prop will hold the boat on the plane at. This is important for rough offshore weather where you can't go fast but want to keep the boat on the plane.

3. Finally, we find the optimum cruise speed for fuel economy and record the revs, speed and fuel economy at that point.

This data gives us a baseline to compare new props to. We then test new props exactly the same way and compare results. It soon becomes obvious if you've got a good prop or not.

The final choice between the props that perform best these tests can then come down to other characteristics such as which prop gives the best lift characteristics you are looking for - some props provide greater stern lift than others, for example.

So, in summary:
1. Check engine height and adjust if required. Many are set too deep which compromises performance.

2. Do the prop tests as described above.

3. Select a prop that gives the best results and gets you up to, or within 100-150rpm, of the max stated rpm for your engine.

[cboise]

Moonlighter,

thanks for the detailed response. I really appreciate it. I will run the boat and see where the anti-vent plate rides on plane. I will go from there with the prop selection.

[2014DF25ARS]

Moonlighter, I've now tested three props on my 13 ft modified V-hull aluminum runabout, with my DF25ARS engine. As you recalled, I had my Suzuki 11-pitch AL prop gauge-measured at a local shop, and the prop measured at 12-pitch. When I had been using the 11-pitch rating in prop calculators, I was getting 2-3% slip. But when I plug in 12 as the pitch, I get 10-11% slip, which everyone seems to feel is more realistic. Makes me think that maybe Suzuki rates their props based on measured performance rather than theoretical geometry. If this were common practice, it might make it easier to predict what prop pitch you need. But given that it's NOT standard practice, it just confuses things IMHO.

During my testing, I mounted a GoPro on the transom of my boat and filmed the motor during a few hole-shot tests. I've just posted one of the videos here: http://www.youtube.com/watch?v=pJacP...ature=youtu.be

Can you check this video and give your opinion on the motor height? I'm not sure what terms are used for the two plates on my motor. I assume you're saying the upper of the two plates should ride above the water when underway.

My latest test was with a Stiletto 12-pitch SS prop. With the Suzuki prop I was getting about 28 MPH GPS @ 5850 max RPM with my 25 HP motor. With the Stiletto, I'm seeing 30 MPH GPS @ 5900-5950 max RPM. I'm out of money, so I think I'll settle here for a while.

[Moonlighter]

We've kinda got two discussions going on here, so I hope that cboise might excuse just one more hijack post! Otherwise 2014, please go back to your original thread and we can continue discussions there.

2014, the "anti-ventilation plate that I am referring to when discussing engine height is the largest plate on the leg, the one directly above the prop.

In your video, I can't see that plate at all, it seems to be a fair way underwater. I also noticed that after you accelerated, I didn't see the engine being trimmed upwards? Does this engine have power trim and tilt? If it does it would be normal to trim up once the boat is on the plane, to achieve optimum revs at cruising throttle setting. I also noticed a fair amount of stray from the leg where the water from under the hull was striking it. Can be a sign of an engine set too deep.

Normally, when we are testing height, we go to WOT then trim the engine up gradually (let the speed catch up as you do this in increments) until the prop "lets go" then trim it down a touch to where it bites again.

That is the revs we record for the WOT throttle check.

If it doesn't have power trim, you might try setting the motor trim one further hole out on the bracket, and check it again.

A trick to help see the anti-vent plate during this height test more easily is to stick some red dots along the top edges, that helps it stand out more clearly in the spray!

It's ok for the plate to be getting splashed and some water onto the top, but it shouldn't be buried under water.

Note: On small light flat or nearly flat bottomed boats, sometimes you do need to set the engine a bit deeper than on larger deeper V hull boats and the plate can then end up a little under water.

However, if your boat currently will take a reasonabe turn at speed without the prop "letting go", then you could lift it one hole higher and test again. It's easy to lift small engines like your 25 and change height - bigger engines like the 250 can be done at home but are probably best done somewhere where you can hook them up to a winch or block and tackle, or a forklift etc.

[cboise]

its all good. learning a lot from this discussion !

[2014DF25ARS]

This morning I had the opportunity to do a little more R&D on my 13-ft modified-V (flat hull @transom) tinny with DF25A motor. I clamped the motor 1 inch higher on the transom and took it out for a run on the local river. As Moonlighter predicted, I gained about 150 RPM - from 5950 to 6100 with my 12-pitch Stiletto prop, and gained about 1 MPH to 31 MPH max. My motor does NOT have power trim/tilt. The effect on porpoising was opposite of what I expected - I had assumed that raising the motor would reduce the lever arm of the motor thrust, and thereby reduce porpoising. But I found that raising the motor increased tendency to porpoise. Go figure...

[Moonlighter]

This morning I had the opportunity to do a little more R&D on my 13-ft modified-V (flat hull @transom) tinny with DF25A motor. I clamped the motor 1 inch higher on the transom and took it out for a run on the local river. As Moonlighter predicted, I gained about 150 RPM - from 5950 to 6100 with my 12-pitch Stiletto prop, and gained about 1 MPH to 31 MPH max. My motor does NOT have power trim/tilt. The effect on porpoising was opposite of what I expected - I had assumed that raising the motor would reduce the lever arm of the motor thrust, and thereby reduce porpoising. But I found that raising the motor increased tendency to porpoise. Go figure...

Did you try adjusting the trim pin one hole in/down? If the prop still had good grip, be daring and go another half-inch higher and see how that goes - you didn't mention if you could see the anti-vent plate now, if it's still under water.....

You should also have seen an improvement in Fuel economy as well.

The flat bottom boats do, in my experience, have an inate tendency to porpoise, all you can do is shift as much weight as you can forward, and trim in a little to see if you can find the happy medium.

[hhall07]

Did you ever figure out the culprit?