Putting it all together.

Thanks for your input Yellowjacket. I actually only played with the idle mixture sitting. A simple enough action, but I ended up screwing up th low power jetting. I would have reset at the lake if the situation was different. The main jet, I left alone. I haven't modified the engine in any way so I have no reason to toy with the main jetting. Talking percentages makes the prop change I did much more dramatic. I wish it was earier in th season. I don't foresee any more tinkering this season. Maybe, I'll get lucky with one last spurt of warmer weather. Absolutely, regarding ony changing one thing at a time.

 

Prop Efficiency?

I just ran some numbers and am perplexed. The first day out at full throttle, I was getting 29 MPH at 6500 RPM indicated on an 11" pitch prop. The second day out, I got 26 MPH at 5800 RPM indicated with a 13" pitch prop. If the props were 90% efficient, I would be getting 61 and 64 MPH respectively.

Working the problem with indicated values, my prop efficiencies would be a pitiful (I think) 43% and 37%, respectively.

The only other possible alternative would be that my tachometer is reading incorrectly. This is certainly a possibility, but I feel like I have verified the calibration with a timing light/tach. I have not checked at full RPM. I have checked to 2000 RPM with full accuracy.

Despite my calibration check on the tach, if we run numbers in the other direction and assuming 90% efficiency on the props, my actual engine speed would be 3100 RPM and 2350 RPM respectively. I have difficulty in believing these values. I also have trouble believing my prop efficiency values, but if I had to choose, I would believe more in the efficiency values.

Is it possible to have such low prop efficiency values?

 

What you are calling efficiency is really slip. Efficiency is how much power gets turned into thrust, slip is how fast you are going compared to how fast you should be going. You are dealing with slip, not efficiency.

Are you taking the gear ratio into account? I believe the gear ratio of your engine is something like 1.67 or 1.77 to one, I forget which it is, but you can look it up. If you aren't taking into account the gear ratio that could be the issue.

There are some prop calculators on line that will let you input the gear ratio as well as the prop pitch and will calculate slip. You should be using one of those.

 

If you assume that it's a 1.77 gear ratio you were getting 21% slip in the first case of 6500 rpm and 29.9 mph, which is pretty reasonable for your type of boat.

In the second case you are seeing almost 35% slip which is saying you have the wrong prop for any one of a number of reasons. This prop is likely designed for a good bit faster boat, and even though it is a 13" pitch it may have cup in the trailing edge that makes it think it's more like a 14. I suspect that is what you have.

Also remember, that although you just twiddled with the idle mixture it still contributes some gas at high speed and you could be running lean at high speed, which would reduce power and cause it to bog.

So number one is to get the engine running right, put the original prop on it and then run make sure it is running to the previous 29.9 mph.

Then change the prop and see where you are.

Then adjust the engine to optimize the speed with that prop by using rev sticks under the saddle, and try different kick out settings.

If you can't get the slip back down and increase the speed to where it should be (30+ mph), get another prop.

And like I said, there are props and there are props, and there are huge differences between different props even though the pitch is the same or near the same.

You said you had a friendly retailer that was letting you try different props. Since it won't cost you anything I'd try a lot of them. You may not be able to get a lot more speed than you had with the 11 inch prop, but you can get the engine speed back down to 6,000 rpm with a reasonable slip, and that might be the best you can do.

 

Once again, invaluable information. Completely forgot about gearing. That will start to put figures where they should be. Thanks.

Good weather today so I'll be taking it out to play.


Additionally: The 11P wasn't the original prop that came on the motor. It was thrown in with the motor deal. According to the seller, this motor was originally on a pontoon boat that the owner decided convert to a fishing dock. There is evidence of the previous owner having toubles with the motor. Looks like a new stator and the wiring harness was redone (albeit incorrectly). The choke and kill leads were reversed. The prop that came with the motor is a 12P and pretty used. I chose the 11P for sea trials because I was concern about the boat being heavy, which it is. It's obvious, I think, that the 13P is not a good fit regardless of whether the motor was running strong or not. 5800 RPM is 5800 RPM and the efficiency/slip numbers speak for themselves. I guess the 11P goes back on and then I'll give the old 12P a shot. My dealer has a 12P 2-blade that might get a try at the cost of smootness.

 

Spring time finally.

The boat got a good rest over the winter, or maybe it was me that got the rest as the pressure was off to get it wet. I got out the finer grits of sand paper and started running the stainless steel trim through it's paces and got most of it polished to an acceptable level. I'm deciding whether to have the bow plates welded or to go with an additional piece that wraps the forward edge. I've got the wrap done, except for final trim and polish. Welding will certainly give a cleaner appearance.

I did a couple mods to the hull to clean up it's shape. I had added some side strakes to widen the bottom aft, so that some lift strake could be added with a wider footprint. The leading edge of a portion of the side strakes would not clear the water until almost 30 MPH. These strakes were trimmed flush at their leading edge and tapered to the transom to reduce drag at slower speeds. I had also installed a keel runner that was 1.5" wide by 0.5" deep. I think that I was getting bad flow off of this in sharp turn and getting some prop ventilation. I tapered the after 3-4" of this up to the hull.

Engine work included cleaning the carbs, replacing fuel pump components and replacing all old fuel line with modern ethanol resistant lines. The 13P prop was also put back on.

Yesterday's outing proved to be successful. The boat got up on plane nicely. I felt there was a touch too much bow up and the trim setting was changed to correct it. Once the motor was warm, I tinkered with the idle richness while idling in gear. I chose to go on the rich end optimum richness. My bogging problem I had last fall was gone.

This time out with the 13P, I was still hitting max RPM at 5800, but now I'm getting the 30 MPH I was getting with the 11P. I think that this is a move in the right direction and now I can look at other things for added performance. I am still tempted to try the 12P, 2 blade that the boat shop has in stock.

My next area of interest is how the motor is sitting relative to the water at speed. On this outing, I noticed a horizontal spray emanating from the lower unit. Closer inspection revealed the cavitation plate riding about an inch below the surface with the horizontal spray coming from a spray rail (?) on the lower unit just above the cavitation plate. Maybe this is the way it is supposed to work and trimming my keel runner changed the flow pattern that I had before. How immersed should the cavitation/ventilation plate be? I am now thinking of experimenting with engine height. Any suggestions on where to start? I think I am running 1" immersed so would raising the motor a 1/2" be overly aggressive? Trimming the keel runner helped with the ventilation while turning. I didn't notice any instances the whole time I was out yesterday.

 

Provided you have the right prop, the higher you raise the motor the faster it will run. The limit being when it stops pumping water. Since you aren't really running that fast a rig, it might not be easy to get a surfacing prop, but at the very least you need to jack the motor up to the point where the cavitation plate is above the bottom of the hull. With the plate below the bottom you are way too deep. I'd put at least an inch of wood under the motor clamp to raise the motor and try that with the 13P. In order to get the motor high enough you may need to add a jackplate, but higher is generally better.

 

I raised the motor 1 1/2". I got an extra MPH out of it, but conditions were not ideal for running at top speed. I did notice that I was getting some prop ventilation. I played with the motor trim and raising the motor only cause more ventilation. It seems to run better in the mid ranges than it did before, but I can't say for sure. I will lower the motor a 1/2" for the next outing. I don't anticipate major performance increases over what I have attained so far. It will be nice to knowing that I have worked all of the angles and maximized the performance potential for this hull and engine combination.