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Posts: 27
| I have an 04 Yamaha 150 four stroke on an 05 Alumacraft tournament sport 175. I bought the boat used and ever since I've owned it it will only get up to about 4200 RPM getting me to about 40mph. I know I should be closer to 6000 rpm wot...right now there is a 13 3/4 x 21 aluminum prop on the motor. I don't care so much about speed but I know its not good to not run your motor at proper rpm. Anyone have an opinion on whether the tach is shot, or if it is that big of a mistake on the size of the prop? Any idea what size I should go to? I guess my biggest weakness right now with the boat is hole shot, it takes a second to plane out. Stainless or Aluminum doesn't matter much to me, would the size be different between the two? And I have checked the height of the motor, the cavitation plate is right where it should be. Thanks in advance guys. |
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| Hiya,
From the sounds of it, the boat seems to be over-propped, but when I used the numbers and checked what kind of slip factor you would have, the numbers do not match up. So...couple of questions: First, is the speed a GPS reading or the dial speedometer on the boat? Second, you mention the anti-ventilation plate is right where it needs to be...where exactly is it in relation to the lowest point on the hull? Third, what kind of load do you normally have in the boat (people, gas, gear, etc)?
One big question here...when you get on plane, are you trimming the motor out to lift hull out of the water more? If no trimming of the motor is taking place, there can be as much as 1000 RPM's gained right there, and that would be huge...
Lots of variables here, and if you can supply a bit more information on these questions, a better idea of what is going on can be had.
Steve |
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Posts: 295
Location: Southern Ontario, Detroit River and Lake StClair | With out actually being there and seeing how thing are set up and running the boat its hard to tell 100% but I'm going to go with your over propped. I've set up up a few boats in the 18' range with 150's on them and pretty much eveyone ran best with a 19p prop. The odd one with a 17p but never one with a 21p. There are a lot of variables though. 4200rpm sounds way low even with a 21p on it. Try Steves suggestion about playing with trim first. Get up on plane with the trim set at 1/3 run at that and see what max speed and RPM are, then trim up till you stop gaining speed or the prop starts to slip and note the speed and RPM there. Make sure its really trimming up also. Check the gauge and have someone watch the motor. Let us know how you make out.
Good Fishin'
Tim
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Posts: 27
| The cavitation plate is exactly at the lowest point of the hull.
4200 RPM is trimmed up at WOT
Usual load is 2 guys, usual muskie gear nothing out of the ordinary.
Speed is on GPS
I have an appt. Monday to check the tach and make sure that is reading right. I talked to a prop guy and this is where he suggested I start due to the huge difference that needs to be gained.
Thanks guys. |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
I would say that is a good idea. With the anti ventilation plate at the lowest point on the hull, it is actually too low on the boat. I run an alumacraft Navigator and can honestly say you could have that motor moved up probably 2 holes and it will perform well. In many cases on boats such as our is the anti-ventilation plate is most efficient when anywhere from 2 to 3 inches above that lowest point on the transom...reason: as soon as the boat passes, water starts to fill the hole the boat made. Moving the motor up will gain you about 200 rpms.
Now...with that motor and boat, 2 guys, gear etc. I would bet going to a 19 pitch prop will be about right (as stated). I would consider the pro-series by yamaha or a ballistic by michigan. The ballistic may even give you an increase in rpm as well since the blades are thinner but still just a durable The pro series is a vented prop and will definitely help hole shot. If you go stainless, move the motor to it's highest mounting hole...as far out of the water as you can get it on the transom. It will hold and you will not have any issues with loss of water pressure...
If you can, I would suggest going out just you and your gear, and see how your numbers show up with a 19 pitch. I would bet you will see right around the 6000 mark. When you get over-pitched, even by 2 inches, it can really hold down your motor since it is working so hard. I tried both a 19 pitch ballistic and a 19 pitch pro series on my boat and my rpms went from 5400 on my 17 pitch ballistic to 4600 trimmed.... It's amazing what can happen when you start to find the sweet spot.
Steve |
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Posts: 1462
Location: Davenport, IA | I believe the general rule of thumb is 250 RPM's for each pitch. Although when I went from a 22 to a 20 on my 115 I only gained 150 RPM's. If you're running a good speed for your setup and getting on plane quickly and everything I would wonder if the tachometer is right. It is not real uncommon for boat tachs to be faulty. |
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Posts: 295
Location: Southern Ontario, Detroit River and Lake StClair | I got thinking about things today at work and that 40mph sound about right for 4200rpm with a 21p prop. That got me thinking that your tach is probably reading right. With a light load and a 21p prop you should be able to get around 5000 rpm with that motor boat set up. One thing you mite want to check is that your throttle linkage is set right and that the motoris getting an actual full throttle when you have the hammer down. If your only getting 4200rpm right now going to a 19p or even a 17p I don't think will get you to 5500rpm which about where you should be running. With the motor off take off the cowl. Have some one give it full throttle while you watch the linkage move. Once its at max try moving the linkage by hand some more and see it moves much. It if does that could very well be your problem.
Good Fishin'
Tim
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Posts: 27
| Thanks Tim....with T storms all day today I will check that out. Its funny, because when I do hammer it down, it feels like there should be a bit more. I really appreciate all the help I've received here. Knowledgeable folks who are willing to help a brother out. |
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Posts: 27
| Got the boat back shop said tach was reading correct and the throttle linkage was right. Some how some way its not getting the HP its supposed to. He thought it sounded like plugs or injector problems. I just changed the plugs and am going to run some Sea foam through it tonight and change the fuel filter....any other thoughts? I guess it all needed to be done anyway, hopefully we see some improvement. |
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Posts: 3508
Location: Elk River, Minnesota | When you do the sea foam, do it two ways...first, give it a deep cleaning by injecting it directly into the carb or injector throat while the motor is on high idle, until it dies. Put a bit more in and let it sit for 20 or so minutes, then dump the rest in the tank and go run the heck out of the motor. That will clean all the gunk out of the engine. Then, replace your plugs (cheap insurance). The fuel filter should be replaced ever other year or so so again a preventative maintenance issue there (another good thing to do as well since the motor is new to you).
I still think you are over pitched on the prop. I'll bet anything if you drop to a 19 pitch with same diameter, and raise that motor a hole or two you'll be sitting pretty. With two guys and gear, a 19 should help out. I'll bet if it is just you in the boat, you'll be really close to top end...add one extra guy and gear, and it can bring rpms down quicker than one might think.
Steve |
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