Prop Size Question

[alpineswampmanla]

Hello,<br />I have an 18' HydraSports 1800CC Center console boat. It has a 1990 Evinrude 120 V4 with the standard V4 gearcase on it. I have a trolling motor with 2 Interstate Trojan batteries under the console that weigh approx 150 pounds total. I normally fish with 2-3 adults which is another 300-450 pounds total. Add another 80-100 pounds for ice/drinks and whatever we catch. <br /><br />It currently has a Raker 13 1/2 x 18 pitch prop on it and the motor turns around 6500 rpm at wide open throttle. The rpm range for the motor is 5500-6000 so I need to bring it down a little. I have tried a 13 x 19 pitch prop and it turns exactly the same rpm as the Raker. I don't necessarily like the Raker as it has too much torque pull to the right, (yes i did adjust the torque tab on the motor)and I cant run anywhere off trimmed all the way down position because unless it is perfectly smooth the boat porpoises(spelling?). With the standard 13 x 19 prop it didnt pull to the right and I could make use of the trim even in rough water but the rpm was out of range. What do you recommend? I read that 2" of pitch brings approximately 300-400 rpm drop. A 13x21 would be approx 6100 rpm using that assumption ,slightly over revving. The only 2 13x22 props I found for this motor was another Raker and a Michigan Wheel Ballistic series prop. Both claim to have bow lifting properties. Most dealers around here dont stock props that will fit this motor, and they wont allow returns on special ordered parts. Please help!<br /><br />Thanks,<br />Brett

 

[walleyehed]

Re: Prop Size Question

First-off, do you have a jackplate??? Where is the engine mounted as far as AV plate in relation to bottom of boat.<br />The torque of the Raker itself is an indication of possible mounting height issues...I've run quite a few Rakers and you don't have the leverage to support the Raker's lift, but the engine may be too low on the transom...Tell us all you can about how it's mounted, the manufacturer of the Tachometer and whatever else you can think may be an issue.

 

[alpineswampmanla]

Re: Prop Size Question

There is no jack plate installed. The motor is mounted with the anti cavitation plate even with the cut-out in the hull of the boat. It does tilt to the left when running. It was more pronounced with the Raker but did it with the standard prop too. I am thinking if installing trim tabs to correct this. I am not sure who made the tach. <br /><br />If I can trim the motor up (almost never that smooth of water) the higher it goes the less the pull.

 

[moderator1]

Re: Prop Size Question

If your engine is mounted directly on the transom at it's lowest point, I think I would try raising it 1 hole at a time until it ventilates, then drop it back one hole. You will find that it requires less positive trim and may help with porpoise.<br />you will need to optimize this set-up before purchasing more props because you will gain RPM and speed by decreasing drag of lower unit.<br />What is the Idle RPM in gear, on the water???

 

[alpineswampmanla]

Re: Prop Size Question

Idle RPM is 800 RPM in gear in the water. There isnt much adjustment to raise the motor off of the transom. It is currently in the slotted holes all the way at the bottom position(resting on the transom). There is only one hole on the motor above the slotted hole. I think total adjustment is approximately 2".<br /><br />Thanks,<br />Brett

 

[Dhadley]

Re: Prop Size Question

Thats a pretty aggressive X dimension. It sounds like a 20" motor on a 25" transom if the AV plate is even with the top of the step. But thats why your performance is as good as it is. And the step is doing what it was intended to do.<br /><br />What youre seeing with the Raker is what we would expect when compared to a standard SSTII. We would also expect to see what youre saying about the steering torque. Thats from the Raker's bite. The torque tab wont have much, if any, effect on steering torque because its basically out of the water. At this point we would install a skeg mounted torque tab. Theres a real good pic and explanation of them at www.bobsmachine.com under "Torque Tamer". They probably sell them here at iboats also. <br /><br />I think youre headed in the right direction but I would suggest we get the rpms verified. And stop worring about "over-revving" that motor. If you are indeed turning 6500, you havent hit the limiter yet. <br /><br />Post your verified top speed and we'll calculate slip %. That will tell us if the tach is off or not. I suspect it may be, slightly.

 

[alpineswampmanla]

Re: Prop Size Question

My GPS says that I am going 42mph @ 6500 rpm with the 13 1/2 x 18 Raker installed. With the standard 13x19 prop top speed was 38mph @ 6500 rpm. How do I measure the transom length? I am pretty sure it is a 20" transom. The motor is a 20" shaft.<br /><br />Brett

 

[Dhadley]

Re: Prop Size Question

Thats about 25% slip so something isnt right. Double check to make sure the tach is set on position 6, not 4.

 

[Dhadley]

Re: Prop Size Question

If the tach was set wrong and the motor was actually turning in the upper 5K range it would make a lot more sense.

 

[alpineswampmanla]

Re: Prop Size Question

Ok I will check the tach when I get home this evening to see where it is set. I have a quick question about the setting. It is a 4 cylinder, does it still have to be set on 6?<br /><br />Brett

 

[Dhadley]

Re: Prop Size Question

Yep, 4 not 6. It has to do with the stator not the number of cylinders.

 

[alpineswampmanla]

Re: Prop Size Question

Dhadley wrote:<br />Yep, 4 not 6. It has to do with the stator not the number of cylinders. <br />Double check to make sure the tach is set on position 6, not 4. <br /><br />Ok now im confused. What position do I set the tach?<br /><br />Brett

 

[Dhadley]

Re: Prop Size Question

My bad. That was too early for me to be typing and trying to answer the phone at the same time. Well, OK -- theres never a good time for that.<br /><br />The tach should be set on position 6 even though it is a 4 cylinder motor.<br /><br />Sorry!!

 

[alpineswampmanla]

Re: Prop Size Question

I removed the tach and here's what I found. It has a 4 position switch on it. They are labeled as follows. The tach was set to position 4.<br /><br />4cyl -switch Position 1<br />6cyl -switch Position 2<br />8cyl -switch Position 3<br />12pole-switch Position 4<br /><br />There is no manufacturer listed on the tach. It has part number 481ANC9 on the side of it. It might be time for a new tach. Any suggestions. The hole size is 3 1/4" The outside lip is 3 1/2".<br /><br />Brett

 

[alpineswampmanla]

Re: Prop Size Question

Ok I took the boat out again today. I replaced the tach. The old one was definitely off. I also raised the motor to its highest adjustment which was 2" higher than before. It was all the way down. The anti-ventilation plate was not above the notch in the transom. I was looking at the front of the motor not the rear so I raised the motor all the way up. Here's what happened. With the same Raker 13 1/2 x 18 at wide open throttle trimmed all the way down my speed was 42mph at 5500 rpm. I could trim up and get about 48mph at 6000 rpm. Any higher on the trim and it would start ventilating. The porpoising is all but gone unless you trim it up way too much and I'm not tilting either. However, I couldnt make any tight turns even slowing down to 25 mph the engine would ventilate\cavitate and over rev. Not a problem in open water, but makes running the bayous difficult. <br />I dropped it down 1 adjustment hole when I got home so I'll see what it does next time out. There are 4 adjustment holes. The bottom one is too low, I think the top one is too high. Hopefully this one will be "JUST RIGHT"!!! One more chance if it isn't though. Thanks for the help! I'll post the results after my next trip out.<br /><br />Brett

 

[moderator1]

Re: Prop Size Question

I think Dhadley will like your new numbers...I do. <br />It's normal to trim down when needed for turning without ventilating....don't let that distract you, it's just another learning process.<br />6mph gain....good work!

 

[alpineswampmanla]

Re: Prop Size Question

Ok I took the boat out again today. Lowering it down 1 adjustment hole makes a big difference. It dropped the rpm's down a bit. Now the most I can get is 5800 rpm fully trimmed out at wide open throttle that's 200 rpm less than before. It also lowered the top speed by about 3 mph. I can still make full use of the trim. It was really windy here today and even heading into a 20mph wind, the front end wasnt porposing unless I trimmed it waaay up. I am able to make sharp turns now without it blowing out and over revving. I think I am going to leave it where it is. I also noticed that since I can make use of the trim without it porposing I can eliminate the torque steer by trimming it up. <br /><br />So with my final adjustments, my top speed is 45mph at 5800 rpm fully trimmed at wide open throttle. Cruising speed at 4000 rpm is 32 mph.<br /><br />Thanks for all of the help and advice!<br />Brett Molaison

 

[Dhadley]

Re: Prop Size Question

Sounds like you found the sweet spot for that prop / hull combination. Very nice job!!<br /><br />Don't ya just love it when it all comes together?

 

[Dhadley]

Re: Prop Size Question

Oops! Forgot to run the numbers for you. Youre right at 10% slip which is not bad at all and pretty much what we see for a Raker thats set up nice. <br /><br />Thanks for letting us know how it turned out!

 

[alpineswampmanla]

Re: Prop Size Question

It is working like a dream. Thanks again for all the advice. My dad bought it new in 1985, put the motor on in 1990 and only used it 4 times. He never really got it adjusted right. It sat unused in his garage for almost 4 years. In 1994, he gave it to me. The bottom carb was completely clogged up and it needed new plugs. I cleaned it out and got it going. The fuel pump went out shortly after. I made almost 20 trips that year and have been using it regularly (every 2 weeks) ever since weather permitting. Last Sept, I changed the water pump and thermostats just for good measure. Other than the prop spinning a hub in January of this year,it has been very reliable. Once I started prop shopping, I decided to get it adjusted right and do away with the porposing and tilting problem. I also wanted to get the right prop not realizing that the tach had gone bad. I'm sure this will help alot as getting up out the water will cut down on fuel and makes it handle alot better too.<br /><br /> Most trips I make are usually 20-25 mile trips(inshore), but if the weather is good I'll make a 45-60 mile trip out to the first oil platforms (about 11-20 miles out in the gulf) for some big fish.<br /><br />Thanks again,<br />Brett Molaison