Please double check the speed/prop calculator info...does it seem right to you?

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

New results to share. I finally determined that one of my 19" pitch props WAS intermittently spinning on the hub under higher loads. It would hold for a while, then after a certain amount of stress, it would break loose. After it broke loose, I could drop rpms on that motor, then bring them back up, which would make it better...but not alot better. Determined this by swapping props from one engine to the other a couple different times.

Decided to go ahead and throw on a pair of 13 3/4"x15" props that came with the boat. (which are in fairly good shape aside from a few minor dings)

Ran down river, then back up...about 30min each way. I was seeing about 21mph at 5400rpm. (which is around 37% slip)
When I pushed it a little farther, it hopped up to about 26mph at 6100rpm (around 27% slip)

Turns out I need to do a little throttle cable adjustment because the port engine wanted to keep going past 6100rpm while the starboard engine just stayed there. Due to some slop in the cable, the throttle butterflies in the starboard engine are only opening about to about 3/4 throttle.

Anyway,
Although the trip was at a snail's pace, I feel like I got some solid and repeatable numbers to work with. I think because the starboard engine wanted to keep climbing, I could expect at least 6400-6500rpm WOT on both engines once I get the throttle cables sorted out.

If that's the case...since I'm looking for 5400-5500rpm top end with my wife and I in the boat, I need to go up by 5-7 inches of pitch, correct? So from a 15" prop, I would be looking at 20-22" pitch being ideal. I guess that puts me looking for a prop with 21" of pitch.

Looks like you guys were much more accurate on your speed expectations than those calculators I was trying to use.


BTW, Is it normal to get lower prop slip numbers as speed increases? I would have thought it would be the other way around.

Thanks!

-Phillip

 

[jestor68]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I had not seen it mentioned anywhere in the discussion; so I thought I'd mention that your WOT rpm range for your motor(s) is 5000 to 6000 rpm.

Since the motor's hp rating is at 5500 rpm, the best workable range is 5500-6000 rpm.

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

BTW, Is it normal to get lower prop slip numbers as speed increases? I would have thought it would be the other way around.

Thanks!

-Phillip

That's usually the way it works. The faster you go, the less boat is in the water and the less drag. That normally results in a lower slip number.

FWIW, when I ran the numbers on your setup I came up with 20" of pitch. If the calculations were correct, 19" of pitch would put you on the high end of the rpm range, 21" somewhere in the middle of the range.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

My owners manual says 4500-5000 for heavy applications such as houseboats...5000-5500 for other lighter applications. Would be interested to know if 6000 is actually the safe top end. (Though I do realize that most engine manufacturers suggest max rpms well below what the engine is capable of.)


Thanks,
Phillip

 

[dan02gt]

Re: Please double check the speed/prop calculator info...does it seem right to you?

No way that calculator it's even close to accurate for your boat. Most 18' 1450 pound bass boats with actual high performance padded-v hulls and modern direct injected 150HP 2 strokes (making around 165hp) just do get into low 60's. I had to Google that boat to even see what it is and even if it could do high 50's on anything other than the truck on the way to the lake no way in hell I would want to drive it that fast and I've driven many 70+ mph boats. It's going to be a scary chine walking nightmare at those speeds as that hull was never meant to go that fast. Then again I don't think it will be a problem because I don't think it's going any faster than about 40-45 with the power you have.

But all this "bench racing" is not worth anything. I would prop it for around 45MPH and see what it does. You got to start somewhere on a setup as unique as this and go from there.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Well I feel like a buttwipe. I had checked the accuracy of my tachs with an old mechanical tach at idle. It was only 200 +/- rpm below what my tachs were saying, so I figured the old tach was off a bit. I just crawled up under the dash of my boat and found that my tachs were set for a 10pole alternator rather than a 12pole, which mine are. Apparently I haven't even touched the upper RPM range of these engines.
I apologize for waisting everyone's time thus far. Will get back to you this weekend with some real numbers.

Thanks,
Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Hey,no problem.Just keep in mind we have been through this a few, lol, times There is a high incidence of tach error either a malfunction or set wrong. Of course its hard to imagine identical failures on two tachs the same age.

 

[jestor68]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Well I feel like a buttwipe. I had checked the accuracy of my tachs with an old mechanical tach at idle. It was only 200 +/- rpm below what my tachs were saying, so I figured the old tach was off a bit. I just crawled up under the dash of my boat and found that my tachs were set for a 10pole alternator rather than a 12pole, which mine are. Apparently I haven't even touched the upper RPM range of these engines.
I apologize for waisting everyone's time thus far. Will get back to you this weekend with some real numbers.

Thanks,
Phillip

No worries mate. Let us know what rpm's they show after re-setting.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Okay...Completely different story to tell after today. Had some friends in town who wanted to go out on the boat, so I had about 900lbs of people, 26gal of gas on board, cooler, ect.

Ended up doing 29mph at 4200rpm, which I believe should be 7% slip.

This was done with my worn 19" props that I reworked a bit...put a little more pitch and cup in them. I also had to pin one of the hubs because it was spun. I know...not the best thing to do, but I had to get a baseline somehow.

Anyway...going to start working on slicking out the hull and seeing how much more speed I can get.

Thanks,
Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

You need to do a run with just you aboard to figure out how well your setup works.
In my opinion your slip is very low, read that not accurate, as rpm goes down slip goes up.
At about 5000 rpm slip would normally be about 10 or 12 % would tend to be higher with a heavy boat
At 4200 your slip would probably be about 18%
Adding pitch and cup may have taken you out of a reasonable rpm range.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

You need to do a run with just you aboard to figure out how well your setup works.
In my opinion your slip is very low, read that not accurate, as rpm goes down slip goes up.
At about 5000 rpm slip would normally be about 10 or 12 % would tend to be higher with a heavy boat
At 4200 your slip would probably be about 18%
Adding pitch and cup may have taken you out of a reasonable rpm range.

Maybe when I added pitch and cup, I turned the props into something similar to a 21"? That would be 17% slip at 29mph.

I know the tachs are accurate now because I have checked them against a GOOD mechanical tach and they match up exactly in all rpm ranges.

I know I wasn't doing less than 29mph because a buddy was following me in his alumacraft which tops out at 27mph with he and his girlfriend in the boat, and we pulled right away from them.
Buddy also checked speed with his gps. It read 29mph when he was following me down river. River was flowing at about 2mph yesterday (according to NWS) which would put him at 27mph water speed...and us at at least 29 because he couldn't keep up.


So I guess that would be the most viable conclusion...I turned my props into 21" pitch.

Thoughts?

Thanks,
Phillip

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Be advised if you added both pitch and cup they're no longer 19p props, meaning the slip calculation isn't accurate. You'd need to measure the pitch (accounting for the extra cup) and use that in the formula, not what the props are stamped.

So, yea, you might have 21p props now (maybe even more).

If 4200 was at WOT (and your tachs are accurate) then you're running too much prop

 

[jestor68]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Do you have pitch blocks? I'm curious as to how you reworked those props yourself and managed to alter pitch and/or get all 3 blades the same.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I run a big fabrication shop so I am very comfortable working with metal.
For this job, I sat down and heald a spare truck tire filled to about 60psi between my knees. I layed the prop against the tire and worked it into shape with a rubber mallet.
Once I had it looking good by eye, I used a square and checked certain points of each prop blade to see how far off they were from eachother. The idea wasn't so much to get it back to what it was, but to get the blades evened out and looking more efficient to my eye.


-Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Be advised if you added both pitch and cup they're no longer 19p props, meaning the slip calculation isn't accurate. You'd need to measure the pitch (accounting for the extra cup) and use that in the formula, not what the props are stamped.

So, yea, you might have 21p props now (maybe even more).

If 4200 was at WOT (and your tachs are accurate) then you're running too much prop

While its very likely over propped his rpm was with 900lbs of people and gear and 26 gallons of gas.
A lightly loaded test run with a gps would sure help tell how the setup works.
Maybe he can remove some cup I've seen some amazing results some time ago on here with increased rpm and speed
after removing some cup.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Should mention for this next part, yesterdays runs were made with three people sitting in the seats toward the stern, my wife and I in the middle, gas tank under the splash well.


Okay...more interesting results. Today my wife and I went out on the boat and had 15+/- gallons of gas with us. No changes to the props or engines. Water conditions also the same.

First Run:
Highest RPMs...5000
Highest speed...26mph
Was plowing a bit.

Second run with the engine trim settings one hole higher.
Highest RPM...5300
Highest speed...29mph
Trim settings raised the bow a bit so it was no longer plowing.


So I lightened the load on the boat (removed 10-11gal of gas and 550-600lbs of people) and the boat starts to plow/has a lower top speed and higher rpm.

Changed the trim settings, have same top speed but much higher RPM.

Correct me if I'm wrong, but it seems that the props aren't far enough in the water and are ventilating when the boat has less of a load on it.

Sound right to you guys?

Thanks,
Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I think a pair of new props may eliminate and venting and will eliminate the unknown of what props have you created.
I find it interesting that the boat plows with a lighter load at roughly the same speed.Perhaps you have created super stern lifting props.
Your a metal guy make yourself a couple of jack plates so you can lower the motors you'll need some setback to lower the motors.
Probably won't have to lower much and the setback will get the props in cleaner water.
You also may want check the bottom for a hook lay a straight edge parallel with and between the keel(s).

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I think it was plowing with a lighter load because I had the trim set on the most forward/lowest hole. I found that before(with just my wife and I and sometimes another couple people, if I went to the second hole back/up, it was raising the bow too much, causing the hull to pound over heavy wake. Now with these prop changes, the ride on the second hole feels very similar to the the way it rode before on the first hole.

Was considering spreading the engines a bit to get them into a more shallow area of the hull. Also because there is a big riveted strake right in front of the centerline of each engine. Thinking that may cause some issue too.

Thanks,
Phillip