115HP Mercury, overpropped?

[meddo]

I got my 75' 1150 inline out for the first time yesterday. WOT was about 5000 which is a bit low. I didnt have my gps with me but it felt like about 25-30 Mph. The prop is chewed up a little bit and is either being replaced or reconditioned depending on your input. I don't know the weight of the boat but I'm' guessing 1200-1500 pounds?.

Serial on the 4865588A1 19p 48-65588A1 making it a 13" x 19" I believe.

The boat is a 17ft Four Winns Candia deck boat. I just want it propped for best engine performance, I'm not trying to get a better holeshot or top speed.

Thanks for your input,
Mike

 

[Texasmark]

Re: 115HP Mercury, overpropped?

I ran a SS 19p on a 115 tower on a BB. Boat was heavy, but padded so it got (most of the hull) out of the water and in eliminating hull drag, the 19 wasn't too much for it.

Just shooting from the hip, if your deck boat is anything like "Deck Boat" brand deck boats, you have a lot of hull in the water and a lot of drag. Add several people (it is a deck boat....means lots of people) and you are really loading it up....hence your lower than desired rpm's.

Depending on the prop you select, you may go down to a 15 to get the kind of performance you want.....but like I said in another thread.....a 15 at the largest diameter you can find to fit your engine. I learned that at the lower rpms and higher loads, diameter has a significant impact upon performance; as much as pitch.

Only way you will really know is to "buy and try". Hustler in here is a good cheap prop.

HTH

Mark

 

[meddo]

Re: 115HP Mercury, overpropped?

Thanks for the reply.
My local marina is letting me try out diferent props rather than buy them. I just got a 13x17 that I will try this weekend. I'll let you know how it performs and we can go from there. I have the speedo hooked up now as well, and will take my gps.

Mike

 

[Texasmark]

Re: 115HP Mercury, overpropped?

You are truly a fortunate man. It is rare to get that opportunity but you will get your desired results.

Good luck.

Mark

 

[ivar]

Re: 115HP Mercury, overpropped?

A chewed up prop will cost both rpms and speed, difficult to say how much. Also mounting height is important, ie how high the cav plate is in relation to the hull. Too low , and you lose efficiency and may cause excessive water spray.

 

[walleyehed]

Re: 115HP Mercury, overpropped?

Let's not get hung on diameter either...diameter allows more engine heigth, and lessens drag...if we can't raise the engine to get max performance, we're waisting time going with the largest diameter you can find...Blade design would be of utmost importance, vs. diameter.

 

[Texasmark]

Re: 115HP Mercury, overpropped?

Point well taken Kenny. That is probably why my 14 1/4 dia performed as well as it did. With my engine jacked up 1 notch and the engine set so far back from the transom, it may be why I did so good with the larger dia.

I think the blade design is the reason for the drastic reduction in rpm's also, so I agree with you there too. Geezzz. Grin.

But I was really surprised at the differences at the lower rpm's as mentioned.
In this particular situation, the larger prop (Hustler) has a pretty impressive list of design criteria even though it is made of aluminum.

Mark

 

[meddo]

Re: 115HP Mercury, overpropped?

I tried the 13x17 today. Wot was about 5100 at around 24-25 Mph speedo. I realize these deckboats have alot of drag but it seems like it should be faster. Other than the size it is the same design as the 13x19. Should I try a 13x19 again that isnt chewed up or go a diferent direction.?

Thanks,
Mike

 

[Texasmark]

Re: 115HP Mercury, overpropped?

Numbers are real low. You are dragging something behind you. You ought to be in the 35 mph arena at least......unless you have a half dozen folks aboard.

Just ran over to the BAM slip calculator and it says that your tower should be running 41 with no slip with the 17 at 5100 rpm's. Punching in 25 for actual speed says that your slip is 40%. That's horrible for a 17' boat at 25mph with 115 hp; even if it is a deck boat......unless you have it really loaded with folks and gear and all.

The other problem is that if the numbers are accurate, with 40% slip your engine should be really unloaded so you'd think your rpm's would be higher with the 15 which should offset the slip and increase your speed. 100 rpm change for 2" of pitch change is a little low.....like half what it should be.....everything else equal.

Confusing.

The 19 at 5000 rpm's assuming you are really running 40%, slip checks in at 27 mph actual which is in line with your assumptions according to BAM.

Mark

 

[walleyehed]

Re: 115HP Mercury, overpropped?

at 5k with the 19, it should have went to nearly 5500 with the 17 just getting into the powerband, maybe a tad more. Are you trimming this out for max RPM? Hows the ride at max trim?
Have we verified the tach?

EDIT: another thought here...did we go from an un-cupped 19 to a cupped 17?
Have we stayed with the same blade design?

 

[meddo]

Re: 115HP Mercury, overpropped?

Blade design is the same. One thing I just noticed is that the 19 isnt much bigger than the 17 at all, its so chewed up. I do not have power trim (yet) and I'm thinking I may have had the pin in a diferent hole on my test with the 17. Speaking of the pin, what hole should I be in? Shoud it just be level or am I looking for an outward angle?

 

[walleyehed]

Re: 115HP Mercury, overpropped?

Put the pin in the hole that gives the best over-all ride.
It'll change with load possibly, so test it in a couple of positions.
"usually" most end up in the second hole from the transom...ocassionally the third.

 

[Texasmark]

Re: 115HP Mercury, overpropped?

Hole depends on transom tilt. Like my current boat has a 25 degree dead rise. I think Boston Whaler has a 0 dead rise as do others I have seen.

I find that paralleliing the antivent plate/hull bottom is a little high for a heavy load. Engine ventilates (may want to) on hole shot.

If tucked in too much, you will get up ok, but you will be nose heavy and not get much speed......which could very well be your problem NOW THAT I REALIZE THAT YOU DON'T HAVE TRIM.

That's the problem with these forums. You are fed a parcel of information. You attempt to answer using your knowledge and personal experiences. A lot of assumptions are made by you that may or may not be true.

Again; very well, your problem could be as simple as plowing (nose down attitude due to engine in too low a hole) and once you push your engine out a hole or two and maybe get a cupped prop (to stop any hole shot ventilation) if you do happen to ventilate on hole shot, you will then be up around the 35 mph where you belong........and it will explain the 40% slip I (BAM) calculated which will now drop to a more reasonable number like maybe 15 or 20 at WOT.

Keep in touch.

Mark

 

[meddo]

Re: 115HP Mercury, overpropped?

Sorry about not mentioning the lack of trim. I think the idea of me plowing is a pretty good assumtion that explains alot. I will keep the 13x17 for this Saturday's test and go out a hole or two. Results to follow.

Thanks,
Mike

 

[Texasmark]

Re: 115HP Mercury, overpropped?

Really looking forward to the results of your testing. I am confused on this one and would really like to see a positive answer.

Mark

 

[meddo]

Re: 115HP Mercury, overpropped?

Finally tested the 13x17 again today. I didnt make it out last weekend and yesterday I was on a no wake lake. I moved it to the 4th hole from the transom, it doesnt plane as fast at this setting but still gets out on plane very quick. Speed was better this time around at 30+ Mph. Rpms were leaning close to 5500 and I think I might have gotten 5600 if I would have given it a chance. I didnt want to run at wot for more than a few moments, as it is a 31 year old motor.

Is 5500 where I want to be or should I be closer to 5800?

Thanks,
Mike

 

[Texasmark]

Re: 115HP Mercury, overpropped?

I don't remember and don't have data for towers (any more).

Buttttt 5500 is no slouchy rpm for any 2 cy OB that I know of and I wouldn't be a bit bashful running it and not be worrying about lugging.

Just checked a '94 sales brochure and all the engines in 3 cyl and up are running 5250 to 5500. The 5250 numbers are for what I would call identical engines which had smaller jets installed for the lower hp rating; like a 75 and 90 are the same engine specs, but the 75 is rated at 5250 (cause it's jetted down) and the 90 at 5500.

HTH

Mark

 

[meddo]

Re: 115HP Mercury, overpropped?

I'm pretty happy with 5500-5600, I've decided to repair the 19er as well and will post my results in a week or so.

Thanks,
Mike

 

[Texasmark]

Re: 115HP Mercury, overpropped?

Sounds good. So the solution to your problem was that your boat was "plowing". Correct?

Mark

 

[meddo]

Re: 115HP Mercury, overpropped?

I think I was definately plowing before, because now It rides better and I got a 10Mph boost in speed.

Ran her again today on the 17, this time with speedo and gps. I also found out I had a bit more throttle left. Still on the fourth hole, 34Mph speedo / 35 Gps at 5600-5700 rpm. The 19 will be repaired by next thursday, I cant wait to try it.

I didnt catch any fish today though...I am 0-3 so far with this boat fishing.