What prop on a 18ft w/a 115 tower of power

[earringboy]

Got a 1979 Sylvan Rodmaster 18ft last fall and just had a 1986 Merc 115 inline six put on. The only prop he had on hand was a 13 3/4 x 15. i think this may be a bit small. I have a 13 3/4x21p and was wondering if anybody thinksd this is to much? Dealer did not water test and I won't be able to hit the lake until next week. Any thoughts or recomendations?

 

[iboatWayne]

Re: What prop on a 18ft w/a 115 tower of power

How funny. Almost the same question I had, except I have a 17' and '78 115. Has a 16 pitch SS on it right now, but I haven't had a chance to get it into the water yet.

 

[steelespike]

Re: What prop on a 18ft w/a 115 tower of power

I would say the 21" is probably too much.Run the 15 see what you get.
Your betrter off with too small as you can throttle back to keep thec revs in the
rpm range.I feel t you will be lugging with a 21 just shouldn't run that way.
If you can't find a prop and the 15 is small you could have it repitched or have cup added.

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

thanks. I kinda thought the 21 might be a bit much. We have a prop shop close to where I work, who knows maybe I can trade the 21 for a 19 or 17. Is stainless worth the extra $$$$?

 

[steelespike]

Re: What prop on a 18ft w/a 115 tower of power

The change to stainless can be tricky. We would need a goopd deal more info to do an accurate estimate of results going to ss.
Results could be as little as a few tenths to several mph.
I would suggest get set up best with an al prop. H can can get you there.
Then it will be a little easier to estimate ss results.Other things to consider are if you boat where hard impact (rocks) is likely the stainless is expensive to repair.
If you are likely to run in sand or mud the ss will usually hold up better than al.
Is it worth the expense?

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

thanks for the info. I have all types of bottom where I fish and have been with my buddy when he put his SS prop in the rocks. Think I will stick with AL, plus I am "cheap". I will know more about how the 15p does after Friday April 16. Provided the lake is calm I will run her up there and take note of my GPS speed. I belive the WOT range on my motor is 5000 to 5500, where is the best place to fall when you have the right prop? I can hardly wait to get on the water. Also is a rebuilt or repaired prop as good as a new one? I have a beat up 17p as well, it might be to far gone though.

 

[Bob_VT]

Re: What prop on a 18ft w/a 115 tower of power

Here is what I ran with my old 115 tower

http://forums.iboats.com/showthread.php?t=177235

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

Thanks Bob, but now I have another question, 3 blades or 4? Raise the motor or leave it in the top hole, motor all the way down

 

[steelespike]

Re: What prop on a 18ft w/a 115 tower of power

A 4 blade is generally used to improve low end performanc.Hole shot,planing and improved cruising.A 3 blade gives good overall performabce with a little better top end.

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

Okay, so I had the boat on the water today. I opened it up and hit 36mph on the gps with the 15p prop, tach showed about 5900 rpm(I think my tach may be off). I throttled down to 5100 rpm and held 31.5 mph. I was out of the hole so fast I dumped my wife right off her chair, bad deal. What do you guys think? I think I would benifit from a steeper pitch. I plan on trying a 21 pitch just because I already own it. Thoughts. What is the best for my motor. If I want to run across the lake at about 35 or so is it best to run wide open or throtled back? Thanks

 

[steelespike]

Re: What prop on a 18ft w/a 115 tower of power

I think the 21 will be too muchj but it will give you another base line to work with.Could knock the rpm down 1200.
Keep in mind a different brand or style can throw those figures off.
Its best to have your motor propped so it can operate within its rpm range.
Generally is agreed to be able to run near the top of the range with todays fuels.
In my opinion find the most efficient cruising speed and run it.
If time is more important to you then efficient speed may be secondary.

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

i think the 21 may be to much as well, but I have it and might as well try it. Both props are Mercury factory props. Motor seemed to hit top end real quick with the 15. Not sure when I will be able to get back out. I am using premium gas, as it is all I can get without ethanol.

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

Okay, i have been thinking about this a bit and just want to make sure my thinking is correct.

15p prop allowed 6000 rpm, Mercury reccomends 5000 to 5500 rpm so I need to reduce the rpms by 500 at least. So it seems 200 rpm per inch of pitch I should be around 18p to give me 5400 rpm, right? And a 19p should put me around 5300 rpm and my 21p down to 4900 rpm.

Am I on the right track?

I see Bob_VT ran a 19p 3 blade al at 5100 rpm going 40.5 mph on a 18ft Forester. I imagine my rig is close to the same weight as Bob's, aluminum boat?

 

[ono_55]

Re: What prop on a 18ft w/a 115 tower of power

I have a 85 115 tower of power on a 16.6 skeeter starfire, running a 17 pitch prop for now i can turn 6300 rpms.This might give you idea of what you my run with a 19 pitch prop.

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

thanks ono_55. I am not sure I would want to run that motor any faster than 5500 rpm's though. How fast did it go?

 

[ono_55]

Re: What prop on a 18ft w/a 115 tower of power

yea 6300rpm is alittle high my mech tells me these motors should run up to 6000rpm.I'am going to go to a 19 pitch prop.Don't know the speed speedo is unhooked.I'am also at 1800ft.

 

[JaseBosto]

Re: What prop on a 18ft w/a 115 tower of power

I dont think the 21p is too high at all for your boat...I think it might be perfect. You will lose some hole-shot but you should be amazed at your top speed compared to before.

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

Bootstrap, I think I might be able to sneek out on Sunday and try the 21, we will see.
ono_55, I called Mercury a few weeks back and they told me 5000 to 5500 was the recomended operating range for my motor, you may want to dig into it a bit more, then again the factory may be playing on the safe side, but I would hate to see anyone hurt their motor by over reving it.My guy said there is no rev limiter on them. I also have no idea how altitude can affect a motor.

 

[Bob_VT]

Re: What prop on a 18ft w/a 115 tower of power

That 21 will probably lug that tower a bit and that's not good. Giving credit where credit is due...... D Hadley guided me with my prop and you might want to drop him a PM

 

[earringboy]

Re: What prop on a 18ft w/a 115 tower of power

I do not want to lug the motor, If I can't get to at least 5400 rpm the prop will come off and I will run the 15 untill I get my hands on a 19 to test. I want it proped right and have read some of H's post where he talks about making the high end of recomended WOT and that being the best for the motor, I need this motor to last for a while. I already own the 21(came with a couple of 50hp mercs I bought for a different boat, guy was moving and gave me a bunch of extras in the deal) so I may as well try it once. The speed is nice to have, some of the lakes I fish are 4 or more miles across, and it is nice to get there and also to get off in a hurry if the weather comes up. The 15p throtteled back to 5100 at 31.5mph is a vast improvement over my last boat, 15ft Starcraft w/25 John, lucky to get 17 mph outta her.