which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of power"?

capn kurt

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which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of power"?

1) Performance issue you are trying to correct.

I would like more speed.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.

It's an aluminum prop, I think it's a quicksilver/mercury prop.

3) Current prop diameter and pitch (required).

14 D 19 P

4) Wide open throttle RPM and speed with an average load (very helpful)

I am currently able to achieve a maximum of 55mph with two people on board, and I am turning (according to built-in tach) around 5200 - 5300 at that speed.

5) Engine/drive make, model, year, and HP

Mercury outboard, 1974 1150 'thunderbolt' ignition 115 hp.

6) Boat make model, year, length and weight

Switzer Craft model 170, 1974, 17', I think 900lbs? (would this be on the plaque mounted on the hull?)

So I would basically like to have another prop to throw on that would get me more top end speed. What would you guys recommend?

Thanks!
 

steelespike

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

The Switzer must be a light fast boat.
I would say that 55 mph is close to spectacular.
Motor is rated at 5,500 rpm.
If your not having any ventilation(cavitation( problems you might be able to raise the motor to a point just before venting is a problem.At 55 that could net a couple of mph.
While it sure appears the motor is tip top are you sure its timed right,carbs are synced
and opening all the way ?
 

Texasmark

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Something is wrong with your numbers. Using the "BAM" prop slip calculator that I pulled off the www and have used successfully for years, (rpm x pitch) / (gear ratio x 1056) yields a theoretical speed of 47.7 mph. Off that comes prop slip, say 10% (at least on an alum prop???? That would be stellar on a high perf SS prop) and you are at (Theo speed - Actual) / Theo speed) = 43 mph.

This is assuming a 2:1 gearbox which I had on my 115 and Merc has used on many engines of that hp. If it were the lower 2.3 that they used on their 90 hp engines the numbers would be even worse.

I say this because I have been there with a 17' Ranger pad hull, a '89 115 tower and a 24P SS Laser prop which the dealer put on the boat initially. First time out, I got 55 out of it but rpm's were way too low and I took the prop back to the dealer and exchanged it for a 19 which worked just right for my boat and allowed me to water ski also. Additionally, I am currently running a '02 90 hp on a very light/fast 17' aluminum with a 24P SS Ballistic XP and 50 + mph is tops at 5800 rpm. (it has the 2.3 gearbox).


Sooooo if you want us to help you, we need some accurate numbers.

Mark
 

iboatWayne

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Wow, I wish I could get my 17' ski boat up to 55mph. I have 17' witchcraft banshee and it tops out at 47mph @ 5500-5600 rpm(gps'd). 1978 Merc 1150 (115hp) with 19P SS solas hr titan. I'm thinking of getting a 20P prop (the 19P was a guess) to get that extra mph. Debating whether I should get the Quicksilver Lightspeed or stay with the HR Titan. The titan is approx. $100 cheaper but *maybe* the lightspeed is better quality?
http://www.iboats.com/Witchcraft__17_Banshee__1978/bp/64b102302r0
Did you GPS that speed?
 

capn kurt

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Hi guys - thanks a LOT for the responses. Okay, I found an online prop calculator and when I punched in my numbers I realized that something was VERY wrong with the info that I gave. So, the issue is that I'm using the onboard instruments to get speed and rpm, which are obviously way off. So, I'll have to get a known good tachometer to stick on it for a test run, and I should be able to use my smart phone with GPS to get an accurate speed number, correct? The boat runs fine as is, but I'll have to check that the carbs are opening all the way and that everything is sync'ed up. The procedure is well documented in my service manual. I'm new to this whole boating thing, and it sounds like 55mph will be pretty much impossible to achieve with this boat anyway. Perhaps I'll just stick with it the way it is and enjoy it. Thanks again!

capn

Oh, and here's a pic of the boat in question:


https://picasaweb.google.com/lh/photo/EqFiP3PdUy-AtqitbCjP5J1drB-Yr-0aTmo-IZWIf88?feat=directlink
 

iboatWayne

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

I did a tune up the other day and I've noticed a slight difference. New spark plugs, new accel blue copper core wires (3009B) and I cleaned up the cap & rotor. Can of seafoam in the tank.
Results were +1 mph (48 up from 47) and smoother running motor. I'm able to bring the rpm up to approx 5800 before it starts to slip. So I'm thinking I can prop up to 20P or even 21P if I dare.
 

Texasmark

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

I'm able to bring the rpm up to approx 5800 before it starts to slip. So I'm thinking I can prop up to 20P or even 21P if I dare.

Too much pitch tain't a good thing if your engine can't handle it. Rpm's can drop to the low end of the band, you can get out of the torque curve so any extra loading and the engine goes to pot like sloppy hole shots that are really hard on the engine. 2 strokes can and will puke all over too much loading.

Might just let her alone unless you have the money to spend and nothing else to do. Butttt looks like you have a light fast hull and if you over prop and it hurts the hole shot, take it to a prop shop and have them port it for you...that will improve your hole shot significantly and then you can have your cake and eat it.

For the record, '74 means that your engine is power head rated (brake hp) at 115 so your prop hp is 10-15% less or worst case about 98 hp. I am running a 90 hp (prop rated) at 50 gps alone at 5800 rpm's with a 24P on a 17' light fast hull. Running some fast numbers on yours and mine, I think you can handle the 20p (SS, high perf prop) and get your 50 and as I said, port it if it loads up on the hole...and give the Sea Foam a little time to clean things up. Running regular gas rather than ethanol can get you some more mph too.

Mark
 

capn kurt

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Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Re: which prop for switzer craft model 170 with 1974 mercury 115hp 1150 "tower of pow

Okay guys, had the boat out today and was able to get an accurate speed using GPS. I was spinning only 5000rpm (again, according to boat's tach), and reached a max of 42mph. Now, that was with four adults and two children on board. Im going to try and get some numbers with me alone tomorrow, so that my calculations will be more accurate. It seems to me that if those rpm numbers are correct, that I cant safely prop it up any more, as I might be bogging the motor too much - what do you think?
 
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