new prop for the new to me boat

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Well that makes more sense. I don't remember where I got the info that the gear ratio was the only thing changed....didn't really make sense for a CT conversion.....you want to run a larger prop at lower rpms...gotta have a bigger gear case to make room for the 14-15" diameter.

While we are on the subject of CT, Remember the Scott Atwater 14 hp CT type engine, 1960-70s time line recalling. I forget the gear ratio and size of the prop but that was some LU for a 14 hp Toon Queen.

Makes a great difference to these by all accounts Mark. So impressed with what it does. I’d imagine the normal gearcase will be obsolete almost on these. The CT is so well suited to this engine. Be madness to get the normal one.
not sure if I mentioned, but there are now guys in the small power boat classes over here replacing their usual V6 2 stroke 2.0 class engines with the pro xs 115 2.1 litre version of this 115. Unbelievably they are competitive with the usual V6 2.0 2/ 150hp motor on the same hull. Incredible. Guys seeing a genuine 60mph out of these things. I think by now everyone is probably aware that they ain’t 115hp or even close to that. Haha.
 

Hunter65

Cadet
Joined
Sep 18, 2020
Messages
20
I finally got the motor raised 2notches and installed the turning point prop, same size as the merc black max, 13.25 x 17. so not really an apples to apples. It takes off fine, gets on plane quick, and I can run 30 mph at 4,000rpm trimmed up pretty good with some trimming left, top speed is about the same 46ish with gps at 6,000rpm
before with the merc prop and down on the bottom notch I could trim into the tilt portion before cavitating, not good...
I may go up one size next time I destroy a prop...but do not want to loose take off speed...maybe up one size with a 4 blade..????
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
I finally got the motor raised 2notches and installed the turning point prop, same size as the merc black max, 13.25 x 17. so not really an apples to apples. It takes off fine, gets on plane quick, and I can run 30 mph at 4,000rpm trimmed up pretty good with some trimming left, top speed is about the same 46ish with gps at 6,000rpm
before with the merc prop and down on the bottom notch I could trim into the tilt portion before cavitating, not good...
I may go up one size next time I destroy a prop...but do not want to loose take off speed...maybe up one size with a 4 blade..????
4 blade will upset the apple cart. Tossing in that 4th blade not only adds a blade but changes the purpose of the prop as I just found out with the Solas SS 4 blade that I just bought. However, the changes it caused were what I wanted to happen and I am happy with it.

My TP 3 blade and 4 blade props have completely different geometry due to this change in intended purpose.....reference: looking at my props. Doing that would just open a new can of worms. I'd just up your pitch if you want to change something. A 19 probably will get you down to your 5500 rpm limit and if slip doesn't change, may or may not change your top speed.....let's see:

Quick peek at go-fast.com prop slip calculator, running 6k with a 17 and 5% slip (for a number) gives you an actual calculated speed of 46. Upping to a 19, same 5% slip assumed, (you staying with the same prop with the only change the pitch), running 5600 (figuring 200 rpm drop per inch) gives you and estimated top speed of 43 and a slightly sloppier hole shot.

Soooooo sir, it looks like you are better off right where you are!
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
4 blade will upset the apple cart. Tossing in that 4th blade not only adds a blade but changes the purpose of the prop as I just found out with the Solas SS 4 blade that I just bought. However, the changes it caused were what I wanted to happen and I am happy with it.

My TP 3 blade and 4 blade props have completely different geometry due to this change in intended purpose.....reference: looking at my props. Doing that would just open a new can of worms. I'd just up your pitch if you want to change something. A 19 probably will get you down to your 5500 rpm limit and if slip doesn't change, may or may not change your top speed.....let's see:

Quick peek at go-fast.com prop slip calculator, running 6k with a 17 and 5% slip (for a number) gives you an actual calculated speed of 46. Upping to a 19, same 5% slip assumed, (you staying with the same prop with the only change the pitch), running 5600 (figuring 200 rpm drop per inch) gives you and estimated top speed of 43 and a slightly sloppier hole shot.

Soooooo sir, it looks like you are better off right where you are!
Wrong answer!!!!!!! I was moving around my open sites just now and in crossing the prop slip calculation window, I noticed a significant error. When I did the 5600 calculation, I didn't/forgot to change the pitch to 19. Once I did that the top speed moved out to 48 so you WILL get an increase in speed but still have a slower hole shot....assuming the torque curve, boat drag characteristics and prop slip remain the same as with the 17 at 6000. I doubt the torque will hold up as torque curves fall as rpms increase in most engines. The other wiggle point is that the 200 is just a popular guess, as is 150, and anywhere inbetween. Depending on the load on the engine and other parameters mentioned this number will be determined.
 
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