3 blade/ 4 blade prop ?

[Dan_o]

Q # 1. I have a 115 hp, in-line 6 cyl. 87 Merc. It has a 13.5 X 19 pitch prop on it. I was given a 4 blade prop as a backup for it when I bought it. I have no idea what size 4 blade prop it is. I don't have it here at this time. Just wondering, is this the right size 4 blade prop that I can use? This is going to be a back-up prop.



Q # 2. I have the above mentioned motor on a 24 ft lone star, that we made into a CC. No Tac yet, but at WOT the speed of the boat was 34.5 mph by GPS. (load= 2 people on flat water.) Does this sound like what it should be or not. Motor runs pretty good at WOT.

Thanks for your input. Dan

 

[weider1717]

Re: 3 blade/ 4 blade prop ?

Q # 1. I have a 115 hp, in-line 6 cyl. 87 Merc. It has a 13.5 X 19 pitch prop on it. I was given a 4 blade prop as a backup for it when I bought it. 4 blade prop is a M-wheel Zoptex 992115 13X19. Just wondering, is this the right size 4 blade prop that I can use? This is going to be a back-up prop.



Q # 2. I have the above mentioned motor on a 24 ft lone star, that we made into a CC. No Tac yet, but at WOT the speed of the boat was 34.5 mph by GPS. (load= 2 people on flat water.) Does this sound like what it should be or not. Motor runs pretty good at WOT.

Thanks for your input. Dan

What is your WOT rpm's?

 

[boat1010]

Re: 3 blade/ 4 blade prop ?

First of all the speed isn't much help to us. The important thing is the RPM when you are at wide open throttle. And as far as the 4 blade prop. It just takes a few min to change the props. Just put it on and see what you think of it. If it is just a stand by prop almost anything will work. You just might like the 4 blade better than the three blade.

 

[Dan_o]

Re: 3 blade/ 4 blade prop ?

I have no idea what the WOT rpm is. I will be hooking up a tach this summer. I will check to see how it is running. This is a back up prop. Thanks for your info.

 

[Blujay96]

Re: 3 blade/ 4 blade prop ?

The four blade will do fine for a backup prop. But like the last post said it may be a better prop for you.

 

[Texasmark]

Re: 3 blade/ 4 blade prop ?

35 on that boat with a prop rated 115 (which your '87 is) sounds good to me assuming you have a reasonable amount of stuff in it. The Lone Star has a flat bottom at the transom with soft chines. Boat weight is not all that much. There is a lot of wetted area, but not a lot of weight so they balance out. 35 is a good speed, nothing to be ashamed of for a rig like that. The trim of the engine and amount of boat you are wetting and the angle of the hull to the water in your picture looks just fine to me.

The 4 blades are usually for fat arsed boats or water skiing where you need some help getting "the mass" on plane. Unless you piled a lot of "stuff" right in front of the transom, you shouldn't have that problem with that boat.

I just did a quick calc with an assumed max rpm of 5500 (should be engine rated max.....was on mine) and with the prop you have on there currently, speed calculates out to be 35.2 mph with no slippage. With that hull I'd guess slippage would be at a minimum so say 5% slippage and you have about 33.5 mph. Your 34.5 gps is mighty close.

I'd say you are right where you want to be and don't need a tach and don't need to put your 4 blade on unless you just want to. I boated half my life without a tach and never lost an engine and had between 0 and few repairs. On the '89 115 I bought new, I had a tach and I ran 6000 WOT constantly and my son-in-law is still running it with only water jacket covers and impellers changed over the years. I think the engine preferred the high rpm's in case you might be worried that your gps speed indicates that your engine rpm's are over 5500. I really liked that engine, loved the smoothness of the 6 cyl, and it was real sweet on gas. Course I bought a USED I/O once but that doesn't count.

Way it looks from here.

Mark