Replacing a prop and need some help...

[bryanj23]

Greetings,

New member here. I have an early 70s Crestliner Muskie 550 15 1/2' trihull boat with a late 60s (68 I think) Mercury Merc 650 65 hp 4 cyl. outboard.

The boat was a gift from my late father in law. Before I got it the motor had been completely gone through but for some reason the prop was not replaced. It was a 13 3/4 x 15 three blade aluminum prop (I think, it was pretty tore up so 13 1/4 - 13 3/4) and I'm pretty sure it was an oem prop though the numbers were too corroded to read. The boat wouldn't plane very well. A friend of ours gave us a 13 3/4 x 15 two blade prop to get me by. With this prop on I top out at 20 mph with myself and my wife and three children in the boat for a total of about 500 lbs. If one of us adults is on a tube with one of the children we can run at about 16 mph.

I would like to use the boat for our family to enjoy days on the lake pulling the tube around and just generally screwing around. I have tried to research the topic and have read many prop reviews but being more of a motorcycle guy myself I feel that I'm best off asking for advice on this one especially considering the cost of many of the props out there. I do not know the wot as the boat has no tach. The only other thing I know is that the prop is 15 spline.

Any help you folks can give me in this matter would be very greatly appreciated.

 

[hwsiii]

Re: Replacing a prop and need some help...

Bryan, I show that boat with an allup weight of about 1,600 pounds and the motor with a 2:1 gear ratio, you should be getting about 30 MPH out of that setup with a 15" Pitch prop. First try and move the motor up at least 1 1/2" , then buy you a Tiny Tach and tell us how many RPM you are turning. My opinion is you have a mtor problem at this time, and you need to find out what it is. Check compression, throttle butterfly opening all the way and plugs.

H

 

[bryanj23]

Re: Replacing a prop and need some help...

Bryan, I show that boat with an allup weight of about 1,600 pounds and the motor with a 2:1 gear ratio, you should be getting about 30 MPH out of that setup with a 15" Pitch prop. First try and move the motor up at least 1 1/2" , then buy you a Tiny Tach and tell us how many RPM you are turning. My opinion is you have a mtor problem at this time, and you need to find out what it is. Check compression, throttle butterfly opening all the way and plugs.

H

Even with all of us in the boat? With just my son and I we probably get 30 out of it. I'm not sure though, I was going to check it tonight but my gps was dead. I'm not sure I have an inch and a half of movement, that's getting pretty close to the top of the transom (or is there an adjustment I'm not seeing?). I apologize, I don't know much about boats.

Gordie told me before he passed that the motor had about 30 hours on a rebuild, and the lower unit less than that. The boat did sit for about 6 years though. I had it in the shop this spring, it had a new pump put in and new control cables put on. The guy at the shop said he gave it a once over but he didn't find anything that needed to be done. That's not to say that he couldn't have missed something, but I know the guy through mutual friends and reputation and I know he knows his stuff.

On the other hand, I imagine this thing is like any other motor and has seals in it, correct? I suppose one of them could be bad having sat for so long. Might not have looked bad at the time but putting them under pressure one could have failed. I haven't noticed anything leaking though.

 

[hwsiii]

Re: Replacing a prop and need some help...

Bryan, these are my numbers for your boat, if you see something that is not right please let me know. I was being conservative with your speed because it is an older trihull.

Boat and Motor

Prop Picker

H

 

[bryanj23]

Re: Replacing a prop and need some help...

Those numbers all make sense. Some are low, some are high. I suppose they average about right. I do have a trolling motor and a big heavy (ridiculously heavy) battery for it. I also carry 9 gallons of fuel. But, we don't carry an ice chest or food; at least we haven't yet. The only thing I didn't see is the number of blades on the prop. Could it be that with this 2 blade prop I'm just not getting the drive to push the boat to 33 mph?

It doesn't sound like there is any formula for finding the diameter and pitch without the wot. Where is a good place to look for a really cheap tach, I really don't have $100+ to throw at a tach on top of buying a new prop if one is needed.

 

[bryanj23]

Re: Replacing a prop and need some help...

Actually now that I think about it we were having some problems with it being kind of finicky to start last time out. Plugs? I'll have to check them and see if I have one fouled out.

 

[hwsiii]

Re: Replacing a prop and need some help...

Bryan, if money is tight just buy the cheapest 3 blade aluminum 13 x 15" Pitch prop you can find that will fit your motor, and it will be fine. Your problem is you don't have enough blade surface area with the two bladed prop to carry the boat and you are having a lot of prop slip because of that.

H

 

[bryanj23]

Re: Replacing a prop and need some help...

Bryan, if money is tight just buy the cheapest 3 blade aluminum 13 x 15" Pitch prop you can find that will fit your motor, and it will be fine. Your problem is you don't have enough blade surface area with the two bladed prop to carry the boat and you are having a lot of prop slip because of that.

H

Thanks a lot for all your help. I would love to put a tachometer on and get it all dialed in but it's another expense. Maybe later. I was looking at props on here and there are several 4 blade props that are comparable in price. What would be the benefits of each? I've read reviews that say 4s are smoother and have faster pick up with little or no loss on top speed. Are these claims true? If so, what would be the advantage of a 3 blade?

Sorry for all the questions, like I said I have very little knowledge of outboards at the moment. I need (and want) to learn a lot more!

 

[hwsiii]

Re: Replacing a prop and need some help...

Bryan, in my opinion your boat doesn't need a 4 blade prop just a new 3 blade aluminum to run correctly. In some larger boats it can help tremendously, but I see no extra benefits for your boat and motor combination and your uses.

H

 

[turbodude51]

Re: Replacing a prop and need some help...

Bryan, in my opinion your boat doesn't need a 4 blade prop just a new 3 blade aluminum to run correctly. In some larger boats it can help tremendously, but I see no extra benefits for your boat and motor combination and your uses.

H

I also need some advice, I have a 1964 Mercury 500 50HP, does not have original prop on it. I counted splines on shaft., seems to be eleven. This prop only has a 12 on it, this number is on inside: 48-38094-12, Motor is on a Mercury 430HD Inflatable, looking for a prop that will give good launch and a decent top end, non of the manuals show much information. This prop seems not to bite well.
Thanks in Advance, Rick

 

[hwsiii]

Re: Replacing a prop and need some help...

Turbo, I need to know the gear ratio of your motor and the manufacturers recommended WOT range. I don't have any information going back that far.

H

 

[turbodude51]

Re: Replacing a prop and need some help...

I looked and could not find ratio in any of the books I have, I know they offered 1:78 but who knows, the original prop that came with that engine was a two blade brass prop. WOT RPMs is 5200-5800
Thanks, Rick

 

[hwsiii]

Re: Replacing a prop and need some help...

Turbo go to the Mercury motor forum and see if anyone knows there.

H

 

[turbodude51]

Re: Replacing a prop and need some help...

In Manual it says Pinion 14 propeller is 23,
thanks, Rick