Which Prop? - Crap Shoot - 1974 Mercury 650 on a 15' tri hull

[Berryn]

Cheers All,

I have spent the last 9 months restoring a 1970 ArrowGlass "Tarpon" Tri Hull. New deck, floatation, transom, paint...etc. Also just rebuilt a 1974 Mercury 650 (65HP) 3 cyl to power the old girl. 120lbs on each cylinder. A couple of questions here....

First, I am running a ProPulse composite prop that my friend Robert gave me. It is adjusted all the way out (17p) and I am able to get 29mph at WOT with myself and one other person - loaded with gas, battery, dry storage and cooler. The engine has no problem getting on plane (Light boat for a fiberglass) and will stay at 5000 RPM no matter where I set the prop. Being in Virginia, I am doing a lot of time on the James and Chickahominy rivers - both have their share of obstacles. This makes me very leary of having a plastic prop as a full-time prop, so I am looking for an aluminum prop. My question is.... available props for this motor range up to 17 pitch, and I would like a little more speed, but I'm not sure if that would be too much and bog down the ol' Merc.

Second, The poppet valve continues to weep a small bit through the weep hole even though all parts of the poppet valve and thermostat are brand new. I have taken it down twice to try to find anything wrong, but to no avail. I'm going broke on gaskets.

I am going to try to post a picture of the boat so you guys/gals can get a better idea of the size (bear with me, I'm a Harley buff). Any suggestions are GREATLY appreciated.. Thanks for your time and thoughts.

 

[jimmbo]

My charts say a 13 or 14 in pitch would be what a 3 cylinder 650 of that era on a 15 ft boat would utilize
Your engine WOT rpm range is 4800 - 5300

I think the helm is too far forward for optimum performance, and without power trim...

 

[Faztbullet]

I thought stick steering was rated at 40hp max.....

 

[Berryn]

Thanks for the reply guys. A 14" prop is about max for the motor so I'm trying to stay under that mark (from the center of the prop shaft to the cavitation plate is approximately 7.25"). Even a 13" prop will spin below the skeg. The helm is placed where it was originally and with 2 people (front and back) planes nice - level and dry. The Ez-Stick is rated for up to 70hp and the boat - up to 80hp. All in all the boat does great. I'm just looking to find the right prop that I don't have to worry about shearing and a little more speed would be nice. I'm thinking 35mph without suffering a loss of rpm.

This is the second boat of this kind I've had. The only difference is the first one was a 1973 ArrowGlass Tarpon with a 50hp (1973) Merc and it would fly. I should never have sold it - so much fun - but I was younger and at the time wanted to go to a bigger boat (family thing). When replacing the floor, stringers, floatation and transom I was careful not to add more weight than original.

I'm really puzzled on this prop thing and don't have enough money to buy a lot of props to find the "perfect" prop (if there is one). I'm thinking of possibly a three blade aluminum somewhere between 11" to 12" with a 15" to 17" pitch.

Thank you all for the responses.

 

[jimmbo]

Props for that engine are in 10 - 11 in diameter range and between 7 to 16 in. pitch

 

[Berryn]

Thanks again Jimmbo. What pitch would you recommend?

 

[jimmbo]

I would start with a 3 blade 11 1/2 X 13, and see what rpms it gets.

 

[Berryn]

So...You think 14 would be too much?

 

[jimmbo]

You asked what I would recommend. I said a 13 would be a good starting point. Props are not that expensive

 

[Berryn]

13 it is. Thank You Jimmbo. I'll keep you posted. You've been GREAT !

 

[WesNewell]

You're running a 17P now at 5000rpm? And getting on plane without problems? Why would you want to go to a 13P prop? That boat looks like its dry weight can't be more than 700#. I'd think you wouldn't go under a 15P prop and that would put you rpm's over max of 5300rpm. Going from a 17p to a 13p will raise rpm ~800rpm. Personally, I'd stay with a 17p prop. and that should get you at least 40 mph with everything set up properly. Bu5t first check yor engine mounting height. Running wot, the AV plate should not be below the waterline. 1.5" below will cost you ~10% max speed.

 

[jimmbo]

Is your tach set properly, and working correctly? You said " will stay at 5000 RPM no matter where I set the prop."

Clarify what you mean "set the prop"

50 hp? that's whats on the cowl

 

[jimmbo]

You're running a 17P now at 5000rpm? And getting on plane without problems? Why would you want to go to a 13P prop? That boat looks like its dry weight can't be more than 700#. I'd think you wouldn't go under a 15P prop and that would put you rpm's over max of 5300rpm. Going from a 17p to a 13p will raise rpm ~800rpm. Personally, I'd stay with a 17p prop. and that should get you at least 40 mph with everything set up properly. Bu5t first check yor engine mounting height. Running wot, the AV plate should not be below the waterline. 1.5" below will cost you ~10% max speed.

His numbers aren't adding up

That engines gear ratio is 13:24

With a 17 " prop, 5000 rpm should be around 41 mph allowing 10% slip
He says he is getting 29 mph with a 17" prop and 5000 rpm.

I suspect a tach problem and an overprop situation.

 

[WesNewell]

That boat only weighs ~500# and is rated for 60hp max. No way should he go below a 17p prop with a good running 65hp. and yes, properly running and mounted he should be getting over 40 mph. Maybe his speedo is off, or he's got the engine mounted way way too low.

 

[Berryn]

The cavitation plate is even with the keel.

The prop is an adjustable ProPulse plastic ("set the prop" - adjustable pitch - between 13 an 17) with very small, narrow blades. I don't believe the ProPulse is doing what a "normal" 17p does. No matter where I set it - 5000 rpm - but started at center position. The only thing that changed, was speed (from 23 to 29). The tach IS a bit iffy - the motor purrs nicely, and up on plane in no time, but may be laboring a bit WOT. I know the tach should have dropped, but it didn't. Only had it out the one time so far. Two people on board (about 400 pounds together), relatively smooth water, 20lbs of ice, 6gal tank (in the aft) one battery (also aft) - 29mph (gps).

The motor is a 65HP but before I rebuilt it I wasn't very hopeful of getting the compression back to snuff, so I put 50hp on it (my joke - they look identical - it actually came back to 120 pounds all).

Gear ratio is 1:64:1. I would like a little more speed, but playing the safety card (these old, small light boats don't handle wakes very well at speed). I'm going to go with Jimmbo on the 13 pitch. Hoping that the bigger, stronger blades will grab more water and reduce slippage. It really is a crap shoot. Like he said, props are cheap. If the 13 doesn't push it at least as fast as present, I'll keep it as a spare until I get another prop.

I will say this.... I'M EXCITED! It is really a FUN little boat. I had one like it before, then went to two Montauks and a 19' Key West - all good, but not nearly as fun for this old geezer - up front with the stick steer on smooth water at 5am with a little fog on the water. We were flipping coins to see who would pilot - not to worry, the back seat is comfortable too and relatively dry.

All these points you guys brought up have been going through my head like a chainsaw. I REALLY REALLY DO thank you all. I'll keep you posted on what the results are. Thanks-A-Million, Dave

 

[WesNewell]

With a 13p prop you will over rev the engine on such a light boat. With the engine rpm not changing with prop pitch, you're probably doing that now. Are you positive you have the tach set to the proper setting for the engine you have? I turn ~5500 rpm with my 60hp and 14P prop on a 17.5' boat that's twice the weight as yours.

 

[jimmbo]

The cavitation plate is even with the keel.

Should be fine with your setup

The motor is a 65HP but before I rebuilt it I wasn't very hopeful of getting the compression back to snuff, so I put 50hp on it (my joke - they look identical - it actually came back to 120 pounds all).

In 74, there was a 50 hp, but it was a 4 cylinder 44 cubic in. block that had the traditional Merc look, metal wrap around cowl, in the late 80s Merc introduced a 3 cylinder 49.8 cubic in. 50 hp that had the clamshell cowl like yours.

FYI, Your engine is a crankshaft rated engine, the 3 cylinder mercs became Propshaft rated in the mid 80s


I would like a little more speed,

To get more speed you either need more power, or best utilize the power you have. Is the hull straight and true(no Hooks or Rockers)? Nice sharp corner at transom-bottom? Is the hull surface smooth? No water logged floatation foam?
Propping at the exact rpm where the peak hp occurs will produce the highest speed. propping on either side of that peak(even though it still running in the WOT range) results in slightly less top speed. The challenge is finding the prop that has the highest efficiency but lets the engine hit the exact rpm where the peak HP occurs. Don't let anyone buffalo you in to thinking switching to a stainless will guarantee you another 5 mph. Switching to a Stainless Steel often requires a complete reevaluation of the propping. Again, most advantages are utilized at speeds Above 40 - 45 mph, One serious disadvantage of a SS is that an Impact with something solid enough to break an Aluminum blade, often bends both the SS blade and the propshaft.

 

[Berryn]

My old 50hp Merc was on the old boat when I bought it almost 20 years ago and sold shortly after. The boat was a 1973 and I always assumed the motor was of or around the same year. It was a 3 cylinder clamshell with a blue MERCURY stripe. I never had to do anything to it except carb kits, fuel lines, impeller and plugs. I did replace the prop, but I purchased another with the same numbers (don't recall numbers).

The tach has more pole settings. I'm not sure that it's set correctly. I tried 6 pole and was WAY off. It's set on 5 pole now, and a friend said he has one I could borrow to compare on my next trip out. The tach is now reading from one of the two yellow wires coming off the stator. I tried the black and white wire from the switch box (top wire - diagram says "tachometer") but got no reading at all.

I would never put a stainless prop on this boat. I would rather break a prop than bend a shaft.

The hull bottom has been gone over - straight, clean and smooth. No bulges, no pulls, even wake. All the old foam was taken out (over 300 pounds) and replaced with 4lb structural floatation foam.

WesNewell, Your comment is the basis for my putting up this post originally. Very real concerns. Also,the crankshaft vs propshaft rating is as much a mystery to me as the pitch. To get the perfect (or near perfect) pitch on such a light and simple boat is still a matter of trial and error as far as I can tell. As I said, I'm not trying to get the maximum speed, just the best feel and performance safely. However I'm thinking now I may have to start looking for a 15 or 17 pitch just to keep the rev limit down - or - maybe not. I'll know more next time out with the new prop and better tach reading.

Thanks again Jimmbo for the helpful insight.

Again, I really appreciate all the input. I'm learning loads of things new to me. Please keep up the comments and Thank you all. Dave

 

[jimmbo]

Bit of history.
1972 was the first year for the 3 cylinder 49.8 cubic inch engine, introduced as a 65hp. It remained a 65 until 1977 when it became a 70hp. In 1984 a 60hp and a 50hp version of the engine was introduced. This was the year Merc switched to prop rating of these engines.
In 1961 Mercury introduced the 4 cylinder 44 cubic inch 50hp. That engine remained in the line as a 50 up till 1984. In 84 it was rebadged as a prop rated 45, and the 3 cylinder 50hp appeared. This 4 cylinder went on to become the 40 ‘Classic’ as well
Colour schemes, in the late 60s, 1968 to be precise, Mercury started using red on its cowl decals. In 1972, the colour changed to blue which was used through 1983. The only exception to the blue in the 70s was on 3 higher performance models offered in 1976, 1977, and maybe 1978. The 65XS(90hp), the 85XS(?hp), and the 150XS(155hp). These were badged in red
In 1984 - 1985 Mercury used an ugly Brown decal on the cowl. In 1986 a Gray scheme decal was used
As to you having a Blue striped clamshell 50 hp. No engine was produced as such. I would say that someone did what you did, placed early 1970s 50 decals on a clamshell covered engine

 

[Sea Rider]

With a 13 pitch prop will know if engine revs at least middle to max wot rpm range factory stated..Post wot results...

Happy Boating