Horsepower - New and Old

[WhalerSailor]

Hi again,
On an earlier post I was telling of a friend who recently repowered his boat. He went from a 1985 Mariner 40 HP to a 1962 Merc 500 (just to keep the correlation between motor and boat). His old Mariner 40 pushed the boat around 30 mph and the lighter weight 4 cylinder Merc 50 only pushes it 9 mph. Both with 3 blade props. The Merc did originally have a two blade that pushed around 23 mph. All props 13 pitch. Still looking for answers. Was the old 50 hp REALLY 50 hp or was the Mariner REALLY something more? Baffeled, Dave

 

[Chinewalker]

Re: Horsepower - New and Old

Sounds like you have a set-up issue. The old 50 ought to go at least as fast as the Mariner 40.

Does the Merc have good spark in all four cylinders? Good compression top to bottom with no more than 5% variance? You're getting full throttle advance? What are your top end RPM? The issue is less about number of blades and more an issue of prop pitch. That motor should be peaking at about 5200-5500 RPM at wide open throttle. What kind of boat is it on?

- Scott

 

[Texasmark]

Re: Horsepower - New and Old

And yes on your question about available hp. The 62 was probably "brake" hp which is power head measured with no accessories. The 85 was probably prop rated and the guys on here use numbers like 12-15% loss. So 15% off 50 hp puts you at 42.5....close enough for a match....all things being equal.

But that's not going to get you that miserable speed. What kind of rpm's are you running on the 50 at 9 mph.......that's not even planing speed.

You have big problems that could be several places and setup could be one, and that could include a 20" lower unit on a 15" transom or vice versa (part of the setup Chine mentioned). Let's start with rpm's.

Mark

 

[Tom P]

Re: Horsepower - New and Old

This may be nothing but I also have a 1985 Merc 500 that I bought last fall, The motor ran fine out of the water. When I put the boat (13’ whaler) in the water it would only go 7 knots (about 9 miles an hour). After a lot of head-banging, I discover the wires where no good. They arched on the engine cover. I got the zap of a life time while removing the cover. It dam near blew me out of the boat. I changed the wires and all was well. BTW – wires for that motor normally need to be hand made but I found out that a kit for a Merc 6 cylinder will fit. Merc part number 84-821945A56.

Hope this helps,

Tom

 

[OldMercsRule]

Re: Horsepower - New and Old

Those old 4 cylinder 500s were very good motors. I would recomend you stick with a 1968 or newer, (especially if you have a problem with this one as it seems from the speed you are getting). I see them on Craig's list for less then $500.00 all the time. The 1968 had the more modern surface gap plugs and trigger fired, [I would stay clear of the point fired units in 1966 and some in 1967, if you can]. Those two engines in good condition should be very similar power, (the 500 slightly more). JR

 

[WhalerSailor]

Re: Horsepower - New and Old

Thanks for all your help guys.... We just took it out today and experimented with props on the lake. The two blade ran about 25 mph as did the three blade but the three blade took about a minute or more to reach WOT. It just doesn't seem to have the power of a 50hp - more like a 30. No hole shot - the two blade cavitates and the three blade bogs. We're still looking for a tach to match the Teleflex "Lido" gauges. It seems there are some other problems with the consistency of the motor's power. Today was only the second time out, but with your help the learning process is not so bad. I'll add to this post as soon as I get a tach installed. Thanks again, Dave

 

[portabuddy]

Re: Horsepower - New and Old

i have a 1967 500 and i had a similar problem where it would run 10km MAX, well as it turns out the throtle cable was broken and not actuating, then all the sudden the botch job that was on there fell off and it ripped open one day, I also run a 3 blade prop abd it seems to just rock... granted it was painted a couple of times and could be smother and cleaner but thats a project for another time...


Question?, will a tach from a 4 stroke aka a car engine work on a 2 stroke? aka. a merc 500?

 

[studlymandingo]

Find the Pitch of your Prop

Find the Pitch of your Prop

We just took it out today and experimented with props on the lake. The two blade ran about 25 mph as did the three blade but the three blade took about a minute or more to reach WOT. It just doesn't seem to have the power of a 50hp - more like a 30. No hole shot - the two blade cavitates and the three blade bogs.

Your problem isn't with the number of blades. You could have a 9 blade prop and it wouldn't get any better; you have to look at the pitch of the props. If you are bogging at takeoff and your RPMs are low at WOT then you need to drop in pitch on the prop. The two blade is likely to be a lower pitched prop; go with a 3 blade of the same pitch as the 2 and you will likely see a significant gain in performance.​

 

[Willyclay]

Re: Horsepower - New and Old

The Mercury website has a link to their prop selection model that may help. I found that if I enter weights instead of using their generic hull types, the results were consistent with existing hull/engine/prop combinations available to me for verification.

 

[WhalerSailor]

Re: Horsepower - New and Old

Thanks again guys,

Studlymandingo, In my first post - at the top I stated that they are both 13 pitch. I thought the same as you. The only reason I put the three blade on in the first place is I understand the three blades have less vibration. But there is definately a BIG difference.

Willyclay, I'll try that. I'll have to give a rough guess at the weight. I would say the boat without motor is in the neighborhood of 600 - 700 lbs.

 

[pcfry]

Re: Horsepower - New and Old

I have a question on this topic. I went to the Mercury Prop Selector, and their recommendation for my setup is a 3blade 9P 12.5" prop. This causes questions to me.

I've got a '75 Galaxie Runabout, 17.5', with a '75 Mercury 40HP Thunderbolt (2stroke), with a 13P 3 blade prop - and my WOT is about 20mph @ +/-3000 RPM. I'm thinking that it should run faster and stronger. But wouldn't a 9P actually make it run slower?

Thanks.

Paul

 

[jheron]

Re: Horsepower - New and Old

She should be running around 5000 RPM at wot, your lugging your engine(very hard on her). If you switch to a 9P it should bring your rpm up and make her feel like a brand new machine! Its like shifting out of overdrive into passing gear.
Regards Jon