Power output

[GonzagaCRO]

Hello,

I have 1974 blue band Merc 500 50HP Thunderbolt on a 2008 16" fiberglass open fisherman who is rated between 40 and 75HP.
Normal load with 2 persons is around 1600lbs (total weight 700kg) and WOT is 5500rpm,it planes without problem but with 3 or more persons it won`t plane.
Cavitation plate is 1cm above keel (lowest point of the boat).
I searched and found out that the power output was rated before at flywheel not at prop shaft, is that true?
How many HP this motor has on prop shaft?
This would explain my problem, I`m just underpowered.

Best regards,

Igor

 

[Dunaruna]

Re: Power output

Yes it is true, the HP ratings for outboards before 1982 (or there abouts) was taken at the flywheel, not prop shaft.

You could expect to loose around 10%, then there is the age of the motor to consider.

You could conceivably be looking at 40hp at the prop.

 

[GonzagaCRO]

Re: Power output

TNX Dunaruna, i suspected that could be the problem. I think I have even less than 40HP.
When I am alone in the boat it runs great, hole shot is good at it planes out immediately but with 3 persons or more on board it struggles and can`t plane out.
I have 12p original mercury prop on it, i borrowed from local mechanic a 13p, 15p and 17p but the problem was only getting worst.

Best regards,

 

[GonzagaCRO]

Re: Power output

OK one more question

What is the minimal power required to push 2000lbs 16" fiberglass open fisherman without problems?

Minimal power to weight ratio calculation maybe? Some "golden rule" ?

Best regards,

 

[Texasmark]

Re: Power output

You were on the right track with your prop switch, just went the wrong way as you noticed by the reduced performance with the substitute props.

Go the other way, like 10P but expect overkill when you are alone: Really fast hole shot and probably full rated rpm's before you get up the speed that you'd like, or that you get now.......I don't remember the numbers but it is somewhere around 200-300 rpm's per inch of pitch.

That would translate into you possibly being able to to overrev the engine while alone and you really don't need to be doing that......Buttttttt 2" x 300 is 600 on your 5500 is only 6100 and on a Merc, I don't call that overrevving and personally wouldn't worry about it.

Some Mercs are rated at 5250 or 5500 rpm max, but in my opinion that is due to deliberate fuel starvation to get only 75 hp out of the same block that delivers 90 hp @ 6000 rpm when allowed to eat and breathe.....an inexpensive way to provide a 75 hp engine for those boats BIA certified for 75 max hp.

Buttttttt the lower the pitch of the prop, usually the larger the diameter (for a given series) which together result in better pushing ability but a decrease in top end performance with a light load and thus a loss of mph; course you gained in this area from your current situation with your heavier load. How much top end variance depends upon the variables in your situation.

So, decide what you want and prop accordingly, or swap props everytime you change from light to heavy loads and vice versa, or change to the lower pitched prop and just adjust to the rig's performance curve when alone.

On your question about what 45 hp can do (50-10%).....not all that much.

HTH,

Mark

 

[sschefer]

Re: Power output

I agree with Mark all the way. You might try adding an inch in circumference and dropping to a 10p but doing it with a 4 blade prop. I personnally like the Stilletto Bay Runner's but there are many to choose from.

I did this with a 100hp Merc/Mariner and I lost a little on the top end but I kept it within the MAX rpm range and the boat came up on plane easily no matter how much my brother ate the night before.

There is another way to compensate and that's by using "Smart Tabs". I think they sell them here on iBoats and if not, several of the online vendors sell them. You are at the bottom end of the HP range so if you decide to go this route just PM me and I'll help you get them setup correctly.

Another way to solve your problem is by putting a little more HP on it. The Merc/Mariner 100hp 2+2 would be a good lightweight fit for your boat. You can check eBay, I think there's one for sale there really cheap right now.

 

[Dunaruna]

Re: Power output

There probably is some sort of calculation to determine minimum HP but it is all relative.

A 50hp 2 stroke on your boat is ideal for trolling and then a slow plane back to the ramp.
A 90hp would be ideal for skiing/tubing etc but too much for trolling.

I think a 70/75 is the perfect match for a 16ft glass boat.

 

[GonzagaCRO]

Re: Power output

TNX all for replies!

When I am alone in the boat and the surface of the sea is smooth, WOT is around 5900rpm but in "normal" conditions it`s 5800rpm.
I noticed that I have around 250rpm drop per person and with 5 persons (driver + 4) WOT is around 4800-4900rpm.
I live in Croatia and selection of props are very limited but there is a SOLAS prop dealer near me so maybe you can recommend something from solas?
I was thinking to do what Texasmark said, just swap the props every time.
I also noticed when going from 12p to 13p rpm decreased 400rpm.


Best regards,