My First Boat!

[Supersportster]

hello all,

A month ago I got a good deal on a boat I was trying to buy all summer.
I have had it out bunch of times, to get used to the boat. Alot of the things I was wondering about I found answers by reading through these fourms.

The boat is a 1989 17 foot Sylvan Supersportster, with a 90 HP Johnson V4 VRO. an electric johnson trolling motor.

The prop was all beat up when i got the boat, and the guy said he had a new prop he would put on. Which he did.

So here is my question, I have read a lot of threads on here about WOT Rpms'. What should they be on my motor? With two other people and me on the boat yesterday, at WOT my tach was reading 6500 rpms. Is this to high?

Thanks

Shane

 

[Gary H NC]

Re: My First Boat!

Welcome to Iboats!
That sounds like to much RPMs.
I think those motors like the 5800 RPM area..

What size and pitch prop is it?

 

[Supersportster]

Re: My First Boat!

Thanks, I look forward to answering questions on here when I can. For a while I will most likely be asking more then I answer...

The Numbers on the prop where 13 1/4 X17

Does this mean I should get a prop that will keep me at 5800 rpms?

Will this change my accleration/top speed?

 

[NelsonQ]

Re: My First Boat!

So the first # is the diameter of the prop, second is the pitch.

You want to match the right prop to your engine to achieve max RPMs without going over.

Each increase in pitch will drop the RPMs by about 200. So you now have a 17P prop.

For example, if you wanted to drop it to 5600-5800 range (assume maximum for your engine is 5700RPMs), you'd need to add a pitch of about 4 to a 21 P to achieve that. Again, you need to find out the maximum RPMs your manufacturer suggests.

I think Gary was pretty close at 5800 (my Johnson V4 115) is around the mid 5000RPMs.

Once you have that, you won't exceed the max RPMs.

As for speed, (the answer is YES, but whether positive or negative) the prop is the only thing you can affect on your engine to achieve speed changes and a lot will depend on the shape of the hull, etc as to how fast you'll go.

Hope this helps.

 

[Supersportster]

Re: My First Boat!

Thanks NelsonQ, it helps TONS.

So now I just need to confirm the max rpms on the 1989 90 HP Johnson, and purchase the proper prop.

Do stainless steal props make a big difference with this motor?

 

[iwombat]

Re: My First Boat!

It should be no problem for your motor to turn 6.5k. It might be getting close to the rev limit built into the power packs though. What you might be suffering from is a loss of top MPH by running a prop that's under pitched. What you're gaining is hole shot. If it were mine, I'd run it for awhile and see how it responds. If you want to trade some hole-shot for top speed, then change props. If not, you're not hurting anything with those RPMs.

 

[NelsonQ]

Re: My First Boat!

iwombat, what is the max RPM for Supersportster's engine? Now you have me thinking (I'm still trying to find the service manual for mine) but I was told mine was in the mid 5000s.

I'll post a separate thread inquiring on mine.

 

[iwombat]

Re: My First Boat!

There is no real max RPMs on these engines. Ask Dhadley where he operates the race motors. At very high RPMs you'll start encountering flow problems and combustion issues and the like, but the internals of the motors should easily handle 7k or more. There's a suggested operating rage, which used to be listed as 5k, but is better at 5.5-6k with today's gas. The power-packs generally have built-in rev-limiters in the upper 6k range.

 

[Supersportster]

Re: My First Boat!

I have been driving it like that for a month or so, it seems fine, by my self in the boat it gets going to 42-45 mph. I like the way it drives, aslong as it is not going to hurt the motor, but if I change the prop to keep it to at around 6 thousand rpm how much top end is there to gain?

 

[iwombat]

Re: My First Boat!

You might get a couple more MPH and maybe some slightly better fuel consumption. If it were me, I wouldn't sweat it too hard. If you want a better prop, then consider more pitch. Otherwise, save the prop $$ for beer and be happy.

 

[Supersportster]

Re: My First Boat!

HAHAHAH IWOMBAT , sound like an exceptionally good plan to me!

as long as it doesnt hurt my motor, then I dont mind, and especially since the prop is brand new, there would be no point in spending the money right now on another one...

I have a few other upgrades in mind .....

 

[Bob547]

Re: My First Boat!

Hi guys, I have the factory manual for 1987 120 and up v4s v6s and v8s. The full throttle operating range is 5000 to 6000 rpms. The max power is rated at 5500 rpms. There is also a section on prop selection. It says "Select a propeller that suits the customer's application and allows the engine to run near the top limit of the full throttle operating range when the boat has a light load in it." I would check to see if your tach is accurate, maybe its off alittle. Find out the factory specs on your motor, and then you can decide to leave it or try a different prop, or maybe borrow a used prop and try it out. Bob