need help with prop and rpm`s

[redlabel1]

good morning new to the site. i have done many searches and can`t quite find the answers i am looking for. I have a new to me used boat it is a 1860 crestliner with a 2001 Bf75 on it.I use it for bowfishing so it has some added weight from the front deck and back deck, generator, batteries and so on. My first question is on it now it has a 13 3/4 x15pitch prop and i can only get 3500 to 3800 rpm`s and 29mph (gps)out of it wide open and if i try to trim it up the boat jumps up and down sometimes at higher speeds so i have to run it with the motor trimmed all the way down.What prop would you recomend because i have to get a new one as i hit a log and broke 2 blades off the current one.And 2nd question is shouldn`t it be running higher rps wide open? thanks for any and all help

 

[etracer68]

Re: need help with prop and rpm`s

If you mean a Mercury Big Foot, I think those are 4 stroke motors. I belive 4 stroke motors dont run as high of RPM's as 2 stroke motors. I would find out what the WOT RPM range is for that motor and then go from there about prop.

 

[etracer68]

Re: need help with prop and rpm`s

I looked up a Mercury 75 big foot, and it said, max rpm 5000 to 6000. If thats correct, I would make sure your tach, rpm is correct. If it is, then I would try to get a demo prop from a dealer, and see how it works for your boat, I would start with a 11 pitch, and see if the rpms go up. They should go up about 400 to 600, maybe more. Soulds like you have alot of weight, also how many people in the boat?

 

[redlabel1]

Re: need help with prop and rpm`s

sorry for not stating but it is a honda 75hp 4 stroke. and usually 4 people in the boat. owners manual says 5000-6000 rmp wot

 

[steelespike]

Re: need help with prop and rpm`s

Your boat is about 530lbs dry,motor 384,4 men ?, gear and fuel etc?
Your 2200 rpm short yet speed seems better than what 3800 rpm would produce
long story short I think your tach is off.Selector switch may be set wrong,or needs to be rotated to clean contacts.You may need a prop change but check the tach first.

 

[redlabel1]

Re: need help with prop and rpm`s

thanks steelespike. is this something that can be done at home or does it need to go to a mechanic?

 

[steelespike]

Re: need help with prop and rpm`s

Some tachs have a selector switch on the back; note the position and rotate it a few times to clean the contacts.Not sure what number it should be set on but a dealer should be able to help.If you bought it from a dealer they may be willing to lend a tach or send the mechanic to check based on your purchase.

 

[redlabel1]

Re: need help with prop and rpm`s

bought a new tach and it is reading the same

 

[steelespike]

Re: need help with prop and rpm`s

Well something is not right. You still have to make up 2200 rpm. You can't do it with a prop
so you need to be sure the motor is at full power.In excellent tune,throttle opening all the way
Bottom clean and smooth.If your running with the motor trimmed all the way down that would account for some of the rpm and speed. It does appear you need more weight forward to hold the bow down when you trim up or less weight so the bow can stay up.You might see how it runs with no weight in the boat and add a little weight at a time trying to keep positive trim as you go.
Raising the motor on the transom can help reduce porpoising.

 

[redlabel1]

Re: need help with prop and rpm`s

after more research i believe i have the wrong tach. i have a new one ordered so i hope this will fix it

 

[steelespike]

Re: need help with prop and rpm`s

Let us know what you find and your results.Your likely to need some fine tuning.
It appears your setup isn't working.

 

[redlabel1]

Re: need help with prop and rpm`s

ok come to find out it had the wrong tach. the old one was model#37250-zv5-950 this tach says universal but it will not work with the bf75-90`s. the new tach is #37250-zw5-010za and it works. my idle is 900 and wot is around 5200. thanks for all help

 

[steelespike]

Re: need help with prop and rpm`s

Great its allways nice to have a success.Thanks for getting the info back to us. you should consider getting your rpm up a little more.
Is 5200 with just you or with a typical load.
Have you considered raising the motor to reduce porpoising? In the exrtreme this could improve hole shot,top speed and reduce porpoising.If you do it yourself its all free.