new to boating and have prop question

[thoruton]

I just traded for a 1988 renken 20 ft spitfire with 5.0 litre mercruise. It takes forever to plane and is at 5000 rpm when doing 25 MPH. I see it has a new prop and assume it has the wrong one. It is a 14 in with 23 pitch. How do I figure the correct one or do I have bigger issues?

Thank you
Thoruton

 

[Bifflefan]

Re: new to boating and have prop question

I just traded for a 1988 renken 20 ft spitfire with 5.0 litre mercruise. It takes forever to plane and is at 5000 rpm when doing 25 MPH. I see it has a new prop and assume it has the wrong one. It is a 14 in with 23 pitch. How do I figure the correct one or do I have bigger issues?

Thank you
Thoruton

A 23p at 5000 should be flying. Is the gear ratio correct? I think it should be 1.50.
Is the prop hub good? It may be a spun hub.
Is the boat in good condition, not water logged.
Is the motor in good condition, good tune and all cylinders firing?

 

[thoruton]

Re: new to boating and have prop question

OK. Figured its going to major issue. How do I tell if waterlogged? Can it be fixed? How can I tell if spun and how big a job is that.


I appreciate the help

Thoruton

 

[Bifflefan]

Re: new to boating and have prop question

How do I tell if waterlogged?

Weight it. go to a local truck scale and get the weight then minus the trl weight.

How can I tell if spun and how big a job is that.

Put it in gear, engine off. Turn the prop by hand, it should click one way and not turn the other. When you find the way it wont turn, really lean on it, if it turns its bad. It may need to be taken to a porp shop to find out for sure.

 

[180shabah]

Re: new to boating and have prop question

23" prop, depending on gear ratio should give you 50-65mph. You definitely need to get the prop hub and coupler checked.

 

[thoruton]

Re: new to boating and have prop question

If its waterlogged would it have high rpm's ? I can understand the not planing. How do I check the ratio? I know it has a replaced outdrive, maybe the wrong one was put in ? I will weigh it later this week when time but I would like to have some ideas of what to be looking for. How would the coupler cause this?


Thank you

Thoruton

 

[EricJRW]

Re: new to boating and have prop question

I confirmed my spun hub by making a mark on the prop that lines up with the cotter pin. When the alignment changed, I knew the hub spun inside the prop. This trick might work for you (assuming you have a castle nut).

 

[180shabah]

Re: new to boating and have prop question

Gear ratio is something we will eventually need to know, but it is NOT you current problem. They do not make one low enough to get 25mph at 5k with a 23" prop.

 

[thoruton]

Re: new to boating and have prop question

does a waterlogged boat make the rpm too high? How do I check the coupler. I am hoping to get home early enough to weigh it today but may have to wait for the weekend. I did take quick look last night and noticed water in the ski storage unit so I am assuming it is water logged but just trying to understand the high rpm

 

[Chaparralboater]

Re: new to boating and have prop question

If the boat is waterlogged, the boat is carrying more weight then needed and causes the speeds to be decreased at a dramatic amount. Water in a ski locked could mean bad drainage but if the floor has soft spots that's usually a good start to the boat having soaked foam..aka water logged. And also a spun prop is giving you high rpm with no speed. The hub is spinning loosely inside the prop that is not moving giving you a slow speed and high rpm..check the prop first as you don't want to burn your engine out. My advice would be go get a new one..aluminum is cheapest and slap it on and you will see a difference..a 14x 21 should give you good hole shot and good top speed. I'm no pro but hope it helps..