Help in choosing a new prop for my 17' Starcraft Bass Boat with a 70HP Evinrude.

[paulf73]

I'm looking for some help in choosing a new prop for my boat. I am not having a real performance problem but would like to improve the performance of the boat overall.

I have read a lot of stories of people switching to a four bladed prop and having performance improve both out of the hole and on the top end. But I have also read horror stories of a four bladed prop killing performance all together, I am really wanting to put a four bladed prop on but don't want to regret it.

Here are the specifics on the boat/motor:

1. I would like to get a bit more speed out of the boat. I consistently see 33 MPH on my GPS. I think there is a little more in this boat than that. It does ok out of the hole but it certainly doesn't leap up on plane like a ski boat. It does get the job done though. I can squeeze another mile per hour or two out of it if I trim it way up but then get into a porpoising situation which is no fun at all.

2. Currently the boat sports the factory Evinrude aluminum 3 bladed prop.

3. Diameter and pitch is 13 1/4" X 17"

4. RPM at WOT is unknown. The boat does not have a tach so I can't say. I can verify that the throttle butterflies are all going wide open though.

5. The engine is a 1981 Evinrude 3 cylinder 70 HP two stroke, the model number is E70ELCIM.

6. The boat is a 1983 Starcraft 17' aluminum bass boat. Weight is unknown.


I have a feeling that just replacing the factory prop with an exact replacement would probably improve the performance of the boat quite a bit. While there are no chunks missing from any of the blades there are a couple of obvious dings. One of the blades is fairly bent and I am sure it is hurting the performance quite a bit. I think it is just a case of a 30 year old prop showing its age.

So what do you experts say? Should I go with a 3 blade or 4 blade? What diameter and pitch on each?

Thank you all very much for any help you can provide.

 

[steelespike]

Re: Help in choosing a new prop for my 17' Starcraft Bass Boat with a 70HP Evinrude.

My info indicates the 81 70 hp is rated at 5,000.
I calculate using 9% slip that your at about 5500.Which is good.
I wouldn't be surprised if your 30 year old prop was tired and could have a higher slip %.
Simply replacing it could result in a little bit of speed.Without any actual rpm can't really tell how well the setup works now,though 33 is respectable.
Usually a 4 bld. won't have as much top end.
I think the same prop is a good bet based on what we have.

 

[paulf73]

Re: Help in choosing a new prop for my 17' Starcraft Bass Boat with a 70HP Evinrude.

My info indicates the 81 70 hp is rated at 5,000.
I calculate using 9% slip that your at about 5500.Which is good.
I wouldn't be surprised if your 30 year old prop was tired and could have a higher slip %.
Simply replacing it could result in a little bit of speed.Without any actual rpm can't really tell how well the setup works now,though 33 is respectable.
Usually a 4 bld. won't have as much top end.
I think the same prop is a good bet based on what we have.

Thank you steelespike-

I don't exactly understand how slip works or is calculated but I do agree that 33 MPH is probably a pretty respectable speed. I think you are right that a direct replacement prop is the safest bet! That way I could have a brand new prop on the motor and have the current one in the storage locker as a spare that I know works.

I may also buy a 4 bladed prop in addition for nothing more than experimental purposes. What diameter and pitch should I go with if I do?

Thanks again!

 

[steelespike]

Re: Help in choosing a new prop for my 17' Starcraft Bass Boat with a 70HP Evinrude.

A 4 blade prop usually improves hole shot,Stays on plane better at reduced throttle,handles better and runs smoother. Top end will likely be slower.
It would be easier to suggest a size if we knew your rpm.You might be able to borrow a shop tach, Or buy a induction tach like a Tinytach or even an optical tach.Both simple to use and don't require permanent installation.
If your motor is runnng at its suggested rpm it is usually normal to reduce one inch in
pitch for a 4 blade. The diameter will probably be smaller than your 3 blade.Don't worry about the diameter unless the difference seems extreme.