Help with prop selection

[Freshtrax]

So the boat is a 14 ft starcraft falcon with a 65 hp merc I had laying around . I put a new transom in it for the long shaft merc, new floors, carpet ect ect... The boat launches hard onto a plane no problem and stays bow up a little until 20-25 mph then it drops down and starts to dig in. My prop is pretty beat up it has a few small chunks missing and I don't think it's helping.
Gross weight.
Boat hull 310 pounds
Motor 250?
People 400-600
Gear gas and battery 120
Total around 1100-1200

It currently has a 13 pitch 11.5 diameter prop on it. I'm thinking of buying a 14 pitch by 11.25 replacement any thoughts? I've never bought a prop before. the motor on there may or may not be 25 hp over the rating ... Is 25 mph all I'm gonna get out of this thing no power trim and setting in the different holes makes no difference at top speed. The cavitation plate is just below the keel -- Merc 650 prop chart http://www.maxrules.com/oldmercs/Propellers/Props/24.pdf

Edited June 12, 2013 by Freshtrax

[Freshtrax]

I don't have a tach. And I'm nowhere near full throttle when it starts to get squirelly

Edited June 11, 2013 by Freshtrax

[aplumma]

Talk to the dealer about a prop loaner program or sign in to iboats.com and do a search they have answered this question sucessfully in the past.

 

 

Art

[Bernie]

Sounds more like a trim problem.

And you will need a tach to get the proper pitch.

[misfish]

I would recommend getting a new prop.13 pitch 11.5 diameter. Without trimming,your pretty much set at what you get. Pretty much an old school set up.

[Freshtrax]

Thanks for chiming in bernie... even with a tach. i cant even get near full rpms with out a neardeath experiance.

 

misfish .. oldschool chart i posted reccomends a 14 ppitch for the length and weight of my boat what makes you reccomend a 13? im just trying to learn.

[rufus]

I had a 40 hp Mercury four stroke that had an auto trim feature. The auto trim would overide my manual setting once I had reached a plane if it thought perhaps the bow was too high. Sounds like what your motor may be doing.

[Freshtrax]

no auto trim.. its a 1975 merc 650...

 

after doing a little research less pitch equals more torque. ok got it. so 13 it is..maby even a 12

Edited June 12, 2013 by Freshtrax

[Sinker]

I'm not sure your going to see much improvement with a prop change on a 65hp. I'd say your way over powered. Try shifting your load around first.

 

S.

[Freshtrax]

i am way overpowered ...when i say it launches onto plane it does...2 secconds tops. i just want to get the bow up while on plane. im going to order an 11 3/4 x12 cupped prop and see what happens

[Freshtrax]

and aplumma ... the crew over at i boats are not to fond of overpowered boats.

[Roy]

and aplumma ... the crew over at i boats are not to fond of overpowered boats.

Neither are insurance companies. Be careful.

[Sinker]

Do you have it trimmed up as high as I will go now?

 

I don't think a prop is going to do what you want.

 

S.

[misfish]

Do you have it trimmed up as high as I will go now?

 

I don't think a prop is going to do what you want.

 

S.

 

There has to be a bar just below the head. This allows you to trim (sorta speak) the motor.

Can you post a sideview of your motor,up close?

[misfish]

Neither are insurance companies. Be careful.

 

What is that boat rated for any ways?

[Terry]

To me you are going the wrong way. A tach would help

But it sounds like you need more power from the prop, not more speed. I would get a bigger peop. Not more pitch. As misfish says more the rod up a hole or two.

[Roy]

What is that boat rated for any ways?

Probably 40HP

[beagle dad]

overpowering a boat could be a big issue

the hull will flex just in front of transom causing a vacuum to the water

at higher speeds the nose of boat will drop ...no matter what you do in this case the boat will just become more dangerous

[Freshtrax]

I have it in the seccond to last position "out". As far as I can go without serious cavitation. I totally agree terry after more reading it is what I need I don't want to go much faster. 30 mph would be fine I'm just trying to get the attitude right while at speed. After allot of reading I also understand that a nicked up prop will give up allot of performance and is bad for the lower unit so I think I'm going to order a tiny tach and go down to my local boat scrap yard and buy as many used props in the right range as I can find and do some testing. before puchacing a brand new prop. thanks for the input guys. This prop stuff gets complicated when you add cupped and slip into the mix. Part science part vodomagic it seems.

[Sinker]

Trim the motor up as high as it will go and see what happens. It might take a little longer to get up on plane, but will keep the bow up as well.

 

I still don't think a prop will make you happy, although by the sounds of it, yours is in pretty rough shape.

 

Pics would really help.

 

S

[davey buoy]

Now my motor is bigger mind you,I ended going to a 17pitch from 21. My advice somewhat came from similar boats and talking to the owners,as well as this site. Hole shot better,but lost a tad on the top end.You must be overpowered,most times when things are right,you keep the motor in the lowest slot.Close to the transom.

[zamboniwaterboy]

Call Jim, he is awesome! http://jandjmarine.com/

[Old Ironmaker]

Over power changes all the dynamics of the hull design, both in the propulsion aspects as well as the metallurgical loads on the hull. You do need a new prop but to tune that boat overpowered without auto trim will be a chore. I have seen videos of over powered aluminum and steel hulls twist like a pretzel. Be careful. In fact I highly recommend a larger boat or smaller powerplant. Notice any leaks lately?

[Freshtrax]

No but I will keep an eye on it... Come on guys it's not like I put a 125 on it. The motor probably weighs less than a 68 40 hp motor if not its close. The way I see it its no different than putting twin pipes and high comp heads on my sled ( 120 mph over ice isn't something I do more than once or twice a season) , or turning the boost up on my Volvo from 8 psi to 12 ( I don't go out and roast the tires every day) . Like I said I haven't come close to full throttle. and don't plan to . I hear ya on the larger boat thing I think I may have 10 ft itis . If I wasn't getting married in two months I would have drug home a sweet 24 ft starcraft islander today.

Edited June 13, 2013 by Freshtrax

[Freshtrax]

Actually my first engine choice was one of these but figured that might be pushing it