Best Prop for 1983 V6 Mercury 150hp for Top Speed?

[Fantasvas]

I just purchased a 1977 17' Witchcraft with a 1983 V6 Mercury 150 Outboard. The prop on there now seems to be an old prop and measures 12 1/2" Diameter and 23" Pitch. Also, it's pretty beat up around all the edges of the fins. I've taken it out and it seems to run okay, but only hits 4500 RPM with two people and 43 MPH WOT trimmed all the way down. I know I need to trim it up to get better numbers, but right now it's so darn cold here in AZ! I'm going to try a Solus Amita 3 with 14.3D x 21P and see how that works. Any other suggestions? Thank you...

 

[Fantasvas]

Here are some pics...

 

[flyingscott]

I would suspect your motor more than the prop or the boat is water logged.

 

[Fantasvas]

flyingscott - the motor runs great - had the carbs rebuilt and a new control box installed - also the boat doesn't take on any water - dry as a bone...

 

[Fantasvas]

flyingscott - I've been reading up on water-logged boats - I guess the best way to check is to drill a hole and pull out some foam - correct? The boat doesn't seem heavy to me when I'm launching or getting it on the trailer - and it doesn't sit low in the water. I guess when I change the prop and run it at the lake I'll get a better idea...thanks for your input...

 

[fhhuber]

"Best" prop varies with the hull and engine.... I don't have the experience to predict best for yours, just one similar boat and 2 props. But changing the prop can make a tremendous difference.

17 ft bass boat (should be similar weight and has similar appearance below water line)
My Johnson 150 came with a SS 14 1/4 X 22 3 blade. (brand not known).
I got lucky and there was a new Apollo 14 1/8 X 23 4 blade in the hull I bought to replace a rotted hull.
The Apollo gives better performance in every way...

 

[Chris1956]

A 23" pitch prop might be a bit tall for you. You want your max RPM up into the mid 5000s or so. Also, some rake would help with performance. An inexpensive prop that has good performance is the 3 blade Turning Point Hustler. I would recommend 21 pitch.

Also, inspect the height of the anti-ventilation plate, while on plane. It should be a bit above the water level, but not more than an inch or so o start. You can vary the height of the motor on the transom to maximize speed. Raising the motor up usually increases speed, but only if she is not too high. Mounting too high causes a decrease in the ability to lift the nose by trimming. It is a custom set-up for each motor/boat combo.

 

[Faztbullet]

That hull does not have a pad to run on it is a round bottom design and takes more HP to create speed. You need a prop that will lift and offer bite in positive trim. You would be better off with a 4blade in 20 pitch and this should put you near 6000rpm and low 50's.

 

[steelespike]

I wouldn't make any changes until you get a wot rpm and gps speed at a maximum up trim.
Your dragging the whole boat through the water.
Bundle up and run just long enough the get a steady reading.

 

[Fantasvas]

Thanks for everyone's help here - I really appreciate it! It looks like I have a lot of different options with everyone's comments. Since I already have the new Solas Amita 3 prop (14.3 x 21) 3-blade aluminum, I'm going to change out the old prop, since it's pretty beat up, and take the boat out with the new prop on and run it WOT trimmed up to get the best possible speed and RPM readings. This should tell me a lot about Top Speed, RPM's, and the boats best performance. Regarding the motor I have on there, my mechanic said I should not exceed 5500 RPM's...It's suppose to warm up soon, so I'll be out on the lake in a few days...I'll keep everyone posted - thanks again for all your input...

 

[steelespike]

If the 21' will deliver about 4900 rpm speed will be about 46. and it will feel peppier.

 

[flyingscott]

With the set-up you have you should be in the 50s. That is not a big boat for a 150 it should have no trouble pushing it even with the prop you have. Right now you are a 1000 rpm low. Rule of thumb is 2" of pitch equals about 200 rpm meaning you need to drop 10" of pitch to get to where you need to be. So either your tach and speedo are way off or by keeping it trimmed down you are making it plow so bad the engine is struggling. I had a 17' runabout with a 125 merc ran 53 with a 21" pitch aluminum prop and I bet my boat was heavier. That's why I think the numbers are kind of off. I would do what steelespike said

 

[steelespike]

1" in pitch is good for about 150 to 200 rpm.
The 21 could add about 400 rpm. At about 4900 rpm it would make about 46 and feel peppier.

 

[flyingscott]

I looked up that boat that hull weighs less than a #1000. That boat should go down the lake like a rocket powered potato chip. That is a mercury v6 150 struggling to push that boat at 43 mph. I hope it's a trim issue and the boat is just plowing like the trailer is still attached. When that boat is set-up rite I bet it will be a blast to drive. I am not going to argue about prop pitch and rpms each prop is different. And if the prop was to much for the motor it should effect the holeshot in a significant way yet he said nothing about that. Seems more like the motor is falling on it's face.

 

[Fantasvas]

flyingscott - I agree with you 100%! This boat should be flying down the lake! Regarding the holeshot, after taking it to a marine shop 3 times to rebuild the carbs, compression check was good (all 6 cylinders were between 110 and 120), install new fuel pump and all new fuel filters and lines, and finally replace the old control box with a new one, the motor is running great - but when I punch it when standing still it has cavitation issues, although once it gets going it planes out fast (trimmed all the way down) - also it feels like the motor wants the boat to go faster (probably because it wasn't trimmed up! 'cause stupid me forgot to tell my son to trim it up at WOT last time we were out - it was freezing cold! LOL!) - Anyway, the cavitation issue and the top speed made me think it may be the prop...and the old prop is pretty beat up...So, I've got the new prop on and I'm going out on Monday to test it out again...thanks for your comments!

I uploaded pics of the old prop on next post...

 

[Fantasvas]

Old boat prop...

 

[steelespike]

Prop looks pretty mundane,a very basic prop.In decent shape for its probable age.The Solas Amita is an entry level prop
with a good reputation.I think it has more sophisticated technology compared to the 23".
You will experience the affects of trimming up and a lower pitch.So the rpm change isn't likely to match my suggestions.

 

[fhhuber]

Get it on plane... trim nose up till it starts bouncing then trim down JUST to make it smoother, but not eliminate the bouncing completely.
Should be top speed.

 

[Scott Danforth]

it was freezing cold! LOL!) -

your in Arizona, put on a jacket. Freezing cold is when you need the axe to cut the ice to get back to the boat launch, or throw the anchor onto the ice to pull it away from the launch to get back in. (BTDT, have the tee-shirt) Boating in the bay of green bay in January in Wisconsin while wearing snowmobile suits and helmets with battery powered socks in your sorell boots and the ambien temperature is 15 degrees is freezing cold.

BTW, your old prop looks fine. a can of paint would make it look new. I am going to lean towards motor issue, water logged hull, or you have the motor trimmed in

 

[SkiDad]

i test drove a similar boat a few years back - 17' checkmate with 135 black max and it hit 48 mph trimmed with 5 people in the boat - it had a 21 pitch stainless. if my memory serves me right it was about 5200 rpm. It was OK out the hole - if i would have bought it i would have dropped to 19"

BTW - NICE BOAT !! I love boats of that vintage - reminds me of being a kid