Insight and evaluation on this boat/motor/prop setup.....

[Swinerton]

Purchased the boat last week, and had it on the water. Boat is a 2004 Polar Kraft Fisherman SC 14', weighing in at 658lbs dry. Total weight on the day we took it out for its maiden run, was about 1300lbs. I know the boat is underpowered for sure, but I would like to maximize what I have first. Motor is a 2005 Johnson 4-stroke with PT/PT.

Top speed measured was 22.1mph topped out on a slightly rippled surface. The boat took a solid 15-20 seconds to get on plane, even with the motor all the way down. ANY trim up at all, and porpoising would take affect. RPM's sounded a little low after 25 years of being around all types of engines. I DO have a tachometer coming in a few days to see where I am at. Hopefully I am under revving a little. Operating RPM range is 5000-5600. I have also heard/read that it's better to run the 4 cycles on the higher end of the RPM range.

Now... I found the info on the Prop. It's a Rascal aluminum in great shape, but I think it may be a tad too aggressive for a 25hp four stroke trying to push 1300lbs. It is a 10.5" X 11 pitch.

I know I wont really know what the next step is, until I get my tach, but I am looking for other insight to see if I am doing what I should be doing. Motor height is good. Hull is clean and waxed etc. Carbs are open all the way @ WOT, and plugs look good.

Sorry for the long post, but I try to include detailed information when I can.

 

[jestor68]

Re: Insight and evaluation on this boat/motor/prop setup.....

The slow time to plane sounds as if a pitch reduction may be in your future.

But, as you said, you'll need the tach to verify things. You'll need that motor to turn at the top of it's rpm range in order to provide peak performance.

Figure on about 200 rpm change with each inch of pitch change.

 

[Swinerton]

Re: Insight and evaluation on this boat/motor/prop setup.....

The slow time to plane sounds as if a pitch reduction may be in your future.

But, as you said, you'll need the tach to verify things. You'll need that motor to turn at the top of it's rpm range in order to provide peak performance.

That's why I am hoping I find it running less than 5100rpm. Then I know I can't go wrong by trying out a 10P.

My 15HP 4-stroke has a 9 X 9 on it. With only 10HP more, a 10.50 X 11P seems like a pretty big jump. Especially with about 600-700lbs more boat weight to move.

 

[emoney]

Re: Insight and evaluation on this boat/motor/prop setup.....

Not to "reduce it to the ridiculous", but is your ballast evenly distributed? Those boats don't take a lot to be stern heavy, so keep that in mind. 15-20 seconds seems like a long time to plane even with a 25hp even if the prop isn't pitched correctly.

 

[Swinerton]

Re: Insight and evaluation on this boat/motor/prop setup.....

Not to "reduce it to the ridiculous", but is your ballast evenly distributed? Those boats don't take a lot to be stern heavy, so keep that in mind. 15-20 seconds seems like a long time to plane even with a 25hp even if the prop isn't pitched correctly.

As evenly as it can be yes. This was with my G/F (136lbs) sitting in the front swivel seat. It pushed her and the bow right up high in the air. I even had the front 15 gal livewell tank full, and our fishing gear up front. 15-20 seconds was an estimate. It very well could have been 12 seconds, but I know it wasn't less than 10.

 

[emoney]

Re: Insight and evaluation on this boat/motor/prop setup.....

Any chance you're getting prop slippage?

 

[Swinerton]

Re: Insight and evaluation on this boat/motor/prop setup.....

Any chance you're getting prop slippage?

I suppose it's possible if the pitch is slightly too aggressive, but by the sound of the motor, it's bogging the motor pretty good trying to move too much water too quickly.

 

[steelespike]

Re: Insight and evaluation on this boat/motor/prop setup.....

I'm assuming The Rascal has a good deal of cup. Props with cup are usually figured at 1" higher pitch.
Another words when calculating a cupped prop a 11" would be calculated at 12".
I'm thinking a Rascal would need to be about 10" for your setup.
Calculating backwards from your speed (not a good idea) guessing 10% slip I get about 4500 rpm
With that load slip could be higher and a tach will help us figure things out.

 

[jestor68]

Re: Insight and evaluation on this boat/motor/prop setup.....

In the mean time, if you put a Doel Fin on your motor, it will help it to plane off.

 

[Swinerton]

Re: Insight and evaluation on this boat/motor/prop setup.....

I'm assuming The Rascal has a good deal of cup. Props with cup are usually figured at 1" higher pitch.
Another words when calculating a cupped prop a 11" would be calculated at 12".
I'm thinking a Rascal would need to be about 10" for your setup.
Calculating backwards from your speed (not a good idea) guessing 10% slip I get about 4500 rpm
With that load slip could be higher and a tach will help us figure things out.

Very good insight right there. I would LOVE to find my motor running between 4600-4800. A good 500rpm increase would help a ton in that case. I could probably get away with a 9 pitch cupped at that point.