interesting prop challenge, 18' deck boat.

[H20Rat]

So I've looking for a backup/secondary prop for my deck boat. Happy with the current performance of the stock 14.25x21p merc prop, but something with more stern lift would be nice.

The boat is an interesting challenge. It is somewhat of a barge to get moving, but once it is up and running the boat moves! (essentially a tri-hull with minimal deadrise.) Because of the heavy back end, I rarely ever trim up at all, it is almost always all the way down. Only at full out WOT can I start to move it up, and even then, it will start porpoising fairly quickly. With the hull design and aft weight, the boat does not like any speed between 10-20 at all, so most of my boat time is either idle speed or low 20's.

1) Performance issue you are trying to correct.
Second prop and better midrange manners.
2/3) Current prop manufacturer, model, aluminum or stainless as a minimum.
Mercruiser 14.25x21p aluminum 3 blade
4) Wide open throttle RPM and speed with an average load (very helpful)
4800+ RPM and 49-50 mph.
5) Engine/drive make, model, year, and HP
Mercruiser 4.3l/4 bbl
6) Boat make model, year, length and weight
1994 Kayot Corso, 18', ~2500lb

So I'm thinking most likely a 4 blade, probably SS. Pitch is a question, considering I'm at or above the top of the rpm range already. Thoughts?

 

[alldodge]

Didn't see with ratio drive you have but thinking you have a 1.47 ratio. If that's the case you have 23 percent slip.

 

[H20Rat]

Yeah going to have to check next weekend. I don't think it was 1.47 but not 100% sure. Didn't the 4.3's/alpha II's generally come with 1.81 or 1.62? 1.47 should be a 5.0l+

 

[alldodge]

Don't know you tell us

 

[steelespike]

1.62 is pretty common. 50 mph at 4900 rpm = 10% slip. Check out the Merc Spitfire in the same pitch.
they are said to not require a pitch change.I read a couple of tests that indicate strong speed comparison.
They retained speed even on an already fast boat.

 

[Maclin]

Does the boat have trim tabs? If not, in my opinion it is a perfect candidate to be helped by those regarding getting out of the hole and planing sooner.

 

[H20Rat]

To circle back on the thread, ratio is 1.84. Original slip was 7% with the 3 blade prop. I bought a 4 blade solex rubex, 21 pitch, hoping it would drop it a couple hundred rpm to 4500 +/- 100 range. Well, the boat liked the prop, but the engine didn't! As far as the boat, I was able to maintain a stable cruise at 18 mph and it handled better overall. The engine though ended up at 4100/4200 rpm at around 42 mph, 7% slip again.

19 pitch rubex 4 is on the way!

 

[H20Rat]

Does the boat have trim tabs? If not, in my opinion it is a perfect candidate to be helped by those regarding getting out of the hole and planing sooner.

I've been really happy with smart tabs on various previous boats, this one just doesn't really have a good spot to mount them. The swim platform supports are in the way for the ideal spot, and the only remaining option would but the tab right behind a chine.

 

[Maclin]

Yeah, was wondering about that. Seem to be the biggest hurdle with getting tabs on.

 

[H20Rat]

Just to close out the thread, I found a prop that works well on the boat... I am currently running a Solex 4 bladed 19 pitch aluminum. The engine is at 4800 rpm, 45 mph on the gps. That puts it at 4% prop slip, so I'm guessing the tach isn't perfectly accurate, but I'm leaning more towards the pitch of the solex props is probably under-rated. (ie, 19 pitch is probably more like 20 pitch.) That would also explain why I took such a dive in rpm going from a 21 pitch 3 blade to a 21 pitch 4 blade.


In any case, summary is that I went from a 3 blade Merc OEM 21 pitch to a 4 blade 19 pitch solex, lost a couple MPH at the top end and maybe -100 rpm, but gained a lot out of the hole and a substantially lower planing speed. The solex also runs smoother and shifts with less of a clunk. (rubber hub)