Low RPM and Porpoising

[wiigelec]

Good day,

VP 4.3 GL on an 18 ft runabout WOT rated RPM 4200-4600.

Tried the following 2 props with results:

Mercury Tempest Plus SS 21” pitch - WOT RPM 4100, top speed ~40mph, cruising speed 3500 RPM 30 mph

Quicksilver 15x17 aluminum - WOT RPM >5000, top speed ~40mph, cruising speed 3500 RPM 25mph

In both cases the boat porpoises with minimal up trim. Looking for suggestions on a prop that will get within specified RPM range and help with porpoising. Considering a 4-blade aluminum 19” pitch.


Thanks

 

[QBhoy]

Are you trimming up ? Remember that tempest can take and hold so much more trim than the average prop. Just give it time to establish itself and keep a grip.

 

[wiigelec]

Yes just a few degrees up from vertical and the bow will start bouncing...

 

[QBhoy]

Throttle past the porpoise point. Usually only happens when trim too high for the speed you are doing. Or when it starts doing it, a nudge down will stop it.
If not, then you have too much weight in the back of the boat.

 

[wiigelec]

There is not much weight in the front it is better with a heavy person up there but usually just the wife and two young kids. I’m not overly concerned about the porpoising if it wants to run with minimal trim I’m ok with that. I have read in a couple places that a 4-blade prop gives more stern lift and might help a bit...

 

[Texasmark]

Rake, blades bent back from the root, is a bow lifting addition favored by Bass Boats whereby they run on a "pad" ....flat portion of hull along the keel area extending from the stern forward a given distance. Purpose is to get the hull out of the water for minimum contact, providing less drag, for 65 MPH performance, and bow lift is necessary to do that. Some bass boats run 4 and 5 blade props but usually have a big mil to turn them and the SS props are tuned for high speed operation on heavy stern BBs.

All 4 blades aren't built the same. As you said, some 4 blades are helpful getting heavy loads up on plane as do the Whale's Tail or trim tabs mounted to the transom. These props are usually larger in diameter, less pitch and no rake. I/Os are stern heavy and usually passengers love to sit in the jump seats adjacent to the engine cover making a problem worse. Flat stern hulls (no dead rise at the transom) tend to run faster with less hp but have a more sensitive trim/porpoising attitude. You have to either change load distribution (move some weight forward) in the boat, drop trim, or increase throttle to stop it. Deep V hulls aren't as susceptible as they ride more in rather than on the water.....all my opinion/experience.

 

[wiigelec]

Please opine on the following props:

Black Max 15.5 x 18
Black Max 14.5 x 19
Spitfire 14 x 19

Obviously not looking at performance props at this time just something to get engine running in appropriate rpm range. Mostly cruising between swimming spots and sight seeing, might pull the kids on a tube at some point.

If it matters boating at 6000’ elevation with a 1.89 gear box...

 

[Texasmark]

Google search specifically for HP reductions in your specific type of engine vs altitude. May wind up with listings from this site as I know it's been kicked around. I don't remember the numbers but at 6k' it's a different world. Sea level rules don't apply.

 

[wiigelec]

Lol yeah not as much air up here!

 

[wiigelec]

14x19 Nemesis 4-blade still a bit high on wot rpm (4800) but very smooth running! When time comes for stainless will go with aggressive 19p quad...

 

[Cortes100]

I've had good luck with the Spitfire/Nemesis 4 blade. FWIW, they're the same prop, Mercury and Quicksilver, one comes with hub kit, and the other does not.
I had a new BlackMax sitting on the shelf, so just for kicks wanted to compare to my Spitfire. Both 21 pitch. When it comes to RPM's, speed, etc., they were almost identical.
But when it comes to actual performance, the 4 blade Spitfire out handled it in every category. Better holeshot, great pulling power, less bow lift, holds plane at lower speed, handles tight turns. Overall, very noticeable difference between the 2.